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Oxygen-Vacancies Activated CO2 Splitting over Amorphous Oxide Semiconductor Photocatalyst Bing Wang, Xiaohui Wang, Lei Lu, Chenguang Zhou, Zhenyu Xin, Jiajia Wang, Xiao-Kang Ke, Guodong Sheng, Shicheng Yan, and Zhigang Zou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02952 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017
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Oxygen-Vacancies Activated CO2 Splitting over Amorphous Oxide Semiconductor Photocatalyst †
Bing Wang,ǁ, Xiaohui Wang,ǁ,‡ Lei Lu,‡ Chenguang Zhou,‡ Zhenyu Xin,‡ Jiajia Wang, Xiao-kang Ke,# Guodong Sheng,& Shicheng Yan,*,‡ Zhigang Zou†,‡ §
†
Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, No. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China ‡
Eco-materials and Renewable Energy Research Center (ERERC), Collaborative Innovation Center of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing University, No. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China. §
College of Mechanics and Materials, Hohai University, Nanjing 211100, P. R. China
#
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China. &
School of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing 312000, P. R. China.
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ABSTRACT: Gaseous oxides generated during industrial processes, such as carbon oxides (COx) and nitrogen oxides (NOx), have important impacts on the Earth’s atmosphere. It is highly desired to develop a low-cost and efficient route to convert them to harmless products. Here, direct splitting of gaseous oxides was proposed based a photocatalysis of amorphous oxide semiconductor. As an example, splitting of CO2 into carbon and oxygen was achieved over amorphous zinc germanate (α-Zn-Ge-O) semiconductor photocatalyst under 300 W Xe lamp irradiation. Electron paramagnetic resonance and 18O isotope labeling indicated that the splitting of CO2 was achieved via photoinduced oxygen vacancies on α-Zn-Ge-O reacted and thus were filled with O of CO2, while the photogenerated electrons reduced the carbon species of CO2 to solid carbon. Under irradiation, such a defect reaction is sustainable by continuous photogenerated hole oxidation of surface oxygen atoms on α-Zn-Ge-O to form oxygen vacancies and to release O2. When we used H2O or NO in place of CO2, H2 and O2, or N2 and O2, was evolved, indicating the same mechanism can also split the H2O or NO.
KEYWORDS: CO2 splitting, photocatalysis, photocorrosion, amorphous photocatalyst, defect reaction
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1. INTRODUCTION There is an urgent need to suppress the ever-growing global warming caused by CO2. A natural method to fix carbon dioxide molecule (CO2) into solid products and releasing oxygen is photosynthesis of plants, which is the most important chemical reaction that converts CO2 and H2O into biomass for remaining the carbon balance in the Earth’s atmosphere.1 Learning from nature, artificial photosynthesis can be constructed based on photocatalysis by using H2O as reductant to convert CO2 into gaseous or liquid reduced carbon-based products (RCPs) and oxygen.2-11 To trigger the light driven reaction of CO2 + H2O → RCPs + O2 over surface of semiconducting photocatalytic materials, the photogenerated hole oxidation of H2O with fourelectron transfer process, which is the rate-limiting step of water splitting due to its high kinetic overpotentials, needed to occur firstly to release O2 and H+. Then photogenerated electrons reduce CO2 to form RCPs such as CH4, a most common product with eight-electron transfer process.12 This means that the photocatalytic reduction of CO2 with H2O faces a major challenge in improvement of reaction kinetics due to the multi-electron process and the requirement of high overpotentials. Recently, physiological metabolism process of bacteria by the bio-enzyme catalysis in place of the photocatalytic CO2 reduction half reaction was coupled to the artificial photosynthesis.13-15 In this reaction, although the kinetics-limited CO2 reduction half reaction was achieved by CO2 and H2 feeding bacteria to excrete liquid RCPs, the high overpotential requirement from overall water splitting to provide hydrogen is remained still. A few routes with relatively easy reaction kinetics, such as electrolysis of molten carbonates into carbon and converting CO2 into solid aluminum oxalate by an O2-assisted Al/CO2 electrochemical cell, have been achieved.16-18 However, there is no report on fixing CO2 into easily storable and more stable solid products by the economical and renewable photocatalysis.
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light light
O2
O*
CO2 +
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M
O*
M
O M
O M
+
C
: Oxygen vacancy (OVs) M: Metallic element O*: Intermediate oxygen species O: Lattice oxygen C: Amorphous carbon
Scheme 1. A scheme to show the proposed route to direct splitting of CO2. The proposed route can directly split CO2 to carbon and O2, and is a sustainable reaction process based on a photocorrosion mechanism.
