Photocatalytic Water Splitting Activity Research Articles

Electrospinning Preparation of GaN:ZnO Solid Solution Nanorods with Visible-Light-Driven Photocatalytic Activity toward H2 Production

The development of a facile method for the synthesis of GaN:ZnO solid solution, an attractive material with a wurtzite-type structure, is vital to enhance its photocatalytic activity toward H2 evolution. Herein, GaN:ZnO solid solution nanorods with diameters of around 180 nm were fabricated by combining the electro-spun method with a sequentially calcinating process. Photocatalytic water-splitting activities of the as-obtained samples loaded with Rh2−yCryO3 co-catalyst were estimated by H2 evolution under visible-light irradiation. The as-prepared GaN:ZnO nanorods at a nitridation temperature of 850 °C showed the optimal performance. Careful characterization of the GaN:ZnO solid solution nanorods indicated that the nitridation temperature is an important parameter affecting the photocatalytic performance, which is related to the specific surface area and the absorbable visible-light wavelength range. Finally, the mechanism of the GaN:ZnO solid solution nanorods was also investigated. The proposed synthesis strategy paves a new way to realize excellent activity and recyclability of GaN:ZnO solid solution nanorod photocatalysts for hydrogen generation.

Electronic and optical properties of nitrogen and sulfur doped strontium titanate as efficient photocatalyst for water splitting: A DFT study

One of the most effective option of photocatalysts for water splitting is doped strontium titanate, SrTiO3. It has a high rate of photo-generated charge transfer and limited photocatalytic activity for water splitting. The search of an appropriate photocatalyst having a high visible light absorption as well as fast charge transportation is extremely needed, however it is a difficult task. The structural, electronic and optical properties of sulfur-doped SrTiO3 and nitrogen-doped SrTiO3 are investigated using calculations based on density functional theory (DFT). According to the band structure calculations, the O-2p states represented the higher levels of the valence band of pure SrTiO3. When S and N atoms are introduced into the SrTiO3 structure on the O site, electronic structure findings indicate that doping the Sulfur (S) atoms reduced the band gap significantly, whereas doping of N atoms increased the bandgap of SrTiO3. According to our results, the N-doped SrTiO3 has a sufficient band gap of 2.03 eV, as well as suitable high visible light absorption and charge carrier transportation. The optical properties showed that N-doped SrTiO3 has good photosensitivity for visible light. In addition, we have found a significant impurity state that differs from O 2p-states, which can increase photocatalytic efficiency. The results of studies of electronic band structure showed that electron-hole transportation was well consistent with the experimental data. Thus, the N-doped SrTiO3 in this study is indeed an attractive candidate for hydrogen evolution throughout the visible light range, providing a logical base for the establishment of innovative photocatalysts.

RGO nanosheets coupled NaNbO3 nanorods based nanocomposite for enhanced photocatalytic and photoelectrochemical water splitting activity

In this present work, reduced graphene oxide (RGO) coupled with hydrothermally grown sodium niobate nanorods (NaNbO3-NRs) have been successfully synthesized. The photocatalytic performance of RGO/NaNbO3-NRs photocatalyst demonstrated faster photodegradation of organic methylene blue (MB) dye than bare NaNbO3-NRs. A ∼6 fold enhancement in the photocatalytic activity of RGO/NaNbO3-NRs nanocomposite than that of NaNbO3-NRs has been demonstrated towards the degradation of MB dye under similar light illumination. Furthermore, the potentiality of the fabricated NaNbO3-NRs and RGO/NaNbO3-NRs nanocomposite photoanodes have been investigated for photoelectrochemical (PEC) water splitting. The fabricated RGO/NaNbO3-NRs nanocomposite photoanode showed ∼4 times higher photocurrent density than the NaNbO3-NRs photoanode. The electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) measurements demonstrated that coupling of RGO nanosheets in the RGO/NaNbO3-NRs nanocomposite reduced the charge transfer resistance (Rct) at the photoanode/electrolyte interface, increased the donor density (Nd), and reduced flat band potential (Vfb) of the RGO/NaNbO3-NRs, thus significantly improving the PEC performance of the RGO/NaNbO3-NRs nanocomposite. The enhancement in the PEC measurements of RGO/NaNbO3-NRs nanocomposite is attributed to the extended absorption of the visible portion of the solar spectra and increased mobility of the photogenerated charge transport in the RGO nanosheets, which improve the separation efficiency and reduce the recombination process. The possible charge transfer mechanism has been proposed responsible for the enhanced photocatalytic and PEC water splitting performance.

