Identifying the Role of Photogenerated Holes in Photocatalytic Methanol Dissociation on Rutile TiO2(110)

Chlorine-functionalized black phosphorus quantum dots induced superoxide anion generation and depletion for efficient chemiluminescence detection

BACKGROUND

Due to their unique optoelectronic properties, environmental friendliness, and excellent biocompatibility, metal-free quantum dots have been a new star in exploring novel chemiluminescence (CL) systems for analytical applications in recent years. However, unknown CL property, relatively weak emission and instability of some of them in water (eg. black phosphorus) often seriously hindered their further applications. Hence, developing a novel QDs-assist CL signal amplification to achieve efficient analyst detection is significant and currently hot topic for researchers.

RESULTS

In this work, we purposely synthesized chlorine-functionalized black phosphorus quantum dots (Cl-BPQDs) with improved stability and rich-hole property, which were demonstrated to exhibit the excellent capability for the activation of ferrate (VI) with large reactive oxygen species generation and leading to enhanced CL signal. The detail mechanism was demonstrated, the unique CL response to the presence of active sites (P-Cl) in Cl-BPQDs, which accelerated ferrate (VI) decomposition and produced a large amount of superoxide anion (•O2-). And then, the radiative recombination of the exogenous electron-donated and existing holes Cl-BPQDs accounting for the strong CL emission. Furthermore, based on the consumption capacity of ascorbic acid (AA) and glutathione (GSH) for •O2-, a direct CL sensing platform of Cl-BPQDs/ferrate (VI) quenching was fabricated to AA and GSH detection. This fabricated assay has broad detection linear ranges (2-200 μM) and low detection limit (GSH: 1.3 μM; AA: 1.7 μM). Compared with the reported CL technique, this new method displayed superior sensitivity and anti-interference capabilities toward transition-metal ions and inorganic anions. The potential analytical application of the new CL system was further demonstrated by the evaluation of total antioxidant capacity (TAC) in diabetic patients.

SIGNIFICANCE

This study proposes a new strategy for enhancing CL signal via Cl-BPQDs triggering •O2- generation and depletion, which provides an innovative tool for ascorbic acid and glutathione detection. This method not only enriches our understanding of the optical characteristics of BP, but also provides a new charge transfer-based path for CL amplification.

Overlooked Role of Photogenerated Holes in Persistent Free Radical Formation on Hematite

Persistent free radicals (PFRs) have garnered considerable attention due to their long lifetime and high reactivity. However, the roles of photogenerated carriers in PFR formation remain underexplored. We compared and analyzed the PFR formation on hematite-SiO2 loaded catechol, combining experimental and theoretical investigations. Significant PFRs were observed only under ultraviolet light irradiation. The PFR concentration on hematite nanoplates (HP, 1.29 × 1017 spins/mg) was higher than those on hematite nanocubes (HC, 9.19 × 1016 spins/mg) and nanorods (HR, 7.02 × 1016 spins/mg). A stronger stability of PFRs on HR (183 h of t1/e) was observed compared with HP (95.4 h of t1/e) and HC (37.7 h of t1/e). Photoelectrochemical analysis and quenching experiments indicated that photogenerated holes, rather than electrons, controlled the PFR formation. Photogenerated holes manipulate the asymmetric distribution of up-spin and down-spin electrons in the p orbital of catechol to regulate PFR formation. Hole quantity and exposed facets caused significant differences in the concentration and stability of PFRs. The high concentration of PFRs on HP is due to abundant holes, while the weak stability of PFRs on HC is due to the exposed {012} facet. This study introduces a novel mechanism for PFR formation regulated by photogenerated holes, contributing to a better understanding of their environmental function and associated risks.

Insight into photocatalytic CO2 reduction on TiO2-supported Cu nanorods: a DFT study on the reaction mechanism and selectivity

Photoreduction of CO2 into hydrocarbons is a potential strategy for reducing atmospheric CO2 and effectively utilizing carbon resources. Cu-deposited TiO2 photocatalysts stand out in this area due to their good photocatalytic activity and potential methanol selectivity. However, the underlying mechanism and factors controlling product selectivity remain less understood. Using first-principles calculations, this study systematically investigates the possible reaction network for CO2 photocatalytic reduction on TiO2 supported Cu-nanorods (nr-Cu/TiO2), driven by the surface-bound *H species generated via a Volmer-like process (H+ + e- + * → *H). Our results reveal that the initial hydrogenation of CO2 on nr-Cu/TiO2 is energetically more favorable via the formate (HCOO) pathway than the carboxyl (COOH) route. Notably, HCOO undergoes further hydrogenation for effective C-O bond cleavage, with H2COOH identified as the key intermediate. Both CO (CO2 → HCOO → H2COOH → H2CO → CO) and CH3OH (CO2 → HCOO → H2COOH → H2CO → CH3OH) production share the H2CO intermediate, with CO formation proceeding via an unexpected "forth-back" mechanism. Energy profiles suggest that CH3OH formation is more favorable than CO formation. Additionally, excess photogenerated electrons were found to enhance CO2 activation and C-O bond cleavage to some extent but have minimal impact on other reaction steps. This study provides atomic-level insights into the CO2 photoreduction mechanism, offering potential guidance for improving product selectivity.

