Noble metal free efficient photocatalytic hydrogen generation by TaON/CdS semiconductor nanocomposites under natural sunlight

Tantalum oxynitride have narrow band gap and its band potentials are suitable for visible light induced hydrogen generation. However, due to fast electron-hole recombination, the efficiency of photocatalytic hydrogen evolution reaction is very low. Herein, we have synthesized semiconductor heterojunction photocatalyst, i.e., TaON/CdS with suitable band positions by a simple precipitation method. Ratio between two semiconductors is optimized to obtain maximum hydrogen evolution. XRD, XPS and TEM analysis demonstrate the formation of heterojunction between these semiconductors. Among the synthesized catalysts, 3% TaON/CdS heterostructure exhibits the highest hydrogen evolution activity with H₂ production rate of 7.5 mmol h⁻¹ under natural solar light, whereas the rate is 11 mmol h⁻¹ under the visible light generated by xenon (Xe) lamp without the addition of any noble metal as the co-catalyst. The CdS and 3% TaON/CdS nanomaterials show an AQE of 5.1% and 12.2%, respectively. Combination of Mott-Schottky, UPS and DR UV–visible spectroscopy studies revealed the formation of S scheme semiconductor heterojunction between these nanomaterials with valence, conduction band positions, i.e., 1.46, −0.78 eV for CdS and 2.19, −0.66 eV for TaON, respectively. These band positions help in efficient e-h pair separation to produce hydrogen from water.     

In situ tuning of band gap of Sn doped composite for sustained photocatalytic hydrogen generation under visible light irradiation

The significance of Sn dopant on the photocatalytic performance of Iron/Titanium nanocomposite towards photocatalytic hydrogen generation by water splitting reaction is investigated. Iron/Titanium nanocomposite modified by Sn⁴⁺ dopant acts as a suitable photocatalyst for induced visible light absorption facilitating pronounced charge separation efficiency. Various characterization techniques reveal the heterojunction formation of hematite Fe₂O₃ with anatase - rutile mixed phase of TiO₂ employing Sn doping, where Sn⁴⁺ dopant accomplishes the phase transformation of anatase to rutile, entering into the TiO₂ lattice. This extended the lifetime of photogenerated charge carriers and enhanced the quantum efficiency of the photocatalyst. The band gap of the nanocomposite is tuned to ~2.4 eV, favoring visible light absorption. A hydrogen generation activity of 1102.8 μmol, approximately five times higher than the lone system (216.5 μmol) is achieved for the 5% Sn doped system for an average of 5 h. Heterojunctions of hematite with anatase-rutile mixed phase, generated as a consequence of tin doping facilitated the enhanced hydrogen generation activity of photocatalyst.     

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.     

Enhancing hydrogen evolution of water splitting under solar spectra using Au/TiO2 heterojunction photocatalysts

An effective improvement of hydrogen evolution from water splitting under solar light irradiation was investigated using quantum dots (QDs) compounds loaded onto a Au/TiO₂ photocatalyst. First, Au/TiO₂ was prepared by the deposition-precipitation method, and then sulfide QDs were loaded onto the as-prepared Au/TiO₂ by a hydrothermal method. QDs were loaded onto Au/TiO₂ to enhance the energy capture of visible light and near-infrared light of the solar spectrum. The results indicated that the as-prepared heterojunction photocatalysts absorbed the energy from the range of ultraviolet light to the near-infrared light region and effectively reduced the electron-hole pair recombination during the photocatalytic reaction. Using a hydrothermal temperature of 120 °C, the as-prepared (ZnS–PbS)/Au/TiO₂ photocatalyst had a PbS QDs particle size of 5 nm, exhibited an energy gap of 0.92 eV, and demonstrated the best hydrogen production rate. Additionally, after adding 20 wt % methanol as a sacrificial reagent to photocatalyze for 5 h, the hydrogen production rate reached 5011 μmol g⁻¹ h⁻¹.     

Enhanced photocatalytic hydrogen evolution from reduced graphene oxide-defect rich TiO2-x nanocomposites

Solar-driven photocatalytic hydrogen generation by splitting water molecules requires an efficient visible light active photocatalyst. This work reports an improved hydrogen evolution activity of visible light active TiO_2-x photocatalyst by introducing reduced graphene oxide via an eco-friendly and cost-effective hydrothermal method. This process facilitates graphene oxide reduction and incorporates intrinsic defects in TiO₂ lattice at a one-pot reaction process. The characteristic studies reveal that RGO/TiO_2-x nanocomposites were sufficiently durable and efficient for photocatalytic hydrogen generation under the visible light spectrum. The altered band gap of TiO_2-x rationally promotes the visible light absorption, and the RGO sheets present in the composites suppresses the electron-hole recombination, which accelerates the charge transfer. Hence, the noble metal-free RGO/TiO_2-x photocatalyst exhibited hydrogen production with a rate of 13.6 mmol h^?1g^?1_cat. under solar illumination. The appreciable photocatalytic hydrogen generation activity of 947.2 μmol h^?1g^?1_cat with 117 μAcm^?2 photocurrent density was observed under visible light (>450 nm).     

