Borate-Mediated Hole Transfer from Irradiated Anatase TiO2 to Phenol

Article pubs.acs.org/JPCC

Borate-Mediated Hole Transfer from Irradiated Anatase TiO2 to Phenol in Aqueous Solution Linlin Hao, Xianqiang Xiong, and Yiming Xu* State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China

Downloaded by SUNY UPSTATE MEDICAL UNIV on September 6, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jpcc.5b03087

S Supporting Information *

ABSTRACT: The development of a highly active TiO2 photocatalyst for energy and environmental use is a great challenge. In this work, we report that the addition of sodium borate to an aqueous suspension of anatase TiO2 at neutral pH can result in a significant enhancement in the rate of phenol degradation. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H2O2 decomposition, and chromate reduction in the presence of phenol. This borate-induced rate increase for phenol degradation was determined not only by the amount of borate adsorption but also by the structure of borate species (pH effect). A (photo)electrochemical measurement with the TiO2 film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O2 were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO2 was negatively shifted by 81 mV. Since the hole oxidation of water to O2 remained unchanged, it is proposed that a borate radical is produced, followed by regeneration through phenol oxidation. This borate-mediated hole transfer would promote the electron transfer to O2 and consequently improve the efficiency of the charge separation for phenol degradation at interfaces. tion is the anion-promoted generation of free •OH radicals in solution, which are more reactive than the •OH radicals bound to the oxide surface.12,13 For this hypothesis, several models have been proposed. The replacement of surface OH− with F− would facilitate the hole oxidation of solvent H2O to form free • OH.11,16 The formation of a fluorine hydrogen bond at interfaces would promote the desorption of surface-bound •OH into solution.17,18 The creation of a surface electrostatic field by F− and PO43− would attract the holes toward the surface and consequently accelerate the hole oxidation of H2O/OH− into • OH.22−24 On the other hand, the observed positive effect of F− is attributed to a negatively shifted flat band potential of TiO2, which would favor the orbital overlap between hvb+ and the substrate and consequently would accelerate the substrate oxidation.19−21 The observed positive effect of PO43− is attributed to an increased adsorption of O2 on the oxide, which would not only increase the rate of O2 reduction but also improve the efficiency of the charge separation.25,26 In all cases, F− anions are claimed to be very stable against hole oxdiation.11 Through spin-trapping EPR (electron paramagnetic resonance), the thermodynamically allowed hole oxidation of PO43− has been also ruled out.27 To date, the mechanisms

1. INTRODUCTION Electronically excited TiO2 with UV light can initiate various redox reactions at room temperature, including O2 reduction to H2O2, water oxidation to O2, and organic degradation to CO2.1−3 This is in accordance with the ultimate goal of utilizing solar energy for water splitting, chemical synthesis, and environmental remediation. It is generally recognized that TiO2 upon band gap excitation will generate an electron (ecb−) and a hole (hvb+) in the conduction and valence bands, respectively. These charge carriers may recombine with heat or migrate to the surface where they are trapped and eventually react with adsorbates. However, the quantum efficiency of TiO2 photocatalysis for a target reaction is usually low.5,6 This is mainly due to the fast recombination of ecb− and hvb+ and/or slow interfacial charge transfer to target substrates. To improve the photocatalytic activity of TiO2, many studies have been made, including the surface modification of TiO2 with cocatalysts. For instance, cobalt phosphate and IrO2 deposited on TiO2 can promote hole transfers for the photoelectrochemical (PEC) oxidation of water7−9 and for the photocatalytic (PC) oxidation of a fluorescent dye in aqueous solution.10 Interestingly, fluoride anions11−21 and phosphate anions22−26 present in the aqueous suspension of TiO2 are also beneficial to the PEC oxidation of water21,24 and to the PC degradation of many organic substrates, including colorless phenol and methanol. However, understanding these anion effects is not straightforward. One common interpreta© XXXX American Chemical Society

Received: March 31, 2015 Revised: August 28, 2015

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DOI: 10.1021/acs.jpcc.5b03087 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C


responsible for the observed effects of F− and PO43− anions remain incompletely understood.28 In this work, we report that the addition of borate anions to an aqueous suspension of TiO2 can also result in a significant enhancement in the rate of phenol degradation. This positive effect of borate was further observed from 2,4-dichlorophenol degradation, spin-trapped •OH formation, H2O2 decomposition, and chromate reduction in the presence of phenol. However, the borate effect on the PC and PEC oxidation of water was negligible, which was quite different from those observed with fluoride and phosphate.21,24 To understand the unique behavior of borate, several measurements were performed, including the adsorption isotherm of borate on TiO2, the effects of borate and proton concentration, and the fates of ecb− and hvb+ with respect to the addition of borate. Furthermore, a plausible mechanism responsible for the borate effect is discussed.

