Photocatalytic Water Splitting Reaction Research Articles

A density functional study of type I to type II crossover in g-C3N4/CoN4 heterostructure in presence of external perturbation

Herein, we report the impact of external perturbation on the photocatalytic performance of g-C3N4/CoN4 heterojunction and concomitant transition from type-I to type-II. The state-of-the-art theoretical modeling presented here is the first study on dynamic role of strain and electric field on g-C3N4/CoN4 heterojunction and its potential application as a solar-driven water splitting reaction. Utilizing the density functional theory (DFT) calculation, V. N. et al. recently reported the photocatalytic activity of g-C3N4/MN4 (M = Mn, Fe and Co) heterojunction [J. Comput. Chem. 2024, 45, 2518-2529, https://doi.org/10.1002/jcc.2746412] and subsequently established g-C3N4/CoN4 as a type I heterojunction for photocatalytic water splitting reaction [Phys. Chem. Chem. Phys. 2024, 26, 21117–21133]. In the present study, we applied external perturbation in the form of mechanical strain, electric field and evaluated the electronic structure properties of the system along with the detailed examination of concomitant optical and magnetic properties of g-C3N4/CoN4 heterojunction. The switching of photocatalytic feature from type I to type II originates due to the crossover between valence band maxima (VBM) of g-C3N4 and CoN4 in presence of external perturbation. Notably, the resulting system acts as a type II photocatalyst for water splitting reaction while applying compressive (negative) strain of −2 to −6% and an electric field of −0.5 and +0.5 V/Å, respectively. Accumulation of photogenerated electrons and holes separately in g-C3N4 and CoN4 units confirms the perceptible separation of charge carriers and thereby reduces the recombination compared to un-perturbed system. A red-shifted absorption maximum (689 nm), superior charge transfer (0.9e), and clear separation of photogenerated electrons and holes are the origin of enhanced photocatalytic efficiency of g-C3N4/CoN4 heterojunction in the presence of external perturbation. We believe that our work will provide enough evidence to the experimentalists to achieve such a system in practice leading to human-friendly applications.

Photocatalytic Hydrogen Production Using TiO2-based Catalysts: A Review.

Photocatalytic water splitting is an environmentally friendly hydrogen production method that uses abundant renewable resources such as water and sunlight. While Titanium dioxide (TiO2) photocatalyst exhibits excellent properties, its high band gap limits absorption to ultraviolet (UV) irradiation, resulting in low photo conversion efficiency. This review explores various modification techniques aimed at enhancing the efficiency of TiO2 under visible light irradiation. Factors influencing the photocatalytic water splitting reaction, such as catalyst structure, morphology, band gap, sacrificial reagents, light intensity, temperature, and potential of Hydrogen (pH)are examined. This review also summarizes different catalyst synthesis methods, and types of photocatalytic reactors, and provides insights into quantum yield. Finally, the review addresses the challenges and future outlook of photocatalytic water splitting.

Tuning the 5d State of Pr3+ in Oxyhalides for Efficient Deep Ultraviolet Upconversion

AbstractVisible‐to‐ultraviolet (UV) upconversion provides a fascinating strategy to achieve deep UV emission through readily accessible visible light. However, the intensity of deep UV emission obtained through visible‐to‐UV upconversion progress is still far from satisfactory, severely constraining its practical applications. Herein, a novel class of praseodymium ion (Pr3+)‐doped rare‐earth oxyhalides (YOCl, YOBr, and LuOBr) to achieve efficient upconverted deep UV emission in the spectral range of 250–350 nm is developed. The upconverted UV emission intensity of LuOBr:Pr3+ is determined to be 56.7 times stronger than that of the well‐established Lu7O6F9:Pr3+. When employed as a photon‐converter to activate photocatalytic water splitting reactions, upconverted deep UV emission enables H2 generation under visible light (λ > 420 nm) excitation from a xenon lamp. The efficient deep UV upconversion stems from tuning 4f15d1 state of Pr3+ by oxyhalide constituent which both facilitates the absorption of excitation photons in long‐lived intermediate 4f2 states and suppress the probability of nonradiative relaxation from 4f15d1 state. These findings not only provide new insights into a mechanistic understanding of the host effect on upconversion process but also make a breakthrough in developing efficient deep upconversion materials that will expand their further applications.

