In Situ Photoconductivity Kinetic Study of Nano-TiO2 during the

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In-situ Photoconductivity Kinetic Study of Nano-TiO during the Photocatalytic Oxidation of Formic Acid: The Effects of New Recombination and Current-Doubling Jingjing Yang, Baoshun Liu, Huan Xie, Xiujian Zhao, Chiaki Terashima, Akira Fujishima, and Kazuya Nakata J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06534 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on September 3, 2015

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The Journal of Physical Chemistry

In-situ Photoconductivity Kinetic Study of NanoTiO2 during the Photocatalytic Oxidation of Formic Acid: The Effects of New Recombination and Current-doubling

Jingjing Yang, aBaoshun liu,*a Huan Xie,a Xiujian Zhao, a Chiaki Terashima,bc Akira Fujishimab and Kazuya Nakata*bc a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology,

Wuhan City, Hubei province 430070, P. R. China b

Research Institute for Science and Technology, Energy and Environment Photocatalyst

Research Division, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. c

Department of Applied Biological Science, Faculty of Science and Technology, Tokyo

University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

ACS Paragon Plus Environment

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ABSTRACT: In the present research, the in-situ photoconductivity (σ) was used to study the electron kinetic of nano-TiO2 films during the photocatalysis of formic acid under UV light irradiation. Some interesting features on the in-situ σ were observed: (a) when the light was turned on, the in-situ σ shows a relatively slow decrease just after the fast increase; (b) when the light was stopped, the in-situ σ decays much faster than that in pure water; (c) the in-situ σ presents an abnormal increase when decaying to dark value, due to the re-injection of electrons to TiO2 CB. We comprehensively studied the effects of formic acid amounts, UV light intensity, UV light irradiation time, and dark pre-adsorption time on the in-situ σ, indicating the presence of the new recombination and the current-doubling effect. It was seen that the new recombination and the current-doubling effect can be weakened by soft water-washing, and the presence of water also is important for the appearance of the new recombination and the currentdoubling effect. Combining with the first-principle calculation, it was confirmed that the weaklyadsorbed formic acid groups near TiO2/water interface should mainly contribute to the new recombination and the current-doubling effect. A kinetic model was proposed to simulate the time dependence of the in-situ σ during the formic acid photocatalysis. The simulation shows that the inclusion of the new recombination and the current-doubling effect well accords to the experimental results. Lastly, the effects of Au deposition on the in-situ σ of TiO2 film during the photocatalysis of formic acid were studied. The interfacial transfer of electrons from TiO2 to Au can be identified by the in-situ σ, which wakens the new recombination and the current-doubling effect.


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INTRODUCTION The pioneer work of Fujishima and Honda

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opens up an era for using solar energy to make

new energy and remove pollutants, which draws much attention world widely for several tens of years. 2-6 Among many kinds of photocatalysts, TiO2 is one of the most important due to its nontoxicity, chemical stability, cheapness, and high photocatalytic activity. In recent years, under the help of nano-technologies, many highly-active nano-TiO2 materials can be prepared, such as the hierarchical nano-structures, 7-11 the nanotube arrays, 12-15 the nanorod arrays, 16-19 the long nanofibers,20 the nanobelts, 21 and the TiO2 nano-crystals with exposed high-energy facets. 22-25 In addition to the preparation, the studies of photocatalytic kinetic are also important for nanoTiO2 materials. Apparently, many photocatalytic oxidations (PO) of organics accord to the quasifirst order mode, which can be described by the classical Langmuir–Hinshelwood (L-H) model. 26

From the microscopic viewpoint, great efforts have been taken to study the kinetics of holes,

as the hole kinetics are directly related to the PO of organics. Due to large specific surface area and many surface defects, the hole kinetics of nano-TiO2 photocatalysis show some special features. Instead of the L-H model, basing on organic-TiO2 interaction, Salvador et al. proposed a direct-indirect (D-I) hole interfacial transfer model to fit the kinetic of PO of organics.

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If

organics can be specifically adsorbed on nano-TiO2 surface, holes can directly transfer to organics from valence band (VB) to organics, which is described by the inelastic interfacial transfer model.

