Photocatalysts For Photoelectrochemical Water Splitting Research Articles

Ultra-thin carbon doped TiO2 nanotube arrays for enhanced visible-light photoelectrochemical water splitting

TiO2 has been widely employed as a photocatalyst for water splitting owing to its high activity, low cost, abundance, safety, and stability. However, the further utilization of TiO2 is still confined to the wide band gap and the rapid recombination speed. Carbon has metallic conductivity and a large electron storage capacity, which can help to absorb photoexcited electrons and enhance charge separation and transfer. Herein, we fabricated the ultra-thin carbon-doped TiO2 nanotube arrays (TNTAs). The as-prepared UTC-doped TiO2@TNT exhibited high photoelectrochemical (PEC) activity, realizing a high H2 evolution rate up to 41.3 μmol cm-2 h−1 (AM 1.5G) and 1.3 μmol cm-2 h−1 (visible light), which exhibit significantly narrowed band gaps and decreased charge resistance. Using ultra-thin carbon doping in TiO2 nanotube arrays opens up a new avenue for optimizing photocatalysts for PEC water splitting.

Effects of Ag doping on LaMnO3 photocatalysts for photoelectrochemical water splitting

As a redox material, perovskite is considered one of the most efficient photovoltaic materials for producing hydrogen via water splitting reactions. This study used a wet chemical method to synthesize La1−xAgxMnO3 nanoparticles (0.00 ≤ x ≤ 0.09). The formation, chemical composition, and morphology of samples were examined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HR-TEM). The optical properties were examined by ultraviolet visible (UV) diffuse reflectance spectroscopy. XRD demonstrated that the samples had a rhombohedral hexagonal structure with a space group R overline{3 } c. The pore volume, pore size, and surface area were calculated and examined. The prepared samples were used as a photoanode in alkaline media for water splitting, and the photocurrent was measured. The photocurrent density recorded (14.01, 12.02, 11.67, and 10.28 μA/cm2) for x = (0.09, 0.06, 0.03, and 0.00) at 1 V vs. Ag/AgCl, respectively. The smaller impedance of the sample (x = 0.09) photoanodes than the sample (x = 0.00), which displayed a considerable decrease in charge transfer resistance. The electron lifetime (τ) increased with increasing Ag concentration, where x = 0.09 has the largest electron lifetime (τ) = 10.04 ms. Therefore, the electron–hole recombination rate of La0.91Ag0.09MnO3 is lower than LaMnO3. The samples demonstrated long-term stability for 1 h and enhanced photoelectrochemical performance.

CuO/g-C3N4 nanocomposite as promising photocatalyst for photoelectrochemical water splitting

CuO/g-C3N4 nanocomposite photocatalyst has been synthesized by co-precipitation method and analyzed its performance in the photoelectrochemical water splitting application. X-ray diffraction analyses and Raman measurement confirms the presence of CuO. Scanning electron microscope image shows rod shape morphology. The CuO/g-C3N4 nanocomposite exhibit the wide range of visible light absorbance and the bandgap of the material is 1.64 eV. Room temperature photoluminescence measurement reveals the presence of CuO charge transfer emission and LUMO – HUMO transition of g-C3N4. The CuO/g-C3N4 composite shows improved photocatalytic water splitting under visible light illumination. This could be attributed to the higher visible light absorbance and reduction in the recombination rate. The CuO/g-C3N4 nanocomposite electrode shows a highly stable photocurrent response.

InP/TiO2 heterojunction for photoelectrochemical water splitting under visible-light

Hydrogen production through photoelectrochemical (PEC) water splitting on photocatalyst is a green and clean method. In this study, we use density functional theory (DFT) calculations to find that the cage-like InP quantum dots (QDs) sensitized TiO2 is an effective photocatalyst for PEC water splitting under visible-light. A 16-ps first-principle molecular dynamics (FPMD) simulation results indicate that the cage-like InP-12, InP-16, InP-20, InP-24, InP-28, and InP-36 QDs are stable at room temperature (300 K). Furthermore, the calculated energy gaps of InP-16, InP-20, InP-24, InP-28, and InP-36 QDs are about 2.0 eV, which are suitable for visible-light absorption. Stable InP-20/TiO2 heterojunction structure was also obtained by FPMD simulation, and the electronic structure calculation result indicates that the InP-20/TiO2 heterojunction has a favorable type-II band aligment, which could prevent the recombination of photoexcited carriers. Finally, the possible reaction pathways of hydrogen production on InP-20/TiO2 heterojunction were investigated. It is found that energy barrier of hydrogen production of the InP-20/TiO2 is 2.56 eV lower than pure TiO2. Our calculations imply that InP QDs sensitized anatase TiO2 is an effective photocatalyst for visible-light PEC water splitting.

WO3–TiO2 nanotubes modified with tin oxide as efficient and stable photocatalysts for photoelectrochemical water splitting

One-step anodization combined with chemical bath deposition was used to prepare ternary hybrid SnO2@WO3–TiO2 photoelectrodes. Various analytical techniques such as X-ray diffraction, field emission scanning electron microscopy, energy-dispersive spectrometry and ultraviolet visible spectroscopy were applied to characterize the synthetic hybrids. Photoelectrochemical water splitting performance of new hybrid photoelectrodes significantly enhanced compared with bare WO3–TiO2 photoelectrodes, as shown by the investigation of the photoelectrochemical properties of electrodes. In comparison with bare WO3–TiO2 sample, WO3–TiO2 immersed in tin chloride for 60 min exhibited a maximum photocurrent density and better photoresponse under light illumination (about 6.5 times improvement in water splitting performance) according to the experimental results. Optimal contents of SnO2 in WO3–TiO2 sample act as mediators for trapping photoinduced electrons, minimize the recombination losses and enhance the transportation of photoinduced electrons in this ternary hybrid photoelectrodes. SnO2@WO3–TiO2 hybrid photoelectrodes were found to be highly stable and recyclable according to reusability experiments. The present work reports efficient, stable and highly active hybrid photocatalysts for hydrogen evolution without the application of precious metal co-catalysts.

