Photocatalytic Water Splitting Technology Research Articles

In-situ construction of N-doped Zn0.6Cd0.4S/oxygen vacancy-rich WO3 Z-scheme heterojunction compound for boosting photocatalytic hydrogen production

Photocatalytic water splitting technology for H2 production represents a promising and sustainable approach to clean energy generation. In this study, a high concentration of oxygen vacancies was introduced into tungsten trioxide (WO3) to create a vacancy-rich layer. This modified WO3 (WO3-x) was then combined with N-doped Zn0.6Cd0.4S through a hydrothermal synthesis, resulting in the formation of a Z-scheme heterojunction composite aimed at enhancing photocatalytic performance. Under visible light, the H2 production activity of the composite reached an impressive 8.52 mmol·g−1 without adding co-catalyst Pt. This corresponds to enhancements of 7.82 and 4.39 times the production yield of pure ZCS and ZCSN, respectively. However, the hydrogen production increased to 21.98 mmol·g−1 when Pt was added as a co-catalyst. Furthermore, an array of characterizations were employed to elucidate the presence of oxygen vacancies and the establishment of the Z-scheme heterojunction. This structural enhancement significantly facilitates the utilization of photo-generated electrons while effectively preventing photo-corrosion of ZCSN, thus improving material stability. Our study provides a new scheme for the incorporation of oxygen-rich vacancy and the construction of Z-scheme heterojunction, demonstrating a synergistic effect that greatly advances photocatalytic performance.

Recent research progress of MOFs-based heterostructures for photocatalytic hydrogen evolution

The growing serious energy crisis and environmental pollution expedite the development of green hydrogen energy. Photocatalytic water splitting technology, converting solar energy into chemical energy directly, provides a clean and economical way for hydrogen evolution. For the past few years, metal–organic frameworks (MOFs) semiconductor materials have become a research hotspot in the field of photocatalytic hydrogen evolution (PHE) due to their multiple advantages, including adjustable structure, high porosity, large specific surface area, abundant active sites, and so forth. Besides this, the construction of heterogeneous structures based on MOFs materials can further improve PHE performance through the effective internal electric field directional transfer of photogenerated electrons and the superior utilization rate of sunlight in comparison to the single MOFs photocatalyst. Herein, in this minireview, we summed up the varieties of synthesis methods of diverse types of MOFs heterojunction complex materials, discussed the H2 evolution efficiency based on this heterostructure photocatalyst in detail, and lastly proposed the advantages, limitations, and outlook of MOFs composites in the PHE field. We sincerely hope this review can provide a reference for future research based on MOFs materials for alleviating the current energy crisis.

Interface engineering of Z-scheme heterojunction for photocatalytic water splitting

Photocatalytic water splitting technology can directly convert solar energy into H2 via a zero-carbon route, offering a sustainable solution for solar utilization and H2 supply. Among various developed photocatalysts, Z-scheme heterojunction mimicking natural photosynthesis by combining two dissimilar semiconductors for redox reactions in series has unequivocally demonstrated its superiority in enhanced charge transfer, robust redox driving force, and wide optical absorption range. A comprehensive understanding on the fundamental principles of interface engineering between semiconductor components is the key to construct an efficient Z-scheme heterojunction. By focusing on different types of semiconductors, this article thoroughly expounds the coupling principles of components in binary mediator-free and ternary solid-mediator Z-scheme heterojunctions for photocatalytic water splitting, from the viewpoint of band structure alignment and interfacial electric field design. In addition to the well summarized research progresses in recent years, perspectives on the challenges and opportunities for developing advanced Z-scheme heterojunctions are provided.

Overall water splitting: What’s next?

Photocatalytic overall water splitting reaction driven by sunlight has been studied as a means of producing renewable solar hydrogen on a large scale. The practicality of this technology will ultimately depend on the cost of the produced solar hydrogen. This cost, in turn, is determined by various factors such as solar-to-hydrogen energy conversion efficiency, scalability, construction/operating costs and the service life of such systems. This article both summarizes the current state of development of photocatalytic overall water splitting technology and presents issues that must be addressed in the future to allow such systems to have real-world utility.