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If organics cannot be adsorbed specifically, holes can be firstly captured by

hydroxyl groups (OH-) or bridged oxygen (2c-O), and then transfer to organics dissolved in water solution. This indirect hole kinetic follows the elastic isoenergetic interfacial transfer model, and can be described by Gerischer-Marcus model.

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It is worthwhile to say that the

indirect interfacial transfer, i.e., holes are captured by OH- on nano-TiO2 surface, leading to the production of hydroxyl free groups (•OH) and the further oxidization of organics, is the widelyaccepted microscopic mechanism.

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The capturing of holes by OH- is fast, and the organic

oxidations by •OH are also fast as most of them belong to single-hole process. 31 However, long lifetime holes are needed for PO of water because it belongs to four-hole process, so the trapped holes should be more important than the free holes, as indicated by other researches. 32, 33 In principle, the photocatalysis is a common result of electrons and holes, so the kinetic behaviors of electrons are the same crucial for PO of organics. However, the investigations on electron kinetics are relatively less in photocatalysis of nano-TiO2. Until now, the main


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knowledge is from the studies of dye-sensitized solar cells (DSSCs), which can be used for reference for studying electron kinetics in photocatalysis. As compared to bulk TiO2, the electron kinetics in nano-TiO2 is different due to the existence of huge amount of defects. It was shown that electron transport in nano-TiO2 follows multi-trapping (MT) mode due to defect modulation, which is 4-6 factors slower than that in bulk TiO2.

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For interfacial transfer, electrons can be

firstly trapped at surface defects, and then transfer to dissolved oxygen or other species, in addition to the direct interfacial transfer through conduction band (CB) states. 35 Which one, the trap states and the CB states, dominate electron interfacial transfer is decided by electron quasiFermi level (EF), as indicated by Bisquert.

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Electron recombination is also affected by defects.

In general, holes are immobilized as they can be quickly trapped. Electrons transport in nanoTiO2 according to the MT mode. Once electrons meet the trapped holes, they will recombine, so the recombination in nano-TiO2 also shows the diffusion feature. Because there are many grain boundaries in nano-TiO2, the increase of grain connection can also promote electron transport as it can provide wide inter-particle transport pathways. 37 Compared to the DSSCs, the electronic kinetic in the PO of organics is more complicated, because it is related to many correlated processes, including the electron generation, the transport, the recombination, and the interfacial transfer. In additional to the intrinsic electronic states (CB states and trap states), the adsorbed organic molecule sometimes can also form special “surface states”, which may have a great effect on the electron kinetic of nano-TiO2. Co-catalyst surface modification is also widely used for increasing the photocatalytic performances of nanoTiO2, as it can provide additional pathway for electron interfacial transfer. To discuss their effect on the electron kinetic of nano-TiO2 photocatalysis, it is needed to perform an in-situ study during the PO of organics. As any processes of electron kinetic in photocatalysis can affect the free electron concentration (nc) in CB of nano-TiO2, in-situ monitoring the change of nc with time as photocatalysis proceeds may be a feasible way to study electron kinetics. As the photoconductivity (σ) is proportional to nc, the full and careful analysis of in-situ σ during the PO of organics may provide useful information. Under the guidance of this thought, the insitu σ was used to study the electron kinetic for the PO of formic acid by porous nano-TiO2 films. After careful resolution of the in-situ σ, a conclusion was drawn that the weakly-adsorbed formic acid groups near nano-TiO2 surface may form special “surface states”. They induce the new recombination and the “current-doubling” effect, having important effects on electronic


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kinetics.

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The noble metals, such as Au nanoparticles, were widely used to modify nano-

TiO2. When Au is used as co-catalysts, it can form electron sink and separate electrons from holes, and in a consequence, the photocatalytic activity can be increased.

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The effect of Au

on the electron kinetics of porous nano-TiO2 films during PO of formic acid were also studied by using the in-situ σ. In additional to this research, we have used the in-situ σ method to study the electron behavior for the PO of methanol by using commercial P25 as photocatalyst.

