Role of Adsorbed Water on Charge Carrier Dynamics in Photoexcited TiO2

Direct Operando Identification of Reactive Electron Species Driving Photocatalytic Hydrogen Evolution on Metal-Loaded Oxides

Performing operando spectroscopy under practical reaction conditions and extracting spectral components correlating with reaction activity are crucial in elucidating the reactive species in photocatalysis. However, the observation of weak signals corresponding to reactive photogenerated species is frequently hampered under reaction conditions owing to intense background signals originating from thermally induced species unrelated to the photoinduced reactions. Herein, by synchronizing the millisecond periodic excitations of photocatalysts with a Michelson interferometer used for FT-IR spectroscopy, we succeeded in significantly suppressing the signals derived from thermally excited electrons and observing the reactive photogenerated electrons contributing to the photocatalytic hydrogen evolution. This demonstration was achieved for metal-loaded oxide photocatalysts under steam methane reforming and water splitting conditions. Although it has long been conventionally believed that loaded metal cocatalysts function as sinks for reactive photogenerated electrons and active sites for reduction reactions, we found that the free electrons in the metal cocatalysts were not directly involved in the reduction reaction. Alternatively, the electrons shallowly trapped in the in-gap states of oxides contributed to enhancing the hydrogen evolution rate upon the loading of metal cocatalysts. We verified that the electron abundance in the in-gap states was clearly correlated to the reaction activity, suggesting that metal-induced semiconductor surface states formed in the periphery of the metal cocatalyst play key roles in the photocatalytic hydrogen evolution. Our microscopic insights shift a paradigm on the traditionally believed role of metal cocatalysts in photocatalysis and provide a fundamental basis for rational design of the metal/oxide interfaces as promising platforms for nonthermal hydrogen evolution.

Distinct Site Motifs Activate O2 and H2 on Supported Au Nanoparticles in Liquid Water

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H2 and sequential reduction of O2-derived intermediates during the formation of H2O2 and H2O. Rates of H2 consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol-1). Selectivities for H2O2 formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H2 activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H2O aids the activation of H2 at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O2 preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H2 and O2 upon Au nanoparticle catalysts within dilute vapor pressures of water (10-4 to 0.1 kPa H2O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H2O) expands catalysis to all surface Au atoms and enables appreciable H2O2 formation.

Probing plasmon-induced surface reactions using two-dimensional correlation vibrational spectroscopy

Surface plasmon resonance (SPR) has the ability to drive catalytic conversion of the reactant molecules via the production of hot electrons, which in general requires high activation energy. The reactions driven by these hot electrons are critical and essential in various heterogeneous surface catalytic reactions. However, there is a need to understand the dynamics of surface reactions and the underlying mechanism, which are influenced by several factors such as the constitution of the nanoparticle, exposure time, and reaction conditions to name a few. However, the effect of solvent in stabilizing the electron-hole pair, the orientation, and the surface coverage of the analyte are poorly understood due to the limitations of current methods. To get deeper insights into the reaction dynamics, we have demonstrated the combined utility of plasmon-enhanced Raman spectroscopy and Two-dimensional correlation spectroscopy (2DCOS) to study the plasmon-driven conversion of 4-nitrothiophenol on the surface of plasmonic nanoparticles. Interestingly, this combined technique provided us with previously unobservable results regarding surface catalysis by conventional spectroscopic analysis alone. Specifically, for the first time, 2DCOS provided critical insights in bridging the gap in our understanding of the interplay of solvent effect, orientation, and surface packing of the analyte molecules. It was observed that certain species like 4,4-dimercaptoazobenzene (DMAB) or 4-aminothiophenol (4-ATP) can be selectively formed based on the ordered or disordered phases of the analytes on the surface, thus paving the way to precisely control light-driven reactions through operando spectroscopy.

Elucidating the role of adsorption during artificial photosynthesis: H2O and CO2 adsorption isotherms over TiO2 reveal thermal effects under UV illumination

Adsorption measurements of CO₂ and H₂O over TiO₂ surfaces in dark and under illumination were carried out to reveal the ensuing bottlenecks of the initial steps of the artificial photosynthesis reaction. When the adsorption isotherms of both CO₂ and H₂O were measured under illumination, the results were comparable to isotherms measured at higher temperatures in dark. This evidence is interpreted as the presence of hot spots, due to charge carrier recombination reactions. Differential heat of adsorption measurements revealed that H₂O adsorption on TiO₂ is stronger, and with a higher coverage than that of CO₂. Dissociation of water is an energetically uphill reaction, and the local hot spots due to charge carrier recombination in indirect bandgap semiconductors can enhance the reaction probability. At higher temperatures, higher reaction probabilities are expected and estimated by a thermodynamic analysis for water splitting reaction. The potential role of these hot spots during natural and artificial photosynthetic reactions is discussed.

