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Controllable synthesis and photocatalytic properties of ZnO hierarchical flower-like porous nanostructures

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Abstract

Two kinds of hierarchical ZnO flower-like porous nanostructures with numerous nanosheets were successfully synthesised through a templatefree and low-cost oxalic acid assisted solvothermal method combined with a heat post-treatment. The obtained products were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM results revealed that the two kinds of products had sword-like and hydrangea-like nanostructures originating from water and water/alcohol mixture solvents, respectively. XRD results confirmed that the growth process of ZnO involved a phase transformation from intermediate compound zinc oxalate hydroxide to ZnO. Furthermore, the photocatalytic properties were also characterised. The results indicated that both samples exhibited high photocatalytic activities for the degradation of rhodamine B, which were mainly ascribed to the porous hierarchical structures.

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Rapid development of nanofabrication techniques has created many different types of advanced nanosized semiconductors. Photocatalytic materials used to degrade organic and inorganic pollutants now include, in addition to TiO2, ZnO, Fe2O3, WO3, MoS2, and CdS. Nanoparticles’ unique properties, e.g. surface to volume ratio and quantum effects, continue to improve and expand photocatalysis’ role in areas like environmental remediation, odor control, sterilization, and renewable energy. Controlling semiconductor size, shape, composition, and microstructure promises to benefit future research and applications in these fields. This review examines recent advances at the interface of nanoscience and photocatalysis, especially pertaining to nanocatalyst enhancements, for current and future environmental applications.
Article
BiTaO4 photocatalyst for methylene blue (MB) degradation, which worked under visible-light irradiation, was systematically investigated for the first time. BiTaO4 possess similar degradation ability of TiO2−XNX under visible-light irradiation (λ > 420 nm). Density functional calculations revealed that Bi 6s orbitals contributed to the formation of valence band, resulting in narrowing the band gap. Moreover, comparing with BiNbO4, we found BiTaO4 showed photocorrosion suppression performance during the photocatalytic process. The differences in the photocorrosion suppression between BiTaO4 and BiNbO4 could be attributed to the conduction curvature and bandwidth, leading to differences in the mobility of photogenerated electrons formed in the conduction bands, which mainly consisted of Ta 5d and Nb 4d orbitals, respectively.
Article
We have demonstrated an environmentally friendly and template-free aqueous synthesis of hierarchically assembled 3D ZnO nanoflakes. The ZnO nanoflakes self-assembled to expose highly interconnected networks of well-defined catalytic active {0001} facets. Well dispersed Pt, Ag and Au metal nanoparticles were loaded to form hybrid ZnO nanoflakes for enhanced photocatalytic activity. The enhanced photocatalytic activity may be attributed to the synergetic effects of well-structured ZnO crystal facets, high metal nanoparticles dispersity, enhanced light absorption and charge-transfer kinetics which leads to high photocatalytic degradation.
Article
The photocatalytic degradation of reactive orange 16 (RO 16) dye has been investigated using thin film tubular flow photoreactor with ZnO catalyst coating. The degradation efficiency of RO 16 dye with immobilized catalyst and freely suspended catalyst are compared. The effect of process parameters such as, aeration, flow rate, amount of catalyst coating, pH and initial concentration of RO 16 dye on the extent of photodegradation of the dye have been optimized. The RO 16 dye completely photo-mineralized into CO2 and simple inorganic ions, such as nitrate, ammonium and sulphate ions. The immobilized catalyst is approximately two times less effective than the freely suspended catalyst. The presence of air positively influences the photocatalytic activation. The optimum reaction parameters are flow rate 9 ml/min, 0.3 g catalyst coating (3.75 mg/cm2, pH 10 and 100 μM RO 16 dye. The rate equation for the degradation based on Langmuir-Hinshelwood model has been proposed.
Article
Graphene based two-dimensional carbon nanostructures serve as a support to disperse catalyst nanoparticles. Reduced graphene oxide is used as a support to anchor semiconductor and metal nanoparticles. Such a design strategy would enable the development of a multifunctional catalyst mat. This Perspective focuses on the interaction between graphene oxide−semiconductor (TiO2, ZnO) and graphene oxide−metal (Au, Pt) nanoparticles and discusses potential applications in catalysis, light energy conversion, and fuel cells.
Article
The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices:  (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utilization of such tailored nanostructures or ordered hybrid assemblies will play an important task in achieving the desired goal of cheap and efficient fuel production (e.g., solar hydrogen production) or electricity (photochemical solar cells).
Article