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Recently, it was also used

to study the electron kinetic of gas photocatalysis by nano-TiO2. In summary, if the suitable kinetic and theoretical analysis can be combined, the measurement of time-dependent in-situ σ can be a powerful method to disclose the electron kinetic in PO of organics, which can be easily used to study the electron kinetic of other materials.

EXPERIMENTAL Material preparation: tetra-butyl titanate (TBOT) and ethanol were purchased from Shanghai Chemical Corp. Diethanolamine (DEA) and Polyvinylpyrrolidone K30 (PVP K30) was purchased from Aldrich. TiO2 films were prepared by sol-gel methods according to the following ways. Firstly, 25 g of PVP K30 was dissolved in 300 mL of ethanol. After that, 0.1 mol TBOT and 5 mL DEA were added, successively. The mixture solution was stirred for several hours at room temperature. Meanwhile, 150 mL ethanol mixed with 7.2 mL H2O was separately prepared and then slowly added to the above mixture solution. Magnetron stirring was kept for long time enough to get a uniform transparent titanium sol. Finally, the sol was aged for at least five days before making TiO2 coatings. A 100×20 mm FTO glasses were used as the substrates for TiO2 film preparation. Firstly, a strip of FTO coating with width ca. 2 mm was removed in the middle of FTO glasses, and then titanium sol was coated on the FTO-removed part of FTO glasses by dip-coating method. After dried at room temperature, the sample was annealed at 500 °C for 1 h to obtain the needed porous nano-TiO2 films for in-situ σ measurement. To decorate TiO2 nanoporous films with Au nanoparticles by thermal deposition method, TiO2 nano-porous films coated on FTO substrate were immersed into an aqueous solution containing 0.005 M HAuCl4 with the temperature at 70 °C, and the solution pH was adjusted 7 by adding 0.2 M NaOH. After a different period of time (from 20 min to 80 min), the samples were washed by DI water, dried


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at room temperature, and calcined at 300 °C for 2 h to generate Au nanoparticles on TiO2 nanoporous films surface. Characterization: Surface morphologies of pure, Au modified nano-TiO2 porous materials were observed by field-emission scanning electron microscopy (FE-SEM; type: S-4800, Hitachi, Tokyo, Japan), which was operated at 5 kV. Crystalline structures of pure TiO2 and Au/TiO2 nanoporous films were characterized by using a X-ray diffraction meter with a 2θ mode from 10° to 80° (XRD; Empyrean, PANalytical, Almelo, Netherland). The operation voltage and current are 40 kV and 40 mA, respectively during XRD measurement, and CuKα radiation was used as the X-ray source. The surface chemical compositions of Au/TiO2 were checked with an X-ray photoelectron spectrometer (XPS; type: VG Multilab 2000, Thermo Scientific, Waltham, America), with the MgKα radiation being used as the X-ray source. The XPS spectra were calibrated using the binding energy (284.6 eV) of C1s electrons. UV-vis absorption spectra of pure TiO2 and Au/TiO2 nano-porous films were measured by using a UV-Vis photo spectrometer within wavelength range from 200 nm to 800 nm (type: UV-1601, Shimadzu, Tokyo, Japan). Theoretical Calculation: Periodic three dimensional calculations were carried out using the VASP 5.3 code with the projector augmented wave method.

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The energy was computed using

the generalized gradient approximation (GGA) of DFT proposed by Perdew, Burke, and Ernzerhof (PBE).

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The electronic states were expanded using a plane wave basis set with a

cutoff of 500 eV. Forces on the ions were calculated through the Hellmann−Feyman theorem, including the Harris−Foulkes correction to forces. Geometry optimization calculation was performed before electronic structure calculation using the conjugated gradient algorithm. Iterative position relaxation of all of atoms was stopped until the forces on atoms were less than 0.01 eV /Å. In our calculation, the plane density functional theory (DFT) was firstly used to relax the atomic structures to equilibrium positions as it can evaluate the lattice parameters of TiO2 unit cells. The 10 3p, 3d, and 4s electrons of each Ti atom, 6 2s and 2p electrons of each oxygen atom, 4 2s and 2p electrons of each carbon atom and 1 1s electron of each hydrogen atom were considered. The Brillouin zone was sampled with the chosen Monkhorst-Pack k-points. 48 The slab model was used to simulate the interaction between that anatase TiO2 (101) surface and formic acid molecule. The surface was represented by four layers slabs made of 4×2 unit cells, which is large enough to assess the influence of the molecular interaction on the result. A vacuum space of a sufficient separation was imposed for the surface interaction and to ensure no


