Identifying the key obstacle in photocatalytic oxygen evolution on rutile TiO2

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 μmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Linking the Doping-Induced Trap States to the Concentration of Surface-Reaching Photoexcited Holes in Transition-Metal-Doped TiO2 Nanoparticles

Transition-metal doping has been demonstrated to be effective for tuning the photocatalytic activity of semiconductors. Nonetheless, the impact of doping-induced trap states on the concentration of surface-reaching photoexcited charges remains a topic of debate. In this study, through time-resolved spectroscopies and kinetic analysis, we found that the concentration of surface-reaching photoholes (Ch+(surf)) in doped TiO2 nanoparticles sensitively relies on the type of dopants and their associated trap states. Among the studied dopants (Fe, Cu, and Co), Fe doping resulted in the most significant increase in Ch+(surf), nearly double that of Co or Cu doping. Fe-doping induced more effective hole trap states, acting as the mediator for interfacial charge transfer, thus accelerating charge separation and consequently enriching Ch+(surf). This work provides valuable insight into understanding and controlling Ch+(surf) in transition-metal-doped TiO2 materials.

Aqueous Titania Interfaces

Water-metal oxide interfaces are central to many phenomena and applications, ranging from material corrosion and dissolution to photoelectrochemistry and bioengineering. In particular, the discovery of photocatalytic water splitting on TiO2 has motivated intensive studies of water-TiO2 interfaces for decades. So far, a broad understanding of the interaction of water vapor with several TiO2 surfaces has been obtained. However, much less is known about liquid water-TiO2 interfaces, which are more relevant to many practical applications. Probing these complex systems at the molecular level is experimentally challenging and is sometimes possible only through computational studies. This review summarizes recent advances in the atomistic understanding, mostly through computational simulations, of the structure and dynamics of interfacial water on TiO2 surfaces. The main focus is on the nature, molecular or dissociated, of water in direct contact with low-index defect-free crystalline surfaces. The hydroxyls resulting from water dissociation are essential in the photooxidation of water and critically affect the surface chemistry of TiO2.

Synergetic Manipulation Mechanism of Single-Atom M-N4 and M-OH (M = Mn, Fe, Co, Ni) Sites for Ozone Activation: Theoretical Prediction and Experimental Verification

Carbon-based single-atom catalysts (SACs) have been gradually introduced in heterogeneous catalytic ozonation (HCO), but the interface mechanism of O3 activation on the catalyst surface is still ambiguous, especially the effect of a surface hydroxyl group (M-OH) at metal sites. Herein, we combined theoretical calculations with experimental verifications to comprehensively investigate the O3 activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN4-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O3 activation pathways was paid particular attention. O3 tends to directly interact with the metal atom on MnN4-NC, FeN4-NC, and NiN4-NC catalysts, among which MnN4-NC has the best catalytic activity for its relatively lower activation energy barrier of O3 (0.62 eV) and more active surface-adsorbed oxygen species (Oads). On the CoN4-NC catalyst, direct interaction of O3 with the metal site is energetically infeasible, but O3 can be activated to generate Oads or HO2 species from direct or indirect participation of M-OH sites. The experimental results showed that 90.7 and 82.3% of total organic carbon (TOC) was removed within 40 min during catalytic ozonation of p-hydroxybenzoic acid with MnN4-NC and CoN4-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O3 activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.

Correlated electron-nuclear dynamics of photoinduced water dissociation on rutile TiO2

Elucidating the mechanism of photoinduced water splitting on TiO2 is important for advancing the understanding of photocatalysis and the ability to control photocatalytic surface reactions. However, incomplete experimental information and complex coupled electron-nuclear motion make the microscopic understanding challenging. Here we analyse the atomic-scale pathways of photogenerated charge carrier transport and photoinduced water dissociation at the prototypical water-rutile TiO2(110) interface using first-principles dynamics simulations. Two distinct mechanisms are observed. Field-initiated electron migration leads to adsorbed water dissociation via proton transfer to a surface bridging oxygen. In the other pathway, adsorbed water dissociation occurs via proton donation to a second-layer water molecule coupled to photoexcited-hole transfer promoted by in-plane surface lattice distortions. Two stages of non-adiabatic in-plane lattice motion-expansion and recovery-are observed, which are closely associated with population changes in Ti3d orbitals. Controlling such highly correlated electron-nuclear dynamics may provide opportunities for boosting the performance of photocatalytic materials.

Accumulation of Long-Lived Photogenerated Holes at Indium Single-Atom Catalysts via Two Coordinate Nitrogen Vacancy Defect Engineering for Enhanced Photocatalytic Oxidation

Visible-light-driven photocatalytic oxidation by photogenerated holes has immense potential for environmental remediation applications. While the electron-mediated photoreduction reactions are often at the spotlight, active holes possess a remarkable oxidation capacity that can degrade recalcitrant organic pollutants, resulting in nontoxic byproducts. However, the random charge transfer and rapid recombination of electron-hole pairs hinder the accumulation of long-lived holes at the reaction center. Herein, a novel method employing defect-engineered indium (In) single-atom photocatalysts with nitrogen vacancy (Nv) defects, dispersed in carbon nitride foam (In-Nv-CNF), is reported to overcome these challenges and make further advances in photocatalysis. This Nv defect-engineered strategy produces a remarkable extension in the lifetime and an increase in the concentration of photogenerated holes in In-Nv-CNF. Consequently, the optimized In-Nv-CNF demonstrates a remarkable 50-fold increase in photo-oxidative degradation rate compared to pristine CN, effectively breaking down two widely used antibiotics (tetracycline and ciprofloxacin) under visible light. The contaminated water treated by In-Nv-CNF is completely nontoxic based on the growth of Escherichia coli. Structural-performance correlations between defect engineering and long-lived hole accumulation in In-Nv-CNF are established and validated through experimental and theoretical agreement. This work has the potential to elevate the efficiency of overall photocatalytic reactions from a hole-centric standpoint.

