Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO₂. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO₂ catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO₂, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO₂. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH₄⁺, NO₃⁻, Cl⁻ and SO₄²⁻.     

Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation

Nitrogen and sulfur co-doped and N-doped TiO₂ anatase TKP 102 (Tayca) were prepared by manual grinding with thiourea and urea, respectively, and annealing at 400 °C. Both materials showed visible-light absorption as measured by Diffuse Reflectance Spectroscopy (DRS). Interstitial N-doping, anionic and cationic S-doping was found when the TiO₂ was doped with thiourea while TiO₂ doped with urea showed only the presence of interstitial N-doping as measured by X-ray Photo-electron Spectroscopy (XPS). The N content on the surface of N-doped TKP 102 photocatalyst was 2.85 at.% and higher than the N content in the N, S co-doped TiO₂ photocatalyst (0.6 at.%).The photocatalytic activity of the doped catalysts was tested using phenol and Escherichia coli as chemical and biological targets, respectively, using N, S co-doped, N-doped TiO₂, undoped Degussa P-25 and undoped TKP 102 powders under simulated solar light. It was found that undoped Degussa P-25 was the photocatalyst with the highest photocatalytic activity towards phenol oxidation and E. coli inactivation. N, S co-doped powders showed almost the same photocatalytic activity as undoped TKP 102 while N-doped TKP 102 was the less active photocatalyst probably due the N impurities on the TiO₂ acting as recombination centers.     

One-pot solvothermal synthesis of 1D plasmonic TiO2@Ag nanorods with enhanced visible-light photocatalytic performance

A plasmonic photocatalyst is designed and synthesized by coating TiO₂ nanoparticles onto the surface of Ag nanorods (Ag NRs), namely one-dimensional TiO₂@Ag NRs photocatalyst. Materials characterizations suggest that the as-prepared catalysts consist of a uniform and high crystalline anatase nanocrystals and highly conductive Ag nanorod. It delivers remarkable visible-light photocatalytic performance in simultaneous H₂ evolution and decomposition of methyl orange (MO) due to the synergetic effects of plasmonic resonance under visible light irradiation, a matched heterojunction, high surface area and large conductivity.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.     

Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.     

Design of cocatalyst loading position for photocatalytic water splitting into hydrogen in electrolyte solutions

Photocatalytic water splitting into hydrogen is a very attractive and desirable technology to realize sustainable and renewable green energy conversion. Up to now, many research results have confirmed that cocatalyst such as Pt is essential for a high efficiency photocatalytic H₂ evolution system. In a traditional view, the cocatalyst should be closely combined with the photocatalyst to achieve a high H₂ or O₂ photo-productive rate. In this work, an unusual point has been put forward that the suitable loading position of cocatalyst Pt for film-type TiO₂ catalyst is on the bare Fluorine-doped tin oxide (FTO) substrate instead of on the surface of TiO₂ in the electrolyte solutions. Especially, in acidic electrolyte, the hydrogen production rate of this new designed catalyst with Pt loaded on FTO (TiO₂-Pt/FTO) reaches 2.4 times that of the common catalyst with Pt loaded on TiO₂ (Pt/TiO₂-FTO). According to the experiment results, it is supposed that this loading way of cocatalyst on the substrate can construct a self-bias photocatalytic electrochemical cell system, drive electrolyte ions' movement directionally, and obtain high photocatalytic H₂ production efficiency. The universality of this innovation has been verified by CdS and CdS@TiO₂ film-type catalysts. This study provides a new guide in exploring high-efficiency film-type photocatalytic system for water splitting into hydrogen in the electrolyte solution.     

Fabrication of ZnO/SrTiO3 nanoarrays and its photoelectrochemical performances

The ZnO/SrTiO₃ nanomaterials were fabricated by a chemical conversion hydrothermal method in order to utilize the high electron transfer rate of one-dimensional ZnO nanorods and photocatalytic activity of SrTiO₃. The technological parameters, such as TiO₂ sol concentration, TiO₂ sol dipping cycle, Sr(NO₃)₂ concentration and reaction temperature, were investigated in the synthetic process and the reaction mechanism of the ZnO/SrTiO₃ nanomaterials was proposed. A photocurrent density of 7.53 mA/cm² was obtained for the as-prepared ZnO/SrTiO₃ photocatalyst, attributed to its improved absorption spectrum and appropriate nanostructure, which indicates a potential application in photoelectrochemical water splitting.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Enhancement of photocatalytic hydrogen production by liquid phase plasma irradiation on metal-loaded TiO2/carbon nanofiber photocatalysts

Enhanced hydrogen production by photocatalytic decomposition was assessed using liquid phase plasma over metal-loaded photocatalysts. Effects of irradiation of the liquid phase plasma were evaluated in the photocatalytic hydrogen production of hydrogen. Carbon nanofiber was introduced as photocatalytic support for the Ni-loaded TiO₂ photocatalyst. The influence of addition of organic reagents into water on hydrogen evolution was also evaluated. The photocatalytic decomposition by irradiation of the liquid phase plasma without photocatalyst produced some hydrogen evolution. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. The carbon nanofiber acted as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanofiber support. Hydrogen evolution was increased significantly by the addition of organic reagents, which acted as a type of sacrificial reagent promoting photocatalysis.     

