Enhanced Photocatalytic Mineralization of Gaseous Toluene over

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Enhanced photocatalytic mineralization of gaseous toluene over SrTiO3 by surface hydroxylation Jiejing Kong, Zebao Rui, and Hong-Bing Ji Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03270 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 5, 2016

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Industrial & Engineering Chemistry Research

Enhanced photocatalytic mineralization of gaseous toluene over SrTiO3 by surface hydroxylation Jiejing Kong 1, 3, Zebao Rui 2, 3*, Hongbing Ji 1, 3* 1

School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P.R. China

2

School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai

519082, P.R. China 3

R&D Center of Waste-gas Cleaning & Control, Huizhou Research Institute of Sun

Yat-sen University, Huizhou 516081, P.R. China

*

Correspondence to the authors can be sent to Z. B. Rui ([email protected] )

and H. B. Ji ([email protected] )

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Abstract Perovskite structured SrTiO3 (STO) was synthesized by a hydrothermal method followed by the second hydrothermal treatment with H2O or NaOH (STO-H2O or STO-NaOH) for the photocatalytic mineralization of gaseous toluene. The second hydrothermal treatment enhances the light absorption and enriches the surface hydroxyl groups of STO. The surface hydroxyls enrichment of STO promotes the generation of hydroxyl radicals and the separation of photocarriers by the combination of hydroxyl with holes, induces a negative shift of its band edge and benefits for the reduction of adsorbed oxygen. The facile generation of reactive radical species, enhanced light absorption and improved photocarriers separation together lead to greatly enhanced photocatalytic efficiency of STO-NaOH. Toluene was completely oxidized into CO2 under UV-light illumination for 6 h at room temperature, demonstrating better performance than STO and commercial P25 catalysts. Such a surface hydroxylation promotion strategy may bring new perception of designing an efficient photocatalyst.

Keywords: SrTiO3, Surface hydroxylation, Photocatalytic oxidation, toluene

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1. Introduction Volatile organic compounds (VOCs) represent an important class of hazardous air pollutants.1 Semiconductor photocatalysis is an environmentally friendly and economically feasible approach for the mineralization of this kind of pollutants at low levels.2-5 The requirement for the effective photocatalysts is the major challenge and limitation of this green technology. In this context, various photocatalysts have been developed, such as TiO2 (or its derivatives) 2, 4, 5 and perovskite structured oxides (e.g. strontium titanate).6-8 As a photocatalyst, SrTiO3 (STO) exhibits high corrosion resistance, chemical stability, low cost and nontoxicity.8 Although STO has a band gap (Eg) close to TiO2 (ca. 3.2 eV),9 the conduction band (CB) edge of STO is about 200 mV more negative than that of TiO2,8 which makes STO an effective photocatalyst for the decomposition of various organic compounds.10-12 However, like TiO2 and other intrinsic photocatalysts, the technological application of STO is restricted due to the relatively low photocatalytic efficiency. One important reason is the low quantum efficiency or the recombination of photogenerated electron–hole pairs greatly inhibits the generation of reactive radical species over STO.9, 13, 14 In addition, the CB edge of STO is more positive than the reduction potential of O2 (e- + O2→O2•−, − 0.33 eV vs. NHE).4 Thus, the generated electrons cannot be consumed by combining with O2 to form oxidant O2•−, an important kind of oxidant. As a result, the photocatalytic degradation of organic compounds over STO by the attack action of reactive radical species, such as O2•− and •OH, is poor, 11 leading to the low photocatalytic efficiency of STO. Various strategies have been performed to overcome these barriers, including noble metal decoration and multicomponent heterojunctions fabrication to enhance

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the photocarriers separation and promote redox reactions,9, 13, 14 morphology tailoring to increase the surface area and active sites15 or to promote light adsorption and photocarriers separation,15, and potentials.7,

