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1.
Proc Natl Acad Sci U S A ; 117(47): 29462-29468, 2020 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-33172992

RESUMO

Using renewable electricity to synthesize ammonia from nitrogen paves a sustainable route to making value-added chemicals but yet requires further advances in electrocatalyst development and device integration. By engineering both electrocatalyst and electrolyzer to simultaneously regulate chemical kinetics and thermodynamic driving forces of the electrocatalytic nitrogen reduction reaction (ENRR), we report herein stereoconfinement-induced densely populated metal single atoms (Rh, Ru, Co) on graphdiyne (GDY) matrix (formulated as M SA/GDY) and realized a boosted ENRR activity in a pressurized reaction system. Remarkably, under the pressurized environment, the hydrogen evolution reaction of M SA/GDY was effectively suppressed and the desired ENRR activity was strongly amplificated. As a result, the pressurized ENRR activity of Rh SA/GDY at 55 atm exhibited a record-high NH3 formation rate of 74.15 µg h-1⋅cm-2, a Faraday efficiency of 20.36%, and a NH3 partial current of 0.35 mA cm-2 at -0.20 V versus reversible hydrogen electrode, which, respectively, displayed 7.3-, 4.9-, and 9.2-fold enhancements compared with those obtained under ambient conditions. Furthermore, a time-independent ammonia yield rate using purified 15N2 confirmed the concrete ammonia electroproduction. Theoretical calculations reveal that the driving force for the formation of end-on N2* on Rh SA/GDY increased by 9.62 kJ/mol under the pressurized conditions, facilitating the ENRR process. We envisage that the cooperative regulations of catalysts and electrochemical devices open up the possibilities for industrially viable electrochemical ammonia production.

2.
Artigo em Inglês | MEDLINE | ID: mdl-32946193

RESUMO

As a favorite descriptor, the size effect of Cu-based catalysts has been regularly utilized for activity and selectivity regulation toward CO2 /CO electroreduction reactions (CO2 /CORR). However, little progress has been made in regulating the size of Cu nanoclusters at the atomic level. Herein, the size-gradient Cu catalysts from single atoms (SAs) to subnanometric clusters (SCs, 0.5-1 nm) to nanoclusters (NCs, 1-1.5 nm) on graphdiyne matrix are readily prepared via an acetylenic-bond-directed site-trapping approach. Electrocatalytic measurements show a significant size effect in both the activity and selectivity toward CO2 /CORR. Increasing the size of Cu nanoclusters will improve catalytic activity and selectivity toward C2+ productions in CORR. A high C2+ conversion rate of 312 mA cm-2 with the Faradaic efficiency of 91.2 % are achieved at -1.0 V versus reversible hydrogen electrode (RHE) over Cu NCs. The activity/selectivity-size relations provide a clear understanding of mechanisms in the CO2 /CORR at the atomic level.

3.
Phys Chem Chem Phys ; 22(26): 14645-14650, 2020 Jul 08.
Artigo em Inglês | MEDLINE | ID: mdl-32572403

RESUMO

Selective hydrogenation of the C[double bond, length as m-dash]O and C[double bond, length as m-dash]C bonds of acrolein on Pt-M-Pt (M = Pt, Cu, Ni, Co) surfaces has been investigated with first-principles calculations to understand the trends of the activity and selectivity of the reaction. On the pristine Pt(111) surface, the results suggest that the production of allyl alcohol (a product of C[double bond, length as m-dash]O bond hydrogenation) is limited by its desorption, which results in the selective hydrogenation of the C[double bond, length as m-dash]C bond. On the other three bimetallic surfaces, the results show that the desorption of the product is no longer rate-limiting, and the reaction should be selective for the C[double bond, length as m-dash]O bond hydrogenation. Although the calculated trends of activity and selectivity agree well with the experiment, the absolute selectivity predicted on the bimetallic surfaces is in contrast with existing experiments. Therefore, other effects such as the steric effect and reactions at other types of active sites may need to be investigated. On the other hand, the scaling relation analysis shows that the formation free energies of the intermediates, except for H, scale well with that of the adsorbed acrolein. This suggests that modifying the binding of H on the surface may be another dimension for the design of more efficient catalysts for the active and selective hydrogenation of the C[double bond, length as m-dash]O bond of acrolein.