Photocatalytic Hydrogen-Future Energy

The rapid increase in energy consumption, driven by the rising vehicle population and travel demand, has led to a surge in greenhouse gas (GHG) emissions. This necessitates the development of sustainable and eco-friendly technologies and fuels to mitigate global and local environmental impacts. Four primary justifications support transitioning to alternative fuels and technologies: dwindling oil reserves worldwide, adverse environmental effects of vehicle pollution, escalating oil production costs, and the complex challenges of maintaining energy security in international markets. To address these concerns, there is a growing global demand for hydrogen as a sustainable energy source. Utilizing photocatalytic and photoelectrochemical (PEC) water-splitting reactions, particularly with graphene quantum dots-based composites, offers promising avenues for efficient and eco-friendly hydrogen generation. The selection of these systems is based on their unique physical and chemical properties, with graphene providing strong support and efficient electron transport, while components like cadmium sulfide (CdS) and zinc oxide (ZnO) enhance light absorption and charge separation. This integrated approach shows potential for efficient and sustainable hydrogen production, further bolstered by advances in metal-containing photocatalysts and collaborative efforts across energy markets.

Enhanced photocatalytic performance of MXene-Modified cation-exchanged CoFe-LDH/CoFeCrO4 heterostructure for Antibiotic degradation and hydrogen production through synergistic charge dynamics

Systemizing an effective charge-transfer channel across a junction interface replete with ample active sites for enhancing the photocatalytic activity of semiconducting materials presents a formidable challenge. Here, we present a novel approach based on an in situ hydrothermal method for synthesizing CoFe-Layered Double Hydroxide (LDH)/CoFeCrO4 heterojunction materials that were partially derived from CoFe-LDH. These materials synergistically interact with Ti3C2 MXene nanosheets facilitating multi-interface interactions. Indeed, the highest tetracycline hydrochloride degradation rate of nearly 92% in 2 h was achieved because of the intense synergy between the CoFe-LDH/CoFeCrO4 heterojunction material optimized with 7.5 wt% of MXene nanosheets (CMC-7.5). This degradation performance was 3.1 times greater than that of the original CoFe-LDH. Further, CMC-7.5 produces the highest H2 gas of 458.79 μmol h−1g−1 from a photocatalytic water splitting reaction. The formation of a Schottky energy barrier between the partially derived CoFe-LDH/CoFeCrO4 unit and MXene promoted the fast transfer of photogenerated electrons from CoFe-LDH/CoFeCrO4 to the surface of MXene, thereby providing a plethora of active sites for photocatalytic reactions. Photoelectrochemical assessments using transient and linear sweep voltammetry along with electrochemical impedance spectroscopy confirmed efficient charge carrier transfer. Moreover, the optimized CMC-7.5 photoelectrode has a superior integral area and exhibited excellent stability over 100 cycles. Finally, the study outlines the construction of several advanced materials with multi-interface heterojunctions involving cation-exchanged LDH derivatives with high photogenerated charge carrier separation efficiency and minimal interfacial migration resistance to serve as stability benchmarks for practical applications.

Reconstructing Ferroelectric Polarization Screening of PbTiO3 by Epitaxial SrTiO3 for Efficient Photocatalytic Overall Water Splitting

AbstractA large depolarization field in single‐domain PbTiO3 is anticipated to drive the spatial separation of photogenerated charge carriers and enable potential active sites for photocatalytic overall water splitting reaction. However, the negatively polarized surface usually cannot provide sufficient active sites for water oxidation to oxygen because of lacking oxygen vacancies, inevitably hindering photocatalytic water splitting reaction. Here, the ferroelectric polarization screening is reconstructed by a heteroepitaxial SrTiO3 film on the positively polarized surface of PbTiO3 nanoplates to generate abundant oxygen vacancies on the negatively polarized surface as active sites for water oxidation. Moreover, it is experimentally and theoretically revealed that the epitaxial SrTiO3 layer on the positively polarized surface is also beneficial for good dispersion of cocatalyst and the formation of low Schottky barrier height at the interface between the heterostructure and the cocatalyst for efficient electron transfer. Compared with pristine PbTiO3, the obtained PbTiO3/SrTiO3 heteroepitaxial structure with the simultaneous regulation of positively and negatively polarized surfaces delivered a greatly enhanced photocatalytic overall water splitting with stoichiometric ratio (2:1) of hydrogen and oxygen evolution by a factor of 15 times. These findings might open up a new avenue for the design of ferroelectric photocatalysts with excellent activity.