17

16

and energy-band engineering to offer adequate redox

Employing these regulations, the photocatalytic performance of

these STO based photocatalysts was enhanced to some extent. However, owing to the difficulty in controlling the precise nanostructures16 and simultaneously achieving the improvement of charge separation and adequate bands position for the reactions by energy band engineering,6 the design of effective STO based photocatalysts continues to be a challenge. Herein, we provide a facile strategy for improving the photocatalytic performance of STO for VOCs degradation by the surface hydroxylation based on the following considerations. Firstly, photocatalysis is a typical surface process,18 and the surface functionalization can regulate the performance of the photocatalysts at no cost of the unique properties of the bulk materials, such as crystal structure.18-20 Secondly, the photogenerated holes can react with the adsorbed H2O or surface hydroxyl groups to form hydroxyl radicals, which are important oxidants. The enrichment of surface hydroxyl groups can promote this reaction, act as hole scavengers to form •OH and promote the separation of electron-hole pairs.21 Thirdly, the hydroxide ions can shift the surface energy band of STO to a more negative level,19 which facilitates the reduction of the adsorbed oxygen to form O2•−, another important kind of oxidant. For example, Ye and the co-workers found that the high alkalinity of the reaction solution could induce a negative shift of the band edge of the surface conduction band in STObased oxide photocatalysts and enhance the photocatalytic performance of H2 evolution.19 Moreover, the wet reaction environment of the VOCs oxidation system is beneficial for the stabilization and action of the surface hydroxyl groups.3, 22 STO was

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synthesized by a hydrothermal process followed by the second hydrothermal treatment with H2O or NaOH (STO-H2O or STO-NaOH) for the photocatalytic mineralization of gaseous toluene, a typical aromatic VOC. The surface enrichment of hydroxyl groups, facile generation of O2•−, HOO•, •OH and holes, enhanced light absorption

and

photocatalytic

improved

photocarriers

mineralization

of

toluene

separation, were

and

obtained

greatly over

enhanced

STO-NaOH,

demonstrating the application potential of the surface hydroxylation promotion strategy.

2. Experimental Procedure 2.1. Materials Synthesis All reagents were in analytical grade for direct use. Pure phase SrTiO3 (STO) was synthesized by a hydrothermal method. In a typical synthesis process, ca.10 mmol Ti(C4H9O)4 was dissolved in 20 mL of ethylene glycol (EG) to form a clear solution, followed by the addition of 20 mL of 0.5 mol/L Sr(NO3)2 aqueous solution drop by drop and 10 mL of 5 mol/L NaOH under magnetic stirring. After stirring for 30 min, the mixture was fed into a 100 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 200 °C for 24 h. The product was collected by centrifugal separation process and washed with deionized water and absolute ethanol several times until the pH reached 7, then dried at 70 °C overnight to obtain the final sample. The as-prepared STO was then modified via the second hydrothermal treatment. In a typical preparation experiment, ca. 0.5 g STO was added into the distilled water or 5 mol/L NaOH aqueous solution. The suspension was ultrasonically treated for 5 min and then sealed in a 100 mL of Teflon-lined stainless steel autoclave and then heated at 150 °C for 10 h. The as-obtained samples were washed with water and absolute

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ethanol until the pH reached 7 and then dried overnight at 70 °C. The as-prepared catalysts were respectively denoted as STO-H2O and STO-NaOH for simplicity.

2.2. Catalysts Characterization The morphologies and crystal analysis were obtained by Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements over a JEOL model JEM2010-HR instrument at an accelerating voltage of 200 kV. X-ray diffraction (XRD) was performed to characterize the crystal structures and compositions of the catalysts by using a D-MAX diffractometer (Cu Kα radiation). Nitrogen adsorption/desorption isotherms at 77 K were measured by Micromeritics ASAP 2020 instrument to estimate the Brunauer-Emmett-Teller surface area, pore size and pore volume of the samples, and the samples were prior degassed at 573 K for 2 h. X-ray photoelectron spectroscopy (XPS) was performed using a ESCALAB 250 spectrometer (Thermo Fisher Scientific, Al Kα, hν = 1486.6 eV) under highvacuum of ~2×10−7 Pa. UV-vis diffuse reflectance spectra (DRS) were recorded on a UV-vis spectrophotometer (UV2450) equipped with an attachment of integrating sphere within the range of 200~800 nm and with BaSO4 as the reflectance standard. Photoluminescence (PL) spectra were characterized by Combined Fluorescence Lifetime and Steady State Spectrometer (FLSP920) with a xenon lamp (excitation wavelength 300 nm) as light source. Fourier transform infrared (FTIR) spectra were recorded on a Fourier transformation infra-red spectrometer (EQUINOX 55). Photocurrents spectra were obtained on an electrochemical analyzer (CHI660E) under full spectrum illumination from a Xe lamp (100 mW cm-2) in a standard threeelectrode system, composed of the as-prepared samples of ca. 2.0 cm2 active area (working electrodes), Pt wire (counter electrode) and Ag/AgCl (saturated KCl)