Enhanced hydrogen production from water splitting by Sn-doped ZnO/BiOCl photocatalysts and Eosin Y sensitization

The construction of semiconductor heterojunction for photocatalytic H₂ production from water splitting is an efficient and environment-friendly technology. In this work, ZnO/BiOCl (ZBC) and Sn-doped ZnO/BiOCl (ZBC-S) photocatalysts with Z-scheme heterojunction were successfully prepared by simple hydrothermal method. The photocatalytic H₂ evolution from water splitting by the as-prepared photocatalysts was investigated. The formation of ZnO/BiOCl heterojunction reduces the recombination probability of the photogenerated carriers. The impurity levels originated from Sn doping reduce the band gap width of ZnO and BiOCl to some extent, thereby enhancing the light absorption ability. The ZBC-S composite exhibits the best photocatalytic activity. In addition, the photocatalytic efficiency of H₂ production was improved by sensitization with Eosin Y (EY) dye. The H₂ production rate under simulated sunlight reaches 4146.77 μmol g^?1 h^?1, which is 27 times higher than that of pure ZnO. Finally, the Z-scheme electron transfer route in ZnO/BiOCl heterojunction was determined, and the photocatalytic H₂ production mechanism of EY sensitized ZBC-S was proposed.     

First-principles investigation of electronic and optical properties of Fe doped in CsBrO3 for enhanced photocatalytic hydrogen production

Self-consistent ab initio calculations carried out using full potential augmented plane wave (FP-LAPW) method were performed to study the electronic, optical and photocatalytic properties of CsBrO₃ and Fe doped CsBrO₃. Ground state and formation energy for CsBrO₃-perovskite are calculated and analyzed. The magnetic moment of Cs, Br, Fe and O are calculated in CsBrO3 and CsBr_0.34Fe_0.66O₃. The band structure, total and partial density of states (DOS) diagrams are discussed.The CsBrO₃ have semiconductor character with a wide direct gap. The value of gap energy is 4.24 eV of CsBrO₃. Fe doped in CsBrO₃ lead to band gap narrowing 1.02 eV for spin up and 1.434 eV for spin dn , which enhances the visible light catalytic activity. The conduction band minimum (CBM) and valence band maximum (VBM) potentials vs. normal hydrogen electrode (NHE) are calculated and analyzed. The general profiles of the optical spectra and the optical properties, including the real and imaginary part of dielectric function, reflectivity, absorption and optical conductivity are discussed. Our results predict that Fe doped CsBrO₃ is a promising visible light photo-catalyst for hydrogen production by water splitting.     

Enhancement of visible light-driven hydrogen production over zinc cadmium sulfide nanoparticles anchored on BiVO4 nanorods

Photocatalytic water splitting to produce H₂ is a promising technology for clean energy generation. However, the use of expensive noble metals, toxicity, low charge separation efficiency and wide band gap of semiconductors hampering the widespread commercialization. Herein, we showed the potential of combining BiVO₄ nanorods with ZnCdS forming a hetero-structure which extend the spectral responsive range, separate the charge carriers effectively and enhances photocatalytic activity compared to single-component materials. The two components of hetero-structure forms an interface contact which also mitigate the problems of lower conduction band position of BiVO₄ and fast recombination of charge carriers of ZnCdS. The BiVO₄–ZnCdS hetero-structure was studied through surface morphology, crystallization properties, elemental analysis and optical properties. Under visible light irradiation, the BiVO₄–ZnCdS heterostructure produced 152.5 μmol g^?1 h^?1 hydrogen from water splitting, which was much higher than that of the individual components and stability of the hydrogen production was observed in three consecutive cycles. The as-obtained heterostructure showed improved visible light harvesting ability, prolong life of charges carriers and charge separation efficiency and Z-scheme mechanism features which results in enhanced photocatalytic activity for water splitting.     

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.     

