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On the synergistic effect of hydrohalic acids in the shape-controlled synthesis of anatase TiO2 single crystals

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Abstract

Cuboid-like anatase TiO2 single crystals bounded by {001} and {100} facets have been successfully synthesized with HCl as a synergistic capping agent together with HF, under the guidance of theoretical calculations. DFT calculations suggest that the preferred replacement of HCl with HF at (100) as well as its favorable adsorption provide a unique stabilization effect on the formation of cuboid-like anatase TiO2.

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... known to produce platelet-shaped particles 17 dominated by {001} facets, whereas chloride can stabilize bi-pyramidal particles with an increased area of the {101} facets or even cuboids, showing the {100} and {001} facets when combined with fluoride. 18 In non-aqueous synthesis, carboxylic acids are used to provide the oxygen needed for TiO 2 synthesis. Rod-shaped particles can then be produced by adding amines to the solution. ...
... This suggests an alternative explanation for the cuboid shaped particles observed in mixtures of HCl and HF, which exhibit only {001} and {100} facets. 18,20 Instead of the face selective substitution of adsorbed chloride by fluoride proposed by Wu et al., 18 our calculations show that the cuboid structure is stabilized thermodynamically by the substitution of surface hydroxyl groups by chloride. ...
... This suggests an alternative explanation for the cuboid shaped particles observed in mixtures of HCl and HF, which exhibit only {001} and {100} facets. 18,20 Instead of the face selective substitution of adsorbed chloride by fluoride proposed by Wu et al., 18 our calculations show that the cuboid structure is stabilized thermodynamically by the substitution of surface hydroxyl groups by chloride. ...
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The ability to synthesize nanoparticles (NPs), here TiO2, of different shapes in a controlled and reproducible way is of high significance for a wide range of fields including catalysis and materials design. Different NP shapes exhibit variations of emerging facets, and processes such as adsorption, diffusion, and catalytic activity are, in general, facet sensitive. Therefore, NP properties, e.g., the reactivity of NPs or the stability of assembled NPs, depend on their shape. We combine computational modeling based on density functional theory with experimental techniques such as transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray powder diffraction to investigate the ability of various adsorbates, including hydrohalic and carboxylic acids, to influence NP shape. This approach allows us to identify mechanisms stabilizing specific surface facets and thus to predict NP shapes using computational model systems and to experimentally characterize the synthesized NPs in detail. Shape-controlled anatase TiO2 NPs are synthesized here in agreement with the calculations in platelet and bi-pyramidal shapes by employing different precursors. The importance of the physical conditions and chemical environment during synthesis, e.g., via competitive adsorption or changes in the chemical potentials, is studied via ab initio thermodynamics, which allows us to set previous and new results in a broader context and to highlight potentials for additional synthesis routes and NP shapes.
... It is thus highly desirable to develop a photoanode based on TiO 2 nanocubods. For this purpose, immense efforts have been devoted to the synthesis of powdered anatase TiO 2 nanocuboids based on solvothermal and hydrothermal methods [8,[14][15][16]. However, owing to the lack of continuous conducting channel, the main limitation in obtained higher photoelectrocatalytic activity in nanopowderbased electrodes is the transport of photogenerated charge across the nanoparticles [17,18]. ...
... mmedia)). Figure 2 the geometrical symmetries of anatase of TiO 2 and FE-TEM analysis, it is undoubtedly that the square top, isosceles trapezoidal and rectangular facets of nanocuboid are {0 0 1}, {1 0 1} and {1 0 0} facets, respectively [14][15][16]23]. ...
Article
Anatase TiO2 film consisting of nanocuboids with co-exposed {1 0 1}, {0 0 1} and {1 0 0} facets have been successfully synthesized via thermally annealing amorphous anodized TiO2 nanotube arrays in ambient fluorine. When employed as a photoanode material in photoelectrochemical water splitting, the film of the clean TiO2 nanocuoboids yields a photocurrent density of up to 0.65 mA cm⁻² at 0.22 V versus the Ag/AgCl electrode with Faradic efficiency of 100% and exhibits excellent stability, which can be attributed to enhanced photogenerated charge separation and transport to the collecting electrode. This film could also potentially be used for other facet-related applications such as TiO2 based dye-sensitized solar cells, sensors and lithium batteries.
... Since the reactivity of nanoparticles is directly related to their surface chemistry [21,22], the stability of highly reactive crystalline planes rather than the stable ones in equilibrium state would further improve the hydrogen adsorption capacity of graphene-nanoparticle systems. In the case of TiO 2 (anatase), the (001) facets are the most reactive surfaces which are rapidly eliminated during the crystal growth to minimize the total surface free energy of the crystal [21,23]. Therefore, utilization of chemical agents such as hydrofluoric acid (HF) that can bind to (001) facets would favor the occurrence of (001) rather than (101) planes in the synthesis of TiO 2 nanocrystals [23]. ...
... In the case of TiO 2 (anatase), the (001) facets are the most reactive surfaces which are rapidly eliminated during the crystal growth to minimize the total surface free energy of the crystal [21,23]. Therefore, utilization of chemical agents such as hydrofluoric acid (HF) that can bind to (001) facets would favor the occurrence of (001) rather than (101) planes in the synthesis of TiO 2 nanocrystals [23]. ...
Article
Homogeneously distributed TiO2 nanoparticles with (001) reactive facets were grown over nitrogen-doped reduced graphene oxide sheets (N-rGO) under solvothermal conditions. Hydrogen storage capacity of the system was significantly improved to 0.91 wt% at room temperature and pressure of 0.8 MPa that is the highest hydrogen storage ever reported for graphene-based nanocomposites at room temperature and low pressures. Importantly, this nanocomposite exhibits ∼91% capacity retention through 5 cycles with more than 88% release of the stored hydrogen at ambient conditions. Enhanced hydrogen uptake and capacity retention were attributed to the synergistic effect of i) reactive facets of TiO2, ii) high dispersion of nanoparticles with the average size of 6 nm, and iii) strong interaction between substrate and TiO2 nanoparticles through polarized C–N bonding of N-rGO sheets.
... Its moderate band gap of about 2.2 eV corresponds to a visible range of the light spectrum which makes it a good candidate for water photodecomposition. [56][57][58][59][60][61][62] (001) 0.98 1.53 (100) 0.58 1.37 1.194 (101) 0. 49 1. 31 28.80 (111) 0.677 (012) 1.06 0.80 (104) 1.45 atomic planar density [nm −2 ] [8] (001) 6.9821 4.5494 (100) 5.7651 10.997 (101) 5.1611 (110) 7.7618 (111) 6.3375 (113) 7.3047 (012) 5.7651 dangling bonds density [nm −2 ] [8] (001) 20.9463 13.6482 (100) 11.5302 21.954 (101) 10.3222 7.7618 (110) 6.3375 (111) (113) 21.9147 (012) 17.3953 ...
... Its moderate band gap of about 2.2 eV corresponds to a visible range of the light spectrum which makes it a good candidate for water photodecomposition. [56][57][58][59][60][61][62] (001) 0.98 1.53 (100) 0.58 1.37 1.194 (101) 0. 49 1. 31 28.80 (111) 0.677 (012) 1.06 0.80 (104) 1.45 atomic planar density [nm −2 ] [8] (001) 6.9821 4.5494 (100) 5.7651 10.997 (101) 5.1611 (110) 7.7618 (111) 6.3375 (113) 7.3047 (012) 5.7651 dangling bonds density [nm −2 ] [8] (001) 20.9463 13.6482 (100) 11.5302 21.954 (101) 10.3222 7.7618 (110) 6.3375 (111) (113) 21.9147 (012) 17.3953 ...
Article
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A relatively new approach to the design of photocatalytic and gas sensing materials is to use the shape-controlled nanocrystals with well-defined facets exposed to light or gas molecules. An abrupt increase in a number of papers on the synthesis and characterization of metal oxide semiconductors such as a TiO2, α-Fe2O3, Cu2O of low-dimensionality, applied to surface-controlled photocatalysis and gas sensing, has been recently observed. The aim of this paper is to review the work performed in this field of research. Here, the focus is on the mechanism and processes that affect the growth of nanocrystals, their morphological, electrical, and optical properties and finally their photocatalytic as well as gas sensing performance.
... Many attempts have been reported to improve the properties of TiO 2 in such a manner as to meet particular requirements relevant to these applications [7,8]. One of the methods is to control the shape of the nanocrystals and expose selected crystallographic facets characterized by different surface energies [9][10][11][12][13]. Nevertheless, the problem of the too wide band gap of TiO 2 (3.2 eV) corresponding only to the absorption of UV light remains unsolved. ...
