Cocatalytic Effect of SrTiO₃ on Ag₃PO₄ toward Enhanced Photocatalytic Water Oxidation

Ag₃PO₄ has been reported to be an excellent photocatalyst for O₂ evolution from aqueous solution, which makes it a promising candidate for designing a Z-scheme water-splitting system. In this work, in order to further improve the photocatalytic activity, a series of SrTiO₃/Ag₃PO₄ composite photocatalysts was constructed by introducing SrTiO₃ (with a less positive valence band minimum) to Ag₃PO₄ and was synthesized by two consecutive hydrothermal processes. The obtained photocatalysts were systematically characterized by XRD, SEM, BET, UV–vis, etc., showing that SrTiO₃/Ag₃PO₄ composites were formed by coating SrTiO₃ onto a Ag₃PO₄ polyhedron. Photocatalytic O₂ evolution results demonstrated that a small amount of SrTiO₃ brought about significant enhancement of photocatalytic activity of Ag₃PO₄ and that the apparent quantum yield at 420 nm reached 16.2% with a molar ratio of SrTiO₃ to Ag₃PO₄ equal to 1/20, which led to the fact that SrTiO₃ could serve as cocatalyst for water oxidation providing both accelerated electron–hole separation by band gap alignment and more active sites by enlarged surface area

<i>In Situ</i> Photochemical Synthesis of Zn-Doped Cu₂O Hollow Microcubes for High Efficient Photocatalytic H₂ Production

Traditionally, Cu ion-based oxide materials are considered not functional as photocatalysts owing to their instability in the photoelectrochemical processes. Here, we report on the light-induced photochemical synthesis of Cu₂O microcubes utilizing CuWO₄ as the precursor. It was found that under light irradiation and in the presence of glucose CuWO₄ could be reduced <i>in situ</i> into Cu₂O with its morphology reassembled from irregular bulk particles to hollow microcubes. Similar morphology transformation could not be observed when CuO or Cu­(NO₃)₂ were used as precursors. More importantly, the <i>in situ</i> photochemical-synthesized Cu₂O naoncubes showed both high activity and excellent stability for glucose reforming under visible light, which overcame the general barrier of Cu₂O instability in photochemical processes. The activity could be remarkably enhanced when 0.1 wt % Zn was doped into the Cu₂O. The excellent performances of the material were related to the existence of hollow microcubes and the modified band structure due to Zn doping

Toward Facet Engineering of CdS Nanocrystals and Their Shape-Dependent Photocatalytic Activities

Controlling the shape or morphology of semiconductor nanocrystals is central to their enhanced physical and chemical properties. Herein, using CdS as a model photocatalyst, we demonstrate that the crystal habit of a visible-light-active semiconductor can be quantitatively controlled through synthesis kinetics. Growth rate control of {0001} facets (<i>r</i>₁) and {101̅1} facets (<i>r</i>_1′) of CdS nanocrystals was achieved by simply employing a syringe pump, which enables us to finely tune the crystal shape from nanocones, to nanofrustums, and further to nanoplates. These shape-controlled samples, showing altered proportions of {0001} to {101̅1} facets, were used to investigate the crystal-facet dependence of solar hydrogen production. The results indicate that CdS nanoplates with the largest {0001} facets showed the highest photocatalytic activity. This work not only advances our knowledge on the growth mechanism of semiconductor crystals but also illustrates a robust method to targeted crystal design of semiconductors toward optimizing their associated catalytic activities

Enhanced Bulk and Interfacial Charge Transfer Dynamics for Efficient Photoelectrochemical Water Splitting: The Case of Hematite Nanorod Arrays