Adsorption Structure-Activity Correlation in the Photocatalytic Chemistry of Methanol and Water on TiO2(110)

ConspectusPhotocatalysis, a process involving light absorption (band gap excitation), charge separation, interfacial charge transfer, and surface redox reactions, has attracted intensive attention because of the potential applications in solar to fuel conversion. Despite the great efforts devoted to the design of materials and optimization of charge separation and overall efficiency, the molecular mechanism of photocatalytic reactions, for example, water oxidation, is still unclear, mainly because of the complexity of powder catalysts and the aqueous environment which prevent the direct experimental detection of adsorption sites, surface species, and charge/energy transfer dynamics. Without direct evidence, the charge transfer and elementary reaction steps remain elusive, and misleading conclusions are sometimes drawn. For instance, the positively charged 5-fold coordinated Ti sites (Ti5cs) on TiO2 surfaces are argued to propel holes and therefore cannot be active sites for oxidative reactions, regardless of the demonstration by scanning tunneling microscopy (STM). Direct site-specific measurements are thus highly demanded. Surface science studies, which rely on well-defined single crystals and ultrahigh vacuum based techniques, can identify the active sites and active species at the catalyst surfaces and measure the interfacial electronic structure and energy of desorbing species for charge transfer analysis, providing direct evidence for investigating the photocatalytic reaction mechanism at the molecular level.In this Account, the elementary photocatalytic chemistry of methanol and water on TiO2, which are investigated by surface science techniques such as atom-resolved STM, ensemble-averaged mass spectrometer based temperature-programmed desorption/time-of-flight spectroscopy, and photoelectron spectroscopy in combination with theoretical calculations, will be described. Both methanol and water can be photocatalytically oxidized at Ti5cs, producing adsorbed formaldehyde and gaseous •OH radicals, respectively, under ultraviolet (UV) light irradiation. The photocatalytic activity shows salient adsorption structure including adsorption site (terminal/bridging), adsorption state (molecular/dissociative) and adsorption configuration (monomer/cluster) dependence, which comes from the ability to generate terminal anions which are capable of capturing photogenerated holes and exhibit superior photocatalytic activity over their parent molecules. These studies reveal the origin of the correlation between photocatalytic activity and adsorption structure of CH3OH and H2O on TiO2 surfaces and suggest that the simple criteria widely used to analyze the feasibility of charge transfer, i.e., the relative position of the band edges and the molecular orbitals of adsorbates, should be replaced by the change of Gibbs free energy of the charge trapping reaction from the thermodynamic point of view. These results contribute to the fundamental understanding of photocatalysis. Based on our research, future state-resolved and time-resolved studies can provide deeper insight into the charge and energy transfer and transient intermediate species, which will benefit the depiction of the overall photocatalytic reactions, for example, the photocatalyzed oxygen evolution reaction from water.

Specific Photocatalytic C-C Coupling of Benzyl Alcohol to Deoxybenzoin or Benzoin by Precise Control of Cα-H Bond Activation or O-H Bond Activation by Adjusting the Adsorption Orientation of Hydrobenzoin Intermediates