Article
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Composites Theory and Practice (czasopismo naukowe Polskiego Towarzystwa Materiałów Kompozytowych) ; ISSN 2084-6096. 2020 vol. 20 no. 3–4, pp. 157–162.
... Belts Cuboid and belt-type titanium dioxide particles were synthesized according to the procedures published by Wu et al. [27] and Zhou et al. [28], respectively. Cuboids were produced using TiCl 4 and HF as a precursor and capping agent, respectively. ...
Article
Tai­lored crys­tals of anatase TiO2 have been syn­the­sized us­ing a hy­drother­mal method with hy­dro­flu­o­ric acid and sodium flu­o­ride as cap­ping agents. Ti­ta­nium diox­ide in the form of sheets, tetrag­o­nal trun­cated bipyra­mids, cuboids and belts of­fer dif­fer­ent ex­posed facets: {001}, {001}/ { 101}, {001}/ { 100} and {100}, re­spec­tively. The strongest oxy­gen ad­sorp­tion was ob­served for crys­tals with ex­posed {001} facets. Ad­sorbed flu­o­rides also im­prove oxy­gen ad­sorp­tion. Our spec­tro­elec­tro­chem­i­cal (SE-DRS) mea­sure­ments of den­sity of elec­tronic states (DOS) re­vealed that the en­ergy of con­duc­tion band edge in­creases in the or­der: {101}<{100}<{001}. Flu­o­ride ions sig­nif­i­cantly in­flu­ence the elec­tronic states dis­tri­b­u­tion, which in the case of sheets in­tro­duce new low en­ergy elec­tronic states. In the case of tetrag­o­nal trun­cated bipyra­mids the ef­fect is op­po­site: low en­ergy states pre­sent in flu­o­ride-free ma­te­ri­als dis­ap­pear upon F− ad­sorp­tion. More­over, grain bound­aries formed be­tween crys­tals in­tro­duce new low en­ergy elec­tronic states, which can act as deep elec­tron traps. Pho­tocur­rent mea­sure­ments have shown that the ox­i­da­tion of methanol oc­curs most ef­fi­ciently at TiO2 sheets of­fer­ing mainly 001 facets. The pho­to­cat­alytic ox­i­da­tion of tereph­thalic acid (test for HO gen­er­a­tion) also de­pends on the ex­posed facets and in­creases in the or­der: cuboids, sheets, tetrag­o­nal trun­cated bipyra­mids, belts. Fur­ther­more, flu­o­ride ions dras­ti­cally de­crease the pho­to­cat­alytic pro­duc­tion of TAOH.
... 26 Han et al. also have synthesized truncated bipyramidal anatase TiO 2 nanoparticles with square or rectangular bases by using K-titanate nanowires as the precursor and urea or ammonium carbonate as the capping agents. 27 In general, the syntheses of anatase crystals with exposed {001} facets and various morphologies, such as sheet, 8 rhombic-shaped, 26 truncated bipyramid, 27 hollow box, 28 sphere, 29 and cuboid, 30 were extensively studied. Hydrofluoric acid 7 or other fluorine-containing species were usually employed as capping agents, such as titanium fluoride, 8 ammonium fluoride, 31 sodium fluoride, 32 ammonium hexafluorotitanate, 33 and sodium fluoroborate. ...
Article
{010}-facet exposed anatase TiO2 crystals exhibit the highest photoreactivity among the exposed facets. To obtain a higher exposure rate of this facet, the work investigated the transformation of the nanosheets with cavities within the layers, derived from the salt-rock structured Li2TiO3 precursor. All the lithium ions were extracted from the precursor by the H+/Li+ ion exchange in HCl aqueous solutions, and after tetramethylammonium ions were intercalated, the precursor can delaminated into the nanosheets. The [TiO3]2- nanosheets were hydrothermally treated under different temperatures and pH values. The results showed that the anatase phase was formed in a wider range of pH and temperature, comparing with using the nanoribbons of [Ti4O9]2- and the nanosheets of [Ti1.73O4]2-. At low pH, the [111]-faceted nanorod-shaped anatase nanocrystals were formed preferentially, and the nanocrystals preferentially growth along the [001] direction with the increase of pH of the solution, leading to a large percentage of {010} facets on their surface. The photocatalytic activity increases with the increase of exposure rate of {010} facets.
... The reactive {0 0 1} facet on F127-TiO 2 photocatalyst structure favors good photocatalysis behavior [30]. Thus, pluronic F127 is effective as a crystallographic controlling agent in enhancing the formation of {0 0 1} facets of anatase, proven by the presence of truncated octahedral bipyramid structures (square shapes) as shown in Fig. 2b, as opposed to less reactive {1 0 1} facet represented by normal bipyramid structures (spheres with blunt edges and rhombuses) as shown in Fig. 2a [31,32]. Ong et al. [33] showed similar observations, whereby the geometrical of a single crystal consisting of flat and square surfaces represents {0 0 1} facets, while trapezoidal surfaces represents {1 0 1} facets. ...
Article
The utilization of triblock copolymer, pluronic F127 as a structure directing agent for the preparation of TiO 2 played an important role in enhancing the photocatalytic degradation rate of atrazine by a factor of 1.7. The mesoporous F127-TiO 2 showed significant modification of morphology, particle and crystallite size, and presence of defect energy belt within the catalyst forbidden band as proven via photolumine-scence spectra and x-ray photon spectroscopy. Hence the photogenerated carriers have longer lifespan to migrate to the catalyst surface for redox activities. Furtherance, surface reactive {0 0 1} facets proven by the formation of new geometrical single crystal of square and rhombus surfaces in F127-TiO 2 facilitates atrazine degradation as well. The increased surface area of F127-TiO 2 promotes greater atrazine absorption, thus governs improved interaction between absorbed atrazine molecules and surface generated active radicals as a prerequisite for good photocatalytic activity. Interestingly, using the same synthesis procedure, it was observed that the addition of pluronic F127 significantly affects anatase crystal structure as opposed to the more thermodynamically stable rutile, generating 61% and 25% of total crystallite size modification for anatase and rutile, respectively. However, there were no changes on the final composition of anatase and rutile crystal structure. In overall, enhancement of the photocatalytic degradation of atrazine is ruled out to the following factors (1) modification of geometrical structures and size, (2) narrowing of band gap due to defect energy belt, (3) longer lifespan of photoexcited charges to the catalyst surface, (4) enhanced surface textural properties and (5) increased exposure of reactive {0 0 1} facets, which were all observed in F127-TiO 2 .
... Inspired by these theoretical findings, high-purity anatase TiO 2 with a large percentage of (001) facets has been synthesized using wet chemical methods with fluorine-containing species [16]. More recently, using different dopants, adsorbates, or solvated species, anatase TiO 2 nanocrystals exposing various crystalline facets have been prepared, including low-index facets such as (100) [18][19][20][21], (010) [22][23][24], (101) [24][25][26][27][28][29][30], (110) [31], and (111) [32], and high-index facets such as (103), (105), (106), (201), (301), and (401) [33][34][35][36][37]. ...
Article
The reactivity of the anatase TiO2 (211) surface is systematically studied by ab initio calculations of the surface energy and water-adsorption energy. We find that anatase (211) has a high surface energy of 0.97 J/m2, close to that of the (001) surface, and the unsaturated fourfold-coordinated Ti4 atom is more reactive than the unsaturated fivefold-coordinated Ti5 atom. Accordingly, for water adsorption on the (211) surface, a dissociative form is favored on Ti4 sites, with a large adsorption energy ΔHH,OH∼1.28 eV, while a nondissociative molecular form is favored on Ti5 sites, with a smaller adsorption energy ΔHH2O∼0.78 eV. Such distinct surface properties lead to a mixed dissociative and molecular adsorption configuration when the coverage is increased from 1/3 to 1 monolayer. These results suggest that, similar to the (001) surface, the anatase (211) surface exhibits high reactivity and may be useful in catalytic and photocatalytic applications as well.