SiI2 monolayer as a promising photocatalyst for water splitting hydrogen production under the irradiation of solar light

The feasibility of SiI₂ monolayer as the candidate for photocatalytic water splitting for hydrogen generation under the irradiation of the solar light is explored. The geometrical structure, the electronic and optical properties, the mobility of carrier and strain engineering of the monolayer are investigated based on the first-principles calculations. The results demonstrate SiI₂ monolayer possesses an indirect gap of 2.33 eV (HSE06), and both the band edge and the bandgap match the redox potential conditions of the water splitting for hydrogen generation. There is an obvious optical absorption in the visible light and near-ultraviolet region and can be enhanced by the compressive strain. Moreover, the mobility of the electron is significantly different from that of the hole, implicating that the effective spatial charge separation is expectable and the ratio of the recombination of the photogenerated charge pairs is low. The primary adsorption site of the water molecule is identified. The Gibbs free energy and the adsorption energies are calculated to demonstrate the H₂ generation from the water molecule splitting on the monolayer. All the considered properties support that SiI₂ monolayer can be achieved as a promising candidate for the photocatalytic water splitting for hydrogen production under the irradiation of the solar light.     

Photocatalytic H2 evolution of porous silicon derived from magnesiothermic reduction of mesoporous SiO2

In this work, porous silicon (PSi) was synthesized by magnesiothermic reduction of mesoporous SiO₂ (MCM-41) and its photocatalytic hydrogen evolution performance was investigated. The unique mesoporous structure of PSi expands the band gap of silicon and shifts its conduction band to a more negative position. As a result, excellent photocatalytic water splitting efficiency of 604.7 μmol h⁻¹ g⁻¹ under visible-light radiation is recorded for the synthesized PSi photocatalysts without loading noble metal cocatalysts. This study presented a promising visible light response photocatalysts for the generation green renewable hydrogen energy basing on PSi material deriving from simple magnesiothermic reduction of mesoporous SiO₂.     

Mesoporous g-C3N4/Zn–Ti LDH laminated van der Waals heterojunction nanosheets as remarkable visible-light-driven photocatalysts

Mesoporous g-C₃N₄/Zn–Ti layered double hydroxide (LDH)-laminated van der Waals heterojunction nanosheets were prepared by a facile one-step in situ hydrothermal method. Due to the strong electrostatic interactions between the positively charged Zn–Ti LDH and negatively charged g-C₃N₄ nanocrystal, a laminated van der Waals heterostructure was successfully formed between Zn–Ti LDH and g-C₃N₄. The mesoporous g-C₃N₄/Zn–Ti LDH-laminated van der Waals heterojunction, which had a narrow bandgap of 2.41 eV extended the photoresponse to the visible light region. The obtained heterojunctions showed excellent visible-light-driven photocatalytic performance for the complete removal of ceftriaxone sodium (up to ∼97%) and a high hydrogen production rate (∼161.87 μmol h⁻¹ g⁻¹). This was mainly attributed to the formation of the laminated van der Waals heterojunctions, which favoured charge separation and the absorption of visible light, and the mesoporous structure, which provided more surface active sites. This facile strategy for preparing mesoporous g-C₃N₄/Zn–Ti LDH-laminated van der Waals heterojunctions offers new insights for the fabrication of high-performance van der Waals heterojunction photocatalytic materials.     

Photocatalytic hydrogen generation by WO3 in synergism with hematite-anatase heterojunction

The present study reports about exploration of a multi-component photocatalytic system comprising of WO₃, TiO₂ and Fe₂O₃ with tandem n-n heterojunctions. The ternary WO₃/TiO₂/Fe₂O₃ nanocomposite with WO₃ nanoparticles over the interfaces of Fe₂O₃ and TiO₂ is synthesized by wet precipitation followed by thermal decomposition. The WO₃/TiO₂/Fe₂O₃ nanocomposite has an enhanced photocatalytic performance towards hydrogen generation by water splitting reaction under visible light irradiation, when compared to the Fe₂O₃/TiO₂ system. A band gap of 2.10 eV, favouring visible light absorption was achieved with the distribution of WO₃ nanopartcles over the interfaces of Fe₂O₃ and TiO_2. The as prepared WTF heterojunction exhibited a maximum hydrogen production rate of 10.2 mL h⁻¹ for a catalyst loading of 0.025 g mL⁻¹. The enhanced photocatalytic performance is tested in presence of various sacrificial agents and proton source. In both cases, the higher photocatalytic efficiency is attributed to the more visible light harnessing ability and pronounced charge separation owing to the tandem n-n heterojunctions generated between TiO₂ with WO₃ and TiO₂ with Fe₂O₃ semiconductors and enhancing the lifetime of the photogenerated electron-hole pairs.     

