Remarkable Visible-Light Photocatalytic Activity Enhancement over Au

Jun 13, 2019 - Remarkable Visible-Light Photocatalytic Activity Enhancement over Au/p-type TiO2 Promoted by Efficient Interfacial Charge Transfer ...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24154−24163

Remarkable Visible-Light Photocatalytic Activity Enhancement over Au/p-type TiO2 Promoted by Efficient Interfacial Charge Transfer Ao Fu,† Xin Chen,‡ Lihang Tong,‡ Defa Wang,†,‡ Lequan Liu,*,†,‡ and Jinhua Ye†,‡,§

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TJU-NIMS International Collaboration Laboratory, Key Lab of Advanced Ceramics and Machining Technology (Ministry of Education) and Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Tianjin 300072, China § International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 3050044, Japan S Supporting Information *

ABSTRACT: Metal-induced photocatalysis has emerged as a promising approach for exploiting visible-light-responsive composite materials for solar energy conversion, which is generally hindered by low photocatalytic efficiency. Herein, for the first time, an Au/p-TiO2 (p-type TiO2) strategy with the hole transfer mechanism is developed, remarkably promoting visible-light photocatalytic performance. An efficient acetone evolution rate (138 μmol·g−1·h−1) in the photocatalytic isopropyl alcohol (IPA) degradation under λex = 500 nm light (light intensity, 5.5 mW/cm2) was achieved over Au/p-TiO2, which is approximately 5 times as high as that over Au/n-TiO2 under the same conditions. Photoluminescence and electrochemical impedance spectroscopy measurements indicate enhanced charge carrier separation and transfer for Au/p-TiO2. In an elaborate study, apparent quantum efficiency and transmission electron microscopy characterization on selective PbO2 deposition over p-TiO2 revealed that visible-light-excited holes other than electrons generated in the Au interband transition transferred to p-TiO2, which is opposite to the general route in Au/n-TiO2 (n-type TiO2). Energetic holes generated in the d band of Au led to a fluent transfer across the Schottky barrier, which is further confirmed by the IPA photodegradation mechanism study with different scavengers over Au/p-TiO2. This discovery opens up new opportunities in designing and developing efficient metal semiconductor composite materials with visible-light response. KEYWORDS: metal-induced photocatalysis, p-type TiO2, Au/p-TiO2, hole transfer, interband transition, IPA degradation

1. INTRODUCTION

Many attempts, including cocatalyst decoration, nanostructure construction, and morphology control, have been made to improve the photocatalytic efficiency of MIP systems.11−14 In our previous work, we also proposed an Au−Cu alloy strategy to promote visible-light photocatalytic efficiency through strengthening interband transitions (d to s).15,16 Despite these efforts, pursuing efficient MIP systems is still at a preliminary stage, and the low metal carrier separation efficiency is one of the major challenges in this field. Thus, enhancing the charge carrier separation efficiency is an effective strategy to improve the photocatalytic activity.17−19 There are at least two reasons for this challenge. On the one hand, the lifetime of plasmonic carriers in the coinage metal is in the femtosecond range, which is about 2 to 3 orders of magnitude shorter than that of semiconductors.20,21 On the other hand, the hot electrons

Photocatalysis is regarded as one of the green technologies for environmental purification and energy generation by using solar energy.1 However, various semiconductors with large band gaps limit photoabsorption to the ultraviolet (UV) region of the solar spectrum, for example, TiO2.2,3 Metal-induced photocatalysis (MIP) known as charge carriers generated by coinage metals (Au, Ag, and Cu NPs) has been attracting substantial attention owing to the superiority for visible-light absorption.4−6 A vast majority of works have been carried out on its application, including photocatalytic and photoelectrochemical (PEC) water splitting, CO2 photoreduction, and activation of inert molecules such as N2, CH4, and so forth.7−10 Much progress has been made in developing metal semiconductor (MSC) materials, yet researchers in this field still face a big challenge in promoting photocatalytic efficiency under visible light. So, exploring novel strategies, especially for enhancing photocatalytic efficiency under visible-light irradiation, will be the key to develop efficient MSC photocatalysts. © 2019 American Chemical Society

Received: April 23, 2019 Accepted: June 13, 2019 Published: June 13, 2019 24154

DOI: 10.1021/acsami.9b07110 ACS Appl. Mater. Interfaces 2019, 11, 24154−24163

Research Article

ACS Applied Materials & Interfaces transfer from the coinage metal nanoparticles to the conduction band (CB) of the semiconductor and participate in visible-light photocatalysis; but the hot electron transition should cross a relatively high Schottky barrier in the metal−semiconductor junction.22,23 It has been confirmed that the excitation of interband transitions, which is the dominant plasmon decay in Au NPs, results in the formation of an asymmetric energy distribution of high-energy holes and low-energy electrons.24 So, there is only a minority of energetic electrons that can cross the Schottky barrier and inject into the semiconductor. In addition, the interband holes created far from the Fermi level with a strong oxidation ability could cause overbarrier injection to the p-type semiconductor in theory.25,26 Herein, based on the above discussion, an Au/p-TiO2 strategy is explored in remarkably promoting visible-light photocatalytic performance via effective utilization of photogenerated holes in Au NPs. The isopropyl alcohol (IPA) degradation rate over Au/p-TiO2 was about 5 times as high as that over Au/n-TiO2. Charge carrier separation and transfer efficiency were studied with photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopy (EIS). Moreover, a combination study of selective photodeposition, relative band position analysis in Au/p-TiO2, and IPA photocatalytic degradation mechanism demonstrated a different route that the interband holes generated in Au transfer to p-TiO2.