... Theoretical studies [13][14][15][16][17] show that the (101) surface is the thermodynamically most stable surface with a small surface energy of 0.49 J/m 2 , while the (001) surface is the highest reactivity surface with a high surface energy of 0.98 J/m 2 . More recently, using different dopants, adsorbates, or solvated species, anatase TiO 2 nanocrystals exposing various crystalline facets have been prepared, including low-index facets such as (001) facets [18,19], (100) [21][22][23][24], (010) [25,26], (101) [27][28][29][30][31][32], (110) [33], and (111) [34], and high-index facets such as (103), (105), (106), (201), (301), and (401) [35][36][37][38][39]. Besides, many researchers have made extensive investigation on surface defects since those defects, e.g., step edges [40][41][42] and O vacancy [43,44] are intrinsic on the surfaces of crystalline materials, which strongly influence the surface chemistry. ...
... Cl − and SO 4 2− anions are reported to preferentially adsorb on anatase planes. 35,36 Lowering the surface energy of the planes would additionally promote the growth of the truncated facet leading to the pyramidal shape of the particles. Since the pyramidal shape of the particles was obtained only after the employment of citric acid, the plausible explanation could be that citrate anions stabilize the anatase planes even better than other anions. ...
Article
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Transformation of layered protonated titanate (H2Ti3O7) nanoribbons to anatase nanoribbons under hydrothermal conditions is a catalyzed process where topotactic transformation and dissolution-recrystallisation compete. In the present work, the transformation was achieved with the enough high thermal input  calcination, or catalysis when performed under hydrothermal conditions. X-ray diffraction, scanning and transmission electron microscopy combined with crystallography of product materials revealed that the catalysis success depended on the suspension’s pH value as well on the counter ions present in the reaction mixture. The process of the transformation from protonated titanate to anatase under hydrothermal conditions is explained for the reaction medium pH range from 1 to 13.7 with the aid of the zeta potential of protonated titanate nanoribbons. Protonated titanate nanoribbons were stable under basic conditions and their transformation to anatase nanoribbons depended on the counter ions present in the reaction mixture and their concnetration as well. With decreasing the pH of the reaction mixture both processes – topotactic transformation and dissolution-recrystallisation – were accelerated. Upon the hydrothermal transformation the nanoribbons served as the substrate for the heterogeneous nucleation of anatase nanocrystals. Sodium titanate nanoribbons were found to be suitable for the transformation to anatase nanoribbons under hydrothermal conditions as well. Presence of water in the reaction mixture was revealed to be crucial for the transformation progress under hydrothermal/solvothermal conditions since used organic solvents served only as a thermal reaction medium and did not promote the catalytic transformation.
... Inorganic nanocrystals with tailored morphologies and specific facets have received much attention in the past decade due to their many intrinsic shape-dependent properties and excellent technological applications in energy and environmental fields [1,2]. As one of the most studied semiconductor metal Materials 2019, 12, 3614; doi:10.3390/ma12213614 ...
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A facile one-pot microwave-assisted hydrothermal synthesis of rutile TiO2 quadrangular prisms with dominant {110} facets, anatase TiO2 nanorods and square nanoprisms with co-exposed {101}/[111] facets, anatase TiO2 nanorhombuses with co-exposed {101}/{010} facets, and anatase TiO2 nanospindles with dominant {010} facets were reported through the use of exfoliated porous metatitanic acid nanosheets as a precursor. The nanostructures and the formation reaction mechanism of the obtained rutile and anatase TiO2 nanocrystals from the delaminated nanosheets were investigated. The transformation from the exfoliated metatitanic nanosheets with distorted hexagonal cavities to TiO2 nanocrystals involved a dissolution reaction of the nanosheets, nucleation of the primary [TiO6]8- monomers, and the growth of rutile-type and anatase-type TiO2 nuclei during the microwave-assisted hydrothermal reaction. In addition, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated through the photocatalytic degradation of typical carcinogenic and mutagenic methyl orange (MO) under UV-light irradiation at a normal temperature and pressure. Furthermore, the dye-sensitized solar cell (DSSC) performance of the synthesized anatase TiO2 nanocrystals with various morphologies and crystal facets was also characterized. The {101}/[111]-faceted pH2.5-T175 nanocrystal showed the highest photocatalytic and photovoltaic performance compared to the other TiO2 samples, which could be attributed mainly to its minimum particle size and maximum specific surface area.
... The reactive {0 0 1} facet on F127-TiO 2 photocatalyst structure favors good photocatalysis behavior [30]. Thus, pluronic F127 is effective as a crystallographic controlling agent in enhancing the formation of {0 0 1} facets of anatase, proven by the presence of truncated octahedral bipyramid structures (square shapes) as shown in Fig. 2b, as opposed to less reactive {1 0 1} facet represented by normal bipyramid structures (spheres with blunt edges and rhombuses) as shown in Fig. 2a [31,32]. Ong et al. [33] showed similar observations, whereby the geometrical of a single crystal consisting of flat and square surfaces represents {0 0 1} facets, while trapezoidal surfaces represents {1 0 1} facets. ...
Article
The utilization of triblock copolymer, pluronic F127 as a structure directing agent for the preparation of TiO 2 played an important role in enhancing the photocatalytic degradation rate of atrazine by a factor of 1.7. The mesoporous F127-TiO 2 showed significant modification of morphology, particle and crystallite size, and presence of defect energy belt within the catalyst forbidden band as proven via photolumine-scence spectra and x-ray photon spectroscopy. Hence the photogenerated carriers have longer lifespan to migrate to the catalyst surface for redox activities. Furtherance, surface reactive {0 0 1} facets proven by the formation of new geometrical single crystal of square and rhombus surfaces in F127-TiO 2 facilitates atrazine degradation as well. The increased surface area of F127-TiO 2 promotes greater atrazine absorption, thus governs improved interaction between absorbed atrazine molecules and surface generated active radicals as a prerequisite for good photocatalytic activity. Interestingly, using the same synthesis procedure, it was observed that the addition of pluronic F127 significantly affects anatase crystal structure as opposed to the more thermodynamically stable rutile, generating 61% and 25% of total crystallite size modification for anatase and rutile, respectively. However, there were no changes on the final composition of anatase and rutile crystal structure. In overall, enhancement of the photocatalytic degradation of atrazine is ruled out to the following factors (1) modification of geometrical structures and size, (2) narrowing of band gap due to defect energy belt, (3) longer lifespan of photoexcited charges to the catalyst surface, (4) enhanced surface textural properties and (5) increased exposure of reactive {0 0 1} facets, which were all observed in F127-TiO 2 .
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A novel fluoride free protocol for highly truncated anatase TiO2 nanocrystals with exposed {001} facets by preferential adsorption of CO3(2-) ions is developed. Experimental observations were corroborated by first principle quantum chemical DFT calculations. The synthesized anatase TiO2 showed improved photocatalytic activity.
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TiO2-SiO2 composites containing 10 wt.%, 20 wt.%, 30% and 40 wt.% of TiO2, obtained by using preformed mesoporous silica nanoparticles MSNs and titanium isopropoxide as titanium source, have been investigated in detail using a variety of techniques. All the samples were characterized by N2-physisorption, X-ray powder diffraction (XRPD), diffusive reflective UV–vis spectroscopy (DRUV-vis), X-ray photoelectron spectroscopy (XPS) and imaged using transmission electron microscopy (TEM). The TiO2-MSN composites, that exhibited a spherical morphology, high specific surface areas and titania in the anatase phase, owing to their specific chemical-physical properties were studied as catalysts in the photocatalytic degradation of Methylene Blue, Methyl Orange and Paracetamol, as examples of polluted wastewaters. The well-defined porous structures of MSNs may offer a special environment for titania nanoparticles, increasing the specific surface area and the thermal stability of the composite, thus modifying the photocatalytic behavior of the materials. The TiO2 loading, the particle size and the surface characteristics were related to the degree of UV absorption and the measured energy band gap of the nanocomposites. A cooperative effect between the two components (TiO2 and SiO2) could be the key factor at the basis of the good photocatalytic performances: nanostructured TiO2 in intimate contact with MSN provides the sites for generation of OH• radicals by oxidation of water and the SiO2 skeleton is able to adsorb the molecules of cationic dyes and prevent poisoning of the TiO2 surface.
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Anatase titanium dioxide (A-TiO2) is one of the most important functional materials and widely used in various energy- and environmental related applications. Over the past decade, great efforts have been devoted to surface engineering of A-TiO2 crystals at the atomic level so as to fundamentally illuminate the relationship between the surface structure and their performance in practical applications. In this review we briefly summarize recent important achievements on the control of specific surface structures of A-TiO2 crystals, focusing on facets with high surface energy (such as {001}, {100}, {10l}) and their combinations. In addition, fascinating performances of A-TiO2 crystals enhanced by these high energy surfaces are examined and discussed through the perspectives of synergistic effects of different facets and surface adsorbates, with additional insights related to some contradictory results. Lastly, we offer a summary and some perspective on current challenge and promising directions in this emerging field. We believe that a comprehensive understanding of surface engineering of A-TiO2 crystals with regard to high energy facets will in the long term help us to rationally design functional nanomaterials with desired performances.
