Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light

The advantage of copper doping onto TiO₂ semiconductor photocatalyst for enhanced hydrogen generation under irradiation at the visible range of the electromagnetic spectrum has been investigated. Two methods of preparation for the copper-doped catalyst were selected – complex precipitation and wet impregnation methods – using copper nitrate trihydrate as the starting material. The dopant loading varied from 2 to 15%. Characterization of the photocatalysts was done by thermogravimetric analysis (TGA), temperature programmed reduction (TPR), diffuse reflectance UV-Vis (DR-UV-Vis), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Photocatalytic activity towards hydrogen generation from water was investigated using a multiport photocatalytic reactor under visible light illumination with methanol added as a hole scavenger. Three calcination temperatures were selected – 300, 400 and 500 °C. It was found that 10 wt.% Cu/TiO₂ calcined at 300 °C for 30 min yielded the maximum quantity of hydrogen. The reduction of band gap as a result of doping was estimated and the influence of the process parameters on catalytic activity is explained.     

Photocatalytic properties of redox-treated Pt/TiO2 photocatalysts for H2 production from an aqueous methanol solution

The influence of redox-treated Pt/TiO₂ photocatalysts on H₂ production is investigated. Catalyst characterizations are performed by TEM, XPS, XRD, BET, and UV–vis/DR spectroscopy techniques. In terms of production rate, the oxidation treatment shows higher reactivity than the reduction treatment. The reduction treatment allows the formation of metallic Pt(0), which more easily catalyzes the transition of TiO₂ from the anatase to the rutile phases. Reduction-treated Pt/TiO₂ photocatalysts have lower S_BET values than oxidation-treated Pt/TiO₂ photocatalysts due to the higher percentage of TiO₂ in the rutile phase. Combining the results of XPS and optical analyses, PtO/TiO₂ shows a higher energy band gap than metallic Pt(0)/TiO₂, indicating that oxidation-treated Pt/TiO₂ is more capable of achieving water splitting for H₂ production. According to the results of this study, the oxidation treatment of Pt/TiO₂ photocatalysts can significantly enhance the reactivity of photocatalytic H₂ production because of their homogenous distribution, lower phase transition, higher S_BET, and higher energy band gap.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

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.     

Direct Z-scheme anatase/rutile bi-phase nanocomposite TiO2 nanofiber photocatalyst with enhanced photocatalytic H2-production activity

Design and preparation of direct Z-scheme anatase/rutile TiO₂ nanofiber photocatalyst to enhance photocatalytic H₂-production activity via water splitting is of great importance from both theoretical and practical viewpoints. Herein, we develop a facile method for preparing anatase and rutile bi-phase TiO₂ nanofibers with changing rutile content via a slow and rapid cooling of calcined electrospun TiO₂ nanofibers. The phase structure and composition, surface morphology, specific surface area, surface chemical composition and element chemical states of TiO₂ nanofibers were analyzed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). By a rapid cooling of 500 °C-calcined electrospun TiO₂ precursor, anatase/rutile bi-phase TiO₂ nanofibers with a roughly equal weight ratio of 55 wt.% anatase and 45 wt.% rutile were prepared. The enhanced H₂ production performance was observed in the above obtained anatase/rutile composite TiO₂ nanofibers. A Z-scheme photocatalytic mechanism is first proposed to explain the enhanced photocatalytic H₂-production activity of anatase/rutile bi-phase TiO₂ nanofibers, which is different from the traditional heterojunction electron–hole separation mechanism. This report highlights the importance of phase structure and composition on optimizing photocatalytic activity of TiO₂-based material.     

