Synergetic Effect of Pt and Borate on the TiO2-Photocatalyzed

Feb 4, 2016 - It has been reported that Pt and borate can promote the TiO2-photocatalyzed degradation of phenol in an aerated aqueous suspension, ...
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Synergetic Effect of Pt and Borate on the TiO2‑Photocatalyzed Degradation of Phenol in Water Xianqiang Xiong and Yiming Xu* State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: It has been reported that Pt and borate can promote the TiO2-photocatalyzed degradation of phenol in an aerated aqueous suspension, respectively. Herein, we report a synergism between Pt and borate greatly improving the photocatalytic activity of anatase TiO2. The rate of phenol degradation was dependent on Pt loading, borate concentration, and solution pH. A maximum rate of phenol degradation was observed at 0.52 wt % Pt, 3.0 mM Na2B4O7, and initial pH 9.0, respectively. Under such conditions, the rates of phenol degradation, upon the addition of Pt, borate, and Pt plus borate, were increased by 1.34, 0.22, and 4.87 times, respectively. This mutual action of Pt and borate were also observed from the photocatalytic degradation of 4chlorphenol, and 2,4-dichlrophenol. A separate experiment with the TiO2 and Pt/TiO2 film electrodes showed that borate could mediate the hole oxidation of phenol, but not water. Furthermore, the electrochemical reduction of O2 over TiO2 and Pt/TiO2, upon addition of borate, became inhibited and promoted, respectively. It is proposed that there a special interaction between Pt and borate facilitating the reduction of O2. These cooperative effects between Pt and borate would improve the charge separation of TiO2 and consequently accelerate the photocatalytic reactions. electrode, fluoride and phosphate have a positive effect,18,23,25 but borate shows no effect.9 It is proposed that borate anion is first oxidized to a borate anion radical, and then regenerated through phenol oxidation. This borate-mediated hole transfer would make the electrons of TiO2 live longer, and consequently promote the reduction of O2. However, under an externally applied potential bias, the electrochemical and photoelectrochemical reduction of O2 over the TiO2 film electrode are both inhibited, upon the addition of borate. This negative effect of borate is due to the reduced surface amount of O2, and the negatively shifted conduction band edge potential of TiO2.9 If the above mechanism is operative, then any action that can speed up the TiO2-photocatalyzed reduction of O2 would further improve the borate-mediated hole oxidation of phenol, as proposed in Scheme 1. In this work, Pt particles and borate anions have been examined as cocatalysts. The photocatalytic degradation of phenol, 4-chlorophenol, and 2,4-dichlorophenol in aqueous solution were used as model reactions, whereas a commercially available anatase TiO2 was used as photocatalyst. The deposition of Pt particles onto TiO2 were made through a photochemical method. In all cases, the rates of organic degradation in the presence of both Pt and borate were much higher than the sum of the rates measured in the presence of

1. INTRODUCTION Titanium dioxide is a well-known photocatalyst that can initiate the degradation and mineralization of various organic pollutants at room temperature.1−3 During the reaction process, O2 is oxidant, and organic substrate is reductant. However, as a potential new technology for environmental remediation, the TiO2 photocatalysis still suffers from a low quantum efficiency. For example, P25, a mixture of anatase and rutile TiO2, is regarded as a benchmark photocatalyst. But its quantum yield at 365 nm is only 0.14, as measured from phenol degradation in aqueous suspension at pH 3.0.4 It is generally recognized that the photogenerated electrons (ecb−) and holes (hvb+) of TiO2 are short-lived. As a result, most of these charge carriers quickly recombine to heat, without net chemical reactions with sorbates on the oxide surface.5 Therefore, to increase the quantum yield of TiO2 photocatalysis, the interfacial charge transfer should be improved. One of the strategies is the introduction of a cocatalyst. For example, noble metals such as Pt, Ag, and Au deposited on TiO2 can catalyze the electron reduction of O2, and consequently promote the counterpart hole oxidation of organics.6−8 Very recently, we have reported that borate anions have positive effect on the photocatalytic degradation of phenol in the aqueous suspension of anatase TiO2.9 On the one hand, this effect of borate is similar to those of fluoride and phosphate anions.10−25 On the other hand, borate is different from fluoride and phosphate. For instance, during the photoelectrochemical oxidation of H2O to O2 over the TiO2 film © XXXX American Chemical Society