Water Splitting on Multifaceted SrTiO3 Nanocrystals: Computational Study

Recent experimental findings suggest that strontium titanate SrTiO3 (STO) photocatalytic activity for water splitting could be improved by creating multifaceted nanoparticles. To understand the underlying mechanisms and energetics, the model for faceted nanoparticles was created. The multifaceted nanoparticles’ surface is considered by us as a combination of flat and “stepped” facets. Ab initio calculations of the adsorption of water and oxygen evolution reaction (OER) intermediates were performed. Our findings suggest that the “slope” part of the step showed a natural similarity to the flat surface, whereas the “ridge” part exhibited significantly different adsorption configurations. On the “slope” region, both molecular and dissociative adsorption modes were possible, whereas on the “ridge”, only dissociative adsorption was observed. Water adsorption energies on the “ridge” ( −1.50 eV) were significantly higher than on the “slope” ( −0.76 eV molecular; −0.83 eV dissociative) or flat surface ( −0.79 eV molecular; −1.09 eV dissociative).

Au tailored on g-C3N4/TiO2 heterostructure for enhanced photocatalytic performance

Inhibiting the recombination of photogenerated charge carriers is of great significance to improve the photocatalytic performance. For hybrid heterostructure photocatalysts, the noble metal is often used to modulate the interfacial charge behavior, which is closed related to metal states. Herein, we construct graphite-like C3N4 and TiO2 (g-C3N4/TiO2) heterostructure with Au nanoparticles tailored on g-C3N4 to form multiple charge separation for high-efficiency charge separation. Due to the enhanced charge separation, Au/g-C3N4/TiO2 heterostructures exhibits excellent photocatalytic activity for pollutant removal and water splitting. The apparent rate constant of Rh6G photodegradation achieves 2 times higher increased compared with different g-C3N4/TiO2 heterostructures with Au modulation. Active species trapping experiments indicate that superoxide radicals (·O2-) play a most important role in catalytic process. This work provides insights into the distinct roles of Au in heterostructure for significantly enhanced charge separation and paves the way forward in designing high-efficiency heterostructure photocatalysts with noble metal decorated.

Fabrication of 0D/2D TiO2 Nanodots/g-C3N4 S-scheme heterojunction photocatalyst for efficient photocatalytic overall water splitting

It is a considerable challenge to design a highly efficient photocatalyst for overall water splitting. Herein, a zero-dimension/two-dimension (0D/2D) TiO2 nanodots/g-C3N4 step-scheme (S-scheme) photocatalyst (TOCN) for overall water splitting was prepared by a facile phase interfacial reaction. TOCN showed higher photocatalytic water splitting activity under simulated sunlight, which is about 2.2 and 1.7 times higher than TiO2 nanodots and g-C3N4, respectively. The enhancement of photocatalytic performance can be mainly attributed to the formation of S-scheme heterojunction, rather than normal type-II heterojunction, which could not only improve charge separation but also remain the redox ability of TiO2 and g-C3N4. This work presents a simple method to construct an efficient photocatalyst with S-Scheme heterojunction for overall water splitting.