Prior to light irradiation, the vessel was kept in the dark for 4 h until an absorption−desorption equilibrium was finally established. Monochromatic light (λ = 500 nm, 300 W Xe lamp equipped with a band-pass filter) was employed as the light source for the photocatalytic reaction. A water filter was set between the lamp and the vessel to avoid the heat effects in the measurements. The light intensity was 5.5 mW/cm2, measured by a spectroradiometer (USR-40; Ushio Inc., Japan). The concentrations of IPA and the products were detected on a gas chromatograph (GC-2014; Shimadzu, Japan) with a flame ionization detector. Apparent quantum efficiencies (AQEs) at various wavelengths were measured by inserting a water filter and various band-pass filters (500, 540, 560, 580, 600, 640, and 710 nm) in front of the reaction cell to obtain the desired incident wavelengths. (2) Photocatalytic H2 or O2 evolution was carried out with 0.1 g of the photocatalyst suspended in 220 mL of water in the presence of 50 mL of methanol as a sacrificial reagent or 270 mL of water in the presence of 0.85 g of AgNO3 as a sacrificial reagent. Pt served as the cocatalyst and was loaded by photodeposition during photocatalytic H2 evolution. Monochromatic light (λ = 500 nm, 300 W Xe lamp equipped with a band-pass filter) was employed as the light source for the photocatalytic reaction. A water filter was set between the lamp and the vessel to protect the vessel. The concentrations of H2 and O2 were analyzed by a gas chromatograph equipped with a TCD detector. (3) Photodegradation mechanism study. To study whether the electrons or holes play a role in the IPA photodegradation, AO, p-BQ, and t-BA served as the scavengers for h+, •O2− and •OH, respectively. To load the AO and p-BQ scavengers on the sample surface, 100 mg of the sample and 10 wt % scavenger were dispersed in 20 mL of ultrapure water in a beaker, and then the suspension was ultrasonically treated for 20 min. Finally, the sample was dried in an oven at 80 °C overnight. For introducing the t-BA scavenger, the liquid t-BA and IPA (molar ratio = 10:90) were mixed well. To study the influence of the t-BA scavenger, the mixed t-BA and IPA gas was injected into the reactor. The evaluation conditions of the photodegradation mechanism study were similar to those mentioned above.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrabutyl titanate [Ti(OC4H9O)4, TBT], ethanol, glycerol, chloroauric acid tetrahydrate (HAuCl4·4H2O), IPA (C3H8O), urea [CO(NH2)2], ammonium oxalate [(NH4)2C2O4·H2O, AO], pbenzoquinone (C6H4O2, p-BQ), and t-butyl alcohol (C4H10O, t-BA) were all purchased from the Tianjin Guangfu Fine Chemical Research Institute, China. 2.2. Synthesis of Au/TiO2. The precursor of p-type TiO2 was prepared through a hydrothermal method, as reported in Zou’s work.27 TBT (2 g) was added into a solution of 60 mL of ethanol and 20 mL of glycerol under magnetic stirring, and then the solution was heated in a 100 mL Teflon-lined autoclave at 180 °C for 24 h. The collected powders were washed with absolute ethanol three times and then dried at 60 °C overnight. X-ray diffraction (XRD) patterns of the precursor are shown in Figure S1 (Supporting Information). The calcination process was different from that in Zou’s work. For the synthesis of ptype TiO2, the precursor were calcined in the oxygen−argon gas mixture (O2/Ar = 30:70) at 470 °C for 1 h with a heating rate of 5 °C/ min. A series of p-type TiO2 with different amounts of Ti vacancies (VTi) were prepared by controlling the ratio of glycerol to ethanol (1:1, 1:2, 1:3, 1:4, and 1:5). As a reference, n-TiO2 was synthesized using the same procedure but without glycerol in the hydrothermal method. The Au/TiO2 photocatalysts were synthetized by the deposition− precipitation method, as shown in our previous work.15 TiO2 (1.0 g) was added to 100 mL of an aqueous solution of HAuCl4 and urea. The Au mass percentage was kept at 1% after optimization (Figure S2). Urea, as the precipitating base, was added with a molar ratio (urea to Au) of ca. 300. Then, the suspension was vigorously stirred at 80 °C for 4 h, and finally, the samples were washed with deionized water several times and centrifuged at 8000 rpm to separate the powders. Then, Au/ TiO2 was obtained after calcination in air at 400 °C for 4 h. The accurate Au contents in Au/p-TiO2 and Au/n-TiO2 detected by inductively coupled plasma−atomic emission spectrometry (Varian 725-ES, American) were 0.63 and 0.72 wt %, respectively (Figure S3). 2.3. Photocatalytic Activity Evaluation.

