A novel photoelectrochemical cell with self-organized TiO2 nanotubes as photoanodes for hydrogen generation

A photoelectrochemical (PEC) cell with an innovative design for hydrogen generation via photoelectrocatalytic water splitting is proposed and investigated. It consisted of a TiO₂ nanotube photoanode, a Pt/C cathode and a commercial asbestos diaphragm. The PEC could generate hydrogen under ultraviolet (UV) light-excitation with applied bias in KOH solution. The Ti mesh was used as the substrate to synthesize the self-organized TiO₂ nanotubular array layers. The effect of the morphology of the nanotubular array layers on the photovoltaic performances was investigated. When TiO₂ photocatalyst was irradiated with UV-excitation, it prompted the water splitting under applied bias (0.6 V vs. Normal Hydrogen Electrode, NHE.). Photocurrent generation of 0.58 mA/cm² under UV-light irradiation showed good performance on hydrogen production.     

Hydrogen evolution by a low cost photocatalyst: Bauxite residue

Bauxite residue or red mud which is an aluminium industry waste has been used as a novel low cost photocatalyst active in visible light for the generation of hydrogen from water. The driving force behind the use of bauxite residue as a photocatalyst is not only the fact that it is widely available but also bauxite residue is a fine grained mixture of oxides and hydroxides (Fe₂O₃, TiO₂, SiO₂, and Al₂O₃, Al(OH)3). The photocatalyst was characterized with respect to BET-SA, UV-DRS, XRD, SEM and EDX. Hydrogen yield of 4600 μmol/h/g of TiO₂ was achieved as compared to hydrogen evolution rate of 164 μmol/h/g of TiO₂ for commercially available titania Degussa P-25. However, the hydrogen evolution was 20.85 μmol/h/g of photocatalyst. The results suggest that bauxite residue appears to be a novel low cost photocatalyst. The various operating conditions of photocatalytic hydrogen generation were studied which include amount of catalyst, illumination intensity, illumination time, effect of various sacrificial donors etc.     

Photocatalytic water splitting ability of Fe/MgO-rGO nanocomposites towards hydrogen evolution

Photocatalytic water splitting has greatly stimulated as an ideal technique for producing hydrogen (H₂) fuel by employing two renewable sources, i.e., water and solar energy. Here, we have adopted a facile hydrothermal approach for the successful synthesis of reduced graphene oxide (rGO) incorporated Fe/MgO nanocomposites followed by thermal treatment at inert atmosphere to investigate their ability for photodegradation and photocatalytic hydrogen evolution via water splitting. Transmission Electron Microscopy images of Fe/MgO-rGO nanocomposite ensured the distribution of Fe/MgO nanoparticles throughout rGO sheets. Notably, all rGO supported nanocomposites, especially the one, thermally treated at 500 °C at Argon (Ar) atmosphere has demonstrated significantly higher photocatalytic efficiency towards the photodegradation of a toxic textile dye, rhodamine B, than pristine MgO and commercially available Degussa P25 titania nanoparticles as well as other composites. Under solar irradiation, Fe/MgO-rGO (500) nanocomposite exhibited 86% degradation of rhodamine B dye and generated almost four times higher H₂ via photocatalytic water splitting compared to commercially available P25 titania nanoparticles. This promising photocatalytic ability of the Fe/MgO-rGO(500) nanocomposite can be attributed to the improved morphological and surface features due to heat treatment at inert atmosphere as well as escalated charge carrier separation with increased light absorption capacity imputed to rGO incorporation.     

Visible light induced photoreduction of water by N-doped mesoporous titania

N-doped mesoporous titania was synthesized by templating method. Three different types of photocatalysts were synthesized by varying chitosan to titania compositions and designated as N-doped mesoporous titania (1:1), (1:2) and (1:3). These synthesized photocatalysts were characterized by XRD, BET-SA, UV-DRS, SEM-EDX and XPS. This photocatalyst is active in visible range with band gap energy of 2.65 eV. Formation of Ti–N bond reveals the decrease in the band gap of TiO₂. The synthesized photocatalysts were screened initially for their photocatalytic activity using water splitting reaction. The maximum hydrogen yield of 2654.57 μmol/h/g of photocatalyst was obtained for N-doped mesoporous titania (1:2). This yield is 16 times higher as compared to the bench mark material Degussa P-25 (161 μmol/h/g of photocatalyst). The best performing photocatalyst N-doped mesoporous titania (1:2) was investigated in detail to study the influence of various operating parameters. Reuse and recycle study results in steady hydrogen yield of 9605.56 μmoles for 30 h.     