Horseradish peroxide (POD), N,N-diethyl-p-phenylenediamine (DPD), and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were purchased from Sigma-Aldrich, and azomethine H was from Alfa Aesar. Other chemicals were obtained from Shanghai Chemicals Inc., including NaF, NaClO4, Na2CO3, Na2SO4, Na3PO4·12H2O, Na2B4O7·10H2O, and phenol. All solutions were prepared with Milli-Q water, and the solution pH was adjusted with a dilute solution of NaOH or HClO4. Photocatalysis and Analysis. Reactions were performed at 25 °C in a Pyrex glass reactor. The light source was a highpressure Hg lamp (300 W, Shanghai Yamin) and was set in front of the reactor at a fixed distance of 10 cm. The light intensity reaching the reactor was 3.30 mW/cm2, as measured with an irradiance meter (Instruments of Beijing Normal University). Unless stated otherwise, experiments were carried out under fixed conditions (1.00 g/L TiO2, 0.43 mM phenol, 8.0 mM Na2B4O7, and initial pH 7.0). The aqueous suspension (50.0 mL) containing the necessary components was stirred in the dark for 30 min 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 immediately analyzed by HPLC (high-performance liquid chromatography) on a Dionex P680 (Apollo C18 reverse column, and 50% CH3OH aqueous solution as an eluent). Hydrogen peroxide was measured on an Agilent 8453 UV− visible spectrophotometer at 553 nm through the PODcatalyzed oxidation of DPD.29 Borate anion was analyzed at 413 nm through its complex with azomethine H.30 The phosphate anion was analyzed at 889 nm through phospho-molybdenum blue.31 The adsorption isotherms of borate and phosphate on TiO2 in aqueous suspension at pH 7.0 were measured at a high TiO2 loading (20.00 g/L). The EPR spectrum of the DMPO−•OH adduct was recorded at the X-band with a Pyrex capillary tube on a Bruker A300 spectrometer equipped with a xenon lamp (150 W, USHIO). (Photo)electrochemical Measurement. The TiO2 film electrode was prepared by the doctor blade method. A gel containing TiO2 and PVA (poly(vinyl alcohol)) was used to coat an indium-doped tin oxide (ITO) substrate, followed by annealing in air at 500 °C for 3 h. The resulting ITO glass was cut into several pieces, with an exposed area of 1 cm × 1 cm. The TiO2 film was then used as the working electrode. Measurements were carried out on a CHI660E Electrochemical

2. EXPERIMENTAL SECTION Materials. Unless stated otherwise, the TiO2 sample purchased from Sigma-Aldrich was used throughout this study. Other TiO2 samples, denoted as AT-A, AT-M, AT-O, and AT-T, were from Shanghai Aladdin Reagent Co., Shanghai Macklin Biochem. Co., Shanghai Chemicals Inco., and Taixing Nanomaterials, China, respectively. The solid was characterized by X-ray diffraction, N2 adsorption, and UV−vis diffuse reflectance spectroscopy (Figure S1, Supporting Information). All TiO2 was in the form of anatase, except AT-T, which was a mixture of anatase (98%) and rutile (2%). The resulting physical parameters for these TiO2 samples are summarized in Table 1. Table 1. Physical Parameters of TiO2 Samplesa samples TiO2 TiO2-A TiO2-M TiO2-O TiO2-T

ds (nm)

Asp (m2/g)

Vp (cm3/g)

dp (nm)

± ± ± ± ±

144 61 7 156 144

0.3128 0.4168 0.0599 0.2724 0.6174

81 239 388 50 144

13.4 19.1 44.7 8.0 10.9

0.2 0.2 2.0 0.2 0.1

a ds, average crystallite size of (101) anatase; Asp, BET surface area; Vp, total pore volume; dp, average pore size.

Figure 1. (A) Time profiles of phenol degradation. (B) Apparent rate constants of phenol degradation obtained with different TiO2 samples. All experiments were performed in an aerated aqueous suspension at initial pH 7.0 (a) in the absence and (b) in the presence of 8.0 mM Na2B4O7. Curve (c) represents the reaction in a homogeneous solution without TiO2. B