Surface photovoltage microscopy for mapping charge separation on photocatalyst particles

Solar-driven photocatalytic reactions offer a promising route to clean and sustainable energy, and the spatial separation of photogenerated charges on the photocatalyst surface is the key to determining photocatalytic efficiency. However, probing the charge-separation properties of photocatalysts is a formidable challenge because of the spatially heterogeneous microstructures, complicated charge-separation mechanisms and lack of sensitivity for detecting the low density of separated photogenerated charges. Recently, we developed surface photovoltage microscopy (SPVM) with high spatial and energy resolution that enables the direct mapping of surface-charge distributions and quantitative assessment of the charge-separation properties of photocatalysts at the nanoscale, potentially providing unprecedented insights into photocatalytic charge-separation processes. Here, this protocol presents detailed procedures that enable researchers to construct the SPVM instruments by integrating Kelvin probe force microscopy with an illumination system and the modulated surface photovoltage (SPV) approach. It then describes in detail how to perform SPVM measurements on actual photocatalyst particles, including sample preparation, tuning of the microscope, adjustment of the illuminated light path, acquisition of SPVM images and measurements of spatially resolved modulated SPV signals. Moreover, the protocol also includes sophisticated data analysis that can guide non-experts in understanding the microscopic charge-separation mechanisms. The measurements are ordinarily performed on photocatalysts with a conducting substrate in gases or vacuum and can be completed in 15 h.

Polar Aromatic Two-dimensional Dion-Jacobson Halide Perovskites for Efficient Photocatalytic H2 Evolution

Polar materials with spontaneous polarization (Ps) have emerged as highly promising photocatalysts for efficient photocatalytic H2 evolution owing to the Ps-enhanced photogenerated carrier separation. However, traditional inorganic polar materials often suffer from limitations such as wide band gaps and poor carrier transport, which hinders their photocatalytic H2 evolution efficiency. Here, we rationally synthesized a series of isostructural two-dimensional (2D) aromatic Dion-Jacobson (DJ) perovskites, namely (2-(2-Aminoethyl)pyridinium)PbI4 (2-APDPI), (3-(2-Aminoethyl)pyridinium)PbI4 (3-APDPI), and (4-(2-Aminoethyl)pyridinium)PbI4 (4-APDPI), where 2-APDPI and 4-APDPI crystalize in polar space groups with piezoelectric constants (d33) of approximately 40 pm V-1 and 3-APDPI adopts a centrosymmetric structure. Strikingly, owing to the Ps-facilitated separation of photogenerated carriers, polar 2-APDPI and 4-APDPI exhibit a 3.9- and 2.8-fold increase, respectively, in photocatalytic H2 evolution compared to the centrosymmetric 3-APDPI. As a pioneering study, this work provides an efficient approach for exploring new polar photocatalysts and highlights their potential in promoting photocatalytic H2 evolution.

Dual Function of Naphthalenediimide Supramolecular Photocatalyst with Giant Internal Electric Field for Efficient Hydrogen and Oxygen Evolution

Organic supramolecular photocatalysts have garnered widespread attention due to their adjustable structure and exceptional photocatalytic activity. Herein, a novel bis-dicarboxyphenyl-substituent naphthalenediimide self-assembly supramolecular photocatalyst (SA-NDI-BCOOH) with efficient dual-functional photocatalytic performance is successfully constructed. The large molecular dipole moment and short-range ordered stacking structure of SA-NDI-BCOOH synergistically create a giant internal electric field (IEF), resulting in a remarkable 6.7-fold increase in its charge separation efficiency. Additionally, the tetracarboxylic structure of SA-NDI-BCOOH greatly enhances its hydrophilicity. Thus, SA-NDI-BCOOH demonstrates efficient dual-functional activity for photocatalytic hydrogen and oxygen evolution, with rates of 372.8 and 3.8 µmol h-1 , respectively. Meanwhile, a notable apparent quantum efficiency of 10.86% at 400 nm for hydrogen evolution is achieved, prominently surpassing many reported supramolecular photocatalysts. More importantly, with the help of dual co-catalysts, it exhibits photocatalytic overall water splitting activity with H2 and O2 evolution rates of 3.2 and 1.6 µmol h-1 . Briefly, this work sheds light on enhancing the IEF by controlling the molecular polarity and stacking structure to dramatically improve the photocatalytic performance of supramolecular materials.

Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.

Crystal Facet Engineering on SrTiO3 Enhances Photocatalytic Overall Water Splitting

Single-crystal semiconductor-based photocatalysts exposing unique crystallographic facets show promising applications in energy and environmental technologies; however, crystal facet engineering through solid-state synthesis for photocatalytic overall water splitting is still challenging. Herein, we develop a novel crystal facet engineering strategy through solid-state recrystallization to synthesize uniform SrTiO3 single crystals exposing tailored {111} facets. The presynthesized low-crystalline SrTiO3 precursors enable the formation of well-defined single crystals through kinetically improved crystal structure transformation during solid-state recrystallization process. By employing subtle Al3+ ions as surface morphology modulators, the crystal surface orientation can be precisely tuned to a controlled percentage of {111} facets. The photocatalytic overall water splitting activity increases with the exposure percentage of {111} facets. Owing to the outstanding crystallinity and favorable anisotropic surface structure, the SrTiO3 single crystals with 36.6% of {111} facets lead to a 3-fold enhancement of photocatalytic hydrogen evolution rates up to 1.55 mmol·h-1 in a stoichiometric ratio of 2:1 than thermodynamically stable SrTiO3 enclosed with isotropic {100} facets.