Photoelectric properties of BiVO4 thin films deposited on fluorine doped tin oxide substrates by a modified chemical solution deposition process

BiVO₄ films deposited on Fluorine doped tin oxide glass substrates were successfully prepared by a modified chemical solution deposition process. Structure and optical spectrum analysis show that the resultant BiVO₄ films consist entirely of monoclinic scheelite structure and have a narrow band gap of ～2.66 eV. The films were investigated by photoelectrochemical and photovoltaic measurements with regard to hydrogen production and solar energy conversion under visible light. The BiVO₄ photoanodes show significantly higher visible light induced photoelectrochemical performance (～1.1 mA/cm² at 1.0 V vs. Ag/AgCl) than those reported ones, which is very promising for splitting water to H₂ and O₂. A Schottky BiVO₄ solar cell was also investigated for comparison with photoelectrochemical measurements. The correlation between the photoelectrochemical and photovoltaic behavior for BiVO₄ was explained. Our research should provide important support for the applications of BiVO₄ films or its modified forms such as doping and nanocomposite in heterojunction photoelectrochemical cells and solar cells with suitable energy level alignment at the interface.     

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.     

g-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C₃N₄ quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C₃N₄, g-C₃N₄/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C₃N₄/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C₃N₄ and BCNQDs with well-matched band structure.     

A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst

The photocatalytic water splitting for generation of clean hydrogen energy has received increasingly attention in the field of photocatalysis. In this study, the Ta₂O₅/g-C₃N₄ heterojunctions were successfully fabricated via a simple one-step heating strategy. The photocatalytic activity of as-prepared photocatalysts were evaluated by water splitting for hydrogen evolution under visible-light irradiation (λ > 420 nm). Compared to the pristine g-C₃N₄, the obtained heterojunctions exhibited remarkably improved hydrogen production performance. It was found that the 7.5%TO/CN heterojunction presented the best photocatalytic hydrogen evolution efficiency, which was about 4.2 times higher than that of pure g-C₃N₄. Moreover, the 7.5%TO/CN sample also displayed excellent photochemical stability even after 20 h photocatalytic test. By further experimental study, the enhanced photocatalytic activity is mainly attributed to the significantly improve the interfacial charge separation in the heterojunction between g-C₃N₄ and Ta₂O₅. This work provides a facile approach to design g-C₃N₄-based photocatalyst and develops an efficient visible-light-driven heterojunction for application in solar energy conversion.     

Interstitial carbon doped of setaria viridis-like Znln2S4 hollow tubes for efficient the performance of photocatalytic hydrogen production

In the photocatalytic water splitting hydrogen production system, the key to efficient use of solar energy is to choose a suitable photocatalyst. As an important ternary sulfide, ZnIn₂S₄ (ZIS) has attracted wide attention because of its narrow band gap (E_g = 2.3–2.8 eV) and wide light absorption range. However, further modification was still needed. Therefore, in this work, the unique C/ZIS hollow tubes with nano-flakes were prepared by a simple solvothermal method. To a large extent, this unique structure increased the utilization of light and active sites. Moreover, the dissolution of PAN during the solvothermal process caused the carbon element to be uniformly doped into the hollow tube framework. After a series of characterization results, C/ZIS-3.0 has the best hydrogen release rate (1241.94 μmol g⁻¹ h⁻¹) and good cyclability under visible light irradiation (λ ≥ 420 nm). And its unique morphology and possible catalytic mechanism were further discussed.     

Photophysical and photocatalytic water splitting performance of stibiotantalite type-structure compounds,SbMO4 (M = Nb,Ta)

Metal oxides with ferroelectric properties are considered to be a new family of efficient photocatalysts. Here, we investigate stibiotantalite type-structure compounds, SbMO₄ (M = Nb, Ta), with layered crystal structures, and ferroelectric properties as photocatalysts for hydrogen generation from the splitting of pure water. Both compounds were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic water splitting performance were characterized and evaluated. Diffuse reflectance analysis showed that both compounds have moderate band gaps of 3.7 eV for SbTaO₄ and 3.1 eV for SbNbO₄ (cf. 3.0 eV for TiO₂). Mott–Schottky analysis reveals that their conduction-band edge potentials are higher than the water reduction (hydrogen evolution) potential (0 V vs. RHE), indicating both compounds can generate hydrogen from water splitting. The photocatalytic water splitting performance was conducted by using pure water and UV-light irradiation, and photocatalytic H₂ production was confirmed for both compounds. After loading RuO₂ cocatalyst, the rates of hydrogen evolution of SbNbO₄ and SbTaO₄ were 24 μmol/g h and 58 μmol/g h, respectively. It was concluded that both compounds can be used as photocatalysts for water splitting under UV irradiation. The photocatalytic activity difference in both compounds was discussed with regard to electronic band structure and dipole moment difference, resulting from their crystal structures.