Ultrasound-assisted synthesis of visible-light-driven Ag/g-C3N4 catalysts in a continuous flow reactor

Size control of supported nanoparticles remains a challenge, especially in the presence of external fields. In this paper, an ultrasound-assisted method was developed for the continuous synthesis of visible-light-driven Ag/g-C3N4 catalysts with controllable sizes and narrow size distributions in a coiled flow inverter reactor. The catalytic performance was characterized by the photocatalytic water splitting, which was considered as a promising method to provide hydrogen fuel. Here, we investigated the synergy effect of output power of ultrasound, residence time, and Ag loading content on the characteristic and catalytic performance of the catalysts. With the increase of ultrasound output power (from 0 W to 120 W) and the decrease of residence time (from 140s to 17.5s), the size of silver nanoparticles becomes smaller, and the distribution is narrower. The higher catalytic activity was achieved for the catalyst under 60 W power, 1.22 times higher than that of the catalyst synthesized without ultrasound. Ag/g-C3N4 catalysts with higher Ag loading exhibited better catalytic activity, despite the bigger size of Ag nanoparticles. The results suggest that 7 wt% Ag/g-C3N4 catalysts synthesized under 60 W ultrasound power for 35s (residence time) exhibited the best photocatalytic activity for water splitting, which was 2.76 times higher than that of using pure g-C3N4.

Superior tunable photocatalytic properties for water splitting in two dimensional GeC/SiC van der Waals heterobilayers

The photocatalytic characteristics of two-dimensional (2D) GeC-based van der Waals heterobilayers (vdW-HBL) are systematically investigated to determine the amount of hydrogen (H2) fuel generated by water splitting. We propose several vdW-HBL structures consisting of 2D-GeC and 2D-SiC with exceptional and tunable optoelectronic properties. The structures exhibit a negative interlayer binding energy and non-negative phonon frequencies, showing that the structures are dynamically stable. The electronic properties of the HBLs depend on the stacking configuration, where the HBLs exhibit direct bandgap values of 1.978 eV, 2.278 eV, and 2.686 eV. The measured absorption coefficients for the HBLs are over ~ 105 cm−1, surpassing the prevalent conversion efficiency of optoelectronic materials. In the absence of external strain, the absorption coefficient for the HBLs reaches around 1 × 106 cm−1. With applied strain, absorption peaks are increased to ~ 3.5 times greater in value than the unstrained HBLs. Furthermore, the HBLs exhibit dynamically controllable bandgaps via the application of biaxial strain. A decrease in the bandgap occurs for both the HBLs when applied biaxial strain changes from the compressive to tensile strain. For + 4% tensile strain, the structure I become unsuitable for photocatalytic water splitting. However, in the biaxial strain range of − 6% to + 6%, both structure II and structure III have a sufficiently higher kinetic potential for demonstrating photocatalytic water-splitting activity in the region of UV to the visible in the light spectrum. These promising properties obtained for the GeC/SiC vdW heterobilayers suggest an application of the structures could boost H2 fuel production via water splitting.

Strain-tunable band alignment and band gap of GaSe/WTe2 heterojunction for water splitting and light-emitting

Van der Waals heterojunctions (vdWHs) based on transition metal dichalcogenides (TMDs) attract much attention due to their unique electrochemical and physical properties. However, most vdWHs in TMD family are type II with indirect band gap. For vdWHs in TMD family, it is difficult to obtain the direct band gap through band arrangement approaches, limiting their applications in optoelectronics and photocatalysis. In present work, a type-I GaSe/WTe2 heterojunction with direct band gap (1.13 eV) is found by first-principles calculations. We find that this heterojunction has dipoleallowed direct band gap transition and large band offsets, which are beneficial to light emission efficiency. The effects of strain on electronic structure and optical properties of GaSe/WTe2 heterojunction are investigated. We find that band gap of type-I GaSe/WTe2 heterojunction can be tuned in large range by strain and it is a promising platform to develop strain-tunable light-emitting device which can emit various wavelengths. The band alignment type of GaSe/WTe2 heterojunction can be tuned from type-I to type-II by strain. Our results show that, in type-II alignment, it has efficient photocatalyticactivity in water splitting and the photocatalytic efficiency can be enhanced by strain. We hope our work can make a contribution to exploring a strain-tuned platform for photocatalytic and light-emitting devices.