Weakened Interfacial Hydrogen Bond Connectivity Drives Selective Photocatalytic Water Oxidation toward H2O2 at Water/Brookite-TiO2 Interface

The formation of H2O2 through the two-electron photocatalytic water oxidation reaction (WOR) is significant but encounters the competition with the four-electron O2 evolution reaction. Recent studies showed a crystal-phase dependence in H2O2 selectivity, where high purity brookite TiO2 (b-TiO2) exhibits remarkable H2O2 selectivity in contrast to the common rutile phase TiO2 (r-TiO2). However, the origin of such a structure-induced selectivity preference remains elusive, primarily due to the complexities associated with the solid-liquid interface system and excited-state chemistry. Herein, we conducted a comprehensive investigation into the selectivity mechanism of WOR at the water/b-TiO2(210) and water/r-TiO2(110) interfaces, employing first-principles molecular dynamics simulations and microkinetic analyses. Intriguingly, our results reveal that the intrinsic catalytic ability of the b-TiO2(210) itself does not enhance H2O2 selectivity compared to r-TiO2(110). Instead, it is the weakened interfacial hydrogen bond connectivity, modulated by the herringbone-like local atomic structure of the b-TiO2(210) surface, that determines the selectivity. Specifically, this weakened H-bond connectivity (i.e., local low water density) at the interface, owing to the strong water adsorption and distinct adsorption orientation, can stabilize the OH• radical and inhibit its deprotonation, leading to an improved H2O2 selectivity. By contrast, the relatively strong interface H-bond connectivity established over r-TiO2(110) accelerates the deprotonation of OH•, with the OH• coverage being 3 orders of magnitude lower than at the water/b-TiO2(210) interface. This study quantitatively demonstrates that the local H-bond structure (water density) at the liquid/solid interface significantly influences photocatalytic selectivity, and this insight may offer a rational approach to enhance the H2O2 selectivity.

Water-Mediated Selectivity Control of CH3 OH versus CO/CH4 in CO2 Photoreduction on Single-Atom Implanted Nanotube Arrays

Controllable methanol production in artificial photosynthesis is highly desirable due to its high energy density and ease of storage. Herein, single atom Fe is implanted into TiO2 /SrTiO3 (TSr) nanotube arrays by two-step anodization and Sr-induced crystallization. The resulting Fe-TSr with both single Fe reduction centers and dominant oxidation facets (001) contributes to efficient CO2 photoreduction and water oxidation for controlled production of CH3 OH and CO/CH4 . The methanol yield can reach to 154.20 µmol gcat -1 h-1 with 98.90% selectivity by immersing all the catalyst in pure water, and the yield of CO/CH4 is 147.48 µmol gcat -1 h-1 with >99.99% selectivity when the catalyst completely outside water. This CH3 OH yield is 50 and 3 times higher than that of TiO2 and TSr and stands among all the state-of-the-art catalysts. The facile gas-solid and gas-liquid-solid phase switch can selectively control CH3 OH production from ≈0% (above H2 O) to 98.90% (in H2 O) via slowly immersing the catalyst into water, where abundant •OH and H2 O around Fe sites play important role in selective CH3 OH production. This work highlights a new insight for water-mediated CO2 photoreduction to controllably produce CH3 OH.

Gradient oxygen doping triggered a microscale built-in electric field in CdIn2S4 for photoelectrochemical water splitting

Construction of a built-in electric field has been identified as an attractive improvement strategy for photoelectrochemical (PEC) water splitting by facilitating the carrier extraction from the inside to the surface. However, the promotion effect of the electric field is still restrained by the confined built-in area. Herein, we construct a microscale built-in electric field via gradient oxygen doping. The octahedral configuration of the synthesized CdIn2S4 (CIS) provides a structural basis, which enables the subsequent oxygen doping to reach a depth of ∼100 nm. Accordingly, the oxygen-doped CIS (OCIS) photoanode exhibits a microscale built-in electric field with band bending. Excellent PEC catalytic activity with a photocurrent density of 3.69 mA cm-2 at 1.23 V vs. RHE is achieved by OCIS, which is 3.1 times higher than that of CIS. Combining the results of thorough characterization and theoretical calculations, accelerating migration and separation of charge carriers have been determined as the reasons for the improvement. Meanwhile, the recombination risk at the doping centers has also been reduced to the minimum via optimal experiments. This work provides a new-generation idea for constructing a built-in electric field from the view point of bulky configuration towards PEC water splitting.

Excitonic Effects of the Excited-State Photocatalytic Reaction at the Molecule/Metal Oxide Interface

Excitonic effects caused by the Coulomb interaction between electrons and holes play a crucial role in photocatalysis at the molecule/metal oxide interface. As an ideal model for investigating the excitonic effect, coadsorption and photodissociation of water and methanol molecules on titanium dioxide involve complex ground-state thermalcatalytic and excited-state photocatalytic reaction processes. Herein, we systemically investigate the excited-state electronic structures of the coadsorption of H2O and CH3OH molecules on a rutile TiO2(110) surface by linear-response time-dependent density functional theory calculations and probe the reaction path for generating HCOOH or CO2, from ground-state and excited-state perspectives. The reaction barriers in excited-state calculations are significantly different from those in ground-state calculations during three processes, with the largest decrease being 0.94 eV for the Ti5c-O-CH2-O-Ti5c formation process.

Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2 O into Ti-OH groups and the deprotonation of pre-existing Ti-OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.