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(reference electrode). The electrolyte was N2-saturated 0.1 mol/L NaOH. The electron paramagnetic resonance (EPR) spectrometer (JESFA 200, JEOL Co.) was used for the measurements of the EPR signals of photoinduced radicals which were spin-trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The EPR signals of the DMPO trapped radicals (DMPO−•OH and DMPO−O2•−) were recorded at ambient temperature. All freshly-prepared suspensions (DMPO: 50 mM, 20 µL; solvent: 5 mL water (for DMPO−•OH, acting as O2•− scavenger) or methanol (for DMPO−O2•−, acting as hole scavenger); mCatal.: 5 mg) were mixed directly and then transferred into a cylindrical quartz cell (length 100 mm, diameter 2 mm). A 500 W high pressure mercury lamp (λ=300~400 nm) was used in situ as a photo-excitation light source. After irradiation for 240 s, the signals of DMPO−•OH and DMPO−O2•− were measured on an EPR spectrometer.

2.3. Photocatalytic Activity Evaluation Photocatalytic reactions were conducted in a stainless-steel reactor equipped 300 W Xe lamp and UV reflector (λ=300~400 nm, optical power density is 80 mW cm-2) under atmospheric pressure, as described in our previous work.3,

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The

photocatalyst (~0.2 g) was dispersed in a 7.0 cm2 of quartz reaction vessel and placed on the bottom of the photocatalytic reactor. Vaporous toluene was injected to the reactor by flowing the simulated air (O2 : N2 = 1 : 3) into a saturator filled with toluene solution. The initial toluene concentration was ~500 ppm, and the relative humidity in the reactor was ca.16%. Before irradiation, the reactor was kept in dark for 30 min to achieve an adsorption–desorption equilibrium. The gas sample in the reaction vessel was withdrawn and analyzed at 30 min intervals. The gaseous toluene content was measured by a gas chromatograph system (GC7900, Tianmei, China),

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equipped with a flame ionization detector (FID). The yield of CO2 was analyzed by another gas chromatograph (GC2060, FID) with a nickel catalyst-based methanizer and a flame ionization detector. Because the intermediate products were all under the detect limit of gas chromatograph, the toluene conversions was calculated according to the yield of CO2 and the carbon balance as % =

,

.,

× 100%

(1)

where , is the initial molar quantity of vaporous toluene in the reactor and

, !

means the corresponding converted toluene molar quantity at irradiation time (t).

3. Results and discussion 3.1. Morphologies and Structures The (HR) TEM images and selected area electron diffraction (SAED) patterns of STO and STO-NaOH are presented in Figure 1. Figure 1a shows STO is mainly composed of irregular nanoparticles with a diameter of 20~50 nm. The SAED pattern in Figure 1a inset reveals that the diffraction pattern can be well recognized and categorized according to cubic structured STO. The lattice spacing estimated from the HRTEM is around 2.76 Å (Figure 1a inset), which corresponds to the (110) planar spacing of cubic structured STO. Figure 1b shows that the second hydrothermal treatment in NaOH solution has no obvious effect on the morphology and surface crystal structure of STO. The XRD patterns in Figure 1c indicate all the characteristic peaks of these samples agree well with the cubic STO phase structure (JCPDS No. 35-0734). The BET surface area, average pore size and pore volume of these samples are listed in Table S1. As listed, the second hydrothermal treatment in H2O has negligible effect on the surface area and pore structure of STO, while the second