The pH effects on H2 evolution kinetics for visible light water splitting over the Ru/(CuAg)0.15In0.3Zn1.4S2 photocatalyst

Much progress has been made in the development of novel visible light photocatalysts that split water into hydrogen (H₂) and oxygen (O₂). In this study, we examine the impact of initial solution pH on H₂ production using an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst under visible light irradiation. In addition, the reaction mechanism was analyzed by examining the oxidation products of the electron donor (I‾) at different solution pH values. The results show that the initial pH significantly influenced the rate of H₂ production and quantum yield (QY). In particular, the photocatalyst yielded the highest apparent QY (∼12.8%) at 420 ± 5 nm and highest H₂ production rate (∼525 μmol h⁻¹) at pH 2; with increasing pH, the H₂ production and QY decreased significantly. The oxidation product of I‾ at pH < 6 was mainly I₃‾, whereas at pH > 6 water splitting did not occur at all, so no IO₃‾ or I₂ were observed.     

Synthesis of mesoporous-assembled TiO2 nanocrystals by a modified urea-aided sol-gel process and their outstanding photocatalytic H2 production activity

Mesoporous-assembled TiO₂ nanocrystals with very high photocatalytic H₂ production activity were synthesized through a modified sol-gel process with the aid of urea as mesopore-directing agent, heat-treated under various calcination temperatures, and assessed for their photocatalytic H₂ production activity via water splitting reaction. The resulting mesoporous-assembled TiO₂ nanocrystals were systematically characterized by N₂ adsorption-desorption analysis, surface area and pore size distribution analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The experimental results showed that the photocatalytic H₂ production activity of the synthesized mesoporous-assembled TiO₂ nanocrystal calcined at 500 °C, which possessed very narrow pore size distribution, was extraordinarily higher than that of the commercially available P-25 TiO₂ and ST-01 TiO₂ powders.     

Solvothermal synthesis of SnNb2O6 nanoplates and enhanced photocatalytic H2 evolution under visible light

Well-defined SnNb₂O₆ nanoplates are synthesized here by a facile template-free solvothermal route in a mixed solvent of water and ethanol without an organic surfactant. The synthesized nanoplates have widths ranging from 200 to 400 nm and thicknesses in a range of 20–30 nm. The nanoplates were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–Vis spectroscopy, Raman spectrometry, and by the Brunauer–Emmett–Teller method. The variation of the lattice parameters and the optical properties of the nanoplates were discussed in detail based on the crystal and electronic structure. The SnNb₂O₆ nanoplates exhibited greatly enhanced photocatalytic activity in terms of the reduction of water for H₂ generation under visible light irradiation as compared to the same compound prepared by a solid–state reaction method. This was mainly attributed to its higher surface area and extremely high two-dimensional anisotropy, which provided a short migration distance along the thickness direction.     

Properties and H2 production ability of Pt photodeposited on the anatase phase transition of nitrogen-doped titanium dioxide

The effect of calcination temperature on the properties and H₂ production ability of nitrogen-doped (N-doped) titanium dioxide (TiO₂) photodeposited with 0.2 wt% Pt (platinum) was studied. The increase in crystallinity of pre-calcinated N-doped TiO₂ initiated at temperatures higher than 131 °C transformed the morphology from anomalous nanostructure to texture composed of nanoparticles and enhanced the specific surface areas. At 200-400 °C, the anatase peaks gradually became sharper and the visible light absorption region decreased due to the growth of crystallites and the decrease of N-doping content, respectively. Maximum H₂ production was reached when N-doped TiO₂ was calcined at 200 °C followed by Pt photodeposition. The maximum condition is brought about by the formation of textures consisting of nanoparticles and a broad absorption region, thus creating superior active sites for photocatalytic H₂ production.     

TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation

The CdS/TiO₂ composites were synthesized using titanate nanotubes (TiO₂NTs) with different pore diameters as the precursor by simple ion change and followed by sulfurization process at a moderate temperature. Some of results obtained from XRD, TEM, BET, UV–vis and PL analysis confirmed that cadmium sulfide nanoparticles (CdSNPs) incorporated into the titanium dioxide nanotubes. The photocatalytic production of H₂ was remarkably enhanced when CdS nanoparticles was incorporated into TiO₂NTs. The apparent quantum yield for hydrogen production reached about 43.4% under visible light around λ = 420 nm. The high activity might be attributed to the following reasons: (1) the quantum size effect and homogeneous distribution of CdSNPs; (2) the synergetic effects between CdS particles and TiO₂NTs, viz., the potential gradient at the interface between CdSNPs and TiO₂NTs.     