Sugars induced exfoliation of porous graphitic carbon nitride for efficient hydrogen evolution in photocatalytic water-splitting reaction

Photocatalytic hydrogen evolution holds great promise for addressing critical energy and environmental challenges, making it an important area in scientific research. One of the most popular photocatalysts is graphitic carbon nitride (gCN), which has emerged as a noteworthy candidate for hydrogen generation through water splitting. However, ongoing research aims to enhance its properties for practical applications. Herein, we introduce a green approach for the fabrication of porous few-layered gCN with surface modifications (such as oxygen doping, carbon deposition, nitrogen defects) with promoted performance in the hydrogen evolution reaction. The fabrication process involves a one-step solvothermal treatment of bulk graphitic carbon nitride (bulk-gCN) in the presence of different sugars (glucose, sucrose, and fructose). Interestingly, the conducted time-dependent process revealed that porous gCN exfoliated in the presence of fructose at 180 °C for 6 h (fructose_6h) exhibits a remarkable 13-fold promotion of photocatalytic hydrogen evolution compared to bulk-gCN. The studied materials were extensively characterized by microscopic and spectroscopic techniques, allowing us to propose a reaction mechanism for hydrogen evolution during water-splitting over fructose_6h. Furthermore, the study highlights the potential of employing a facile and environmentally friendly fructose-assisted solvothermal process to improve the efficiency and stability of catalysts based on graphitic carbon nitride.

Advances in the structural engineering of layered bismuth-based semiconductors for visible light-driven photocatalytic water splitting.

Hydrogen production via the photocatalytic water splitting reaction on semiconductors presents a promising avenue to directly achieve solar energy conversion and storage. Bismuth-based semiconductors with layered structures, a hierarchical arrangement of components stacked in the form of two-dimensional extended layers where the atoms within each layer are typically strongly bonded, while the interactions between the layers are relatively weak, have emerged as an important series of photocatalyst candidates. In this review, we focus on the new emerging layered bismuth-based semiconductors with structures in Sillén, Aurivillius, Sillén-Aurivillius and bismuth chromate systems primarily employed in the photocatalytic water splitting reaction. From a crystal structure-oriented view, we delve into discussions on how the component and unit of a crystal structure influence the intrinsic properties, including light absorption and photogenerated charge transfer and separation, of materials as well as the corresponding photocatalytic performance of the water splitting reaction. The strategies for modulating the ferroelectricity and surface modification of these layered bismuth-based semiconductors are also involved. We also discuss the limitations of these materials accompanied by a forward-looking perspective, and we hope to provide some insights from the view of rational material design and engineering for the fabrication of high-efficiency photocatalytic water splitting systems.

Computational study based prediction of new photocatalysts for water splitting by systematic manipulation of MXene surfaces

Photocatalytic water splitting reactions (HER and OER) on the asymmetric surfaces of Janus MXenes with the help of intrinsic electric field (Ei).

Photocatalytic overall water splitting endowed by modulation of internal and external energy fields.

The pursuit of sustainable and clean energy sources has driven extensive research into the generation and use of novel energy vectors. The photocatalytic overall water splitting (POWS) reaction has been identified as a promising approach for harnessing solar energy to produce hydrogen to be used as a clean energy carrier. Materials chemistry and associated photocatalyst design are key to the further improvement of the efficiency of the POWS reaction through the optimization of charge carrier separation, migration and interfacial reaction kinetics. This review examines the latest progress in POWS, ranging from key catalyst materials to modification strategies and reaction design. Critical analysis focuses on carrier separation and promotion from the perspective of internal and external energy fields, aiming to trace the driving force behind the POWS process and explore the potential for industrial development of this technology. This review concludes by presenting perspectives on the emerging opportunities for this technology, and the challenges to be overcome by future studies.