2.4. PEC Measurements. PEC properties were measured using an Autolab potentiostat/galvanostat (Model PGSTAT) in a threeelectrode cell with a Pt wire as the counter electrode and an Ag/ AgCl as the reference electrode. Na2SO4 (0.1 M) was used as the electrolyte solution. EIS measurements were carried out with a sinusoidal ac perturbation of 10 mV applied over the frequency range of 0.01 to 105 Hz. 2.5. Theoretical Simulation. The absorption spectrum of Au was simulated based on the Mie theory. The dielectric constant for Au was taken from Johnson and Christy’s report. In the absorption spectra of Au NPs with a diameter of 3 nm, an overlap of the surface plasmon resonance band and the interband transition absorption edges was observed. After clarifying the contribution of the localized surface plasmon resonance (LSPR), the interband transition spectrum was obtained. 2.6. Structure Characterizations. XRD patterns were obtained using a D/MAX-2500 X-ray diffractometer equipped with Cu Kα radiation at 40 kV and 140 mA at a scanning rate of 5°/min. Transmission electron microscopy (TEM) analysis was performed using a Tecnai G2F-20 transmission electron microscope with a fieldemission gun operating at 200 kV. UV−visible (UV−vis) diffuse reflectance spectra were obtained on a UV−vis spectrophotometer (UV-2700, Shimadzu, Japan) and then converted into absorption spectra via the Kubelka−Munk transformation. The electron spin resonance (ESR) characterization was performed by a JFS-FA200 (JEOL, Japan) spectrometer. X-ray photoelectron spectrum (XPS) measurements were performed on a PHI 5000 Versa Probe (Φ ULVAC-PHI, Inc., Japan/USA) model X-ray photoelectron spectrometer instrument with monochromatized Al Kα radiation (1486.6 eV) as an X-ray anode operated at 50 W. The pressure was 10−7

(1) IPA photodegradation. Typically, 50 mg of the catalyst was spread uniformly in a quartz vessel with an irradiation area of 7.06 cm2. Then, the vessel was pretreated by artificial air [v(N2)/ v(O2)] = 4:1 for 10 min to remove absorbed gaseous impurities. A certain amount of gaseous IPA was injected into the vessel. 24155

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Figure 1. (a) XRD patterns of four samples; (b) low temperature (123 K) ESR spectra of p-TiO2 and n-TiO2; (c) Ti 2p3/2 XPS spectra of p-TiO2 and nTiO2; and (d) positron annihilation spectra of Au/p-TiO2 and Au/n-TiO2.

vacancies (VTi).29,30 In the ESR study, a remarkably strong ESR signal at g = 1.998 corresponding to p-TiO2 is observed under both room temperature (298 K) and low temperature (123 K), which is caused by Ti vacancies (VTi).29,30 On the other hand, nTiO2 has only a weak signal at g = 1.990 at 123 K, which can be assigned to the existence of trace Ti3+−oxygen vacancy associates (Figures 1b and S5).30−32 In addition, the presence of titanium vacancies was further confirmed by XPS characterization. The Ti 2p3/2 XPS peak of n-TiO2 is located at 457.7 eV associated with Ti3+ (accompanied by VO), whereas p-TiO2 has a peak located at 458.7 eV.29,30 The increased binding energy is because the neighboring O and Ti atoms of VTi get less electrons compared with those on n-TiO2 (Figures 1c and S6).33 The amount of Ti−OH groups studied by XPS in n- and p-TiO2 was determined to be 6.3 and 5.7%, respectively (Figure S7). Moreover, the I−t curve with a sharp cathodic current peak appeared upon the turn-on of the light (Figure S8), whereas nTiO2 did not show any cathodic current under the same conditions (Figure S9), which is also consistent with the feature of p-type TiO2.29 The negative slope of Mott−Schottky plots gives out more solid evidence to confirm the p-type TiO2 (Figure S10). The maintenance of p-TiO2 after Au loading was demonstrated by PALS. The positron annihilation lifetime has been proven to be a useful method for determining the intrinsic defects in the semiconductor, especially for investigating vacancy-type defects.34 Three positron lifetime components were observed for Au/p-TiO2 and Au/n-TiO2 (Figure 1d and Table S1). The shorter lifetime component (τ1) was attributed to the free annihilation of positrons in a defect-free crystal. In a disordered system, shallow positron traps (e.g., oxygen vacancies) or small vacancies (e.g., monovacancies) can reduce the surrounding electron density and hence increase the τ1 lifetime.35 It has been reported that the shorter lifetime components (τ1) for defected TiO2 (287.0 ps) and normal TiO2 (264.0 ps) correspond to the existence of titanium vacancies and oxygen vacancies, respectively.29 τ1 values for Au/

Pa, and the binding energy was referenced to C 1s peaks (284.8 eV) of contaminated carbon. PL spectra were measured by a Horiba JobinYvon Fluorolog spectrometer with the excitation light at 480 nm. Positron annihilation lifetime spectroscopy (PALS) was performed with a fast/slow coincidence ORTEC system with a time resolution of ∼201 ps full width at half-maximum. A 5 × 10−5-Bq source of 22Na was sandwiched between two identical samples (10 × 10 mm2, 1 mm thickness).