Benzyl alcohol (BA) is a major biomass derivative and can be further converted into deoxybenzoin (DOB) and benzoin (BZ) as high-value products for industrial applications through photocatalytic C-C coupling reaction. The photocatalytic process contains two reaction steps, which are (1) the C-C coupling of BA to hydrobenzoin (HB) intermediates and (2) either dehydration of HB to DOB or dehydrogenation of HB to BZ. We found that generation of DOB or BZ is mainly determined by the activation of Cα-H or O-H bonds in HB. In this study, phase junction CdS photocatalysts and Ni/CdS photocatalysts were elaborately designed to precisely control the activation of Cα-H or O-H bonds in HB by adjusting the adsorption orientation of HB on the photocatalyst surfaces. After orienting the Cα-H groups in HB on the CdS surfaces, the Cα-H bond dissociation energy (BDE) at 1.39 eV is lower than the BDE of the O-H bond at 2.69 eV, therefore improving the selectivity of the DOB. Conversely, on Ni/CdS photocatalysts, the O-H groups in HB orient toward the photocatalyst surfaces. The BDE of the O-H bonds is 1.11 eV to form BZ, which is lower than the BDE of the Cα-H bonds to the DOB (1.33 eV), thereby enhancing the selectivity of BZ. As a result, CdS photocatalysts can achieve complete conversion of BA to 80.4% of the DOB after 9 h of visible light irradiation, while 0.3% Ni/CdS photocatalysts promote complete conversion of BA to 81.5% of BZ after only 5 h. This work provides a promising strategy in selective conversion of BA to either DOB or BZ through delicate design of photocatalysts.

Stretching vibration driven adiabatic transfer kinetics for photoexcited hole transfer from semiconductor to adsorbate

Interfacial hole transfer from a photoexcited semiconductor to surface adsorbates is pivotal for initiating solar-to-chemical energy conversion, yet the atomic-level transfer kinetics remains elusive. Using the methoxy/TiO2(110) system as an archetype, here we elucidate the hole transfer mechanism from hole-trapping lattice oxygen to the methoxy adsorbate at gas/solid and liquid/solid interfaces through molecular dynamics simulations and static minimum energy path calculations. Instead of direct nonadiabatic hopping, we uncover an adiabatic migration pathway adapted to local substrate relaxation, driven by a bond-stretching mechanism supported by stronger Ti-O stretching vibrations. Notably, this mechanism persists at the aqueous methoxy/TiO2(110) interface, albeit hindered by interfacial water and coadsorbates. Surprisingly, the hole transfer barriers across various photoexcited adsorbate/TiO2 interfaces correlate more closely with the vertical excitation energies of the adsorbates rather than their redox potentials, indicating an early-type transition-state nature. These insights deepen our understanding of elementary hole transfer kinetics in surface photochemistry.

Bismuth niobate/g-C3N4 heterojunction for maximised visible light photocatalytic removal of Bisphenol A

The occurrence of xenobiotic pollutants in the aquatic environment troubling the present and future generation. Persistent Organic Pollutants (POPs) is one such class of xenobiotic that was dominant in that category. In the present paper, a competent visible light driven heterojunction photocatalyst combining Bismuth niobate and g-C3N4 was developed for the effective removal of Bisphenol A (BPA), a notable POP. Before constructing the heterostructure the calcination temperature for bismuth niobate synthesis was optimised for achieving most proficient photocatalysis. A phase change in the crystal structure of bismuth niobate was apparent. The Bi3NbO7 at 300-500 °C transformed to Bi5Nb3O15 at 600-700 °C and to orthorhombic BiNbO4 at 900 °C as the temperature was enhanced. With the increment in the temperature the light absorbance of the materials enhanced in UV and reduced in visible light. Thus, the bismuth niobate obtained by calcining at 500 °C demonstrated highest BPA removal under sunlight was chosen for heterojunction construction. After the heterojunction construction with g-C3N4 the crystal lattice strain was observed to be reduced for all composites, and a greater mobility of charge carriers was observed within the composite. The presence of either of the materials resulted in a different band structure and thus Type II and Z-scheme pathway was inferred. A commendable photocatalytic activity was observed for B1.5G and BG1.5 under sunlight and LED light respectively. Hight amount of g-C3N4 in the BG1.5 resulted in maximum absorbance in LED light. Superoxide radicals (*O2-) radicals were observed as major radicals for B1.5G composite, whereas both *O2- and holes (h+) were the major radicals in case of BG1.5.

Catalytic mechanisms for As(III) oxidation by H2O2 over TiO2 surfaces, and effects of support, vacancy and photoirradiation

