Photoinduced Water Research Articles

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.

Synergistic effect of carbon dopants on photoinduced water−splitting on graphitic carbon nitride

Graphitic carbon nitride (g-CN), composed of heptazines, holds promise as a photocatalyst for hydrogen evolution. But its suboptimal visible light absorption and poor electrical conductivity limit photocurrent density and catalytic activity. Carbon atom doping, specifically carbon-modified g-CN (C/g-CN), enhances photocatalytic performance. This study investigates the impact of carbon dopants at the three-coordinated nitrogen atom (N3C) and the two-coordinated nitrogen atom (N2C) sites on the reaction mechanism of water-splitting in the ground and excited states. Nonadiabatic dynamics simulations are employed on water hydrogen-bonded to C-doped heptazines (H2O⋯C/heptazine) to unveil the molecular-level decomposition mechanism in the excited states. The simulations reveal that the C dopants at N2C improve the activity for water-splitting in the ground state. The C dopants at N3C improve the H atom detachment from H2O via the electron-driven proton transfer (EDPT) process. The synergistic effect of carbon dopants on enhanced catalytic activity of C/g-CN is elucidated.

Bioinspired hydrogels: polymeric designs towards artificial photosynthesis.

Aquatic environments host various living organisms with active molecular systems, such as the enzymes in the thylakoid membrane that realise photosynthesis. Various challenges in achieving artificial photosynthesis, such as water splitting, have been studied using both inorganic and organic molecules. However, several problems persist, including diffusion-limited reactions and multiple redox reactions in the liquid phase. In this Feature Article, we discuss the significant challenges in using polymer networks as active mediators for photoinduced water splitting. In the creation of artificial chloroplasts, polymer networks offer various advantages, such as stable dispersions of multiple types of functional molecules and close molecular arrangements. To incorporate these features, stepwise synthesis and integration can be utilized during the hierarchical construction of polymer networks. The constituent molecules such as ruthenium complex and platinum nanoparticles in the photoinduced electron transfer circuits are closely arranged to smoothly operate forward reactions by polymer networks. The quantum efficiency of photoinduced H2 generation in gel systems is considerably higher than that of conventional solution systems. Additionally, a thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) network of microgels can be used to integrate catalytic nanoparticles into the inside by using the electrostatic interaction and the mesh size changes. By focusing on the redox changes of copolymerised molecules that induce swelling/shrinking at a constant temperature, active electron transfer can be precisely achieved using the coil-globule transition of the PNIPAAm having viologen. This article highlights the potential of polymer networks to develop strategies for active electron transfer and energy conversion systems similar to those found in living organisms.

A computational study photophysical properties of a series phenothiazine-linked-BODIPY as D-D-π-a photosensitizer unit for photo-induced hydrogen production from water

Phenothiazine-derivatives are designed as sensitizers which covalently-linked with BODIPY for application in a system of dye-sensitized photoelectrochemical cells (DSPECs). A series of phenothiazine-linked-BODIPY as D-D-π-A chromophore have been investigated by a computational approach for studying the capability of the photosensitizers for application in photoinduced water splitting hydrogen production in DSPECs. The geometries of sensitizers at the ground state and excited state were optimized by the CAM-B3LYP/6-31G (d,p) level method. The theoretical study has been done in order to investigate the photophysical properties such as absorption profile, HOMO-LUMO energy and surface plots of the photosensitizer. The result of calculation for HOMO-LUMO energy level of each dyes showed that the LUMO energy are ranging more than -2.8 - (-2.86) eV which is higher than -4.0 eV of conduction band of TiO2 as semiconductor in DSPEC, meaning that all these dyes could be used as photosensitizers in DSPECs. In addition, the oscillator strength data of each dyes showed the dyes have absorption profile in visible region, meaning the dyes could harvest higher intensity of sunlight to induce water splitting reaction to produce hydrogen.