NiS Protective Layer for Repelling Chloride Ion Effectively for Water Oxidation on Photocatalytic Seawater Splitting

Due to the increase of global power use, the shortage of energy supply and global warming have become the main obstacles to social and economic development. Today’s society is gradually developing cleaner and greener energy represented by solar energy to alleviate the current energy shortage. Among them, semiconductor photocatalytic water splitting technology is one of the most important methods. However, global freshwater resources are in short supply; the design and synthesis of proper photocatalysts for seawater splitting has been regarded as one of the most powerful approaches solving the dilemma of a shortage of freshwater resources and utilization of solar energy. Based on that, the quaternion Pt/Co3O4–NiS/STO composites for photocatalytic water splitting in a seawater system were successfully synthesized by a simple hydrothermal method and photodeposition method. Noticeably, the loading of Pt and Co3O4 cocatalysts greatly improved the separation efficiency of photogenerated electrons and holes. At the same time, the surface modification of NiS made the photocatalysts able to resist the toxicity of a large number of chloride ions in seawater. The oxygen yield for seawater splitting was nearly twice that of untreated samples and about half of the amount of that in pure water. Our work introduced an ideal modification material structure to demonstrate the feasibility of seawater splitting, which would guide other photocatalytic seawater splitting systems in the future.

The three-dimensionally ordered microporous CaTiO3 coupling Zn0.3Cd0.7S quantum dots for simultaneously enhanced photocatalytic H2 production and glucose conversion

Glucose conversion assisted photocatalytic water splitting technology to simultaneously produce H2 and high value-added chemicals is a promising method for alleviating the energy shortage and environmental crisis. In this work, we constructing type II heterojunction by in-situ coupling Zn0.3Cd0.7S quantum dots (ZCS QDs) on three-dimensionally ordered microporous CaTiO3 (3DOM CTO) for photocatalytic H2 production and glucose conversion. The DFT calculations demonstrate that substitution of Zn on the Cd site improves the separation and transmission of photogenerated carriers. Therefore, 3DOM CTO-ZCS composite exhibits best H2 production performance (2.81 mmol g-1h−1) and highest apparent quantum efficiency (AQY) (5.56 %) at 365 nm, which are about 47 and 18 times that of CTO nanoparticles (NPs). The improved catalytic performance ascribed to not only good mass diffusion and exchange, highly efficient light harvesting of 3DOM structure, but also the efficient charges separation of type Ⅱ heterojunction. The investigation on photocatalytic mechanism indicates that the glucose is mainly converted to gluconic acid and lactic acid, and the control reaction step is gluconic acid to lactic acid. The selectivity for gluconic acid on 3DOM CTO-ZCS is 85.65 %. Our work here proposes a green sustainable method to achieve highly efficient H2 production and selective conversion of glucose to gluconic acid.

Enhanced photocatalytic H2 production of graphitic carbon nitride via implantation of lithium and potassium ions

Photocatalytic water splitting technology using sunlight to produce hydrogen is considered to be an effective method to solve the energy crisis. Graphitic carbon nitride (g-C3N4) as an attractive two-dimensional semiconductor with visible-light response, has been widely applied in water splitting. However, poor light absorption and fast carrier recombination limit the potential application. Herein, the novel g-C3N4 (MCN) embedded by lithium and potassium ions were prepared by the facile molten salt method. Detailed characterizations have proved that the presence of Li+ and K+ could improve the crystallinity and decrease the band gap of g-C3N4, thus effectively promoting carrier separation and expanding light absorption. Theoretical calculation results also have shown that the appearance of Li+ and K+ could significantly reduce the over potential of hydrogen evolution. The MCN exhibits a higher photocatalytic hydrogen evolution rate (239.41 μmol/h), which is 18.2 times than pure g-C3N4 (13.1 μmol/h). This work provides a new idea for the design and preparation of novel 2D photocatalysts.