Design and preparation of CdS/H-3D-TiO2/Pt-wire photocatalysis system with enhanced visible-light driven H2 evolution

In recent years, tremendous efforts have been devoted to develop new photocatalyst with wide spectrum response for H₂ generation from water or aqueous solution. In this work, CdS nanoparticles (NPs) have been immobilized on hydrogenated three-dimensional (3D) branched TiO₂ nanorod arrays, resulting in a highly efficient photocatalyst, i.e, CdS/H-3D-TiO₂. In addition, electrochemical reduction of H⁺ ion is identified as a limiting step in the photocatalytic generation of H₂ at this catalyst, while here a Pt wired photocatalysis system (CdS/H-3D-TiO₂/Pt-wire) is designed to overcome this barrier. Without the application of potential bias, visible light photocatalytic hydrogen production rate of CdS/H-3D-TiO₂/Pt-wire is 18.42 μmol cm^?2 h^?1, which is 11.2 times that of CdS/H-3D-TiO₂ without Pt (1.64 μmol cm^?2 h^?1). The Pt wire acts as an electron super highway between the FTO substrate and H⁺ ions to evacuate the generated electrons to H⁺ ions and catalyze the reduction reaction and consequently generate H₂ gas. This work successfully offers a novel direction for dramatic improvement in H₂ generation efficiency in photocatalysis field.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

High quantum efficiency Pt/TiO2 catalyst for sacrificial water reduction

Platinum photodeposition on TiO₂ from methanolic solution of chloroplatinic acid (CPA) is investigated to determine the conditions that give optimum photocatalytic activity towards dehydrogenation of methanol. Conditions favoring nucleation of Pt islets rather than their autocatalytic growth enhance the catalytic activity. Photoplatinization from idoplatinic acid, adsorbing more strongly on TiO₂ than CPA produced more active Pt/TiO₂ catalysts. The best catalyst prepared from CPA yielded H₂ from 12.5% methanol solution at a quantum efficiency of 23.9% whereas for idoplatinic acid based catalysts, the quantum efficiency increased to 42.5%.     

Visible light induced dye-sensitized photocatalytic hydrogen production over platinized TiO2 derived from decomposition of platinum complex precursor

Pt/TiO₂ derived from complete decomposition of the surface-anchored Pt(dcbpy)Cl₂ (dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) precursor (denoted as C-Pt/TiO₂) was prepared to serve as photocatalyst in visible light region. For dye-sensitized hydrogen production experiments, the photocatalyst was sensitized by Ru(2,2′-bipyridine-4,4′-dicarboxylic)₂(NCS)₂ (the N3 dye) and Ru(2,2′bipyridyl-4,4′-dicarboxylic) (4,4′- dinonyl-2,2′bipyridine) (NCS)₂ (the Z907 dye) to induce hydrogen evolution in the presence of sacrificial electron donor, triethanolamine (TEA). The hydrogen generation results showed that C-Pt/TiO₂ was found to be a much more active photocatalyst when compared to P-Pt/TiO₂, prepared by conventional method of photochemical deposition of H₂PtCl₆ (denoted as P-Pt/TiO₂). For further investigation, the photodegradation experiments in visible region were also confirmed the better photocatalytic activity of C-Pt/TiO₂. The enhanced catalytic activity is due to efficient interparticle electron transfer with the small-size and high-disperse platinum particles generated from photodeposition of Pt(dcbpy)Cl₂, which was verified by the transmission electron microscopy (TEM) measurement.     

CdS@Mg(OH)2 core/shell composite photocatalyst for efficient visible-light photocatalytic overall water splitting

Coating a protective agent or promoter on the surface of the photocatalyst is a proven good strategy to realize photocatalytic hydrogen production from pure water, but remains still a considerable challenge. Herein, a novel CdS@Mg(OH)₂ core/shell composite nanorods photocatalyst was synthesized by coating Mg(OH)₂ on CdS surface by hydrothermal and precipitation processes. The coated-Mg(OH)₂ layer did not change the structure of CdS, and the photocatalytic overall water splitting performance of the CdS@Mg(OH)₂ under visible light irradiation was improved obviously. After loading nano-Pt via the photodeposited method, the hydrogen production rate and stability of Pt/CdS@Mg(OH)₂ were 3.3 and 2.4 times that of the Pt/CdS under the visible light irradiation, respectively. The surface Mg(OH)₂ layer improved the hydrophilicity and stability of the core/shell composites and increased the amount of active sites, thus improving the photocatalytic properties. It is believed that Mg(OH)₂ can be used as a new co-catalyst to enhance the performance of photocatalytic overall water splitting.     