Role of reduced graphene oxide in boosting visible-light-driven photocatalytic activity of BiVO4 nanostructures

Although monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising photocatalyst due to its low band gap (Eg = 2.4-2.6 eV), significant visible light absorption, and its valence band potential is positive enough for water splitting and pollutants degradation, it has some drawbacks hindering its sole usage in photocatalysis. These drawbacks include low surface conductivity, fast electron-hole (e-/h+) pair recombination, low surface area, and low solubility in the aqueous medium. Therefore, m-BiVO4 is composited with reduced graphene oxide (r-GO) to mitigate these drawbacks. r-GO has an extremely large surface area, a high electrical conductivity and can accept and trap electrons from m-BiVO4 via its delocalized conjugated 𝜋-system. Such traps lengthen the electron / hole (e-/h+) pair lifetime on m-BiVO4 increasing the photocatalytic reactions efficiency on its surface. In addition, the presence of oxygen-containing groups on r-GO helps in anchoring m-BiVO4 particles on the r-GO layer so the m-BiVO4 particles are more dispersed and display a larger surface area. These oxygenated groups ease the solubilization of anchored m-BiVO4 particles in water by forming hydrogen bonds. In this mini-review, m-BiVO4–r-GO composite applications in photocatalytic water splitting, pollutants degradation, and other reactions will be briefly discussed. Generally, these composites showed remarkable results in reactions that rely on the valence band holes of m-BiVO4, whereas the reactions that depend on conduction band electrons required morphology and size modification for the m-BiVO4 before its compositing with r-GO.

Density functional theory-based modeling of the half-metallic g-C3N4/CoN4 heterojunction for photocatalytic water splitting reaction.

Using density functional theory (DFT), we have investigated the structural, optical, electronic and magnetic properties of a graphitic carbon nitride (g-C3N4) and CoN4 composite to explore the effect of the heterojunction on the photocatalytic performance of g-C3N4. The structure of g-C3N4 is modified while complexing with CoN4 and the corresponding stabilization is confirmed through adhesion energy calculation. The phonon spectra analysis furthermore guaranteed the lattice-dynamic stability of the CoN4 bulk and the CoN4 slab. Pristine g-C3N4 is a wide band gap semiconductor, which becomes half metallic upon CoN4 inclusion. The metallicity in the g-C3N4/CoN4 composite originates from the spin down channel, keeping the spin up channel in a semiconducting state. The charge density analysis and work function calculation suggest a substantial amount of charge transfer from g-C3N4 to the CoN4 unit in the g-C3N4/CoN4 heterojunction. The model heterojunction of the g-C3N4/CoN4 composite can enhance the utilization ratio of visible light for the g-C3N4 photocatalyst. In g-C3N4/CoN4, the valence band maximum (VBM) has a more positive potential compared to O2/H2O (+1.23 V) on the normal hydrogen electrode (NHE) scale. However, the conduction band minimum (CBM) displays a more negative potential compared to H+/H2 (0 V) on the NHE scale. The details of the band structure, density of states and band edge position determining calculations confirm that the g-C3N4/CoN4 composite forms a type 1 heterojunction, making it a suitable photocatalyst for water splitting reaction. The practical application of the g-C3N4/CoN4 heterostructure as a photocatalyst was substantiated in the presence of polar solvent (water) by calculating the band gap, charge transfer interaction and charge density difference. There is a significant decrease of charge transfer and thereby charge density difference in the g-C3N4/CoN4 heterojunction in the presence of water; however, it still holds potential for use as a photocatalyst for water splitting reaction. The state-of-the-art theoretical modeling of the g-C3N4/CoN4 heterojunction is the first theoretical study incorporating the CoN4 crystal.