4.
Environ Sci Technol ; 54(13): 8022-8031, 2020 07 07.
Artigo em Inglês | MEDLINE | ID: mdl-32412745

RESUMO

Iron-based nanosized ecomaterials for efficient Cr(VI) removal are of great interest to environmental chemists. Herein, inspired by the "mixed redox-couple" cations involved in the crystal structure and the quantum confinement effects resulting from the particle size, a novel type of iron-based ecomaterial, semiconducting chalcopyrite quantum dots (QDs), was developed and used for Cr(VI) removal. A high removal capacity up to 720 mg/g was achieved under optimal pH conditions, which is superior to those of the state-of-the-art nanomaterials for Cr(VI) removal. The mechanism of Cr(VI) removal was elucidated down to an atomic scale by combining comprehensive characterization techniques with adsorption kinetic experiments and DFT calculations. The experimental results revealed that the material was a good electron donor semiconductor attributed to the existence of "mixed redox couple of Cu(I)-S-Fe(III)" in the crystal structure. With the size-dependent quantum confinement effect and the high surface area, the semiconducting chalcopyrite QDs could effectively remove Cr(VI) from aqueous solution through a syngenetic photocatalytic reduction and adsorption mechanism. This study not only reports the design histogram of the iron-based CuFeS2 QD ecomaterial for efficient Cr(VI) removal but also paves the way for understanding the atomic-scale mechanism behind the syngenetic effects of using the QD semiconducting material for Cr(VI) removal.


Assuntos
Pontos Quânticos , Poluentes Químicos da Água , Adsorção , Cromo/análise , Cobre , Compostos Férricos , Oxirredução , Poluentes Químicos da Água/análise
5.
ChemSusChem ; 13(13): 3524-3529, 2020 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-32274880

RESUMO

Solar-driven conversion of CO2 with H-terminated silicon has recently attracted increasing interest. However, the molecular mechanism of the reaction is still not well understood. A systematic study of the mechanism has been carried out with first-principles calculations. The formation energies of the intermediates are found to be insensitive to the structure of the surface. On the fully H-terminated Si(111) surface, several pathways for the conversion of CO2 into CO at a coordinatively saturated Si site are studied, including the conventional COOH* pathway and the direct insertion of CO2 into Si-H and Si-Si bonds. Although the barrier of the COOH* pathway is lowest among the three pathways, it is higher than that for OH* elimination, which suggests that CO2 should be converted by other types of active site. The reaction at the isolated and dual coordinatively unsaturated (CUS) Si sites, which can be generated by light illumination, heat, and Pd loading, are then studied. The results suggest that the most efficient pathway to convert CO2 is to convert it into CO and O* at an isolated CUS Si site before O* reacts with a terminating H* to form adsorbed OH* and generate new isolated CUS Si sites. Therefore, the CUS Si site catalyzes the reaction until all H* is converted into OH*. The results provide new insight into the mechanism of the reaction and should be helpful for the design of more efficient Si-based catalysts for CO2 conversion.

6.
Inorg Chem ; 59(4): 2379-2386, 2020 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-32009398

RESUMO

Ultrathin two-dimensional (2D) nanosheets with efficient light-driven proton reduction activity were obtained through the exfoliation of novel metal-organic frameworks (MOF), which were synthesized by using a bis(4'-carboxy-2,2':6',2″-terpyridine) ruthenium complex as a linker and 3d transition-metal (Mn, Co, Ni, and Zn) anions as nodes. The nanosheet of the Ni2+ node exhibits a photocatalytic hydrogen evolution rate of 923 ± 40 µmol g-1 h-1 at pH = 4.0, without the presence of any cocatalyst or cosensitizer. A combined experimental and theoretical study suggests a reductive quenched pathway for the photocatalytic hydrogen evolution by the nanosheet. The transition-metal nodes at the edge of the nanosheets are proposed as the active sites. Density functional theory (DFT) calculations attributed the different catalytic activities of the nanosheets to the discrepancy of H adsorption free energy at various transition-metal nodes.