Received: December 6, 2015 Revised: January 17, 2016

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PtO2.27−30 Importantly, upon the Pt laoding, the TiO2 phase remained unchanged, in terms of the crystal structure, crystallite size, and band gap energy. However, the surface area, total pore volume, and average pore size of Pt/TiO2 were a little larger than those of bare TiO2. This is probably indicative of the formation of fine Pt particles on TiO2. Photocatalysis and Analysis. Reactions were carried out in a Pyrex-glass reactor, and Light source was a high pressure mercury lamp (300 W, Shanghai Yamin). The light intensity reaching the external surface of the reactor was 3.30 mW/cm2, as measured with an irradiance meter (Instruments of Beijing Normal University). Unless states otherwise, the experimental conditions were set at 1.00 g/L catalyst, initial pH 9.0, 0.43 mM phenol, 0.30 mM 4-CP, 0.30 mM 2,4-DCP, and 3.0 mM Na2B4O7. A suspension (50 mL) containing necessary components was stirred in the dark for 1 h, and then irradiated with UV light. At given intervals, 2.0 mL of the suspension was withdrawn, and filtered through a 0.22 μm membrane. The filtrate was analyzed by HPLC (high performance liquid chromatography) on a Dionex P680 (Apollo C18 reverse column). The eluent was a mixture of CH3OH and H2O at a volume ratio of 5:5 for phenol, and 6:4 for 4-CP, and 7:3 for 2,4-DCP, followed by addition of 0.1% acetic acid. Hydrogen peroxide was measured on an Agilent 8453 UV− visible spectrophotometer at 553 nm through the PODcatalyzed oxidation of DPD.31 Hydroxyl radical (HO•) was measured on a Shimadzu F-2500 spectrofluorometer through reaction with coumarin to form a fluorescent 7-hydroxycoumarin.32 Borate anion was analyzed at 413 nm through its complex with Azomethine H.33 Electrode Fabrication and Measurement. The TiO2 film electrode was prepared by the doctor blade method. An indium-doped tin oxide (ITO) substrate was coated with a gel containing 0.8 wt % TiO2 and 2.9 wt % PVA, and then sintered in air at 500 °C for 3 h. After that, the ITO substrate was cut into pieces. Each piece had an exposed area of 1 × 1 cm2, whereas the other part was sealed by an epoxy resin. To prepare the Pt/TiO2 film electrode, the above TiO2 film electrode was immersed in an aqueous solution containing 0.24 wt % H2PtCl6 and 2 M CH3OH for 30 min. After the electrode was taken out from the solution, it was irradiated with a 300 W Hg lamp for 10 min. These coated ITO glass were used as the working electrode. Measurement was carried out on a CHI660E Electrochemical Station (Chenghua, Shanghai), using a saturated calomel electrode (SCE) as the reference electrode, a platinum gauze as the counter electrode. The supporting electrolyte was 0.5 M NaClO4. The working electrode was illuminated through a quartz window with a 500 W Xe lamp from the electrode/electrolyte side.

Scheme 1. Possible Mechanism for the Cooperative Effect of Pt and Borate.

individual Pt and borate. To optimize the conditions, the effect of Pt loading, borate concentration, and the solution pH were examined. To understand the interfacial charge transfer, a (photo)electrochemical measurement was separately made with a TiO2 and Pt/TiO2 film. Furthermore, a possible mechanism responsible for the observed synergistic effect between Pt and borate is discussed.