Oxysulfide-Based Photocatalyst Sheet Modified with Silica Layer for Z-Scheme Overall Water Splitting at Atmospheric Pressure

Photocatalytic water splitting using particulate photocatalysts is considered as an ideal route to produce economically cheap renewable, and transportable H2 utilizing sunlight [1]. Photocatalyst sheets are directly scalable and have recently gained interest because a benchmark STH conversion efficiency of 1.0% was achieved at near ambient pressure using the one composed of Rh, La-codoped SrTiO3 and Mo-doped BiVO4 (BVO:Mo) as the H2- and O2- evolution photocatalysts (HEP and OEP), respectively. However, such systems absorb only a little portion of the sunlight due to the weak visible light absorption of the doped SrTiO3 [2]. To utilize longer wavelength visible light, our group has recently studied photocatalyst sheets composed of Ga-doped La5Ti2Cu0.9Ag0.1O7S5 (LTCA:Ga) (HEP) and BVO:Mo (OEP) with Au as conducting layer. The combination of LTCA:Ga/Au/BVO:Mo modified with Rh/Cr2O3 and CoO x as reduction and oxidation cocatalysts, respectively, achieved an STH efficiency of 0.4% under reduced pressure, but this was far lower than the oxide-based sheet system and the activity was decreased sharply against background pressure [3]. Considering intense visible light absorption of LTCA (λabs ~ 700 nm) and the associated maximum theoretical energy conversion efficiency for this combination, improvements in the net STH efficiency and activation under atmospheric pressure is highly indispensable.In this study, LTCA photocatalysts with different dopants were applied to the sheet system in combination with hydrothermally synthesized BVO:Mo and the resulting sheets were modified with CoO x /Cr2O3/Rh cocatalysts in an attempt to upgrade the water splitting activity. Moreover, surface modification onto LTCA-based photocatalyst sheets with a thin amorphous TiO2 and SiO2 layer were evaluated at elevated pressure and high temperature.Doped LTCA were synthesized by a conventional solid-state reaction in sealed evacuated quartz tubes similar to previous studies [3]. Precursors were blended inside a glove box. BVO:Mo (Mo 0.1 mol%) was synthesized by a hydrothermal method [4]. Photocatalyst sheet was fabricated by the particle transfer method [3]. SiO2 and TiO2 layers functioning as molecular sieves were deposited onto the cocatalyst-modified sheet from titanium tetraisopropoxide (TTIP) or tetraethyl orthosilicate (TEOS) by photo-decomposition and hydrolysis method, respectively, to prevent backward reactions at elevated pressure [5]. Photocatalytic water splitting reaction was carried out at different temperatures and pressures using a closed gas circulation system.Rh cocatalyst was first adsorbed and photo-deposited onto the doped LTCA/Au/BVO:Mo(h) sheet [3]. Gas evolution rates were significantly enhanced upon deposition of Cr2O3 layer as the oxygen reduction reaction (ORR), a backward reaction, was majorly suppressed promoted by uncovered Rh surface under the measured condition. Photocatalyst sheet composed of doped LTCA/Au/BVO:Mo(h) with optimal dopant amounts and cocatalysts loadings showed the H2 and O2 evolution rates of 110 and 54 µmol/h, respectively, under visible light irradiation and yielded an STH of 0.67% at 4 kPa and temperature 301 K [6]. The water splitting activity was improved by 1.6 times under reduced pressure compared to the previously studied LTCAGa/Au/BVO:Mo. However, this activity was highly sensitive against background pressure as nearly 90% reduced upon increasing pressure from 2 to 90 kPa owing to the predominating ORR occurring on LTCA and exposed Au surface. To suppress this backward reaction, TiO2 layer was coated to suppress ORR but the sheet failed to work at a temperature higher than 318 K due to the condensation of such layer. However, SiO2 layer coated sheet effectively suppressed ORR backward reaction and stably work under hot water and elevated pressure [6]. Consequently, under optimized condition SiO2 deposited sheet achieved an STH of 0.4% at 90 kPa at temperature 333 K.Doped LTCA materials were applied to the photocatalyst sheet systems as HEP along with BVO:Mo and Au layer as OEP and conductive layer, respectively, for Z-scheme pure water splitting under visible light. This combination achieved an STH of 0.67% at reduced pressure. This STH is considered as one of the highest values recorded for OWS systems involving non-oxide photocatalysts. The activity of the photocatalyst sheet was found negatively dependent onto background pressure due to the ORR on LTCA and Au layer. Surface modifications with SiO2 layer activated the sheet under atmospheric pressure. These findings will contribute to the improvement in the STH for other systems under realistic condition where OWS is likely to be conducted at an ambient pressure. References Nandy et al. Chem. Sci. 2021, 12, 9866-9884.Wang et al. Nat. Mater. 2016, 15, 611-615.Chen et al. ACS Catal. 2023, 13, 3285-3294.Li et al. Energy Environ. Sci., 2014, 7, 1369-1376.Pan et al. Angew. Chem. Int. Ed. 2015, 54, 2955-2959.Nandy et al. Joule, 2023, under revision.