a b s t r a c t Ground-based in-situ measurements were performed to investigate the spectral optical properties and hygroscopicity of light scattering and absorbing aerosols in pollution and dust plumes in Northeast Asia. The scattering Ångström exponents of pollution plumes are high, ranging from 0.8 to 1.8, because light scattering aerosols consist of relatively fine particles such as sulfate, nitrate, ammonium and carbona-ceous particles. Pollution case having high organic carbon (OC) exhibits a relatively high absorption Ångström exponent (AAE), in the range of 1.0e1.5 due to the absorption by refractory OC at near-UV spectral region. Among the different pollution cases, the AAE is the highest in a polluted dust plume; this can be attributed to the strong wavelength dependence of light absorption by dust particles. The mass absorption cross section (MAC), which represents the ability of light-absorbing aerosols to absorb a photon, is 4.2e8.3 m 2 g À1 , and it is high when black carbon is mixed with OC and dust particles. The MAC decreases with increasing relative humidity, especially for pollution plume with a high OC content and for dust plume. Under humid conditions, hygroscopic chemical species can absorb water vapor, thus growing in size and enhancing aerosol light scattering, but the MAC decreases because of the possible shielding effects of absorbing aerosols with water uptake.
Article
Two kinds of different ZnO nanostructures, nanotubes, and nanoflowers were synthesized using an aqueous solution approach. It was demonstrated that the addition of Al3+ and ZnO sol causes the different growth behavior of ZnO nanostructures, and leads to the formation of a nanotube array as well as self-assembled nanoflowers, respectively. The effects of reaction parameters on the morphology of ZnO nanostructures were investigated. The present route reports the possibility for morphology control and assembly of ZnO nanostructures on arbitrary substrates at a low temperature on a large scale.
Article
This review focuses on the heterogeneous photocatalytic treatment of organic dyes in air and water. Representative studies spanning approximately three decades are included in this review. These studies have mostly used titanium dioxide (TiO2) as the inorganic semiconductor photocatalyst of choice for decolorizing and decomposing the organic dye to mineralized products. Other semiconductors such as ZnO, CdS, WO3, and Fe2O3 have also been used, albeit to a much smaller extent. The topics covered include historical aspects, dark adsorption of the dye on the semiconductor surface and its role in the subsequent photoreaction, semiconductor preparation details, photoreactor configurations, photooxidation kinetics/mechanisms and comparison with other Advanced Oxidation Processes (e.g., UV/H2O2, ozonation, UV/O3, Fenton and photo-Fenton reactions), visible light-induced dye decomposition by sensitization mechanism, reaction intermediates and toxicity issues, and real-world process scenarios.
Article
The aim of this study was to investigate the efficiency of the electrochemically assisted photocatalytic process in the degradation of C. I. Reactive Blue 19 on a Ti/Ru0,3Ti0,7O2 electrode. By using the quartz device and the Ti-supported noble metal oxide coating, decolorizations ratios higher than 95% and TOC reduction of about 52% were observed during a period of 120 min. Comparing this value with the sum of the decolorization ratios obtained by a single application of electrochemical and photochemical procedures, it is possible to observe a significant synergic effect between both processes. The electrochemical process was conduced at 28°C (refrigeration with air) and pH of 11.0, and with a constant potential of 1.8 V versus Ag/AgCl. With these conditions, the mean current was 23 mA. The voltammetric results also confirm the synergic effect of the simultaneous process.
Article
A TiO₂/FTO (FTO=fluorine-doped tin oxide) electrode was prepared by dip-coating FTO in a suspension of TiO₂ prepared from a sol-gel method and was used as a photoanode to split an aqueous solution of formic acid to produce hydrogen. The surface of the TiO₂/FTO film was covered with assemblies of TiO₂ nanoparticles with a diameter of approximately 20 nm. Under irradiation by using a Xe lamp, splitting of formic acid was performed at different applied current densities. Compared to splitting water or utilizing FTO and Pt foil as the anode, the splitting voltage is much lower and can be as low as -0.27 V. The results show that the splitting voltage is related to the concentration of free formate groups. The evolution rate of hydrogen measured by using gas chromatography is 130 μmol h⁻¹ at a current density of 20 mA cm⁻² and the energy-conversion efficiency can be 1.79 %. Photoelectrolysis of formic acid has the potential to be an efficient way to produce hydrogen with a high energy-conversion efficiency.
Article
Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.
Article
The microbiological quality of coastal or river waters can be affected by faecal pollution from human or animal sources. An efficient MST (Microbial Source Tracking) toolbox consisting of several host-specific markers would therefore be valuable for identifying the origin of the faecal pollution in the environment and thus for effective resource management and remediation. In this multidisciplinary study, after having tested some MST markers on faecal samples, we compared a selection of 17 parameters corresponding to chemical (steroid ratios, caffeine, and synthetic compounds), bacterial (host-specific Bacteroidales, Lactobacillus amylovorus and Bifidobacterium adolescentis) and viral (genotypes I-IV of F-specific bacteriophages, FRNAPH) markers on environmental water samples (n = 33; wastewater, runoff and river waters) with variable Escherichia coli concentrations. Eleven microbial and chemical parameters were finally chosen for our MST toolbox, based on their specificity for particular pollution sources represented by our samples and their detection in river waters impacted by human or animal pollution; these were: the human-specific chemical compounds caffeine, TCEP (tri(2-chloroethyl)phosphate) and benzophenone; the ratios of sitostanol/coprostanol and coprostanol/(coprostanol+24-ethylcopstanol); real-time PCR (Polymerase Chain Reaction) human-specific (HF183 and B. adolescentis), pig-specific (Pig-2-Bac and L. amylovorus) and ruminant-specific (Rum-2-Bac) markers; and human FRNAPH genogroup II.