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO₄ has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO₄ yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO₄ photoanode coupled with FeOOH (W–BiVO₄/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO₄. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO₄/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm², which is seven folds higher than that of the pristine BiVO₄ photoanode (0.31 mA/cm² 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO₄/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO₄/FeOOH photoanode.     

Solar photocatalytic removal of herbicides from real water by using sol–gel synthesized nanocrystalline TiO2: Operational parameters optimization and toxicity studies

A comparative study of the photocatalytic activity of two different TiO₂ catalysts in solar photocatalytic oxidation, mineralization and detoxification of waters containing herbicides 2,4-dichlorophenoxyacetic acid (2,4-D), bentazon and toxic intermediates was performed in a pilot plant scale photoreactor. Commercial TiO₂ (Degussa P25) and TiO₂ synthesized by citrate sol–gel method (ECT-1023t) were selected as photocatalysts. The optimal basic operational parameters to eliminate these herbicides and toxic intermediates were established for both catalysts at laboratory scale. ECT-1023t showed better photocatalytic activity than the commercial Degussa P25 at solar pilot plant scale with both herbicides in real water at natural pH (6.8–7.8) without any additive. The toxicity of the treated solutions was evaluated using the Microtox test based on the inhibition of bioluminescence of the bacteria Vibrio fischeri. The toxic effect of the main intermediate of 2,4-D, the 2,4-dichlorophenol (2,4-DCP), was higher than the parental herbicide. Acute toxicity of 2,4-D and intermediates (2,4-DCP) was reduced during the photocatalytic treatment by using ECT-1023t as photocatalyst. Longer times were necessary to obtain similar results when using P25 as photocatalyst. No inhibitory growth effect of the herbicide bentazon and its photoproducts on Vibrio fischeri bacteria bioluminescence was observed for either photocatalyst in any of the irradiated samples collected at predetermined times using an initial concentration of 0.1325 mM of the herbicide.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Rational design and fabrication of surface tailored low dimensional Indium Gallium Nitride for photoelectrochemical water cleavage

Currently several type of energy sources exist in the modern world. The energy makes people's life more comfortable, easy, time savings, fast transformation of information and various modes of transmission. Because of large demand of energy, efforts on production of energy increases day by day which subsequently increase serious environmental concerns such as pollution and lack of existing natural resources. In this respect, several attempts have been proposed for new type of renewable and chemical energy systems to overcome the economic burden, global warming and environmental problems caused by the use of conventional fossil fuels. Hydrogen production via water splitting is a promising and ideal route for renewable energy using the most abundant resources of solar light and water. Cost effective photocatalyst for Photoelectrochemical (PEC) water splitting using semiconductor materials as light absorbers have been extensively studied due to their stability and simplicity. Over the past few decades, various metal oxide photocatalysts for water splitting have been developed and their photocatalytic application was studied under UV irradiation. Alternative semiconductor photocatalyst should harness solar energy in the visible light, one such semiconductor material is indium gallium nitride (InGaN), owing to its suitable and tunable energy band-gap, chemical resistance and notable photoelectrocatalytic activity. This review article is initiated with the brief introduction about the origin and methods of production of hydrogen gas from both renewable and nonrenewable energy sources. Multi-functional properties and applications of InGaN are described along with past and recent efforts of InGaN materials for hydrogen evolution by several investigators are provided in detail. In addition, future prospects and ways to improve the PEC performance of InGaN are presented at the end of this review.     

Carbon-incorporated TiO2 microspheres: Facile flame assisted hydrolysis of tetrabutyl orthotitanate and photocatalytic hydrogen production

We report a novel and facile energy-saving method to prepare microspherical carbon-incorporated titania powders by flame assisted hydrolysis of tetrabutyl orthotitanate. The as-prepared samples were fully characterized by XRD, SEM, XPS, UV-Vis absorption spectra and photocatalytic hydrogen production. Anatase TiO₂ can be obtained directly without any post heat treatment. The as-prepared TiO₂ is formed of microspheres with sizes in a range of 0.5∼2.0 μm. XPS measurement shows the presence of carbon species which come from the incomplete combustion of organic compounds. Enhanced photocatalytic hydrogen production rates were observed for the as-prepared samples. A maximum hydrogen production rate was 8.1 μmol h⁻¹, which was 1.8 times larger than that of Degussa P25. The improved photocatalytic activity is attributed to enhanced light absorption behavior, which is caused by carbon incorporation and microspherical structure. This work demonstrates a novel and efficient strategy to synthesize microspherical anatase TiO₂ photocatalyst without any special equipment or setup.     

