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.     

Photocatalytic hydrogen production from aqueous solutions over novel Bi0.5Na0.5TiO3 microspheres

Metal oxide compounds containing bismuth are considered as potential candidates for photocatalysis in both contaminant degradation and H₂ generation, due to the interesting lone electron pairs and the band gap narrowing effect of Bi³⁺. Quaternary perovskite oxide Bi_0.5Na_0.5TiO₃ was thus synthesized at low temperature via a soft chemical route. The influence of alkaline concentrations on the structure, morphology, and optical properties of the samples has been systematically investigated. All samples existed as hierarchical microspheres, which are consisted of cubic nanocrystallines. For the first time, the photocatalytic water splitting for H₂ evolution over Bi_0.5Na_0.5TiO₃ has been studied. A high H₂ evolution rate of 325.4 μmol h⁻¹ g cat⁻¹ under the irradiation of a 500 W xenon lamp was obtained. More importantly, no decrease in the catalytic performance was observed after three consecutive runs of 15 h, suggesting new possibility in designing multi-component photocatalysts for future applications.     

Sonochemistry synthesis of Bi2S3/CdS heterostructure with enhanced performance for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution from water splitting is an efficient, eco-friendly method for the conversion of solar energy to chemical energy. A great number of photocatalysts have been reported but only a few of them can respond to visible-light. Metal sulfides, a class of visible-light response semiconductor photocatalysts for hydrogen evolution and organic pollutant degradation, receive a lot of attention due to their narrow band gaps. Herein, we report the sonochemical synthesis of Bi₂S₃/CdS nanocrystal composites with microsphere structure at mild temperature. The phases of Bi₂S₃ and CdS can be observed obviously in HRTEM image. The heterostructure consisting of the two species of nanocrystals plays a key role in separating photo-generated charge carriers. Photocatalytic activities for water splitting are investigated under visible-light irradiation (λ > 400 nm) and an enhanced photocatalytic activity is achieved. The initial rate of H₂ evolution is up to 5.5 mmol h⁻¹ g⁻¹ without resorting to any cocatalysts.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2

Photocatalysts CuS/TiO₂ for hydrogen production were synthesized by hydrothermal method at high temperature and characterized by XRD, UV–visible DRS, XPS, EDX, SEM and TEM. When TiO₂ was loaded with CuS, it showed photocatalytic activities for water decomposition to hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the one with 1 wt% CuS-loaded TiO₂ showed the maximum photocatalytic activity for water splitting, which indicated CuS could improve the separation ratio of photoexcited electrons and holes. What's more, the amounts of the produced hydrogen was about 570 μmol h⁻¹, which had exceeded pure titania (P25) 32 times. In the present paper, it is proven that CuS can act as an effective co-catalyst to enhance the photocatalytic H₂ production activity of TiO₂.     

Photocatalytic hydrogen production over CuO and TiO2 nanoparticles mixture

Cheap and efficient photocatalysts were fabricated by simply mixing TiO₂ nanoparticles (NPs) and CuO NPs. The two NPs combined with each other to form TiO₂/CuO mixture in an aqueous solution due to the opposite surface charge. The TiO₂/CuO mixture exhibited photocatalytic hydrogen production rate of up to 8.23 mmol h⁻¹ g⁻¹ under Xe lamp irradiation when the weight ratio of P25 to CuO was optimized to 10. Although the conduction band edge position of CuO NPs is more positive than normal hydrogen electrode, the TiO₂/CuO mixture exhibited good photocatalytic hydrogen production performance because of the inter-particle charge transfer between the two NPs. The detailed mechanism of the photocatalytic hydrogen production is discussed. This mixing method does not require a complicated chemical process and allows mass production of the photocatalysts.     

Facile fabrication of Bi2O3/TiO2-xNx nanocomposites for excellent visible light driven photocatalytic hydrogen evolution

Mesoporous Bi₂O₃/TiO_2−xN_x nanocomposites (BiNT) were synthesized by soft chemical template free homogeneous co-precipitation technique. XRD, XPS, TEM, UV-Vis DRS and photoluminescence studies were adapted to determine the structural, electronic and optical properties. The photocatalytic activities of the catalysts were evaluated for water splitting to generate clean hydrogen fuel under visible light irradiation (λ ≥ 400 nm). BiNT-400 catalyst showed highest results towards hydrogen production (198.4 μmol/h) with an apparent quantum efficiency of 4.3%. The pronounced activity of BiNT-400 sample towards hydrogen production was well consistent with high crystallinity, large surface area, proper excitation by N doping and Bi₂O₃ sensitization.     