Water-repelling, vapor-resisting, self-cleaning, oil–water-separating, water-purifying, antibacterial Hanji cellulose papers coated with Teflon and ZnO particles via supersonic spraying

Cellulose-based Korean mulberry paper, called Hanji, has been used as a functional material in heaters, supercapacitors, sensors, and light-emitting diodes because of its low weight, high mechanical strength, antibacterial activity, and eco-friendliness. Imparting water resistance property to these functional papers is critical to protect them against potential moisture-associated damage and to ensure their longevity. This study applied supersonic spraying technique to deposit superhydrophobic Teflon and antibacterial ZnO particles on Hanji. The results show that functionalized Hanji deposited with Teflon/ZnO exhibited water-repelling, anti-fogging, and antibacterial properties. Furthermore, the photoinduced water purification and self-cleaning features of functionalized Hanji has been demonstrated. Hanji enabled the passage of purple-colored hexane; however, blue-colored water could not permeate the Hanji filter, demonstrating the oil–water separation capability of Hanji.

TiO2–SiO2 Self-Standing Materials bearing Hierarchical Porosity: MUB-200(x) Series toward 3D-Efficient VOC Photoabatement Properties

Three-dimensional photoactive self-standing porous materials have been synthesized through the integration of soft chemistry and colloids (emulsions, lyotrope mesophases, and P25 titania nanoparticles). Final multiscale porous ceramics bear 700-1000 m2 g-1 of micromesoporosity depending on the P25 nanoparticle contents. The applied thermal treatment does not affect the P25 anatase/rutile allotropic phase ratio. Photonic investigations correlated with the foams' morphologies suggest that the larger amount of TiO2 that is introduced, the larger the walls' density and the smaller the mean size of the void macroscopic diameters, with both effects inducing a reduction of the photon transport mean free path (lt) with the P25 content increase. A light penetration depth in the range of 6 mm is reached, thus depicting real 3D photonic scavenger behavior. The 3D photocatalytic properties of the MUB-200(x) series, studied in a dynamic "flow-through" configuration, show that the highest photoactivity (concentration of acetone ablated and concentration of CO2 formed) is obtained with the highest monolith height (volume) while providing an average of 75% mineralization. These experimental results validate the fact that these materials, bearing 3D photoactivity, are paving the path for air purification operating with self-standing porous monolith-type materials, which are much easier to handle than powders. As such, the photocatalytic systems can now be advantageously miniaturized, thereby offering indoor air treatment within vehicles/homes while drastically limiting the associated encumbrance. This volumetric counterintuitive acting mode for light-induced reactions may find other relevant advanced applications for photoinduced water splitting, solar fuel, and dye-sensitized solar cells while both optimizing photon scavenging and opening the path for the miniaturization of the processes where encumbrance or a foot-print penalty would be advantageously circumvented.

Spin-Polarized Photoemission from Chiral CuO Catalyst Thin Films

The chirality-induced spin selectivity (CISS) effect facilitates a paradigm shift for controlling the outcome and efficiency of spin-dependent chemical reactions, for example, photoinduced water splitting. While the phenomenon is established in organic chiral molecules, its emergence in chiral but inorganic, nonmolecular materials is not yet understood. Nevertheless, inorganic spin-filtering materials offer favorable characteristics, such as thermal and chemical stability, over organic, molecular spin filters. Chiral cupric oxide (CuO) thin films can spin polarize (photo)electron currents, and this capability is linked to the occurrence of the CISS effect. In the present work, chiral CuO films, electrochemically deposited on partially UV-transparent polycrystalline gold substrates, were subjected to deep-UV laser pulses, and the average spin polarization of photoelectrons was measured in a Mott scattering apparatus. By energy resolving the photoelectrons and changing the photoexcitation geometry, the energy distribution and spin polarization of the photoelectrons originating from the Au substrate could be distinguished from those arising from the CuO film. The findings reveal that the spin polarization is energy dependent and, furthermore, indicate that the measured polarization values can be rationalized as a sum of an intrinsic spin polarization in the chiral oxide layer and a contribution via CISS-related spin filtering of electrons from the Au substrate. The results support efforts toward a rational design of further spin-selective catalytic oxide materials.