Ferro/Nonferroelectric Vertical Heterostructure Superlattice as a Visible-Light-Responsive Photocatalyst: A DFT Prediction

Developing narrow-band-gap ferroelectric semiconducting photocatalysts is a promising strategy for efficient photocatalytic water splitting with high energy conversion efficiency. Within this context, six ferro/nonferroelectric vertical heterostructure superlattices (VHSs) are constructed in this work by stacking ferroelectric SiS or GeS with nonferroelectric layered organic photocatalysts (C2N, g-C3N4, and melon), layer by layer. The geometry and electronic structures of these six VHSs are systematically investigated by density functional theory calculations. Consequently, four VHSs (SiS/g-C3N4, GeS/C2N, GeS/g-C3N4, and GeS/melon) are predicted to simultaneously possess several important and highly desirable features for photocatalytic water splitting, namely excellent visible-light adsorption, remarkable spontaneous polarization (0.49-0.70 C/m2), spatial charge separation, as well as suitable band-edge positions, thus serving as potential candidates for photocatalytic water splitting to produce hydrogen. This work not only provides a new strategy to use narrow-band-gap ferroelectric semiconductors for photocatalytic water splitting but also offers inspiration for developing photocatalysts with high energy conversion efficiency.

Interface-confined intermediate phase in TiO2 enables efficient photocatalysis

As a prototypical photocatalyst, TiO[Formula: see text] has been extensively studied. An interesting yet puzzling experimental fact was that P25-a mixture of anatase and rutile TiO[Formula: see text]-outperforms the individual phases; the origin of this mysterious fact, however, remains elusive. Employing rigorous first-principles calculations, here we uncover a metastable intermediate structure (MIS), which is formed due to confinement at the anatase/rutile interface. The MIS has a high conduction-band minimum level and thus substantially enhances the overpotential of the hydrogen evolution reaction. Also, the corresponding band alignment at the interface leads to efficient separation of electrons and holes. The interfacial confinement additionally creates a wide distribution of the band gap in the vicinity of the interface, which in turn improves optical absorption. These factors all contribute to the enhanced photocatalytic efficiency in P25. Our insights provide a rationale to the puzzling superior photocatalytic performance of P25 and enable a strategy to achieve highly efficient photocatalysis via interface engineering.

Fully Conjugated 2D sp2 Carbon-Linked Covalent Organic Frameworks for Photocatalytic Overall Water Splitting

Covalent organic frameworks (COFs) hold great promise for solar-driven hydrogen production. However, metal-free COFs for photocatalytic overall water splitting remain elusive, primarily due to challenges in simultaneously regulating their band structures and catalytic sites to enable concurrent half-reactions. Herein, two types of π-conjugated COFs containing the same donor-acceptor structure are constructed via Knoevenagel condensation and Schiff base reaction to afford cyanovinylene- and imine-bridged COFs, respectively. The difference in the linkage leads to a remarkable difference in their photocatalytic activity toward water splitting. The 2D sp2 carbon-linked COF exhibits notable activity for photocatalytic overall water splitting, which can reach an apparent quantum efficiency of 2.53% at 420 nm. In contrast, the 2D imine-linked COF cannot catalyze the overall water-splitting reaction. Mechanistic investigations reveal that the cyanovinylene linkage is essential in modulating the band structure and promoting charge separation in COFs, thereby enabling overall water splitting. Moreover, it is further shown that crystallinity substantially impacts the photocatalytic performance of COFs. This study represents the first successful example of developing metal-free COFs with high crystallinity for photocatalytic overall water splitting.

Prompt Hole Extraction Suppresses V5+ Dissolution and Sustains Large-Area BiVO4 Photoanodes for Over 2100 h Water Oxidation

Nanostructured bismuth vanadate (BiVO4) is at the forefront of emerging photoanodes in photoelectrochemical tandem devices for solar water splitting owing to the suitable band edge position and efficient charge separation capability. However, the (photo)chemical corrosion involving V5+ dissolution limits the long-term stability of BiVO4. Herein, guided by DFT calculations, we introduce an ALD-derived NiOx catalyst layer on BiVO4 to stabilize the surface Bi-O bonds, facilitate hole extraction, and thus suppress the V5+ dissolution. At the same time, the ALD NiOx catalyst layer could efficiently suppress the surface recombination and accelerate the surface OER kinetics, boosting the half-cell applied bias photon-to-current efficiency of BiVO4 to 2.05%, as well as a fill factor of 47.1%. By adding trace NaVO3 to the electrolyte, the NiOx/BiVO4 photoanode with an illumination area of 10.5 cm2 shows a record operational stability of more than 2100 h.

Gradient tungsten-doped Bi3TiNbO9 ferroelectric photocatalysts with additional built-in electric field for efficient overall water splitting

Bi3TiNbO9, a layered ferroelectric photocatalyst, exhibits great potential for overall water splitting through efficient intralayer separation of photogenerated carriers motivated by a depolarization field along the in-plane a-axis. However, the poor interlayer transport of carriers along the out-of-plane c-axis, caused by the significant potential barrier between layers, leads to a high probability of carrier recombination and consequently results in low photocatalytic activity. Here, we have developed an efficient photocatalyst consisting of Bi3TiNbO9 nanosheets with a gradient tungsten (W) doping along the c-axis. This results in the generation of an additional electric field along the c-axis and simultaneously enhances the magnitude of depolarization field within the layers along the a-axis due to strengthened structural distortion. The combination of the built-in field along the c-axis and polarization along the a-axis can effectively facilitate the anisotropic migration of photogenerated electrons and holes to the basal {001} surface and lateral {110} surface of the nanosheets, respectively, enabling desirable spatial separation of carriers. Hence, the W-doped Bi3TiNbO9 ferroelectric photocatalyst with Rh/Cr2O3 cocatalyst achieves an efficient and durable overall water splitting feature, thereby providing an effective pathway for designing excellent layered ferroelectric photocatalysts.