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An interface between toluene and water was utilized to synthesize ca. 10 nm thick of anatase TiO2 nanosheets (NSs) with 82% exposure of {001} facets. In this procedure, highly corrosive and toxic HF, which was generally used to prepare TiO2 NSs with largely exposed high energy facets, was avoided. Furthermore, the surfaces of the NSs were quite clean as suggested by XPS analysis. Serving as anode materials in lithium-ion batteries, these as-prepared anatase TiO2 NSs manifested a low initial irreversible capacity loss (12.5% at 1 C), an excellent capacity retention at 10 C charge-discharge rate (101.9 mA h g-1 after 100 cycles), and enhanced rate performance at 0.5-10 C current rates in compared with Degussa P25 TiO2 nanoparticles (NPs). Their excellent electrochemical performances were mainly derived from the large proportion of {001} exposed facets and a very short diffusion pathway, which allowed fast and efficient Li+ transportation in the electrodes.
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Owing to its scientific and technological importance, crystallization as a ubiquitous phenomenon has been widely studied over centuries. Well-developed single crystals are generally enclosed by regular flat facets spontaneously to form polyhedral morphologies because of the well-known self-confinement principle for crystal growth. However, in nature, complex single crystalline calcitic skeleton of biological organisms generally has a curved external surface formed by specific interactions between organic moieties and biocompatible minerals. Here we show a new class of crystal surface of TiO2, which is enclosed by quasi continuous high-index microfacets and thus has a unique truncated biconic morphology. Such single crystals may open a new direction for crystal growth study since, in principle, crystal growth rates of all facets between two normal {101} and {011} crystal surfaces are almost identical. In other words, the facet with continuous Miller index can exist because of the continuous curvature on the crystal surface.
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A facile hydrothermal route employing H2O2 as structure-directing agent was explored to fabricate anatase microflowers with dominant {101} facets and anatase microspheres with exposed {001} facets. The influence of H2O2 concentration on crystal structure, morphology, and facet composition of TiO2 was investigated in detail. H2O2 plays a crucial role in determining the crystal structure, morphology, and exposed facets of TiO2. The presence of H2O2 favors the formation of anatase phase. When the concentration of H2O2 was in the range 0.7–3.3 M, anatase microflowers with dominant {101} facets were produced. In contrast, when the concentration of H2O2 was higher than 6.6 M, anatase microspheres with exposed {001} facets were formed. A mechanism was proposed to account for the influence of H2O2 on crystal structure and morphology of TiO2. Photocatalytic degradations of rhodamine B and 2,4-dichlorophenol indicated that anatase microspheres with exposed {001} facets showed much higher photocatalytic activity than anatase microflowers with dominant {101} facets.
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Uniform rhombic anatase TiO2 nanoplates with selectively exposed {010} facets were successfully fabricated using a novel solvothermal method without adding surfactant or templates. In our synthetic approach, the coordination effect of acetic acid (HAc) and N,N−dimethyl formamide (DMF) regulated the reaction rate and contributed to the formation of TiO2 nanoplates. The TiO2 samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectra (IR), UV-visible diffuse reflectance spectroscopy (DRS), Brunauer–Emmett–Teller (BET) specific surface areas and photoluminescence spectra (PL). The TEM characterization results suggested that the growth of rhombic nanoplates was governed by a nucleation–crystallization–dissolution growth mechanism. The mechanism demonstrated that both the reaction time and the HAc/DMF ratio played crucial roles in controlling size and shape of TiO2 nanocrystals. By tuning the ratio of HAc to DMF, reaction time and reaction temperature, the photocatalytic performance of the as−synthesized TiO2 samples was improved. The enhanced photocatalytic activity could be ascribed to the exposed {010} facets and the high surface areas of the TiO2 nanoplates .
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Titanium dioxide (TiO2) is one of the most widely investigated metal oxides due to its extraordinary surface, electronic and catalytic properties. However, the large band gap of TiO2 and massive recombination of photogenerated electron-hole pairs limit its photocatalytic and photovoltaic efficiency. Therefore, increasing research attention is now being directed towards engineering the surface structure of TiO2 at the most fundamental and atomic level namely morphological control of {001} facets in the range of microscale and nanoscale to fine-tune its physicochemical properties, which could ultimately lead to the optimization of its selectivity and reactivity. The synthesis of {001}-faceted TiO2 is currently one of the most active interdisciplinary research areas and demonstrations of catalytic enhancement are abundant. Modifications such as metal and non-metal doping have also been extensively studied to extend its band gap to the visible light region. This steady progress has demonstrated that TiO2-based composites with {001} facets are playing and will continue to play an indispensable role in the environmental remediation and in the search for clean and renewable energy technologies. This review encompasses the state-of-the-art research activities and latest advancements in the design of highly reactive {001} facet-dominated TiO2via various strategies, including hydrothermal/solvothermal, high temperature gas phase reactions and non-hydrolytic alcoholysis methods. The stabilization of {001} facets using fluorine-containing species and fluorine-free capping agents is also critically discussed in this review. To overcome the large band gap of TiO2 and rapid recombination of photogenerated charge carriers, modifications are carried out to manipulate its electronic band structure, including transition metal doping, noble metal doping, non-metal doping and incorporating graphene as a two-dimensional (2D) catalyst support. The advancements made in these aspects are thoroughly examined, with additional insights related to the charge transfer events for each strategy of the modified-TiO2 composites. Finally, we offer a summary and some invigorating perspectives on the major challenges and new research directions for future exploitation in this emerging frontier, which we hope will advance us to rationally harness the outstanding structural and electronic properties of {001} facets for various environmental and energy-related applications.
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This paper proposes a new simple approach for synthetic of shape-defined anatase nanocuboids TiO2 by using combination of titanium tetra isopropoxide (TTIP) and orthotitanic acid (H4TiO4) as titanium precursor. In first step by adding precursor to ethanol solution mixed with HCl the primary sal has been prepared, therefore two other different films by adding Methylcellulose (MC) as a carbon containing material and adding TiO2 nanopowder as nanofiller have been prepared and properties of thin film as a function of parameters of annealing temperature and additives, have been discussed. The films were deposited on glass substrates and characterized by using UV-vis spectroscopy and scanning electron microscopy (SEM). SEM results show that optical properties of TiO2 thin films were changed by baking in 300,400 and 500 degrees C as annealing temperature. Moreover, they indicate that the additives have strong effect on anatase structure and therefore influence the optical properties.
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Crystalline titanium dioxide was synthesised under mild conditions by the thermal degradation of peroxotitanic acid in the presence of a number of fluoride-containing surface modifying agents (NH4F, NH4BF4, NH4PF6, NBu4F, NBu4BF4, NBu4PF6). The resulting materials were characterised by PXRD, SEM, HRTEM, XPS and NEXAFS. Particle phase, size, and surface area were noticeably affected by the choice of surface modifying agent. Both the cation and anion comprising the modifying agent affect the surface Ti3+ population of the materials, with two apparent trends observed: F− > BF4− > PF6− and NBu4+ > NH4+. All materials displayed evidence of fluorine doping on their surfaces, although no evidence of bulk doping was observed.
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In this work, we have succeeded in preparing rutile and anatase TiO2 mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single crystal structures, including rutile mesoporous TiO2 nanorods with tunable sizes and anatase mesoporous TiO2 nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size and phase of the TiO2 mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO2 mesoporous single crystals, photocatalytic activity was tested. The resultant TiO2 mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single crystal nature, the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {110} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO2 single crystals hold great promise for building energy conversion devices and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO2.
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Clean and efficient hydrogen production is of great interest because hydrogen is envisioned as the fuel of the future. In particular, hydrogen production from biomass-derived alcohols has attracted great attention because of the potential application in fuel cells. In this short review, the major results obtained in the last years by the Material, Environment and Energy (MEE) research group at the University of Trieste (Italy) in the photocatalytic production of hydrogen are summarized. Our attention has been devoted to the use of biomass-derived oxygenated compounds (mainly ethanol and glycerol) as sacrificial agents to improve hydrogen production. Various synthetic techniques (sol–gel, hydrothermal synthesis etc.) have been adopted to prepare nanostructured TiO2-based photocatalysts with different phase composition and/or morphology in the form of powders. Different strategies have been adopted to improve the performances of TiO2-based materials, especially favoring the photocatalytic activity under simulated sunlight. Metal nanoparticles (Cu, Pt, Au, Pd), self-doping of TiO2 and hierarchically organized nanocomposite with carbon nanotubes strongly improve the hydrogen production. The results will highlight the role of different parameters (phase composition, morphology, doping and nanocomposite formulation) in the improvement of photocatalytic hydrogen production.