Loofah-like carbon nitride sponge towards the highly-efficient photocatalytic transfer hydrogenation of nitrophenols with water as the hydrogen source

The vigorous development of photocatalytic water splitting technology has laid the foundation for the photocatalytic transfer hydrogenation of organic substrates to produce the high value-added chemicals using water as hydrogen source. Nevertheless, the high dissociation energy of the O–H bond impedes its academic progress and the practical applications. Herein, we synthesize a 3D hierarchical porous loofah-like carbon nitride sponge (LCN) with ultrathin thickness via the supramolecular pre-organization coupling with the oxidation etching process, in which the heterogeneous oxygen atoms and the nitrogen vacancies are in-situ engineered. On top of the adorable photocatalytic H2 evolution (4812 μmol h−1 g−1), LCN associated with Pt cocatalyst reveals a conversion rate of 96.5 % towards the hydrogenation of 4-nitrophenol, substantially superior to the reference experiment (8.3 %). Further based on the isotope-labeling tests and the density functional theory calculations, the photo-generated H0 from water is clarified to be the direct reducing agent, tactfully skipping the hydrogen extraction step in the traditional path. This work provides a green and sustainable methodology to transfer the solar energy to the valuable fine chemicals, as well as highlights the importance of the 3D hierarchical porous structure to the catalytic activity.

Band offset engineering at C2N/MSe2 (M = Mo, W) interfaces.

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology. The key parameters in such heterostructure configurations are the valence and conduction band offsets at the interface, which determine the device performance. Here, based on density functional theory calculations, the bandgap and band offsets at C2N/MSe2 (M = Mo, W) interfaces have been engineered. The main findings demonstrate that the C2N monolayer interacts with both MoSe2 and WSe2 monolayers through weak van der Waals interactions. These heterostructures possess a narrower indirect bandgap and a typical type-II heterostructure feature, being suitable for promoting the separation of photogenerated electron–hole pairs. The calculated Gibbs free energy of hydrogen adsorption demonstrates a reduction in the overpotential, towards the hydrogen evolution reaction, upon forming heterostructures. To further tune the bandgap values and band offsets of heterostructures, the external perturbations are included through a vertical strain and finite electric field. It is found that both the vertical strain and electric field strongly modulate the bandgap values and the magnitude of the band offsets, while the typical type-II band alignment remains preserved. It is noticeable that the band offset magnitudes of the C2N/MoSe2 and C2N/WSe2 heterostructures are more sensitive to an external electric field than to a vertical interlayer strain.

Green hydrogen production by solar photocatalysis using Pt-TiO2 nanosheets with reactive facets

Green production of hydrogen is essential for the development of a hydrogen economy. In this study, the photocatalytic water-splitting technology is developed to harness solar energy for production of renewable hydrogen. Pt-TiO2 nanosheets were fabricated by a facile hydrothermal method, followed by photo-reduction of Pt(acac)2 on B,F-codoped TiO2 with reactive facets. The as-prepared photocatalysts were characterised by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and UV-vis diffuse reflectance spectroscopy (DRS). The photocatalytic hydrogen production performance was systematically investigated under UV-visible irradiation. The parametric results indicate that the TiO2 nanosheet structure, Pt loading and photocatalyst concentration have significant impacts on the photocatalytic hydrogen production. The highest hydrogen production rate obtained is 5,086 μmol h-1g-1.

Potential Visible-Light Driven PtO₂/GaN vdW Hetero-Bilayer Photocatalysts for Water Splitting Using First-Principles