2. EXPERIMENTAL SECTION Materials. Anatase TiO2, chloroplatinic acid, and coumarin were purchased from Sigma−Aldrich. Other chemicals in analytical grade were purchased from Shanghai Chemicals, Inc., including Na2B4O7, phenol, 4-chlorophenol (4-CP), 2,4dichlorophenol (2,4-DCP) and poly(vinyl alcohol) (PVA). Milli-Q ultrapure water was used, and the solution pH was adjusted with a dilute solution of HClO4 or NaOH. Synthesis and Characterization. The Pt loaded TiO2 (Pt/TiO2) was prepared by a photochemical deposition method.26 An aqueous suspension containing 18 g/L TiO2, 2 M CH3OH, and a certain amount of H2PtCl6 was irradiated with a 300 W mercury lamp for 3 h. Then the solid was filtered and washed thoroughly with distilled water, and dried in an oven at 80 °C. The amount of H2PtCl6 remaining in the filtrate was measured by an inductively coupled plasma mass spectroscopy, from which the weight percent of Pt (wt %) in Pt/TiO2 was calculated. The solid was characterized with X-ray diffraction (XRD), diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), and N2 adsorption (Figure S1, Supporting Information). The physical parameters of those samples are listed in Table 1. The XRD peak due to metallic Pt (PDF #65Table 1. Physical Parameters of TiO2 and Pt/TiO2a catalyst TiO2 0.26% 0.52% 1.08% 2.24%

Pt/TiO2 Pt/TiO2 Pt/TiO2 Pt/TiO2

ds (nm)

Asp (m2/g)

Vp (cm3/g)

dp (nm)

Eg (eV)

13.6 13.4 13.5 13.6 13.5

140.8 143.5 141.4 142.1 145.9

0.334 0.379 0.353 0.355 0.375

80.7 91.7 87.1 90.2 86.1

3.20 3.20 3.20 3.20 3.20

3. RESULTS AND DISCUSSION Phenol Degradation. Photoreactions were carried out under UV light at wavelengths longer than 320 nm. Under such conditions, all of the model substrates (phenol, 4-CP, and 2,4DCP) in aqueous solution were stable against direct photolysis. Figure 1A shows the results of phenol degradation over TiO2 and 0.52 wt % Pt/TiO2, measured in an aerated aqueous suspension at initial pH 9.0. As the irradiation time increased, the concentration of phenol in aqueous phase decreased. All of the kinetic curves obtained in the absence and presence of borate were satisfactorily fitted to the pseudo-first-order rate eq (Figure S2), and the resulting apparent rate constants of phenol degradation (kobs) are listed in Table 2. The rate of phenol

a

ds, crystallite size estimated by XRD from the (101) anatase; Asp, BET surface area; Vp, total pore volume; dp, average pore size; and Eg, band gap energy.

2868) was observed only at a Pt loading higher than 1 wt %. The sample was gray in color, and had visible light absorption increasing with the Pt loading. In the XPS spectrum of 2.24 wt % Pt/TiO2, there were three peaks at 70.4, 73.8, and 75.8 eV, respectively. Two of them are assigned to Pt 4f 7/2 and 4f5/2 of metallic Pt, whereas the peak at 75.8 eV is attributed to B

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Figure 1. Photocatalytic degradation of (A) phenol, (B) 4-CP, and (C) 2,4-DCP, in aqueous solution at initial pH 9.0. Experiments were carried out under the different conditions of (a) TiO2, (b) TiO2 + 3.0 mM borate, (c) 0.52 wt % Pt/TiO2, and (d) 0.52 wt % Pt/TiO2 + 3.0 mM borate.