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much higher than that (7−9) obtained in the absence of borate. Since HQ is the hydroxylated product of phenol, it follows that borate may have a positive effect on the generation of •OH and/or on the generation of phenol+•, followed by hydrolysis to form HQ. Nevertheless, the increased formation of HQ in aqueous solution may result from the borate-induced desorption of HQ from TiO2. However, a separate experiment in the dark, with 1.00 g/L TiO2 and 40 μM HQ or BQ in aqueous solution at pH 7.0, showed that the organic adsorption was weak (10%) and changed only by 1% upon addition of borate (8.0 mM). Therefore, the borate-induced enhancement in the photogeneration of oxidative species is highly possible, which will be discussed in the Possible Mechanism section. Phenol in aqueous solution weakly adsorbed onto TiO2 (3%). Then, a question arose regarding whether borate has an inhibitive effect on the adsorption and degradation of a highly adsorptive substrate over TiO2. With this concern, 2,4dichlorophenol (DCP) was used as a model substrate. Figure S5 shows the isotherm of DCP adsorption on TiO2 in aqueous solution at pH 7.0, where Ceq and qe represent the concentrations of DCP in the aqueous phase and on the oxide surface at equilibrium, respectively. The isotherm well fit the Langmuir adsorption equation, qe/qmax = KCeq/(1 + KCeq), from which the maximum amount of adsorption (qmax) and the adsorption constant (K) were calculated to be 27.3 μmol/g (0.19 μmol/m2) and 4.0 × 104 M−1, respectively. In the presence of 8.0 mM Na2B4O7, the isotherm of DCP adsorption on TiO2 was also Langmuir-type, but the values of qmax and K were decreased to be 17.8 μmol/g (0.12 μmol/m2) and 3.2 × 104 M−1, respectively. This means that in aqueous solution, borate competes with DCP for the sorptive sites of TiO2. In general, the rate of organic degradation is proportional to the amount of organic adsorption on the oxide.33 However, in aqueous solution, the rate of DCP degradation over the irradiated TiO2 was increased by approximately 2.2 times upon addition of borate (Figure 2). These observations indicate that borate has a positive effect on both the TiO2-photocatalyzed degradation of a weakly and strongly adsorptive phenol in aqueous solution. Furthermore, all of the time profiles of phenol and DCP degradation fit well to the pseudo-first-order rate eq (Figure S6). In general, organic degradation is initiated by various reactive species (Ox) generated from the irradiated TiO2,


Station (Chenhua, Shanghai) using a saturated calomel electrode (SCE) as the reference electrode and platinum gauze as the counter electrode. The supporting electrolyte was 0.5 M NaClO4, and the light source was a 150 W Xe lamp. Linear sweep voltammetry (LSV) was recorded at a scan rate of 10 mV/s. A Mott−Schottky (M−S) plot was obtained by following a literature method.32 Briefly, a set of cyclic voltammetry curves were measured at different scan rates (v = 0.02−0.25 V/s). Then, at given potential (E), the current (i) was plotted against v, resulting in the differential capacity (C = i/v). Through a plot of (1/C)2 against E, the flat band potential of TiO2 was calculated. In this study, all of the potentials are presented against the normal hydrogen electrode (NHE) through the equation E(vs NHE) = E(vs SCE) + 0.24 V.

3. RESULTS AND DISCUSSION Phenol Degradation. In the present study, all of the photoreactions were carried out under UV light at wavelengths longer than 320 nm. Under these conditions, both phenol and sodium borate in aqueous solution at pH 7.0 were stable against photolysis because they did not absorb the incident light (Figure S2A). In such a solution, phenol adsorption on TiO2 was also negligible (less than 3%). Figure 1A shows the result of phenol degradation in the aerated aqueous suspensions of TiO2 at initial pH 7.0. As the irradiation time increased, the concentration of phenol in the aqueous phase decreased, which is indicative of phenol degradation initiated by TiO2 photocatalysis. Strikingly, the rate of phenol degradation became notably increased upon addition of borate. This beneficial effect of borate is not due to its effect on the optical property of TiO2. A mixture of TiO2 and borate showed an absorption spectrum nearly overlapping that of parent TiO2 (Figure S2B). Since TiO2 was only the light-absorbing species, it follows that borate anions may have participated in the TiO2photocatalyzed degradation of phenol in aqueous suspensions. The positive effect of borate was also observed with other anatase TiO2 samples (Figure 1B). With each TiO2, the rate of phenol degradation in the presence of borate was always larger than that in the absence of borate. However, the borate-induced rate enhancement of phenol degradation did not correlate well with the photocatalytic activity of TiO2 used. Moreover, when P25 was used as a photocatalyst, the rate of phenol degradation upon addition of borate was increased by only 3.4% (Figure S3). On one hand, the photocatalytic activity of TiO2 is greatly dependent on its physical properties (Table 1).1−3 On the other hand, the interaction between TiO2 and borate may change from one TiO2 sample to another. A detailed discussion of these discrepancies among the samples is not the purpose of this work. According to our knowledge, this positive effect of borate on the anatase-photocatalyzed reaction is first reported in TiO2 photocatalysis. During phenol degradation (Figure 1A), hydroquinone (HQ) and benzoquinone (BQ) were identified as the major organic intermediates (Figure S4A). Note that catechol is often produced from phenol degradation in acidic aqueous solution,11 but it was not found here in the neutral solution. Regardless of borate being present or not, the total amount of HQ and BQ produced was always much less than the amount of phenol that disappeared. At the same time, the suspension pH also gradually decreased with time (Figure S4B). These observations indicate that most of the phenol has degraded into some small fragments. Interestingly, the mole ratio of HQ to BQ obtained in the presence of borate was 19−36, which was

