Electrospun TiO2/SnO2 nanofibers with innovative structure and chemical properties for highly efficient photocatalytic H2 generation

Innovative TiO₂/SnO₂ nanofibers were fabricated via electrospinning an innovated precursor solution and used for photocatalytic H₂ generation. The nanofibers exhibited greatly enhanced H₂ evolution rate compared to bare TiO₂ nanofiber and P25. The enhanced efficiency of the TiO₂/SnO₂ nanofibers was attributed to its excellent synergistic properties: (1) its good mesoporosity; (2) the red-shift of absorbance spectra to enhance light absorbance capability; (3) its long nanofibrous structure and (4) anatase TiO₂ – rutile TiO₂ – rutile SnO₂ ternary junctions favorable for the separation of electrons and holes. Based on our experimental results, the optimum ratio of TiO₂/SnO₂ nanofibers with 3% Sn demonstrated the highest efficiency in H₂ generation.     

Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water

The photocatalytic activity for H₂ evolution from pure water over Pd loaded TiO₂ prepared by gardenia extract (Pd-Gardenia-TiO₂) is systematically investigated. The as-prepared photocatalysts are characterized by X-ray diffraction, high resolution transmission electron microscopy, Fourier transform infrared spectra, and X-ray photoelectron spectroscopy. Gardenia extract functions as reducing and stabilizing agents simultaneously. The mean size of the as-prepared Pd nanoparticles is in the range of 2.3 ± 0.5 nm based on TEM images. The Pd-Gardenia-TiO₂ catalyst exhibits good photocatalytic activity for H₂ evolution (93 μmol · h⁻¹ · g⁻¹), which is much higher than that of Pd photodeposited on TiO₂. Possible factors for its photocatalytic activity from pure water are also investigated.     

CuCr2O4/TiO2 heterojunction for photocatalytic H2 evolution under simulated sunlight irradiation

CuCr₂O₄/TiO₂ heterojunction has been successfully synthesized via a facile citric acid (CA)-assisted sol-gel method. Techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectrum (UV-vis DRS) have been employed to characterize the as-synthesized nanocomposites. Furthermore, photocatalytic activities of the as-obtained nanocomposites have been evaluated based on the H₂ evolution from oxalic acid solution under simulated sunlight irradiation. Factors such as CuCr₂O₄ to TiO₂ molar ratio in the composites, calcination temperature, photocatalyst mass concentration, and initial oxalic acid concentration affecting the photocatalytic hydrogen producing have been studied in detail. The results showed that the nanocomposite of CuCr₂O₄/TiO₂ is more efficient than their single part of CuCr₂O₄ or TiO₂ in producing hydrogen. The optimized composition of the nanocomposites has been found to be CuCr₂O₄·0.7TiO₂. And the optimized calcination temperature and photocatalyst mass concentration are 500 °C and 0.8 g l⁻¹, respectively. The influence of initial oxalic acid concentration is consistent with the Langmuir model.     

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.     

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.     

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.     

Enhanced photocatalytic hydrogen production by improving the Pt dispersion over mesostructured TiO2

Among other potential applications, mesoporous titania with high surface area and crystalline framework is attractive in photocatalytic hydrogen production. The mesoporous structure with pore walls formed by nanocrystals of anatase would provide a shorter distance of the electron–hole pairs to reach the photocatalyst surface and a higher surface area to deposit modifiers of its photocatalytic activity. In this work, we have successfully applied a hard-templating pathway to obtain ordered mesoporous titania (m-TiO₂) with high surface area and anatase as main crystalline phase. Subsequently, various amounts of metallic Pt have been deposited using different impregnation methods. All reactions performed exhibit, at short times, a rapid increase in the hydrogen production rate until a point in which a nearly constant value is achieved. The material prepared by the “citrate method”, based on reduction and encapsulation with sodium citrate of Pt nanoparticles before the photocatalytic reaction, leads to the highest hydrogen production rates with the shortest time to reach the change on the trend of the activity curve. The reason of this result is that citrate method provides very good dispersion and, specially, because the Pt nanoparticles are deposited and reduced preferentially within the pores of m-TiO₂, leading to stronger interactions than the other two explored dispersion routes (wetness impregnation and photodeposition). Thus, despite introducing less than half of the theoretical amount of Pt, citrate method produces close to twice the amount of hydrogen obtained by the other dispersion routes. This production capacity is even higher when the amount of Pt loaded is increased, with the optimal concentration being determined as 2% (w/w).     

Cobalt sulfide quantum dots modified TiO2 nanoparticles for efficient photocatalytic hydrogen evolution

Cobalt sulfide quantum dots (CoSx QDs) modified TiO₂ nanoparticles are prepared with a precipitation-deposition method using TiO₂, cobalt acetate and sodium sulfide as the precursors. CoSx QD acts as an effective cocatalyst, which accelerates the transfer of the photo-generated electrons and serves as the active site for the reaction between electrons and H₂O, thus enhancing the separation of the e⁻/h⁺ pairs and the photocatalytic H₂ production activity of TiO₂. The amount of CoSx exhibits an optimum value at about 5% (mole ratio to TiO₂), at which the H₂ production rate achieves 838 μmol h⁻¹ g⁻¹ using ethanol as the sacrificial reagent. This exceeds that of the pure TiO₂ by more than 35 times.     