Superlative photoelectrochemical properties of 3D MgCr-LDH nanoparticles influencing towards photoinduced water splitting reactions

In the present work, we report the synthesis of single system three-dimensional (3D) open porous structure of MgCr-LDH nanoparticles in a substrate-free path by using one-step formamide assisted hydrothermal reaction followed by visible light irradiation for significant photoelectrochemical (PEC) properties that manifest towards photocatalytic H2 and O2 production. The as-prepared nanostructured materials were characterized by various physico-chemical characterization techniques. Moreover, this unique synthetic approach produces 3D open porous network structure of MgCr-LDH nanoparticles, which were formed by stacking of numerous 2D nanosheets, for effective light harvestation, easy electronic channelization and unveil superlative PEC properties, including high current density (6.9 mA/cm2), small Tafel slope of 82 mV/decade, smallest arc of the Nyquist plot (59.1 Ω cm−2) and photostability of 6000 s for boosting water splitting activity. In addition, such perfectly self-stacked 2D nanosheets in 3D MgCr-LDH possess more surface active defect sites as enriched 50% oxygen vacancy resulting a good contact surface within the structure for effective light absorption along with easy electron and hole separation, which facilitates the adsorption of protons and intermediate for water oxidation. Additionally, the Cr3+ as dopant pull up the electrons from water oxidation intermediates, thereby displaying superior photocatalytic H2 and O2 production activity of 1315 μmol/h and 579 μmol/h, respectively. Therefore, the open 3D morphological aspects of MgCr-LDH nanoparticles with porous network structure and high surface area possess more surface defect sites for electron channelization and identified as distinct novel features of this kind of materials for triggering significant PEC properties, along with robustly enhance the photocatalytic water splitting performances.

Role of surface morphology and terminating groups in titanium carbide MXenes (Ti3C2Tx) cocatalysts with engineering aspects for modulating solar hydrogen production: A critical review

• Current progress of Ti 3 C 2 MXene as prominent materials for hydrogen production is reviewed. • Insight into structural and catalytic properties of Ti 3 C 2 MXene are summarized. • Design of Ti 3 C 2 MXene with 0D, 1D, 2D and 3D dimensions are deliberated. • The control of surface termination groups in Ti 3 C 2 MXene are discussed. • Photocatalytic reaction engineering improvement through morphological modulation are systematically reviewed. Hydrogen production through photoinduced water splitting is regarded as one of the best alternatives in providing clean and renewable energy sources. Recently, MXene as co-catalysts are considered promising green structured materials to replace expensive noble metals to promote both efficiency and photostability. The utilization of titanium carbide (Ti 3 C 2 T x ) MXene as a co-catalyst has shown great promise for assisting solar to hydrogen conversion owing to their structural flexibility, surface morphological, and tailorable termination groups. The surface functional groups, morphological developments and engineering aspects are important factors to optimize Ti 3 C 2 T x as the best co-catalyst in solar energy applications. This review provides recent advances in morphological design of Ti 3 C 2 T x MXene with critical analysis on the role of termination groups for promoting photoactivity and products selectivity. A deep focus is given to the engineering design of Ti 3 C 2 T x MXene with special regard to quantum dots, monolayer, and hierarchical structures. Firstly, the general overview of Ti 3 C 2 T x MXene is introduced with insights into their catalytic properties and formation of surface termination groups to gain profound understanding of their basic catalytic structure. Next, the effects of tailoring the morphology of Ti 3 C 2 T x MXene into 2D accordion-like structure, monolayer, hierarchical, quantum dots, and nanotubes structure to expedite their role to promote solar-based photoactivity are critically discussed. In addition, specific considerations are given on the effects of reaction parameters, challenges, and synthesis strategies for tailoring the morphology of Ti 3 C 2 T x MXene with controlled functional groups. Finally, future perspectives are provided for the potential use of Ti 3 C 2 T x MXene as green materials in reaction engineering and environment applications.

