Noble-metal-free Z-Scheme MoS2–CdS/WO3–MnO2 nanocomposites for photocatalytic overall water splitting under visible light

To fabricate an efficient two-component Z-scheme system for visible light induced overall water splitting, CdS/WO₃ nanocomposites, with cubic CdS nanoparticles grown on the surface of hexagonal WO₃ nanorods, were prepared via a facile precipitation of Cd²⁺ with S²⁻ in the presence of pre-obtained hexagonal WO₃ nanorods. MnO₂ and MoS₂, the co-catalysts for O₂ and H₂ generation respectively, were selectively deposited on WO₃ and CdS in the CdS/WO₃ nanocomposites. The resultant MoS₂–CdS/WO₃–MnO₂ composites show photocatalytic activity for overall water splitting under visible light, with an optimized performance observed over 2.0%MoS₂-0.2 CdS/WO₃-1.0%MnO₂. The visible light induced overall water splitting over MoS₂–CdS/WO₃–MnO₂ nanocomposites can be attributed to the presence of a Z-scheme charge transfer pathway in the CdS/WO₃ nanocomposites, ie, the transfer of the photo-generated electrons from the CB of WO₃ to the VB of CdS to recombine with the photo-generated holes through an efficient interface between cubic CdS and hexagonal WO₃. The left photo-generated holes in VB of WO₃ and the photo-generated electrons in CB of CdS therefore can accomplish the water oxidation and water reduction simultaneously, with the assistance of the surface deposited cocatalysts (MnO₂ and MoS₂). This work demonstrated the great potential of fabricating the two-component direct Z-scheme photocatalytic systems for overall water splitting from two semiconductors with a staggered band structure.     

Hydrogen production from water–methanol solution over visible light active indium–titanium oxide photocatalysts modified with copper oxide

Titanium oxide coupled with different amount of indium oxides were studied for production of hydrogen under visible light irradiation from water–methanol solution. The photocatalysts were prepared by co-precipitation and characterized by surface area and pore analysis, X-ray diffraction, field emission scanning electron microscopy, UV–Vis diffuse reflectance spectra, and photoluminescence spectroscopy. With increases in indium oxide content, the surface area, visible light absorption and separation of photogenerated electron-holes were enhanced. For binary catalysts, the activity was highest for 16.7 at.% indium with hydrogen production of 1829 μmol/g/h. Incorporation of copper oxide further enhanced the activity with hydrogen production of 2149 μmol/g/h. The higher hydrogen production for ternary catalyst can be attributed to the synergistic effects of higher surface area, stronger absorption in visible light region and enhanced separation of photogenerated charge carriers. The hydrogen generation was attributed to partial oxidation of methanol to formaldehyde thereby producing pure hydrogen.     

Photocatalytic overall water splitting under visible light over an In–Ni–Ta–O–N solid solution without an additional cocatalyst

A sort of mixed nitrogen oxides, with a general formula In–Ni–Ta–O–N, has been synthesized with a solid state reaction method and used as the catalyst for visible light (λ > 400 nm) driven overall water splitting without the extra loading of a cocatalyst. Hydrogen and oxygen are evolved with a high rate in an ideal ratio of 2 to 1. The non-nitrided mixed oxide and individual oxide or nitride do not show any activity under the same reaction conditions. The elemental ratio plays a key role and the composition with the best activity is found as In:Ni:Ta:O:N = 0.9:0.1:1:3.21:0.774, with which the sample has an absorption edge at 550 nm.     

Efficient and stable photocatalytic hydrogen production from water splitting over ZnxCd1–xS solid solutions under visible light irradiation

A simple co-precipitation method was employed to synthesize a series of cubic zinc-blende phase of Zn_xCd_1−xS photocatalysts using Na₂S as the S source. Structural, morphological and optical properties of the samples have been investigated by XRD, SEM, EDS, XRF, ICP, N₂ physisorption and UV–vis diffuse reflectance techniques. The Zn_xCd_1−xS solid solution is not a simple compound mixture of ZnS and CdS, its XRD patterns show new structural peaks instead of mixture of original peaks. The lattice parameter a measured from the XRD patterns of the Zn_xCd_1−xS samples exhibits a slightly nonlinear relationship with the Zn mole fraction, which is slightly inconsistent with Vegard's law, thus suggesting that a nonhomogeneous alloy structure exists in Zn_xCd_1−xS solid solution. The photocatalytic H₂ evolution from water splitting in the sacrificial reagents of 0.25 M Na₂S/0.35 M K₂SO₃ under visible light at 30 °C and 55 °C were also examined in the study. It is found that Zn_xCd_1−xS solid solution with composition x = 0.4–0.5 shows the highest photocatalytic H₂ production performance. The studied Zn_xCd_1−xS exhibits at least 50 h stable photocatalytic activity under outdoor sunlight irradiation.     

Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods

Stability and efficiency are important to realize the practical applications of photocatalysts for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods with a wide absorption range were designed and synthesized by a two-step route. The ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water splitting into hydrogen in the presence of S²⁻ and SO₃²⁻ as sacrificial reagents. Furthermore, the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO₂ co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO and CdS, core–shell and nanorod structures employed are considered to contribute to the favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.     

TiO2–graphene nanocomposites for photocatalytic hydrogen production from splitting water

TiO₂ (P25)–graphene (P25–GR) hybrids were prepared via solvothermal reaction of graphene oxide and P25 using ethanol as solvent. The as-prepared P25–GR nanocomposites were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, photoluminescence emission spectroscopy and ultraviolet-visible (UV–vis) diffuse reflectance spectroscopy. The results indicated that P25–GR nanocomposites possessed enhanced light absorption ability and charge separation efficiency. As photocatalysts, P25–GR hybrids were much better than the bare P25, when they were used in the hydrogen evolution from aqueous methanol solution under Xe-lamp illumination. A significant enhancement in the rate of hydrogen production was achieved through using P25–GR as photocatalysts, comparing to bare P25. The optimum mass ratio of GR to P25 in the hybrids was 0.5 wt%. The higher mass ratio of GR in P25–GR would decrease the photocatalytic activity of P25.     

Visible light driven nanocrystal anatase TiO2 doped by Ce from sol–gel method and its photoelectrochemical water splitting properties

Visible light driven nanocrystal anatase TiO₂ was prepared by doping rare earth element Ce through sol–gel method. UV–Vis diffusion reflectance spectrum indicated its absorption edge extended to about 550 nm, red shifting about 170 nm compared with that without doping. Ce doping TiO₂ showed obvious anodic photocurrent effect for water splitting under visible light irradiation (λ > 420 nm) in photoelectrochemical measurement with three electrodes configuration. Ce doping TiO₂ showed higher photocurrent density than that of without doping TiO₂ under full arc irradiation. Furthermore, the electronic structures for CeO₂ and TiO₂ were analyzed theoretically based on the first principle calculation. As a result, the electronic structure for Ce doping TiO₂ is proposed as the overlap and some degree of hybridization among splitting occupied Ce 4f and unoccupied Ce 4f with O 2p and Ti 3d respectively. The visible light responsive property is mainly due to the transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d.     

Enhanced photocatalytic hydrogen production over In-rich (Ag–In–Zn)S particles

The ratio of ZnS to AgInS₂ is usually adjusted to tune the band gaps of this quaternary (Ag–In–Zn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag] ratio was kept constant. The hydrogen production rate was enhanced by increasing the content of indium sulfide. Compared to the steady H₂ evolution rate obtained with equal moles of indium and silver ([In]/[Ag] = 1, 0.64 L/m² h), that obtained with In-rich photocatalyst ([In]/[Ag] = 2, 3.75 L/m² h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the content of indium. The indium-rich samples did not induce phase separation between Ag_xIn_xZn_yS_2x+y and AgIn₅S₈, instead forming a single-phase solid solution. Although the photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played a critical role in the hydrogen production rate. This study demonstrates the significant effect of In₂S₃ on this unique (Ag–In–Zn)S photocatalyst.     