7.
Proc Natl Acad Sci U S A ; 117(23): 12572-12575, 2020 06 09.
Artigo em Inglês | MEDLINE | ID: mdl-31980521

RESUMO

Electrochemical CO reduction can serve as a sequential step in the transformation of CO2 into multicarbon fuels and chemicals. In this study, we provide insights on how to steer selectivity for CO reduction almost exclusively toward a single multicarbon oxygenate by carefully controlling the catalyst composition and its surrounding reaction conditions. Under alkaline reaction conditions, we demonstrate that planar CuAg electrodes can reduce CO to acetaldehyde with over 50% Faradaic efficiency and over 90% selectivity on a carbon basis at a modest electrode potential of -0.536 V vs. the reversible hydrogen electrode. The Faradaic efficiency to acetaldehyde was further enhanced to 70% by increasing the roughness factor of the CuAg electrode. Density functional theory calculations indicate that Ag ad-atoms on Cu weaken the binding energy of the reduced acetaldehyde intermediate and inhibit its further reduction to ethanol, demonstrating that the improved selectivity to acetaldehyde is due to the electronic effect from Ag incorporation. These findings will aid in the design of catalysts that are able to guide complex reaction networks and achieve high selectivity for the desired product.

8.
Nat Nanotechnol ; 15(2): 131-137, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31907442

RESUMO

To date, effective control over the electrochemical reduction of CO2 to multicarbon products (C ≥ 2) has been very challenging. Here, we report a design principle for the creation of a selective yet robust catalytic interface for heterogeneous electrocatalysts in the reduction of CO2 to C2 oxygenates, demonstrated by rational tuning of an assembly of nitrogen-doped nanodiamonds and copper nanoparticles. The catalyst exhibits a Faradaic efficiency of ~63% towards C2 oxygenates at applied potentials of only -0.5 V versus reversible hydrogen electrode. Moreover, this catalyst shows an unprecedented persistent catalytic performance up to 120 h, with steady current and only 19% activity decay. Density functional theory calculations show that CO binding is strengthened at the copper/nanodiamond interface, suppressing CO desorption and promoting C2 production by lowering the apparent barrier for CO dimerization. The inherent compositional and electronic tunability of the catalyst assembly offers an unrivalled degree of control over the catalytic interface, and thereby the reaction energetics and kinetics.

9.
Phys Chem Chem Phys ; 22(3): 1187-1193, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31848529

RESUMO

Photocatalytic synthesis of organic compounds has attracted more and more attention recently. In this work, we present a theoretical study on the molecular mechanism of the photocatalytic reduction of nitrobenzene to aniline on the rutile TiO2(110) surface. We have studied the adsorption and conversion of nitrobenzene at both the surface Ti site and the oxygen vacancy (Ov) site. The full reaction pathways at these two sites were calculated. The rate-limiting step and possible intermediates were identified. The results suggest that Ov is more active in the adsorption and conversion of nitrobenzene. Interestingly, we found that the chemistry of nitrobenzene on the rutile TiO2(110) surface, especially the breaking of the N-O bond, is closely related to the number of excess electrons available. Based on the calculation, we have proposed a full molecular mechanism which is compatible with the existing experiments. The results should be helpful for the design of more efficient photocatalysts for the conversion of nitrobenzene.