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hydrothermal treatment in NaOH solution leads to the decrease in BET-area and pore volume of STO to some extent, probably due to the collapse or plugging of the small pores during the treatment process. Figure 2a shows the FTIR spectra of the as-synthesized catalysts. Strong and wide band located between 745 and 557 cm−1 is a combination of the following bands: a band at 745 cm−1 which is assigned to stretching vibrations of SrO and a band at 557 cm−1 which refers to stretching vibrations of TiO2.23 These bands confirm the surface STO structure. The weak band around 1385 cm−1 can be considered as C-OH bending vibrations,24 probably caused by the tiny organic residuals during the synthesis process. The band around 1466 cm−1 is assigned to normal polymeric O-H stretching vibration of H2O in Sr–Ti–O lattice.25 The bands at 3419 and 1626 cm−1 are assigned to O-H stretching and bending vibrations.24 As compared, the treated samples show more intense OH vibration bands than STO, among which STONaOH holds the highest surface OH amount. The XPS spectra of Sr 3d and Ti 2p for STO, STO-H2O and STO-NaOH (Figure S1) agree very well with each other. The curves of Sr 3d region contain two peaks at 132.9 eV for Sr 3d5/2 and 134.6 eV for Sr 3d3/2, respectively. The peak at 132.9 eV agrees well with those reported for SrTiO3 perovskite material and the peak at ~134.6 eV can be assigned to SrO complexes.26, 27 The Ti 2p peaks around 458.3 and 464.1 eV refers to Ti4+.27 In Figure 2b, two distinct O 1s peaks were detected. The first peak centered around 529.5 eV was ascribed to oxygen bound to Sr and Ti,

28

while the

higher one (~531.7 eV) revealed the surface hydroxyl species.29, 30 It should be noted that the higher O1s BE peak was ascribed to the chemisorbed oxygen in some cases.31 Because the existence of oxygen vacancies or negatively charged metals is necessary for the formation of the chemisorbed oxygen,31 we can exclude the effect of surface

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chemisorbed oxygen on the assignment O1s XPS peaks in consideration with the hydrothermal treating process and the chemical states of Sr2+ and Ti4+ (Figure S1). The relative amount of surface hydroxyl species was estimated by calculating the peak area ratio of OH (AO−OH) to Sr/Ti-O (AO2−latt). As summarized in Table S2, there is a sequence of STO < STO-H2O STO-H2O > STO-NaOH, indicating less electron–hole pair recombination happens over the modified samples, especially STO-NaOH. The electron-hole separation efficiency of these samples was further investigated by the photoelectrochemical method. Figure 2f shows the transient photocurrent responses at a fixed potential of 0.4 V via four on–off cycles of STO, STO-H2O and STO-NaOH under simulated sunlight irradiation. The samples all generate transient photocurrents with a reproducible response to on–off cycles, indicating the effective separation and transfer of photoinduced electron–hole pairs. Obviously, the photocurrent density of STO-NaOH (0.28 µA·cm-2) is much larger than that of STO (0.12 µA·cm-2) and STO-H2O (0.20 µA·cm-2), demonstrating the more efficient simulated sunlight absorption and separation of photogenerated electrons-holes over STO-NaOH, which agrees well with the DRS and PL results above.

3.3. EPR Study The electron paramagnetic resonance (EPR) signals of DMPO trapped radicals were recorded to determine the formation of •OH and O2•−. As shown in Figure 3, there is no EPR signal in both the aqueous suspension and methanol suspension in the dark. However, the characteristic quartet peaks with an intensity ratio of 1:2:2:1 appear as the consequence of photo-excitation with UV light irradiation in the aqueous suspension (Figure 3a), which is consistent with the previous studies for •OH adducts.33 The production of •OH adducts is in order of STO < STO-H2O < STO-NaOH. For the measurements performed in the methanol suspension, the quartet characteristic peaks with an intensity ratio of 1:1:1:1, which are assigned to DMPOO2•− adducts,34 are observed over STO-H2O and STO-NaOH with UV light irradiation

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(Figure 3b), and the O2•− adducts production of STO-NaOH is higher than that of STO-H2O. However, these signals cannot be detected on STO. The EPR results confirm that both •OH and O2•− radicals can be formed on STO-H2O and STO-NaOH under UV light irradiation, which are active oxidation species responsible for the photodegradation of toluene.