Role of P in improving V-doped SrTiO3 visible light photocatalytic activity for water splitting: A first – Principles study

In this work, the +3-valent P-doped SrTiO₃ has been shown for the first time to be theoretically achievable and to improve the photocatalytic performance of SrTiO₃ in co-doping with metal ions. The effects of P and V doping on the electronic structure, optical properties and photocatalytic activity of the SrTiO₃ system were systematically investigated using a first-principles calculation method. The results show that when V is singly doped with SrTiO₃, although the band gap width is greatly reduced and the optical absorption performance is significantly enhanced, it is difficult to photocatalytic split water for hydrogen production due to the high VBM. When one electron provided by +3-valent P as a donor is transferred near +5-valent V, the orbital hybridization between V-3d and O-2p will lead to a suitable reduction in the band gap width of SrTiO₃. The band edge alignment respect to water redox potentials proves that its band edges match well with the redox potentials of water. It was also found that P and V co-doping could reduce the complexation of photogenerated carriers and create a new optical absorption peak in the visible range. The calculated results imply that the P and V co-doping could improve the visible light photocatalytic activity of perovskite SrTiO₃.     

Electronic structure and water splitting under visible light irradiation of BiTa1−xCuxO4 (x = 0.00–0.04) photocatalysts

A series of photocatalysts, BiTa_1?xCu_xO₄ (x = 0.00–0.04), were synthesized by the conventional solid-state reaction method and their electronic structures and photocatalytic activities were investigated. The electron microscope observations revealed that the particle sizes of BiTaO₄:Cu crystals were smaller and the surface with many characteristic steps was more obvious than that of the nondoped BiTaO₄, which lead to the increase of photocatalytic activity of water splitting. The UV–vis spectra indicate that the Cu²⁺ ions doping not only enhanced the photocatalytic activity under ultraviolet–visible (λ > 300 nm) light irradiation but also induced the visible light (λ > 400 nm) response. The photocatalyst doped with 2 mol% Cu²⁺ and loaded with 0.3 wt% RuO₂ co-catalyst was found to have the highest activity. New band gap in the visible light range is obtained by copper-doped BiTaO₄, which is attributed to the transition from the donor level resulting from the Cu impurity to the conduction band of BiTaO₄ doped with copper on the basis of the result of DFT calculation.     

Reasonable regulation of kinetics over BiVO4 photoanode by Fe–CoP catalysts for boosting photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using earth-abundant semiconductor offer a promising strategy to produce the sustainable clean energy. Herein, we successfully engineered BiVO₄ photoanode with Fe-doped CoP oxygen evolution catalysts (BiVO₄–Fe/CoP) for the first time. Fe/CoP catalysts could significantly break the kinetic limitations of BiVO₄, contributing to the enhanced charge injection efficiencies and charge carrier density. The rational heterostructure has boosted the photocurrent density and incident photon-to-current conversion efficiency (IPCE) to 2.16 mA/cm² and 43% (7 times than that of the bare BiVO₄), respectively. Meanwhile, the BiVO₄–Fe/CoP photoanode also exhibited the desirable long-term stability during PEC water oxidation for 4 h. The results prove the feasibility of BiVO₄–Fe/CoP configuration, which further provided a novel approach towards the development of efficient PEC water oxidation system.     

Hydrogen generation from splitting water with Al–Bi(OH)3 composite promoted by NaCl

In this paper, a novel Al–Bi(OH)₃ system hydrogen-generating material is investigated. Hydrolysis experiments show that the hydrolysis properties of the Al–10 wt% Bi(OH)₃ composite are significantly improved by doping with sodium chloride, and the Al–10 wt% Bi(OH)₃–5 wt% NaCl composite has a low activation energy (10.4 kJ mol⁻¹). With the further optimization of milling time, the hydrogen yield of Al–10 wt% Bi(OH)₃–5 wt% NaCl composite reaches 1000 mL g⁻¹ in 1 min. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive spectroscopy and thermogravimetric analysis are applied to characterize the composite and explore the hydrolysis mechanism. The characterization results show that the activation of aluminum mainly comes from three factors: (1) The formation of alumina during ball milling plays an important role in preventing the agglomeration between Al–Bi, Al–Al and Bi–Bi; (2) Bismuth generated during ball milling can form micro-galvanic cell with aluminum to promote the corrosion of aluminum; (3) Sodium chloride as a grinding aid contributes to crush aluminum powder, and chloride ions facilitate the corrosion of aluminum in the hydrolysis process. In addition, the drying method and initial water temperature have a great influence on by-products. The composite is expected to be used in mobile emergency fuel cell due to its rapid hydrogen production capacity.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Ni2P(O)–Fe2P(O)/CeOx as high effective bifunctional catalyst for overall water splitting