3. RESULTS AND DISCUSSION p-Type TiO2 (p-TiO2) and n-type TiO2 (n-TiO2) were prepared via the solvothermal method by controlling the ratio of glycerol−ethanol and only ethanol solution, respectively, and then calcined in a mixture of oxygen and argon at 470 °C for 1 h. The XRD patterns of p-TiO2 optimized in different oxygen and argon ratios are presented in Figure S4. When the ratio of oxygen to argon is lower than 30:70, p-TiO2 was in the form of anatase phase. The anatase p-TiO2 gradually converted into the rutile phase as the ratio of oxygen to argon increased from 30:70 to 90:10. Au was loaded onto TiO2 with the deposition− precipitation method using urea as the precipitating base.15 Then, Au NPs were deposited on the TiO2 surface after calcining at 400 °C for 4 h, and tightly contacted Au/p-TiO2 composites were obtained. Previous studies have shown that the interaction between Au and TiO2 in Au/TiO2 prepared by the deposition−precipitation method is stronger than that prepared by other deposition methods such as photodeposition, which is favorable for charge separation.27,28 XRD results indicate that both anatase p-TiO2 and n-TiO2 have good crystallinity with 2θ diffraction peaks of 25.3, 37.7, and 48.0°, corresponding to (101), (004), and (200) crystal planes, respectively. A diffraction peak of Au(200) at 44.4° can be observed in both Au/p-TiO2 and Au/n-TiO2 (Figure 1a). ESR spectra, XPS spectra, transient current (I−t) curve and Mott−Schottky plot curve were employed to confirm the successful preparation of p-TiO2. It has been demonstrated earlier that common TiO2 (n-TiO2) possessed oxygen vacancies (VO), whereas defected TiO2 (p-TiO2) were rich in titanium 24156

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Figure 2. (a) UV−vis absorption spectrum of Au/p-TiO2, Au/n-TiO2, p-TiO2, and n-TiO2; (b) room-temperature ESR spectra of Au/p-TiO2 with different amounts of Ti vacancies; (c) acetone evolution rates of IPA photodegradation over the Au/p-TiO2 samples with different amounts of Ti vacancies; and (d) IPA photodegradation over Au/p-TiO2, Au/n-TiO2, p-TiO2, and n-TiO2 (catalyst, 1 g; light source, λex = 500 nm; light intensity = 5.5 mW/cm2).

p-TiO2 gradually increased, which is reflected in the intensity of the ESR signal. During this process, marked improvement of the acetone evolution rate over Au/p-TiO2 is achieved with the amount of VTi increasing with the change in ratio from 1:5 to 1:2 (Figure 2c). When the ratio of glycerol to ethanol is 1:2, a stable IPA degradation rate with 138 μmol·g−1·h−1 was achieved over Au/p-TiO2, which is 5-fold as high as that over Au/n-TiO2 (Figure 2d). The Ti/O ratio in p-TiO2 was about 0.90:2 (Ti0.90O2) determined by the chemical titration method and XPS measurement. Further increasing the amount of VTi decreased the photocatalytic activity and charge separation efficiency of Au/p-TiO2 (Figures 2c and S13). It may be caused by the excessive titanium vacancies serving as carrier recombination sites, which hamper the charge carriers’ separation process. Recycle experiments evaluating the photocatalyst stability over Au/p-TiO2 were also carried out. The amount of acetone evolution decreased by 7.6% after four cycles (2 h per cycle), which demonstrates a relatively good stability and repeatability of Au/p-TiO2 for photocatalytic IPA degradation (Figure S14). The main product of IPA photocatalytic degradation is acetone, which should be mainly caused by the following two aspects: (1) further oxidizing acetone to CO2 involves a complex multiphoton reaction, which makes it more difficult as compared with the photocatalytic degradation of IPA to acetone36,37 and (2) the initial concentration of IPA is high. To clarify that charge carriers (practically holes in Au/pTiO2) possess the oxidation potential to oxidize acetone into CO2, a control experiment was carried out by reducing the amount of IPA. As shown in Figure S15, acetone production reaches a maximum at 70 min and decreases thereafter, whereas CO2 production keeps increasing all through the evaluation, indicating that acetone can be further oxidized to CO2 over Au/ p-TiO2 under 500 nm light irradiation. For further comparison, photocatalytic water splitting was also carried out under λex = 500 nm light irradiation (light intensity = 5.5 mW/cm2) (Figures 3a and S16). Au/p-TiO2