Role of Ti3C2 MXene as Prominent Schottky Barriers in Driving Hydrogen Production through Photoinduced Water Splitting: A Comprehensive Review

Role of Ti₃C₂ MXene as Prominent Schottky Barriers in Driving Hydrogen Production through Photoinduced Water Splitting: A Comprehensive Review

Cover Feature: Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles (Chem. Eur. J. 68/2021)

Fiber gels made of Ru(bpy)32+-decorated chitosan and incorporating IrO2 nanoparticles (see SEM image in the background) can effectively perform photoinduced water oxidation with better performance than isolated components. In particular, a significantly higher quantum yield for water oxidation and a better stability of the system are reported. More information can be found in the Full Paper by G. La Ganga, S. Campagna et al. (DOI: 10.1002/chem.202102032).

CoS@TiO2 S-scheme heterojunction photocatalyst for hydrogen production from photoinduced water splitting

To synthesize durable catalysts, cobalt sulfide (CoS) microsheet particles are woven into a polyhedral cage shape to prevent dissolution and light corrosion during photochemical reactions. Titanium dioxide (TiO2) nanoparticles with excellent stability against water and light are used to enclose the cage. The surface of the dodecahedral CoS particle is positively charged, and the surface of the spherical TiO2 particles is negatively charged, resulting in a core–shell shaped xCoS@yTiO2 heterojunction particle. The xCoS@yTiO2 heterojunction catalyst has a stronger ability to absorb visible light and greater photocatalytic hydrogen evolution activity than pure TiO2 or CoS catalysts: When lactic acid is used as an electronic sacrificial agent, the hydrogen production of 1CoS@2TiO2 reaches 1945 μmol g−1 for 10 h, and the catalytic activities are 114.4 and 13.2 times those of CoS and TiO2, respectively. Spin-trapping ESR results reveal that charge transfer in the core–shell shaped xCoS@yTiO2 heterojunction follows the S-scheme mechanism. The excellent catalytic activity of the core–shell shaped xCoS@yTiO2 heterojunction catalyst is because of its favorable bandgap location for water splitting, higher photocurrent density, lower photoluminescence, and having strong redox sites. These factors ultimately delay the recombination of photo-induced electron/hole pairs and, as a result, high catalytic efficiency is stably maintained up to the fifth recycling test.

Cobalt-Based Metal-Organic Cages for Visible-Light-Driven Water Oxidation.

Water oxidation to molecular oxygen is indispensable but a challenge for splitting H2O. In this work, a series of Co-based metal-organic cages (MOCs) for photoinduced water oxidation were prepared. MOC-1 with both bis(μ-oxo) bridged dicobalt and Co-O (O from H2O) displays catalytic activity with an initial oxygen evolution rate of 80.4 mmol/g/h and a TOF of 7.49 × 10-3 s-1 in 10 min. In contrast, MOC-2 containing only Co-O (O from H2O) in the structure results in a lower oxygen evolution rate (40.8 mmol/g/h, 4.78 × 10-3 s-1), while the amount of oxygen evolved from the solution of MOC-4 without both active sites is undetectable. Isotope experiments with or without H218O as the reactant successfully demonstrate that the molecular oxygen was produced from water oxidation. Photophysical and electrochemical studies reveal that photoinduced water oxidation initializes via electron transfer from the excited [Ru(bpy)3]2+* to Na2S2O8, and then, the cobalt active sites further donate electrons to the oxidized [Ru(bpy)3]3+ to drive water oxidation. This proof-of-concept study indicates that MOCs can work as potential efficient catalysts for photoinduced water oxidation.