Synergistic effect of Cu2O/TiO2 heterostructure nanoparticle and its high H2 evolution activity

Cu₂O/TiO₂ nanoparticles were prepared by solvothermal method, which formed the heterostructure of Cu₂O/TiO₂. Due to the heterostructure, the H₂ evolution rate under simulated solar irradiation was increasingly promoted. Meanwhile a certain amount of Cu particles which were confirmed by Transmission Electro Microscopy (TEM) and X-Ray Photoelectron Spectroscopy (XPS), formed on the surface of Cu₂O/TiO₂, and the photoactivity was accordingly further enhanced. The stabilized activity was maintained after many times irradiation. It is interesting that after a few hours irradiation the amount of Cu particles on the surface kept unchanged in the presence of Cu₂O and TiO₂. The Cu particles that formed during hydrogen generation reaction play a key role in the further enhancement of the hydrogen production activity. In this study, it is the first time to study the details on the formation of the stable ternary structure under simulated solar irradiation and their synergistic effect on the photoactivity of the water splitting.     

Factors affecting the production of H2 by water splitting over a novel visible-light-driven photocatalyst GaFeO3

A d¹⁰ photocatalyst, GaFeO₃ having a band gap of ∼2.7 eV, exhibits significant activity for the overall splitting of water under visible light (>395 nm) irradiation, in the absence of sacrificial reagent or a noble metal co-catalyst. The doping of an anion led to considerable enhancement in activity, the S-doped catalysts displaying better activity compared to the samples containing nitrogen. Even though the H₂/O₂ yields were affected by preparation-dependent grain morphology, no direct relationship was observed between the photoactivity of a sample and its specific surface area. The techniques of HRTEM, SEM, XPS, Laser Raman, UV–visible and photoluminescence spectroscopy have enabled to demonstrate that, besides the grain morphology, certain lattice imperfections and microstructure may also play a crucial role in water splitting activity of a photocatalyst. The factors responsible for catalyst deactivation are examined.     

H2 evolution from a photoelectrochemical cell with n-Cu2O photoelectrode under visible light irradiation

H₂ evolution was observed for the first time from a photoelectrochemical cell using an n-type Cu₂O photoelectrode under visible light irradiation. Three-electrode configuration was used in the photoelectrochemical cell to observe H₂ evolution. AgCl/Ag calomel electrode and a platinum plate were used as the reference and counter electrodes, respectively. Fe²⁺/Fe³⁺ redox couple was used as the electrolyte. H₂ evolution was visible on the platinum electrode in the photoelectrochemical cell.     

Yb-doped WO3 photocatalysts for water oxidation with visible light

Yb-doped WO₃ photocatalysts were prepared by co-sputtering WO₃ and Yb, followed by annealing in air for water oxidation with visible light. All the obtained photocatalysts were monoclinic with sputtering power of Yb up to 10 W and displayed no optical absorption red shift. In photoelectrochemical (PEC) studies, the photocurrent densities were improved with up to 0.34 at.% Yb in WO_3, with the highest photocurrent of 1.3 mA/cm² (1.2 V vs. Ag/AgCl) achieved with <0.1 at.% Yb. Electrochemical impedance spectroscopy (EIS) measurements showed that optimized Yb doping reduced charge transfer resistance and increased donor density of WO₃ photocatalyst. The improvement in photocurrent density was attributed to enhanced conductive carrier path, increased oxygen vacancies and 4f¹³ orbital configuration due to Yb³⁺ substitution of W⁶⁺.     