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interaction with the lowest layer of the upper slab. The lowest layer of the supercell was fixed during the calculation to prevent surface deformation. The formic acid and TiO2 surface was simulated in box 20 Å in Z direction. The absorption energy, Eads, was calculated according to the expression E

(1)

A positive value of Eads>0 indicates the stable adsorption. In addition, after the static selfconsistent calculation, the differential charge densities (DCD) maps and densities of states were performed.

Figure 1. Photocatalytic cell diagram for measuring the in-situ photoconductivities of nano-TiO2 porous film during the photocatalytic oxidation of formic acid Measurement of in-situ photoconductivity: Figure 1 shows the diagram of photo-chemical cell for the in-situ σ measurement during the PO of formic acid, the PO of methanol, and in pure water. The as-prepared nano-porous TiO2 coated FTO glass was inserted to pass through the photochemical cell, with two ends of FTO glass substrates being left outside the photochemical cell. The two-ends of FTO glass were connected to an electrochemical workstation (CHI 760E) for in-situ σ measurement. Five 365 nm fluorescent tubes (Philips, 15 W) were used as the light source, which illuminate the nano-TiO2 porous films along the normal direction, as shown in Figure 12. 100 ml water that contains different amounts of formic acid was firstly added in the photochemical cell. Before starting the experiments, the formic acid solution was kept in dark for ca. 60 min to allow the full adsorption of formic acid on nano-TiO2 surface. The intensity of UV


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light that reached TiO2 surface was varied by changing the distance between lamps and experimental setup and by turning on different number of UV lamps. For measuring the in-situ σ under condition of gas phase, a closed gas phase photocatalytic cell with the volume of 400 mL was used. 4 µL formic acid was firstly injected to the gas photo-catalytic. The in-situ σ during the PO of formic under gas condition was measured after keeping in dark for ca. 60 min, with the light intensity was set as ca. 2.0 mW/cm2. In this experiment, the light intensity was determined by Optical power meter (Newport PMKIT-05-01 ).

Figure 2 (A) surface FE-SEM image of pure nano-TiO2 porous film; (B) cross-sectional FE-SEM image of nano-TiO2 porous film; (C) surface FE-SEM image of Au/TiO2 film (thermal deposition time is 40 min) RESULTS AND DISCUSSION Material characterization: Figure 2 shows the surface FE-SEM images of pure TiO2 (A), Au/TiO2 (C) nano-porous films, as well as the cross-sectional FE-SEM image (B) of pure TiO2 film. The pure TiO2 film consists of uniform inter-connected nano-TiO2 grains, with tiny pores being within the film. Our previous study showed that this TiO2 film has high photocatalytic


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activity for removing acetone due to the good crystallinity, the good grain inter-connection, and the nano pores. It can be seen from Figure 2 (B) that the TiO2 film is flat and has a thickness of ~ 200 nm. Many bright spots present on the surface after Au deposition, and the energy dispersive X-ray spectroscopy (EDX) map on the top-right corner of Figure 2(C) shows the existence of Au. The Au nanoparticles are uniform and have a size of below 10 nm. The atomic ratios of Au to Ti are 5.1 %, 9.7 %, 23 %, and 27% as the loading time increase from 20 min to 80 min.

Figure 3. (A) XRD patterns of pure TiO2 and Au/TiO2 nano-porous films (thermal deposition time is 40 min); (B) Highly-resolved core-level XPS peak of O1s; (C) Highly-resolved core-level XPS peak of Ti2p; (D) Highly-resolved core-level XPS peak of Au4f The X-ray diffraction (XRD) patterns of the pure TiO2 and the Au/TiO2 nano-porous TiO2 films were shown in Figure 3 (A). All of the XRD peaks of TiO2 films can be indexed to anatase TiO2. By taking the (101) diffraction peak as reference, the grain size was calculated to be ~10 nm according to Scherrer formula, which is in accordance with the FE-SEM results. We can see the presence of metal Au for the Au/TiO2 sample. To investigate the chemical compositions and the valence state of the pure TiO2 and Au/TiO2 films. High resolution O1s, Ti2p, and Au4f corelevel XPS were recorded, as shown in Figure 3 (B), (C), and (D), respectively. The Ti2p and