Table 2. Apparent Rate Constants (kobs, 10−3 min−1) of Phenol Degradationa catalyst

kobs (phenol)

kobs (4-CP)

kobs (2,4-DCP)

TiO2 TiO2 + borate Pt/TiO2 Pt/TiO2 + borate

2.53 3.08 5.92 14.85

3.14 6.70 7.07 31.23

5.47 5.22 17.17 32.45

the positive effect of carbonate dissolved in an alkaline aqueous solution at pH 9.0. Upon the addition of 3.0 mM Na2CO3, the rate of phenol degradationover TiO2 was increased by 7.6%. But the rate of phenol degradation over 0.52 wt % Pt/TiO2 in the presence of 3.0 mM Ba2B4O7 was decreased by 14% upon the addition of 3.0 mM Na2CO3 (Figure S3). Similar results were also obtained from the photocatalytic degradation of 4-CP and 2,4-DCP (Figure 1B and C). All of the reaction rates measured in the presence of both borate and Pt were much higher than the sum of the rates obtained in the presence of individual borate and Pt (Table 2). However, different from phenol, both 4-CP and 2,4-DCP in aqueous solution had some adsorption on TiO2 and Pt/TiO2. This would help organic degradation over the photocatalysts. Upon the addition of borate, the adsorption of 2,4-DCP was obviously decreased, due to the surface occupation of borate. As a result, the rate of 2,4-DCP degradation over TiO2 was decreased. Impressively, the rate of 2,4-DCP degradation over Pt/TiO2 upon the addition of borate was still increased by 4.93 times. There is indeed a cooperative effect between borate and Pt facilitating the TiO2-photocatalyzed reactions. Effect of Variables. To optimize the conditions, the effects of Pt loading, borate concentration, and the solution pH were examined. In all cases, the catalyst was fixed at 1.00 g/L. Figure 2 shows the results of phenol degradation in an aerated aqueous suspension. As the weight percent of Pt in Pt/TiO2 increased, the rate of phenol degradation increased, and then decreased after reaching a maximum at 0.52 wt % Pt (Figure 2A). The former gives a support that Pt is beneficial to the TiO2-photocatalyzed degradation of phenol. The latter is

a

Reactions were performed in the aqueous suspension of 1.00 g/L TiO2 or 0.52 wt % Pt/TiO2 at initial pH 9.0. Conditions: borate (Na2B4O7), 3.0 mM; phenol, 0.43 mM; 4-CP, 0.30 mM; and 2,4-DCP, 0.30 mM.

degradation over Pt/TiO2 was higher than that over TiO2, whereas the rate of phenol degradation obtained in the presence of borate was higher than that measured in the absence of borate. Control experiments without TiO2 showed that both Pt and borate were not active for phenol degradation, either in the dark or under UV light. These observations confirm that Pt and borate do have a positive effect on the TiO2-photocatalzyed degradation of phenol, respectively. However, upon the addition of Pt, borate, and Pt plus borate, the rates of phenol degradation over TiO2 were increased by 1.34, 0.22, and 4.87 times, respectively. That is, the rate of phenol degradation obtained in the presence of both borate and Pt was much larger than the sum of the rates measured in the presence of individual Pt and borate. Since the TiO2 phase remains unchanged upon the Pt loading (Table 1), it follows that there is an synergistic effect between Pt and borate promoting the TiO2-photocatalyzed degradation of phenol. Note that the observed positive effect of borate is not due to

Figure 2. Photocatalytic degradation of phenol in an aerated aqueous suspension, measured under the different conditions: (A) at initial pH 9.0, (a) in the absence and (b) presence of 3.0 mM Na2B4O7; (B) at initial pH 9.0, with (c) TiO2, and (d) 0.52 wt % Pt/TiO2; (C) 0.52 wt % Pt/TiO2, and 3.0 mM Na2B4O7. C

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Figure 3. (A) Emission intensity of 7-hydroxycoumarin, monitored at 457 nm. (B) Photocatalytic formation of H2O2 in the presence of 0.43 mM phenol. Experiments were carried out in aerated aqueous suspension at initial pH 9.0 under the conditions of (a) TiO2, (b) TiO2 + 3.0 mM Na2B4O7, (c) 0.52 wt % Pt/TiO2, and (d) 0.52 wt % Pt/TiO2 + 3.0 mM Na2B4O7.