The development of cost-effective bifunctional catalysts with excellent performance and good stability is of great significance for overall water splitting. In this work, NiFe layered double hydroxides (LDHs) nanosheets are prepared on nickel foam by hydrothermal method, and then Ni₂P(O)–Fe₂P(O)/CeO_x nanosheets are in situ synthesized by electrodeposition and phosphating on NiFe LDHs. The obtained self-supporting Ni₂P(O)–Fe₂P(O)/CeO_x exhibit excellent catalytic performances in alkaline solution due to more active sites and fast electron transport. When the current density is 10 mA cm^?2, the overpotential of hydrogen evolution reaction and oxygen evolution reaction are 75 mV and 268 mV, respectively. In addition, driven by two Ni₂P(O)–Fe₂P(O)/CeO_x electrodes, the alkaline battery can reach 1.45 V at 10 mA cm^?2.     

Overall photocatalytic water splitting on Na2ZrxTi6−xO13 (x = 0, 1) nanobelts modified with metal oxide nanoparticles as cocatalysts

In this work, we present the preparation of Na₂Zr_xTi_6?xO₁₃ (x = 0, 1) nanobelts through a rapid solvocombustion method. The phases exhibited stable photocatalytic activity for overall water splitting under UV light. Effect of the annealing temperature on the physicochemical properties and the catalytic performance of the materials were studied. Na₂ZrTi₅O₁₃ exhibited a higher rate of H₂ evolution compared to Na₂Ti₆O₁₃, and it was attributed to the incorporation of Zr⁴⁺ in the structure, which generates a distortion in the octahedral sites of the structure. This distortion promoted an enhanced charge transport and a reduction in the recombination of the free carriers and a higher photocatalytic activity. The nanobelts were superficially modified through the deposition of metal oxide nanoparticles as cocatalyst, MO (M = Ni, Cu). The incorporation of metal oxide nanoparticles improved the charge separation process and the overall efficiency. An integral study of the structural, morphological, textural, optical and photoelectrochemical properties of the materials is presented and a charge transference mechanism on the semiconductor interface is proposed. The highest catalytic activity was obtained by Na₂ZrTi₅O₁₃ modified with CuO (2909 μmol g?¹ h^?1), and corresponds to an increase of 13.6 times the activity of the bare photocatalyst. This was attributed to an improved charge separation at the interface of n-type Na₂ZrTi₅O₁₃ and p-type CuO semiconductors. For the best of our knowledge, the activity exhibited for overall water splitting of Na₂Zr_xTi_6?xO₁₃ (x = 0, 1) nanobelts prepared by solvocombustion method and modified with the addition of MO nanoparticles in this work is higher compared to the reported in previous works.     

H2 production of (CdS–ZnS)–TiO2 supported photocatalytic system

Mixed semiconductor (CdS–ZnS)–TiO₂(1 : 1 : 1) mixture system over different supports like MgO, CaO, γ-Al₂O₃, SiO₂ and modified MgO and CaO, have been prepared, characterized and tested for H₂ production in a S²⁻⧸SO²⁻₃ mixture solution. (CdS–ZnS)–TiO₂(D) over MgO support wherein the TiO₂ taken is from Degussa (D) sample gives 206.7 μmol⧸h of H₂ production and this catalyst sustain the H₂ production rate for longer durations. Dopants like Li₂O, Cs₂O or K₂O make MgO and CaO supports act like super basic oxide when they are doped and in turn increase the photocatalytic activity. The (CdS–ZnS)–TiO₂(I) system wherein the TiO₂ taken from Titanium Isopropoxide, supported on 20 wt% Li₂O doped CaO is found to give 209.8 μmol⧸h rate of H₂ production. Characterization studies like UV-Visible spectra, X-ray Diffraction spectra and Scanning Electron Microscope photographs were taken for all the catalysts and the data generated over these samples is evaluated. A scheme of H₂S photocatalytic decomposition of ZnCdS–TiO₂(D⧸I) over different supports in the presence of S²⁻⧸SO²⁻₃ substrate, is proposed indicating the formation of thiosulfate cycle at this heterojunction. © 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.     