Electrocatalytic oxidation of antidiabetic drug metformin adsorbed on intercalated MXene

Two-dimensional (2D) Ti₃C₂T_x transition metal carbide (MXene) nanosheets intercalated with sodium ions (SI-Ti₃C₂T_x MXene) were used in the adsorption and electrochemical regeneration process for removal of the antidiabetic drug metformin (MF) as a model emerging pollutant. After MF adsorption, SI-Ti₃C₂T_x MXene oxidized the MF on its surface through its electrocatalytic activity at very low current density and cell potential. For complete oxidation the optimum parameters were 0.525 C g^-1, 0.005 mA cm^-2, and pH 6 in absence of NaCl or 26.25 C g^-1 and 0.5 mA cm^-2 in the presence of 2.5 w/v% NaCl. The overall regeneration of SI-Ti₃C₂T_x is governed by a combined mechanism, i.e., desorption followed by degradation. The degradation mechanism, such as direct electron transfer or indirect oxidation, depends on the applied operating conditions. Thus, the investigation suggests that these 2D sheets are good nanoadsorbents as well as good electrocatalysts and proves their usefulness in practical water-treatment applications.

Quantum Anharmonic Calculations of Vibrational Spectra for Water Adsorbed on Titania Anatase(101) Surface: Dissociative versus Molecular Adsorption

The interaction of water molecules and hydroxyl groups with titanium dioxide (TiO₂) surfaces is ubiquitous and very important in anatase nanoparticle photocatalytic processes. Infrared spectroscopy, assisted by ab initio calculations of vibrational frequencies, can be a powerful tool to elucidate the mechanisms behind water adsorption. However, a straightforward comparison between measurements and calculations remains a challenging task because of the complexity of the physical phenomena occurring on nanoparticle surfaces. Consequently, severe computational approximations, such as harmonic vibrational ones, are usually employed. In the present work we partially address this complexity issue by overcoming some of the standard approximations used in theoretical simulations and employ the Divide and Conquer Semiclassical Initial Value Representation (DC-SCIVR) molecular dynamics. This method allows to perform simulations of vibrational spectra of large dimensional systems accounting not only for anharmonicities, but also for nuclear quantum effects. We apply this computational method to water and deuterated water adsorbed on the ideal TiO₂ anatase(101) surface, contemplating both the molecular and the dissociated adsorption processes. The results highlight not only the presence of an anharmonic shift of the frequencies in agreement with the experiments, but also complex quantum mechanical spectral signatures induced by the coupling of molecular vibrational modes with the surface ones, which are different in the hydrogenated case from the deuterated one. These couplings are further analyzed by exploiting the mode subdivision performed during the divide and conquer procedure.

Photocatalytic hydrogen evolution over cyanine-sensitized Ag/TiO2

Sensitization of TiO2 by dyes such as cyanine and their derivatives is used as a technique to improve potency for the production of hydrogen gas as an alternative green fuel. These dyes shift the spectrum of TiO2 from the UV region to the visible region, enabling it to harvest as much sunlight as possible. Herein, four different derivatives of cyanine (labelled C1, C2, C3, and C4) were prepared and doped in Ag/TiO2 via the impregnation method. The properties of the prepared photocatalysts were studied by XRD, SEM-EDS, FTIR, and UV-visible spectroscopy. The sensitized photocatalysts exhibited a similar morphology, nanoscale particle size, and good absorbance in the visible region. The rate constant for the photocatalytic activity of Ag/TiO2 showed a great enhancement for hydrogen evolution after sensitization from 0.088 to 0.33 μmol min-1. Doping of the C2 derivative in Ag/TiO2 promoted the photocatalytic and sonophotocatalytic rates of H2 production by 7.5 and 9 times, respectively. Also, the amount of photocatalyst had a significant effect on the photocatalytic activity of the sensitized Ag/TiO2, where 0.14 g was the optimum dose, giving the maximum yield at both the initial rate and 300 min. One of the important factors causing the efficiency to reach high levels is the inhibition of photogenerated electron/hole recombination. This was achieved by adding a small quantity of methanol, which increased the rate by 9 times. The stability of the prepared photocatalysts was tested, which gave good results even after their 5th use. All the results confirmed that the sensitization of metal oxides is a promising solution in industry to produce clean energy (H2) in high quantities over highly stable photocatalysts.