Figure 4. (A) Linear sweep voltammetry of Pt/TiO2 film electrode in the presence of 0.40 mM phenol, recorded in the dark (dotted lines), and under UV light (solid lines), at a scan rate of 10 mV/s. (B) Photocurrent at 0.6 V vs. SCE, measured under UV light. Experiments were carried out under N2 in 0.5 M NaClO4 at pH 9.0, (a) in the absence, and (b) the presence of 3.0 mM Na2B4O7.

As the initial pH of the suspension increased, the rate of phenol degradation over 0.52 wt % Pt/TiO2 in the presence of 3.0 mM Na2B4O7 increased, and then decreased (Figure 2C). A maximum of phenol degradation was observed at initial pH 9.0. This optimum pH of 9.0 was higher than that of 7.0, observed from bare TiO2.9 It is known that the structure of borate in aqueous solution is greatly dependent on borate concentration, the solution pH, and temperature.34 In a very acidic and alkaline aqueous solution, borate would exist in the forms of B(OH)3 and B(OH)4−, respectively. In aqueous solution at pH 9.0, various boron species are present, including B3O3(OH)52− and B4O5(OH)42−. In the present case, the exact structure of boron species is not known. However, the interaction between Pt and borate may have influence on the hydrolysis and structure of boron species. As a result, the TiO2 and Pt/TiO2 systems showed different effects of borate concentration and solution pH. As a reference, borate anions could adsorb onto TiO2 in aqueous solution at pH 3−11.9 A maximum adsorption of borate on TiO2 was located at pH 9.0. As the equilibrium concentration of borate in aqueous solution at pH 9.0 increased, the amount of borate adsorption increased toward saturation (Figure S4). The limit adsorption of borate on 0.52% Pt/TiO2 was about 10% lower than that on TiO2. In aqueous solution at pH 9.0, the surface of TiO2 would be negatively charged. Then, in such solution, the observed adsorption of borate anions on TiO2 and Pt/TiO2 is not due to a columbic interaction.

ascribed to the excess Pt particles that absorb and shield the incident light reaching TiO2 (Figure S1), consequently slowing down the TiO2-photocatalyzed reaction. Similar effect of Pt loading was also observed in the presence of 3.0 mM Na2B4O7. As it is expected, the rates of phenol degradation over different Pt/TiO2 were all increased upon the addition of borate. However, the borate-induced rate increase of phenol degradation was dependent on Pt loading. With 0.26, 0.52, 1.08, and 2.24 wt % Pt/TiO2, the rates of phenol degradation upon the addition of borate were increased by 1.51, 1.62, 1.74, and 1.94 times, respectively. Since the same borate concentration was used, these observations imply that a cooperative effect between Pt and borate increases with the Pt loading. As the concentration of borate added in the suspension increased, the rate of phenol degradation over TiO2 increased toward saturation (Figure 2B), as observed early.9 However, the rate of phenol degradation over 0.52 wt % Pt/TiO2 first increased with the borate concentration, and then decreased after reaching a maximum at 3.0 mM Na2B4O7. Furthermore, at given borate concentration, the borate-induced rate increase of phenol degradation measured from Pt/TiO2 is notably larger than that obtained from TiO2. Again, this is indicative of a cooperative effect between Pt and borate that promotes the TiO2-photocatalyzed degradation of phenol in an aerated aqueous suspension. At a borate concentration higher than 3.0 mM, the observed rate decrease of phenol degradation over Pt/ TiO2 with the borate concentration is probably due to changes in the borate structure, which will be discussed below. D

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electrode.9 Both of the systems indicate that borate is capable of mediating the hole transfer for phenol oxidation, but not for water oxidation. Such borate-mediated hole transfer would promote the charge separation of Pt/TiO2, and consequently increase the photocurrent of phenol oxidation. The second experiment was to examine the Pt-mediated reduction of O2. Figure 5 shows the dark current−voltage