An infinite chain, {[Ni(tn)2]3[Fe(CN)4(μ-CN)2]2}n,a new catalyst for electrochemical-driven water splitting and photochemical-driven hydrogen evolution from water under blue light

A new catalyst for both water reduction and oxidation, based on an infinite chain, {[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n, is formed by the reaction of NiCl₂, 1,3-propanediamine (tn) and K₃ [Fe(CN)₆]. {[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n can electro-catalyze hydrogen evolution from a neutral aqueous buffer (pH 7.0) with a turnover frequency (TOF) of 1561 mol of hydrogen per mole of catalyst per hour (H₂/mol catalyst/h) at an overpotential (OP) of 837 mV {[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n also can electro-catalyze O₂ production from water with a TOF of ～45 mol O₂ (mol cat)^?1s^?1 at an OP of 591 mV. Under blue light (λ = 469 nm), together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, {[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n can photo-catalyze hydrogen generation from an aqueous buffer (pH 4.0) with a turnover number (TON) of 11,450 mol H₂ per mole of catalyst (mol of H₂ (mol of cat)^?1) during 10 h irradiation. The average of apparent quantum yield (AQY) is as high as 40.96% during 10 h irradiation. Studies indicate that {[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n exists in two forms: a cyano-bridged chain ({[Ni(tn)₂]₃ [Fe(CN)₄ (μ-CN)₂]₂}_n) in solid, and a salt ([Ni(tn)₂]₃ [Fe(CN)₆]₂) in aqueous media; Catalytic reaction occurs on the nickel center of [Ni(tn)₂]²⁺, and the introduction of [Fe(CN)₆]^3- can improve the catalytic efficiency of [Ni(tn)₂]²⁺ for H₂ or O₂ generation. We hope these findings can afford a new method for the design of catalysts for both water reduction and oxidation.     

Photocatalytic system with water soluble nickel complex of S,S′-bis(2-pyridylmethyl)-1,2-thioethane over CdS nanorods for hydrogen evolution from water under visible light

In this paper, we report a new nickel complex, [(bpte)NiCl₂] (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane) that can serve as a catalyst both for electrochemical and photochemical driven hydrogen production from water. As an electrocatalyst, [(bpte)NiCl₂] can electrocatalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 555.78 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) at an overpotential (OP) of 837.6 mV. Together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, the nickel complex also can photocatalyze hydrogen evolution in heterogeneous environments and can work for 107 h. Under an optimal condition, the photocatalytic system can afford 24900 mol of H₂ per mole of catalyst during 83 h irradiation, with a TOF of 300H₂ per catalyst per hour. The average value of apparent quantum yield (AQY) is ～24% at 420 nm.     

Fabrication of In(OH)3–In2S3–Cu2O nanofiber for highly efficient photocatalytic hydrogen evolution under blue light LED excitation

This study synthesized the In(OH)₃–In₂S₃ nanosheet via a simple hydrothermal method. The different amounts of In(OH)₃–In₂S₃ nanosheet used to synthesize In(OH)₃–In₂S₃–Cu₂O composite by a wet chemical method. FESEM, FETEM, XRD, XPS, BET, UV–vis DRS, and PL spectroscopy characterized the In(OH)₃–In₂S₃–Cu₂O composite. Compared with In(OH)₃–In₂S₃ nanosheet, In(OH)₃–In₂S₃–Cu₂O composite can exhibit the synergistic effects of higher specific surface area, higher light-harvesting capacity, and accelerating the separation and migration of photogenerated charge carriers. In addition, In(OH)₃–In₂S₃–Cu₂O nanofiber was fabricated via a facile electrospinning process at the different amounts of In(OH)₃–In₂S₃ nanosheet. The manufacturing process offers many advantages, such as simplicity, low temperature, and without templates. The effect of operational parameters (such as the reaction conditions of photocatalysts, pH values, sacrificial reagents, and light sources) on the photocatalytic hydrogen production of In(OH)₃–In₂S₃–Cu₂O nanofiber was investigated. The results indicate that the appropriate reaction conditions of In(OH)₃–In₂S₃–Cu₂O nanofiber can reveal higher efficient photocatalytic water splitting than commercial TiO₂ or ZnO nanofiber under blue light LED excitation. Furthermore, In(OH)₃–In₂S₃–Cu₂O nanofiber can provide a simple fabrication process, high photocatalytic activity, and high reusability shall be beneficial for the practical application of photocatalytic hydrogen production.