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Photodeposition of Au Nanoclusters for Enhanced Photocatalytic Dye Degradation over TiO 2 Thin Film

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Au nanoparticles (NPs) decorated heterogeneous TiO2 catalysts are known to be effective in the degradation of various organic pollutants. The photocatalytic performance of such Au-TiO2 structures remarkably depends on size, morphology and surface coverage of Au NPs decorating TiO2. Here we propose an effective way of preparing highly active Au nanoclusters (NCs) decorated TiO2 thin film by a novel photo-deposition method. By altering the solvent type and as well as the illumination time we achieved well-controlled surface coverage of TiO2 by Au NCs, which directly influences the photocatalytic performance. Here Au NCs coverage affects both the electron store capacity and the optical absorption of the hybrid Au-TiO2 system. At low surface coverage, 19.2-29.5 % Au NCs seem to enhance significantly the optical adsorption of TiO2 at UV wavelengths therefore leads to a higher photocatalytic performance.
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... This enables controlling the size and the geometry of deposited Au structures by simply altering the UV light intensity and irradiation time. Subsequently it was also found that the dielectric constant and the pH of the medium drastically influence the morphology of structures during the process which can range from globular rounded to almost ultra-sharp needle-like structures [24]. Basically, the method is not limited to only Au and it allows patterning of TiO 2 with various types of other metallic ions [25]. ...
... Au clusters were grown on the TiO 2 thin films by a modified photodeposition method as reported previously [23,24]. The starting materials, gold chloride (HAuCl 4 ) and non-ionic surfactant (Triton X-100) were purchased from Alfa Aesar. ...
... It has been shown that the use of HAuCl 4 without any surfactant led to the formation of sharp needle-like structures with a low nucleation rate [23]. In our recent study we have shown that dielectric constant and as well as the acidity of the solvent govern the morphology of photo-deposited Au structures [24]. Here by adding non-ionic surfactant to HAuCl 4 we achieved a higher nucleation rate which is crucial to form densely distributed Au structures on TiO 2 . ...
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... A continuous TiO 2 nanofilm provides a long diffusion length (reservoir) for electrons, and inhibits the recombination of electron-hole pairs, compared to the conventional supported NP [21]. To date, only a few works were reported on the deposition of Au NP on TiO 2 film, e.g., electrostatic self-assembly in colloid [22], evaporation of island-like film [23][24][25], and photocatalytic deposition [26,27]. ...
... Fig. S3 shows chemical structure of Au/TiO 2 nanofilm (S2) by XPS spectra. A peak at 83.4 eV of Au 4f signal is assigned metallic Au 0 for Au/ TiO 2 , compared to none of that for TiO 2 [26,39]. It suggests that an oxidized state (Au δ+ ) is reduced to metallic state (Au 0 ) after calcination. ...
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... To remove organic pollutants, various strategies have been implemented such as adsorption [7,8], filtration [9], photocatalytic degradation [10,11], and catalytic reduction [12]. Among these approaches, photocatalytic degradation has received tremendous attention from researchers due to its efficacy and cheap and effective process is widely used to purify water and decompose organic pollutants into nontoxic substances [13][14][15][16]. Recently, carbon-based materials have attracted increased attention for photocatalytic applications due to their unique chemical and physical properties. ...
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... [47] The unique surface-plasmon resonance (SPR) effect of Au offers a large local electric field and tunable resonance wavelength, which further improve the separation efficiency of photogenerated charges, extend the visible light absorption, and efficiently enhance the photocatalytic activity. [43,48,49] In this study, aiming to facilitate the utilization of renewable energy such as biomass and solar energy, we report herein the synthesis of Au/HT photocatalysts by one-step solvothermal method under mild conditions. SPR effect was observed for H 2 generation. ...
... Au clusters were deposited on the TiO 2 thin lms by a slightly modied photocatalytic deposition method as schematically shown in Fig. 1. 18,24,25 The starting materials, gold(III) chloride (AuCl 3 ) was purchased from Alfa Aesar. The TiO 2 thin lms were dipped into a quartz cuvette, lled with 6.5 ml of an aqueous AuCl 3 (0.8 Â 10 À3 M). ...
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Photocatalytic technologies have received extensive attention as green catalysts. Here, we successfully designed gold (Au) core–Palladium (Pd) shell-structured nanoparticles (NPs) as a nanoreactor, in which the Pd shell provides the active sites and Au core offers strong plasmonic properties. We then used the seed-mediated growth approach to fabricate the Au core–Pd shell-structured nanoparticles on a polyvinyl alcohol (PVA) nanofibrous mat as a highly active photocatalytic composite material for hydrogenation reactions under room temperature. Our results show that the Au core–Pd shell-structured nanoparticles are more effective in catalyzing the hydrogenation of styrene than the Pd nanoparticles. The photocatalytic experimental results show that the yields of ethylbenzene under light conditions are higher than that under 60 °C in the dark. This shows that the PVA/Au NP/Pd NP composite material can efficiently utilize heat from light to promote catalytic reactions via photothermal effects. The PVA/Au NP/Pd NP composite material not only exhibits a very high photocatalytic efficiency, but can also easily recover from the reaction. The work offers a simple and efficient strategy for recyclable and highly active catalysts for light-driven organic reactions.
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In this study, we applied cluster beam deposition (CBD) as a new approach for fabricating efficient plasmon-based photocatalytic materials. Au nanoclusters (AuNCs) produced in the gas phase were deposited on TiO₂ P25-coated silicon wafers with coverage ranging from 2 to 8 atomic monolayer (ML) equivalents. Scanning Electron Microscopy (SEM) images of the AuNCs modified TiO₂ P25 films show that the surface is uniformly covered by the AuNCs that remain isolated at low coverage (2 ML, 4 ML) and aggregate at higher coverage (8 ML). A clear relationship between AuNCs coverage and photocatalytic activity towards stearic acid photo-oxidation was measured, both under ultraviolet and green light illumination. TiO₂ P25 covered with 4 ML AuNCs showed the best stearic acid photo-oxidation performance under green light illumination (Formal Quantum Efficiency 1.6 × 10-6 over a period of 93 h). These results demonstrate the large potential of gas-phase AuNCs beam deposition technology for the fabrication of visible light active plasmonic photocatalysts.