Figure 2. Photocatalytic degradation of 2,4-dichlorophenol in the aqueous suspension of TiO2 at initial pH 7.0 (a) in the absence and (b) in the presence of 8.0 mM Na2B4O7. Curve (c) refers to the homogeneous reaction without TiO2. C


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Figure 3. Effects of (A) borate concentration at initial pH 7.0 and (B) solution pH at 2.0 mM Na2B4O7 on the rate increase of phenol degradation (kobs,B/kobs, solid symbols), where kobs and kobs,B represent the apparent rate constants of phenol degradation obtained in the absence and presence of borate, respectively. The amount of B4O72− adsorption on TiO2 (qe,B, open symbols) was separately measured with 20.0 g/L TiO2 (A) at initial pH 7.0 and (B) at 8.0 mM Na2B4O7.

including ecb−, hvb+, O2−•, and •OH. The reaction between Ox and the organic substrate (S) can occur either on the oxide surface or in solution. Under a fixed condition (TiO2 loading, light intensity, etc.), the concentration of Ox would be constant. However, the kinetic analysis of four different pathways results in similar rate equations of the Langmuir− Hinshelwood type.34 In all cases, the rate of organic degradation is first order in S at low [S] and zeroth order in S at high [S]. Accordingly, the rate equation alone is silent concerning the real mechanism.35,36 In the present case, the substrate concentration was low, and thus first-order kinetics in phenol was observed. Upon addition of borate, the kinetics remained unchanged, and the reaction rate increased. This is indicative of an increase in the concentration of Ox. Such an enhancement in the generation of Ox implies that borate has participated in the TiO2-photocatalyzed reaction. Since the reaction kinetics is still first order in phenol, it is highly possible that the borate concentration remains constant during the reaction process. In other words, borate seems to act as a catalyst, promoting the generation of Ox. Effect of Borate Concentration and Solution pH. Figure 3A shows the results of phenol degradation in aqueous solution as a function of borate concentration, measured at a fixed initial pH (7.0) and TiO2 loading (1.0 g/L). As the concentration of borate in the suspension increased, the rate of phenol degradation increased, and then approached a limit (Figure S7A). Similar changes were also observed in the amount of borate adsorption on TiO2 (qe,B) and in the borateinduced rate enhancement of phenol degradation (kobs,B/kobs, Figure 3A). Interestingly, the relationship between kobs,B/kobs and qe,B was approximately linear (Figure S7B), which implies that the boron species adsorbed on the oxide is responsible for the observed positive effect of borate. In the saturation region of qe,B, the free borate in solution accounted for more than 90% of total borate added to the suspension. However, in this region, the rate enhancement of phenol degradation upon addition of borate was very small. Therefore, the observed positive effect of borate is ascribed to the adsorbed borate on the oxide, other than the free borate in solution. Figure 3B shows the results of phenol degradation in aqueous solution as a function of the initial pH, measured at a fixed borate concentration and TiO2 loading. As the initial pH of the suspension increased, the rate of phenol degradation

increased and then decreased after reaching a maximum (Figure S7C). However, the initial pH effect observed in the presence of borate was much larger than that measured in the absence of borate. It implies that the borate-induced rate enhancement of phenol degradation is not simply due to the pH effect. In fact, the rate increase of phenol degradation upon addition of borate (kobs,B/kobs) was observed only at pH values of 5−9 (Figure 3B). Furthermore, the value of kobs,B/kobs proportional to the value of qe,B was observed only at pH values of 5.0−7.0 (Figure S7D). Above pH 7.0, an increase in qe,B resulted in a decrease in kobs,B/kobs. These observations confirm the importance of borate adsorption on one hand and imply the structure effect of boron species on the other hand. It is known that the solution chemistry of metal borate is greatly dependent on the boron concentration, solution pH, and temperature.37 In aqueous solution at low and high pH, boron species would mainly exist in the forms of H3BO3 and B(OH)4−, respectively. In a neutral solution, boron species would exist in a polymeric form including B3O3(OH)4− and B5O6(OH)4−. In the present solution, the exact structure of boron species is unknown. However, the positive effect of borate observed only around neutral pH implies that the polymeric boron species might be more efficient than the monomeric boron species in participating in the TiO2-photocatalzyed degradation of phenol. On the other hand, the positive effect of borate observed at pH 5−9 would be very relevant to water treatment. Comparison to Other Anions. Inorganic anions are widely present in an aquatic environment, and their effect on the TiO2-photocatalyzed reaction has been examined probably since 1990.38 Figure 4 shows the result of phenol degradation over TiO2 in an aqueous suspension at initial pH 7.0. On the addition of perchlorate and sulfate, the rates of phenol degradation changed a little, but on the addition of phosphate, fluoride, and borate, the rates of phenol degradation were increased by 0.54, 0.72, and 2.22 times, respectively. Since the reaction was carried out under air, the carbonate dissolved in aqueous solution may have an influence on the TiO2−PC reaction. Upon addition of 8.0 mM Na2CO3, the rate of phenol degradation was increased by 0.21 times. However, the rate of phenol degradation measured in the presence of Na2CO3 and Na2B4O7 was nearly the same as that measured in the presence of Na2B4O7. These observations confirm that borate does have a positive effect on the TiO2−PC degradation of phenol. On D