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

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

Boosting photocatalytic water oxidation by surface plasmon resonance of AgxAu1−x alloy nanoparticles

Surface plasmon resonance (SPR) induced photocatalysis with multi-metallic nanoparticles stays indistinct, especially in photo-induced water oxidation reaction. Herein, we report a strategy of engineering band structure by noble metal alloying (Ag x Au 1−x ). The modulated composition of AgAu alloy nanoparticles loaded on rutile TiO 2 to selectively tune SPR wavelengths and d-band position. We found that SPR-induced water oxidation activity varies as a function of the ratio of Au and Ag in Ag x Au 1−x /TiO 2 photocatalysts, exhibiting“volcano-like” relationship between the oxygen evolution production rate and the metal-composition. The Ag 0.6 Au 0.4 /TiO 2 photocatalyst shows superior water oxidation performance about 3-folds of individual Ag and Au. Photoemission spectra (PES) of alloy are in correlation with density of states, revealing the sensitivity of d-band onset energies and SPR excitation wavelengths. Furthermore, surface-photovoltage (SPV) response of alloy signifies the superiority of alloy in interfacial charge separation of hot carriers. Density functional theory (DFT) calculations validate the experimental results by revealing complex AgAu alloy-semiconductor (Ti-O-Au-Ag) for oxygen evolution along with electrodynamic simulations to trace distribution of electric-field components. This work demonstrates critical role of d-band tuning across alloy composition for boosting catalytic fraction through the integrated effects of light absorption, Schottky barrier height and d-band position of the plasmonic photocatalysts. In surface plasmon induced photocatalyst, both optical and electronic properties are important for eventual photocatalytic efficiency. For photocatalytic water oxidation reaction, the stability of plasmonic metal is also a critical issue. In this work, an alloyed Ag x Au 1−x /TiO 2 plasmon-induced water oxidation photocatalyst is employed as a model system to study the structure-function relationship. We demonstrated that a three-in-one photocatalyst, i.e. efficient visible-light absorption, suitable electronic structure and high-stability, could be achieved through alloy components adjustment. • Simple chemical approach for compositional tuning of Ag x Au 1−x alloy across SPR wavelengths of electromagnetic spectrum. • Enhancement in plasmon-induced water oxidation activity achieved by tuning of d-band structure and Schottky barrier height. • SPV confirms efficient charge separation across Ag 0.6 Au 0.4 /TiO 2 photocatalyst. • DFT calculation confirms that noble metal as an active may increase photocatalytic OER performance.

State-of-the-art progress in overall water splitting of carbon nitride based photocatalysts

Converting solar energy into hydrogen (H2) by photocatalytic water splitting is a promising approach to simultaneously address the increasing energy demand and environmental issues. Half decade has passed since the discovery of photo-induced water splitting phenomenon on TiO2 photoanode, while the solar to H2 efficiency is still around 1%, far below the least industrial requirement. Therefore, developing efficient photocatalyst with a high energy conversion efficiency is still one of the main tasks to be overcome. Graphitic carbon nitride (g-C3N4) is just such an emerging and potential semiconductor. Therefore, in this review, the state-of-the-art advances in g-C3N4 based photocatalysts for overall water splitting were summarized in three sections according to the strategies used, and future challenges and new directions were discussed.

Wavelength-Dependent Water Oxidation on Rutile TiO2(110).

Understanding the microscopic mechanism of water photocatalysis on TiO2 is of great value in energy chemistry and catalysis. To date, it is still unclear how water photocatalysis occurs after the initial light absorption. Here we report the investigation of the photoinduced water dissociation and desorption on a R-TiO2(110) surface, at different wavelengths (from 250 to 330 nm), using temperature-programmed desorption and time-of-flight techniques. Primary photooxidation products, gas phase OH radicals and surface H atoms, were clearly observed at wavelengths of ≤290 nm. As the laser wavelength decreases from 290 to 250 nm, the relative yield of H2O oxidation increases significantly. Likewise, photoinduced H2O desorption was also observed in the range of 320-250 nm, and the relative yield of H2O desorption also increases with a decrease in wavelength. The strong wavelength-dependent H2O photooxidation and photodesorption suggest that the energy of charge carriers is important in these two processes. More importantly, the result raises doubt about the widely accepted photocatalysis model of TiO2 in which the excess energy of charge carriers is useless for photocatalysis. In addition, the H2O photooxidation is more likely initiated by nonthermalized holes and is accomplished on the ground state potential energy surface via a non-adiabatic decay process.