Synthesis of g-C3N4/TiO2 with enhanced photocatalytic activity for H2 evolution by a simple method

A photocatalyst composed of graphite-like carbon nitride (g-C₃N₄) and TiO₂ was fabricated by a simple method to calcine the mixture of melamine and TiO₂ precursor. The photocatalyst has enhanced photoactivity for hydrogen evolution from water. Characterization by XRD, FTIR, SEM and elemental analysis showed that the crystal structure and morphologies of composites were affected by the amount of melamine in the composite. The UV–Vis characterization displayed that the optical absorption range of g-C₃N₄/TiO₂ hybrid was broadened with a synergistic effect. The photoactivity for H₂ evolution was shown that the best result obtained from the composite with 67 wt% melamine has about 5 times improvement compared with bare TiO₂ or pure g-C₃N₄. The enhanced photoactivity might be related with the favorable structure resulted from heat-treatment temperature, and the content of g-C₃N₄ participating in wide optical absorption, separation and transportation of electronic-holes, as well as morphology of composite.     

Enhanced hydrogen generation from methanol aqueous solutions over Pt/MoO3/TiO2 under ultraviolet light

The photocatalytic activity in hydrogen production from methanol reforming can be significantly enhanced by Pt/MoO₃/TiO₂ photocatalysts. Compared with Pt/P25, the photocatalytic activity of optimized Pt/MoO₃/TiO₂ shows an evolution rate of 169 μmol/h/g of hydrogen, which is almost two times higher than that of Pt/P25. XRD and Raman spectra show that MoO₃ are formed on the surface of TiO₂. It is found that with the bulk MoO₃ just formed, the catalyst shows the highest activity due to a large amount of heterojunctions and the high crystallinity of MoO₃. The HRTEM image showed a close contact between MoO₃ and TiO₂. It is proposed that the Z-scheme type of heterojunction between MoO₃ and TiO₂ is responsible for the improved photocatalytic activity. The heterojunction structure of MoO₃/TiO₂ does not only promote the charge separation, but also separates the reaction sites, where the oxidation (mainly on MoO₃) and reduction (on TiO₂) reactions occurred.     

Hydrogen production using Pt-loaded TiO2 photocatalysts

A series of synthesised TiO₂-based and commercial photocatalysts were modified by Pt photodeposition and a study made of their photocatalytic activity in hydrogen production. The modified commercial photocatalysts were Evonik P25, Kronos vlp7000 and Hombikat UV-100, and the other modified photocatalysts were synthesised by our group using sol–gel and sol–gel hydrothermal processes (SG400, SG750 and HT). Pt weight percentages used in the study were 0.5, 1.0 and 2.1 wt.% (Pt/TiO₂). The photocatalysts were extensively characterised by X-ray diffraction (XRD), UV–vis diffuse reflectance, Brunauer–Emmett–Teller (BET) surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR) and laser light dispersion. Methanol (25% vol.) was used as sacrificial agent over the 8 h of the hydrogen production tests and measurements were taken of the final concentrations of formaldehyde and formic acid as well as initial and final TOC. Photoactivity of all photocatalysts increased in the presence of Pt. The most efficient of the synthesised photocatalysts was SG750 and of the commercial photocatalysts P25. Maximum production of SG750 was 1846 μmol h⁻¹ at 1.0 wt.% Pt and its production per surface unit was notably higher than that of P25.     

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%.     

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.     

Controllable O2 oxidization graphene in TiO2/graphene composite and its effect on photocatalytic hydrogen evolution

TiO₂/reduced graphene oxide composite (T-rGO) was synthesized and its performance was evaluated with photocatalytic hydrogen evolution. It was found that the hydrogen evolution rate of T-rGO increased significantly after injecting small amount of air into the vacuum pumped and UV irradiated sealed reaction cell. The IR, XPS, Raman and ESR spectra analysis indicated that the O₂^•−, which generated from the reaction of photoinduced electrons and the injected O₂ can moderately and controllably increase the oxygen groups on graphene planar of T-rGO at ambient condition. The amount of oxygen groups on graphene planar of T-rGO will affect the p-doping concentration of graphene, thus affect the p–n junction and the performance of T-rGO for photocatalytic hydrogen evolution.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Quantifying influence of operational parameters on photocatalytic H2 evolution over Pt-loaded nanocrystalline mesoporous TiO2 prepared by single-step sol–gel process with surfactant template

The photocatalytic evolution of hydrogen from water was investigated under various conditions over 0.6 wt% Pt-loaded nanocrystalline mesoporous TiO₂ photocatalyst prepared by a single-step sol–gel process with a surfactant template. The highly crystalline photocatalyst possessed a mesoporous characteristic with high surface area and narrow monomodal pore size distribution. More specifically, the influence of the following operational parameters, namely sacrificial reagent type, initial solution pH, photocatalyst concentration, initial sacrificial reagent concentration (of the best sacrificial reagent studied), and irradiation time, was the main focus. The hydrogen evolution was experimentally found to be strongly affected by all of the above parameters. The optimum values of initial solution pH, photocatalyst concentration, and sacrificial reagent concentration, as well as the appropriate type of sacrificial reagent, were obtained. The results showed that the utilization of the photocatalyst with the proper selection of optimum operational conditions could lead to considerably high photocatalytic hydrogen evolution activity.     