Hollow In2O3 nanotubes decorated with Cd0.67Mo0.33Se QDs for enhanced photocatalytic hydrogen production performance

Novel Cd0.67Mo0.33Se/In2O3 hollow nanotubes were prepared for photocatalytic hydrogen production application. Under visible light irradiation, Cd0.67Mo0.33Se/In2O3 hollow nanotubes showed enhanced photocatalytic performance. And the apparent quantum efficiency of 34.86% was obtained when irradiated with 420 nm monochromatic light. The modification of Cd0.67Mo0.33Se QDs on the surface of In2O3 hollow nanotubes effectively improved the utilization rate of light absorption, increased the separation and migration rate of electrons, inhibited the recombination of photo-generated electron and hole pairs, thus enhancing the photocatalytic activity of water splitting to produce hydrogen. It would be an efficient photocatalyst for hydrogen production application in future.

Introducing the novel composite photocatalysts to boost the performance of hydrogen (H2) production

Endeavor has been made in this work to develop a supported Si photocatalyst to efficiently split water into hydrogen under irradiation with visible light. Hydrothermal and solid phase reaction method were used to synthesize of Si/CNTs photocatalysts and characterized by using UV–visible optical absorption spectra (UV–Vis), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Scanning electron microscope (SEM) and etc. By examining their properties, it was found that the two types of surface dangling bonds existed on the surface of CNTs which greatly impacts on reaction efficiency. The generation of hydrogen (H2) onto supported Si catalyst may take place on hydroxyl and hydrogen bond with Si. The bandgap estimated from the reflection spectra was 2.2 eV for Si/CNTs photocatalyst. The highest generation of H2 of Si/CNTs was observed as 648 μmol h−1 which is greater than pristine Si without adding any hole-scavengers. The outcomes demonstrated that CNTs had a significant impact on photocatalytic water splitting activity because of high conductivity on remarkable net-like 2D structure. No apparent decrease in H2 production was detected after three successive runs representing the stability of the catalyst. Surface functions hold to achieve high efficiency in such a photocatalytic framework.

Towards Computer-Aided Graphene Covered TiO2-Cu/(CuxOy) Composite Design for the Purpose of Photoinduced Hydrogen Evolution

In search a hydrogen source, we synthesized TiO2-Cu-graphene composite photocatalyst for hydrogen evolution. The catalyst is a new and unique material as it consists of copper-decorated TiO2 particles covered tightly in graphene and obtained in a fluidized bed reactor. Both, reduction of copper from Cu(CH3COO) at the surface of TiO2 particles and covering of TiO2-Cu in graphene thin layer by Chemical Vapour Deposition (CVD) were performed subsequently in the flow reactor by manipulating the gas composition. Obtained photocatalysts were tested in regard to hydrogen generation from photo-induced water conversion with methanol as sacrificial agent. The hydrogen generation rate for the most active sample reached 2296.27 µmol H2 h−1 gcat−1. Combining experimental and computational approaches enabled to define the optimum combination of the synthesis parameters resulting in the highest photocatalytic activity for water splitting for green hydrogen production. The results indicate that the major factor affecting hydrogen production is temperature of the TiO2-Cu-graphene composite synthesis which in turn is inversely correlated to photoactivity.