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Au4f XPS spectra split into two peaks due to the spinning orbital-coupling effect. The O1s XPS spectrum of the pure TiO2 is asymmetrical, which consists of two peaks. The peak at 529.36 eV is from the lattice oxygen (Ti-O-Ti) and the shoulder at 531.9 eV is due to surface-covered hydroxyl groups. 49,50 The position of O1s spectrum produces a small positive shift, after the Au deposition. The symmetrical Ti2p3/2 peak is at 458.06 eV, indicating the Ti element in the TiO2 film is +4 valence, deposition of Au has not obvious effect on chemical states of TiO2. The Au4f XPS peaks in Figure 3 (D) show that the Au mainly exist in metal state, 51 so the bright spots on the TiO2 nano-porous film surface (Figure 2(C)) are metal Au nanoparticles. UV-Vis absorption spectra were also used to check the deposition of Au nanopartices, which were obtained by subtracting the UV-Vis absorption spectra of Au/TiO2 films by that of pure TiO2 films, as shown in Figure 4. It can be seen that the absorption of the Au nanoparticles are near 550 nm, which is due to the localized surface plasmon resonance of Au nanoparticles (LSPR).

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With the increase of Au loading time, the localized surface plasmon resonance

absorption increases, indicating the increase of Au amount on the TiO2 nano-porous film surface, in accordance with the EDX analysis.

Figure 4 UV-Vis absorption spectra of Au nanoparticles on nano-TiO2 porous surface after thermal deposition for different period of time


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Figure 5. (A) Dependences of in-situ photoconductivities on time during PO of formic acid, methanol, and in pure water; (B) normalized decays of photoconductivities after turning off the light in pure water, formic acid water solution, and methanol water solution In-situ photoconductivities of pure TiO2 film: Figure 5 (A) shows the time-dependences of insitu σ of pure nano-TiO2 films during the PO of formic acid, methanol, and in pure water. In pure water, under UV light irradiation, the σ increases as compared to that in dark due to the increase of nc as the holes can be quickly trapped on OH-. When the light was turned off, the insitu σ slowly decays due to the recombination, because there is no photocatalytic reaction in pure water. 45 When formic acid was added, the dependence of in-situ σ on time is different. The σ is higher than that in pure water, indicating that the capturing of hole by formic acid is so fast that electrons cannot be extracted timely, resulting in the accumulation in CB of TiO2. The steadystate σ is also very high even after long time UV irradiation, meaning that the formic acid is stable and cannot be decomposed quickly. In addition, it was also seen that, just after light was turned on, the σ shows a sharp increase and then starts to decrease after reaching the maximum, as labeled as (a) in Figure 5 (A), which is different from that in pure water. Two possible reasons can induce this slow decrease of the in-situ σ: (1) the decrease of formic acid amount at liquid/solid (L/S) interfaces because the PO reduces the amount of formic near TiO2 surface; (2) the new recombination via formic acid (if formic acid can form the new recombination centers). After the light was stopped, the in-situ σ in formic acid solution shows a much faster decay than that in pure water. As the intrinsic electron-hole recombination of nano-TiO2 was prohibited by the PO of formic acid, this in-situ σ sharp decrease may be due to the electron interfacial transfer


