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Post-illumination Activity in a Single Phase Photocatalyst of Modoped TiO Nanotube Array from Its Photocatalytic "Memory" 2

Fan Feng, Weiyi Yang, Shuang Gao, Caixia Sun, and Qi Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04845 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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ACS Sustainable Chemistry & Engineering

Post-illumination Activity in a Single Phase Photocatalyst of Mo-doped TiO2 Nanotube Array from Its Photocatalytic “Memory” Fan Feng,†,‡,# Weiyi Yang,†,# Shuang Gao,§ Caixia Sun,¶,∆ and Qi Li*,† †

Environment Functional Materials Division, Shenyang National Laboratory for Materials

Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

§

Division of Energy and Environment, Graduate School at Shenzhen, Tsinghua University,

Shenzhen 518055, P. R. China ¶

Key Laboratory of New Metallic Functional Materials and Advanced Surface Engineering in

Universities of Shandong, Qingdao Binhai University, Qingdao 266555, P. R. China ∆

School of Mechanical and Electronic Engineering, Qingdao Binhai University, Qingdao

266555, P. R. China *To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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KEYWORDS: Photocatalytic “memory” activity; Mo-doped TiO2 nanotube array; Single phase photocatalyst; H2O2 production in the dark; Electron trapping/release

ABSTRACT: Several composite photocatalysts with photocatalytic “memory” effect had recently been developed, which could possess post-illumination activity for an extended period of time in dark for many potential applications. Here, a single phase photocatalyst of Mo-doped TiO2 nanotube array was developed for the first time with the post-illumination photocatalytic “memory” effect, which could eliminate the requirement of building composite photocatalysts with heterojunctions and largely broaden the material selection for this interesting photocatalytic “memory” effect. Due to the proper electronic band gap structure and variable valences of Modopants, photogenerated electrons could transfer from TiO2 to Mo-dopants and be trapped there by reducing Mo6+ to Mo5+ under UV-irradiation. When UV-irradiation was switched off, these trapped electrons could be released from Mo-dopants and react with O2 through the two-electron O2 reduction process to produce H2O2 in the dark. Thus, Mo-doped TiO2 nanotube array could remain active in the dark as demonstrated by its effective disinfection of E. coli cells when UVirradiation was turned off. This work demonstrated that photocatalysts with post-illumination photocatalytic “memory” effect were not limited to composite photocatalysts with heterojunctions. Various single phase photocatalysts could also possess this interesting photocatalytic “memory” effect through doping metal elements with variable valences.

INTRODUCTION Photocatalysis has great potentials for technical applications in environmental pollution control and clean energy production.1-8 In the past a few decades, various photocatalysts had been


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developed, which largely enhanced the utilization efficiency of solar energy for photocatalytic reactions.9-16 By proper illumination, photocatalysts could generate various kinds of reactive oxygen species (ROSs), which resulted in the disinfection of microorganisms and degradation of organic pollutants.17-20 Generally, photocatalysts are only functional under illumination due to their requirement of continuous illumination to generate electron-hole pairs for their production of ROSs.21,22 Thus, most photocatalysts rapidly lost their activities when the illumination was switched off, while continuous activity is required for many potential applications in the dark for an extended period of time. To solve this problem, several photocatalysts with an interesting post-illumination photocatalytic “memory” effect23-28 had recently been developed since it was firstly found in a composite photocatalyst of PdO nanoparticles modified nitrogen-doped TiO2 in 2008.23 It was found that part of their photoactivity could be stored in “memory” under illumination, so they could remain active from this “memory” after the illumination was switched off for an extended period of time. To possess this interesting photocatalytic “memory” effect, these photocatalysts should contain both the light absorbing component and the electron storing component. So photogenerated electrons could transfer from the light absorbing component to the electron storing component and be stored there under light illumination. Then, they could be gradually released for the production of ROSs through the reaction with O2 to maintain activity when the light illumination was switched off.23-28 Till now, reported photocatalysts with this postillumination photocatalytic “memory” effect for an extended period of time were all composed of

composite

photocatalysts,

including

TiON/PdO,

Cu2O/TiO2,

Cu2O/SnO2,

I/TiO2,

Au@Cu7S4/TiO2, and CNT-Gr/g-C3N4.23-28 Their light absorbing components and electron storing components were in different phases. Thus, it required appropriate energy band matching