Ag3PO4 photocatalyst: Hydrothermal preparation and enhanced O2 evolution under visible-light irradiation

Visible-light-driven semiconducting photocatalysts of Ag₃PO₄ were prepared by a hydrothermal method, and were optimized by adjusting reaction conditions, i.e., temperature, pH of reaction solution, concentration of feedstock, and time of hydrothermal process. The obtained photocatalysts were then systematically characterized by different instruments, such as XRD, UV–vis, FESEM, and BET, to reveal the physicochemical properties. Furthermore, activities of photocatalysts for visible-light-driven O₂ evolution were evaluated, demonstrating that the photocatalytic activity of Ag₃PO₄ prepared by hydrothermal reaction (initial rate of O₂ evolution, 1156 μmol g⁻¹ h⁻¹) was more than two times as that of sample prepared by room-temperature reaction (initial rate of O₂ evolution, 533 μmol g⁻¹ h⁻¹), which could be attributed to its better ability to utilize visible light and more regulated morphology.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

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.     

Remarkable promotion of photocatalytic hydrogen evolution from water on TiO2-pillared titanoniobate

TiO₂-pillared titanoniobate TiO₂/HTiNbO₅ as an efficient photocatalyst was prepared via an exfoliation–restacking route. The as-prepared nanohybrid is mesoporous with a high specific surface area of 171 m²/g and a gallery height of 1.55 nm. Under a 300 W Xe lamp irradiation, the nanohybrid exhibited a high photocatalytic activity of 219 μmol/h/(g cat) in splitting water into hydrogen, which is 12 times as high as its parent HTiNbO₅ (18 μmol/h/g) and 24 times as TiO₂ (9 μmol/h/g). Enlarged surface area and effective electronic coupling between the host and the guest components contribute to the high photocatalytic activity of TiO₂/HTiNbO₅. Its photocatalytic activity was further improved through platinizing, and 5 wt% Pt-loaded TiO₂/HTiNbO₅ gave a remarkable hydrogen evolution rate of 4735 μmol/h/g. A photoexcitation model of the semiconductor–semiconductor pillared photocatalyst was proposed based on the results of XPS and UV–vis.     

Fabrication of graphene/CaIn2O4 composites with enhanced photocatalytic activity from water under visible light irradiation

A series of graphene/CaIn₂O₄ composites were synthesized using a facile solvothermal method to improve the photocatalytic performance of CaIn₂O₄. The reduction of graphene oxide to graphene and the deposition of CaIn₂O₄ nanoparticles on the graphene sheets can be achieved simultaneously during the solvothermal process. The photocatalytic activities of as-prepared graphene/CaIn₂O₄ composites for hydrogen evolution from CH₃OH/H₂O solution were investigated under visible light irradiation. It was found that graphene exhibited an obvious influence on the photocatalytic activity of CaIn₂O₄. The graphene/CaIn₂O₄ composite reached a high H₂ evolution rate of 62.5 μmol h⁻¹ from CH₃OH/H₂O solution when the content of graphene was 1 wt%. Furthermore, the 1 wt% graphene/CaIn₂O₄ composite did not show deactivation for H₂ evolution for longer than 32 h. This work could provide a new insight into the fabrication of visible light driven photocatalysts with efficient and stable performance.     

Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

This work focused on hydrogen production from the photocatalytic water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO₂–SiO₂ mixed oxide photocatalysts, of which the mesoporous-assembled TiO₂–SiO₂ mixed oxides with various TiO₂-to-SiO₂ molar ratios were synthesized by a sol–gel process with the aid of a structure-directing surfactant. The effects of SiO₂ content, calcination temperature, and phase composition of the mixed oxide photocatalysts were investigated. The experimental results showed that the TiO₂–SiO₂ mixed oxide photocatalyst with the TiO₂-to-SiO₂ molar ratio of 97:3 and calcined at 500 °C provided the maximum photocatalytic hydrogen production activity. The characterization results supported that the 0.97TiO₂–0.03SiO₂ mixed oxide photocatalyst (with the suitable SiO₂ content of 3 mol%) possessed superior physicochemical properties for the photocatalytic reaction as compared to the pure TiO₂, particularly higher specific surface area, lower mean mesopore diameter, higher total pore volume, and lower crystallite size, which led to an enhanced photocatalytic activity.     