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irradiated TiO2 occurs via sequential one-electron-transfer steps, with the concurrent oxidation of H2O to O2,39 and that the rate of Cr(VI) reduction is not influenced by O2 present in solution.40 An experiment was carried out in an aerated aqueous suspension of TiO2 at an initial pH of 7.0. As the irradiation time increased, the concentration of Cr(VI) in the solution phase decreased. However, upon addition of borate, the rate of Cr(VI) reduction slightly decreased (Figure S9). This observation indicates that borate does not have a positive effect on the electron reduction of Cr(VI) to Cr(III) or on the hole oxidation of H2O to O2. The observed slight decrease in the rate of Cr(VI) reduction upon addition of borate is mostly due to a slight decrease in the amount of Cr(VI) adsorption on TiO2. However, in the presence of phenol, the rate of Cr(VI) reduction was notably increased upon addition of borate (Figure 5A). Meanwhile, the rate of phenol degradation was also greatly increased (Figure 5B). Control experiments without TiO2 under similar conditions showed that Cr(IV) did not react with phenol in the dark or under UV light. These observations indicate that the enhanced reduction of Cr(VI) upon addition of borate is due to the enhanced degradation of phenol over irradiated TiO2. Since the electrons and holes of TiO2 are generated in a pair, it is highly possible that there is a borate-mediated hole transfer to phenol, which facilitate the charge separation and consequently accelerates Cr(VI) reduction. This issue concerning the possible role of borate will be further discussed in the mechanism section. Formation of Oxygen Reactive Species. It is widely accepted that •OH radicals can be produced from the hole oxidation of OH−/H2O on TiO2. The detection of •OH radicals is usually made by using DMPO spin-trapping EPR.41 Figure 6 shows the EPR spectrum and signal intensity as a function of time, measured in the aerated aqueous suspension of TiO2 at initial pH 7.0. In the dark, there was no signal. Under the suspension irradiated with UV light, a quartet signal was observed due to the formation of the DMPO−•OH adduct. However, as the irradiation time was further increased, this signal intensity began to decrease, probably due to the hole oxidation of the DMPO−•OH adduct.42 Interestingly, both the signal increase and decay with time became faster upon addition of borate. These observations indicate that borate has a positive effect on both the formation and decomposition of the DMPO−•OH adduct, similar to those observed from


Figure 4. Apparent rate constants of phenol degradation over TiO2 in aqueous suspensions at initial pH 7.0, measured in the presence of different inorganic anions (8.0 mM).

the other hand, the different effects of these anions are not simply due to their difference in adsorption on TiO2. For example, in aqueous solution at pH 7.0, fluoride adsorption on TiO2 was rather weak, but its positive effect on phenol degradation was still very strong, as argued in the early study.16,17 Moreover, in aqueous solution at pH 7.0, both phosphate and borate ions strongly adsorbed onto TiO2 (Figure S8). The measured values of qmax and K were 139 μmol/g (0.96 μmol/m2) and 3.41 × 103 M−1 for phosphate and 41.5 μmol/g (0.29 μmol/m2) and 167 M−1 for borate (B4O72−), respectively. Although phosphate in aqueous solution had a large and strong adsorption on TiO2, its positive effect on phenol degradation was small, as compared to that of borate under similar conditions. It should be pointed out that the anion effect on the TiO2−PC degradation of the organic substrate is greatly dependent on the anion concentration, the solution pH, and the surface properties of TiO2, as observed from the fluoride effect.11−21 Therefore, on the basis of the above results, it cannot be concluded that borate anions are superior to fluoride and phosphate anions. Reduction of Cr(VI). Most organic degradation occurs through the valence hole of TiO2 either directly or indirectly. Then a question arose regarding whether the electron transfer of TiO2 is influenced by borate. With this concern, the photocatalytic reduction of Cr(VI) to Cr(III) was used as a model reaction. It is known that chromate reduction over

Figure 5. (A) Chromate reduction and (B) phenol degradation measured in the N2-purged aqueous suspension of TiO2, chromate, and phenol at initial pH 7.0 (a) in the absence and (b) in the presence of 8.0 mM Na2B4O7. Curve (c) refers to the homogeneous reaction without TiO2. E


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Figure 6. (A) EPR spectra of the DMPO−•OH adducts at 40 s, and (B) EPR intensity at 3500 G, measured in the suspensions of TiO2 at initial pH 7.0 (a) without and (b) with 8.0 mM Na2B4O7.