A green and facile synthesis of TiO2/graphene nanocomposites and their photocatalytic activity for hydrogen evolution

This work reports a green and facile approach to synthesize chemically bonded TiO₂/graphene sheets (GS) nanocomposites using a one-step hydrothermal method. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and ultraviolet visible (UV-Vis) diffuse reflectance spectra. The photocatalytic activity was evaluated by hydrogen evolution from water splitting under UV-Vis light illumination. An enhancement of photocatalytic hydrogen evolution was observed over the TiO₂/GS composite photocatalysts, as 1.6 times larger for TiO₂/2.0 wt%GS than that of Degussa P25. This fabrication process features the reduction of graphene oxide and formation of TiO₂ simultaneously leading to the well dispersion of generated TiO₂ nanoparticles on the surface of GS.     

The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation

A novel photocatalyst was designed from the inspiration of natural forest's high efficient on light harvesting and energy conversion. This novel “forest-like” photocatalyst was successfully synthesized by a facile continuously-conducted three steps methods: electrospinning TiO₂ nanofiber acts as the trunks, hydrothermal growth ZnO nanorods on the surface of TiO₂ nanofiber acts as the branches, while photodeposition of Cu nanoparticles on the surface of TiO₂ nanofiber and ZnO nanorods act as the leaves. This novel photocatalyst demonstrated higher photocatalytic hydrogen generation rate than most of semiconductor catalysts and many newly developed catalysts such as Pt/TiO₂ catalyst and artificial leaves Pt/N–TiO₂ catalyst in a water/methanol sacrificial reagent system under the light irradiation as a result of its enhanced light absorption ability, enlarged specific surface area promoting mass transfer and providing more reaction sites and its potential on anti-recombination of electrons and holes. Meanwhile, it is interesting to note that the photocatalytic hydrogen generation activity has a liner relationship with the hierarchy of materials, which means higher hierarchy materials display higher photocatalytic hydrogen generation activity. It is reasonable to believe that this natural mimic photocatalyst without noble metals will benefit the energy generation and novel materials development.     

Cd0.5Zn0.5S/Ni2P noble-metal-free photocatalyst for high-efficient photocatalytic hydrogen production: Ni2P boosting separation of photocarriers

Establishing efficient co-catalytic loaded semiconductors for efficient charge separation is a hopeful way for enhance photocatalytic water splitting hydrogen evolution. Herein, we successfully constructed the Cd_0.5Zn_0.5S/Ni₂P (CZS/Ni₂P) nanocomposites via two-step hydrothermal method. The CZS/Ni₂P composites show much improved activity than the origin CZS for photocatalytic H₂ generation. When the content of Ni₂P loaded on the Cd_0.5Zn_0.5S (CZS) is 0.3 mol%, the photocatalyst achieves the highest photocatalytic hydrogen generation rate of 41.26 mmol g⁻¹ h⁻¹ under visible light. The Ni–S bonds on the close contact interface between CZS and Ni₂P can be act as electron-bridge to provide a channel for electron transfer. During the photocatalysis processing, Ni₂P can be used as electron traps to attract electrons from CZS, resulting in the improvement of the photocatalytic performance.     

Separation of hot electrons and holes in Au/LaFeO3 to boost the photocatalytic activities both for water reduction and oxidation

Construction of plasmon-based nanostructures is an effective way to enhance the photocatalytic activities of semiconductor photocatalysts for water-splitting. However, the synergistic effect of plasmon-related hot electrons and holes for water splitting in the plasmon-hybrid photocatalyst is rarely considered. Herein, we construct a plasmon-based Au/LaFeO₃ composite photocatalyst to investigate the complex roles of hot electrons and holes for solar water splitting. Benefiting from the formation of Schottky junction and surface plasmon resonance effect of the Au nanoparticles, the synthesized photocatalyst exhibits an excellent photocatalytic activity for each half-reaction of water splitting, and the rates for H₂ and O₂ generation are obtained as high as 202 μmol g⁻¹ h⁻¹ and 23 μmol g⁻¹ h⁻¹, respectively. Moreover, an in-depth investigation reveals that the improved hydrogen evolution is caused by the hot electron injection from Au to LaFeO₃, and the hot holes in Au induced by the separation of hot charges can initiate the water oxidation directly on the surface of gold. Thus, this work provides a new insight into the synergistic effect of plasmon-related hot electrons and holes for boosting the photocatalytic reactions.     