Novel two-dimensional (2D) PtO2/GaN van der Waals (vdW) hetero-bilayers (HBL) are studied here for photocatalytic water splitting (PWS) application under first-principles density functional theory (DFT). We proposed six HBLs due to the atomic orientational variations and two of them are found dynamically stable confirmed by phonon dispersion curves. The two stable HBLs, HBL1, and HBL6 also show negative binding energy depicted by the interlayer distance-dependent binding energy curves. Among them, HBL1 has the lowest binding energy, suggesting the exothermic practicability of the material. Electronically both materials show a visible ranged indirect bandgap of ~2.65 (2.69) eV for HBL 1 (HBL6), lowered by ~2 times compared to their intrinsic constituents (2D PtO2, 2D GaN). The bandgaps also have type-II band orientation, which is highly required for efficient spatial carrier separation in photocatalytic water splitting (PWS) applications. The optical properties of the HBLs were also calculated, and it’s found that the HBLs have $\sim 2\times 10 ^{5}{\mathrm {cm}}^{-1} $ of perovskite material-like absorption coefficient in the visible spectrum, a key requirement for efficient photocatalysis. Reflectivity is as low as ~7% in the visible spectrum, suggesting the low-loss nature of the materials. Photocatalytic band-edges with type-II band alignments show sufficient kinetic overpotential for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both HBLs, suggesting effective water-splitting capacity. Moreover, we have explored the biaxial strain-induced tunability of the electronic bandgap, absorption coefficients, and photocatalytic band edges. They all found responsive due to homogeneous biaxial strain and show bandgap-lowering, absorption coefficient visible shifting, and band-edges tuning from compressive to tensile strains in the −6 % to +6% range. These studies suggest that the novel PtO2/GaN vdW layered material can be a probable efficient material for visible-light-driven photocatalytic water-splitting technology.

Photocatalytic Water-Splitting Coupled with Alkanol Oxidation for Selective N-alkylation Reactions over Carbon Nitride.

Photocatalytic water splitting technology (PWST) enables the direct use of water as appealing "liquid hydrogen source" for transfer hydrogenation reactions. Currently, the development of PWST-based transfer hydrogenations is still in an embryonic stage. Previous reports generally centered on the rational utilization of the in situ generated H-source (electrons) for hydrogenations, in which photogenerated holes were quenched by sacrificial reagents. Herein, the fully-utilization of the liquid H-source and holes during water splitting is presented for photo-reductive N-alkylation of nitro-aromatic compounds. In this integrate system, H-species in situ generated from water splitting were designed for nitroarenes reduction to produce amines, while alkanols were oxidized by holes for cascade alkylating of anilines as well as the generated secondary amines. More than 50 examples achieved with a broad range scope validate the universal applicability of this mild and sustainable coupling approach. The synthetic utility of this protocol was further demonstrated by the synthesis of existing pharmaceuticals via selective N-alkylation of amines. This strategy based on the sustainable water splitting technology highlights a significant and promising route for selective synthesis of valuable N-alkylated fine chemicals and pharmaceuticals from nitroarenes and amines with water and alkanols.

Recent progress on visible active nanostructured energy materials for water split generated hydrogen

Solar-driven water splitting is one amongst the fastest growing areas of research for hydrogen evolution. The technology employs application of nanostructured energy materials with featured charge-separation and light absorption characteristics. The high charge-carrier recombination and wide band-gap of energy materials are major material-related bottlenecks that restricted the large-scale implementation. Besides it, the co-catalyst, electron mediator and sacrificial electron donor (SED) were the other major components for further improving the H2 yield. A water splitting reactor with well-optimized photon and mass transfer properties is mandatory for studying hydrogen evolution at large scale. The present review compiles various strategies that were presented for reducing charge-carrier recombination and extending visible light absorption of materials, insights for loading earth-abundant co-catalysts on photocatalyst surface, recently developed reversible redox and non-noble metal-based solid-state electron mediators and advantage of applying organic contaminant and biomass-derived waste as SEDs. The discussions on several engineered reactor designs for the photocatalytic water splitting technology were also presented. A cost-effective way of designing a photocatalytic water splitting system with functionalized nanostructured energy materials, the earth-abundant co-catalysts, organic waste-based electron donors and efficient electron mediators for achieving improved hydrogen evolution rate is comprehensively discussed.