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and high quality interfaces between their components for efficient photogenerated electron transfer, which could limit the selection of photocatalyst material systems for the photocatalytic “memory” effect. Thus, it would be interesting to examine if a single phase semiconductor could also obtain such a post-illumination photocatalytic “memory” effect, which could overcome the difficulties in building composite photocatalysts with heterojunctions and largely broaden the selection of photocatalysts with this “memory” effect for various applications. Herein, we report for the first time that a single phase photocatalyst with the post-illumination photocatalytic “memory” effect could be successfully obtained by doping a metal element with variable valences of Mo into a single anatase phase TiO2 nanotube array. It was found that the Mo-dopant could create a shallow defect level located at ~ 0.06 VNHE below the conduction band of TiO2, and its chemical state could exchange between Mo6+ and Mo5+ by trapping photogenerated electrons under UVirradiation and releasing them to react with O2 to produce H2O2 in the dark by two-electron reduction of O2 when UV-irradiation was turned off.29 The post-illumination photocatalytic “memory” effect of Mo-doped TiO2 nanotube array was demonstrated by its effective disinfection of E. coli cells in the dark when UV-irradiation was turned off, and the proposed working mechanism was verified by the measurement of the stored photogenerated electron amount and the production of H2O2 through their release. EXPERIMENTAL SECTION Synthesis of Mo-doped TiO2 nanotube array Ti/Mo (Mo/Ti atomic ratio at ~ 5.26%) alloy was obtained by firstly smelting Ti and Mo together in vacuum arc furnace. To form a single-phase microstructure, Ti/Mo alloy was then quenched in 15 wt.% NaCl aqueous solution after its homogenizing treatment at 1000 oC for 4 h.


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Mo-doped TiO2 nanotube array samples were synthesized by an anodization process at room temperature with the Ti/Mo alloy foil. Before anodization, the Ti/Mo alloy foil was grinded and polished by abrasive paper and polishing cloth, rinsed with DI water, and blow-dried under nitrogen purge. A two-electrode electrochemical system with the platinum foil as the cathode was used for anodization. The Ti/Mo alloy foil was anodized in an ethylene glycol solution containing 0.3 wt.% NH4F and 3 vol.% H2O at 30 V for 2 h. After anodization, the obtained sample was soaked in DI water for 8 h to remove residual organics and then dried in air. After drying, it was calcinated at 550 oC for 2 h in air to crystallize Mo-doped TiO2 nanotube array. For comparison purpose, pure TiO2 nanotube array was also synthesized by the same anodization process with the pure titanium foil at room temperature followed by heat-treatment under same experimental conditions. Characterization of Mo-doped TiO2 nanotube array Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to observe morphologies of the as-prepared samples. FESEM images were obtained with a SUPRA55 FESEM (Carl Zeiss NTS GmbH, Germany). To enhance their surface conductivity, SEM samples were sputtered with gold for 120 s before imaging (Cressington 208HR Sputter Coater, UK). TEM observations were conducted on a JEOL 2100F TEM (JEOL Ltd., Japan), and a thin film of the samples was dispersed on Cu grids to prepare TEM samples. The crystal structures of samples were analyzed on a D8 ADVANCE X-ray diffractometer (Bruker Corporation, Germany). An ESCALAB250 X-ray photoelectron spectrometer (Thermo, USA) was used for XPS measurements. A UV-2550 spectrophotometer (Shimadzu Corporation, Japan) was used to measure the optical absorbance spectra of these samples. A GPH287T5L/4P14W UV lamp was used as the photoirradiation source (WONDER-LIGHT,


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USA). The open circuit potential-time (OCPT) curves and capacitance of the Mo-doped TiO2 nanotube array electrode were measured by a CHI 660D electrochemical workstation (Shanghai Chenhua Instrument Ltd., China) in a 0.2 M Na2SO4 solution. The counter electrode was the platinum wire electrode, and reference electrode was the Ag/AgCl electrode. Detection of hydrogen peroxide (H2O2) concentration The colorimetric DPD method was used to detect in situ photogenerated H2O2, which was based on the horseradish peroxidase (POD) catalyzed oxidation of N,N-diethyl-p-phenylenediamine (DPD). In a typical experiment, 0.1 g N,N-diethyl-p-phenylenediammonium sulfate was dissolved in 10 mL H2SO4 solution (0.05 M), and 10 mg POD was dissolved in 10 mL DI water. They were stored in a refrigerator at 4 oC in the dark. To detect the concentration of H2O2 in the solution, 5 mL aliquot of the test solution was put into a 10 mL test tube, and 0.5 mL phosphate buffer solution (0.5 M KH2PO4 and 0.5 M K2HPO4) was added into it to yield a mixture with pH at ~ 6.0. Then, 50 µL DPD solution and 50 µL POD solution were added into the mixture solution in sequence, and it was shaken for another 10 s. Before the UV-vis spectrum measurement, the solution was settled for 30 s. The H2O2 concentration could be obtained quantificationally through measuring the absorption maximum at λmax of 551 nm by the UV-2550 spectrophotometer. Post-illumination photocatalytic “memory” disinfection of Escherichia coli (E. coli) bacteria in the dark To demonstrate the post-illumination photocatalytic “memory” effect of Mo-doped TiO2 nanotube array, its disinfection effect on wild type E. coli (CGMCC E. coli 8099, China General Microbiological Culture Collection Center, Beijing, China) was conducted in the dark. E. coli cells were cultured overnight and then diluted to a cell suspension (ca.107 cfu/mL) in the buffer