10.
Nat Commun ; 10(1): 32, 2019 01 03.
Artigo em Inglês | MEDLINE | ID: mdl-30604776

RESUMO

We present a microkinetic model for CO(2) reduction (CO(2)R) on Cu(211) towards C2 products, based on energetics estimated from an explicit solvent model. We show that the differences in both Tafel slopes and pH dependence for C1 vs C2 activity arise from differences in their multi-step mechanisms. We find the depletion in C2 products observed at high overpotential and high pH to arise from the 2nd order dependence of C-C coupling on CO coverage, which decreases due to competition from the C1 pathway. We further demonstrate that CO(2) reduction at a fixed pH yield similar activities, due to the facile kinetics for CO2 reduction to CO on Cu, which suggests C2 products to be favored for CO2R under alkaline conditions. The mechanistic insights of this work elucidate how reaction conditions can lead to significant enhancements in selectivity and activity towards higher value C2 products.

11.
Environ Sci Technol ; 51(6): 3278-3286, 2017 03 21.
Artigo em Inglês | MEDLINE | ID: mdl-28245121

RESUMO

Interaction of phenol and naphthol with reduced graphene oxide (rGO), and their competitive behavior on rGO were examined by batch experiments, spectroscopic analysis and theoretical calculations. The batch sorption showed that the removal percentage of phenol or naphthol on rGO in bisolute systems was significantly lower than those of phenol or naphthol in single-solute systems. However, the overall sorption capacity of rGO in bisolute system was higher than single-solute system, indicating that the rGO was a very suitable material for the simultaneous elimination of organic pollutants from aqueous solutions. The interaction mechanism was mainly π-π interactions and hydrogen bonds, which was evidenced by FTIR, Raman and theoretical calculation. FTIR and Raman showed that a blue shift of C═C and -OH stretching modes and the enhanced intensity ratios of ID/IG after phenols sorption. The theoretical calculation indicated that the total hydrogen bond numbers, diffusion constant and solvent accessible surface area of naphthol were higher than those of phenol, indicating higher sorption affinity of rGO for naphthol as compared to phenol. These findings were valuable for elucidating the interaction mechanisms between phenols and graphene-based materials, and provided an essential start in simultaneous removal of organics from wastewater.


Assuntos
Grafite/química , Fenol , Adsorção , Naftóis , Óxidos , Fenóis/química
12.
Angew Chem Int Ed Engl ; 56(12): 3289-3293, 2017 03 13.
Artigo em Inglês | MEDLINE | ID: mdl-28194910

RESUMO

Noble-metal-free bimetal-based electrocatalysts have shown high efficiency for water oxidation. Ni and/or Co in these electrocatalysts are essential to provide a conductive, high-surface area and a chemically stable host. However, the necessity of Ni or Co limits the scope of low-cost electrocatalysts. Herein, we report a hierarchical hollow FeV composite, which is Ni- and Co-free and highly efficient for electrocatalytic water oxidation with low overpotential 390 mV (10 mA cm-2 catalytic current density), low Tafel slope of 36.7 mV dec-1 , and a considerable durability. This work provides a novel and efficient catalyst, and greatly expands the scope of low-cost Fe-based electrocatalysts for water splitting without need of Ni or Co.

13.
J Am Chem Soc ; 138(49): 15896-15902, 2016 12 14.
Artigo em Inglês | MEDLINE | ID: mdl-27960337

RESUMO

Photocatalytic reduction of CO2 into organic molecules is a very complicated and important reaction. Two possible pathways, the fast-hydrogenation (FH) path and the fast-deoxygenation (FdO) path, have been proposed on the most popular photocatalyst TiO2. We have carried out first-principles calculations to investigate both pathways on the perfect and defective anatase TiO2(101) surfaces to provide comprehensive understanding of the reaction mechanism. For the FH path, it is found that oxygen vacancy on defective surface can greatly lower the barrier of the deoxygenation processes, which makes it a more active site than the surface Ti. For the FdO path, our calculation suggests that it can not proceed on the perfect surface, nor can it proceed on the defective surface due to their unfavorable energetics. Based on the fact that the FH path can proceed both at the surface Ti site and the oxygen vacancy site, we have proposed a simple mechanism that is compatible with various experiments. It can properly rationalize the selectivity of the reaction and greatly simplify the picture of the reaction. The important role played by oxygen vacancy in the new mechanism is highlighted and a strategy for design of more efficient photocatalysts is proposed accordingly.