fluoride and phosphate.13,22,23 However, the DMPO−•OH adduct can result not only from the •OH attack of DMPO but also from the hole oxidation of DMPO, followed by hydrolysis.41,43 On the other hand, the electron reduction of O2, followed by the photolysis and electron reduction of the product H2O2, can also generate •OH radicals. Therefore, this study of the DMPO−•OH adduct cannot be taken as confident evidence that borate has a beneficial effect on the formation of • OH and/or on the hole oxidation of OH−/H2O over the irradiated TiO2. The electron reduction of O2 to H2O2 over the irradiated TiO2 was then examined with a colorimetric method, and the result is shown in Figure 7 (solid symbols). In this case, excess

rate of H2O2 generation upon addition of borate is due to the increased rate of H2O2 decomposition. To practice this possibility, the TiO2−PC decomposition of H2O2 was examined in the absence of phenol, and the result is shown in Figure 7 (open symbols). Interestingly, the rate of H2O2 decomposition was greatly increased upon addition of borate. Such an enhancement in the rate of H2O2 decomposition is not due to changes in the dark adsorption of H2O2 on TiO2 or in the rate of H2O2 photolysis (Figure S10). These observations indicate that borate has a positive effect on the TiO2−PC decomposition of H2O2. However, it is still difficult to verify the positive effect of borate on the TiO2−PC reduction of O2 to H2O2. On one hand, upon addition of borate, the rate of H2O2 decomposition was increased only by 109%, not significantly exceeding the rate decrease of H2O2 formation (63%), as estimated for the first 5 min. On the other hand, experiments for the formation and decomposition of H2O2 were carried out under different conditions. This issue of O2 reduction in the absence and presence of borate will be further investigated below. (Photo)electrochemical Measurement. The first experiment was to examine the reduction of O2 on a TiO2 film electrode without illumination. Figure 8 shows the dark current−voltage curves of a TiO2 film electrode, measured in

Figure 7. Photocatalytic formation (solid symbols) and decomposition (open symbols) of H2O2 in the presence and absence of 0.43 mM phenol, respectively. The experiment was carried out in an aerated aqueous suspension of TiO2 at initial pH 7.0 (a) without and (b) with 8.0 mM Na2B4O7.

phenol was used as a hole scavenger. As the irradiation time increased, the amount of H2O2 in the aqueous phase increased and then decreased due to the formation and decomposition of H2O2, respectively. However, upon addition of borate, the formation and decomposition rates of H2O2 became decreased and increased, respectively. This makes the judgment complicated. It is possible that the observed decrease in the

Figure 8. Current−voltage curves of the TiO2 film electrode in 0.5 M NaClO4 at initial pH 7.0, measured in the dark under N2 (dotted lines) and under air (solid lines) (a) in the absence and (b) in the presence of 8.0 mM Na2B4O7. F


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Figure 9. Time profiles of open circuit potentials measured with the TiO2 film electrode in 0.5 M NaClO4 at initial pH 7.0 (A) under N2 and (B) under air (a) in the absence and (b) in the presence of 8.0 mM Na2B4O7. Two different TiO2 electrodes were used for (A) and (B), respectively.

Figure 10. (A) Current−voltage curves of the TiO2 film electrodes measured under N2 in the dark (dotted lines) and under UV light (solid lines). Conditions: (a) TiO2, (b) TiO2 + 8.0 mM Na2B4O7, (a′) TiO2 + 0.27 mM phenol, and (b′) TiO2 + 0.27 mM phenol + 8.0 mM Na2B4O7. The electrode was first used for (a) and (a′) and then for (b) and (b′) after washing with water several times. (B) Time profiles of the current at 0.94 V vs NHE, measured with a new TiO2 film electrode in the presence of (c) 0.43 mM phenol and (d) 0.43 mM phenol + 2.0 mM Na2B4O7. All experiments were carried out in 0.5 M NaClO4 at initial pH 7.0.

In this case, the number of electrons photogenerated on the TiO2 film electrode in the presence of borate was obviously greater than that in the absence of borate. A greater number of electrons remaining on the electrode indicates a slower rate of O2 reduction. Both sets of OCP results obtained under N2 and under air indicate that the interfacial electron transfer of TiO2 is slowed upon the addition of borate. The oxidation of water by the photogenerated hole of TiO2 was examined under N2 through linear sweep voltammetry, and the result is shown in Figure 10A. In the dark, the anodic current of the electrode was very weak. Under UV light, the electrode current was significantly increased due to the hole oxidation of water. However, the curves obtained in the absence and presence of borate nearly overlapped, which means that borate has little effect on the hole oxidation of water, as observed from chromate reduction (Figure S9). When phenol was used as a hole scavenger, the photocurrent became significantly increased due to phenol oxidation being easier than water oxidation. Interestingly, the photocurrent was further increased upon addition of borate. These observations confirm that borate is beneficial to the hole oxidation of phenol. Moreover, during the repeated experiments, the photocurrent recorded at 0.94 V vs NHE remained nearly constant, either in the absence or presence of borate (Figure 10B). Since phenol is