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solution (0.05 M KH2PO4 and 0.05 M K2HPO4, pH 7.0) before disinfection experiments. All solid or liquid materials were autoclaved for 30 min at 121 oC before use. The Mo-doped TiO2 nanotube array sample (with the effective mass ~ 10 mg) was firstly illuminated by the UV lamp for 8 h. Then, the UV lamp was switched off and the sample was used for disinfection experiment in the dark over 6 mL fresh E. coli cell suspension. For comparison purpose, the survival ratios of E. coli cells in dark without the presence of photocatalyst sample and with the presence of Mo-doped TiO2 nanotube array sample without pre-illumination were also investigated under same experimental conditions. Post-illumination photocatalytic “memory” degradation of methyl orange (MO) by Modoped TiO2 nanotube array in the dark The degradation of methyl orange (MO) by Mo-doped TiO2 nanotube array was conducted to further demonstrate its post-illumination photocatalytic “memory” effect. For comparison purpose, the MO degradation experiments were conducted under different reaction conditions, including under UV light illumination without photocatalyst, in the dark treated by Mo-doped TiO2 nanotube array without pre-illumination of UV light, in the dark treated by Mo-doped TiO2 nanotube array with pre-illumination of UV light for 8 h, and treated by Mo-doped TiO2 nanotube array under UV light illumination, respectively. The Mo-doped TiO2 nanotube array sample (with the effective mass ~ 10 mg) was added into 15 mL MO solution (3 ppm). At every time interval, 3 mL solution was collected from the suspension and immediately centrifuged to separate photocatalysts, and the light absorption of the clear solution was measured by the UV2550 spectrophotometer. RESULTS AND DISCUSSION


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Morphology, microstructure and composition of Mo-doped TiO2 nanotube array Self-organized Mo-doped TiO2 nanotube array samples were synthesized by anodization of Ti/Mo (Mo/Ti atomic ratio at ~ 5.26%) alloy foil in a fluoride-based electrolyte followed with the crystallization in air at 550 oC for 2 h. Figure 1a shows the FESEM image of the top view of Mo-doped TiO2 nanotube array and the insert image in Figure 1a shows its TEM image of the cross-section view, both of which clearly demonstrated its nanotube structure. The average diameter of these Mo-doped TiO2 nanotubes was ~ 100 nm, and their average wall thickness was ~ 20 nm. Figure 1b shows the TEM image of the Mo-doped TiO2 nanotube array sample and its corresponding distribution maps of Ti, O, and Mo elements, which suggested that Ti, Mo, and O elements were distributed uniformly in the Mo-doped TiO2 nanotube array. Figure 2a compares the GAXRD patterns of pure TiO2 nanotube array and Mo-doped TiO2 nanotube array. Diffraction peaks of alpha Ti could be observed in the GAXRD patterns of both samples due to its existence in substrates under both nanotube arrays. The observed diffraction peak intensity differences between these samples could be attributed to the microstructural differences between pure Ti and Ti-Mo alloy foils in crystal size and texture.30 Only the anatase phase could be observed for both samples, while no diffraction peaks of Mo-species could be observed. It was found that the diffraction peaks of the anatase phase in Mo-doped TiO2 nanotube array moved to higher 2θ values, compared to that of the anatase phase in pure TiO2 nanotube array. As a representative, Figure 2b compares their anatase (101) diffraction peaks at a higher magnification. The anatase (101) diffraction peak position of TiO2 nanotube array was 25.44o, while that of Mo-doped TiO2 nanotube array sample increased to 25.48o. The ionic radius of Mo6+ is 0.59 Å,31 and that of Ti4+ is 0.605 Å.32 Due to the closeness of their sizes, it is not difficult for Mo6+ to replace Ti4+ in the anatase lattice. It could then slightly decrease the anatase


