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Localization and stabilization of photogenerated electrons at TiO2 nanoparticle surface by oxygen at ambient temperature Junxian Gao, Jinze Lyu, Ji Li, Junwei Shao, Yanhong Wang, Weiqi Ding, Rui Chen, Siyu Wang, and Zebin He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01011 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018
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Localization and stabilization of photogenerated electrons at TiO2 nanoparticle surface by oxygen at ambient temperature
Junxian Gaoa, Jinze Lyua, b*, Ji Lia, b, Junwei Shaoa, Yanhong Wanga, Weiqi Dinga, Rui Chena, Siyu Wanga, Zebin Hea a
School of Environment and Civil Engineering, Jiangnan University, Wuxi, Jiangsu, 214122, China
b
Jiangsu Key Laboratory of Anaerobic Biotechnology, Jiangnan University, Wuxi, Jiangsu, 214122,
China
Corresponding Author: Jinze Lyu School of Environment and Civil Engineering, Jiangnan University, Wuxi, Jiangsu, 214122, China Email:
[email protected] Tel: +86 510 85197210 Fax: +86 510 85197210
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Abstract: Understanding the mechanism by which oxygen adsorption influences the separation behavior of charge carriers is important in the photocatalytic removal of air pollutants. In this study, we performed steady-state surface photovoltage and surface photocurrent spectroscopy combined with an atmosphere control system to determine the effect of oxygen on the charge separation behavior at the surface of anatase TiO2 nanoparticles at ambient temperature. Results showed that photogenerated electrons were movable in N2 atmosphere but were localized in O2 atmosphere. O2 obviously enhanced the stabilization of the localized photogenerated electrons when the surface defects of TiO2 were fully occupied by adsorbed O2. Moreover, O2 adsorption increased the energy demand for exciting electrons from the valence band to localized surface defect states and reduced the density of band tail states. These findings suggest us that the effect of gaseous species on the mobility and stability of charge carriers should be considered to understand the photocatalytic degradation of air pollutants.
Keywords: Surface defect; Visible light; Surface photovoltage; Surface photocurrent; Oxygen vacancy
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1. INTRODUCTION Photocatalytic technologies based on TiO2 and other semiconductor nanoparticles have increasing application prospect in air pollution1-4 and self-cleaning5-7. In these fields, oxygen commonly acts as the reactant of fundamental reactions. The effect of the interaction between oxygen and TiO2 surface states on the separation of photogenerated charge carriers is the key to understand photocatalysis.8 Surface band bending theory is usually applied to explain adsorbent-induced charge separation.9 Photoluminescence results show that O2 enhances the separation of charge carriers at low temperatures (mostly 77–87 K).10-11 O2 as an electron acceptor was considered to increase the upward bending degree of TiO2 surface bands and thus enhances the separation efficiency of charge carriers and the migration of holes from bulk to surface.9-10 However, the physical properties, such as conductivity, surface states, and carrier–phonon interaction, of TiO2 at low temperatures significantly differ from those at ambient temperature. Therefore, the mechanism underlying oxygen-induced charge separation at ambient temperature remains unclear. Recent studies have employed scanning tunneling microscopy (STM) and density function theory (DFT) calculations to elucidate the mechanism by which oxygen affects the surface state of TiO2.12-15 O vacancy hardly exists at the TiO2 surface in the presence of O2 or H2O at ambient temperature. O2 preferentially adsorbs on the site of
near subsurface O vacancy on the anatase (101) surface.16 Under the influence of
adsorbed O2, the subsurface O vacancy migrates to the surface and unites with O2 to form a new surface state.17 These studies inspired us to understand the effect of O2 on the surface state and charge separation on the basis of the localized adsorption sites and adsorption mode. However, experimental data on the O2 adsorption-determined charge separation under real conditions remain lacking. 3
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In this work, surface photovoltage (SPV) and surface photocurrent (SPC) spectroscopy combined with an accurate atmosphere control system were conducted to study the interaction between oxygen and photogenerated charge carriers of anatase TiO2 nanocrystals at room temperature (298 K). SPV characterizes the potential variation of semiconductor surface caused by illumination. The higher the SPV, the more the charge carriers migrate to the semiconductor surface, and the higher the separation efficiency of the carriers. The phase result corresponding to SPV qualitatively indicates the migration direction of photogenerated charge carriers. SPC was measured to study the mobility of photogenerated charge carriers in the presence of oxygen. SPV and SPC are also sensitive to the surface states, thereby providing detailed information about the effect of oxygen on the surface states and the corresponding charge separation.
2. EXPERIMENTAL SECTION 2.1. Preparation of materials. Anatase TiO2 (25 nm, Aladdin) was labeled “Anatase”. Anatase TiO2 powder was calcined at 773 K for 4 h in a muffle furnace to reduce defects. The obtained sample was labeled “L-Anatase”.
2.2. Characterization of catalysts. X-ray diffraction (XRD) was performed using a D8 Advance (Bruker, Germany) with Cu Kα radiation. The surface composition of the prepared samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, USA) with Al Kα as X-ray source. The XPS curves were calibrated by adventitious carbon signal at 284.6 eV. The adsorption of N2 and O2 was measured using a BELSORP-max Surface Area Analyzer (MicrotracBEL, Japan). The temperatures for N2 and O2 adsorption were 77 K and 298 K, respectively. High-resolution transmission electron microscopy (HRTEM) was conducted with a Joel 4
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2100F instrument (FEI, USA). Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) mapping were determined with an S4800 instrument (Hitachi, Japan). UV-vis diffuse reflectance spectroscopy (DRS) was recorded using a UV-1800APC spectrometer (Macy, China). 2.3. Characterization of photogenerated charge carriers. The photogenerated charge carriers were characterized using a self-assembled atmosphere surface photovoltage/photocurrent spectroscopy system (ASPS system). As shown in Figure S1, the structure of the ASPS system consists of a xenon light source (XHA-500, 500 W, China), a monochromator (Omni-λ300, Zolix, China), a light chopper (SR540, Stanford Research Systems, USA), an atmosphere tank, a lock-in amplifier (SR830, Stanford Research Systems, USA), a gas source, and a computer. The light from the xenon lamp was adjusted to monochromatic light by the monochromator, modulated by the light chopper to 23 Hz, and then induced to the atmosphere tank. The modulated light irradiated the sample surface through an indium tin oxide (ITO) glass, thereby producing a periodic electric signal. The signal was translated into SPV, SPC, and phase by the lock-in amplifier. SPV and SPC experiments were conducted with the same equipment, except for the electrode structure. A schematic of the electrode structure is shown in Figure S1. For the SPV measurement, the prepared sample was placed between ITO glass and copper base. The bias voltage between the ITO glass and the copper base was adjustable in the region of −1 to +1 V. A comb-like electrode with ITO coating was used for SPC measurements. The conducting coating in the middle of the ITO glass was etched off. The prepared sample covered this disconnected area, and a bias of 10 V was applied between the two sides of the comb-like electrode. Thus, the photogenerated charge carriers would separate and form a current under light irradiation. The wavelength of SPV and SPC in this work was set between 5
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300 and 500 nm (between 300 and 800 nm in particular cases). The scan of induced light started from a long wavelength to a short wavelength. ITO glass absorbs light at wavelength