Formation of Oxygen Reactive Species. It has been proposed that various reactive species can be produced from TiO2 photocatalysis, including •OH and H2O2. In general, the • OH radicals are detected indirectly through a spin trapping electron paramagnetic resonance spectroscopy, and/or through a fluorescence spectroscopy. In the present study, coumarin was used as a probe, because it can react with •OH to form a fluorescent 7-hydroxycoumarin.32 Figure 3A shows the results of fluorescence measurement, performed with TiO2 and 0.52 wt % Pt/TiO2 in the absence and presence of 3.0 mM Na2B4O7, respectively. As the irradiation time increased, the emission intensity of 7-hydroxycoumarin at 457 nm increased toward limit. At given time, the emission intensity increased in the order of Pt/TiO2 + borate > Pt/TiO2 > TiO2 + borate > TiO2. Impressively, the borate-caused emission enhancement of Pt/ TiO2 was much larger than that of TiO2. However, 7hydroxycoumarin may result from the hole oxidation of coumarin, followed by hydrolysis. Then, the enhanced formation of 7-hydroxycoumarin is not surely indicative of the enhanced generation of OH radicals from TiO2 photocatalysis. Figure 4B shows the result of H2O2 production, measured in an aerated aqueous suspension at initial pH 9.0. In this case, excess phenol was present, so as to consume the photogenerated holes of TiO2. In the absence of borate, the amount of H2O2 obtained from 0.52 wt % Pt/TiO2 was much larger than that obtained from TiO2. Upon the addition of borate, the amount of H2O2 was increased either on TiO2 or on Pt/TiO2. However, the borate-caused enhancement of H2O2 production over Pt/TiO2 was much smaller than that over TiO2. Control experiments in the dark showed a fast decomposition of H2O2 over Pt/TiO2 (Figure S5). There is a competition between the formation and decomposition of H2O2. As a result, the amount of H2O2 produced from Pt/TiO2 began to decrease with time, after reaching a maximum at 10 min. Interestingly, in the presence of borate, such decay of H2O2 concentration with time was not observed. This is probably due the interaction between Pt and borate inhibiting the (photo)decomposition of H2O2 over Pt/TiO2. Nevertheless, the observed trends for the photocatalytic formation of 7-hydroxycoumarin and H2O2 are nearly in agreement with that for the photocatalytic degradation of phenol (Figure 1A).

Figure 5. Dark current−voltage curves of (a) TiO2, (b) TiO2 + 3.0 mM Na2B4O7, (c) Pt/TiO2, and (d) Pt/TiO2 + 3.0 mM Na2B4O7, measured under N2 (dotted lines), and under O2 (solid lines). Experiments were carried out with the film electrode in 0.5 M NaClO4 at pH 9.0, with a scan rate of 10 mV/s.

curves of TiO2 and Pt/TiO2 film electrodes, measured in 0.5 M NaClO4 at initial pH 9.0. First, the reduction current under air was obviously larger than that under N2, indicative of the O2 reduction being a dominant process. Second, under air, the onset potentials for the TiO2 and Pt/TiO2 film electrodes were about −0.17 and +0.15 V vs SCE, respectively. This observation indicates that the overpotential for the reduction of O2 is decreased by 300 mV due to Pt loading. Third, with the TiO2 film electrode, the current of O2 reduction upon the addition of borate was decreased, due to decrease in the rate of O2 reduction, and/or in the amount of O2 adsorption on the electrode surface.9 Forth, with the Pt/TiO2 film electrode, the current of O2 reduction was larger than that measured with the TiO2 film electrode. This is in agreement with the conclusion that Pt can promote the reduction of O2 over TiO2.35−37 Interestingly, the electrode current upon the addition of borate was further increased. A separate experiment with a Pt wire or mesh as the working electrode also showed an enhancement in the reduction of O2 upon the addition of borate (Figure S7). These observations clearly indicate that there is a special interaction between Pt and borate, which facilitates the reduction of O2 on the electrode.