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lattice spacing and make the anatase diffraction peak positions move to higher 2θ values subsequently as shown in Figure 2b. Both the absence of Mo species diffraction peaks and the higher anatase diffraction peak 2θ values in Mo-doped TiO2 nanotube array than that in pure TiO2 nanotube array suggested that part of Ti4+ cations were replaced by Mo6+ dopants in the anatase lattice of Mo-doped TiO2 nanotube array.30,33 XPS analysis was conducted to analyze the chemical composition and element valence states in Mo-doped TiO2 nanotube array. The XPS survey spectrum of the Mo-doped TiO2 nanotube array sample clearly demonstrated the existence of Ti, Mo, and O in the sample (see Figure 3a). C 1s peak could also be observed in the XPS survey spectrum, which was from the widespread existence of carbon in the environment. Figure 3b shows the high resolution XPS scan over Ti 2p peak, in which the peaks located at 464.5 eV and 458.8 eV could be attributed to the Ti4+ 2p1/2 and Ti4+ 2p3/2 orbitals, respectively.34,35 Figure 3c shows the high resolution XPS scan over O 1s peak, which could be best fitted by the combination of two peaks centered at 530.1 eV (the lattice oxygen) and 531.0 eV (the adsorbed oxygen), respectively.35 Figure 3d shows the high resolution XPS spectrum over Mo 3d peak. The peaks located at 235.6 eV and 232.4 eV could be attributed to the Mo6+ 3d3/2 and Mo6+ 3d5/2 orbitals, respectively.36 XPS analysis results indicated that the main valence state of Ti in the sample was Ti4+ and that of Mo was Mo6+, respectively. Optical properties of Mo-doped TiO2 nanotube array The optical properties of Mo-doped TiO2 nanotube array was investigated by the diffuse reflectance measurement, and its light absorbance spectrum could be approximated by the Kubelka-Munk function as given by Eq. (1): F(R) = (1-R)2 / 2R

(1)


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where R is the diffuse reflectance.37 Figure 4a compares the light absorbance spectra of pure TiO2 nanotube array and Mo-doped TiO2 nanotube array, and both nanotube arrays demonstrated light absorbance from UV to visible light range. The relatively low visible light absorbance of pure TiO2 nanotube array could be attributed its relatively higher crystal lattice distortion from its specific nanotube architecture.38 Compared with TiO2 nanotube array, Mo-doped TiO2 nanotube array demonstrated a clear red-shifted light absorbance further into visible light region, which could be attributed to Mo doping.32 Figure 4b shows Tauc Plots ((F(R)*hv)0.5 vs hv) of both samples,39 and their band gap values could be obtained by the extrapolation of the linear region to the photon energy axis. The band gap value of pure TiO2 nanotube array was determined at ~ 3.15 eV and that of Mo-doped TiO2 nanotube array was determined at ~ 2.80 eV, consistent with their light absobance performances. The band gap of Mo-doped TiO2 nanotube array was 0.35 eV narrower than that of pure TiO2 nanotube array sample, which could be attributed to the defect level in the band gap created by Mo-dopant.32,40 The conduction band of anatase TiO2 was located at ~ -0.29 VNHE,41 so that the defect level created by Mo-dopant was located at ~ 0.06 VNHE. Thus, the electronic structure of the band gap of Mo-doped TiO2 nanotube array could allow photogenerated electrons to move from TiO2 to Mo-dopants.42 Photogenerated electron trapping and release in Mo-doped TiO2 nanotube array As we demonstrated in our previous work,23-25 photocatalysts with the “memory” effect in dark must allow the trapping of photogenerated electrons under illumination and its subsequent release in the dark to produce radicals.23-25 Thus, the trapping and release of photogenerated electrons in Mo-doped TiO2 nanotube was first investigated. Due to the XPS instrument limitation, real time XPS analysis could not be conducted on the sample with UV illumination. Figure 5 shows the high resolution XPS spectrum over Mo 3d peaks in Mo-doped TiO2 nanotube