As(III) is much more toxic than As(V) while shows apparently lower affinity at minerals surfaces. Oxidation of As(III) to As(V) by H2O2 over anatase surface provides an attractive avenue for pollution control, and the chemocatalytic and photocatalytic mechanisms are unraveled by means of the DFT + D3 approach. Impacts of anatase as support, O2c/O3c vacancy, photoirradiation are addressed as well. As(III) oxidation under various reaction conditions leads to As(V) through dual electron transfers, while energy barriers differ substantially and decline as 1.80 (direct oxidation) > 1.35 (anatase as support) > 1.24 (O3c vacancy) > 0.50 (chemocatalysis) > 0.28 (photocatalysis) ≥ 0.26 (O2c vacancy) eV. Anatase as support promotes the reaction through bonding with H2O2/As(OH)3 and electron transfers, and its close participation during chemocatalysis produces the TiOOH active site that causes As(III) oxidation to proceed facilely under ambient circumstances. TiOOH exists in two forms (monodentate and bidentate mononuclear) and is critical for chemocatalysis, while its destruction for O3c vacancy exhibits strongly adverse effects to As(III) oxidation. Photoirradiation readily generates the OH• radicals, and corresponding mechanism is plausible while less preferred than the newly posed mechanism based on the Ti(H2O2) active site. Synergism among a number of surface atoms conduces to the superior activity for O2c vacancy and photocatalysis. Results provide a comprehensive understanding for As(III) oxidation to As(V) by H2O2, and facilitate catalysts design for As(III) oxidation that alleviates environmental pollution.

How Doping Regulates As(III) Adsorption at TiO2 Surfaces: A DFT + U Study

The efficient adsorption and removal of As(III), which is highly toxic, remains difficult. TiO2 shows promise in this field, though the process needs improvement. Herein, how doping regulates As(OH)3 adsorption over TiO2 surfaces is comprehensively investigated by means of the DFT + D3 approach. Doping creates the bidentate mononuclear (Ce doping at the Ti5c site), tridentate (N, S doping at the O2c site), and other new adsorption structures. The extent of structural perturbation correlates with the atomic radius when doping the Ti site (Ce >> Fe, Mn, V >> B), while it correlates with the likelihood of forming more bonds when doping the O site (N > S > F). Doping the O2c, O3c rather than the Ti5c site is more effective in enhancing As(OH)3 adsorption and also causes more structural perturbation and diversity. Similar to the scenario of pristine surfaces, the bidentate binuclear complexes with two Ti-OAs bonds are often the most preferred, except for B doping at the Ti5c site, S doping at the O2c site, and B doping at the O3c site of rutile (110) and Ce, B doping at the Ti5c site, N, S doping at the O2c site, and N, S, B doping at the O3c site of anatase (101). Doping significantly regulates the As(OH)3 adsorption efficacy, and the adsorption energies reach -4.17, -4.13, and -4.67 eV for Mn doping at the Ti5c site and N doping at the O2c and O3c sites of rutile (110) and -1.99, -2.29, and -2.24 eV for Ce doping at the Ti5c site and N doping at the O2c and O3c sites of anatase (101), respectively. As(OH)3 adsorption and removal are crystal-dependent and become apparently more efficient for rutile vs. anatase, whether doped at the Ti5c, O2c, or O3c site. The auto-oxidation of As(III) occurs when the As centers interact directly with the TiO2 surface, and this occurs more frequently for rutile rather than anatase. The multidentate adsorption of As(OH)3 causes electron back-donation and As(V) re-reduction to As(IV). The regulatory effects of doping during As(III) adsorption and the critical roles played by crystal control are further unraveled at the molecular level. Significant insights are provided for As(III) pollution management via the adsorption and rational design of efficient scavengers.

Insight into mechanism for remarkable photocatalytic hydrogen evolution of Cu/Pr dual atom co-modified TiO2

The development of high-activity photocatalysts is crucial for the current large-scale development of photocatalytic hydrogen applications. Herein, we have developed a strategy to significantly enhance the hydrogen photocatalytic activity of Cu/Pr di-atom co-modified TiO2 architectures by selectively anchoring Cu single atoms on the oxygen vacancies of the TiO2 surface and replacing a trace of Ti atoms in the bulk with rare earth Pr atoms. Calculation results demonstrated that the synergistic effect between Cu single atoms and Pr atoms regulates the electronic structure of Cu/Pr-TiO2, thus promoting the separation of photogenerated carriers and their directional migration to Cu single atoms for the photocatalytic reaction. Furthermore, the d-band center of Cu/Pr-TiO2, which is located at -4.70 eV, optimizes the adsorption and desorption behavior of H*. Compared to TiO2, Pr-TiO2, and Cu/TiO2, Cu/Pr-TiO2 displays the best H* adsorption Gibbs free energy (-0.047 eV). Furthermore, experimental results confirmed that the photogenerated carrier lifetime of Cu/Pr-TiO2 is not only the longest (2.45 ns), but its hydrogen production rate (34.90 mmol g-1 h-1) also significantly surpasses those of Cu/TiO2 (13.39 mmol g-1 h-1) and Pr-TiO2 (0.89 mmol g-1 h-1). These findings open up a novel atomic perspective for the development of optimal hydrogen activity in dual-atom-modified TiO2 photocatalysts.