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Wire grid polarizers (WGPs), periodic nano-optical meta-surfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. We show that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. We elucidate why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior material candidates in this spectral range. To demonstrate this, we present the design, fabrication and optical characterization of a titanium dioxide WGP. At a wavelength of 193 nm an unprecedented extinction ratio of 384 and a transmittance of 10 % is achieved.
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The low quantum yields and lack of visible light utilization hinder the practical application of TiO2 in high-performance photocatalysis. Herein, we present a design of TiO2 nanopillar arrays (NPAs) decorated with both Au and Pt nanoparticles (NPs) directly synthesized through successive ion layer adsorption and reaction (SILAR) at room temperature. Au/Pt NPs with sizes of ~4 nm are well-dispersed on the TiO2 NPAs as evidenced by electron microscopic analyses. The present design of Au/Pt co-decoration on the TiO2 NPAs shows much higher visible and ultraviolet (UV) light absorption response, which leads to remarkably enhanced photocatalytic activities on both the dye degradation and photoelectrochemical (PEC) performance. Its photocatalytic reaction efficiency is 21 and 13 times higher than that of pure TiO2 sample under UV-vis and visible light, respectively. This great enhancement can be attributed to the synergy of electron-sink function of Pt and surface plasmon resonance (SPR) of Au NPs, which significantly improves charge separation of photoexcited TiO2. Our studies demonstrate that through rational design of composite nanostructures one can harvest visible light through the SPR effect to enhance the photocatalytic activities initiated by UV-light, and thus realize more effectively utilization of the whole solar spectrum for energy conversion.
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This work highlights a strategy for the one-step synthesis of FeAu nanoparticles by the pulsed laser ablation of alloy targets in the presence of different solvents. This method allows particle generation without the use of additional chemicals; hence, solvent-metal interactions could be studied without cross effects from organic surface ligands. A detailed analysis of generated particles via transmission electron microscopy in combination with EDX elemental mapping could conclusively verify that the nature of the used solvent governs the internal phase structure of the formed nanoparticles. In the presence of acetone or methyl methacrylate, a gold shell covering a non-oxidized iron core was formed, whereas in aqueous media, an Au core with an Fe3O4 shell was generated. This core-shell morphology was the predominant species found in >90% of the examined nanoparticles. These findings indicate that fundamental chemical interactions between the nanoparticle surface and the solvent significantly contribute to phase segregation and elemental distribution in FeAu nanoparticles. A consecutive analysis of resulting Fe@Au core-shell nanoparticles revealed outstanding oxidation resistance and fair magnetic and optical properties. In particular, the combination of these features with high stability magnetism and plasmonics may create new opportunities for this hybrid material in imaging applications.
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Heterogeneous photocatalysis using TiO2 can effectively remove antibiotics from water using UV light; however, its performance is notably reduced under sunlight irradiation. The use of metallic nanoparticles deposited on TiO2 may result in the photo-activation of the catalyst within the visible spectrum. In this study mono- (Au, Ag and Cu) and bi-metallic Au-Ag and Au-Cu nanoparticles were deposited on TiO2 to photocatalytically degrade the antibiotic ciprofloxacin in pure water using either UV-C or simulated sunlight. The optimal loading of mono-metallic nanoparticles on TiO2 was determined as 1.5wt.% for Au and Ag, and 1.0wt.% for Cu; first order degradation rates (k app ) of 0.06, 0.117 and 0.072min-1, respectively, were determined for these materials. In UV-C tests, the complete degradation of ciprofloxacin was achieved upon 90min of irradiation, whilst complete mineralization was reached in <180min for all of the tested catalysts. In simulated sunlight photocatalysis, ciprofloxacin was only partially removed upon 360min of irradiation when using mono-metallic materials, while complete mineralization was achieved when bi-metallic nanoparticles on TiO2 were tested. A group of by-products were identified and degradation paths were elucidated for photolysis and photocatalysis. Toxicity tests using V. fischeri showed the non-toxicity of the by-products remaining after 360min of simulated sunlight irradiation. Even though toxicity was low, ciprofloxacin by-products showed some residual antibiotic activity. No catalyst deactivation was observed after 3 consecutive reaction cycles.
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M–Au/TiO2 (M = Ag, Pd, Pt) composites were prepared through a facile one-pot photodeposition synthesis and evaluated for solar water splitting (SWS) with and without a sacrificial agent. The M–Au combination exhibits a dominant role in augmenting the H2 generation activity by forming a bi-metallic system. Degussa P25 was used as a TiO2 substrate to photodeposit Au followed by Au + M (M = Ag/Pd/Pt). The SWS activity of the M–Au/TiO2 was determined through photocatalytic H2 production in the presence of methanol as a sacrificial agent under one sun conditions with an AM1.5 filter. The highest H2 yield was observed for Pt0.5–Au1/TiO2 and was around 1.3 ± 0.07 mmol h−1 g−1, with an apparent quantum yield (AQY) of 6.4%. Pt0.5–Au1/TiO2 also demonstrated the same activity for 25 cycles of five hours each for 125 h. Critically, the same Pt0.5–Au1/TiO2 catalyst was active in overall SWS (OSWS) without any sacrificial agent, with an AQY = 0.8%. The amount of Au and/or Pt was varied to obtain the optimum composition and it was found that the Pt0.5–Au1/TiO2 composition exhibits the best activity. Detailed characterization by physico-chemical, spectral and microscopy measurements was carried out to obtain an in-depth understanding of the origin of the photocatalytic activity of Pt0.5–Au1/TiO2. These in-depth studies show that gold interacts predominantly with oxygen vacancies present on titania surfaces, and Pt preferentially interacts with gold for an effective electron–hole pair separation at Pt–Au interfaces and electron storage in metal particles. The Pt in Pt0.5–Au1/TiO2 is electronically and catalytically different from the Pt in Pt/TiO2 and it is predicted that the former suppresses the oxygen reduction reaction.