(Invited) Dye-Sensitized Oxide Nanosheets for Visible-Light Water Splitting

Metal oxide nanosheets derived from the layered counterparts have attracted attention in various fields due to their unique structural properties. The anisotropic feature of nanosheets, having a thickness of ~1 nm and lateral dimensions ranging from several hundred nanometers to a few micrometer, is advantageous for heterogeneous photocatalysis, as the diffusion length of photogenerated carriers to the surface is shortened, leading to higher activity. It is also known that certain nanoparticulate metals or metal oxides on a semiconductor photocatalyst work as cocatalysts to promote water reduction and/or oxidation. In heterogeneous photocatalysis, the effect of cocatalyst size on the water-splitting performance had not been examined at sizes smaller than 1 nm due to the lack of an effective preparation method and a suitable photocatalyst. We have demonstrated that metal nanoclusters (such as Pt) of <1 nm in size could be deposited on the interlayer nanospace of KCa2Nb3O10 using the electrostatic attraction between a cationic metal complex and a negatively charged Ca2Nb3O10 – sheet, without the aid of any additional reagent. The material obtained exhibited 8 times greater photocatalytic activity for overall water splitting under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of <1 nm Pt nanoclusters for photocatalytic overall water splitting. With further modification by a ruthenium(II) photosensitizer, this material also worked as a H2 evolution photocatalyst in visible-light-driven Z-scheme water splitting, in combination with a WO3-based O2 evolution photocatalyst and a triiodide/iodide redox couple. Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous AlO x clusters demonstrated a remarkable photocatalytic activity with a dye-based turnover number and frequency for H2 evolution of 4580 and 1960 h–1, respectively, which were by far the highest among dye-sensitized nonsacrificial photocatalytic systems ever reported.

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.

Novel S-scheme WO3/RP composite with outstanding overall water splitting activity for H2 and O2 evolution under visible light

Elemental red phosphorus (RP) has an ultra-low activity of splitting water for H2 and O2 evolution. In this study, RP was decorated by trace amount of WO3 to greatly enhance the photocarriers separation and photocatalytic activity for overall water splitting under visible light. The 2.5% WO3/RP composite achieved the highest activity to produce H2 and O2, and the activity was 7.0 times that of pure RP under visible light (λ ≥ 420 nm). The significantly increased photocarriers separation of WO3/RP was ensured by transient photocurrent, electrochemical impendence, photoluminescence, and time-resolved photoluminescence characterizations. More importantly, WO3/RP maintained high stability when it was persistently used for 20 cycles. Based on the reactive oxygen species (O2− and OH) detected during the photocatalytic process, an S-scheme mechanism was proposed for WO3/RP composites to efficiently separate the photocarriers. This work provides a notion for improving the overall water splitting activity of RP via an S-scheme photocarriers separation mode.

Understanding the synergistic effect of Co-loading and B-doping in g-C3N4 for enhanced photocatalytic activity for overall solar water splitting

An effective approach of loading of metal (Co) and doping of non-metal (B) together has been considered to enhance the photocatalytic activity of tri-s-triazine based g-C3N4. Here, pristine and functionalized g-C3N4 with B-doping at bay C site (BC1–C3N4), corner C site (BC2–C3N4), Co-loading (Co–C3N4) and Co-loading and B-doping together [(Co–B)C1–C3N4 and (Co–B)C2–C3N4] have been studied for structural, electronic, optical and photocatalytic properties using hybrid density functional theory. B-doping in g-C3N4 formed p-type semiconductor while Co-loading revealed formation of n-type semiconductor with formation of Co-3d intermediate states while both together indicated partial charge compensation. All the functionalized films show lower overpotential value for oxygen evolution reaction (OER) as compared to g-C3N4 while for hydrogen evolution reaction (HER) lower overpotential values are obtained for Co–C3N4, (Co–B)C1–C3N4 and (Co–B)C2–C3N4. The present work, highlights the synergistic effects of Co-loading and B-doping in g-C3N4 in terms of formation of impurity states, high absorption, charge redistribution and lower OER and HER overpotential along with suitable redox potentials than g-C3N4 and suggests (Co–B)C1–C3N4 as potential candidate for overall photocatalytic water splitting.

Elucidating the mechanistic origins of P dopants triggered active sites and direct Z-scheme charge transfer by P-MoS2@WO3 heterostructures for efficient photocatalytic hydrogen evolution

MoS2 has been proven to be an environmental-friendly and economic photocatalyst for clean energy generation. However, the energy conversion efficiency still needs to be more enhanced for practical application due to the limited number of active sites on the surface and the high recombination rate of electron-hole pairs. Herein, P-doping was utilized to increase the active sites, and the heterojunction was constructed with WO3 to improve the charge separation rate. Our results show that the heterojunction P-MoS2@WO3 photocatalyst has a stable and durable photocatalytic activity in water splitting, which shows H2 evolution rate of 73.8 μmol h–1 g−1. This good photocatalytic performance of P-MoS2@WO3 is attributed to increased active sites at the thermodynamically stable basal (002) plane of MoS2 by the P-doping and the enhanced charge separation rate by the heterojunction with a direct Z-scheme mode. This study offers an efficient way to increase the active sites and separate charge carriers in MoS2-based photocatalysts for H2 generation.