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As an important metal oxide, titanium dioxide (TiO2) materials, particularly in anatase phase, have been extensively investigated and utilized in many technological applications due to its excellent physicochemical properties. The surface properties, as well as the performance in various applications are largely determined by external surfaces exposed, and thus crystal shape engineering is paramount for this type of materials. This review summarizes the recent progress in designing and fabricating anatase TiO2 crystals from the perspectives of thermodynamics and kinetics controlled routes. Furthermore, the bio-inspired technique is also highlighted which would open a new field for the growth of single crystals with unique morphology. The strength of crystal shape engineering of anatase TiO2 in biomedical applications is also identified that opportunities for continued research and specific areas where significant advancements are needed.
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We report a facile organic-free hydrothermal strategy to fabricate rutile TiO2 single crystals with a new type of curved surfaces which consist of high-energy-facet-bound nanohills rather than quasi-continuous miller-index microfacets. Moreover, by solely altering the concentration of F-, the proportion of facets as well as the aspect ratio of TiO2 could be readily controlled. This journal is
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TiO2 films with exposed {001} facets and uniform nanopores in the crystal facets were fabricated on a Ti substrate using the hydrothermal process followed by calcination at 600 °C. The effects of pretreatment of the Ti substrate and the hydrothermal conditions (volume ratio of water/isopropanol, hydrothermal time, temperature and HF concentration) on the formation of nanopores in the facets of the TiO2 crystals were investigated. The sizes of the nanopores can be controlled by adjusting the hydrothermal temperature. TG/DTA-MS analysis indicates that HF and isopropanol adsorbed on the Ti substrate are combusted into gas and released during calcination. The effects of cleaning and heat treatment of the hydrothermally treated Ti substrate indicate that the presence of residual organic matter and HF in the TiO2 facets and their transformation and reaction during calcination are essential for nanopore formation on the various facets of the TiO2 crystals. The as-prepared nanoporous TiO2 films with exposed {001} facets exhibit considerably higher UV photocatalytic activity for the degradation of pharmaceuticals and personal care products than non-porous TiO2 films with exposed {001} facets and P25 coated on the titanium sheet, revealing that the rational design and fabrication of the nanoporous TiO2 films is an effective method to improve the photocatalytic activity.
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Anatase TiO2 nanosheets (NSs) with high surface area have been prepared via a one-step thermal decomposition of titanium tetraisopropoxide (TTIP) in oleylamine (OM), and their adsorption capacities and photocatalytic activities are investigated by using methylene blue (MB) and methyl orange (MO) as model pollutants. During the synthesis procedure, only one type of surfactant, oleylamine (OM), is used as capping agents and no other solvents are added. Structure and properties of the TiO2 NSs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption analysis, UV-vis spectrum, X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) methods. The results indicate that the TiO2 NSs possess high surface area up to 378m2 g-1. The concentration of capping agents is found to be a key factor controlling the morphology and crystalline structure of the product. Adsorption and photodegradation experiments reveal that the prepared TiO2 NSs possess high adsorption capacities of model pollutants MB and high photocatalytic activity, showing that TiO2 NSs can be used as efficient pollutant adsorbents and photocatalytic degradation catalysts of MB in wastewater treatment.
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Cuboid‐shaped anatase TiO2 nanocrystals with co‐exposed {101} and [111]‐facets (or {101} and {100} facets), diamond‐shaped, rod‐like and irregular anatase TiO2 nanocrystals with dominant {010} facets were controllably synthesized from titanium (IV) ethoxide (TEOT) using HF as a capping agent via a facile sol‐gel procedure, and characterized by X‐ray powderdiffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), nitrogen adsorption, photoluminescence (PL) spectra, UV‐vis diffuse reflectance spectra,and electrochemical impedance spectroscopy (EIS). For the as‐prepared anatase TiO2 nanocrystals, the 0.5HF‐TiO2 revealed the best photocatalytic efficiency under UV light irradiation: 94.1 % of methylene blue (MB) was photodegraded within 90 min. The reaction rate constant k(min−1) was 1.39, 1.47, 1.59, 1.66, 1.92, 2.75, and 30.5 times higher than that of 2.0HF‐TiO2 (2.20×10−2 min−1), 1.5HF‐TiO2 (2.07×10−2 min−1), 1.0HF‐TiO2 (1.92×10−2 min−1), 2.5HF‐TiO2 (1.84×10−2 min−1), 3.0HF‐TiO2 (1.59×10−2 min−1), Bodi‐TiO2 (1.11×10−2 min−1), and the blank sample (0.10×10−2 min−1), respectively. Compared to other as‐prepared anatase TiO2 nanocrystals with the similar co‐exposed crystal facets, the highest efficiency of 0.5HF‐TiO2 could be attributed to the minimum crystal size,narrower pore size distribution, the maximum specific surface area, the lowest PL intensity and charge‐transfer resistance, andthe smallestrecombination resistance. Anatase TiO2 nanocrystals with co‐exposed {101}, {010}/{100} and [111]‐facets was successfully achieved using sol‐gel method. The 0.5HF‐TiO2exhibited the highest photocatalytic degradation efficiency (94.1 %), which was about 1.08, 1.10, 1.12, 1.12, 1.18, 1.45, and 10.01 times higher than that of the 2.0HF‐TiO2 (87.0 %), 1.5HF‐TiO2 (85.9 %), 1.0HF‐TiO2 (84.3 %), 2.5HF‐TiO2 (84.2 %), 3.0HF‐TiO2 (79.5 %), Bodi‐TiO2 (64.9 %), and the blank sample (9.4 %), respectively.
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Improving the performance of mostly employed anatase TiO2 photocatalysts by properly controlling their crystal shape represents a big challenge to improve their efficiency in photocatalytic applications. After the synthesis, reported in 2008, of anatase microcrystals enriched in high-energy {001} facets, many efforts have been made aimed at tuning the crystal morphology of anatase, by means of either fluorine-mediated or more environmentally friendly methods, producing a deviation from its regular crystal growth. In this relatively new field of investigation, controversial opinions emerged concerning the role of each type of facet and its relative amount in relation to photoefficiency optimization. This review addresses this topic by presenting a critical survey of selected literature reports. After a brief introduction on the main synthetic strategies adopted to obtain shape-controlled anatase photocatalysts, the attention is focused on the methods employed for their comprehensive characterization, including the identification and quantification of exposed facets and the assessment of their influence on bulk and surface properties relevant to photoactivity. Potential interferences, derived from synthetic routes and possibly affecting the conclusions of facet-dependent photoactivity investigations, are also discussed. Key examples of test reactions actually demonstrating how both the type and/or the amount of specific facets influence photocatalytic activity are finally reported, aiming at providing rational bases for the design of better performing shape-controlled anatase photocatalysts.
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F-doped TiO2 nanorods (F-TiO2 NRs) were prepared through a simple hydrolysis of titanium chloride (TiCl4) and a refluxing process at 100 °C and atmospheric pressure in the presence of sodium fluoride (NaF). The prepared samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis spectrophotometry and N2 adsorption analysis. It is found that F⁻ ions are incorporated into the lattice of TiO2 and absorbed on the surface of TiO2 NRs. The NaF/TiCl4 molar ratio is found to play an important role in the formation of the anatase F-TiO2 NRs. When the molar ratio of NaF/TiCl4 is 4.0/1.0, the prepared F-TiO2 NRs show the highest photocatalytic activity for the degradation of methylene blue (MB) under UV and visible irradiation, showing that F-TiO2 NRs are efficient photocatalytic degradation catalysts of MB pollutant in wastewater treatment.
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As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single- or dual-layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO2/TiO2 GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO2 and TiO2 networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO2 network to the high electron mobility SnO2 network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO2 ETL constructed under the comparable conditions.