3.4. Photocatalytic Performance The toluene adsorption curves over STO, STO-H2O and STO-NaOH without light illumination were first tested (Figure S2). The equilibrium toluene adsorption amounts over these samples are all close to 0.3 µmol/g under the same initial toluene concentration employed in the photocatalytic experiments. No obvious toluene degradation was found over STO without light irradiation (Figure S3) or under UVlight in the absence of a photocatalyst (Figure 4a) during the six hours reaction time. As sown in Figure 4a, STO-NaOH exhibits the highest photocatalytic activity among all the samples including STO, modified STO and commercial P25, which can completely convert toluene into CO2 after UV-light illumination for 6 h. To make a quantitative photocatalytic activity comparison among these samples, the apparent reaction rate constants (k) for photocatalytic toluene degradation was regressed from the ln(C0/Ct) versus t plot for each sample, as plotted in Figure 4a inset. The asregressed reaction rate constants for STO, STO-H2O, P25 and STO-NaOH are 0.0042, 0.0042, 0.0072 and 0.012 min−1, respectively, indicating the highest degradation rate of STO-NaOH. In all cases, R2 (correlation coefficient) values are higher than 0.99, confirming the proposed kinetics for photocatalytic mineralization of toluene. In addition, UV-light photocatalytic degradation of toluene over STO, STO-H2O and STO-NaOH for 3 h was studied at various temperatures of 30, 45, 60, 90, and

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120 °C and shown in Figure S4. As shown, toluene conversion efficiency increases with the increase in temperature over these samples. For instance, the toluene conversion reaches ~100% over STO-NaOH after UV-light illumination for 2 h at 90~120 °C. Similarly, pseudo-first order reaction kinetics was used to analyze the reaction process, and the corresponding apparent reaction rate constant was linear regressed from the ln(C0/Ct) versus t plot for all the samples, as plotted in Figure S4 and summarized in Table S3. As listed, the as-regressed reaction rate constants increase with the increase in temperature. At a high temperature, the migration of the photogenerated electros and holes speeds up, leading to less recombination before they move to the particle surface to react with other species. Moreover, at a high temperature, the desorption of adsorbed water molecules on the sample surface may create more unoccupied reactive sites, which are available for photodegradation of toluene and/or intermediates.35 The apparent activation energy (Ea) based on the linear fitting of ln k versus 1000/T (Figure 4b) was then regressed by the Arrhenius equation. The Arrhenius plots for STO, STO-H2O and STO-NaOH all lead to two Ea values, and the Ea1 values in the low temperature region (30~60

o

C) are larger than those

corresponding values (Ea2) in the high temperature region (60~120 oC) (Table S4), indicating the photocatalytic reaction over these samples is more sensitive to temperature at a low reaction temperature. This is probably because the reaction transfers from surface reaction control to mass transfer control with an increase in temperature and conversion.31 The Ea1 values in the low temperature region has an order of STO-NaOH (24 kJ/mol) < STO-H2O (32 kJ/mol) = STO (32 kJ/mol), indicating the better photocatalytic ability of STO-NaOH. The cyclic stability of STO-

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NaOH in Figure 4c shows good performance stability in toluene conversion over 5 cycles at 30 °C and with UV-light irradiation for 6 h for each cycle.