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

TiO2 Phase Ratio's Contribution to the Photocatalytic Activity

Photocatalysis is one of the approaches for solving environmental issues derived from extremely harmful pollution caused by industrial dyes, medicine, and heavy metals. Titanium dioxide is among the most promising photocatalytic semiconductors; thus, in this work, TiO2 powders were prepared by a hydrothermal synthesis using titanium tetrachloride TiCl4 as a Ti source. The effect of the hydrochloric acid (HCl) concentration on TiO2 formation was analyzed, in which a thorough morpho-structural analysis was performed employing different analysis methods like XRD, Raman spectroscopy, SEM/TEM, and N2 physisorption. EPR spectroscopy was employed to characterize the paramagnetic defect centers and the photogeneration of reactive oxygen species. Photocatalytic properties were tested by photocatalytic degradation of the rhodamine B (RhB) dye under UV light irradiation and using a solar simulator. The pH value directly influenced the formation of the TiO2 phases; for less acidic conditions, the anatase phase of TiO2 crystallized, with a crystallite size of ≈9 nm. Promising results were observed for TiO2, which contained 76% rutile, showing a 96% degradation of RhB under the solar simulator and 91% under UV light after 90 min irradiation, and the best result showed that the sample with 67% of the anatase phase after 60 min irradiation under the solar simulator had a 99% degradation efficiency.

Determining the Transformation Kinetics of Water Oxidation Intermediates on Hematite Photoanode

The oxygen evolution reaction (OER) from water is a sequential oxidation reaction process, involved in transformation of multiple reaction intermediates. For photo(electro)catalytic OER, revealing the intermediates transformation kinetics is quite challenging due to its coupling with photogenerated charge dynamics. Herein, we specifically study the transformation kinetics of the OER intermediates in rationally thin hematite photoanodes through increasing the ratio between surface intermediates and photogenerated charges in bulk. We directly identify the formation and consumption kinetics of one-hole OER intermediate (FeIV═O) in photoelectrochemical water oxidation using operando transient absorption (TA) spectroscopy. The microsecond formation kinetics of the FeIV═O species are sensitively changed by the excitation mode of Fe2O3. The subsecond consumption kinetics are closely dependent on surface FeIV═O species density, demonstrating that the cooperation of FeIV═O intermediates is the key to accelerating water oxidation kinetics on the Fe2O3 surface. This work provides insight into understanding and controlling water oxidation reaction kinetics on Fe2O3 surface.

Facilitating two-electron oxygen reduction with pyrrolic nitrogen sites for electrochemical hydrogen peroxide production

Electrocatalytic hydrogen peroxide (H₂O₂) production via the two-electron oxygen reduction reaction is a promising alternative to the energy-intensive and high-pollution anthraquinone oxidation process. However, developing advanced electrocatalysts with high H₂O₂ yield, selectivity, and durability is still challenging, because of the limited quantity and easy passivation of active sites on typical metal-containing catalysts, especially for the state-of-the-art single-atom ones. To address this, we report a graphene/mesoporous carbon composite for high-rate and high-efficiency 2e^- oxygen reduction catalysis. The coordination of pyrrolic-N sites -modulates the adsorption configuration of the *OOH species to provide a kinetically favorable pathway for H₂O₂ production. Consequently, the H₂O₂ yield approaches 30 mol g^-1 h^-1 with a Faradaic efficiency of 80% and excellent durability, yielding a high H₂O₂ concentration of 7.2 g L^-1. This strategy of manipulating the adsorption configuration of reactants with multiple non-metal active sites provides a strategy to design efficient and durable metal-free electrocatalyst for 2e^- oxygen reduction.

Understanding oxygen evolution mechanisms by tracking charge flow at the atomic level

Current classifications of oxygen evolution catalysts are based on energy levels of the clean catalysts. It is generally asserted that a LOM-catalyst can only follow LOM chemistry in each electron transfer step and that there can be no mixing between AEM and LOM steps without an external trigger. We use ab initio theory to track the charge flow of the water-on-catalyst system and show that the position of water orbitals is pivotal in determining whether an electron transfer step is water dominated oxidation (WDO), lattice-oxygen dominated oxidation (LoDO), or metal dominated oxidation (MDO). Microscopic photo-catalytic pathways of TiO₂ (110), a material whose lattice oxygen bands lie above the metal bands, show that viable OER pathways follow either all AEM steps or mixed AEM-LOM steps. The results provide a correct description of redox chemistries at the atomic level and advance our understanding of how water-splitting catalysts produce desorbed oxygen.

Unconventional rate law of water photooxidation at TiO2 electrodes

Photoelectrochemical oxidation of water over semiconductors is a promising route for the production of sustainable solar fuels. TiO₂ water photooxidation has been intensively studied over the past 50 years, but its rate law and mechanism are still undetermined. The main challenges are that there is no appropriate reaction kinetic model currently, and that both the reaction rate constant and reactant photohole concentration/density are not readily quantified with respect to conventional chemical reactions. Here we report that the rate law and photohole transfer mechanism could be determined by a combination of multiple (photo-) electrochemical techniques. We demonstrate that the kinetics of TiO₂ water oxidation by photogenerated holes can be well-described by a model of surface state mediating charge transfer and recombination. The rate law, in terms of steady-state photocurrent, is the product of the surface hole density exponential dependent rate constant and the surface hole density, with first order for all the surface hole densities studied. This reactant concentration dependent rate constant is conceptually unexpected for an elementary step in conventional chemical reactions. In addition, we find that hydroxyl ions in bulk solutions are involved in the reaction as indicated by observation of the solution pH dependent apparent rate constant. This study may thus lead to key insights both for strategies to evaluate and/or enhance photoelectrochemical performances and for understanding reaction mechanisms.