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Crystal growth rates depend on the density of kink sites along step edges, ρ. The supersaturation-dependence of the kink density, ρ(S), is often ignored despite available non-equilibrium models for centrosymmetric crystals. The aim of this work is to derive ρ(S) from a rather simple 1D nucleation framework in a manner that accounts for multi-height kinks. These become more prevalent at higher S and/or lower kink energies where the step front roughens (i.e., higher ρ). We arrive at a density of kinetic kinks that contribute to the step velocity and make comparisons with other models and simulations. Our model almost entirely eliminates systematic errors in predicted step velocities and crystal growth rates.
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Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called “materials gap” and “pressure gap” when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the “materials gap” and “pressure gap” between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO2, TiO2 and Cu2O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO2, TiO2 and Cu2O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible.
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Anatase titanium dioxide, as one of the most well-known functional materials, is widely utilized in various fields of the environment and energy. In recent decades, much attention has been attracted to the surface engineering of the anatase crystal at a microscopic level for a better understanding of the relationship between surface and photocatalytic activity. In this review, a brief description is presented for the synthesis of the low-index facets ({0 0 1}, {1 0 0}, {1 0 1}, {1 1 0} and {1 1 1}) and high-index facets. Besides, the comparison between the photocatalytic activity of the faceted anatase and common types of TiO2 (mostly P25) is performed in this review. Meanwhile, a comprehensive study for the comparison between photocatalytic performance (organic pollutants photodegradation and water splitting) of the various types of the low-index facets and high-index facets is performed. The location of the band edge and band gap value in semiconductors during photocatalytic reaction play an important role in energy requirements for electron transfer and the light absorption respectively; so, band gap and band edge location in various types of anatase TiO2 facets are investigated. The recombination rate of the photogenerated electron–hole pairs is commonly described as one of the main limiting factors for the photocatalytic activity of the TiO2. In this review, the effect of the tailored facets of anatase crystal on the charge carrier’s separation as a promising strategy is completely described. Finally, the novel ideas for future studies on the facets engineering of the anatase crystals are also suggested.
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The synthesis of mesoporous TiO2 nanosheets is reported using Ti(IV) Isopropoxide as Ti(IV) precursor. A sol-gel process combined with microwave activation is used. Three different halogenhydric acids (HX), were used to peptise the sol: HF(ac), HCl (ac) and HBr (ac). The three obtained TiO2-I(HX) samples were characterized by XRD, XRF, N2-adorption, SEM, TEM, DRS and XPS. The three synthesized samples have high values of specific surfaces (between 100 m²/g and 200 m²/g) and similar band gap values (3.2-3.3 eV). The analysis of the surface composition by XPS confirms the presence of the halogenated species (F, Cl or Br) on the surface of each ones of the samples. The nanometric size (ca 5 nm) of the particles for each of the three samples was confirmed by XRD and by TEM. On the other hand, the nature of the halogenated acid used plays a role in the composition of the phases. While the TiO2-I (HF) sample was 100% anatase, the other samples turned out to be biphasic, showing anatase/rutile in the TiO2-I(HCl) sample and anatase/brookite in the TiO2-I(HBr) sample. The samples were tested under two illumination conditions (UV and visible light) using rhodamine B and caffeine. The indirect role of the halide agent on the photocatalytic activities thereof is discussed.
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In this work, a poly(ethylene glycol)-b-poly(1H,1H,7H-dodecafluoroheptyl methacrylate) (PEG-b-PDFMA) block copolymer was first synthesized by the reversible addition−fragmentation chain transfer (RAFT) polymerization. Then a novel facile approach was developed to fabricate oval cuboid TiO2 particles with mesoporous structure by using the PEG-b-PDFMA block copolymer as a template and titanium tetrabutoxide (TBOT) as a precursor, followed by evaporation-induced self-assembly (EISA) process and calcination process. The results show that the PEG-b-PDFMA block copolymer can control the oriented assembly of nanoparticles and act as templates for the formation of a mesopore. It is found that the mass ratio of TBOT/PEG-b-PDFMA and water content in the solution have a significant influence on the morphology of TiO2 particles. When the mass ratio of TBOT/PEG-b-PDFMA is 0.25/1, oval cuboid TiO2 particles with mesopores are obtained, which exhibits a high photocatalytic activity for the degradation of methylene blue (MB) dye under UV light irradiation.
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This work reports on a simple hydrothermal method for tuning the shape and exposed face of TiO2 nanocrystal by using sodium titanate as a precursor in the presence of different alkali metal ion salts. The {101}-exposed anatase TiO2 was synthesized in lithium salt solution. The formation of anatase TiO2 with exposed high-index {301} facet, obtained in the presence of sodium ions, has never before been reported upon. When the reaction proceeded in potassium salt solution, the obtained anatase TiO2 with a high aspect ratio was elongated in the <001> direction. Anatase TiO2 with a high aspect ratio was obtained in hydrothermal process in the presence of low charge density high atomic number alkali metal ions, which were not easily absorbed by the {001} planes of anatase, so the growth along this direction was not restrained. The photodegradation of methylene blue, assisted by anatase TiO2 with various exposed faces reveals that the photoactivity of anatase {101} planes is larger than that of {301} planes.
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We present an ab initio density-functional investigation of the structure and energetics of several stoichiometric 1×1 low-index surfaces of anatase, a TiO2 polymorph ∼9% less dense and ∼1.2 kcal/mol less stable than rutile. Although our calculations do not reproduce the relative ordering of the two phases that is observed experimentally, the calculated bulk structural and elastic properties of both polymorphs are in excellent agreement with the experiment, suggesting that surface relaxations are correctly described as well. As expected, the surface energies of anatase appear to be related to the presence of undercoordinated Ti atoms: the surfaces with fourfold-coordinated Ti atoms have a larger energy than those with fivefold-coordinated Ti. Furthermore, we find that the average surface energy of a TiO2 anatase macroscopic crystal is smaller than that of rutile. Finally, patterns in the relaxation of the surface atoms which are common to different surfaces are analyzed.
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Single crystalline anatase TiO(2) rods with dominant reactive {010} facets are directly synthesized by hydrothermally treating Cs(0.68)Ti(1.83)O(4)/H(0.68)Ti(1.83)O(4) particles. The nanosized rods show a comparable conversion efficiency in dye-sensitized solar cells (DSSCs), and a superior photocatalytic conversion of CO(2) into methane to the benchmark P25 TiO(2) nanocrystals.
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Crystal facet engineering of semiconductors has become an important strategy for fine-tuning the physicochemical properties and thus optimizing the reactivity and selectivity of photocatalysts. In this review, we present the basic strategies for crystal facet engineering of photocatalysts and describe the recent advances in synthesizing faceted photocatalysts, in particular TiO(2) crystals. The unique properties of faceted photocatalysts are discussed in relation to anisotropic corrosion, interaction dependence of adsorbates, photocatalytic selectivity, photo-reduction and oxidation sites, and photocatalytic reaction order. Ideas for future research on crystal facet engineering for improving the performance of photocatalysts are also proposed.
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Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. A typical example is titanium dioxide (TiO2), which has promising energy and environmental applications. Most available anatase TiO(2) crystals are dominated by the thermodynamically stable {101} facets (more than 94 per cent, according to the Wulff construction), rather than the much more reactive {001} facets. Here we demonstrate that for fluorine-terminated surfaces this relative stability is reversed: {001} is energetically preferable to {101}. We explored this effect systematically for a range of non-metallic adsorbate atoms by first-principle quantum chemical calculations. On the basis of theoretical predictions, we have synthesized uniform anatase TiO(2) single crystals with a high percentage (47 per cent) of {001} facets using hydrofluoric acid as a morphology controlling agent. Moreover, the fluorated surface of anatase single crystals can easily be cleaned using heat treatment to render a fluorine-free surface without altering the crystal structure and morphology.
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Highly uniformed tetragonal faceted-nanorods (TFNRs) of anatase TiO2 enclosed by active {100} facets with narrow size distribution are obtained with high-yield (>90%). In addition, hierarchically structured anatase TiO2 which consist of TFNRs are also prepared, and the sizes of the TFNR components could be tuned.
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THE large-scale use of photovoltaic devices for electricity generation is prohibitively expensive at present: generation from existing commercial devices costs about ten times more than conventional methods1. Here we describe a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits a commercially realistic energy-conversion efficiency. The device is based on a 10-µm-thick, optically transparent film of titanium dioxide particles a few nanometres in size, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. Because of the high surface area of the semiconductor film and the ideal spectral characteristics of the dye, the device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to electrical current (more than 80%). The overall light-to-electric energy conversion yield is 7.1-7.9% in simulated solar light and 12% in diffuse daylight. The large current densities (greater than 12 mA cm-2) and exceptional stability (sustaining at least five million turnovers without decomposition), as well as the low cost, make practical applications feasible.