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Switched photocurrent direction in photoelectrodes is a very interesting phenomenon and has demonstrated their potentials in important applications including photodiodes, phototransistors, light-driven sensors and biosensors. However, the design and mechanism understanding of such photoelectrodes remain challenging to date. Here we report a new phenomenon of sequence-driven the photocurrent direction on a simple bilayer structure of 5 nm thick Au and 10 nm TiO2 under visible-light irradiation. It is found that when Au layer are deposited as the outer layer on TiO2 coated fluorine doped tin oxide (FTO) substrate (designated as FTO/TiO2/Au), anodic photocurrent is obtained due to the band bending formed at the electrode-electrolyte interface. Interestingly, simply swapping the deposition sequence of Au and TiO2 leads to cathodic photocurrent on FTO/Au/TiO2 electrode. Characterization and calculations on the photoelectrode reveals that the photogenerated electrons can be easily trapped in the energy well formed between the band bending and the Schottky contact, which allows electronic tunnelling through the 1.6 nm thick space charge layer, resulting in a unique anodic to cathodic photocurrent conversion. The understanding of this new phenomenon can be important for designing new generation optoelectronic converting devices in a low-cost and facile manner.
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The role of localized surface plasmon resonance (LSPR) in UV-Vis light irradiated Au/TiO2 photocatalysis systems has been investigated, and it is demonstrated experimentally for the first time that both pros and cons of LSPR exist simultaneously for this photocatalytic reaction. We have proved that when operating under mixed UV and green light irradiation, the LSPR injected hot electrons (from the Au nanoparticles to TiO2 under green light irradiation) may surmount the Schottky barrier (SB) formed between the Au nanoparticles and TiO2, and flow back into the TiO2. As a result, these electrons may compensate for and even surpass those transferred from TiO2 to the Au nanoparticles, thus accelerating the recombination of UV excited electron-hole pairs in TiO2. This is the negative effect of LSPR. On the other hand, more hot electrons existing on the surface of the Au nanoparticles due to LSPR would favor the photocatalytic reaction, which accompanied by the negative effect dominates the overall photocatalytic performance. The presented results reveal the multi-faceted essence of LSPR in Au/TiO2 structures, and is instructive for the application of metal-semiconductor composites in photocatalysis. Moreover, it is confirmed that the extent to which the above pros and cons of LSPR dominate the overall photocatalytic reaction depends on the intensity ratio of visible to UV light.
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Pure TiO2 thin films were deposited onto quartz substrates using a ceramic TiO2 target at an elevated substrate temperature of 573 K by RF magnetron sputtering, and an analysis of structural, optical and photoluminescence characteristics of the films upon phase transformation is reported in this paper. Structural investigations using X-ray diffraction revealed that the as-deposited film was amorphous in nature. Thermal annealing for 2 h at 873 K in air resulted in the formation of anatase phase, and a phase transformation to rutile was observed at 1073 K. An increase in grain size and an improvement in crystallinity were also observed on annealing. Rod- like rutile crystallites were observed in the SEM images of the film annealed at 1273 K. As-deposited films and films annealed up to 1073 K were highly transparent in the visible region with a transparency >80%. Optical band gap of the films decreased upon thermal annealing which is attributed to phase transformation from amorphous to anatase and then to rutile. Optical parameters such as refractive index, optical conductivity and optical dielectric constant increased with increase in annealing temperature. Since rutile is the optically active phase, the superior refractive index of the film annealed at 1073 K along with its high transparency in visible region suggests the application of this film in antireflective coatings. Photoluminescence emission of maximum intensity was observed for the film annealed at 873 K, which exhibits anatase phase. Intense blue emission observed in this film makes it suitable for use in optoelectronic display devices.
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Studies of the optical properties and catalytic capabilities of noble metal nanoparticles (NPs), such as gold (Au) and silver (Ag), have formed the basis for the very recent fast expansion of the field of green photocatalysis: photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum. The reason for this growth is the recognition that the localised surface plasmon resonance (LSPR) effect of Au NPs and Ag NPs can couple the light flux to the conduction electrons of metal NPs, and the excited electrons and enhanced electric fields in close proximity to the NPs can contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. Previously the LSPR effect of noble metal NPs was utilized almost exclusively to improve the performance of semiconductor photocatalysts (for example, TiO2 and Ag halides), but recently, a conceptual breakthrough was made: studies on light driven reactions catalysed by NPs of Au or Ag on photocatalytically inactive supports (insulating solids with a very wide band gap) have demonstrated that these materials are a class of efficient photocatalysts working by mechanisms distinct from those of semiconducting photocatalysts. There are several reasons for the significant photocatalytic activity of Au and Ag NPs. (1) The conduction electrons of the particles gain the irradiation energy, resulting in high energy electrons at the NP surface which is desirable for activating molecules on the particles for chemical reactions. (2) In such a photocatalysis system, both light harvesting and the catalysing reaction take place on the nanoparticle, and so charge transfer between the NPs and support is not a prerequisite. (3) The density of the conduction electrons at the NP surface is much higher than that at the surface of any semiconductor, and these electrons can drive the reactions on the catalysts. (4) The metal NPs have much better affinity than semiconductors to many reactants, especially organic molecules. Recent progress in photocatalysis using Au and Ag NPs on insulator supports is reviewed. We focus on the mechanism differences between insulator and semiconductor-supported Au and Ag NPs when applied in photocatalytic processes, and the influence of important factors, light intensity and wavelength, in particular estimations of light irradiation contribution, by calculating the apparent activation energies of photo reactions and thermal reactions.
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The TiO2/UV photocatalytic degradation of methylene blue (MB) has been investigated in aqueous heterogeneous suspensions. In addition to a prompt removal of the color, TiO2/UV-based photocatalysis was simultaneously able to oxidize the dye, with an almost complete mineralization of carbon and of nitrogen and sulfur heteroatoms into CO2, NH4+, NO3− and SO42−, respectively. A detailed degradation pathway has been determined by a careful identification of intermediate products, in particular aromatics, whose successive hydroxylations lead to the aromatic ring opening. These results suggest that TiO2/UV photocatalysis may be envisaged as a method for treatment of diluted waste waters in textile industries.