High-efficiency hydrogen evolution reaction photocatalyst for water splitting of Type-II β-AsP/g-C3N4 van der Waals heterostructure

Photocatalytic for water splitting to produce hydrogen is recognized as a low-cost, promising and attractive method to solve environmental problems and energy crises, but finding a high-performance photocatalyst is a big challenge. In this work, we designed a type-II β-AsP/g-C₃N₄ van der Waals heterostructure as an efficient photocatalyst and had the first principles calculations to analyze its stability, electronic properties, and photocatalytic performance. The results showed that the photocatalyst of β-AsP/g-C₃N₄ heterostructure met the proper band gap and band edge of the redox potential of water splitting, had effective charge separation of photogenerated electronic holes, and efficient visible light response. Importantly, our research showed that the β-AsP/g-C₃N₄ heterostructure could proceed spontaneously in thermodynamics and had an excellent photocatalytic performance in further study. It had quite good hydrogen evolution performance with the Gibbs free energy of ?0.02 eV, which is closer to zero than ?0.09 eV of Pt (111). The overpotential of its oxygen evolution reaction is as low as 0.57 V. This work showed excellent development prospects for β-AsP/g-C₃N₄ heterostructure in the field of photocatalysts, which will promote the development of g–C₃N₄–based photocatalytic for water splitting.     

Stability of an H2-producing photocatalyst (Ru/(CuAg)0.15In0.3Zn1.4S2) in aqueous solution under visible light irradiation

Efficient photocatalytic water-splitting systems require stable photocatalysts that have photocatalytic activity with repeated consecutive use. This study investigated H₂ production under visible light irradiation with an Ru/(CuAg)_0.15In_0.3Zn_1.4S₂ photocatalyst and KI as an electron donor. In addition, the stability and reusability of the catalyst were evaluated over multiple cycles of H₂ production and catalyst regeneration. The results show that sintering temperature influenced the crystallinity and photocatalytic activity, as indicated by the X-ray diffraction analyses and H₂ production rates. In particular, the catalyst sintered at 873 K yielded the highest quantum yield of 4.6% at 420 ± 5 nm of wavelength. After seven consecutive reaction cycles, the quantum yield decreased from 4.6% to 3.0% at the end of the seventh cycle. The decrease probably occurred because (1) particles of the catalyst underwent pronounced aggregation, which led to the increase in particle size; and (2) a release of significant metal ions was observed during H₂ production, which led to a loss of the catalyst mass and potential changes in the photocatalytic activity. This study will help facilitate a search of stable photocatalysts for water splitting.     

Hydrogen evolution by photocatalytic decomposition of water under ultraviolet–visible irradiation over K2La2Ti3−xMxO10+δ perovskite

New layered transition metal substituted perovskite-type oxides K₂La₂Ti_3−xM_xO_10+δ (M = Fe, Ni, W; δ varies with different M) were synthesized with high-temperature solid state reaction, and characterized with X-ray diffraction (XRD) and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). The photocatalytic activity of these catalysts was studied under ultraviolet and visible light irradiation. The results indicated that substitution of a part of Ti⁴⁺ with Fe³⁺ and W⁶⁺ resulted in a marked increase in water splitting activity. The activity of these catalysts for water splitting decreased in the order: Fe³⁺ > W⁶⁺ > Ni²⁺ ≥ no substitution. The effect of different amounts of Ti⁴⁺ with Fe³⁺ substitution on water splitting was also evaluated. The highest hydrogen evolution was observed for perovskite composition having a Fe:Ti molar ratio of 1:14 (1:12 weight ratio) and this hydrogen evolution was over 4 times higher than the Fe-free K₂La₂Ti₃O₁₀ composition. Fe was also found to be a promising component for photo activity under visible light irradiation. Finally, the effect of Na₂S/Na₂SO₃ system as the sacrificial agent on the photocatalytic activity was also studied.     