14.
ACS Appl Mater Interfaces ; 8(51): 35132-35137, 2016 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-27966849

RESUMO

Efficient water splitting through electrocatalysis has been studied extensively in modern energy devices, while the development of catalysts with activity and stability comparable to those of Pt is still a great challenge. In this work, we successfully developed a facile route to synthesize graphene-like layered carbon (GLC) from a layered silicate template. The obtained GLC has layered structure similar to that of the template and can be used as support to load ultrasmall Ru nanoparticles on it in supercritical water. The specific structure and surface properties of GLC enable Ru nanoparticles to disperse highly uniformly on it even at a large loading amount (62 wt %). When the novel Ru/GLC was used as catalyst on a glass carbon electrode for hydrogen evolution reaction (HER) in a 0.5 M H2SO4 solution, it exhibits an extremely low onset potential of only 3 mV and a small Tafel slope of 46 mV/decade. The outstanding performance proved that Ru/GLC is highly active catalyst for HER, comparable with transition-metal dichalcogenides or selenides. As the price of ruthenium is much lower than platinum, our study shows that Ru/GLC might be a promising candidate as an HER catalyst in future energy applications.

15.
Nat Commun ; 7: 11981, 2016 06 16.
Artigo em Inglês | MEDLINE | ID: mdl-27306541

RESUMO

Highly active and low-cost electrocatalysts for water oxidation are required due to the demands on sustainable solar fuels; however, developing highly efficient catalysts to meet industrial requirements remains a challenge. Herein, we report a monolayer of nickel-vanadium-layered double hydroxide that shows a current density of 27 mA cm(-2) (57 mA cm(-2) after ohmic-drop correction) at an overpotential of 350 mV for water oxidation. Such performance is comparable to those of the best-performing nickel-iron-layered double hydroxides for water oxidation in alkaline media. Mechanistic studies indicate that the nickel-vanadium-layered double hydroxides can provide high intrinsic catalytic activity, mainly due to enhanced conductivity, facile electron transfer and abundant active sites. This work may expand the scope of cost-effective electrocatalysts for water splitting.

16.
Environ Sci Technol ; 50(7): 3658-67, 2016 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-26978487

RESUMO

Graphene oxide (GO) has attracted considerable attention because of its remarkable enhanced adsorption and multifunctional properties. However, the toxic properties of GO nanosheets released into the environment could lead to the instability of biological system. In aqueous phase, GO may interact with fine mineral particles, such as chloridion intercalated nanocrystallined Mg/Al layered double hydroxides (LDH-Cl) and nanocrystallined Mg/Al LDHs (LDH-CO3), which are considered as coagulant molecules for the coagulation and removal of GO from aqueous solutions. Herein the coagulation of GO on LDHs were studied as a function of solution pH, ionic strength, contact time, temperature and coagulant concentration. The presence of LDH-Cl and LDH-CO3 improved the coagulation of GO in solution efficiently, which was mainly attributed to the surface oxygen-containing functional groups of LDH-Cl and LDH-CO3 occupying the binding sites of GO. The coagulation of GO by LDH-Cl and LDH-CO3 was strongly dependent on pH and ionic strength. Results of theoretical DFT calculations indicated that the coagulation of GO on LDHs was energetically favored by electrostatic interactions and hydrogen bonds, which was further evidenced by FTIR and XPS analysis. By integrating the experimental results, it was clear that LDH-Cl could be potentially used as a cost-effective coagulant for the elimination of GO from aqueous solutions, which could efficiently decrease the potential toxicity of GO in the natural environment.