Aerosol-Assisted Deposition for TiO2 Immobilization on Photocatalytic Fibrous Filters for VOC Degradation

Atomization and spraying are well-established methods for the production of submicrometer- and micrometer- sized powders. In addition, they could be of interest to the immobilization of photocatalytic nanoparticles onto supports because they enable the formation of microporous films with photocatalytic activity. Here, we provide a comparison of aerosol-assisted immobilization methods, such as spray-drying (SD), spray atomization (SA), and spray gun (SG), which were used for the deposition of TiO₂ dispersions onto fibrous filter media. The morphology, microstructure, and electronic properties of the structures with deposited TiO₂ were characterized by SEM and TEM, BET and USAXS, and UV-Vis spectrometry, respectively. The photocatalytic performances of the functionalized filters were evaluated and compared to the benchmark dip-coating method. Our results showed that the SG and SA immobilization methods led to the best photocatalytic and operational performance for the degradation of toluene, whereas the SD method showed the lowest degradation efficiency and poor stability of coating. We demonstrated that TiO₂ sprays using the SG and SA methods with direct deposition onto filter media involving dispersed colloidal droplets revealed to be promising alternatives to the dip-coating method owing to the ability to uniformly cover the filter fibers. In addition, the SA method allowed for fast and simple control of the coating thickness as the dispersed particles were continuously directed onto the filter media without the need for repetitive coatings, which is common for the SG and dip-coating methods. Our study highlighted the importance of the proper immobilization method for the efficient photocatalytic degradation of VOCs.

VK. One – Pot Hydrothermal Preparation and Defects Enhanced Photocatalytic Activity of Bi doped CdWO4 Nanospheres

In the field of photocatalysis, the suppression of electron-hole recombination through various defects has been an emerging trend to enhance the photocatalytic activity. The separation efficiency of electron-hole recombination of...

In situ investigation of water on MXene interfaces

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K+) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li+) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li+) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K+) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

The Effect of Photoinduced Surface Oxygen Vacancies on the Charge Carrier Dynamics in TiO2 Films

Metal-oxide semiconductors (MOS) are widely utilized for catalytic and photocatalytic applications in which the dynamics of charged carriers (e.g., electrons, holes) play important roles. Under operation conditions, photoinduced surface oxygen vacancies (PI-SOV) can greatly impact the dynamics of charge carriers. However, current knowledge regarding the effect of PI-SOV on the dynamics of hole migration in MOS films, such as titanium dioxide, is solely based upon volume-averaged measurements and/or vacuum conditions. This limits the basic understanding of hole-vacancy interactions, as they are not capable of revealing time-resolved variations during operation. Here, we measured the effect of PI-SOV on the dynamics of hole migration using time-resolved atomic force microscopy. Our findings demonstrate that the time constant associated with hole migration is strongly affected by PI-SOV, in a reversible manner. These results will nucleate an insightful understanding of the physics of hole dynamics and thus enable emerging technologies, facilitated by engineering hole-vacancy interactions.

A Review of Recent Developments in Molecular Dynamics Simulations of the Photoelectrochemical Water Splitting Process

In this review, we provide a short overview of the Molecular Dynamics (MD) method and how it can be used to model the water splitting process in photoelectrochemical hydrogen production. We cover classical non-reactive and reactive MD techniques as well as multiscale extensions combining classical MD with quantum chemical and continuum methods. Selected examples of MD investigations of various aqueous semiconductor interfaces with a special focus on TiO2 are discussed. Finally, we identify gaps in the current state-of-the-art where further developments will be needed for better utilization of MD techniques in the field of water splitting.

Selectivity in the Ligand Functionalization of Photocatalytic Metal Oxide Nanoparticles for Phase Transfer and Self-Assembly Applications