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Rietveld refinements of corundum, a rutile and anatase nanocrystalline synthetic mixture, and gypsum, on laboratory energy dispersive X-ray diffraction (EDXD) data are reported. Cell parameters, positional and displacement parameters are in reasonable agreement with single-crystal reference data, despite the rather poor resolution of EDXD data. In particular, good results were obtained for gypsum (unrestrained refinement) with counting times as short as 1000 s. © 2001 International Union of Crystallography Printed in Great Britain - all rights reserved.
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Photoemission spectromicroscopy was used to investigate the electronic structure of TiO 2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as‐grown crystals, as‐deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals. In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap. As‐grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 0.8 eV, which is attributed to Ti<sup>3+</sup> (3d) defect states. Annealing the crystals at high temperatures in O 2 or subsequent thermal reduction in an Ar–H 2 mixture (95%–5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti<sup>3+</sup> states. In addition, some redistribution of the spectral weight is observed in the VB spectra.
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In recent years, heterogenous photocatalysis gained an enormous interest due to continuously increasing environmental pollution. Here we propose a facile approach to synthesize cauliflower-like CeO2-TiO2 hybrid structures by magnetron reactive sputtering, exhibiting an extremely high photocatalytic activity. While heating and air-quenching of sputter deposited TiO2 thin film (first layer) triggered the formation of nanocrack network, second heat treatment led to transformation of CeO2 film (second layer) into CeO2 nanoclusters (NCs). We address the resulting high photocatalytic activity to the confined structure of the CeO2 NCs and CeO2-TiO2 interface, which allows Ce³⁺/Ce⁴⁺ dynamic shifting. In addition to high photocatalytic activity in an aqueous medium, the prepared CeO2-TiO2 hybrid structures exhibited a significant self-cleaning property in air (non-aqueous).
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Hydrogen (H2) production via photocatalytic water splitting is one of the most promising technologies for clean solar energy conversion to emerge in recent decades. The achievement of energy production from water splitting would mean that we could use water as a fuel for future energy need. Among the various photocatalytic materials, titanium dioxide (TiO2) is the dominant and most widely studied because of its exceptional physico-chemical characteristics. Surface decoration of metal/non-metal on TiO2 nanoparticles is an outstanding technique to revamp its electronic properties and enrich the H2 production efficiency. Metal dopants play a vital role in separation of electron-hole pairs on the TiO2 surface during UV/visible/simulated solar light irradiation. In this paper, the basic principles, photocatalytic-reactor design, kinetics, key findings, and the mechanism of metal-doped TiO2 are comprehensively reviewed. We found that Langmuir-Hinshelwood kinetic model is commonly employed by the researchers to demonstrate the rate of H2 production. Copper (Cu), gold (Au) and platinum (Pt) are the most widely studied dopants for TiO2, owing to their superior work function. The metal dopants can amplify the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance (SPR), generation of gap states by interaction with TiO2 VB states. The recent advances and important consequences of 2D materials, perovskites, and other novel photocatalysts for H2 generation have also been reviewed.
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The present monograph attempts to unify these diverse and exciting new developments within a common framework. First, the physical principles underlying heterogenous electron-transfer processes are outlined in a concise way and are compared to the homogeneous counterpart. This analysis includes the notion of the Fermi level in liquids and solids as well as the distribution of electronic energy levels in solids and liquids. A comparison is made between the salient kinetic features of homogeneous and heterogeneous electron transfer reactions. This establishes the basis for the subsequent treatment of the transduction of excitation energy and photo-initiated electron transfer in organized molecular assemblies, such as micelles, vesicles and monolayers. Transmembrane redox processes are critically reviewed. Particular attention is given to semiconductor electrodes and particles. This includes a discussion of quantum size effects, the nature of space charge layers as well as surface states and the dynamics of charge carrier-induced redox reactions at the semiconductor solution interface. These processes are of fundamental importance in such diverse fields as photochromism, electrochromic displays, electroreprography and photography, information storage, photocatalysis, photodegradation of paints, and solar energy conversion.
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We present a facile method, combining sputtering and gas aggregation techniques, to prepare a photocatalytic TiO2 thin film decorated with stable Al plasmonic nanoparticles (NPs) to reveal the localized surface plasmon resonance (LSPR) effect on TiO2 photocatalysis under UV irradiation. We demonstrate for the first time the negative and positive influences of LSPR on UV photocatalysis by irradiating Al NPs/TiO2 hybrid structures at two different UV wavelengths: both at and above the plasmonic absorption of Al NPs. These findings open the door to designing low-cost Al-TiO2 photocatalytic hybrid surfaces which function in a broad spectrum range from deep UV to VIS wavelengths.
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Hierarchical gold (Au) structures with sharp edges garner a strong interest for nanoelectronics, nanoenergy harvesting devices, and nano‐biomedical applications due the exceptional strong electric field (hot spot) enhancement at their tips. Herein, a facile method to synthesize hierarchical Au needle clusters (HAuNCs) on highly active titanium oxide (TiO2) thin film is reported. Different from surfactant‐directed photochemical‐assisted synthesis methods, a photocatalytic deposition approach is demonstrated, which allows positioning and patterning of HAuNCs on TiO2 target without using any surfactant or stabilizer. This green synthesis approach enables to control the size and the geometry of deposited HAuNCs by simply altering the photocatalytic activity of TiO2 target, UV light intensity, and irradiation time.