Low-Temperature Oxidation of Methane on Rutile TiO2(110): Identifying the Role of Surface Oxygen Species

Understanding the microkinetic mechanism underlying photocatalytic oxidative methane (CH4) conversion is of significant importance for the successful design of efficient catalysts. Herein, CH4 photooxidation has been systematically investigated on oxidized rutile(R)-TiO2(110) at 60 K. Under 355 nm irradiation, the C-H bond activation of CH4 is accomplished by the hole-trapped dangling OTi- center rather than the hole-trapped Ob- center via the Eley-Rideal reaction pathway, producing movable CH3• radicals. Subsequently, movable CH3• radicals encounter an O/OH species to form CH3O/CH3OH species, which could further dissociate into CH2O under irradiation. However, the majority of the CH3• radical intermediate is ejected into a vacuum, which may induce radical-mediated reactions under ambient conditions. The result not only advances our knowledge about inert C-H bond activation but also provides a deep insight into the mechanism of photocatalytic CH4 conversion, which will be helpful for the successful design of efficient catalysts.

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.

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.

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.

Enhanced immobilization of Arsenic(III) and Auto-oxidation to Arsenic(V) by titanium oxide (TiO2), due to Single-Atom vacancies and oxyanion formation

Pollution control of As(III), a naturally occurring carcinogen, has recently gained a global attention, while due to the dominance of neutral H3AsO3 over a wide pH range, As(III) immobilization by most minerals is not efficient as As(V) immobilization. TiO2 shows promise for controlling As(III) pollution, and herein, a comprehensive study about As(III) adsorption by TiO2 and oxyanion formation is conducted by means of DFT + D3 methods. Both anatase and rutile are effective for As(III) adsorption, while As(III) adsorption affinities differ significantly and are -1.48 and -3.79 eV for pristine surfaces, ascend to -3.85 and -5.08 eV for O vacancies, and further to -5.37 and -5.26 eV for Ti vacancies, respectively. The bidentate binuclear complexes dominate for pristine surfaces, and O vacancies prefer OAs insertion into TiO2 lattice, while for Ti vacancies, all As(III) centers are auto-oxidized to As(V). Ti-3d, O-2p or/and As-4p rather than other orbitals contribute significantly to As adsorption, and O and Ti vacancies promote adsorption through stronger orbital hybridization. The superior adsorption for Ti vacancies originates from As(V) formation instead of bonding interactions. The formation of As oxyanions, which may occur spontaneously at pristine surfaces and is greatly promoted by O and Ti vacancies, enhances As(III) adsorption pronouncedly and becomes a viable strategy for As(III) immobilization. H2AsO3- and HAsO32- dominate for pristine surfaces and O vacancies, and for Ti vacancies, H2AsO4- and HAsO42- dominate over anatase whereas AsO43- also makes an important contribution over rutile. Results rationalize experimental observations available, and provide significantly new insights about the migration, bioavailability and fate of As(III) over TiO2 surfaces that facilitate the exploration of scavengers for As and other pollutants.

Tuning the water-splitting mechanism on titanium dioxide surfaces through hydroxylation

Experimental research demonstrates that surface hydroxyl groups can boost TiO₂'s ability to split water but the water splitting mechanism and roles of hydroxyl groups are still not clear. The hydroxyl groups formed by H₂O or H₂ cracking on pure TiO₂ surfaces are represented by types I (OH1) and II (OH2), respectively. Six types of hydroxylated TiO₂ surfaces of anatase (101), rutile (110), and brookite (210) with OH1 and OH2 hydroxyl groups were constructed. The mechanism of the water oxidation process on the hydroxylated TiO₂ surfaces was systematically investigated through density functional theory calculations. The variation and significant roles of hydroxyl groups in the mechanism of the oxygen evolution reaction (OER) and product selectivity were discussed. All hydroxylated TiO₂ surfaces eventually tend to produce oxygen through a four-electron/proton process, which is fundamentally different from the OER process on pure Ti₂O surfaces from a thermodynamic standpoint. The lowest surface overpotential of R-110-OH1 is 0.53 V, the highest surface overpotential of B-210-OH2 is 1.49 V, and the surface overpotentials of other hydroxylated TiO₂ are between 0.5 and 1.5 V. Rutile (110) and brookite (210) have hydroxyl groups of the OH1-type that are more conducive to the OER process. This study investigates the mechanism of water splitting on the surface of hydroxylated TiO₂, allowing for a deeper understanding of the function of surface hydroxyl groups in the OER process as well as providing instructions for future research into the development of effective water-splitting catalysts based on hydroxylated TiO₂ surfaces.