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to solution. If the new recombination exists, it can also induce the sharp decrease of the in-situ σ. Importantly, it was interesting to see that a new in-situ σ peak appears (labeled as (b), not observed in other studies) after the sharp decrease, showing that electrons can be re-injected to the CB of TiO2. As only nano-TiO2 and formic acid were present, the thermal release of electrons from deep localized states to the CB of nano-TiO2 and the current-doubling effect of formic acid can both induce this re-injected peak. To further confirm the physical reason of the first σ decrease and the second σ decrease, the in-situ σ of nano-TiO2 during the PO of methanol was also studied for comparison. It was seen that the in-situ σ is also higher than that in pure water due to the capturing of holes by methanol. However, there is no significant decrease of in-situ σ after the sharp increase when the light was turning on. Since the methanol concentration at L/S interface can also be reduced due to the PO process, so the first in-situ σ decrease during the PO of formic acid cannot be only ascribed to the decrease of formic acid concentration. Figure 5 (B) shows the normalized in-situ σ decay, after the light was turned off. Firstly, it was seen that the decay of in-situ σ in methanol solution is slower than that in formic acid solution; secondly, no electron re-injected peak can be seen. As the intrinsic recombination in nano-TiO2 is also limited by the PO of methanol, the insitu σ decrease in methanol solution can be mainly due to the interfacial transfer of electrons to solution. The much faster decay of in-situ σ indicates that electron kinetic of nano-TiO2 during the PO of formic acid not only includes the interfacial transfer of electrons, but also contains the new recombination. As the electron re-injected peak was not seen for the PO of methanol, so this peak is not from the thermal release of electrons from TiO2 localized states, but from formic acid, i.e. current-doubling effect. To further study the observed electron kinetics during the PO of formic acid, the effects of formic acid concentrations and UV light intensity on the in-situ σ were studied, as shown in Figure 6(A) and (B). Figure 6(A) shows that, the first decrease of the in-situ σ and the re-injected peak become stronger and stronger with the increase of formic acid concentrations. After turning on the light, we know that the decrease degree of formic acid amount at L/S interface can be reduced by increasing the formic acid concentrations. However, we can see that the first decrease of in-situ σ becomes higher as the formic acid concentrations increase. From this viewpoint, the first decrease of in-situ σ cannot be fully contributed to the decrease of formic acid amounts at


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L/S interface, and the new recombination should also play an important role, which is in accordance with the above analysis. It is easy to understand that the increase of formic acid concentrations increase the number of new recombination centers, which results in the increase of the new recombination. At the same time, the current-doubling effect can be also increased because higher formic acid amount can lead to more active formic acid. Figure 6(B) shows that the in-situ σ of nano-TiO2 during the PO of formic acid in cases of different UV light intensities. The first decrease of in-situ σ tends to be higher and higher with the increase of light intensities, indicating the increase of the new recombination. It can also be seen that the current-doubling effect firstly increase and then decreases as the UV light intensity increase. Firstly, the increase of light intensity can increase the number of active formic acids, which subsequently leads to the increases of the current-doubling effect. On the contrary, as indicated by the increase of steadystate σ, the further increase of light intensity will increase electron density in CB of TiO2, leading to the limitation of the current-doubling effect due the increase of the new recombination. According to the above explanation for the reason of the change of in-situ σ with time, the appearance of the new recombination and the current-doubling effect should depend on the amount of formic acid locating at/near the nano-TiO2 surface. From this view of point, extending the time for formic acid pre-adsorption in dark will lead to more formic acid at/near TiO2 surface, which should increase the effects of new recombination and current-doubling effect. Figure 6(C) shows the dependences of the in-situ σ on time during the PO of formic acid after dark preadsorption for different periods of time. It can be seen that the first decreases of the in-situ σ and the second decrease of the in-situ σ both becomes faster and faster with the increase of the dark pre-adsorption time. Firstly, almost no current-doubling effect was seen for zero time dark preadsorption, and then it starts to appear and becomes clearer and clearer as the dark pre-adsorption time increases. This result also told us how formic acid induces the new recombination and the current-doubling effect is dependent on the accumulation of formic acid at/near the nano-TiO2 surface. As shown in Figure 6(D), the new recombination and the current-doubling effect become stronger and stronger as the UV light irradiation time increases. The increase of UV light irradiation time can produce more active formic acid at/near TiO2 surface or alter the adsorption mode of formic acid, leading to the increase of the new recombination and the current-doubling effect.