TiO2 nanosheets loaded with Cu: A low-cost efficient photocatalytic system for hydrogen evolution from water

Rutile TiO₂ nanosheets were prepared by a simple solvothermal process, and Cu was loaded on the surface of TiO₂ nanosheets using the in situ photo-deposition method. Meanwhile, photocatalytic H₂ evolution from water over the as-prepared TiO₂ nanosheets loaded with Cu was explored using methanol as a sacrificial reagent. The results indicate that the TiO₂ nanosheets loaded with Cu is an efficient photocatalyst under UV irradiation. During the first 5 h, a rate of H₂ evolution of approximately 22.1 mmol g⁻¹ h⁻¹ was achieved under optimal conditions. Furthermore, for practical purposes, the photocatalytic hydrogen evolution was studied as a function of content of Cu, pH of solution, concentration of methanol and dosage of photocatalyst, respectively. At last, the photocatalytic mechanism was preliminarily discussed.     

Low-temperature preparation of AgIn5S8/TiO2 heterojunction nanocomposite with efficient visible-light-driven hydrogen production

AgIn₅S₈ and AgIn₅S₈/TiO₂ heterojunction nanocomposite with efficient photoactivity for H₂ production were prepared by a low-temperature water bath deposition process. The resultant AgIn₅S₈ shows an absorption edge at ∼720 nm, corresponding to a bandgap of ∼1.72 eV, and its visible-light-driven photoactivity (100.1 μmol h⁻¹) for H₂ evolution is 9 times higher than that (11 μmol h⁻¹) of the product derived from a hydrothermal process, while the obtained AgIn₅S₈/TiO₂ heterojunction nanocomposites prepared by using commercially available TiO₂ nanoparticles (P25) as TiO₂ source exhibit remarkably improved photoactivity as compared to the pristine AgIn₅S₈, and the AgIn₅S₈/TiO₂ nanocomposite with molar ratio of 1:10 shows a maximum photocatalytic H₂ evolution rate (371.1 μmol h⁻¹), which is 4.3 times higher than that (85 μmol h⁻¹) of the corresponding AgIn₅S₈/TiO₂ nanocomposite derived from a hydrothermal method. This significant enhancement in the photocatativity of the present AgIn₅S₈/TiO₂ nanocomposite can be ascribed to the better dispersion of the AgIn₅S₈ formed on TiO₂ nanoparticle surfaces and the more intimate AgIn₅S₈/TiO₂ heterojunction structure during the water bath deposition process under continuously stirring as compared to the corresponding nanocomposite derived from a hydrothermal method. This configuration of nanocomposite results in fast diffusion of the photogenerated carriers in AgIn₅S₈ towards TiO₂, which is beneficial for separating spatially the photogenerated carriers and improving the photoactivity. The present findings shed light on the tuning strategy of spectral responsive region and photoactivity of photocatalysts for efficient light-to-energy conversion.     

A red La(OH)3/TiO2:B,N composite photocatalyst for broad-band visible-light-driven hydrogen evolution

A red La(OH)₃/TiO₂:B,N composite photocatalyst with broad-band visible-light-response was successfully synthesized through a three-step synthetic route. Structural, morphologic, optical and surface characteristics of the as-prepared photocatalysts were characterized by XRD, SEM, TEM, UV–vis, BET and XPS measurements. The 1.5% La added sample exhibited a high photocatalytic performance (48 μmol h⁻¹) in the presence of K₂CO₃ as the sacrificial agent under visible-light irradiation (λ > 400 nm), which is almost a 24-fold increase compared with the sample without La added.     

Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach

We report a green and facile approach for the preparation of carbon-modified (C-modified) TiO₂ composite materials by hydrothermal synthesis followed by pyrolytic treatment. The resultant materials were characterized by powder X-ray diffraction (XRD), nitrogen physisorption studies, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). The photocatalytic performances of these materials were evaluated by calculating the amount of hydrogen evolved from the decomposition of water under solar simulated irradiation conditions. An improvement was achieved from no H₂ evolution at all with the bare TiO₂, to an evolution of 0.21 mL g⁻¹ h⁻¹ from a composite material modified with an optimum carbon loading of 3.62%. These results suggested that the interaction of carbon with predominantly rutile form of TiO₂ can promote shallow trapping of photogenerated electrons in the oxygen vacancies. This phenomenon consequently enhances the photocatalytic activity by minimizing charge carrier recombination, a characteristic demonstrated by fluorescence quenching of the TiO₂ emission.