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This Progress Report highlights recent developments in dye-sensitized solar cells composed of both liquid electrolytes and solid-state hole transport materials. The authors discuss and review the present understanding of and recent developments in the operational processes, such as charge generation, transport, recombination, and E charge collection. Also, the merits and challenges of alternative device approaches are discussed, including extremely thin absorber cells, devices containing inorganic p-type hole-transporters and non-TiO2 mesoporous metal-oxide electrodes employed in dye-sensitized solar cells.
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Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed. This paper starts with a summary of the wide variety of technical fields where TiO2 is of importance. The bulk structure and bulk defects (as far as relevant to the surface properties) are briefly reviewed. Rules to predict stable oxide surfaces are exemplified on rutile (1 1 0). The surface structure of rutile (1 1 0) is discussed in some detail. Theoretically predicted and experimentally determined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear planes—CSPs) are discussed, as well as the image contrast in scanning tunneling microscopy (STM). The controversy about the correct model for the (1×2) reconstruction appears to be settled. Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different TiO2(1 1 0) surfaces are given. The structure of the TiO2(1 0 0)-(1×1) surface is discussed and the proposed models for the (1×3) reconstruction are critically reviewed. Very recent results on anatase (1 0 0) and (1 0 1) surfaces are included.
Article
Uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets over a wide size range (60-830 nm in length) with controllable aspect ratios were solvothermally synthesized through hydrolysis of titanium tetraisopropoxide (TTIP) using acetic acid (HAc) as the solvent and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) as the capping agent. The size and aspect ratio of the anatase TiO2 nanocuboids can be readily adjusted by changing the composition parameters including the contents of [bmim][BF4], water, and HAc in the quaternary solution system. It was revealed that [bmim][BF4] played an important role in stabilizing both the {100} and {001} facets of the anatase TiO2 nanocuboids. On the one hand, [bmim][BF4] acted as a fluoride source to release F-ions for stabilizing the {001} facets; on the other hand, the [bmim](+) ions acted as effective capping ions to preferentially stabilize the {100} facets. The obtained near-monodisperse anatase TiO2 nanocuboids exhibited an interesting self-assembly behavior during deposition. These single-crystalline anatase nanocuboids showed extremely high crystalline phase stability, retaining the pure phase of anatase as well as the morphology even after being calcined at 900 degrees C. Moreover, the anatase nanocuboids exhibited considerably enhanced photocatalytic activity owing the wholly exposed active {100} and {001} facets.
Article
Control over faceting in nanocrystals (NCs) is pivotal for many applications, but most notably when investigating catalytic reactions which occur on the surfaces of nanostructures. Anatase titanium dioxide (TiO(2)) is one of the most studied photocatalysts, but the shape dependence of its activity has not yet been satisfactorily investigated and many questions still remain unanswered. We report the nonaqueous surfactant-assisted synthesis of highly uniform anatase TiO(2) NCs with tailorable morphology in the 10-100 nm size regime, prepared through a seeded growth technique. Introduction of titanium(IV) fluoride (TiF(4)) preferentially exposes the {001} facet of anatase through in situ release of hydrofluoric acid (HF), allowing for the formation of uniform anatase NCs based on the truncated tetragonal bipyramidal geometry. A method is described to engineer the percentage of {001} and {101} facets through the choice of cosurfactant and titanium precursor. X-ray diffraction studies are performed in conjunction with simulation to determine an average NC dimension which correlates with results obtained using electron microscopy. In addition to altering the particle shape, the introduction of TiF(4) into the synthesis results in TiO(2) NCs that are blue in color and display a broad visible/NIR absorbance which peaks in the infrared (λ(max) ≈ 3400 nm). The blue color results from oxygen vacancies formed in the presence of fluorine, as indicated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) studies. The surfactants on the surface of the NCs are removed through a simple ligand exchange procedure, allowing the shape dependence of photocatalytic hydrogen evolution to be studied using monodisperse TiO(2) NCs. Preliminary experiments on the photoreforming of methanol, employed as a model sacrificial agent, on platinized samples resulted in high volumes of evolved hydrogen (up to 2.1 mmol h(-1) g(-1)) under simulated solar illumination. Remarkably, the data suggest that, under our experimental conditions, the {101} facets of anatase are more active than the {001}.
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The shape control of noble metal nanocrystals is crucial to their optical properties and catalysis applications. In this Progress Report, the recent progress of shape-controlled synthesis of Pd and Pt nanostructures assisted by small adsorbates is summarized. The use of small strong adsorbates (e.g., I(-) , CO, amines) makes it possible to fabricate Pd and Pt nanostructures with not only well-defined surface structure but also morphologies that have not been achieved by other synthetic strategies. The roles of small adsorbates in shape control of Pd and Pt nanocrystals are discussed in the Report. Also presented in the Report are unique optical and catalytic properties of several Pd and Pt nanostructures (e.g., ultrathin Pd nanosheets, concave Pt octapod, concave Pd tetrahedra), as well as their bioapplications, to demonstrate the power of using small strong adsorbates in the shape control of Pt and Pd nanostructures.
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Using bottom-up chemistry techniques, the composition, size, and shape in particular can now be controlled uniformly for each and every nanocrystal (NC). Research into shape-controlled NCs have shown that the catalytic properties of a material are sensitive not only to the size but also to the shape of the NCs as a consequence of well-defined facets. These findings are of great importance for modern heterogeneous catalysis research. First, a rational synthesis of catalysts might be achieved, since desired activity and selectivity would be acquired by simply tuning the shape, that is, the exposed crystal facets, of a NC catalyst. Second, shape-controlled NCs are relatively simple systems, in contrast to traditional complex solids, suggesting that they may serve as novel model catalysts to bridge the gap between model surfaces and real catalysts.
Article
The properties of nanomaterials for use in catalytic and energy storage applications strongly depends on the nature of their surfaces. Nanocrystals with high surface energy have an open surface structure and possess a high density of low-coordinated step and kink atoms. Possession of such features can lead to exceptional catalytic properties. The current barrier for widespread industrial use is found in the difficulty to synthesise nanocrystals with high-energy surfaces. In this critical review we present a review of the progress made for producing shape-controlled synthesis of nanomaterials of high surface energy using electrochemical and wet chemistry techniques. Important nanomaterials such as nanocrystal catalysts based on Pt, Pd, Au and Fe, metal oxides TiO(2) and SnO(2), as well as lithium Mn-rich metal oxides are covered. Emphasis of current applications in electrocatalysis, photocatalysis, gas sensor and lithium ion batteries are extensively discussed. Finally, a future synopsis about emerging applications is given (139 references).
Article
Put your best face forward: The performance of TiO2 anatase crystals in energy and environmental applications is normally correlated with the TiO2 crystal facets exposed, and increasing the percentage of highly reactive surfaces is extremely important. A new gas-phase oxidation process using TiCl4 as precursor now yields anatase TiO2 single crystals with primarily high-index {105} facets, which can cleave water photocatalytically. (Figure Presented)
Article
Many facets: A simple synthetic route, which is based on reduction in aqueous solution, results in Pt concave nanocubes (see picture) enclosed by high-index facets such as {510}, {720}, and {830}. The nanocrystals exhibit electrocatalytic activity (per unit surface area) that is 3.5 times higher than the commercial Pt/C catalyst in the oxygen reduction reaction.
Article
When used as a photocatalyst, titanium dioxide (TiO2) absorbs only ultraviolet light, and several approaches, including the use of dopants such as nitrogen, have been taken to narrow the band gap of TiO2. We demonstrated a conceptually different approach to enhancing solar absorption by introducing disorder in the surface layers of nanophase TiO2 through hydrogenation. We showed that disorder-engineered TiO2 nanocrystals exhibit substantial solar-driven photocatalytic activities, including the photo-oxidation of organic molecules in water and the production of hydrogen with the use of a sacrificial reagent.
Article
The development of high-performance nanocatalysts relies essentially on the generation of stable and active surface sites at the atomic scale through synthetic control of the size, shape, and chemical composition of nanoscale metals and metal oxides. One promising route is to induce the exposure of catalytically active high-index facets of nanostructures through shape-controlled syntheses. We have designed and prepared two types of Pd nanoshells that are enclosed by high-index {730} and {221} facets through heteroepitaxial growth on high-index-faceted Au nanocrystals. The turnover numbers per surface atom of the high-index-faceted Pd nanoshells have been found to be 3-7 times those of Pd and Au-Pd core-shell nanocubes that possess only {100} facets in catalyzing the Suzuki coupling reaction. These results open up a potential for the development of inexpensive and highly active metal nanocatalysts.