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Figure 6 (A) Dependence of in-situ photoconductivities on time during the PO of formic acid in the case of different amounts; (B) Dependence of in-situ photoconductivities on time during the PO of formic for different UV light intensities；(C) Dependence of in-situ photoconductivities on time during the PO of formic acid after different period of pre-adsorption time in dark ; (D) Dependence of in-situ photoconductivities on time during the PO of formic after different periods of UV light irradiation time. (Please see the Supporting information for clearer Figures) Analysis of possible mechanism: As discussed above, the electron kinetic of nano-TiO2 porous films during the PO of formic acid shows the new recombination and the current-doubling effect. As indicated by the Fourier transform infrared spectroscopy analysis and the first-principle calculations,

53-56

formic acid can chemically bond on TiO2 surface via bidentate and

monodentate combinations. At the same time, formic acids can also weakly combine with TiO2 surface, and mainly locate at the S/L interface near TiO2 surface. To confirm which one, the chemically-bonded or the weakly-adsorbed formic acids, contribute to the new recombination


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and the current-doubling effect, two additional experiments were conducted. For the first one, the nano-TiO2 porous film was placed in formic acid solution for long time to allow full chemically and weakly adsorption, which produces the obvious current-doubling and the recombination effect, as shown in Figure 7(A) (solid line). Later, this nano-porous TiO2 film was softly washed by water, and then the in-situ σ was measured again. It was seen that the new recombination and the current-doubling effect couldn’t be observed (dashed line). As the chemically-bonded formic acid is impossible to be removed by soft washing, so we concluded that the new recombination and the current-doubling effect are not from the chemically-bonded formic acids, but from the weakly-adsorbed formic acid at S/L interface. In order to investigate the function of water, the in-situ σ was also measured during the PO of formic acid under the condition of gas phase (not in aqueous solution), as shown Figure 7(B). Under the continuous UV light irradiation, it was seen that the in-situ σ firstly increases and then decreases to the dark value after long time (red line), meaning that the formic acid can be completely photodegraded. No similar first decrease of in-situ σ was seen when the light was turned on. If the light was stopped after a period time of irradiation, the in-situ σ shows a sharp decrease and no current-doubling effect (re-injected insitu σ peak) can be observed. Therefore, the water is also important for producing the currentdoubling effect as it can lead to the formation of S/L interface.

Figure 7 (A) Dependence of in-situ photoconductivities on time during the PO of formic acid before and after washed by water; (B) Dependence of in-situ photoconductivities on time during the PO of formic under the condition of gas phase


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To elucidate the function of the chemically-bonded formic acid, three kinds of adsorption of formic acid molecule on anatase (101) surface were modeled, as shown in Figure 8. The anatase TiO2 (101) surface also includes unsaturated ions, fivefold-coordinated Ti (5c-Ti) and twofold-coordinated O (2c-O), in additional to saturated ions (6c-Ti and 3c-O). We took three possibly stable adsorbents into consideration: (1) the ketonic group of formic acid bonds with 5cTi, and the hydroxyl group (OH) bonds with the 2c-O next to the 5c-Ti, as shown in Figure 8 (A); (2) the ketionic oxygen bonds with 5c-Ti, and the OH bonds with the 2c-O of the 5c-Ti by H bond, as shown in Figure 8 (B); (3) the formic acid bonds with TiO2 surface via the dissociated mode, with the H atom moving to the nearest 2c-O, as shown in Figure 8 (C). The first two cases belong to the monodentate combination of formic acid on TiO2 surface, and the last one is the bidentate combination. The adsorption energies of these adsorption modes were calculated, and they are 0.86, 0.64, and 0.73 eV, respectively, showing that these chemical bonding is stable thermodynamically.

Figure 8 Three physical models for formic acid chemically-bonding on anatase (101) surface ((A) the ketonic group of formic acid bonds with 5c-Ti, and the hydroxyl group (OH) bonds with the 2c-O next to the 5c-Ti; (B) the ketionic oxygen bonds with 5c-Ti, and the OH bonds with the 2cO of the 5c-Ti by H bond; (C) the formic acid bonds with TiO2 surface via the dissociated mode, with the H atom moving to the nearest 2c-O) Figure 9 (A), (C), and (E) shows the differential charge density (DCD) maps of the above three cases. The adsorptions of formic acid molecule on TiO2 surface leads to the charge redistribution between formic acid molecule and the atoms on TiO2 surface, indicating the