3.5. Discussion On the basis of the estimated band gap and valence band edge values of STObased photocatalysts, the relative positions of conduction band (CB) and valence band (VB) for them are shown in Figure 5a. The CB position of STO is located at ca. −0.32 eV, which is slightly more positive than the electrochemical potential of the desired reaction (O2/O2•− = −0.33 eV).3 Thus, it is difficult to reduce the adsorbed oxygen to O2•− over STO, as demonstrated by DMPO-EPR in Figure 3. After the second hydrothermal treatment with H2O and NaOH, the CB of STO-H2O and STONaOH shift to more negative potentials of −0.40 and −0.52 eV, respectively, which agrees well with the previous finding by Ouyang et al.19 The more negative potentials supply a strong potential for adsorbed oxygen reduction. Meanwhile, the large amount of hydroxyl groups over STO-H2O and STO-NaOH surface are beneficial to generate hydroxyl radicals by combination with holes, which promotes the separation of photocarriers. In addition, it was reported that surface hydroxyl groups facilitated adsorption and activation of O2 molecules bound to nearby oxygen vacancies.30 All these factors are beneficial for the photocatalytic oxidation of toluene. The photocatalytic mechanism of STO-H2O and STO-NaOH was proposed and illustrated in Figure 5b. In comparison with STO-H2O, the more negative level of CB and the larger amount of surface hydroxyl groups of STO-NaOH promote more O2•−, HOO•, •OH and holes generation, which can participate in the photocatalytic reactions to directly decompose toluene. As a result, the photocatalytic mineralization of toluene over STO-NaOH is greatly promoted.

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4. Conclusions Perovskite structured SrTiO3 (STO) was synthesized by a hydrothermal method, and then modified via the second hydrothermal treatment with H2O or NaOH (STOH2O or STO-NaOH) for the photocatalytic mineralization of gaseous toluene. The second hydrothermal treatment, especially with NaOH solution, enriched surface hydroxyl groups, induced a negative shift of conduction band edge, and promoted more O2•−, HOO•, •OH and holes generation. Moreover, the modified STO exhibited enhanced light absorption and improved photocarriers separation. Finally, greatly enhanced photocatalytic mineralization of toluene over STO-NaOH was obtained. Toluene was completely oxidized into CO2 after UV-light illumination for 6 h at ambient temperature, demonstrating better performance than STO and commercial P25 catalyst. In short, the development of effective photocatalysts by facile surface hydroxylation method and their potential environmental application are demonstrated. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. BET surface area, pore size, pore volume, XPS data, reaction rate constants, activation energies, pre-exponential factors of the samples; toluene adsorption curve; toluene decomposition curves over STO with and without UV-light irradiation; toluene degradation at various temperatures and their ln(C0/Ct) versus t plots.

Acknowledgements The work was supported by the Natural Science Foundation of China (21576298 and 21425627), the Science and Technology Plan Project (2013B090500029 and 2014B090902006) and

Natural Science

Foundation

15


(2014A030313135

and


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2014A030308012) of Guangdong Province.