Membrane electrode assembly-based photoelectrochemical cell for hydrogen generation

A novel photoelectrochemical cell (PEC) for generation of hydrogen via photocatalytic water splitting is proposed and investigated. At the heart of the PEC is a membrane electrode assembly (MEA) integrated with Degussa P25 TiO₂ powder as a model photocatalyst for the photoanode and Pt catalyst powder for the dark cathode, respectively. It serves as a compact photocatalytic reactor for water splitting as well as an effective separator for the generated hydrogen and oxygen. The unique characteristic of the MEA-based PEC is that the use of co-catalyst, sacrificial reagent and supporting electrolyte in the cell is totally not required. The novel PEC can be operated without addition of water in the cathode compartment resulting in improved photo conversion efficiency. In addition, the application of a Degussa P25/BiVO₄ mixed photocatalyst was found to significantly enhance the hydrogen generation. Further improvements for the MEA-based PEC utilizing solar energy are also proposed.     

Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles

Photo-induced reforming of methanol, ethanol, glycerol and phenol at room temperature for hydrogen production was investigated with the use of ultra-small Pt nanoparticles (NPs) loaded on TiO₂ nanotubes (NTs). The Pt NPs with diameters between 1.1 and 1.3 nm were deposited on TiO₂ NTs by DC-magnetron sputtering (DC-MS) technique. The photocatalytic hydrogen rate achieved an optimum value for a loading of about 1 wt% of Pt. Apparent quantum yield for hydrogen generation was measured for methanol and ethanol water solutions reaching a maximum of 16% under irradiation with a wavelength of 313 nm in methanol/water solution (1/8 v/v). Pt NPs loaded on TiO₂ NTs represented also a true water splitting catalyst under UV irradiation and pure distilled water. DC-MS method appears to be a technologically simple, ecologically benign and potentially low-cost process for production of an efficient photocatalyst loaded with ultra-small NPs with precise size control.     

ZnO–Bi2O3/graphene oxide photocatalyst with high photocatalytic performance under visible light

Abstract

Photocatalytic nanomaterials are attracting more and more attention because of their potential for solving environmental problems. ZnO, as one of the most promising photocatalysts, can only be excited by ultraviolet (UV) or near UV radiation. The objective of the study is to describe an efficient visible light driven ZnO based photocatalyst. In this regard, we communicate the preliminary research on the synthesis, characterisation and photocatalytic properties of ZnO–Bi₂O₃/graphene oxide (GO) composite materials. It was found that the photodegradation of methylene blue in the presence of ZnO–Bi₂O₃/GO reached 99·62% after irradiation with visible light for 2 h. The presence of GO enhances the stability of ZnO–Bi₂O₃ and reduces the recombination of charge carriers. ZnO–Bi₂O₃/GO also shows high photocatalytic activity for the degradation of acid blue, acid yellow, reactive red, acid red, reactive yellow and reactive blue under visible light irradiation. The novel aspect is the combination of GO and Bi₂O₃ doped ZnO. The use of GO enhances the efficiency of photocatalysis, and Bi₂O₃ doping ZnO excites the absorption of visible light. The impact of the research concerns the study of ZnO–Bi₂O₃/GO, which can be used as a promising photocatalyst for the treatment of textile wastewater.     

Green synthesis of fibrous hierarchical meso-macroporous N doped TiO2 nanophotocatalyst with enhanced photocatalytic H2 production

A new approach to prepare hierarchical and fibrous meso-macroporous N-doped TiO₂ is attempted at room temperature without using templates by the addition of titanium isopropoxide droplets to the ammonia solution. The catalysts are thoroughly characterized by physico-chemical and spectroscopic method to explore the structural, electronic and optical properties. The photocatalytic activities of the catalyst were evaluated with hydrogen generation. NTP catalyst calcined at 400 °C (NTP-400) exhibited 602.7 μmol/3 h H₂ generation from 10 vol.% methanol under visible light. The excellent photocatalytic activity for NTP-400 is attributed to the porous networks existing in our system with uniform N dispersion throughout the catalyst. The hierarchical and fibrous structures allow easy channelization of electron as in the case of nanotubes for effective surface charge transfer. Along with macroporosity, nitrogen incorporation and mesoporosity play some important roles for enhanced photoactivities.     

Single crystalline TiO2 nanorods with enhanced visible light activity for solar hydrogen generation

Single crystalline TiO₂ nanorods and polycrystalline nanotubes were fabricated with same length to investigate the effects of their nanostructures on photocatalytic properties for splitting water. In order to enhance the visible light absorbance, TiO₂ nanorods and nanotubes were sensitized with semiconductor nanoparticles such as CdS, CdSe, and CdS/CdSe, and compared in viewpoint of solar hydrogen generation. It was observed that single-crystalline nanorods showed superior photocatalytic properties to polycrystalline nanotubes, and also the potential level of the nanorods with rutile phase was measured as lower than that of the nanotubes with mixture of anatase and rutile. Further improvement of photo-conversion efficiency was obtained by subsequent heat treatments of the sensitized photoelectrodes. It turns out that the improvement is attributed to the improved crystallinity and the increased size of the nanoparticles during the post-annealing treatments. It was demonstrated that TiO₂ nanorods with lower potential level and a single crystalline phase on FTO glass were advantageous for effective charge injection from the sensitized nanoparticles and transport without recombination lost at grain boundaries.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.