Precisely Constructed Metal Sulfides with Localized Single-Atom Rhodium for Photocatalytic C-H Activation and Direct Methanol Coupling to Ethylene Glycol

Although there are many studies on photocatalytic environmental remediation, hydrogen evolution, and chemical transformations, less success has been achieved for the synthesis of industrially important and largely demanded bulk chemicals using semiconductor photocatalysis, which holds great potential to drive unique chemical reactions that are difficult to implement by the conventional heterogeneous catalysis. The performance of semiconductors used for photochemical synthesis is, however, usually unsatisfactory due to limited efficiencies in light harvesting, charge-carrier separation, and surface reactions. The precise construction of heterogeneous photocatalysts to facilitate these processes is an attractive but challenging goal. Here, single-atom rhodium-doped metal sulfide nanorods composed of alternately stacked wurtzite/zinc-blende segments are successfully designed and fabricated, which demonstrate record-breaking efficiencies for visible light-driven preferential activation of C-H bond in methanol to form ethylene glycol (EG), a key bulk chemical used for the production of polyethylene terephthalate (PET) polymer. The wurtzite/zinc-blende heterojunctions lined regularly in one dimension accelerate the charge-carrier separation and migration. Single-atom rhodium selectively deposited onto the wurtzite segment with photogenerated holes accumulated facilitates methanol adsorption and C-H activation. The present work paves the way to harnessing photocatalysis for bulk chemical synthesis with structure-defined semiconductors.

Photocatalytic Oxidative Dehydrogenation of Propane for Selective Propene Production with TiO2

Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C₃H₆) with lower energy consumption in chemical industry. However, the selectivity of the C₃H₆ product is always poor because of overoxidation. Herein, the ODHP reaction into C₃H₆ on a model rutile(R)-TiO₂(110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C₃H₈) into C₃H₆ can occur efficiently on R-TiO₂(110) at 90 K via a stepwise manner, in which the initial C-H cleavage occurs via the hole coupled C-H bond cleavage pathway followed by a radical mediated C-H cleavage to the C₃H₆ product. An exceptional selectivity of ∼90% for C₃H₆ production is achieved at about 13% propane conversion. The mechanistic model constructed in this study not only advances our understanding of C-H bond activation but also provides a new pathway for highly selective ODHP into C₃H₆ under mild conditions.

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.

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.

Hydration of TiO2 Facets Regulates As(III) Adsorption: DFT and DRIFTS Study

Hydration of TiO₂ facets controls the reactions occurring at the mineral-water interfaces. However, the underlying mechanism of the facet-dependent hydration and the effect of hydration on contaminant adsorption are still ambiguous. Herein, arsenite [As(III)] adsorption on hydrated {001}, {100}, {101}, and {201} TiO₂ was explored by integrating multiple characterizations and density functional theory (DFT) calculations. Our macroscopic adsorption results show an As(III) adsorption density order of {201} > {100} > {101} > {001}, though As(III) on each facet formed a bidentate binuclear structure, as evidenced by the extended X-ray absorption fine structure analysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy analysis identified distinctive surface hydroxyls on four-faceted TiO₂ upon water adsorption. The hydrated surface regulated the subsequent As(III) adsorption, giving an As(III) adsorption energy order of {201} (-0.95 eV) < {100} (-0.38 eV) < {101} (-0.005 eV) < {001} (0.04 eV) according to DFT calculations. The As(III) adsorption energy on hydrated facets was linearly correlated with the macroscopical As(III) adsorption density (R² = 0.99, p < 0.05), revealing that the impregnable water binding highly suppressed the exchange of As(III) molecules with adsorbed water. Our study provided a novel insight into the facet-dependent interfacial adsorption.

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...

Cooperative Motion in Water-Methanol Clusters Controls the Reaction Rates of Heterogeneous Photocatalytic Reactions

Detailed information about the influences of the cooperative motion of water and methanol molecules on practical solid-liquid heterogeneous photocatalysis reactions is critical for our understanding of photocatalytic reactions. The present work addresses this issue by applying operando nuclear magnetic resonance (NMR) spectroscopy, in conjunction with density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, to investigate the dynamic behaviors of heterogeneous photocatalytic systems with different molar ratios of water to methanol on rutile-TiO₂ photocatalyst. The results demonstrate that methanol and water molecules are involved in the cooperative motions, and the cooperation often takes the form of methanol-water clusters that govern the number of methanol molecules reaching to the active sites of the photocatalyst per unit time, as confirmed by the diffusion coefficients of the methanol molecule calculated in the binary methanol-water solutions. Nuclear Overhauser effect spectroscopy experiments reveal that the clusters are formed by the hydrogen bonding between the -OH groups of CH₃OH and H₂O. The formation of such methanol-water clusters is likely from an energetic standpoint in low-concentration methanol, which eventually determines the yields of methanol reforming products.