A Conformal Carbon Nanolayer Coated Fe2 O3 Cocatalyst for the Promoted Activity of Plasma-Sputtered BiVO4 Photoanode

To simultaneously improve the hole extraction ability of the BiVO₄ photoanode and accelerate the surface reaction kinetics, herein, a carbon nanolayer conformally coated Fe₂ O₃ (C-Fe₂ O₃ ) as oxygen evolution catalyst (OEC) is loaded on the H₂ plasma treated nanoporous BiVO₄ (BVO(H₂ )) surface by a hydrothermal reaction. It is found that the H₂ plasma induced vacancies in BVO remarkably increases the conductivity, and the C-Fe₂ O₃ enables hole extraction from the bulk to the surface as well as efficient hole injection to the electrolyte. As a result, the C-Fe₂ O₃ /BVO(H₂ ) photoanode achieves a photocurrent density of 4.4 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and an ABPE value of 1.5 % at 0.68 V vs. RHE, which are 4.8-fold and 13-fold higher than that of BVO photoanode, respectively.

Linking the Photoinduced Surface Potential Difference to Interfacial Charge Transfer in Photoelectrocatalytic Water Oxidation

Charge transfer at the semiconductor/solution interface is fundamental to photoelectrocatalytic water splitting. Although insights into charge transfer in the electrocatalytic process can be gained from the phenomenological Butler-Volmer theory, there is limited understanding of interfacial charge transfer in the photoelectrocatalytic process, which involves intricate effects of light, bias, and catalysis. Here, using operando surface potential measurements, we decouple the charge transfer and surface reaction processes and find that the surface reaction enhances the photovoltage via a reaction-related photoinduced charge transfer regime as demonstrated on a SrTiO₃ photoanode. We show that the reaction-related charge transfer induces a change in the surface potential that is linearly correlated to the interfacial charge transfer rate of water oxidation. The linear behavior is independent of the applied bias and light intensity and reveals a general rule for interfacial transfer of photogenerated minority carriers. We anticipate the linear rule to be a phenomenological theory for describing interfacial charge transfer in photoelectrocatalysis.

Recent Advances of Core-Shell Cu-Based Catalysts for the Reduction of CO2 to C2+ Products

Copper is a key metal for carbon dioxide (CO₂ ) reduction reaction, which can reduce CO₂ to value-added products. The core-shell structure can effectively promote the C-C coupling process due to its strong synergistic effect originated from its unique electronic structure and interface environment. Therefore, the combination of copper and core-shell structure to design an efficient Cu-based core-shell structure catalyst is of great significance for electrocatalytic CO₂ reduction (CO₂ RR). In this review, we first briefly summarize the basic principle of CO₂ RR. In addition, we outline the advantages of core-shell structure for catalysis. Then, we review the recent research progresses of Cu-based core-shell structures for the selective reduction of multi-carbon (C₂₊ ) products. In the end, the challenges of using core-shell catalyst for CO₂ RR are described, and the future development of this field is prospected.

Activation of CO2 by Direct Cleavage Triggered by Photoelectrons on Rutile TiO2(110)

The initial activation of the inert CO₂ is a key step in its photoreduction to valuable chemicals. This process was proposed to proceed mainly by CO₂ accepting a photoelectron to form a CO₂^•- radical or by CO₂ accepting two photoelectrons and a proton to form the HCOO^- anion on the prototypical rutile TiO₂(110) surface. Here, we reveal a new mechanism, in which CO₂ is directly cleaved to CO and the adsorbed O^2- anion under the trigger of two photoelectrons, by using density functional theory calculations with the HSE06 hybrid functional. The newly revealed mechanism is more favorable than the two previously proposed pathways. Furthermore, our results show that the deficiency of photoelectrons on the catalyst surface is a potential reason for the current low efficiency of CO₂ photoreduction.

Untangling product selectivity on clean low index rutile TiO2 surfaces using first-principles calculations

Computational modeling of metal oxide surfaces provides an important tool to help untangle complex spectroscopy and measured catalytic reactivity. There are many material properties that make rational catalytic design challenging, and computational methods provide a way to evaluate possible structural factors, like surface structure, individually. The mechanism of water oxidation or oxygen evolution is well studied on some anatase surfaces and the rutile TiO₂ (110) surface but has not yet been mapped on other low-index Miller rutile surfaces that are present in most experimental nano-titania catalysts. Here first principles calculations provide new insights into water oxidation mechanisms and reactivity of the most common low-index Miller facets of rutile TiO₂. The reactivity of three surfaces, (101), (010), and (001), are explored for the first time and the product selectivity of multistep electron transfer on each surface is compared to the well-studied (110) surface. Density functional theory shows that a peroxo, O^(p), intermediate is more favorable for water oxidation on all facets. The ˙OH radical formation is favored on the (001) facet resulting in a high overpotential for oxygen evolution reaction (OER). The (101) and (110) facets have low overpotentials, ∼0.3 V, and favor two-electron proton-coupled electron transfer to produce H₂O₂. The only facet that prefers direct OER is (001), leading to O₂ evolution in a four-electron process with an overpotential of 0.53 V. A volcano plot predicts the selectivity and activity of low-index Miller facets of rutile TiO₂, revealing the high activity of the peroxo OER mechanism on the (010) facet.