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(PHOTO)ELECTROCHEMICAL MEASUREMENT In this study, all of the measurements were made with a Pt/ TiO2 film electrode in 0.5 M NaClO4 at initial pH 9.0. The first experiment was to examine the borate-mediated hole oxidation of phenol. Figure 4 shows the linear sweep voltammetry of Pt/ TiO2, measured under N2 and in the presence of phenol. In the dark, the electrode current was very weak, either in the absence or presence of borate. Under UV light, the anodic current was notably increased, indicative of phenol oxidation by the photogenerated holes of Pt/TiO2. As the applied external potential increased, the photocurrent increased, due to enhancement in the efficiency of charge carrier separation. Upon the addition of borate, interestingly, the photocurrent was further increased. Moreover, during the five repeated experiments, the photocurrent at given applied potential remained nearly constant (Figure 4B). This is indicative of the high stabilities of Pt/TiO2 and borate. However, in the absence of phenol, the electrode photocurrent due to H2O oxidation was not increased upon the addition of borate (Figure S6). These observations from the Pt/TiO2 film electrode were similar to those reported for the TiO2 film

4. CONCLUSIONS In this work, we have presented that the coaddition of Pt and borate into the irradiated aqueous suspension of TiO2 can result in significant enhancement in the rate of phenol degradation. During the reaction process, Pt and borate function as the electron and hole mediators of TiO2, respectively. Since the electrons and holes of TiO2 are photogenerated in a pair, the mutual action of Pt and borate would make the holes and electrons live much longer, as compared to those in the presence of individual Pt and borate. As a result, both the rates of O2 reduction and phenol oxidation are greatly enhanced. Moreover, there is also a special E