Photo-Generated Charge Carrier Dynamics in Metal Oxide Photocatalysts

Solar water splitting is one of the most attractive energy carrier generation processes to utilize solar energy. Light generated hydrogen is a clean energy carrier, since their consumption for energy generation produces only water. Moreover, generated hydrogen can be stored and used whenever required. Hydrogen is mostly used as a feedstock in the chemical and petrochemical industries, e.g. production of ammonia, methanol and fertilisers.[1]The photocatalytic solar water splitting process has been studied extensively over the last half century. However, despite the efforts, no ideal photocatalyst has yet been identified to meet three main requirements for commercialisation: (1) low cost, (2) high solar to hydrogen (STH) efficiency and (3) durability. Over the last two decades, considerable efforts have focused on developing visible light active photocatalysts, and some promising photocatalysts with relatively high water splitting efficiency were discovered.[2] In contrast to search of appropriate photocatalyst (nano)materials, mechanisms of photocatalytic water splitting reactions, in particular charge carrier dynamics of photocatalysts, were little studied. Understanding charge carrier dynamics in photocatalysts will certainly provide an opportunity to design high efficiency photocatalysts.In this presentation, we will show dynamics of photo-generated charge carries in metal oxide photocatalyst by employing a series of transient absorption spectroscopies covering from 150 fs to 100 s over UV-VIS-NIR wavelength ranges.[2] Correlation of the charge carrier dynamics with photocatalysis reactions will be discussed.This work was partly supported by ARC DP fund (DP180103815) and ARC LIEF funds (LE170100235 and LE200100051), Australia, and the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant number 2023-45 and 2022-99), Japan. We also acknowledge supports from School of Engineering, RMIT University, and Forefront Research Center, Faculty of Science, Osaka University.

Enhanced Visible Light-Driven Photocatalytic Water-Splitting Reaction of Titanate Nanotubes Sensitised with Ru(II) Bipyridyl Complex.

The ion exchange of Na+ cations was used to photosensitise titanates nanotubes (Ti-NTs) with tris(2,2'-bipyridine)ruthenium(II) cations (Ru(bpy)32+); this yielded a light-sensitised Ti-NTs composite denoted as (Ru(bpy)3)Ti-NTs, exhibiting the characteristic absorption of Ru(bpy)32+ in visible light. Incident photon-to-current efficiency (IPCE) measurements and the photocatalytic reduction of methyl viologen reaction confirmed that in the photosensitisation of the (Ru(bpy)3)Ti-NTs composite, charge transfer and charge separation occur upon excitation by ultraviolet and visible light irradiation. The photocatalytic potential of titanate nanotubes was tested in the water-splitting reaction and the H2 evolution reaction using a sacrificial agent and showed photocatalytic activity under various light sources, including xenon-mercury lamp, simulated sunlight, and visible light. Notably, in the conditions of the H2 evolution reaction when (Ru(bpy)3)Ti-NTs were submitted to simulated sunlight, they exceeded the photocatalytic activity of pristine Ti-NTs and TiO2 by a factor of 3 and 3.5 times, respectively. Also, (Ru(bpy)3)Ti-NTs achieved the photocatalytic water-splitting reaction under simulated sunlight and visible light, producing, after 4 h, 199 and 282 μmol×H2×gcat-1. These results confirm the effective electron transfer of Ru(bpy)3 to titanate nanotubes. The stability of the photocatalyst was evaluated by a reuse test of four cycles of 24 h reactions without considerable loss of catalytic activity and crystallinity.