Light-Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

Splitting of alcohols into hydrogen and corresponding carbonyl compounds, also called acceptorless alcohol dehydrogenation, is of great significance for both synthetic chemistry and hydrogen production. Light-Driven Alcohol Splitting (LDAS) by heterogeneous photocatalysis is a promising route to achieve such transformations, and it possesses advantages including high selectivity of the carbonyl compounds, extremely mild reaction conditions (room temperature and irradiation of visible light) and easy separation of the photocatalysts from the reaction mixtures. Because a variety of alcohols can be derived from biomass, LDAS can also be regarded as one of the most sustainable approaches for hydrogen production. In this Review, recent advances in the LDAS catalyzed by the heterogeneous photocatalysts are summarized, focusing on the mechanistic insights for the LDAS and aspects that influence the performance of the photocatalysts from viewpoints of metallic co-catalysts, semiconductors, and metal/semiconductor interfaces. In addition, challenges and prospects have been discussed in order to present a complete picture of this field.

Unraveling the Photogenerated Electron Localization on the Defect-Free CH3NH3PbI3(001) Surfaces: Understanding and Implications from a First-Principles Study

The localization of photogenerated electrons in photovoltaic and photocatalytic materials is crucial for reducing the electron-hole recombination rate. Here, the photogenerated electron localization is systematically investigated on the CH₃NH₃PbI₃ (MAPbI₃) perovskite using first-principles calculations. It is found that under vacuum conditions, the photogenerated electron is delocalized in the MAPbI₃ bulk as well as on the stochiometric MAPbI₃(001) surface with the CH₃NH₃I (MAI) termination, while it is trapped on the defect-free PbI₂-terminated surface. Our ab initio molecular dynamics simulations reveal that the introduction of solutions will prompt the formation of localized electronic states. The photogenerated electron is discovered to be localized on both the MAI- and PbI₂-terminated surfaces in the presence of solutions with different concentrations of HI, from pure water to the saturated solution. We demonstrate that the Pb-I bond weakening or breaking resulting in an unsaturated coordination of a Pb site is the prerequisite to trap the photogenerated electron.

Solvent Water Controls Photocatalytic Methanol Reforming

Understanding the role of different solvent molecules for practical solid-liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reaction. In this study, the operando nuclear magnetic resonance (NMR) method, combined with density functional theory (DFT) calculations, was employed to evaluate the control effect of solvent water in the photocatalytic reforming mechanism of methanol with a Pt-TiO₂ catalyst. Results indicate that the presence of water effectively promotes the formation of the HCHO intermediate but inhibits the H₂ evolution originating from the switch of the hydrogen source of the H₂ formation from CH₃OH to H₂O. More interestingly, as detected directly in the ab initio molecular dynamics simulation, a small amount of H₂O can dissociate, and the evolved -OH species at Ti_5c site can greatly reduce the C-H activation barrier of -CH₃O, contributing to the formation of oxidation products (e.g., HOCH₂OH and CH₃OCH₂OH) on the Pt-TiO₂ surface.

Theoretical study of kinetics of proton coupled electron transfer in photocatalysis

Photocatalysis induced by sunlight is one of the most promising approaches to environmental protection, solar energy conversion, and sustainable production of fuels. The computational modeling of photocatalysis is a rapidly expanding field that requires to adapt and to further develop the available theoretical tools. The coupled transfer of protons and electrons is an important reaction during photocatalysis. In this work, we present the first step of our methodology development in which we apply the existing kinetic theory of such coupled transfer to a model system, namely, methanol photodissociation on the rutile TiO₂(110) surface, with the help of high-level first-principles calculations. Moreover, we adapt the Stuchebrukhov-Hammes-Schiffer kinetic theory, where we use the Georgievskii-Stuchebrukhova vibronic coupling to calculate the rate constant of the proton coupled electron transfer reaction for a particular pathway. In particular, we propose a modified expression to calculate the rate constant, which enforces the near-resonance condition for the vibrational wave function during proton tunneling.

Identifying the role of excess electrons and holes for initiating the photocatalytic dissociation of methanol on a TiO2(110) surface

As a prototype for the catalytic oxidation of organic contaminants, photocatalytic methanol dissociation on rutile TiO₂(110) has drawn much attention, but its reaction mechanism remains elusive.

Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges

Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO₂ photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO₂ photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO₂ photocatalysis at a molecular level is highlighted. Insights into the structures of TiO₂ and the basic principles of TiO₂ photocatalysis are discussed first, which provides the basic concepts of TiO₂ photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO₂ surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO₂ photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO₂ photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.

Visible light-driven C-H activation and C-C coupling of methanol into ethylene glycol

The development of new methods for the direct transformation of methanol into two or multi-carbon compounds via controlled carbon–carbon coupling is a highly attractive but challenging goal. Here, we report the first visible-light-driven dehydrogenative coupling of methanol into ethylene glycol, an important chemical currently produced from petroleum. Ethylene glycol is formed with 90% selectivity and high efficiency, together with hydrogen over a molybdenum disulfide nanofoam-modified cadmium sulfide nanorod catalyst. Mechanistic studies reveal a preferential activation of C−H bond instead of O−H bond in methanol by photoexcited holes on CdS via a concerted proton–electron transfer mechanism, forming a hydroxymethyl radical (⋅CH₂OH) that can readily desorb from catalyst surfaces for subsequent coupling. This work not only offers an alternative nonpetroleum route for the synthesis of EG but also presents a unique visible-light-driven catalytic C−H activation with the hydroxyl group in the same molecule keeping intact.

Direct transformation of methanol into two- or multi-carbon compounds is extremely attractive but remains a challenge. Here, the authors report an efficient photocatalytic route to the transformation of methanol into ethylene glycol and hydrogen over a molybdenum disulfide nanofoam-modified cadmium sulfide nanorod catalyst.

Unique Trapped Dimer State of the Photogenerated Hole in Hybrid Orthorhombic CH3NH3PbI3 Perovskite: Identification, Origin, and Implications

Revealing the innate character and transport of the photogenerated hole is essential to boost the high photovoltaic performance in the lead-based organohalide perovskite. However, knowledge at the atomic level is currently very limited. In this work, we systematically investigate the properties of the photogenerated hole in the orthorhombic CH₃NH₃PbI₃ using hybrid functional PBE0 calculations with spin-orbit coupling included. An unexpected trapping state of the hole, localized as I₂^- (I dimer), is uncovered, which was never reported in photovoltaic materials. It is shown that this localized configuration is energetically more favorable than that of the delocalized hole state by 191 meV and that it can highly promote the diffusion of the hole with an energy barrier as low as 131 meV. Furthermore, the origin of I dimer formation upon trapping of the hole is rationalized in terms of electronic and geometric effects, and a good linear correlation is found between the hole trapping capacity and the accompanying structural deformation in CH₃NH₃PbX₃ (X = Cl, Br, and I). It is demonstrated that good CH₃NH₃PbX₃ materials for the hole diffusion should have small structural deformation energy and weak hole trapping capacity, which may facilitate the rational screening of superior photovoltaic perovskites.

Toward a Comprehensive Understanding of Enhanced Photocatalytic Activity of the Bimetallic PdAu/TiO2 Catalyst for Selective Oxidation of Methanol to Methyl Formate

Photocatalytic selective oxidation of alcohols over titania supported with bimetallic nanoparticles represents an energy efficient and sustainable route for the synthesis of esters. Specifically, the bimetallic PdAu/TiO₂ system was found to be highly active and selective toward photocatalytic production of methyl formate (MF) from gas-phase methanol. In the current paper, we applied the electronic structure density functional theory method to understand the mechanistic aspects and corroborate our recent experimental measurements for the photocatalytic selective oxidation of methanol to MF over the PdAu/TiO₂ catalyst. Our theoretical results revealed the preferential segregation of Pd atoms from initially mixed PdAu nanoclusters to the interface of PdAu/TiO₂ and subsequent formation of a unique structure, resembling a core@shell architecture in close proximity to the interface. The analysis of the calculated band gap diagram provides an explanation of the superior electron-hole separation capability of PdAu nanoparticles deposited onto the anatase surface and hence the remarkably enhanced photocatalytic activity, in comparison to their monometallic counterparts. We demonstrated that facile dissociation of molecular oxygen at the triple-point boundary site gives rise to in situ oxidation of Pd. The in situ formed PdO/TiO₂ is responsible for total oxidation of methanol to CO₂ (no MF formation) in the gas phase. Our investigation provides theoretical guidance for designing highly selective and active bimetallic nanoparticles-TiO₂ catalysts for the photocatalytic selective oxidation of methanol to MF.