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array sample pre-illuminated by UV light for 8 h and stored in the dark for 1 h. It demonstrated that the Mo 3d XPS signal could be best fitted by the combination of four peaks of Mo6+ 3d3/2 peak centered at 235.6 eV, Mo6+ 3d5/2 peak centered at 232.4 eV,36 Mo5+ 3d3/2 peak centered at 234.7 eV, and Mo5+ 3d5/2 peak centered at 231.5 eV,43 respectively, while only Mo6+ peaks existed in the high resolution XPS spectrum over Mo 3d peaks in Mo-doped TiO2 nanotube array sample without UV illumination (see Figure 3d). The XPS analysis result demonstrated that ~ 12.4% Mo6+ in the Mo-doped TiO2 nanotube array was reduced to Mo5+ after 8 h UV-irradiation followed by 1 h dark time, while the chemical states of Ti and O elements remained unchanged from their high resolution XPS spectra (see Figure S1 in the Supporting Information). Under UV illumination, anatase TiO2 could be activated to produce photogenerated electron-hole pairs. The XPS analysis result clearly suggested that photogenerated electrons could move from TiO2 to Mo-dopants due to the proper electronic band gap structure of Mo-doped TiO2 nanotube array, and Mo-dopant had an effective photogenerated electron trapping capability under UV illumination due to its chemical state change from +6 valence to +5 valence. Electrochemical tests were carried out to further demonstrate the photogenerated electron trapping and release in Mo-doped TiO2 nanotube array. Figure 6a shows the open circuit potential-time (OCPT) curves of the Mo-doped TiO2 nanotube array electrode versus the Ag/AgCl electrode under different conditions. Without UV illumination, its OCPT curve was mostly constant at ~ 0.10 V in the dark. Upon UV-irradiation, however, its OCPT curve sharply dropped at the beginning of the UV illumination and then remained at ~ -0.005 V, which could be attributed to the equilibrium state between the generation/transfer of photogenerated electrons to the Mo-doped TiO2 nanotube array surface and their reaction with electron acceptors (mostly O2). The sample’s OCPT curve was further measured in dark after being previously irradiated by


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UV illumination for 8 h, which showed a gradual increase behavior with the increase of time in dark towards its OCPT value in dark without previous UV illumination. When the previous UV illumination was off, the equilibrium state between the generation/transfer of photogenerated electrons to the Mo-doped TiO2 nanotube array surface and their reaction with electron acceptors could not be maintained. During the dark time, no more photogenerated electrons could be generated and transferred to the Mo-doped TiO2 nanotube array surface, while electron acceptors (mostly O2) could react with previously trapped electrons to induce the observed OCPT curve increase behavior towards its OCPT value in dark without previous UV illumination. This observation clearly indicated that photogenerated electrons trapped on Mo-dopant could be gradually released in dark for several hours after the UV-irradiation was shut off.44 Figure S2 in the Supporting Information shows the open circuit potential-time (OCPT) curves of the pure TiO2 nanotube array electrode versus the Ag/AgCl electrode under different conditions. It demonstrated that it had very close OCPT curves in the dark with or without previous UV illumination for 8 h, which clearly indicated that no photogenerated electron release happened in the dark on pure TiO2 nanotube array after previous UV illumination because photogenerated electron trapping could not happen on pure TiO2 nanotube array under UV illumination when Mo dopant was not in presence. The quantitative analysis of the amount of photogenerated electrons trapped/released in Mo-doped TiO2 nanotube array From its photogenerated electron trapping and release capability, the Mo-doped TiO2 nanotube array electrode could be considered as a capacitor. Thus, a quantitative analysis of photogenerated electrons trapped in Mo-doped TiO2 nanotube array could be obtained by its capacitance and OCPT curve. The capacitance of the Mo-doped TiO2 nanotube array electrode


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was obtained by chronopotentiometry with the constant current charge-discharge test.45,46 Figure 6b shows its constant current charge-discharge curve, in which the charging current density was set at 2 A g-1 and the effective mass of the Mo-doped TiO2 nanotube array electrode was determined at ~ 0.5 mg. The capacitance of the electrode could be determined by Eqs. (2) and (3): C = Q/∆U

(2)

Q = I*t

(3)

where C is the capacitance, Q is the electric quantity, ∆U is the voltage difference, I is the discharging current, and t is the discharging time. From its potential-time curve (∆U = 0.5 V, t = 82.5 s), the capacitance of Mo-doped TiO2 nanotube array electrode was determined as 0.165 F. The number of Mo-dopant (nMo) in the Mo-doped TiO2 nanotube array electrode (Mo/Ti atomic ratio at ~ 5.26%) could be determined as 1.83*1017. The number of electrons trapped/released from the Mo-doped TiO2 nanotube array electrode in the dark after being previously irradiated by UV illumination for 8 h could be obtained from its OCPT curve and capacitance value by Eqs. (4) and (5): Qr = ∆Ur*C