Tunable Photocatalytic Properties of Planar GaN/GeC Hetero-Bilayer: Production of H₂ Fuel

Novel GaN-based van der Walls (vdW) hetero-bilayer (HBL) is systemically studied through the first-principles density functional theory (DFT) framework to explore its optoelectronic property facilitated photocatalytic activity for hydrogen (H2) fuel production. The vdW concept of planar GaN with GeC monolayer, together making stacked hetero-bilayer, shows unique and tunable optoelectronic properties that fulfill all the photocatalysis attributes. The confirmation of chemical and dynamical stabilities is made with negative interlayer binding energy and non-negative frequency in phonon spectra, respectively. Among the dynamically stable hetero-bilayers, binding energy in HBL 2 (AB1) is the lowest and interlayer distance ~2.954Å is the smallest. The HBLs are sensitive to the stacking arrangements, where HBL 2 demonstrates type-I direct bandgap and HBL 6 (AB6) is Type-II direct bandgap. The intrinsic HBLs show superior carrier mobility as their effective masses are lower from their constituent monolayers. Also, the intrinsic absorption coefficient of the HBLs increase as high as ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , 10 times greater than conventional perovskite materials, which show outperforming optoelectronic conversion efficiency. Additionally, the HBLs are highly tunable in both bandgap and type due to biaxial strains and cross-plane electric fields. With biaxial strains, from compressive to tensile, both HBLs show monotonous bandgap lowering with a dominant direct-indirect bandgap splitting in HBL 2. Interestingly, for all the biaxial strains within the -6% to +6% range, both HBLs have sufficient kinetic overpotential for photocatalytic water splitting and an increase in visible peak by 3.5 (2) times in the HBL 6 (HBL 2). Electric fields also increase the photocatalytic water splitting performance with a net bandgap lowering and increasing the visible absorption peak by ~1.5 times compared with intrinsic HBLs. These promising properties that we have found in the planar GaN/GeC vdW hetero-bilayer are highly suggestive for the impending use of this material in photocatalytic water splitting technology to produce H2 fuel.

Highly efficient visible-light photocatalytic H2 evolution over 2D–2D CdS/Cu7S4 layered heterojunctions

Converting solar energy into clean and sustainable chemical fuels is a promising strategy for exploiting renewable energy. The application of photocatalytic water splitting technology in hydrogen production is important for sustainable energy development and environmental protection. In this study, for the first time, 2D Cu7S4 co-catalysts were coupled on the surface of a CdS nanosheet photocatalyst by a one-step ultrasonic-assisted electrostatic self-assembly method at room temperature. The as-fabricated 2D–2D CdS/Cu7S4 layered heterojunctions were demonstrated to be advanced composite photocatalysts that enhance the water splitting efficiency toward hydrogen production. The highest hydrogen evolution rate of the 2D–2D CdS/2%Cu7S4 binary heterojunction photocatalyst was up to 27.8 mmol g−1 h−1 under visible light irradiation, with an apparent quantum efficiency of 14.7% at 420 nm, which was almost 10.69 times and 2.65 times higher than those of pure CdS nanosheets (2.6 mmol g−1 h−1) and CdS-2%CuS (10.5 mmol g−1 h−1), respectively. The establishment of the CdS/Cu7S4 binary-layered heterojunction could not only enhance the separation of photogenerated electron–hole (e−–h+) pairs, improve the transfer of photo-excited electrons, and prolong the life-span of photo-generated electrons, but also enhance the light absorption and hydrogen-evolution kinetics. All these factors are important for the enhancement of the photocatalytic activity. Expectedly, the 2D–2D interface coupling strategy based on CdS NSs can be extensively exploited to improve the hydrogen-evolution activity over various kinds of conventional semiconductor NSs.

Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations.

In addition to the significance of photocatalytic hydrogen evolution, the utilization of the in situ generated H/D (deuterium) active species from water splitting for artificial photosynthesis of high value‐added chemicals is very attractive and promising. Herein, photocatalytic water splitting technology is utilized to generate D‐active species (i.e., Dad) that can be stabilized on anchored 2nd metal catalyst and are readily for tandem controllable deuterations of carbon–carbon multibonds to produce high value‐added D‐labeled chemicals/pharmaceuticals. A highly crystalline K cations intercalated polymeric carbon nitride (KPCN), rationally designed, and fabricated by a solid‐template induced growth, is served as an ultraefficient photocatalyst, which shows a greater than 18‐fold enhancement in the photocatalytic hydrogen evolution over the bulk PCN. The photocatalytic in situ generated D‐species by superior KPCN are utilized for selective deuteration of a variety of alkenes and alkynes by anchored 2nd catalyst, Pd nanoparticles, to produce the corresponding D‐labeled chemicals and pharmaceuticals with high yields and D‐incorporation. This work highlights the great potential of developing photocatalytic water splitting technology for artificial photosynthesis of value‐added chemicals instead of H2 evolution.