The functionalization of photocatalytic metal oxide nanoparticles of TiO₂ , ZnO, WO₃ and CuO with amine-terminated (oleylamine) and thiol-terminated (dodecane-1-thiol) alkyl-chain ligands was studied under ambient conditions. A high selectivity was observed in the binding specificity of a ligand towards nanoparticles of these different oxides. It was observed that oleylamine binds stably to only TiO₂ and WO₃ , whereas dodecane-1-thiol binds stably only to ZnO and CuO. Similarly, polar-to-nonpolar solvent phase transfer of TiO₂ and WO₃ nanoparticles could be achieved by using oleylamine, but not dodecane-1-thiol, whereas the opposite holds for ZnO and CuO. The surface chemistry of ligand-functionalized nanoparticles was probed by attenuated total reflectance (ATR)-FTIR spectroscopy, which enabled the occupation of the ligands at the active sites to be elucidated. The photostability of the ligands on the nanoparticle surface was determined by the photocatalytic self-cleaning properties of the material. Although TiO₂ and WO₃ degrade the ligands within 24 h under both UV and visible light, ligands on ZnO and CuO remain unaffected. The gathered insights are also highly relevant from an application point of view. As an example, because the ligand-functionalized nanoparticles are hydrophobic in nature, they can be self-assembled at the air-water interface to give nanoparticle films with demonstrated photocatalytic as well as anti-fogging properties.

Hydrogen Production through Glycerol Photoreforming on TiO2/Mesoporous Carbon: Influence of the Synthetic Method

This article explores the effect of the synthetic method of titanium dioxide (TiO₂)/C composites (physical mixture and the water-assisted/unassisted sol-gel method) on their photocatalytic activity for hydrogen production through glycerol photoreforming. The article demonstrates that, apart from a high surface area of carbon and the previous activation of its surface to favor titania incorporation, the appropriate control of titania formation is crucial. In this sense, even though the amount of incorporated titania was limited by the saturation of carbon surface groups (in our case, ca. 10 wt.% TiO₂), the sol-gel process without water addition seemed to be the best method, ensuring the formation of small homogeneously-distributed anatase crystals on mesoporous carbon. In this way, a ca. 110-fold increase in catalyst activity compared to Evonik P25 (expressed as hydrogen micromole per grams of titania) was achieved.

Accelerated decomposition of the fungicide, iprodione, on TiO2 surface under solar irradiation: experimental study and DFT mechanisms

In the present work we have studied photo-induced decomposition of iprodione on silica support with different additions of titanium dioxide. Both the experimental and theoretical (DFT) approaches have been applied. It was found that 16 hours visible light exposure of the samples with 0.1% and 1.0% of TiO₂ leads respectively to 48.28% and 21.05% of residual amounts of iprodione in these samples. A number of intermediates and end products were identified by means of GS-MS and LC-MS chromatography. The iprodione isomer (RP 30228) and its decay product 1-(3,5-dichlorophenyl)-5-isopropyl biuret (RP 36221) were identified among them. Our DFT calculations have revealed the detailed mechanisms of formation of the above products and the mechanism of accelerated proton-induced decomposition of iprodione molecules adsorbed on the TiO₂ surface. Also, the intra-molecular reasons for iprodione stability in acidic media were clarified together with the mechanism of hydantoin cycle opening under the action of hydroxyl anions.

Improving Glycerol Photoreforming Hydrogen Production Over Ag2O-TiO2 Catalysts by Enhanced Colloidal Dispersion Stability

Solar-driven photocatalytic reforming of biomass-derived resources for hydrogen production offers a sustainable route toward the generation of clean and renewable fuels. However, the dispersion stability of the catalyst particles in the aqueous phase hinders the efficiency of hydrogen production. In this work, a novel method of mixing Ag2O-TiO2 photocatalysts with different morphologies was implemented to promote colloidal dispersion stability, thereby improving hydrogen production performance. A series of Ag2O-TiO2 nanoparticles with different morphologies were synthesized, and their dispersion stabilities in aqueous phase were investigated individually. Two types of Ag2O-TiO2 particles with different morphologies under certain proportions were mixed and suspended in glycerol aqueous solution without adding any dispersant for enhancing dispersion stability while reacting. From the results, photocatalytic hydrogen production was found to be strongly correlated to colloidal dispersion stability. The mixed suspension of Ag2O-TiO2 nanosphere and nanoplate achieved an excellent colloidal dispersion stability without employing any additives or external energy input, and the photoreforming hydrogen production obtained from this binary component system was around 1.1-2.3 times higher than that of the single-component system. From the calculated hydrogen production rate constants between continuous stirring and the binary system, there was only <6% difference, suggesting an efficient mass transfer of the binary system for photoreforming hydrogen production. The proposed method could provide some inspiration to a more energy-efficient heterogeneous catalytic hydrogen production process.