(4)

ne = Qr/e

(5)

where Qr is the electric quantity released, ∆Ur is the increment of voltage caused by the electron release, ne is the number of electrons released, and e is the electron charge. From its OCPT curve (∆Ur = 0.0364 V) and capacitance value (0.165 F), the electric quantity released (Qr) from the Mo-doped TiO2 nanotube array electrode in the dark after being UV-irradiated for 8 h was determined as 6.01*10-3 C, and the number of electrons released (ne) was determined as 3.75*1016. Thus, ~ 20.5% Mo6+ in the Mo-doped TiO2 nanotube array could transform to Mo5+


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after 8 h UV illumination by trapping these photogenerated electrons. This calculation result was consistent with the XPS analysis shown in Figure 5, where ~ 12.4% Mo-dopant in the Mo-doped TiO2 nanotube array was still Mo5+ after part of these trapped electrons had been released in the dark when the UV illumination had been shut down for 1 h. The production of H2O2 in the dark after previous UV illumination by Mo-doped TiO2 nanotube array The demonstrated electron trapping on and release from Mo-dopant in Mo-doped TiO2 nanotube array should allow the production of reactive oxygen species in the dark after the UV illumination was switched off. The defect level created by Mo-dopant in the anatase band gap was determined at ~ 0.06 VNHE, which was positive than the potential of one-electron reduction of O2 (-0.33 VNHE).47 Thus, photogenerated electrons released from Mo dopant could not produce •

O2– in the dark through the one-electron reduction of O2. However, it was more negative than

the potential of two-electron reduction of O2 (0.695 VNHE), which made it possible to produce reactive H2O2 in the dark through the two-electron reduction of O2.29,47 Therefore, continuous activity could be expected on Mo-doped TiO2 nanotube array in the dark after the UV illumination was switched off. Figure 7 shows the investigation of H2O2 production by Modoped TiO2 nanotube array in test solutions through a colorimetric DPD method48 under different experimental conditions. Under UV illumination, H2O2 production was observed by Mo-doped TiO2 nanotube array (the red curve) as the photocatalysis was going on. After 1 h UV illumination, the H2O2 concentration in the test solution was determined at ~ 35.2 µM. No obvious H2O2 production was observed by Mo-doped TiO2 nanotube array in the dark without previous UV illumination (the black curve), which was due to the lack of photogenerated electron-hole pairs to produce radicals under this situation. As expected, H2O2 was obviously


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produced by Mo-doped TiO2 nanotube array in the dark with previous UV illumination (the blue curve) due to the release of trapped electrons from Mo-dopant in the dark and their subsequent reaction with O2 in the two-electron reduction process. After 1 h in dark, the H2O2 concentration in the test solution was determined at ~ 2.59 µM. The UV light pre-illumination time could have a significant effect on the H2O2 production in the dark of Mo-doped TiO2 nanotube array and its subsequent photocatalytic “memory” activity. Figure S3 in the Supporting Information shows the H2O2 productions by our sample in the dark after a series of UV light pre-illumination time from 2 h, 4 h, 8 h to 12 h, respectively. The H2O2 concentrations in test solutions after 1 h in dark were determined at ~ 1.80, 2.29, 2.59, and 2.66 µM for the UV light pre-illumination time of 2 h, 4 h, 8 h and 12 h, respectively. With the increase of the pre-illumination time, the trapped electron amount could increase, which subsequently could result in more H2O2 production and a better photocatalytic “memory” activity. When the pre-illumination time increased over a certain value, the trapped electron amount got saturated and no further increase of the H2O2 production was observed. From Figure S3, the trapped electron amount got saturated at ~ 8 h, and we chose it as the UV light pre-illumination time used in the following study. For comparison purpose, the production of H2O2 by pure TiO2 nanotube array was also investigated under different experimental conditions through the colorimetric DPD method, and the results were demonstrated in Figure S4 in the Supporting Information. Without Mo-dopant, pure TiO2 nanotube array could not produce H2O2 in the dark no matter a previous UV illumination was conducted or not. This observation was consistent with its OCTP measurement result (see Figure S2), which further suggested that no photogenerated electron release happened in the dark on pure TiO2 nanotube array after previous UV illumination because photogenerated