Construction of an arsenene/g-C3N4 hybrid heterostructure towards enhancing photocatalytic activity of overall water splitting: A first-principles study

Using optical energy to split water on heterostructured photocatalysts is one of the most promising methods to solve current environmental problems. And the key challenge to achieve this goal is to get an excellent water splitting photocatalyst. Herein, the structure, electronic structure, optical performance and thermodynamic feasibility of arsenene/g-C3N4 catalyst for water splitting is investigated. It is showing that arsenene/g-C3N4 heterostructure, with type-II semiconductor characteristics, possess a suitable band gap (2.03 ​eV), potentials at valence band maximum (VBM) and at conduction band minimum (CBM), and thus it can effectively drive overall water splitting. Meanwhile, the arsenene/g-C3N4 heterostructure possess an excellent property to absorb visible light, and the solar-to-hydrogen (STH) efficiency reaches 17.08%. Furthermore, the free energy of the water splitting reaction is downhill in a certain pH range (3.78–6.49), indicating that overall water splitting can happen spontaneously. Therefore, the arsenene/g-C3N4 heterostructure is a promising candidate for photocatalytic overall water splitting.

Constructing a novel metal-free g-C3N4/g-CN vdW heterostructure with enhanced visible-light-driven photocatalytic activity for water splitting

With the increasing environmental pollution and energy crisis, developing and designing environmentally friendly, low-cost, metal-free and efficient photocatalyst for water splitting is vitally important. Herein, a novel g-C3N4/g-CN van der Waals (vdW) heterostructure is proposed and its structural, electronic and optical properties are systematically investigated based on first-principles method. The results show that the g-C3N4/g-CN vdW heterostructure is a type-II semiconductor, which satisfies band gap and band edge requirements of the redox potential of water splitting. In addition, the oxygen evolution reaction (OER) on g-C3N4 side and the hydrogen evolution reaction (HER) on g-CN side can proceed spontaneously in thermodynamics. Most notably, the electronic, thermodynamic and optical properties of the g-C3N4/g-CN vdW heterostructure can be modulated by applying biaxial strain to achieve the purpose of better catalytic water splitting. Also, the good visible-light absorption intensity of the g-C3N4/g-CN vdW heterostructure can also be exhibited without strain and it is much better under strain of −2%. Consequently, these findings suggest that the g-C3N4/g-CN vdW heterostructure has a promising potential as a photocatalyst for water splitting.

Nonlayered Tin Thiohypodiphosphate Nanosheets: Controllable Growth and Solar-Light-Driven Water Splitting

As a promising candidate in various fields, including energy conversion and electronics, layered van der Waals metal phosphorus trichalcogenides (MPX3) have been widely explored. In addition to the layered structures, MPX3 comprising post-transition metals (i.e., Sn and Pb) are known to form a unique 3D framework with nonlayered structure. However, the nonlayered two-dimensional (2D) crystals of this family have remained unexplored until now. Herein, we successfully synthesized 2D nonlayered tin thiohypodiphosphate (Sn2P2S6) nanosheets, having an indirect bandgap of 2.25 eV and a thickness down to ∼10 nm. The as-obtained nanosheets demonstrate promising photocatalytic water splitting activity to generate H2 in pure water under simulated solar light (AM 1.5G). Moreover, the ultrathin Sn2P2S6 catalyst shows auspicious performance and stability with a continuous operation of 40 h. This work is not only an expansion of the MPX3 family, but it is also a major milestone in the search for new materials for future energy conversion.