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While there is continuous progress in development of new photocatalytic thin films and coatings, the lack of a reliable and standard procedure for measuring the photocatalytic performance of such active surfaces makes it difficult to compare results between research groups and different measurement setups. Here, a comparative study was carried out to demonstrate the high photocatalytic activity of sputter-deposited TiO 2 film with self-organized nanocrack networks by using two different analytical approaches: (i) bleaching of a thin Methylene Blue (MB) solid layer on photocatalytic TiO 2 thin film (in-air) and (ii) the decolorization of a MB aqueous solution in the presence of TiO 2 thin film (in-solution). While the decolorization of aqueous MB solution provides an indirect observation of the photocatalytic effect imposed by the TiO 2 film, the use of a solid MB layer as an indicator allows monitoring of photocatalytic reactions at the solid-air interface directly. We showed the applicability of this approach as a complementary and a fast analysis method to reveal the photocatalytic efficiency of thin films by comparing it with the state of the art inks (based on MB and other similar organic dyes) used as photocatalysis indicator.
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We believe this to be the first monograph devoted to the physicochemical properties of solutions in organic solvent systems. Although there have 1 been a number of books on the subject of non-aqueous solvents - 4, they have been devoted, almost entirely, to inorganic solvents such as liquid ammonia, liquid sulphur dioxide, etc. A variety of new solvents such as dimethylformamide, dimethylsulphoxide and propylene carbonate have become commercially available over the last twenty years. Solutions in these solvents are of technological interest in connection with novel battery systems and chemical synthesis, while studies of ion solvation and transport properties have fostered academic interest. This monograph is primarily concerned with electrolytic solutions although discussion of non-electrolyte solutions has not been excluded. We have deliberately omitted consideration of the important area of solvent extraction, since this has been adequately covered elsewhere. Our contributors were asked to review and discuss their respective areas with particular reference to differences in technique necessitated by use of non-aqueous solvents while not reiterating facts well-known from experience with aqueous solutions. We have striven to build their contributions into a coherent and consistent whole. We thank our con­ tributors for following our suggestions so ably and for their forebearance in the face of our editorial impositions.
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Nanoscale gold-titania (Au/TiO2) catalysts may provide the right combination of electronic structure, structural dynamics, and stability to affect the activity and selectivity towards wide ranging chemical transformations, including reactions for utilization of renewable energy sources such as biomass and fuel cells. The Au/TiO2–based systems have also emerged as promising photocatalysts capable of promoting light-induced production of hydrogen and other renewable hydrocarbon-based fuels. This perspective summarizes some of the fundamental aspects and concepts built over the last 30 years that help explain the catalytic and photocatalytic performance of Au/TiO2 materials. The application of emerging operando methods, based on synchrotron experimental techniques, is also briefly highlighted within the context of key catalytic reactions that may have fundamental importance in renewable energy production and storage.
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Snowman-like Au-TiO2 nanoassemblies comprising of clusters of Au nanostructures (nanospheres and nanorods) on TiO2 nanoparticles have been synthesized using a controlled hydrolysis method and their photocatalytic activity under both ultraviolet radiation and visible light was investigated. The photocatalytic activity was evaluated by monitoring the degradation of a model dye, methylene blue (MB) and a colorless hazardous fungicide, carbendazim (CZ). The Au nanoclusters play the role of co-catalyst when ultraviolet radiation was used, while it plays the role of light harvester when visible light was used for photocatalysis. In both the cases, these clustered Au on TiO2 snowman-like nanoassemblies possess enhanced photocatalytic activity compared to bare TiO2 and their superior performance can be ascribed to the efficient charge separation of the photo generated electrons and holes at the Au-TiO2 interface, facilitated by the close proximity of the Au nanostructures in the cluster. In essence, this study illustrates that the concept of having a cluster of noble metal nanoparticles on to a semiconductor oxide provides distinctive advantages in enhancing the photocatalytic performance of hybrid nanoassemblies under both ultraviolet and visible light irradiations.
Article
In the present investigation, TiO2 modified with a different geometry and size of gold particles, such as nanospheres (NSPs), nanostars (NSTs) and nanorods (NRs), were prepared by the immobilization method. The effect of the gold shape, size and TiO2 matrix type (TiO2 microspheres or rutile TIO-6_TiO2) were systematically investigated. The obtained photocatalysts were thoroughly characterized by UV-Vis diffuse-reflectance spectroscopy (DRS), BET surface area measurements, scanning electron microscopy (SEM), scanning transmission microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity under visible light (λ > 420 nm) has been estimated in phenol degradation reaction in an aqueous phase. The significantly high photocatalytic activity under visible light irradiate as demonstrated by the TiO2 sample modified by spheres of gold. The average rate of phenol decomposition was 1.9 μmol·dm-3·min−1 and was three-times higher compared to the pristine TiO2 amorphous microspheres. On the other hand the photocatalytic activity was relatively lower and was 0.38 and 0.27 μmol·dm−3·min−1 for nanorods and nanostars deposited on the amorphous form of TiO2 microspheres, respectively. The visible light activity decreased in following order: (NSPs)> (NRs)> (NSTs). The obtained photocatalytic efficiency of samples was ascribed to the geometry and the size effect of the enhanced and the possible mechanism for this was discussed in detail. Furthermore, in this work we show the effect of calcination temperature on the structure of gold NPs, NRs and NSs before and after modification on the morphology and photocatalytic activity of Au-TiO2.
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Mesoporous SiO2 templates deposited TiO2 nanocrystals are synthesized via a sol-gel route, and Au nanoparticles (NPs) are deposited in the tubular mesopores of the templates by a photodeposition method (Au/SiO2-TiO2). The photocatalytic characteristics of Au/SiO2-TiO2 are discussed with the action spectra of photoreactions of 2-propanol and methylene blue. Photocatalytic activities of SiO2-TiO2 under individual ultraviolet (UV) and visible (Vis) light illumination are enhanced by deposition of Au NPs. Furthermore, Au/SiO2-TiO2 shows higher photocatalytic activities under simultaneous irradiation of UV and Vis light compared to the activity under individual UV and Vis light irradiation. Since the photocatalytic activity under simultaneous irradiation is almost the same as the total activities under individual UV and Vis light irradiation, it is concluded that the electrons and the holes generated by lights of different wavelengths are efficiently used for photocatalysis without carrier recombination.