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electron trapping could not happen on pure TiO2 nanotube array under UV illumination unless Mo dopant was introduced into TiO2. The production of H2O2 in the dark after previous UV illumination by Mo-doped TiO2 nanotube array indicated that it could possess the postillumination photocatalytic “memory” effect to be active in the dark even after the UV illumination was switched off. Post-illumination photocatalytic “memory” disinfection of Escherichia coli (E. coli) bacteria by Mo-doped TiO2 nanotube array in the dark The post-illumination photocatalytic “memory” effect of Mo-doped TiO2 nanotube array was demonstrated in this study by its disinfection of Escherichia coli (E. coli) bacteria in the dark after UV illumination was switched off. In this experiment, Mo-doped TiO2 nanotube array sample was firstly illuminated by UV light source for ~ 8 h. Then, UV illumination was switched off and the sample was used to conduct the disinfection experiment in the dark on fresh E. coli cell suspensions (ca. 107 cfu/mL). Figure 8 shows the E. coli cell survival ratio in the dark treated by pre-illuminated Mo-doped TiO2 nanotube array sample, compared with that by Mo-doped TiO2 nanotube array sample without pre-illumination and that with no presence of photocatalysts. It demonstrated clearly that the survival ratio of E. coli cells only had a slight decrease to ~ 90% in the dark when there was no photocatalyst in the E. coli cell suspension or treated by Mo-doped TiO2 nanotube array sample without pre-illumination, which suggested that Mo-doped TiO2 nanotube array was not toxic to E. coli cells itself while it could not produce active H2O2 in the dark without previous UV illumination. When Mo-doped TiO2 nanotube array had been preilluminated by UV light, a clear disinfection on E. coli cells in the dark was observed as expected due to the production of active H2O2 from the release of Mo-dopant trapped electrons after UV illumination was switched off. The survival ratio of E. coli cells continuously dropped with the


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increase of the treatment time to several hours in the dark. After 4 h treatment in the dark, for example, it dropped to ~ 50%. Thus, the post-illumination disinfection capability of Mo-doped TiO2 nanotube array in the dark was not from the photocatalytic material itself. It relied on its “memory” of prior UV light illumination (trapping of photogenerated electrons) so it could produce active H2O2 in the dark afterwards from the release of these trapped electrons. Post-illumination photocatalytic “memory” degradation of methyl orange (MO) by Modoped TiO2 nanotube array in the dark The post-illumination photocatalytic “memory” effect of Mo-doped TiO2 nanotube array was further demonstrated by its degradation of methyl orange (MO) in the dark after UV illumination was switched off. Figure S5 in the Supporting Information shows the MO degradation curves under different conditions. It demonstrated clearly that UV illumination itself could not effectively degrade MO without the presence of photocatalyst. Without UV illumination, Modoped TiO2 nanotube array sample without pre-illumination also could not degrade MO because it could not produce active H2O2 in the dark without previous UV illumination. The slight decrease of the MO concentration at the beginning could be attributed to its adsorption on our sample. When Mo-doped TiO2 nanotube array had been pre-illuminated by UV light for 8 h, a clear MO degradation in the dark was observed as expected due to the production of active H2O2 from the release of Mo-dopant trapped electrons after UV illumination was switched off. The MO degradation in the dark was not very fast because the reactive oxygen specie generated from the photocatalytic “memory” effect was the relatively mild oxidant of H2O2 and its concentration was not high. Stability and reusability of Mo-doped TiO2 nanotube array on its post-illumination photocatalytic “memory”


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The stability and reusability of Mo-doped TiO2 nanotube array on its post-illumination photocatalytic “memory” were investigated by examining the H2O2 productions by Mo-doped TiO2 nanotube array in the dark after being pre-illuminated under UV light for 8 h for four continuous runs and the results were summarized in Figure S6 in the Supporting Information. The results demonstrated that the H2O2 concentrations in the test solution after 1 h in dark were determined at ~ 2.59, 2.56, 2.40, and 2.50 µM for these four runs, respectively. Thus, the similar H2O2 production behaviors of these four runs indicated that our sample had a good stability and reusability, which is critical for its potential environmental remediation applications. In summary, a single phase semiconductor photocatalyst of Mo-doped TiO2 nanotube array with post-illumination photocatalytic “memory” effect was successfully created by doping a metal element of Mo with variable valences into the anatase phase TiO2 nanotube array. Due to the shallow defect level created by Mo-doping (at ~ 0.06 VNHE below the conduction band of TiO2), photogenerated electrons could transfer from TiO2 to Mo-dopant under UV illumination, while part of them could be trapped there through the valence change of Mo-dopant from Mo6+ to Mo5+. These trapped electrons could be released from Mo-dopant after the UV illumination was switched off, and react with O2 to produce active H2O2 in the dark by two-electron reduction of O2 to induce the observed activity of Mo-doped TiO2 nanotube array in the dark as demonstrated by its effective disinfection of E. coli cells. The trapping and release of photogenerated electrons on Mo-dopant was verified by both the XPS analysis and electrochemical tests, and the amount of trapped/released electrons responsible for the observed post-illumination photocatalytic “memory” effect was obtained for the first time by measuring the OCPT curve and capacitance of Mo-doped TiO2 nanotube array electrode. It was found that ~ 20% of Mo-dopants in Mo-doped TiO2 nanotube array paticipated in the trapping and release of