0.5 M NaClO4 at initial pH 7.0. The current obtained under air was much larger than that measured under N2. This observation indicates that the reduction of O2 on the electrode is the dominant process. However, upon addition of borate to the electrolyte, the cathodic current was notably decreased. Since the measurement was made with the same electrode, the observed current decrease upon addition of borate is indicative of a decrease in the amount of O2 adsorption on the TiO2 film and/or in the rate constant of O2 reduction. The reduction of O2 by the photogenerated electrons of TiO2 was investigated with an open circuit potential (OCP) technique, and the result is shown in Figure 9. After the electrode was illuminated with UV light, the electrons of TiO2 were generated. After the light was blocked off, the number of electrons on the electrode began to decrease with time due to recombination with the trapped holes and/or reactions with oxidants in the electrolyte. In general, the electron density on the electrode exponentially increases with OCP. A fast decay of OCP with time means a rapid interfacial electron transfer. The experiment was first carried out under N2 (Figure 9A). Obviously, the decay of OCP with time in the presence of borate was slower than that in the absence of borate. It means that borate has an inhibitive effect on the electron transfer of the TiO2 film. The experiment was then carried out under air. G


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rate of chromate reduction in the presence of phenol is increased (Figure 5), and the rate of O2 reduction should be increased as well. Since borate is in net recycle, with constant formation of the extra reactive species, the kinetics of phenol degradation is first order in phenol, as observed in the absence of borate (Figures 1 and 2). In thermodynamics, the oxidation of H2O by B(OH)4• to produce O2 is also possible. The four-electron redox potential for H2O in aqueous solution is 0.82 V vs NHE. But no enhancement of water oxidation to O2 was observed upon addition of borate (Figures S9 and 10). On one hand, the multielectron reduction of O2 is very slow, and it needs an overpotential probably exceeding 0.5 V. On the other hand, the PC and PEC oxidation of water over TiO2 is a complicated issue. It has been debated that O2 evolution from the irradiated aqueous solution of TiO2 occurs primarily through a Lewis acid−base-type reaction instead of an electron-transfer-type pathway.46 On the basis of the electronic structure of water, the hole oxidation of H2O/OH− on the surface of TiO2 or in aqueous solution to •OH is also claimed to be impossible.47 In combination with those debates, the borate-induced increase in DMPO−•OH adduct formation (Figure 6B) would be ascribed to the borate-mediated hole oxidation of DMPO, followed by hydrolysis.


present in excess, it follows that both TiO2 and borate are stable during the period of time investigated. This result of PEC is in agreement with the above kinetic study that the borate concentration remains constant during the TiO2−PC degradation of phenol in aqueous suspensions (Figures 1 and 2). Possible Mechanism. To further elucidate the mechanism, the flat band potential of TiO2 (Efb) was measured in 0.5 M NaClO4 at initial pH 7.0,32 and the result is shown in Figure S11. Through the M−S plot, the values of Efb in the absence and presence of 8 mM Na2B4O7 were calculated to be −0.185 and −0.266 V vs NHE, respectively. In other words, the flat band potential of TiO2 upon addition of borate was negatively shifted by 81 mV. In the energy diagram of TiO2, the conduction and valence bands will shift upward by 81 mV (Scheme 1). This will decrease the overlap between the Scheme 1. Energy Diagram for TiO2 in the Absence (Solid Lines) and Presence (Dotted Lines) of Boratea

4. CONCLUSIONS In this work, we have shown that borate anions present in the irradiated aqueous suspension of TiO2 at neutral pH are beneficial to phenol degradation. This positive effect of borate looks similar to those of fluoride and phosphate reported in the literature. However, borate has some effects, obviously different from those of fluoride and phosphate. For example, the PEC oxidation of water to O2 is greatly enhanced by the addition of fluoride or phosphate,21,24 but borate has no obvious effect on water oxidation, either with chromate as an electron scavenger (Figure S9) or under an externally applied potential bias (Figure 10). Second, for the TiO2-photocatalyzed decomposition of H2O2, fluoride has a negative effect14,48 but borate has a positive effect (Figure 7). Then, the mechanism of the borate effect would be different from those proposed for the effect of fluoride and phosphate. (See the literature survey in the Introduction section.) Accordingly, we propose that there is a borate radical produced from the hole oxidation of borate. This kind of borate radical is reactive to phenol, DCP, H2O2, and DMPO but not to water. The borate-mediated enhancement in the hole oxidation of phenol is supported by an LSV experiment (Figure 10). On the other hand, the flat band potential of TiO2 upon addition of borate is negatively shifted, similar to those observed from the fluoride effect.16,19−21 As a result, the interfacial electron transfer from the irradiated TiO2 to O2 is slowed down, as indicated by LSV and OCP experiments (Figures 8 and 9). However, in TiO2 photocatalysis, the electrons and holes are photogenerated and consumed in a pair. Then, the borate-mediated hole transfer to phenol would compensate and promote the electron reduction of O2. As a result, the efficiency of the charge separation of TiO2 is improved, and the rate of the interfacial reaction is increased. Nevertheless, the intermediates produced from phenol degradation may remain on the oxide surface, competing with phenol for the reactive species. Due to the fluoride adsorption on TiO2, it is suspected that the fluorideinduced rate increase of phenol degradation is due to the reduced adsorption and inhibition of these intermediates.11,21

a

On the right side are shown the redox potentials for relevant couples in aqueous solution, where ph and ph+• represent phenol and its cation radical, respectively.