p-TiO2 (297.0 ps) and Au/n-TiO2 (312.3 ps) are longer than those of defected TiO2 and normal TiO2, respectively. These results indicate that Au/p-TiO2 and Au/n-TiO2 possess titanium vacancies and oxygen vacancies, respectively; meanwhile, Au decreases the annihilation rate and increases the positron lifetime correspondingly. In addition, this conclusion was also confirmed by ESR tests (Figure S11). 3.1. Photophysical and Photocatalytic Properties. UV−vis diffusion reflectance spectra are shown in Figure 2a. Pure anatase TiO2 possessing a band gap of 3.2 eV can only be excited by UV light. Visible-light absorption, composed of strong absorption in the 400−500 nm range and an obvious LSPR peak centered at about 560 nm, is obtained after introducing Au NPs. For Au/TiO2 prepared with the same method, Au NPs should have similar shapes and sizes in Au/pTiO2 and Au/n-TiO2. Therefore, the shift of the plasmonic peak position can be attributed to the different dielectric constants of p-TiO2 and n-TiO2. The IPA photodegradation was adopted to study the photocatalytic performance under λex = 500 nm irradiation by using a band-pass filter (light intensity = 5.5 mW/cm2), and the lamp was equipped with a heat-absorbing filter to avoid the heat effect in evaluation. A primary study was carried out by evaluating the acetone evolution rate over as-prepared samples when the ratio of glycerol to ethanol in p-TiO2 was 1:3 (Figure S12). Pure p-TiO2 and n-TiO2 showed negligible activity. The acetone evolution rate of Au/n-TiO2 was still fairly low (28 μmol·g−1·h−1), whereas Au/p-TiO2 exhibited significantly enhanced activity with 99 μmol·g−1·h−1 under the same conditions. This result experimentally proves that the proposed Au/p-TiO2 strategy of introducing Au deposited on p-TiO2 with titanium vacancies is promising. To optimize Au/p-TiO2, a series of Au/p-TiO2 with different amounts of Ti vacancies (VTi) were prepared by controlling the ratio of glycerol to ethanol (1:5, 1:4, 1:3, 1:2, and 1:1) (Figure 2b). With the increase in the ratio of glycerol to ethanol, the amount of Ti vacancies (VTi) in 24157

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Figure 3. (a) Time course of O2 production; (b) LSV curves of Au/TiO2 and Au/n-TiO2 under λex = 500 nm light irradiation and in the dark; (c) transient photocurrent responses at 0.3 V vs Ag/AgCl under λex = 500 nm light irradiation and in the dark; (d) AQEs of IPA degradation over Au/pTiO2 at various light wavelengths.

Figure 4. (a) PL spectra of Au/p-TiO2 and Au/n-TiO2; (b) fluorescence decay of Au in Au/p-TiO2 and Au/n-TiO2; (c) EIS Nyquist plot of samples under λex = 500 nm light irradiation. Inset in (c): enlarged EIS patterns of Au/p-TiO2 and Au/n-TiO2.

the Au NPs were obtained by a sputter-coating method. The thickness of Au deposited by the sputtering technique is about 3 nm detected by a NX-10 atomic force microscope (Park System) (Figure S17). Linear sweep voltammetry (LSV) curves of the as-prepared samples under λex = 500 nm light irradiation and in the dark within the potential range of −0.05 to 0.7 V versus Ag/AgCl are shown in Figure 3b. Compared with Au/nTiO2, the photocurrent of Au/p-TiO2 increased significantly when the light was turned on, indicating that Au/p-TiO2 harvests light better than Au/n-TiO2. In addition, the photocurrent response of Au/p-TiO2 presents a remarkable enhancement, reaching about 2.3-fold that of Au/n-TiO2 (Figure 3c). As

shows the photocatalytic performance with an O2 release rate of 17.4 μmol·g−1·h−1 and a H2 release rate of 7.8 μmol·g−1·h−1, which is about 1.7 times and 1.4 times as much as that of Au/nTiO2, respectively. These results further confirm that the photocatalytic efficiency of Au/p-TiO2 is superior to that of Au/ n-TiO2. PEC performance over composited materials was also studied by a conventional three-electrode configuration in an aqueous solution containing 0.1 M Na2SO4. The Au/TiO2 film served as the photoelectrode. The p-TiO2 and n-TiO2 films coated on fluorine-doped tin oxide (14 Ω per square, Nippon sheet Glass Group, Japan) were prepared by the hydrothermal method, and 24158

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transfer resistance in both cases, which suggests that the charges can quickly transfer to the interface and be consumed by the surface reaction. 3.3. Clarifying Electron-Transfer Direction. It is generally accepted that a direct electron transfer from the metal to the n-type semiconductor represents the common transmission path in MIP systems under visible light.15,44 Nevertheless, the type of charge carrier transfer may be different in the composite between Au and p-type semiconductor (electron transfers from Au to p-type semiconductor, or hole transfers from Au to p-type semiconductor).26,45,46 Selective photodeposition of Ag and PbO2 nanoparticles were carried out for both Au/p-TiO2 and Au/n-TiO2 under λex = 500 nm light irradiation to clarify the charge-transfer route (Figure 5). If