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(11) Minero, C.; Mariella, G.; Maurino, V.; Vione, D.; Pelizzetti, E. Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 2. Competitive Reactions of Phenol and Alcohols on a Titanium Dioxide-Fluoride System. Langmuir 2000, 16, 8964− 8972. (12) Mrowetz, M.; Selli, E. Enhanced Photocatalytic Formation of Hydroxyl Radicals on Fluorinated TiO2. Phys. Chem. Chem. Phys. 2005, 7, 1100−1102. (13) Mrowetz, M.; Selli, E. H2O2 Evolution During the Photocatalytic Degradation of Organic Molecules on Fluorinated TiO2. New J. Chem. 2006, 30, 108−114. (14) Vohra, M. S.; Kim, S.; Choi, W. Effects of Surface Fluorination of TiO2 on the Photocatalytic Degradation of Tetramethylammonium. J. Photochem. Photobiol., A 2003, 160, 55−60. (15) Park, H.; Choi, W. Effects of TiO2 Surface Fluorination on Photocatalytic Reactions and Photoelectrochemical Behaviors. J. Phys. Chem. B 2004, 108, 4086−4093. (16) Xu, Y.; Lv, K.; Xiong, Z.; Leng, W.; Du, W.; Liu, D.; Xue, X. Rate Enhancement and Rate Inhibition of Phenol Degradation on Irradiated Anatase and Rutile TiO2 on the Addition of NaF: New Insight into the Mechanism. J. Phys. Chem. C 2007, 111, 19024− 19032. (17) Cong, S.; Xu, Y. Enhanced Sorption and Photodegradation of Chlorophenol over Fluoride-Loaded TiO2. J. Hazard. Mater. 2011, 192, 485−489. (18) Cheng, X.; Leng, W.; Liu, D.; Xu, Y.; Zhang, J.; Cao, C. Electrochemical Preparation and Characterization of Surface-Fluorinated TiO2 Nanoporous Film and Its Photoelectrochemical and Photocatalytic Properties. J. Phys. Chem. C 2008, 112, 8725−8734. (19) Monllor-Satoca, D.; Gomez, R. Electrochemical Method for Studying the Kinetics of Electron Recombination and Transfer Reactions in Heterogeneous Photocatalysis: The Effect of Fluorination on TiO2 Nanoporous Layers. J. Phys. Chem. C 2008, 112, 139−147. (20) Monllor-Satoca, D.; Lana-Villarreal, T.; Gomez, R. Effect of Surface Fluorination on the Electrochemical and Photoelectrocatalytic Properties of Nanoporous Titanium Dioxide Electrodes. Langmuir 2011, 27, 15312−15321. (21) Zhao, D.; Chen, C.; Wang, Y.; Ji, H.; Ma, W.; Zang, L.; Zhao, J. Surface Modification of TiO2 by Phosphate: Effect on Photocatalytic Activity and Mechanism Implication. J. Phys. Chem. C 2008, 112, 5993−6001. (22) Sheng, H.; Li, Q.; Ma, W.; Ji, H.; Chen, C.; Zhao, J. Photocatalyttic Degradation of Organic Pollutants on Surface Anionized TiO2: Common Effect of Anions for High Hole-Availability by Water. Appl. Catal., B 2013, 138−139, 212−218. (23) Jing, L.; Zhou, J.; Durrant, J. R.; Tang, J.; Liu, D.; Fu, H. Dynamics of Photogenerated Charges in the Phosphate Modified TiO2 and the Enhanced Activity for Photochemical Water Splitting. Energy Environ. Sci. 2012, 5, 6552−6558. (24) Cao, Y.; Jing, L.; Shi, X.; Luan, Y.; Durrant, J. R.; Tang, J.; Fu, H. Enhanced Photocatalytic Activity of nc-TiO2 by Promoting Photogenerated Electrons Captured by the Adsorbed O2. Phys. Chem. Chem. Phys. 2012, 14, 8530−8536. (25) Jing, L.; Cao, Y.; Cui, H.; Durrant, J. R.; Tang, J.; Liu, D.; Fu, H. Acceleration Effects of Phosphate Modification on the Decay Dynamics of Photogenerated Electrons of TiO2 and Its Photocatalytic Activity. Chem. Commun. 2012, 48, 10775−10777. (26) Kim, J.; Lee, C. W.; Choi, W. Platinized WO3 as an Environmental Photocatalyst that Generates OH Radicals under Visible Light. Environ. Sci. Technol. 2010, 44, 6849−6854. (27) Nie, L.; Yu, J.; Li, X.; Cheng, B.; Liu, G.; Jaroniec, M. Enhanced Performance of NaOH-Modified Pt/TiO2 toward Room Temperature Selective Oxidation of Formaldehyde. Environ. Sci. Technol. 2013, 47, 2777−2783. (28) Li, F. B.; Li, X. Z. The Enhancement of Photodegradation Efficiency Using Pt-TiO2 Catalyst. Chemosphere 2002, 48, 1103−1111. (29) Yu, J. G.; Qi, L. F.; Jaroniec, M. Hydrogen Production by Photocatalytic Water Splitting over Pt/TiO2 Nanosheets with Exposed (001) Facets. J. Phys. Chem. C 2010, 114, 13118−13125.

interaction between Pt and borate, which not only affects the structure of boron species (Figure 2), but also promotes the reduction of O2 over TiO2 (Figure 5). However, the exact reasons for them are not known. This synergetic effect between Pt and borate would be very useful in the fields of semiconductor photocatalysis, electrochemistry, and environmental technology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11923. XRD patterns, diffuse reflectance spectra, XPS spectrum, N2 adsorption−desorption isotherms, first-order kinetics for phenol degradation, carbonate effect, borate adsorption, H2O2 decomposition, current−voltage curves for water oxidation on Pt/TiO2 and for O2 reduction on Pt (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.Xu.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21377110). REFERENCES

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