Preparation strategy of bimetallic MOF hollow photocatalysts for hydrogen evolution

Hollow structure materials have been proved to be excellent photocatalysts due to their advantages in the separation of electron-hole pairs. However, the synthesis of such materials is complex, time-consuming, and poor versatility. In this study, Cu-DHTP/ZIF-67 (DHTP = 2,5-dihydroxyterephthalic acid) hollow materials containing both Co and Cu were designed and synthesized by a simple one-step etching and ligand exchange strategy. It is worth noting that the bimetallic hollow catalyst can be synthesized in the presence of both Cu(Ⅱ) and DHTP within 5 min at room temperature. The synthesized hollow bimetallic catalyst has improved photoresponse performance and electron-hole pair separation ability, which results in the best catalytic performance compared with DHTP/ZIF-67, Cu/ZIF-67, and ZIF-67 when used in the photocatalytic water splitting reaction. The hydrogen evolution yield of Cu-DHTP/ZIF-67 can reach 39.7 mmol g−1, which is 6 times than that of pure ZIF-67. This study demonstrates the promising potential of Cu-DHTP/ZIF-67 photocatalyst in the field of photocatalytic water reduction and hydrogen evolution, and provides a new idea for designing and preparing more hollow structure bimetallic photocatalysts with excellent performance.

Quantum geometric spin frustration of antiferromagnetic CuFeO2 enables photocatalytic applications

Photocatalysis, containing a potential solution to the global crises of energy shortage and the greenhouse effect, has been studied extensively in recent years. The CuFeO2 (abbreviated as CFO) is a material with a Delafossite structure, and has also been widely studied because of its triangular lattice antiferromagnets. The triangular lattice composed of Fe3+ ions, along with strong antiferromagnetic spin coupling between the Fe3+ ions, leads to a geometric spin frustration. Furthermore, CFO is also a potential candidate for the photocatalytic material due to its narrow band gap of 1.1–1.6 eV, electrochemical stability in an aqueous environment, high charge carrier mobility, flat-band potential positioned at ∼ 1 eV, and contains an abundance of raw materials in the earth. In this study, we successfully grew the size and highly-quality CFO single crystals using the optical floating-zone technique. Powder x-ray diffraction (XRD) and Rietveld refinement indicate that the CFO is a rhombohedral structure with a space group of R3̅m, consisting of edge-shared FeO6 octahedron to form the FeO2 layer, and the Cu cations occupied between the two FeO2 layers. The core-level electron energy loss (EEL) spectra found the valence of Fe and Cu in CFO is +3 and +1, respectively. The low-loss EEL spectrum observed two volume plasmons at ∼5 eV and ∼23 eV contributed from the collective oscillation of Cu-O antibonding states σCu−O* and mixed nonbonding Cu 3dxy, σCu−O bonding states, and Fe-O, respectively, based on the density function theory calculation. The optical absorption spectrum measured the energy bandgap for CFO is about 1.5 eV. The Raman spectrum shows significant Eg and A1 g vibration modes corresponding to the in-plane Fe-O and out-of-plane Cu-O vibrations, respectively. Both magnetic susceptibility, χ(T), with the applied magnetic field along the c-axis and specific heat capacity reveal the two magnetic phase transition temperatures at low temperatures: TN1 = 13.5 K when transition from paramagnetic (PM) phase to partially disordered antiferromagnetic, and TN2 = 10.5 K when complete transition from partially disordered antiferromagnetic to antiferromagnetic phase. The additional calculation of magnetic entropy change is 0.15, which is roughly close to 0.2, meaning that only 1/5 of the Fe3+ spins are disordered when transforming from PM states to partially disordered at T < TN1 in this study. The presence of instantaneous ferromagnetic spin arrangement (↑↑↓) extends the residence time of H2O on the surface of CFO, facilitating electron exchange with CFO and promoting photocatalytic water splitting reactions.