the samples have the same preparation process, the enhanced PEC performance should be caused by the formation of a titanium vacancy, which can efficiently promote charge transfer and separation. An AQE study, which is used to validate the intrinsic driving force in photocatalysis, was carried out over Au/p-TiO2 for IPA degradation (Figure 3d). As can be seen, the main part of the AQE curve with high values bears strong resemblance to the interband transition band, indicating that the driving force for IPA degradation over Au/p-TiO2 under visible light mainly comes from these interband transitions, which is consistent with our previous report.15 This phenomenon is also supported by the theoretical research that direct interband excitation is the dominant pathway for plasmon decay in Au nanoparticles.38,39 For comparison, an AQE of Au/n-TiO2 was also tested. AQEs of 0.22 and 0.08% at 500 nm over Au/p-TiO2 and Au/n-TiO2 were obtained, respectively. Moreover, a comparison of AQEs over metal semiconductor composites is summarized in Table S2. As can be seen, Au/p-TiO2 achieves a high level AQE in photodegradation at around 500 nm among metal semiconductor systems. These results confirmed that the Au/pTiO2 strategy could remarkably promote visible-light photocatalytic performance via effective utilization of photogenerated holes in Au NPs. 3.2. Carrier Separation Efficiency. Quick recombination of charge carriers is one of the main reasons limiting the photocatalytic performance of MIP. In order to gain insight into the enhancement of photocatalytic activity, charge-carrier separation was studied with PL spectroscopy (Figure 4a). The emission peak at about 530 nm is due to the interband transition between the d bands and s bands of Au NPs.40−43 The intensity for Au/p-TiO2 decreased dramatically as compared with that for Au/n-TiO2, that is, Au/p-TiO2 possesses more efficient carrier separation efficiency. Fluorescence decay measurements gave out more solid evidence on the enhanced separation (Figure 4b). The lifetime of Au in Au/p-TiO2 shows faster fluorescence decay, 0.32 ns, which is about 52% shorter than that in Au/nTiO2 (0.67 ns), indicating that the charge-carrier separation efficiency of Au/p-TiO2 is obviously promoted. In EIS, the radius of arc of Nyquist plots can be used to evaluate the chargetransfer resistance at the semiconductor/electrolyte interface, and the smaller arc radius implies smaller charge-transfer resistance. As shown in Figures 4c and S18, same trends of curvature radius are observed at both 325 and 500 nm. Figure 4c shows the EIS Nyquist plot of samples under λex = 500 nm light irradiation. In this case, only Au NPs can be excited while titanium dioxide cannot. The position of energetic holes generated in the deep d band of Au is more positive (relative to NHE) than the Schottky barrier in Au/p-TiO2, so the holes can fluently transfer across the Schottky barrier toward the valence band (VB) of p-TiO2, leading to efficient hot carriers separation from each other. Therefore, the charge-transfer resistance over Au/p-TiO2 is smaller as compared with Au/nTiO2. At 325 nm (Figure S18), the TiO2 semiconductor is excited. For Au/n-TiO2, the hot electrons produced by n-TiO2 should cross a higher Schottky barrier to transfer to gold, whereas the hot electrons produced by p-TiO2 in Au/p-TiO2 can transfer to gold without crossing the upward bending Schottky barrier to achieve carrier separation. This is the main reason why Au/p-TiO2 demonstrates smaller arc radius in the EIS measurement under UV irradiation. So, the same trend was observed in both cases while the mechanism is different. Therefore, Au/p-TiO2 shows the minimum interfacial charge-

Figure 5. Charge-transfer route of Au/p-TiO2 and Au/n-TiO2. TEM images of Au/p-TiO2 photodeposited with PbO2 nanoparticles (a,b) and Au/n-TiO2 photodeposited with Ag nanoparticles (c,d) under λ = 500 nm light irradiation.

photoexcited electrons in Au transfer to p-TiO2, Ag+ will be mainly reduced on the p-TiO2 surface. On the contrary, when photoexcited holes in Au transfer to p-TiO2, PbO2 will be oxidized from Pb(NO3)2 on the p-TiO2 surface. TEM studies were carried out on Au/p-TiO2 and Au/n-TiO2 after being irradiated with λex = 500 nm for 0.5 h in the presence of Pb(NO3)2 and AgNO3, respectively. Generally, lager sizes of PbO2 and Ag NPs can be distinguished from Au NPs with the average size of 3 nm (Figures 5 and S19). Lattice fringe spacing analysis further confirmed the PbO2 and Ag particles. TEM images of the photodeposition of PbO2 nanoparticles on p-TiO2 are shown in Figure 5a,b. The larger particles with lattice fringe spacings of 0.313 nm are assigned to the (110) plane of PbO2,47 that is, PbO2 nanoparticles were selectively photodeposited onto p-TiO2, which implies that the interband holes transfer from Au to p-TiO2. Ag nanoparticles with a lattice fringe spacing of 0.235 nm are observed on n-TiO2 (Figures 5c,d and S20), indicating that electrons transfer from Au to n-TiO2. This result is consistent with the previous report that interband electrons transfer from gold to SrTiO3 in visible-light water oxidation.15 These charge-tracking results manifest that Au/p-TiO2 and Au/ n-TiO2 have entirely different transfer routes under visible-light irradiation, and the photoexcited hole transfer from Au to pTiO2 should be dominant in the metal and p-type semiconductor system. To theoretically demonstrate the feasibility of holes transferring from Au to p-TiO2, XPS analysis was carried out (Figures 6a and S21). The VB levels of p-TiO2 and n-TiO2 are demonstrated to be 1.03 and 2.40 eV below their corresponding Fermi levels, respectively. Compared with the n-type semiconductor, the Fermi level (EF) of the p-type semiconductor is closer to the VB energy (EVB). For Au nanoparticles, a 5d-band 24159

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Figure 6. (a) VB XPS spectrum of Au/p-TiO2 and Au/n-TiO2; (b) schematic illustration of the energy band position in the Au/p-TiO2 composite nanostructure.