Assuntos
Grafite/química , Nanopartículas Metálicas/química , Poluentes Químicos da Água/química , Purificação da Água/métodos , Adsorção , Hidróxido de Alumínio/química , Grafite/isolamento & purificação , Concentração de Íons de Hidrogênio , Hidróxido de Magnésio/química , Microscopia Eletrônica de Varredura , Óxidos/química , Espectroscopia Fotoeletrônica , Soluções/química , Espectroscopia de Infravermelho com Transformada de Fourier , Poluentes Químicos da Água/isolamento & purificação , Purificação da Água/instrumentação
17.
J Am Chem Soc ; 137(30): 9515-8, 2015 Aug 05.
Artigo em Inglês | MEDLINE | ID: mdl-26186284

RESUMO

Under local plasmonic excitation, Raman images of single molecules can now surprisingly reach subnanometer resolution. However, its physical origin has not been fully understood. Here we report a quantum-mechanical description of the interaction between a molecule and a highly confined plasmonic field. We show that when the spatial distribution of the plasmonic field is comparable to the size of the molecule, the optical transition matrix of the molecule becomes dependent on the position and distribution of the plasmonic field, resulting in a spatially resolved high-resolution Raman image of the molecule. The resonant Raman image reflects the electronic transition density of the molecule. In combination with first-principles calculations, the simulated Raman image of a porphyrin derivative adsorbed on a silver surface nicely reproduces its experimental counterpart. The present theory provides the basic framework for describing linear and nonlinear responses of molecules under highly confined plasmonic fields.


Assuntos
Nanoestruturas/química , Teoria Quântica , Modelos Moleculares , Análise Espectral Raman
18.
Phys Chem Chem Phys ; 15(13): 4625-33, 2013 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-23423429

RESUMO

Local structures and adsorption energies of a formic acid molecule and its decomposed intermediates (H, O, OH, CO, HCOO, and COOH) on highly electrocatalytically active mushroom-like Au-core@Pd-shell@Pt-cluster nanoparticles with two atomic layers of the Pd shell and stoichiometric Pt coverage of around half-monolayer (Au@2 ML Pd@0.5 ML Pt) have been investigated by first principles calculations. The adsorption sites at the center (far away from the Pt cluster) and the edge (close to the Pt cluster) are considered and compared. Significant repulsive interaction between the edge sites and CO is observed. The calculated potential energy surfaces demonstrate that, with respect to the center sites, the CO2 pathway is considerably promoted in the edge area. Our results reveal that the unique edge structure of the Pt cluster is responsible for the experimentally observed high electrocatalytic activity of the Au@Pd@Pt nanoparticles toward formic acid oxidation. Such microscopic understanding should be useful for the design of new electrochemical catalysts.


Assuntos
Formiatos/química , Ouro/química , Nanopartículas Metálicas/química , Paládio/química , Platina/química , Teoria Quântica , Adsorção , Catálise
19.
ACS Nano ; 6(8): 7066-76, 2012 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-22793258

RESUMO

It has been a long-term desire to fabricate hybrid silicon-molecular devices by taking advantages of organic molecules and the existing silicon-based technology. However, one of the challenging tasks is to design applicable functions on the basis of the intrinsic properties of the molecules, as well as the silicon substrates. Here we demonstrate a silicon-molecular system that produces negative differential resistance (NDR) by making use of the well-defined intrinsic surface-states of the Si (111)-√3 × âˆš3-Ag (R3-Ag/Si) surface and the molecular orbital of cobalt(II)-phthalocyanine (CoPc) molecules. From our experimental results obtained using scanning tunneling microscopy/spectroscopy, we find that NDR robustly appears at the Co(2+) ion centers of the CoPc molecules, independent of the adsorption configuration of the CoPc molecules and irrespective of doping type and doping concentration of the silicon substrates. Joint with first principle calculations, we conclude that NDR is originated from the resonance between the intrinsic surface-state band S(1) of the R3-Ag/Si surface and the localized unoccupied Co(2+)d(z(2)) orbital of the adsorbed CoPc molecules. We expect that such a mechanism can be generally used in other silicon-molecular systems.


Assuntos
Modelos Químicos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Silício/química , Simulação por Computador , Impedância Elétrica , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
20.
J Am Chem Soc ; 134(24): 9978-85, 2012 Jun 20.
Artigo em Inglês | MEDLINE | ID: mdl-22658233

RESUMO

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.

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