On the origin of multihole oxygen evolution in haematite photoanodes

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. We show here that the OER rate has a third-order dependence on the surface hole density. We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Identifying Photocatalytic Active Sites of C2H6 C-H Bond Activation on TiO2 via Combining First-Principles Ground-State and Excited-State Electronic Structure Calculations

The activation of C-H bonds at low temperatures has attracted widespread interest in heterogeneous catalysis, which involves complex thermocatalytic and photocatalytic reaction processes. Herein, we systematically investigate the photothermal catalytic process of C-H bond activation in C₂H₆ dehydrogenation on rutile TiO₂(110). We demonstrate that the photochemical activity of the C₂H₆ molecule adsorbed on TiO₂(110) is site-sensitive and that C₂H₆ is more easily adsorbed at the Ti_5c site with a lower dehydrogenation energy barrier. The first C-H bond activation of the C₂H₆ adsorbed at the Ti_5c site tends to occur in the ground state, whereas O_br-adsorbed C₂H₆ is more photoactive during the initial adsorption. During the dehydrogenation of C₂H₆, the photogenerated electrons are always located at the Ti⁴⁺ sites of the TiO₂ substrate while the photogenerated holes can be captured by C₂H₆ to activate the C-H bond.

Insights into the Mechanism of Ozone Activation and Singlet Oxygen Generation on N-Doped Defective Nanocarbons: A DFT and Machine Learning Study

N-doped defective nanocarbon (N-DNC) catalysts have been widely studied due to their exceptional catalytic activity in many applications, but the O₃ activation mechanism in catalytic ozonation of N-DNCs has yet to be established. In this study, we systematically mapped out the detailed reaction pathways of O₃ activation on 10 potential active sites of 8 representative configurations of N-DNCs, including the pyridinic N, pyrrolic N, N on edge, and porphyrinic N, based on the results of density functional theory (DFT) calculations. The DFT results indicate that O₃ decomposes into an adsorbed atomic oxygen species (O_ads) and an ³O₂ on the active sites. The atomic charge and spin population on the O_ads species indicate that it may not only act as an initiator for generating reactive oxygen species (ROS) but also directly attack the organics on the pyrrolic N. On the N site and C site of the N₄V₂ system (quadri-pyridinic N with two vacancies) and the pyridinic N site at edge, O₃ could be activated into ¹O₂ in addition to ³O₂. The N₄V₂ system was predicted to have the best activity among the N-DNCs studied. Based on the DFT results, machine learning models were utilized to correlate the O₃ activation activity with the local and global properties of the catalyst surfaces. Among the models, XGBoost performed the best, with the condensed dual descriptor being the most important feature.

Achieving Theory-Experiment Parity for Activity and Selectivity in Heterogeneous Catalysis Using Microkinetic Modeling

Microkinetic modeling based on density functional theory (DFT) energies plays an essential role in heterogeneous catalysis because it reveals the fundamental chemistry for catalytic reactions and bridges the microscopic understanding from theoretical calculations and experimental observations. Microkinetic modeling requires building a set of ordinary differential equations (ODEs) based on the calculation results of thermodynamic properties of adsorbates and kinetic parameters for the reaction elementary steps. Solving a microkinetic model can extract information on catalytic chemistry, including critical reaction intermediates, reaction pathways, the surface species distribution, activity, and selectivity, thus providing vital guidelines for altering catalysts.

However, the quantitative reliability of traditional microkinetic models is often insufficient to conclusively extrapolate the mechanistic details of complex reaction systems. This can be attributed to several factors, the most important of which is the limitation of obtaining an accurate estimation of the energy inputs via traditional calculation methods. These limitations include the difficulty of using static DFT methods to calculate reaction energies of adsorption/desorption processes, often rate-controlling or selectivity-determining steps, and the inadequate consideration of surface coverage effects. In addition, the robust microkinetic software is rare, which also complicates the resolution of complex catalytic systems.

In this Account, we review our recent works toward refining the predictions of microkinetic modeling in heterogeneous catalysis and achieving theory–experiment parity for activity and selectivity. First, we introduce CATKINAS, a microkinetic software developed in our group, and show how it disentangles the problem that traditional microkinetic software has and how it can now be applied to obtain kinetic results for more sophisticated reaction systems. Second, we describe a molecular dynamics method developed recently to obtain the free-energy changes for the adsorption/desorption process to fill in the missing energy inputs. Third, we show that a rigorous consideration of surface coverage effects is pivotal for building more realistic models and obtaining accurate kinetic results. Following a series of studies on acetylene hydrogenation reactions on Pd catalysts, we demonstrate how this new approach can provide an improved quantitative understanding of the mechanism, active site, and intrinsic structural sensitivity. Finally, we conclude with a brief outlook and the remaining challenges in this field.

Unraveling Charge-Separation Mechanisms in Photocatalyst Particles by Spatially Resolved Surface Photovoltage Techniques

The photocatalytic conversion of solar energy offers a potential route to renewable energy, and its efficiency relies on effective charge separation in nanostructured photocatalysts. Understanding the charge-separation mechanism is key to improving the photocatalytic performance and this has now been enabled by advances in the spatially resolved surface photovoltage (SRSPV) method. In this Review we highlight progress made by SRSPV in mapping charge distributions at the nanoscale and determining the driving forces of charge separation in heterogeneous photocatalyst particles. We discuss how charge separation arising from a built-in electric field, diffusion, and trapping can be exploited and optimized through photocatalyst design. We also highlight the importance of asymmetric engineering of photocatalysts for effective charge separation. Finally, we provide an outlook on further opportunities that arise from leveraging these insights to guide the rational design of photocatalysts and advance the imaging technique to expand the knowledge of charge separation.

Core-Shell Nanostructured Ru@Ir-O Electrocatalysts for Superb Oxygen Evolution in Acid

The design of highly active and durable catalysts for the sluggish anodic oxygen evolution reaction (OER) in acid remains an urgent yet challenging goal in water electrolysis. Herein, a core-shell nanostructured Ru@Ir-O catalyst with tensile strains and incorporated oxygens is introduced in the Ir shell that holds an extremely low OER overpotential of 238 mV at 10 mA cm^-2 in acid. The material also shows a remarkable 78-fold higher mass activity than the conventional IrO₂ at 1.55 V in 0.5 M H₂ SO₄ . Structural characterization and theoretical calculations reveal that the core-shell interaction and tensile strain cause band position shift and charge redistribution. These electronic factors furthermore optimize the bonding strength of O* and HOO* intermediates on the surface, yielding significantly boosted OER activity relative to the conventional IrO₂ .