Photocatalytic hydrogen generation through water splitting on nano-crystalline LaFeO3 perovskite

Visible light active ABO₃ type photocatalyst with LaFeO₃ composition was synthesized by sol-gel method. The photocatalyst was characterized by different techniques such as X-ray diffraction, BET surface area analysis, particle size analysis, scanning electron microscopy, UV–visible diffuse reflectance spectroscopy (UV–Visible DRS), and photoluminescence spectroscopy. LaFeO₃ photocatalyst exhibited an optical band gap of 2.07 eV with the absorption spectrum predominantly in visible region of the spectrum. The BET surface area of photocatalyst LaFeO₃ was observed as 9.5 m²/g, with the crystallite size of 38.8 nm as calculated by the Debye-Scherer equation. The photocatalytic activity of LaFeO₃ was investigated for hydrogen generation through sacrificial donor assisted photocatalytic water splitting reaction by varying conditions in feasible parametric changes using visible light source, ethanol as a sacrificial donor and Pt solution of H₂PtCl₆ as a co-catalyst. The rate of photocatalytic hydrogen evolution was observed to be 3315 μmol g⁻¹ h⁻¹ under optimized conditions and using 1 mg dose of photocatalyst with reaction time of 4 h and illumination of 400 W.     

Novel B-site substituted KCuTa3-xNbxO9 solid solution photocatalysts with modulated band structure for visible-light-driven hydrogen evolution

Perovskite-like metal oxides (PLMOs), featuring unique structural and optical properties, exhibit great potential in photocatalytic water splitting field. However, the wide bandgap and strong carrier recombination severely suppress their photocatalytic hydrogen production activity. Thus, design and development of novel PLMO photocatalyst with extended photo-response range and enhanced photo-generated charge separation/transport efficiency remains an ongoing challenge. Herein, a series of novel B-site substituted KCuTa_3-xNb_xO₉ solid solution photocatalysts were synthesized via a simple solid-state reaction method. With an increased content of Nb, a distinct red-shifted of the optical absorption edge of KCuTa_3-xNb_xO₉ solid solution was observed, leading to a decreased bandgap (from 2.69 to 1.91 eV), and a positive shift of the conduction band bottom (from −0.54 to −0.49 eV vs RHE). All of the Nb-substituted KCuTa₃O₉ solid solutions exhibit enhanced separation efficiency of photoinduced charge carriers, which leads to increased hydrogen evolution activity, among which KCuTa_0.75Nb_2.25O₉ exhibits the highest hydrogen evolution rate of 2.16 μmol h⁻¹ under the visible light irradiation (λ > 420 nm), which is approximately 7-fold higher than that of the pure KCuTa₃O₉. This study demonstrates the potential of modulating band structure through constructing solid solutions for efficient perovskite-like metal oxides photocatalysis.     

Fabrication of nano N-doped In2Ga2ZnO7 for photocatalytic hydrogen production under visible light

N-doped In₂Ga₂ZnO₇ photocatalysts were fabricated by solid state reaction route. All the prepared photocatalysts were successfully characterised by PXRD, optical absorption spectra, SEM, TEM, XPS, BET surface area and photoresponse studies. The formation of In₂Ga₂ZnO₇ was confirmed by the PXRD pattern. Optical absorption spectra showed that the visible light absorption of all the photocatalysts were enhanced by nitrogen doping. Among all the prepared photocatalysts, 1 wt% Pt loaded N-GaInZn-500 showed enhanced photocatalytic activity towards hydrogen evolution under visible light irradiation in presence of 10 vol% methanol solution as sacrificial agent. The excellent photocatalytic activity of N-GaInZn-500 is in agreement with N-content, bandgap energy, PL intensity and Surface area.     

TiO2 nanotubes for hydrogen generation by photocatalytic water splitting in a two-compartment photoelectrochemical cell

Highly ordered TiO₂ nanotube arrays for hydrogen production have been synthesized by electrochemical anodization of titanium sheets. Under solar light irradiation, hydrogen generation by photocatalytic water splitting was carried out in the two-compartment photoelectrochemical cell without any external applied voltage. The hydrogen gas and oxygen generated on Pt side and on TiO₂ nanotubes side respectively were efficiently separated. The effect of anodization time on the morphology structures, photoelectrochemical properties and hydrogen production was systematically investigated. Due to more charge carrier generation and faster charge transfer, a maximum photoconversion efficiency of 4.13% and highest hydrogen production rate of 97 μmol h⁻¹cm⁻² (2.32 mL h⁻¹cm⁻²) were obtained from TiO₂ nanotubes anodized for 60 min.