Prediction model of type and band gap for photocatalytic g-GaN-based van der Waals heterojunction of density functional theory and machine learning techniques

Applying suitable two-dimensional (2D) heterojunctions to photocatalytic water cracking reaction can obtain excellent catalytic performance. However, due to many candidate materials and complex interface effects, finding suitable heterojunctions combinations has become a challenge for the above applications. Based on about 1000 pieces of material data in the computational 2D material database, we adopted a simple energy band shift hypothesis to creatively build a machine learning prediction model of g-GaN based 2D Van der Waals (vdW) heterostructures’ type with good performance, and carried out a first-principle calculation to verify the hypothesis. The results show that the band shift hypothesis is valid for the g-GaN based vdW heterojunctions combination without the participation of elements locatedin thefirsttransitional period and this classification model with the area under curve (AUC) value of 0.93. In addition, we further built a regression prediction model for the band gap value of type Ⅱ g-GaN based vdW heterojunctions in line with the band edge position of photocatalytic water-splitting reaction, with a mean absolute error (MAE) of 0.24 eV. This work establishes a machine learning screening process for g-GaN based vdW heterojunction applied in the field of photocatalysis, which greatly improves the research efficiency.

(Invited) Creation of Active Water-Splitting Photocatalysts By Controlling Cocatalysts Using Atomically Precise Metal Nanoclusters

With global warming and the depletion of fossil resources, our fossil-fuel-dependent society is expected to shift to one that instead uses hydrogen (H2) as a clean and renewable energy. Water-splitting photocatalysts can produce H2 from water using sunlight, which are almost infinite on the earth. However, further improvements are indispensable to enable their practical application. To improve the efficiency of the photocatalytic water-splitting reaction, in addition to improving the semiconductor photocatalyst, it is extremely effective to improve the cocatalysts (loaded metal nanoclusters, NCs) that enable the reaction to proceed on the photocatalysts. We have thus attempted to strictly control metal NCs on photocatalysts by introducing the precise-control techniques of metal NCs established in the metal NC field into research on water-splitting photocatalysts. Specifically, the cocatalysts on the photocatalysts were controlled by adsorbing atomically precise metal NCs on the photocatalysts and then removing the protective ligands by calcination. This work has led to several findings on the electronic/geometrical structures of the loaded metal NCs, the correlation between the types of the loaded metal NCs and the water-splitting activity, and the methods for producing high water-splitting activity. We expect that the obtained knowledge will lead to clear design guidelines for the creation of practical water-splitting photocatalysts and thereby contribute to the construction of a hydrogen-energy society. Figure 1

Photo/electrocatalytic hydrogen evolution using Type-II Cu2O/g-C3N4 Heterostructure: Density functional theory addresses the improved charge transport efficiency

One of the most efficient ways for the photogenerated charge carriers is by the development of heterojunction between p-type and n-type semiconductors, which creates an interfacial charge transfer between two semiconductors. By enhancing the bifunctional characteristics for hydrogen generation via photocatalytic and electrocatalytic water splitting reaction, we report the type-II Cu2O/g-C3N4 heterostructure in this article. Due to significantly increased catalytically active sites for the hydrogen evolution reaction (HER) reaction during electrocatalysis and decreased charge transfer resistance, the as-prepared heterostructure exhibits a lower overpotential of 47 and 72 mVdec-1 for the HER and oxygen evolution reactions (OER), respectively, when compared to alone g-C3N4. In addition, Cu2O/g-C3N4 heterostructures have a higher photocatalytic hydrogen evolution of 3492 µmol gcat-1 in the presence of Triethanolamine as a sacrificial agent, which is nearly 2-fold times greater than g-C3N4 (1818 µmol gcat-1) after 5 h of continuous light-irradiation. Moreover, produced heterostructure exhibits 81% of Faradaic efficiency and 18% of apparent quantum yield. This work successfully explains how the rise in water splitting is induced by the transfer of photogenerated electrons in a cascade way from p-type Cu2O to the n-type g-C3N4 using density functional theory (DFT) calculations.