Figure 7. IPA photodegradation over Au/n-TiO2 (a) and Au/p-TiO2 (b) without and with different reactive species scavengers (catalyst, 1 g; light source, λex = 500 nm; light intensity, 5.5 mW/cm2); (c) time-dependent absorption spectra of the DPD/POD-Au/p-TiO2 solution; (d) ESR spectra of the DMPO−•O2− adduct measured in the methanol solution of Au/n-TiO2 exposed to λex = 500 nm light. POD: Peroxidase DPD: N,N-diethyl-1,4phenylenediammonium sulfate DMPO: 5,5-dimethyl-1-pyrroline N-oxide.

charge-transfer routes, which helps to easily understand the efficient photocatalytic IPA degradation over Au/p-TiO2. Herein, the species of Au/p-TiO2 and Au/n-TiO2 for the gaseous IPA photodegradation were studied via a method of introducing scavengers. p-BQ, AO, and t-BA were adopted and served as scavengers for the species •O2−, h+, and •OH, respectively.50,51 Figure 7a illustrates the IPA photodegradation over Au/n-TiO2 without and with different scavengers. As can be seen, after introducing t-BA, AO, and p-BQ, the photocatalytic activity for oxidizing IPA to acetone reduced significantly, indicating that all of the active species •OH, h+, and •O2− participated in the initial oxidization of IPA, but •O2− dominated this reaction. For Au/p-TiO2 (Figure 7b), the addition of AO and t-BA significantly decreases the acetone evolution rate, whereas the presence of p-BQ has almost no

edge lies at about 1.95 eV relative to its Fermi level, where the onset of the interband transition occurs.15,48 When p-TiO2 and Au NPs come into contact, a new equilibrium state will build up and cause a positive shift (relative to NHE) of the aligned EF, considering that a slight band bending will happen after contact (ca. 0.3 eV).49 So, a rough schematic illustration of Au/p-TiO2 band energy can be obtained (Figure 6b). As can be seen, under the λex = 500 nm light irradiation which corresponds to 2.48 eV, from both energy and relative band positions, the holes from interband transitions in Au NPs have sufficient energy (1.95 > 1.03 eV) to transfer into the VB of p-TiO2, even considering the band bending. Therefore, the hot interband holes in the d-band of Au NPs can cause effective overbarrier injection to p-TiO2. 3.4. Photocatalytic Degradation Mechanism. The photocatalytic degradation mechanism can further confirm the 24160

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Figure 8. Proposed mechanism for IPA degradation over Au/n-TiO2 (a) and Au/p-TiO2 (b) under λex = 500 nm light irradiation.

the photocatalytic IPA degradation (Figure 8a). The remaining holes on Au can also participate in the IPA oxidation. In the case of Au/p-TiO2, the active radicals are only generated from the hot holes, which would oxidize the absorbed H2O to generate •OH or oxidize the IPA directly, whereas the hot electrons in Au NPs are mainly consumed to form H2O2 which is inactive for IPA oxidation (Figure 8b). However, the IPA degradation efficiency of Au/p-TiO2 is more than that of Au/n-TiO2. It is because the presence of the Schottky barrier at the interface of Au/n-TiO2 can hinder the injection of hot electrons from Au NPs to the CB of n-TiO2, whereas energetic holes generated in the deep d band of Au can lead to a fluent transfer across the Schottky barrier toward the VB of p-TiO2, leading to efficient hot carriers separating from each other. A comparison of the mechanism study reveals that Au/p-TiO2 has an independent carriertransfer mechanism in which holes transfer from Au to p-TiO2, which agrees well with the TEM study of selective deposition. There are two types of excitation models that can take place in the MIP process, namely intraband excitation and interband excitation. Theoretical research shows that the interband excitation is the dominant plasmon decay absorption pathway in Au NPs, along with an asymmetric energy distribution of high-energy holes and low-energy electrons. In the traditional MIP systems of gold and n-type semiconductor, the interband electrons need to be excited to a relatively high position in order to cross the Schottky junction to reach the semiconductor and realize the separation of charge carriers. On the one hand, the interband electron transition from d states to s states above the Fermi level consume much energy, and only a minority of them can cross the Schottky junction, resulting in a low charge carrier separation efficiency. On the other hand, the interband holes created far from the Fermi level have very strong oxidation ability and cannot be effectively utilized. In this work, the Au/pTiO2 strategy effectively takes advantage of the fact that the VB of the p-type semiconductor is closer to its Fermi level, so that the energetic interband holes generated in gold can smoothly transfer to the VB of the semiconductor and be consumed. The mechanism is confirmed by the TEM study of selective photocatalytic deposition and the IPA photodegradation mechanism study. This strategy not only improves the carrier separation efficiency but also effectively utilizes the interband holes.