g-C3N4/TiO2可见光催化降解磺胺甲恶唑的研究

以TiO₂颗粒和三聚氰胺为原料,采用高温煅烧法制备g-C₃N₄/TiO₂复合光催化材料,研究其对仿生生态系统中磺胺类抗生素的去除效果。利用扫描电子显微镜（SEM）、X射线衍射仪(XRD)、傅里叶变换红外光谱仪（FTIR）、紫外可见分光光度计（UV-vis DRS）对g-C₃N₄/TiO₂进行表征,并研究在可见光条件下g-C₃N₄/TiO₂对溶液中磺胺甲恶唑（SMX）的光催化降解效果。结果表明,g-C₃N₄/TiO₂具有良好的光催化活性,在可见光照射下,当g-C₃N₄/TiO₂投加量为0.2 g·L^-1时,对初始质量浓度为200 μg·L^-1的SMX的去除率可达84.3%。在相同条件下,而g-C₃N₄和TiO₂只能分别去除21.0%和16.0%的SMX,同时在仿生系统中12.37 g·m^-2 g-C₃N₄/TiO₂可以去除95.35%的SMX。通过质谱分析推测,SMX可能的降解路径分别为S—N键断裂、C—N键断裂、S—C键断裂、SMX的羟基化和SMX上氨基的硝化反应,两种可能的中间产物分别为对氨基苯磺酰胺和3-氨基-5-甲基异恶唑。     

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.     

The size and valence state effect of Pt on photocatalytic H2 evolution over platinized TiO2 photocatalyst

Photocatalytic hydrogen production from water or organic compounds is a promising way to resolve our energy crisis and environmental problems in the near future. Over the past decades, many photocatalysts have been developed for solar water splitting. However, most of these photocatalysts require cocatalyst to facilitate H₂ evolution reaction and noble metals as key cocatalysts are widely used. Consequently, the condition of noble metal cocatalyst including the size and valence state etc plays the key role in such photocatalytic system. Here, the size and valence state effect of Pt on photocatalytic H₂ evolution over platinized TiO₂ photocatalyst were studied for the first time. Surprisingly, it was found that Pt particle size does not affect the photoreaction rate with the size range of several nanometers in this work, while it is mainly depended on the valence state of Pt particles. Typically, TOFs of TiO₂ photodeposited with 0.1–0.2 wt% Pt can exceed 3000 h⁻¹.     

Hydrogen production by mechano-chemical reaction of Ti2O3 in water

The purpose of this study is to clarify the physicochemical properties of Ti₂O₃ for hydrogen production. The hydrogen production properties of Ti₂O₃ before and after milling in pure water were investigated in a flat-bottomed reaction vessel made of a Pyrex glass cell which was attached to a closed-gas circulation system under an Ar atmosphere at room temperature using three different types of stirring rod. Ti₂O₃ was found to be a new material for mechano-chemical hydrogen production, although it did not exhibit photocatalysis or mechano-catalysis for hydrogen production.     

Large impact of heating time on physical properties and photocatalytic H2 production of g-C3N4 nanosheets synthesized through urea polymerization in Ar atmosphere

The effect of heating time on the polymerization processes of urea into g-C₃N₄ nanosheets was studied in Ar atmosphere. It was found that heating time had a great influence on the crystalline quality, specific surface area (S_BET) and photocatalytic H₂ production of the obtained g-C₃N₄ nanosheets. G-C₃N₄ nanosheets with some degree of disorders in crystal structure were formed within 1 h at 550 °C, and these structural disorders maintained after 4 h, however disorders disappeared after extended heating of 6 h. G-C₃N₄ nanosheets with similar disorders in the crystal structure hold similar S_BET and exhibit comparable H₂ production rates. The highest H₂ production rate of 1.4 mmol h⁻¹ g⁻¹ occurs after 8 h heating, corresponding to 2.6% quantum efficiency at 420 nm.