Oxo dicopper anchored on carbon nitride for selective oxidation of methane

Selective conversion of methane (CH₄) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu₂@C₃N₄) as advanced catalysts for CH₄ partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H₂O₂) and oxygen (O₂) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH₄ with O₂ achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu⁻¹h⁻¹. Mechanistic studies reveal that the high reactivity of Cu₂@C₃N₄ can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C₃N₄ substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H₂O₂ and O₂ activation in thermo- and photocatalysis, respectively.

Selective conversion of methane into value-added chemicals is a promising approach for utilization of hydrocarbon sources. Here the authors develop dimeric copper centers supported on graphitic carbon nitride (denoted as Cu₂@C₃N₄) with >10% conversion and >98% selectivity toward methyl oxygenates in both thermo- and photo- catalytic reactions.

A TiO2-x nanobelt array with oxygen vacancies: an efficient electrocatalyst toward nitrite conversion to ammonia

Electrocatalytic nitrite reduction not only holds significant potential in the control of nitrite contamination in the natural environment, but also is an attractive approach for sustainable ammonia synthesis. In this communication, we report that a TiO_2-x nanobelt array with oxygen vacancies on a titanium plate is able to convert nitrite into ammonia with a high faradaic efficiency of 92.7% and a large yield of 7898 μg h^-1 cm^-2 in alkaline solution. This monolithic catalyst also shows high durability with the maintenance of its catalytic activity for 12 h. Theoretical calculations further reveal the critical role of oxygen vacancies in nitrite electroreduction.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Hole utilization in solar hydrogen production

In photochemical production of hydrogen from water, the hole-mediated oxidation reaction is the rate-determining step. A poor solar-to-hydrogen efficiency is usually related to a mismatch between the internal quantum efficiency of photon-induced hole generation and the apparent quantum yield of hydrogen. This waste of photogenerated holes is unwanted yet unavoidable. Although great progress has been made, we are still far away from the required level of dexterity to deal with the associated challenges of wasted holes and its consequential chemical effects that have placed one of the greatest bottlenecks in attaining high solar-to-hydrogen efficiency. A critical assessment of the hole and its related phenomena in solar hydrogen production would, therefore, pave the way moving forward. In this regard, we focus on the contextual and conceptual understanding of the dynamics and kinetics of photogenerated holes and its critical role in driving redox reactions, with the objective of guiding future research. The main reasons behind and consequences of unused holes are examined and different approaches to improve overall efficiency are outlined. We also highlight yet unsolved research questions related to holes in solar fuel production.

Optimally Selecting Photo- and Electrocatalysis to Facilitate CH4 Activation on TiO2(110) Surface: Localized Photoexcitation versus Global Electric-Field Polarization

Photo- and electrocatalytic technologies hold great promise for activating inert chemical bonds under mild conditions, but rationally selecting a more suitable method in between to maximize the performance remains an open issue, which requires a fundamental understanding of their different catalytic mechanisms. Herein, by first-principles calculations, we systematically compare the activation mechanisms for the C-H bond of the CH₄ molecule on TiO₂(110) under the photo- and electrocatalytic modes without or with water involved. It quantitatively reveals that the activation barrier of the C-H bond decreases dramatically with a surprising 74% scale by photoexcitation relative to that in thermocatalysis (1.12 eV), while the barrier varies with a maximum promotion of only 5% even under -1 V/Å external electric field (EEF). By detailed geometric/electronic analysis, the superior photocatalytic activity is traced to the highly oxidative lattice O_br ^•- radical excited by a photohole (h ⁺), which motivates the homolytic C-H bond scission. However, under EEF from -1 V/Å to 1 V/Å, it gives a relatively mild charge polarization on the TiO₂(110) surface region and thus a limited promotion for breaking the weakly polar C-H bond. By contrast, in the presence of water, we find that EEF can facilitate CH₄ activation indirectly assisted by the surface radical-like OH* species from the oxidative water cleavage at high oxidative potential (>1.85 V vs SHE), which explains the high energy cost to drive electrocatalytic CH₄ conversion in experiment. Alternatively, we demonstrate that more efficient CH₄ activation could be also achieved at much lower oxidative potential when integrating the light irradiation. In such a circumstance, EEF can not only promote the h ⁺ accumulation at the catalyst surface but also help H₂O deprotonation to form hydroxide, which can serve as an efficient hole-trapper to generate OH^• radical (OH^- + h ⁺ → OH^•), unveiling an interesting synergistic photoelectrocatalytic effect. This work could provide a fundamental insight into the different characteristics of photo- and electrocatalysis in modulating chemical bond cleavage.

Enhancement of metal ion-induced hole transfer on water oxidation performance of BiVO4 photoanode

The introduction of metal ions as carrier transport media in acidic electrolytes significantly enhanced the water oxidation performance of the BiVO4 photoanode. In particular, the BiVO4 electrode in the Co2+...

Theoretical and experimental studies of high efficient all-solid Z-scheme TiO2-TiC/g-C3N4 for photocatalytic CO2 reduction via dry reforming of methane

All-solid Z-scheme heterojunction TiO2-TiC/g-C3N4 was proposed and synthesized successfully by a facile calcination method and used for photocatalytic CO2 reduction in the presence of CH4. Under sub-atmospheric pressure and room...