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ZnO is a promising catalyst for hydrogen and oxygen production due to the position of the conduction and valence bands. Nevertheless, there are some limitations to the efficiency due to its n-type character. Once in contact with the electrolyte, an extraction barrier for electrons is formed. The purpose of this work was to create an extraction site for electrons by synthesizing small pyramidally shaped ZnO nanocrystals (∼10 nm), which consist of a predefined site for the growth of a gold particle at the tip. Photoelectrochemical deposition of gold on ZnO was performed to yield the hybrid structure. Photoluminescence (PL) studies of the relative change of intensities of band gap versus defect state relaxation showed electron transfer from the conduction band of ZnO to Au. Using cyclic voltammetry, Au-mediated charge extraction from Au-ZnO hybrids was demonstrated, which circumvents the electron extraction barrier in ZnO. Thus, this work demonstrates the nanoscale design of hybrid structures for photocatalytic applications.
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The thermal and physicochemical properties of protic ionic liquids (PILs) are reported. It is highly evident that there has been an extensive range of alkylammonium, imidazolium, and heterocyclic cations paired with many organic and inorganic anions that have been employed to prepare PILs. There has been strong interest in modifying the properties of PILs through the addition of water or other molecular solvents. For many applications, the presence of some water in the PILs is not detrimental, and instead leads to enhanced solvent properties such as lower viscosity, higher conductivities, and lower melting points. It remains an issue of definition though of how to refer to these resulting protic solutions. There is also an ongoing difficulty surrounding how to describe the proton activity in the PILs, analogous to pH in aqueous systems. For a broad range of applications, it has been reported that the acidity/basicity of the PIL or PIL-solvent system is crucial for their beneficial properties. It is expected that the fundamental properties of PILs will continue to be explored, along with continued interest in many existing and new applications, such as in electrochemistry, organic and inorganic synthesis, and biological applications. In particular, there has been a significant interest in a broad- range of PILs for use as electrolytes and incorporation in polymer electrolytes for fuel cells, and other energy storage devices.
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We apply high-energy proton ion-implantation to modify TiO2 nanotubes selectively at their tops. In the proton-implanted region we observe the creation of intrinsic co-catalytic centers for photocalalytic H2-evolution. We find proton implantation to induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also on anatase single crystal (001) surfaces. Nevertheless, for TiO2 nanotubes a strong synergetic effect between implanted region (catalyst) and implant-free tube segment (absorber) can be obtained.
Article
X-ray photoelectron (XPS) experiments at normal and grazing emission are performed, demonstrating the labile nature of the anatase TiO2 (101) surface after argon cluster ion sputtering, and the propensity of oxygen vacancies to migrate subsurface at room temperature. Near-ambient XPS (NAP-XPS) shows that molecular water adsorbs on the anatase TiO2 (101) surface at pressures of 0.6 mbar and above, at room temperature, in a mixed molecular and dissociated state. Water adsorbs in a similar fashion on both defected and stoichiometric surfaces and reaches a saturation point between 0.6 and 1.8 mbar. This means there is little difference in reactivity with regards to water adsorption on both sputtered and defected surfaces, giving credence to the theory that anatase has superior photocatalytic activity over rutile due to the tendency of oxygen vacancies to lie subsurface, and therefore being able to contribute to photocatalysis without being quenched by adsorbates.
Article
Emission of photoexcited hot electrons from plasmonic metal nanostructures to semiconductors is key to a number of proposed nanophotonics technologies for solar harvesting, water splitting, photo catalysis and a variety of optical sensing and photodetector applications. Favorable materials and catalytic properties make systems based on gold and TiO2 particularly interesting but the internal photoemission efficiency for visible light is low because of the wide bandgap of the semiconductor. We investigated the incident photon-to-electron conversion efficiency of thin TiO2 films decorated with Au nanodisk antennas in an electrochemical circuit and found that incorporation of a Au mirror beneath the semiconductor amplified the photo response for light with wavelength  = 500 - 950 nm by a factor 2-10 compared to identical structures lacking the mirror component. Classical electrodynamics simulations showed that the enhancement effect is caused by a favorable interplay between localized surface plasmon excitations and cavity modes that together amplify the light absorption in the Au/TiO2 interface. The experimentally determined internal quantum efficiency for hot electron transfer decreases monotonically with wavelength, similar to the probability for interband excitations with energy higher than the Schottky barrier obtained from a density functional theory band structure simulation of a thin Au/TiO2 slab.
Article
In the present investigation, Au/TiO2 nanocomposites were prepared by in situ, photodeposition (PD) and SMAD method. The synthesized photocatalysts were evaluated for their photocatalytic activity in hydrogen generation. The hydrogen evolution rate was observed to be 1200 μmols h−1, 920 μmols h−1 and 1600 μmols h−1 for in situ, photodeposition (PD) and SMAD, respectively under UV-vis light illumination. However, under purely visible light illumination the highest hydrogen evolution of 32.4 μmols h−1 was observed in case of Au/TiO2 prepared by PD followed by SMAD (6.9 μmols h−1). The significantly high photocatalytic activity as demonstrated by Au/TiO2 synthesized by photodeposition method under purely visible light irradiation proves the potential of photocatalyst in efficient solar energy conversion. The poly disperse particles of AuNPs on TiO2 by photodeposition method enhances plasmonic effects. The catalyst was thoroughly characterized by powdered X-ray diffraction, UV-vis DRS and TEM for understanding the plasmonic properties. TEM images further substantiated the deposition of gold on TiO2 matrix of heterogeneous particle size with an average size of 8-10 nm (smaller particles) and 53-70 nm (bigger particles). The photodeposition of AuNPs on different titania supports was also studied.