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.     

Significant improvement of TiO2 photocatalytic hydrogen generation by photothermic synergistic action and underlying mechanism

This article reported an extremely easy method of optical radiation-assisted thermal excitation to dramatically increase photocatalytic hydrogen generation ability of water splitting with P25 as a model compound. This method compensated for the time waste, high cost and operational complexity of traditional catalytic material modification methods, and largely improved the photocatalytic hydrogen production ability of photocatalytic materials. The hydrogen generation rates at room temperature is 1090 μmol/g/h. At 50 °C, the rate increase to 10670 μmol/g/h. The quantum rates at room temperature and 50 °C are 6.5 and 63.3, respectively. It is clear that appropriate low-temperature heating could largely accelerate the hydrogen generation rate of P25.This work presents the detailed mechanism how this method largely enhances photocatalytic hydrogen generation of P25 as well as the laws. The new method offers some evidences and reference for research on how the photothermic synergistic action facilitates the photocatalytic hydrogen generation of catalytic materials.     

CdS@Mg(OH)2 core/shell composite photocatalyst for efficient visible-light photocatalytic overall water splitting

Coating a protective agent or promoter on the surface of the photocatalyst is a proven good strategy to realize photocatalytic hydrogen production from pure water, but remains still a considerable challenge. Herein, a novel CdS@Mg(OH)₂ core/shell composite nanorods photocatalyst was synthesized by coating Mg(OH)₂ on CdS surface by hydrothermal and precipitation processes. The coated-Mg(OH)₂ layer did not change the structure of CdS, and the photocatalytic overall water splitting performance of the CdS@Mg(OH)₂ under visible light irradiation was improved obviously. After loading nano-Pt via the photodeposited method, the hydrogen production rate and stability of Pt/CdS@Mg(OH)₂ were 3.3 and 2.4 times that of the Pt/CdS under the visible light irradiation, respectively. The surface Mg(OH)₂ layer improved the hydrophilicity and stability of the core/shell composites and increased the amount of active sites, thus improving the photocatalytic properties. It is believed that Mg(OH)₂ can be used as a new co-catalyst to enhance the performance of photocatalytic overall water splitting.     

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.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

Hybrid of AgInZnS and MoS2 as efficient visible-light driven photocatalyst for hydrogen production

We present a simple two-step hydrothermal method to prepare AgInZnS/MoS₂ nanocomposite. The morphology and compositional characteristics of the sample were investigated by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The as-synthesized heterostructures exhibited superior photocatalytic activity for hydrogen evolution under visible-light irradiation and the optimum loading amount of MoS₂ is at 0.5 wt%. In an attempt to explain this phenomenon, a possible mechanism was also proposed. The enhanced photocatalytic activities toward water splitting arise from the boosted active sites for hydrogen generation and the enhanced charge transfer. This work may contribute to the design and construction of highly efficient visible-light responsive photocatalyst for sustainable energy harvesting and conversion.     

Enhanced photocatalytic hydrogen evolution over 0D/3D NiTiO3 nanoparticles/CdIn2S4 microspheres heterostructure photocatalyst

Constructing S-scheme heterojunction is regarded as an effective mode to motivate excellent photocatalytic performance for hydrogen generation. This paper prepares NiTiO₃/CdIn₂S₄ S-scheme heterostructure photocatalyst by hydrothermal method successfully. In the experiments, 20 wt% NiTiO₃/CdIn₂S₄ has the supreme photocatalytic activity with the H₂ generation rate of 5168.6 μmol g⁻¹ h⁻¹ and the apparent quantum yield (AQY) of 5.14% at 420 nm, approximately 7.7 times of pure CdIn₂S₄. Through the phase characterization analyses, NiTiO₃ and CdIn₂S₄ successfully compounded, with NiTiO₃ nanoparticles wrapping around CdIn₂S₄ microspheres to form the irregular clumps. Further analyses of performance reveal the larger specific surface area, wider absorption region, faster charge transfer rate, outstanding photostability and recyclability for 20 wt% NiTiO₃/CdIn₂S₄, all of which play the significant role in photocatalytic hydrogen evolution activity. Finally, a plausible S-scheme photocatalytic mechanism for NiTiO₃/CdIn₂S₄ is proposed. This study provides a novel and effective S-scheme photocatalyst for hydrogen generation from water splitting.