RGO-α-Fe2O3/β-FeOOH ternary heterostructure with urchin-like morphology for efficient oxygen evolution reaction

RGO-α-Fe2O3/β-FeOOH ternary heterostructure composite has been developed as an active photocatalyst for photoelectrochemical water splitting. RGO-α-Fe2O3/β-FeOOH has been successfully synthesized through one-pot hydrothermal-ionic liquid assisted route using tetrabutyl ammonium L-methioninate [TBA][L-Met] and graphene oxide (GO). The electrochemical studies were carried out to demonstrate the oxygen evolution reaction (OER) and photocatalytic activities. The overpotential reduction by rGO-α-Fe2O3/β-FeOOH sample was higher than the nanobean α-Fe2O3/β-FeOOH and pristine α-Fe2O3 samples. The photocurrent density enhanced for rGO-α-Fe2O3/β–FeOOH urchin-like structure and obtained as 0.62mA·cm−2. The data evidences the idea that the β–FeOOH layer in the modified morphology acts as an electron donor and the donor density increased substantially. Results from this study indicate that the rGO-α-Fe2O3/β-FeOOH can improve OER performance significantly with a peak shift of 0.13V in the potential window, revealing its excellent potential as a higher rate and lower energy needed candidate.

Ta3N5/Co(OH)x composites as photocatalysts for photoelectrochemical water splitting.

Ta3N5 nanotubes (NTs) were obtained from nitridation of Ta2O5 NTs, which were grown directly on Ta foil through a 2-step anodization procedure. With Co(OH)x decoration, a photocurrent density as high as 2.3 mA cm-2 (1.23 V vs. NHE) was reached under AM1.5G simulated solar light; however, the electrode suffered from photocorrosion. More stable photoelectrochemical (PEC) performance was achieved by first loading Co(OH)x, followed by loading cobalt phosphate (Co-Pi) as double co-catalysts. The Co(OH)x/Co-Pi double co-catalysts may act as a hole storage layer that slows down the photocorrosion caused by the accumulated holes on the surface of the electrode. A "waggling" appearance close to the "mouth" of Ta2O5 NTs was observed, and may indicate structural instability of the "mouth" region, which breaks into segments after nitridation and forms a top layer of broken Ta3N5 NTs. A unique mesoporous structure of the walls of the Ta3N5 NTs, which is reported here the first time, is also a result of the nitridation process. We believe that the mesoporous structure makes it difficult for the nanotubes to be fully covered by the co-catalyst layer, hence rationalizing the remaining degradation by photocorrosion.

Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe2O3 Photoanode

AbstractThe last few decades’ extensive research on the photoelectrochemical (PEC) water splitting has projected it as a promising approach to meet the steadily growing demand for cleaner and renewable energy in a sustainable and economically viable fashion. Among many potential photocatalysts, hematite (α‐Fe2O3) emerges as a highly promising photoanode material with favorable characteristics including visible light absorption (a suitable band gap energy), earth abundance, chemical stability, and low cost. A pronounced disadvantage of α‐Fe2O3 is its low photovoltage together with an extremely short hole diffusion length and a low electrical conductivity, which limit its PEC water oxidation performance. To make α‐Fe2O3 as a viable photocatalyst for PEC water splitting, one needs to rectify these unfavorable characteristics of α‐Fe2O3 by elaborated multiple modifications. In this review article, we introduce various modification strategies of hematite with emphasis on surface modifications to achieve low onset potential as well as high photocurrent approaching the theoretical value for solar water splitting.

Rutile-Anatase Core-Shell TiO2 Nanostructured Array for Photoelectrochemical Water Oxidation and CO2 Photoconversion

In this work, TiO2-based nanostructured arrays with the core-shell structure composed of the anatase NPs (ANP) on the surface of rutile nanostructures arrays are used to be photocatalysts for photoelectrochemical water splitting and CO2 photoconversion. A superior PEC performance in alkaline electrolyte can be achieved in the hierarchical ANPs/rutile TiO2 nanodendrite (RND) array photoanode due to the formation of type-II heterojunction of anatase/rutile TiO2. However, the anodic-shift onset potential and decreased saturation photocurrent density are shown in the J-V curve of the ANP/RND array photoanode obtained in neutral electrolyte. An ultra-thin In2S3 layer is further deposited on the surface of the ANP/RND array photoanode to construct additional type-II heterojuntion for the enhancement of the charge separation of the ANP/RND array array photoanode in neutral electrolyte. By the surface modification of the photoanode with the transparent In2S3 layer, a photocurrent density of 1.6 mA cm− 2 at 1.23 V vs. RHE is attained using the In2S3/ANP/RND array photoanode in neutral electrolyte under illumination of AM 1.5G (100 mWcm−2). On the other hand, the rutile TiO2 NR array (RNR) shows an ability to convert CO2 and H2O into CH4, CO, and H2 under Hg-lamp irradiation. Compared to the RNR array, ANP/RNR array shows superior photocatalytic activity for CO2 photoconversion. Both the overall conversion efficiency and the selectivity of CH4 production are improved as the formation of core-shell structure. The details will be discussed in this presentation.