Hydrogen and electricity production in a light-assisted microbial photoelectrochemical cell with CaFe2O4 photocathode

A microbial photoelectrochemical cell (MPEC) was designed with a p-type CaFe2O4 semiconductor as the photoelectrode for simultaneous hydrogen and electricity production under light illumination. The CaFe2O4 photoelectrode was synthesized by the sol–gel method and well characterized by x-ray diffraction, field emission scanning electron microscope, and UV–Vis–NIR spectrophotometer. The linear sweep voltammogram of the CaFe2O4 photoelectrode presented the cathodic photocurrent output. For the MPEC, with an external resistance of 2000 Ω, the maximum power density of 143 mW·m−2 was obtained. Furthermore, with an external resistance of 100 Ω, the maximum hydrogen production rate of 6.7 μL·h−1·cm−2 could be achieved. The MPEC with CaFe2O4 photocathode was compared to MPEC with other photocathodes as well as photocatalytic water splitting technology.

Recent developments in the global titanium dioxide markets

Photocatalytic water splitting technology has broad prospect in dealing with the global energy shortage and environmental problems. TiO2 semiconductor is the most widely applied photocatalyst. However, weak response to visible light and rapid recombination of electron-hole limite its photocatalytic performance. TiO2 coupling with carbon materials can improve this problems, which are attributed to electrical conductivity and abundant pore structure of carbon materials. Based on this, the methods to enhance the visible response of TiO2 are summarized and the deep understanding of carbon nanotubes, graphene, activated carbon, carbon quantum dots, fullerene 60, carbon fibers, and biochar in photocatalytic process are proposed. DFT simulation is used to calculate the mechanism of electron migration and catalytic reaction in carbon/TiO2. The scheme of using water/methanol solution to promote the hydrogen production performance of photocatalysts is described in detail. Finally, the challenges and development directions of carbon-based TiO2 materials are summarized.

Striving Toward Visible Light Photocatalytic Water Splitting Based on Natural Silicate Clay Mineral: The Interface Modification of Attapulgite at the Atomic-Molecular Level

Development of visible light driven photocatalytic water splitting catalysts for hydrogen production with both high activity and stability has remained a challenge to renewable energy research in the past decades. Herein, for the first time, the interface modified attapulgite (ATP) at the atomic-molecular level was employed as a photocatalyst for hydrogen production from water under visible light illumination. Because of the enhancement of the surface hydroxyl, the increase of dye adsorption, and high efficiency of photoexcited electron injection ability, the maximal hydrogen generation rate over the modified ATP coupled with Eosin Y is 2.9 times higher than that of original ATP, and the highest apparent quantum efficiency reached 28.6% at 490 nm. This work will pave a new pathway for the modification of natural catalysts for visible light driven photocatalytic water splitting technology.

High surface area optofluidic microreactor for redox mediated photocatalytic water splitting

Photocatalytic water splitting is a promising approach for hydrogen generation, but the low efficiency of current photoreactors limits its widespread exploitation and commercialization. Recent developments in optofluidic microreactors open a window for advancing photocatalytic water splitting technology. Nevertheless, existing optofluidic microreactors with the planar design show the low active surface area and rate of mass transport, thereby restricting the hydrogen production performance. In this work, we proposed an optofluidic microreactor with staggered micro-pillars in the reaction micro-chamber. Such design not only enlarges the surface area to load catalyst but also induces perturbation to the liquid flow and shortens the transport length, which increases the active surface area and enhances the mass transfer and eventually boosts the hydrogen production rate. To evaluate the performance of this new optofluidic microreactor, a redox mediated water splitting reaction was implemented. Results showed that the developed microreactor with micro-pillar structure exhibited a higher reaction rate. As compared to the conventional planar optofluidic microreactor, the maximal increment of the reaction rate could reach 56%.