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compound enabled by Cr2O3 barrier layer coated on Rh/SrTiO3. Chem. Commun. 2016, 52, 9636-9639. (13) Guan, X.J.; Guo, L.J. Cocatalytic effect of SrTiO3 on Ag3PO4 toward enhanced photocatalytic water oxidation. ACS Catal. 2014, 4, 3020-3026. (14) Yu, T.; Hu, W.l.; Jia, L.X.; Tan, X.; Huang, J.R.; Huang, X. Enhanced photoelectrochemical performance of coaxial-nanocoupled strontium-Rich SrTiO3/TiO2 {001} nanotube arrays. Ind. Eng. Chem. Res. 2015, 54, 8193-8200. (15) Kuang, Q.; Yang, S.H. Template synthesis of single-crystal-like porous SrTiO3 nanocube assemblies and their enhanced photocatalytic hydrogen evolution. ACS appl.mater. Interfaces 2013, 5, 3683-3690. (16) Wang, B.; Shen, S.H.; Guo, L.J. SrTiO3 single crystals enclosed with highindexed {023} facets and {001} facets for photocatalytic hydrogen and oxygen evolution. Appl. Catal. B-Environ. 2015, 166-167, 320-326. (17) Wang, D.F.; Kako, T.; Ye, J.H. Efficient photocatalytic decomposition of acetaldehyde over a solid-solution perovskite (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3 under visible-light irradiation. J. Am. Chem. Soc. 2008, 130, 2724-2725. (18) Fox, M.A.; Dulay, M.T. Heterogeneous photocatalysis. Chem. Rev. 1993, 93, 341-357. (19) Ouyang, S.X.; Tong, H.; Umezawa, N.; Cao, J.Y.; Li, P.; Bi, Y.P.; Zhang, Y.J.; Ye, J.H. Surface-alkalinization-induced enhancement of photocatalytic H2 evolution over SrTiO3-based photocatalysts. J. Am. Chem. Soc. 2012, 134, 1974-1977. (20) Zheng, Z.K.; Huang, B.B.; Lu, J.B.; Wang, Z.Y.; Qin, X.Y.; Zhang, X.Y.; Dai, Y.; Whangbo, M.H. Hydrogenated titania: synergy of surface modification and morphology improvement for enhanced photocatalytic activity. Chem. Commun. 2012, 48, 5733-5735. (21) Fu, X.L.; Huang, D.W.; Qin, Y.; Li, L.F.; Jiang, X.L.; Chen, S.F. Effects of preparation method on the microstructure and photocatalytic performance of ZnSn(OH)6. Appl. Catal. B-Environ. 2014, 148-149, 532-542. (22) Yang, T.F.; Huo, Y.; Liu, Y.; Rui, Z.B.; Ji, H.B. Efficient formaldehyde oxidation over nickel hydroxide promoted Pt/γ-Al2O3 with a low Pt content, Appl. Catal. B-Environ. 2017, 200, 543–551. (23) George, C.N.; Thomas, J.; Jose, R.; Kumar, H.P.; Suresh, M.; Kumar, V.R.; Wariar, P.S.; Koshy, J. Synthesis and characterization of nanocrystalline strontium titanate through a modified combustion method and its sintering and dielectric properties. J. Alloys Compd. 2009, 486, 711-715. (24) Ashiri, R. Detailed FT-IR spectroscopy characterization and thermal analysis of synthesis of barium titanate nanoscale particles through a newly developed process. Vib. Spectrosc. 2013, 66, 24-29.

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Figure captions

Figure 1.TEM images and SAED patterns of (a) STO and (b) STO-NaOH, and (c) XRD patterns of STO, STO-H2O and STO-NaOH. Figure 2. (a) FTIR spectra, (b) O 1 s XPS spectra, (c) UV-vis diffuse reflectance spectra & dependence of (αhν)2 on the photon energy (inset), (d) XPS valence band spectra and (e) photoluminescence spectra of STO, STO-H2O and STO-NaOH, and (f) Time-dependent photocurrents of various electrodes under on-off simulated sunlight exposure pulse of 80 s with a constant bias of 0.4 V vs. Ag/AgCl electrode. Figure 3. DMPO-EPR spin-trapping spectra of (a) DMPO-•OH and (b) DMPO-O2•− adducts formed in the suspension of STO, STO-H2O and STO-NaOH after irradiation under UV light irradiation (λ=300~400 nm) for 240 s. Figure 4. (a) UV-light photocatalytic decomposition of toluene over STO, STO-H2O and STO-NaOH at 30 oC and its ln(C0/Ct) versus t plot (inset); (b) Arrhenius plots for UV-light photocatalytic degradation of toluene over the samples; (c) Cyclic stability test of STO-NaOH for UV-light photocatalytic decomposition of toluene at 30 oC for 6 h. Figure 5. (a) Eg values (in eV) and position of CB and VB for STO, STO-H2O and STO-NaOH at pH = 7 vs. NHE; (b) Mechanism of photocatalytic toluene mineralization over STO-H2O and STO-NaOH.

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c

♣ ♣

Relative Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

Tausonite STO-NaOH

♣ ♣

♣

♣

♣

♣

STO-H2O

STO 20

30

40

50

60

70

80

2θ (deg.)

Figure 1.TEM images and SAED patterns of (a) STO and (b) STO-NaOH, and (c) XRD patterns of STO, STO-H2O and STO-NaOH.

20


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a

b

O 1s

529.5 531.7

3419 ν (O-H)

Intensity (a.u.)