Article
Titania thin films with ordered nanostructures are of general interest in fields of photocatalysis, gas sensors, energy storages, energy conversions, etc. In this study, we report a low‐temperature crystallization and the simultaneously occurred morphology change of titania thin films in a dilute H2SO4 solution. Amorphous titania nanowire arrays were fabricated by a Ti–H2O2 interaction, which transformed to crystallized nanoflower arrays through a dissolution–precipitation route during the subsequent acid treatments. The nanoflowers were doped with nitrogen and also incorporated with sulfate ions. An increasing H2SO4 concentration resulted in larger nanoflowers with higher anatase content; but the crystallinity reduced. The low‐temperature‐derived nanoflower arrays possessed high density of surface hydroxyl groups and defect VO‐Ti3+ sites, which contributed to a high absorption and enhanced photodegradation efficiency of rhodamine B in water under the illumination of UV–visible light during the first several runs of photocatalytic evaluations. The beneficial surface defects diminished gradually with increasing runs; however, the average reaction rate constants of the acid treated films are still superior to that of the calcinated one, which can be attributed to several structural parameters such as the nanoflower morphology, incorporation of sulfate ions, and the coexistence of anatase and rutile.
Article
These spectra compare the Ti 2p core-level XPSline shape for atomically clean, stoichiometric samples of Ti, Ti2O3, and TiO2; the latter two are UHV-cleaved or fractured single crystals. The Ti0 and Ti4+ peaks of Ti metal and TiO2, respectively, exhibit the simple shapes expected for such core levels. The Ti3+ peaks of Ti2O3, however, are more complex, although there are no other valence states of Ti in the sample.
Article
Nanomaterials are showing great potential for the improvement of water treatment technologies. In recent years, catalysis and photocatalysis processes using gold nanoparticles (Au-NPs) have received great attention due to their effectiveness in degrading and mineralizing organic compounds. This paper aims to review and summarize the recently published works and R & D progress in the field of photocatalytic oxidation of various water pollutants such as toxic organic compounds (i.e. azo dyes and phenols) by Au-NPs/TiO2 under solar, visible and UV irradiation. Extensive research which has focused on the enhancement of photocatalysis by modification of TiO2 employing Au-NPs is also reviewed. Moreover, the effects of various operating parameters on the photocatalytic activity of these catalysts, such as size and loading amount of Au-NPs, pH and calcination, are discussed. The support type, loading amount and particle size of deposited Au-NPs are the most important parameters for Au/TiO2 catalytic activity. Our study showed in particular that the modification of TiO2, including semiconductor coupling, can increase the photoactivity of Au/TiO2. In contrast, doping large gold NPs can mask or block the TiO2 active sites, reducing photocatalytic activity. The optimized loading amount of Au-NP varied for each experimental condition. Finally, research trends and prospects for the future are briefly discussed.
Article
Although TiO2 is one of the most efficient photocatalysts, with the highest stability and the lowest cost, there are drawbacks that hinder its practical applications like its wide band gap and high recombination rate of the charge carriers. Consequently, many efforts were directed toward enhancing the photocatalytic activity of TiO2 and extending its response to the visible region. To head off these attempts, modification of TiO2 with noble metal nanoparticles (NMNPs) received considerable attention due to their role in accelerating the transfer of photoexcited electrons from TiO2 and also due to the surface plasmon resonance which induces the photocatalytic activity of TiO2 under visible light irradiation. This insightful perspective is devoted to the vital role of TiO2 photocatalysis and its drawbacks that urged researchers to find solutions such as modification with NMNPs. In a coherent context, we discussed here the characteristics which qualify NMNPs to possess a great enhancement effect for TiO2 photocatalysis. Also we tried to understand the reasons behind this effect by means of photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra, and Density Functional Theory (DFT) calculations. Then the mechanism of action of NMNPs upon deposition on TiO2 is presented. Finally we introduced a survey of the behaviour of these noble metal NPs on TiO2 based on the particle size and the loading amount.
Article
Analysis of the shape of the curve of reflected x-ray intensity vs glancing angle in the region of total reflection provides a new method of studying certain structural properties of the mirror surface about 10 to several hundred angstroms deep. Dispersion theory, extended to treat any (small) number of stratified homogeneous media, is used as a basis of interpretation.Curves for evaporated copper on glass at room temperature are studied as an example. These curves may be explained by assuming that the copper (exposed to atmospheric air at room temperature) has completely oxidized about 150A deep. If oxidation is less deep, there probably exists some general reduction of density (e.g., porosity) and an electron density minimum just below an internal oxide seal. This seal, about 25A below the nominal surface plane, arrests further oxidation of more deeply-lying loose-packed copper crystallites.All measurements to date have been carried out under laboratory atmospheric conditions which do not allow satisfactory separation or control of the physical and chemical variables involved in the surface peculiarities. The method, under more controlled conditions of preparation and treatment of the surface, promises to be useful.
Article
Here we demonstrate the photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 nm to 800 nm, by manipulating the shape of the decorated Au nanostructures. The samples were prepared by delicately depositing Au nanoparticles (NPs), Au nanorods (NRs) and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays. As compared to bare TiO2, Au NP-decorated TiO2 nanowire electrodes exhibited significantly enhanced photoactivity in both UV and visible regions. For Au NR-decorated TiO2 electrodes, the photoactivity enhancement was however noted in visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO2 nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV-visible region. Monochromatic incident photon-to-electron conversion efficiency (IPCE) measurements indicated that excitation of surface plasmon resonance (SPR) of Au is responsible for the enhanced photoactivity of Au nanostructures-decorated TiO2 nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO2 in UV region was attributable to the effective surface passivation of Au NPs. Furthermore, three-dimensional finite-difference time domain (FDTD) simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO2 upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO2 in UV region was partly due to the increased optical absorption of TiO2 associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems that can effectively harvest the entire UV-visible light for solar fuel production.
Article
X-ray photoelectron spectroscopy (XPS) investigations of supported nanoparticles smaller than 10 nm show a significant shift of the electron binding energy of core levels compared with the bulk values. In this work, such shifts were examined at differently supported and prepared gold nanoparticles for the 4f electron level. Special attention was paid to the influence of reducing pretreatment in hydrogen and, moreover, the influence of different oxide supports. Surprisingly, in most cases, lower binding energies than the Au 4f7/2of 84.0 eV were observed depending on the oxidic support as well as the pretreatment conditions. The origin of these differences of the core level values are discussed in terms of different models like electron transfer from the support to the particles, size and geometric effects. It seems that especially geometric factors like the particle shape play an important role.