Synergetic effects of defects and acid sites of 2D-ZnO photocatalysts on the photocatalytic performance

Regulation of defects and surface acidic sites of photocatalysts is an efficient approach to improve the photocatalytic activity. Ultrathin 2D-ZnO photocatalysts were prepared to uncover the synergetic effects of defects and surface acidic sites on the photocatalytic activity. The reaction constant for photocatalytic degradation of MB upon ZnO-S is 2.26, 2.82, 12.2 times higher than that of SH-500, SO-500, and ZnO-R, respectively. The results revealed that the surface defects, hydroxyl group and chemisorbed water played pivotal roles in the generation of reactive oxygen species (ROS). Although the limited improvement of visible absorption was achieved after introduction of oxygen vacancy (V_O), the overall photocatalytic activity decreased due to the reduced ROS production capacity shown by density functional theory (DFT) calculations. Hydroxyl radical is the key ROS in degradation of organics, and electron contributes a little bigger than hole in the generation of hydroxyl radical. Importantly, the decrease in surface acidic sites resulted in the decreased photocatalytic activity, proven by the dynamics of photoinduced carriers. This study reveals that the improved photocatalytic activity of 2D-ZnO photocatalysts can be attributed to the synergetic effects of surface defects and acidic sites rather than the enhanced visible absorption resulted from the V_O introduction.

Alkaline treatment as a means to boost the activity of TiO2 in selective photocatalytic processes

In this work, the activity enhancement of TiO₂ photocatalysts by alkaline treatment has been investigated.

Initial Approximation to the Design and Construction of a Photocatalysis Reactor for Phenol Degradation with TiO2 Nanoparticles

A photoreactor was designed, built, and optimized to carry out the degradation of phenol. To achieve this, phenol concentration was used as the reference to compare the photocatalysis reaction efficiency obtained through this research with results from other studies. Additionally, during the building process, different types of glass were evaluated with the objective of finding a functional and economic material to build the photoreactor. It was found that Pyrex glass was the most suitable material to work with. As a UV light source to build the photoreactor, a dry gel nail lamp was used with 9 W, λ = 365 nm bulbs. On the other hand, the effects of different parameters (such as the catalyst mass (TiO₂ Degussa P-25), stirring speed (RPM), UV lamps, and temperature) over the photocatalysis reaction rate were analyzed. Also, the reaction's thermodynamic parameters were determined and found to be similar to those found in other investigations. Finally, the homogeneity in the distribution of TiO₂ particles inside the reactor when stirred at 475 rpm was verified using a COMSOL Multiphysics computer fluid dynamics simulation, which showed the theoretical trajectory of particles inside the reactor depending on the stirring rate of the reactor.

Wettability Recovery Behavior Governed by Desorption of Hydroxyl Species in Steel

The radiation induced surface activation (RISA) effect which occurs at the bilayer of metal and oxide due to irradiation contributes to improving the materials surface wettability and enhancing electrothermal characteristics and corrosion resistance. The purpose of the present study is to reveal the correlation between the wettability recovery behavior and the microstructural features of the adsorbed water and/or hydroxyl species on the surfaces. The wettability change due to the RISA effect was investigated in the oxidized austenitic stainless steel with various holding times under darkness after UV irradiation. The microstructure features of the adsorbed water and/or hydroxyl species on the surfaces were characterized by Raman spectroscopy and scanning probe microscopy (SPM). Results show that the contact angle of the specimen after UV irradiation parabolically increased with increasing holding time, regardless of the chemical composition within these two steels. The absorbed water layer was observed in both specimens by Raman analysis, and its intensity decreased with increasing holding time. From the force-distance curves of SPM analysis, the hydrophilic component was rapidly decreased at a holding time below 200 h, then gradually saturated when the holding time increased above 200 h from the SPM analysis. These results reveal that the wettability recovery behavior in oxidized austenitic stainless steels after UV irradiation can be ascribed to two kinds of mechanisms. One is the desorption of the hydrophilic components which are predominant when the contact angle is less than 30°, and the other is the absorption of the hydrophobic components which works mainly when the contact angle is above 30°.

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising carbon-free approach to produce hydrogen from water. A photoelectrochemical cell consists of a semiconductor photoelectrode in contact with an aqueous electrolyte. Its performance is sensitive to properties of the photoelectrode/electrolyte interface, which may be tuned through functionalization of the photoelectrode surface with organic molecules. This can lead to improvements in the photoelectrode's properties. This Minireview summarizes key computational investigations on using molecular functionalization to modify photoelectrode stability, barrier height, and catalytic activity. It is discussed how first-principles density functional theory, first-principles molecular dynamics, and device modeling simulations can provide predictive insights and complement experimental investigations of functionalized photoelectrodes. Challenges and future directions in the computational modeling of functionalized photoelectrode/electrolyte interfaces within the context of experimental studies are also highlighted.