Trans. Intensity (a.u)

STO-NaOH

δ (O-H)1626

529.5 531.7

STO

529.5 1466 ν (O-H)

STO-H2O STO-NaOH 3000

2000

Wavenumber (cm-1)

1000

525

(αhν)2

531

2.76 eV

STO-H2O

3

STO

2 1 0 2.8

3.0

3.2

3.4

3.6

3.8

4.0

hν (eV)

0.2

534

d

STO-NaOH

4

528

Binding Energy (eV)

5

0.4

STO

ν (Ti-O)557

c 0.6

Abs. (a.u.)

531.7

745

ν (Sr-O)

STO (NaOH)

STO-NaOH

Relative Intensity (a.u.)

4000

STO-H2O

1385 δ (C-OH)

2.85 eV

STO-H2O

STO (H2O) 0.0

200

2.98 eV

STO 300

400

500

600

700

10

800

8

Wavelength (nm)

e

f STO STO-H2O STO-NaOH

6

4

STO

2

0

Binding Energy (eV)

0.4 STO-NaOH

~ 385 nm

STO-H2O STO

0.3

J (µA / cm2)

PL Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.1

360

380

λ(nm)

400

0.0 0

420

60

120

180

240

300

360

420

Time (s)

Figure 2. (a) FTIR spectra, (b) O 1 s XPS spectra, (c) UV-vis diffuse reflectance spectra & dependence of (αhν)2 on the photon energy (inset), (d) XPS valence band spectra and (e) photoluminescence spectra of STO, STO-H2O and STO-NaOH, and (f) Time-dependent photocurrents of various electrodes under on-off simulated sunlight exposure pulse of 80 s with a constant bias of 0.4 V vs. Ag/AgCl electrode.

21



a

STO-H2O B

Intensity (a.u.)

STO-NaOH

STO

dark 318

321

324

UV-light on for 4 min 327

318

321

A

324

327

A

Magnetic (mT)

b Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

STO-NaOH

STO-H2O

STO

dark 318

321

324

UV-light on for 4 min 327

318

321

A

324

327

A

Magnetic (mT)

Figure 3. DMPO-EPR spin-trapping spectra of (a) DMPO-•OH and (b) DMPO-O2•− adducts formed in the suspension of STO, STO-H2O and STO-NaOH after irradiation under UV light irradiation (λ=300~400 nm) for 240 s.

22


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-3.5

P25 STO-H2O STO No Catalyst

-4.0

60

-4.5

STO-NaOH 2

40

STO-NaOH

P25

ln(C0/Ct)

Conversion (%)

80

b

STO-NaOH

lnk

a100

STO-H2O

STO-H2O

STO

-5.0

1

STO 20 0 0

30

60

90

120

150

180

-5.5

Time (min)

0 0

60

120

180

240

300

360

2.6

Time (min)

c100

2.8

3.0

1000/T (K-1)

3.2

80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0

1

2

3

4

5

Cycles

Figure 4. (a) UV-light photocatalytic decomposition of toluene over STO, STO-H2O and STO-NaOH at 30 oC and its ln(C0/Ct) versus t plot (inset); (b) Arrhenius plots for UV-light photocatalytic degradation of toluene over the samples; (c) Cyclic stability test of STO-NaOH for UV-light photocatalytic decomposition of toluene at 30 oC for 6 h.

23



a Potential (eV vs. NHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1

STO

STO-H2O

STO-NaOH •-

O2/O2 O2/HO2•

0 1

3.30 eV

3.30 eV

3.30 eV -

•OH/OH

2

•OH/H2O

O 2p

3

Figure 5. (a) Eg values (in eV) and position of CB and VB for STO, STO-H2O and STO-NaOH at pH = 7 vs. NHE; (b) Mechanism of photocatalytic toluene mineralization over STO-H2O and STO-NaOH.

24


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For TOC only

Highlights Surface hydroxylation enhances light absorption and charge separation of SrTiO3. Surface hydroxylation induces a negative band edge shift of SrTiO3.

Surface

hydroxylation promotes the formation of hydroxyl radicals and superoxide radicals. Surface hydroxylation enhances photocatalytic mineralization of toluene.

25


Enhanced Photocatalytic Mineralization of Gaseous Toluene over

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