Molecular Organic Sensitizers for Photoelectrochemical Water Splitting

The photoelectrochemical approach to photoinduced water splitting is gaining increasing interest for solar fuels generation. Dye‐sensitized photoelectrochemical cells (DS‐PEC) have been studied to improve the collection of the Vis range of solar radiation. Metal complexes have so far been the most investigated dyes but in recent years metal‐free organic dyes have emerged as a new frontier for the evolution of the sector. This Minireview systematically describes the use of organic sensitizers and has been organized in terms of dye‐sensitized photoactive electrode(s): photoanode; photocathode; tandem DS‐PEC with photoanode and photocathode. The main classes of dyes are introduced and described in their key properties, device performance, and water splitting data. Thanks to the increasing number of articles, this review highlights the growing importance of organic sensitizers and their molecular design for a more efficient solar generation of clean fuels.

Theory of chemical bonds in metalloenzymes XXIII fundamental principles for the photo-induced water oxidation in oxygen evolving complex of photosystem II

Molecular quantum mechanics (MQM) investigations have been performed for elucidation of fundamental principles of the photo-induced water oxidation in oxygen evolving complex (OEC) of photosystem II (PSII). To this end, as a first theoretical step, broken symmetry (BS) quantum mechanics (QM) and QM(BS)/molecular mechanics (MM) calculations have been conducted for elucidation of geometrical, electronic and spin structures of the CaMn4Ox (X = 5, 6) cluster in the five steps Si (i = 0∼4) of the Kok cycle for water oxidation. The QM and QM/MM calculations have provided full optimised geometries of short-lived key intermediates and transition state structure for the O-O bond formation in the native solar-energy conversion. The interplay between theory and experiment have clearly indicated that the CaMn4O5 cluster in OEC of PSII exhibits typical physicochemical properties of strong correlation electron system (SCES) confined with effective protein field. Our QM and QM/MM computational results for key intermediates and transition structure for the O-O bond formation in the Kok cycle are now plentiful for derivation of fundamental principles (FP) for understanding of photo-induced water oxidation in OEC of PSII. We summarize twenty nine fundamental principles (FPs) in systematic manner by QM and QM/MM calculations for understanding of water oxidation in Oxygen Evolving Complex(OEC) of Photosystem II(PSII). One of our final aims is theoretical design of the next-generation artificial photosystem materials composed of abundant metal ions.

Photo-induced water oxidation via cascade charge transfer on nanostructured BiVO4/TiO2 modified with dye and co-catalyst molecules

A titania-coated BiVO4 photoanode surface-modified with ruthenium dye Rutbipy and molecular water oxidation co-catalyst CuTCPP is fabricated for the first time. The coating of BiVO4 with transparent TiO2 nanoparticle and the subsequent deposition of Rutbipy and CuTCPP onto the TiO2 layer improves the photocurrent at the thermodynamic water oxidation potential (0.82 V vs. NHE pH 7) under 160 mW cm−2 illumination. The controlled-potential experiments under illumination reveal that the photoelectrocatalytic water oxidation activity by the dye and co-catalyst combination increases by two-fold in comparison with the co-catalyst only. A spectral monitoring of BiVO4/TiO2-Rutbipy film following a controlled-potential experiment shows the accumulation of Ru3+ formed via hole transfer from BiVO4 to the titania-bound Rutbipy. Hole transfer from Rutbipy to CuTCPP is also evident from a controlled-potential experiment conducted in the titania-only system. This study provides experimental evidence and energetic grounds for the cascade hole transfer from BiVO4 to Rutbipy and to the eventual CuTCPP, which suppress the electron-hole recombination rate and improve the water oxidation kinetics in the photoanode system.