Article
Au–TiO2/bentonite samples were prepared via deposition–precipitation method and calcined at different temperatures. These samples were characterized by X-ray diffraction (XRD), UV–vis diffusion reflectance spectroscopy (DRS), BET method, X-ray photoelectron spectroscopy (XPS) and TEM. The photocatalytic activities of the samples were tested by photodegradation of sulforhodamine B (SRB) under ultraviolet (UV) and visible light irradiation. The result showed that Au–TiO2/bentonite catalysts exhibited higher efficiency for mineralizing SRB than the well-known commercial TiO2 photocatalyst P25 in terms of COD changes. The most important advantage of Au–TiO2/bentonite over P25 was that it could be readily separated from aqueous suspensions by sedimentation after the reaction. It can maintain almost the same activity after being repeatedly used for 12 times. Possible mechanisms for SRB photoreaction in the presence of Au–TiO2/bentonite were proposed in this paper.
Article
TiO2 nanoparticles when subjected to UV irradiation exhibit blue coloration as electrons are stored within the particles. Upon contact with gold nanoparticles, a partial disappearance of the blue color is seen as the stored electrons are transferred from TiO2 to Au nanoparticles. The charge distribution between the semiconductor and metal nanoparticles causes the Fermi level to shift to more negative potentials. By employing C60/C60•- as a probe−redox couple, we were able to estimate the apparent Fermi levels of TiO2 and TiO2/Au nanoparticles. A Fermi level shift of −22 mV observed for the Au−TiO2 nanocomposite is indicative of improved charge separation in semiconductor−metal systems and demonstrates its usefulness for improving the efficiency of photocatalytic reactions.
Article
X-ray photoelectron spectroscopy (XPS) was employed to study the surface composition and electronic structure of Au/TiO2 catalysts in comparison with TiO2 (anatase) and to reveal time-dependent X-ray irradiation damage of the samples. The occurrence of Au nano-sized particles on a TiO2 support was found to result in a slight shift of Ti 2p core-level spectrum and in changes of the valence band and X-ray induced Auger spectra, compared to TiO2-only. It was shown that for different means of energy referencing the charge-corrected Au 4f7/2 binding energy in Au/TiO2 catalysts was 0.15–0.45 eV lower than that in pure bulk Au. Exposure to X-rays of Au/TiO2 catalysts and pure TiO2 caused a reduction of Ti 4+ oxidation state and desorption of oxygen from the surface. As a result, the surface chemical composition and electronic structure of the samples changed with time. The X-ray irradiation affected charge transfer processes in Au/TiO2 so that the pattern of X-ray induced damage in the Au-based catalyst turned out to be quite different from that in TiO2, with some characteristics displaying the very opposite features. Decreasing of the Au 4f7/2 binding energy and concurrent increasing of the fraction of Ti3+ species observed in the beginning of X-ray irradiation of Au/TiO2 may be taken as direct evidence for charge transfer from oxygen vacancies created by irradiation to Au particles.
Article
Cetylpyridinium chloride(CPC)-stabilized gold organosol in toluene has been prepared by using a two-phase (water−toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of the surfactant, CPC. The surfactant-stabilized gold nanoparticles were exploited to examine their optical properties when exposed to various solvent systems and ligands by measuring the changes in the localized surface plasmon resonance (LSPR) spectrum. It was seen that the position of the surface plasmon band of metal nanoparticles is greatly influenced by the solvents and the ligands under consideration. The surface plasmon absorption maxima modulates/varies between 520 and 550 nm for gold nanoparticles, depending on the refractive index of the solvent. The significant discovery presented here is that λmax of the LSPR shifts to the blue by 3 nm for the increase of one carbon atom in the alcohol chain. Cationic and anionic surfactants of different chain lengths induce changes in the optical properties of gold nanoparticles, whereas zwitterionic amino acid molecules do not incite remarkable changes in the LSPR spectrum. The λmax of the LSPR gradually shifts to the red with the increase in chain length for both the cationic and anionic surfactants indicating specific binding of the surfactant molecules around the gold particles. Binding of three model compounds (1-dodecylamine, 1-dodecanol, and 1-dodecanethiol) indicates their relative affinity toward the gold surface that corroborate the HSAB (Hard−Soft Acid−Base) principle.
Article
The degradation (oxidation) of methylene blue assisted by TiO2 photocatalysis (with an irradiation wavelength of 365 nm) was investigated in aqueous suspension. The rate constants for this heterogeneous photocatalysis were evaluated as a function of the concentration of the dye, the amount of TiO2 and the pH. The reaction is found to conform to a Langmuir adsorption isotherm and the rate vs. pH profile exhibits a linear increase with pH in acidic solutions with a maximum at around pH 6.9.
Article
Gamma-Al2O3, ZrO2, and TiO2 gold supported model catalysts have been synthesized by laser vaporization. Structural characterization using Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy experiments have shown that the gold clusters deposited on the different supports have similar distribution of size centered around 3 nm and are in the metallic state. However, X-ray photoemission measurements also indicate lower binding energies than the usual Au 4f(7/2) at 84.0 eV for both alumina and titania supported catalysts, indicating a modification of the electronic structure of the metal. One has taken benefit of these features to study the influence of the nature of the support toward CO oxidation activities without being hindered by particle size or gold oxidic species effects. By comparing the activities of the different catalysts, it is concluded that the nature of the support directly affects the activity of gold. The following tendency is observed: titania and zirconia are superior to alumina as supports, titania being slightly better than zirconia. From XPS and activity results we can conclude that the existence of negatively charged clusters is not the key point to explain the high activity observed for Au/ZrO2 and Au/TiO2 catalysts and also that metallic Au is the major catalytically active phase. Hence, due to their very nature, titania and to a less extent zirconia should participate to the catalytic process.
Plasmon Enhanced Internal Photoemission in Antenna
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