A robust ALD-protected silicon-based hybrid photoelectrode for hydrogen evolution under aqueous conditions

Hybrid systems combining molecular catalysts with inorganic materials is a promising solution towards cheap yet efficient and stable photoelectrochemical hydrogen production.

Hydrogen production through direct sunlight-driven water splitting in photo-electrochemical cells (PECs) is a promising solution for energy sourcing. PECs need to fulfill three criteria: sustainability, cost-effectiveness and stability. Here we report an efficient and stable photocathode platform for H₂ evolution based on Earth-abundant elements. A p-type silicon surface was protected by atomic layer deposition (ALD) with a 15 nm TiO₂ layer, on top of which a 300 nm mesoporous TiO₂ layer was spin-coated. The cobalt diimine–dioxime molecular catalyst was covalently grafted onto TiO₂ through phosphonate anchors and an additional 0.2 nm ALD-TiO₂ layer was applied for stabilization. This assembly catalyzes water reduction into H₂ in phosphate buffer (pH 7) with an onset potential of +0.47 V vs. RHE. The resulting current density is –1.3 ± 0.1 mA cm^–2 at 0 V vs. RHE under AM 1.5 solar irradiation, corresponding to a turnover number of 260 per hour of operation and a turnover frequency of 0.071 s^–1.

Influence of Surface-Related Phenomena on Mechanism, Selectivity, and Conversion of TiO2 -Induced Photocatalytic Reactions

Heterogeneous photocatalysis is the result of an inextricable connection of several factors differently contributing to the overall process. Photon absorption is the "sine qua non" condition for the reaction to occur. In fact, photons can be considered as immaterial reactants, and all of the phenomena related to the interaction of light-matter play a prominent role. However, other factors contribute in a concerted way to address the reaction, so that the relative contribution of each of them is often difficult to evaluate. In this framework, the present paper highlights some aspects of the interaction of TiO₂ surface-adsorbate species that could be underestimated and their influence on the conversion, selectivity, and mechanisms of photocatalytic reactions. To this aim, some paradigmatic examples on the adsorption of water and organics on TiO₂ are reported.

Water-Assisted Hole Trapping at the Highly Curved Surface of Nano-TiO2 Photocatalyst

Heterogeneous photocatalysis is vital in solving energy and environmental issues that this society is confronted with. Although photocatalysts are often operated in the presence of water, it has not been yet clarified how the interaction with water itself affects charge dynamics in photocatalysts. Using water-coverage-controlled steady and transient infrared absorption spectroscopy and large-model (∼800 atoms) ab initio calculations, we clarify that water enhances hole trapping at the surface of TiO2 nanospheres but not of well-faceted nanoparticles. This water-assisted effect unique to the nanospheres originates from water adsorption as a ligand at a low-coordinated Ti-OH site or through robust hydrogen bonding directly to the terminal OH at the highly curved nanosphere surface. Thus, the interaction with water at the surface of nanospheres can promote photocatalytic reactions of both oxidation and reduction by elongating photogenerated carrier lifetimes. This morphology-dependent water-assisted effect provides a novel and rational basis for designing and engineering nanophotocatalyst morphology to improve photocatalytic performances.

Role of Dissociatively Adsorbed Water on the Formation of Shallow Trapped Electrons in TiO2 Photocatalysts

The mismatch between short lifetimes of free charge carriers and slow kinetics of surface redox reactions substantially limits the efficiency of most photocatalytic systems. Hence, the knowledge of trapping and recombination of photogenerated electrons and holes at different time scales is key for a rational optimization of photocatalytic materials. In this study, we used subsecond time-resolved diffuse-reflectance FTIR spectroscopy to investigate how energy and intensity of the incident irradiation affect the dynamics of photogenerated charge carriers in TiO₂ P25 photocatalysts subjected to different pretreatments and how shallow trapped electrons (STE) are formed under these conditions. Intensity-dependent measurements demonstrated that electrons and holes generated by 325 and 409 nm irradiation undergo bimolecular and trap-assisted recombination, respectively. Analysis of characteristic times of photogenerated electron absorption rise and decay indicated that the apparent charge carrier dynamics at the time scale of seconds to minutes relate to chemical trapping of photogenerated electrons and holes. The presence of dissociatively adsorbed water on the oxide surface was required for efficient STE formation. This suggests that STE form at the seconds-minutes time scale upon surface-mediated self-trapping of electrons.