influence on the degradation rate compared with those without using scavengers. These results imply that h+ and •OH are the main oxidation species and that •O2− does not play an important role in the IPA photodegradation over Au/p-TiO2. By further considering their band gaps, it is the interband holes (h+) that transfer from Au to the valence bland of p-TiO2, whose potential is positive enough for oxidizing IPA to acetone (IPA/acetone = +0.14 V vs NHE).52 At the same time, as the potential of the VB maximum in p-TiO2 is higher than the energy potential for oxidization of H2O (•OH/H2O = +2.68 V vs NHE),53 h+ could oxidize the absorbed H2O to •OH. So, the addition of t-BA affected the degradation rate of IPA, whereas the addition of AO almost inhibited the degradation of IPA. However, the leaving electrons on Au do not have enough potential to reduce oxygen and to produce superoxide radicals (e− + O2+ H+ → HO2, −0.046 V vs NHE),37 which is confirmed by the fact that the superoxide radicals (•O2−) are not involved in the oxidation of IPA over Au/p-TiO2 under λex = 500 nm light. The excited electrons might participate in another reaction with O2 to form H2O2 (O2 + 2H+ + 2e− → H2O2, +0.695 V vs NHE)54 because the reduction potentials of O2/H2O2 are more positive (relative to NHE) than those of O2/•O2−. Further experiments were carried out to prove the capability of Au/p-TiO2 to generate H2O2 under λex = 500 nm light irradiation. Figure 7c shows that the evolution of H2O2 increases with the increase of irradiation time under the presence of Au/p-TiO2, indicating that the photoexcited electrons on Au/p-TiO2 can indeed reduce O2 to form H2O2. These results are supported by the theoretical research that the excitation of interband transitions on Au NPs generates an asymmetric energy distribution of holes and electrons.38 The electrons generated in the interband transition, whose energy is consumed during the transition from d states to s states above the Fermi level, can only reach the reduction potential of oxygen to form H2O2 rather than •O2−. Different from Au/p-TiO2, over which photocatalytic IPA degradation mainly involves h+, photocatalytic IPA degradation over Au/nTiO2 mainly involves •O2−. ESR characterization was carried out to further confirm •O2− radical generation (Figure 7d). Signals corresponding to DMPO−•O2− were not observed for Au/pTiO2 under λex = 500 nm light irradiation. In contrast, Au/nTiO2 created ESR signals attributed to DMPO−•O2−, which were formed by the reduction of O2, Figure 7d.55,56 This demonstrates that the electrons transferred from Au to the CB minima of n-TiO2, where the potential is more negative than the potential to produce superoxide radicals.57 Overall, based on the results described above, the transfer routes of carriers and the mechanism of IPA degradation over Au/n-TiO2 and Au/p-TiO2 are proposed (Figure 8). For Au/nTiO2, the d electrons of Au were excited and subsequently injected into the CB of n-TiO2 through the Schottky barrier to generate •O2−, which is the dominant active species involved in

4. CONCLUSIONS In summary, a remarkable visible-light photocatalytic performance of IPA photodegradation was achieved over Au/p-TiO2, which was about 5 times as high as that of Au/n-TiO2. Effectively, charge carrier separation in the Au/p-TiO 2 composite was demonstrated by PL and EIS. Both the enhanced activity and efficient charge carriers’ separation efficiency were 24161

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ACS Applied Materials & Interfaces mainly attributed to the fluent over-barrier transfer of energetic interband holes from the d-band of Au NPs to p-TiO2, which is revealed based on the TEM study on selective PbO2 deposition over p-TiO2 and the relative band position analysis in Au/pTiO2. Furthermore, the IPA photodegradation mechanism study confirms that the holes created in the Au interband transition can fluently cross the Schottky barrier from Au to pTiO2. The Au/p-TiO2 strategy described here not only provides a feasible route to enhance the efficiency of visible-light photocatalytic processes driven by the metal and p-type semiconductor but also opens up new opportunities in designing and preparing visible-light responsive photocatalysts.

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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/acsami.9b07110. Detection of H2O2 and •O2−; XRD patterns of p-TiO2 and n-TiO2 precursors; XRD patterns of p-TiO2 calcined in different oxygen concentrations; ESR spectra of pTiO2, n-TiO2, Au/p-TiO2, and Au/n-TiO2; O 1s XPS spectra of p-TiO2 and n-TiO2; transient photocurrent curve of p-TiO2 at −0.40 V vs Ag/AgCl under visible light (λ > 420 nm) and in the dark; positron lifetime parameters of samples; acetone evolution rate over the four samples when the ratio of glycerol to ethanol is 1:3; time course of H2 production under λex = 500 nm light irradiation; particle size distribution of Au in Au/p-TiO2 and Au/n-TiO2; TEM images of Au/n-TiO2; and total XPS spectrum of Au/p-TiO2 and p-TiO2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Defa Wang: 0000-0001-7196-6898 Lequan Liu: 0000-0003-3837-6831 Jinhua Ye: 0000-0002-8105-8903 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21673157, 21503145, and 21633004) and the Beiyang Reserved Academic Program of the Tianjin University.

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