conduction band and the unfilled orbital of O2 but increase the overlap between the valence band and the filled orbital of phenol. As a result, the rates of O2 reduction and phenol oxidation will be decreased and increased, respectively, as observed in Figures 9 and 10. If this holds, the hole oxidation of water upon addition of borate should be enhanced, as proposed for the fluoride effect.19−21 However, in practice, neither PC nor PEC oxidation of water changed with the addition of borate (Figures S9 and 10A). In the presence of borate, there may be additionally formed some reactive species which can react with phenol and H2O2 but are not capable of H2O oxidation. This unknown species would be a kind of borate radical. To our knowledge, there is only one report showing that the reduction potential of B(OH)4• in aqueous solution at pH 11.5 is +1.4 V vs NHE.44 This potential is more negative than the valence band edge potential of anatase TiO2, which is 2.67 V vs NHE in aqueous solution at pH 7.0. Therefore, the hole oxidation of B(OH)4− to B(OH)4• is thermodynamically possible. This borate radical can oxidize phenol to phenol•+ with the recovery of the borate anion, but it is not capable of H2O oxidation to •OH. The redox potentials for the phenol•+/ phenol and •OH/H2O couples in aqueous solution at pH 7.0 are 1.03 and 2.40 V vs NHE, respectively.4,45 Then, the hydrolysis of phenol•+ in aqueous solution will result in the formation of HQ (Figure S4A). This borate-mediated hole transfer would make the electrons longer-lived. As a result, the H


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(10) Tachikawa, T.; Zhang, P.; Bian, Z.; Majima, T. Efficient Charge Separation and Photooxidation on Cobalt Phosphate-Loaded TiO2 Mesocrystal Superstructures. J. Mater. Chem. A 2014, 2, 3381−3388. (11) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Photocatalytic Transformation of Organic Compounds in the Presence of Inorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer Reactions of Phenol on Titanium Dioxide-Fluoride System. Langmuir 2000, 16, 2632−2641. (12) 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. (13) Mrowetz, M.; Selli, E. Enhanced Photocatalytic Formation of Hydroxyl Radicals on Fluorinated TiO2. Phys. Chem. Chem. Phys. 2005, 7, 1100−1102. (14) Mrowetz, M.; Selli, E. H2O2 Evolution During the Photocatalytic Degradation of Organic Molecules on Fluorinated TiO2. New J. Chem. 2006, 30, 108−114. (15) 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. (16) Park, H.; Choi, W. Effects of TiO2 Surface Fluorination on Photocatalytic Reactions and Photoelectrochemical Behaviors. J. Phys. Chem. B 2004, 108, 4086−4093. (17) 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. (18) Cong, S.; Xu, Y. Enhanced Sorption and Photodegradation of Chlorophenol over Fluoride-Loaded TiO2. J. Hazard. Mater. 2011, 192, 485−489. (19) 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. (20) 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. (21) 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. (22) 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. (23) 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. (24) 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. (25) 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. (26) 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. (27) Brusa, M.; Grela, M. A. Experimental Upper Bound on Phosphate Radical Production in TiO2 Photocatalytic Transformations in the Presence of Phosphate. Phys. Chem. Chem. Phys. 2003, 5, 3294− 3298.

However, in the present case, the TiO2-photocatalyzed decomposition of H2O2 without organics was still enhanced upon addition of borate (Figure 7). Therefore, the observed positive effect of borate is surely ascribed to the boratemediated hole transfer. This hypothesis of borate radical involvement might be relevant to further study of the phosphate, sulfate, and carbonate effect on the TiO2-photocatalyzed reactions. Moreover, the borate-induced rate enhancement of phenol degradation is determined not only by the surface concentration of borate on TiO2 but also by the structure of boron species. However, the structure effect of boron species is not clear and needs to be examined further.

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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.5b03087. XRD pattern, N 2 adsorption isotherm, substrate absorption spectra, M−S plots, and kinetic data for photocatalytic reactions and their correlation with borate adsorption (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-571-87951895. Tel: +86571-87952410. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (no. 21377110) and the National Basic Research Program of China (no. 2011CB936003).

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REFERENCES

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Borate-Mediated Hole Transfer from Irradiated Anatase TiO2 to Phenol

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