Charge transport in the bulk and across the semiconductor/electrolyte interface is one of the major issues that limits photoelectrochemical (PEC) performance in hematite photoelectrodes. Efficient charge transport in the entire hematite is of great importance to obtaining high photoelectrochemical properties. Herein, to reach this goal, we employed both TiO₂ underlayer and overlayer deposition on hematite nanorod films, followed by a fast annealing treatment. The TiO₂ underlayer and overlayer not only serve as dopant sources for carrier density increase but also reduce charge recombination at the fluorine-doped tin oxide (FTO)/hematite interface and accelerate charge transfer across the hematite/electrolyte interface. This synergistic doping and interface modifying effects give rise to an enhanced photoelectrochemical water oxidation performance of hematite nanorod arrays, generating an impressive photocurrent density of 1.49 mA cm^–2 at 1.23 V vs RHE. This is the first report on using both underlayer and overlayer modification with the same material to improve charge transport through the entire electron transport path in hematite, which provides a novel way to manipulate charge transfer across the semiconductor interface for a high-performance photoelectrode

<i>In Situ</i> Photochemical Synthesis of Zn-Doped Cu₂O Hollow Microcubes for High Efficient Photocatalytic H₂ Production

Traditionally, Cu ion-based oxide materials are considered not functional as photocatalysts owing to their instability in the photoelectrochemical processes. Here, we report on the light-induced photochemical synthesis of Cu₂O microcubes utilizing CuWO₄ as the precursor. It was found that under light irradiation and in the presence of glucose CuWO₄ could be reduced <i>in situ</i> into Cu₂O with its morphology reassembled from irregular bulk particles to hollow microcubes. Similar morphology transformation could not be observed when CuO or Cu­(NO₃)₂ were used as precursors. More importantly, the <i>in situ</i> photochemical-synthesized Cu₂O naoncubes showed both high activity and excellent stability for glucose reforming under visible light, which overcame the general barrier of Cu₂O instability in photochemical processes. The activity could be remarkably enhanced when 0.1 wt % Zn was doped into the Cu₂O. The excellent performances of the material were related to the existence of hollow microcubes and the modified band structure due to Zn doping

Experimental and Numerical Studies on a One-Step Method for the Production of Mg in the Silicothermic Reduction Process

In this paper, a new efficient one-step technical method was first developed for the production of magnesium in the industry. The one-step method could combine the two processes of dolomite decomposition and magnesium reduction in the magnesium reduction retort. Thus, the high-temperature carbon dioxide produced by the dolomite decomposition process could be collected in a timely manner instead of being emitted into the atmosphere, and excessive heat loss caused by the two separate processes also could be almost completely avoided. This paper presents an experimental study on the intrinsic chemical kinetics mechanisms of this new efficient one-step technology. By applying each of the most likely solid-state kinetic models, the kinetic parameters of the two reactions that reacted during the dolomite decomposition stage and magnesium reduction stage were evaluated, and the kinetic models that best verify the experimental data were attempted. For the dolomite decomposition stage of the one-step technology, the equation of the chemical kinetic model can be represented by α²/2 = <i>k</i>_D1τ in the temperature range of 1173–1473 K, and the apparent activation energy was determined to be 160.6 kJ mol^–1. For the magnesium reduction stage of the one-step technology, the surface reaction chemical kinetic model 1 – (1 – β)^1/3= <i>k</i>_Sτ described very satisfactorily the experimental values for the different reduction temperature. Then, a one-step model incorporating the chemical reaction kinetics of the dolomite decomposition stage and the magnesium reduction stage and heat conduction was first developed. The simulations of the impact of heating temperature on the dolomite decomposition stage and magnesium reduction stage were carried out in the reduction retorts of the furnace utilizing this model. The distribution of dolomite decomposition extent in the retorts, the total extent of dolomite decomposition with time, the distribution of magnesium reduction extent in the retorts, and the total extent of magnesium reduction with time were studied in detail. The analysis showed that the one-step technology is effective in not only reducing the cycle time of dolomite decomposition stage and magnesium reduction stage but also saving energy

A Multistep Ion Exchange Approach for Fabrication of Porous BiVO₄ Nanorod Arrays on Transparent Conductive Substrate