Article
(Figure Presented) Amazing anatase: Anatase TiO2 tubular structures made up of microcrystallites with a high percentage of {001} facets were synthesized by a simple one-step hydrothermal process with ZrO2 fibers as a template (see graphic). The morphologies, structures and growth procedures were systematically investigated, and a plausible mechanism for the formation of these structures was proposed.
Article
In this work, we describe a solution-based synthesis of monodisperse Cu(2)O nanocrystals with controllable sizes in the nanoscale regime. Two types of nanocrystals, cubes and rhombic dodecahedra unifaceted with either {001} or {110} crystal planes, have been prepared at a 100% morphological yield. In particular, synthetic parameters and formation processes of the Cu(2)O nanocrystals have been investigated in detail, and a range of well-oriented supercrystals/superlattices built from the two types of nanobuilding blocks have been attained for the first time. It has been revealed that n-hexadecylamine used in the present work plays multiple roles: it serves as a chelating ligand to form [Cu(NH(2)C(16)H(33))(4)](2+) complex precursor, as a phase-transferring agent to transfer divalent Cu(2+) ions into the organic phase, as a reducing agent to generate monovalent Cu(+) (i.e., Cu(2)O), as a passivating adsorbate to control crystal morphology, and as a surface capping agent to generate self-assemblies of nanocrystals via van der Waals interaction. Apart from synthesis and self-assembly, disassembly and reassembly of Cu(2)O nanocrystals have also been investigated. The disassembly processes are accompanied with aggregative growths of nanocrystals, which can be attributed to a combined process of "oriented attachment" and Ostwald ripening, leading to permanent engagement and enlargement of nanocrystals. Finally, our self-assembled nanocrystals of Cu(2)O show a lower detection limit, lower operating temperature, and higher sensitivity in ethanol vapor detection, compared with other Cu(2)O-based alcohol sensors reported in the recent literature. A greater depletion layer of carrier and a relatively small contact potential may account for the observed sensing enhancement in the sensors made from the organized Cu(2)O nanocrystals.
Article
Tetragonal faceted-nanorods of single-crystalline anatase TiO(2) with a large percentage of higher-energy {100} facets have been synthesized by hydrothermal transformation of alkali titanate nanotubes in basic solution.
Article
The TiO2 microspheres which are a new functional material with dominant (001) facets that offers both large surface area and good recyclability has been demonstrated. TiO2 microspheres have been studied owing to their unique optical properties and ease of recovery in practical applications. The X-ray diffraction (XRD) pattern of the products was obtained from the mixture of Ti(OBu)4 (10 ml), absolute ethanol (40 ml), and a solution 40% HF in deionized water (1.2 ml) for the reaction time tR=5.5 hours. Owing to their high surface area, recyclability, and excellent photocatalytic properties, the TiO2 micro- spheres are promising for practical applications as solar cells, catalyst supports, and water treatment engineering. The synthesized TiO2 microspheres exhibit a superior photocatalytic activity compared with Degussa P25.
Article
This paper reports a facile synthesis of anatase TiO(2) nanocrystals with exposed, chemically active {001} facets. The nanocrystals were prepared by digesting electrospun nanofibers consisting of amorphous TiO(2) and poly(vinyl pyrrolidone) with an aqueous acetic acid solution (pH = 1.6), followed by hydrothermal treatment at 150 degrees C for 20 h. The as-obtained nanocrystals exhibited a truncated tetragonal bipyramidal shape with 9.6% of the surface being enclosed by {001} facets. The use of electrospinning is critical to the success of this synthesis as it allows for the generation of very small particles of amorphous TiO(2) to facilitate hydrothermal crystallization, an Ostwald ripening process. The morphology of the nanocrystals had a strong dependence on the pH value of the solution used for hydrothermal treatment. Low pH values tended to eliminate the {001} facets by forming sharp corners while high pH values favored the formation of a rodlike morphology through an oriented attachment mechanism. When acetic acid was replaced by inorganic acids, the TiO(2) nanocrystals further aggregated into larger structures with various morphologies.
Article
Owing to wide-ranging industrial applications and fundamental importance, tailored synthesis of well-faceted single crystals of anatase TiO(2) with high percentage of reactive facets has attracted much research interest. In this work, high-quality anatase TiO(2) single-crystal nanosheets mainly dominated by {001} facets have been prepared by using a water-2-propanol solvothermal synthetic route. The synergistic functions of 2-propanol and HF on the growth of anatase TiO(2) single-crystal nanosheets were studied by first-principle theoretical calculations, revealing that the addition of 2-propanol can strengthen the stabilization effect associated with fluorine adsorption over (001) surface and thus stimulate its preferred growth. By measuring the (*)OH species with terephthalic acid scavenger, the as-prepared anatase TiO(2) single-crystal nanosheets having 64% {001} facets show superior photoreactivity (more than 5 times), compared to P25 as a benchmarking material.
Article
The reconstructed anatase TiO2(001) surface has been investigated by low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and angle-resolved mass spectroscopy of recoiled ions (AR-MSRI). Prior investigations have observed or considered only a (1x1) unreconstructed termination for this surface with no detailed structural analysis. Our LEED results indicate a previously unobserved two-domain (1x4) reconstruction after sputtering and annealing the (1x1) surface. The XPS data for this reconstruction indicate the presence of only Ti4+. Simulations of the AR-MSRI experimental data indicate a best fit for a microfaceted surface, revealing both (103) and (1;03) surface planes.
Article
ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
Article
The effects of surface chemistry on the morphology and phase stability of titanium dioxide nanoparticles have been investigated using a thermodynamic model based on surface free energies and surface tensions obtained from first principles calculations. It has been found that surfaces representing acidic and alkaline conditions have a significant influence on both the shape of the nanocrystals and the anatase-to-rutile transition size. The latter introduces the possibility of inducing phase transitions by changing the surface chemistry.
Article
Methanol adsorption on clean and hydrated anatase TiO(2)(001)-1 x 1 is studied using density functional theory calculations and first principles molecular dynamics simulations. It is found that (i) dissociative adsorption is favored on clean TiO(2)(001) at both low and high methanol coverages; (ii) on the partially hydrated surface, methanol dissociation is not affected by the coadsorbed water and can still occur very easily; (iii) the dissociative adsorption energy of methanol is always larger than that of water under similar conditions. This implies that water replacement by methanol is energetically favored, in agreement with recent experimental observations on colloidal anatase nanoparticles.
Article
The shapes of noble metal nanocrystals (NCs) are usually defined by polyhedra that are enclosed by {111} and {100} facets, such as cubes, tetrahedra, and octahedra. Platinum NCs of unusual tetrahexahedral (THH) shape were prepared at high yield by an electrochemical treatment of Pt nanospheres supported on glassy carbon by a square-wave potential. The single-crystal THH NC is enclosed by 24 high-index facets such as {730}, {210}, and/or {520} surfaces that have a large density of atomic steps and dangling bonds. These high-energy surfaces are stable thermally (to 800 degrees C) and chemically and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface areas for electro-oxidation of small organic fuels such as formic acid and ethanol.
Article
Over the past decades, the tremendous effort put into TiO2 nanomaterials has resulted in a rich database for their synthesis, properties, modifications, and applications. The continuing breakthroughs in the synthesis and modifications of TiO2 nanomaterials have brought new properties and new applications with improved performance. Accompanied by the progress in the synthesis of TiO2 nanoparticles are new findings in the synthesis of TiO2 nanorods, nanotubes, nanowires, as well as mesoporous and photonic structures. Besides the well-know quantum-confinement effect, these new nanomaterials demonstrate size-dependent as well as shape- and structure-dependent optical, electronic, thermal, and structural properties. TiO2 nanomaterials have continued to be highly active in photocatalytic and photovoltaic applications, and they also demonstrate new applications including electrochromics, sensing, and hydrogen storage. This steady progress has demonstrated that TiO2 nanomaterials are playing and will continue to play an important role in the protections of the environment and in the search for renewable and clean energy technologies.
Article
Tungsten oxide loaded with nanoparticulate platinum is demonstrated to exhibit high activity for the decomposition of organic compounds both in liquid and gas phases; the activity was almost comparable to that of TiO2 under UV light irradiation and much higher than that of nitrogen-doped TiO2 under visible irradiation.
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