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formation of chemical bonding among them. The DCD maps show that the O atom of formic acid bonds with the Ti atom of TiO2 surface through p-d π hybridization, and the H atom of formic acid bonds with the 2c-O through s-p σ hybridization. Both of the adsorption energies and the DCD analysis show stable chemical bonds are formed between formic acid molecule and TiO2. The densities of states (DOS) were also calculated, as shown Figure 9 (B), (D), and (F). It can be seen that the chemical adsorption of formic acid on TiO2 surface does not form the intermediate states within TiO2 forbidden band for all of the above three cases. The electronic orbitals of formic acid hybridize with the orbitals of (101) surface of anatase TiO2. The energy levels mainly locate at the VB and the CB of TiO2, which do not form the new gap states. In a consequence, our theoretical calculations indicate that the mono- and bidentate formic acid molecule on TiO2 surface cannot capture the holes efficiently, so the chemically bonded formic acid cannot contribute to the new recombination and the current-doubling effect, which is also in accordance with the above experimental results.


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Figure 9 differential charge density maps and density of states of the above three cases ((A) and (B) are corresponding to (A) in Figure 8; (C) and (D) are corresponding to (B) in Figure 8; (E) and (F) are corresponding to (C) in Figure 8) Basing on the above experiments and the theoretical calculations, a possible mechanism for the new recombination and the current-doubling effect was proposed, as shown in Figure 10. In the water solution, formic acid molecule dissociates as H+ and HCOO-, and the nano-TiO2 surface is also covered with OH- and water molecule. The HCOO- can combine with the OH- or water molecule through hydrogen bonds, which makes the HCOO- accumulate on nano-TiO2 surface. By referring to literature,

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it can be known that the redox potential of HCOO-/HCOO•

locates within the forbidden band of TiO2. Under UV light irradiation, the OH- changes to •OH after capturing a hole, which can further lead to the oxidization of HCOO- as HCOO•. The Coulomb attractions among •OH and HCOO- groups, or OH- and HCOO• groups can further


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strengthen the accumulation of HCOO- and HCOO• at the S/L interface of nano-TiO2. The above in-situ σ experiments confirmed that The HCOO• at S/L interface of TiO2 plays two important roles: (1) it can capture an electron from the CB of nano-TiO2, leading to the new recombination; (2) it can also re-inject an electron to the CB of TiO2, i.e., current-doubling effect, as discussed by other researches. 38-41

Figure 10 Proposed physical model for the new recombination and the current-doubling effect Kinetic Simulation: According to our proposed physical model, a kinetic simulation was performed to compare with the experimental results. According to Figure 9, in order to simulate the in-situ σ, a step function was used to describe UV light illumination G G

(2)

Eq. (2) means that the light is turned on at time 0, and turned off at time t0, with u(t) being the unit step function. The nano-TiO2 porous film thickness is termed as d. The absorption coefficient of TiO2 at 365 nm is described as α, which is set as 1.0×104 cm-1. The G0 can be written as Eq. (3) for a normal illumination on TiO2 surface, with I0 being the initial intensity of light reaching on TiO2 surface, and light reflection is not taken into consideration.


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"

G ! 1 "

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(3)

The possible reactions can processes are listed as follows (we assumed that the capturing of holes by OH- is very fast, and almost all of holes were captured by OH- under weak UV light irradiation) ∙ h %& '()) → '()) ; υ -./ ['()) ]

(4)

e 3& )4 → )4 ; υ5 - 6

(5)

∙ ∙ ∙ e 3& '()) → '()) ; υ4 -7 6 ['()) ]

(6)

;υ; -" ['())∙ ] HCOO → ()4

(7)

Eq. (4) is the capturing of holes by HCOO- with a rate constant of kht; Eq. (5) denotes the interfacial transfer of electrons from CB to O2, and kcb is the rate constant; Eq. (6) is the recombination via formic acid with the rate of v2, Eq. (7) show the current-doubling of formic acid by injecting an electron to CB of TiO2. The variation of nc and [HCOO•] with time can be described as:

In Situ Photoconductivity Kinetic Study of Nano-TiO2 during the

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