BiVO₄ is recognized as a promising semiconductor for photoelectrochemical (PEC) applications. However, the lack of synthesis methods to prepare high-quality nanostructural BiVO₄ film is rarely reported in the PEC field. In this study, we report a novel synthesis approach to prepare one-dimensional BiVO₄ nanowire arrays using a multistep ion exchange approach through a solvothermal–hydrothermal–annealing process. The resulting BiVO₄ electrodes showed a nanorod structure with high porosity. In particular, the aspect ratio surface of the BiVO₄ nanostructure was found favorable for its PEC application. The BiVO₄ nanostructure with an optimized synthesized condition showed efficient PEC water oxidation with a photocurrent of 1.67 mA/cm² at 1.83 V (vs RHE)

Understanding Hematite Doping with Group IV Elements: A DFT+<i>U</i> Study

Si, Ge, or Sn doped hematite (α-Fe₂O₃) photoanodes show significantly enhanced efficiency for photo-oxidization of water. We employed DFT+<i>U</i> to study the doping of α-Fe₂O₃ with group IV elements, i.e., Si, Ge, and Sn. From the calculated formation energies and chemical potentials, three key points are concluded. (1) Low oxygen pressure is favored for doping both substitutional and interstitial dopants. (2) Substitutional doping of the Fe atom at the lattice site is more stable than interstitial doping in the octahedral vacancies. (3) Most interestingly, Ge doping is found to be easiest among the three dopants. This result contradicts intuition based on atomic size and indicates that Ge doping should be more efficient than Si and Sn doping in increasing the charge carrier concentration. Incorporation of the dopants at the Fe site generates an electron polaron and the dopant with the +4 valence state by spontaneous transfer of one electron from the dopant atom to a surrounding Fe atom, according to the analyses of charge transition energy levels and density of states. We identify the factors affecting the charge transfer process. The study elucidates the dopants role in increasing the electrical conductivity of α-Fe₂O₃ and provides guidelines for designing new efficient photoanodes

CdS/CdSe Core–Shell Nanorod Arrays: Energy Level Alignment and Enhanced Photoelectrochemical Performance

Novel CdS/CdSe core–shell nanorod arrays were fabricated by a chemical bath deposition of CdSe on hydrothermally synthesized CdS nanorods. The CdS rods were hexagonal phase faced and the top of the rod was subulate. After the chemical bath deposition approach, CdS nanorod arrays were encapsulated by a uniform CdSe layer resulting enhanced absorbance and extended absorption edges of the films. A tandem structure of the energy bands of CdS/CdSe was also formed as a result of the Fermi level alignment, which is a benefit to the efficient separation of photogenerated charges. CdS/CdSe core–shell arrays gave a maximum photocurrent of 5.3 mA/cm², which was 4 and 11 times as large as bare CdS and CdSe, respectively

Facile Fabrication of Sandwich Structured WO₃ Nanoplate Arrays for Efficient Photoelectrochemical Water Splitting

Herein, sandwich structured tungsten trioxide (WO₃) nanoplate arrays were first synthesized for photoelectrochemical (PEC) water splitting via a facile hydrothermal method followed by an annealing treatment. It was demonstrated that the annealing temperature played an important role in determining the morphology and crystal phase of the WO₃ film. Only when the hydrothermally prepared precursor was annealed at 500 °C could the sandwich structured WO₃ nanoplates be achieved, probably due to the crystalline phase transition and increased thermal stress during the annealing process. The sandwich structured WO₃ photoanode exhibited a photocurrent density of 1.88 mA cm^–2 and an incident photon-to-current conversion efficiency (IPCE) as high as 65% at 400 nm in neutral Na₂SO₄ solution under AM 1.5G illumination. To our knowledge, this value is one of the best PEC performances for WO₃ photoanodes. Meanwhile, simultaneous hydrogen and oxygen evolution was demonstrated for the PEC water splitting. It was concluded that the high PEC performance should be attributed to the large electrochemically active surface area and active monoclinic phase. The present study can provide guidance to develop highly efficient nanostructured photoelectrodes with the favorable morphology