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photogenerated electrons after UV illumination for 8 h. This work demonstrated that photocatalysts with post-illumination photocatalytic “memory” effect were not limited to composite photocatalysts with heterojunctions. By proper material design, various single phase photocatalysts could also possess post-illumination photocatalytic “memory” effect through doping metal elements with variable valences, which could have a broad range of environmental application potentials for their continuous activities in the dark for an extended period of time.


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FIGURES

Figure 1. (a) FESEM image of the top view of Mo-doped TiO2 nanotube array (Note, insert image in Figure 1a shows its TEM image of the cross-section view). (b) TEM image of Modoped TiO2 nanotube array and its corresponding distribution maps of Ti, O, and Mo elements.

Figure 2. (a) GAXRD patterns of pure TiO2 nanotube array and Mo-doped TiO2 nanotube array, respectively. (b) The anatase (101) diffraction peaks of pure TiO2 nanotube array and Mo-doped TiO2 nanotube array, respectively, at a higher magnification.


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Figure 3. (a) XPS survey spectrum of Mo-doped TiO2 nanotube array. (b) to (d) High resolution XPS spectra over Ti 2p, O 1s and Mo 3d peaks, respectively.

Figure 4. (a) The light absorbance spectra of pure TiO2 and Mo-doped TiO2 nanotube array, respectively. (b) Tauc Plots ((F(R)*hv)0.5 vs hv) constructed from Figure 4a.


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Figure 5. High resolution XPS spectrum over Mo 3d peaks in Mo-doped TiO2 nanotube array sample pre-illuminated by UV light for 8 h and stored in the dark for 1 h.

Figure 6. (a) The open circuit potential-time (OCPT) curves of the Mo-doped TiO2 nanotube array electrode versus the Ag/AgCl electrode in the dark environment without UV-irradiation, upon UV-irradiation, and in the dark after being UV-irradiated previously for 8 h, respectively. (b) The constant current (2 A g-1) charge-discharge curve of the Mo-doped TiO2 nanotube array electrode versus the Ag/AgCl electrode.


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Figure 7. The absorption spectra of the colorimetric DPD solutions after 1 h treatment by the Mo-doped TiO2 nanotube array sample under UV light illumination, in the dark without previous UV-irradiation on the sample, and in the dark with previous UV-irradiation on the sample for 8 h, respectively.

Figure 8. E. coli cell survival ratio in the dark treated by pre-illuminated Mo-doped TiO2 nanotube array sample, compared with that by Mo-doped TiO2 nanotube array sample without pre-illumination and that with no presence of photocatalysts.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. High resolution XPS spectra over Ti 2p and O 1s peaks in Mo-doped TiO2 nanotube array sample with and without previous UV illumination, open circuit potential-time (OCPT) curves of pure TiO2 nanotube array electrode, production of H2O2 by Mo-doped TiO2 nanotube array sample with different UV light pre-illumination time, production of H2O2 by pure TiO2 nanotube array, degradation curves of MO by Mo-doped TiO2 nanotube array sample under different conditions, production of H2O2 by Mo-doped TiO2 nanotube array sample with 8 h UV preillumination for 4 continuous runs (PDF) AUTHOR INFORMATION Corresponding Author *Qi Li. E-mail: [email protected]. Tel: +86-24-83978028. ORCID Qi Li: 0000-0002-0735-8860 Author Contributions #

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This study was supported by the National Natural Science Foundation of China (Grant No. 51672283 and 51602316), the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No. Y4N56R1161 and Y5N56F2161), and the Natural Science Foundation of Shandong Province, P. R. China (Grant No. ZR2017MEM017).


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TOC/Abstract Graphic

SYNOPSIS Mo-doped TiO2 nanotube array photocatalyst with continuous activities in the dark through a “memory” effect had potentials for environmental applications.


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Postillumination Activity in a Single-Phase ... - ACS Publications

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