Directly splitting CO2 into carbon and oxygen may be an effective method to overcome the kinetic limitations, owing to that this process does not need additional chemical reducing agent and only requires four-electron transfer. Recently, experimental evidence indicated that CO2 can be decomposed into C and O2 under irradiation with above 11.44 eV photons,19 which is 10.4 eV higher than the thermodynamic requirement of 1.04 eV for CO2 splitting into C and O2 (as the heat of formation of CO2 from C is -393.5 kJ mol-1). This means that light driven splitting of CO2 is feasible and needs to decrease the reaction activation energy by catalysis. Previous results suggested that metastable oxygen vacancies (OVs) in magnetite from hydrogen reduction seem to be able to react with O of CO2 and to incorporate oxygen in the form of O2- into the oxygen sublattice of the magnetite.20 However, this reaction did not proceed when the OVs in magnetite were consumed completely, meaning that it is a challenge to in-situ regenerate the OVs for achieving a sustainable splitting of CO2. Recently, heating metal oxides
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(MOn) at elevated temperature from concentrated solar radiation, an OVs generation reaction of MOn → MOn-δ + δ/2O2 was achieved under argon atmosphere.21,
22
Subsequently, a solar
thermochemical CO2 splitting into CO can occur via a reaction of MOn-δ + δCO2 → MOn + CO. Although the OVs generation on the MOn is recyclable by temperature swing, the elevated temperature requirement and complexity of device is still the challenge to large-scale applications. As is well known, under irradiation, photocorrosion of oxide photocatalysts by photogenerated hole oxidation is able to in-situ generate oxygen vacancies. 23 This inspired us to use the in-situ photoinduced OVs to achieve a defect reaction to split CO2 over oxide photocatalyst under a mild reaction condition that does not require high temperature and complicated equipment. To achieve this purpose, a major challenge is how to generate high concentration of OVs on surface of photocatalyst. Here, we proposed to use amorphous semiconductor photocatalyst with weak lattice constraint via photocorrosion to facilitate the generation of OVs, which can react with oxygen atoms of CO2. Simultaneously, photogenerated electrons reduce the carbon species of CO2 to solid carbon. Such a defect reaction is sustainable by continuous photogenerated hole oxidation of surface oxygen atoms on amorphous semiconductor to form OVs and O2 (Scheme 1). Our findings offer a light driven method to directly fix CO2 into solid carbon by combining photoinduced defect reaction and photocatalysis. The same mechanism is also useful to split other molecules such as H2O and NO. 2. EXPERIMENTAL DETAILS Synthesis of amorphous α-Zn-Ge-O photocatalyst. All the raw materials used in this study were analytical grade reagents and were used without further purification. Amorphous ZnGe-O was synthesized by an ion exchange route in presence of strong reducing agent. Firstly, Na2GeO3 solid powders were prepared by heating a stoichiometric mixture of GeO2 and Na2CO3
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in muffle furnace at 900 °C for 12 h. In a typical experiment, zinc nitrate hexahydrate powders (1.5 g) were added into Na2GeO3 aqueous solution (5.8 mM, 10.36 mL). After that, a freshly prepared sodium borohydride solution (25 mM, 5.2 mL) was injected for obtaining the amorphous Zn-Ge-O photocatalyst. After the suspension was magnetically stirred continuously for 20 h, the resulting powders were collected by centrifugation, washed with distill water for three times, and freeze-dried at -50 oC in vacuum for 24 h. Characterization. The composition of α-Zn-Ge-O was determined by energy dispersive Xray (EDX) analysis on a Hitachi S-4800 field-emission scanning electron microscope (Hitachi, Japan), and was further confirmed by wavelength-dispersive X-ray fluorescence (XRF) spectrometer (ARL-9800, Switzerland) operated at 50 kV and 50 mA. The α-Zn-Ge-O powders (50 mg) were pressured into a pellet for XRF analysis. Semi-quantitative analysis method was used to quantify the elemental content. The crystallographic phase of the α-Zn-Ge-O was identified by powder X-ray diffraction (XRD, RigakuUltima III, Japan) operated at 40 kV and 40 mA with Cu-Kα radiation. XRD pattern was recorded for 2θ values ranging from 10 to 80o with a scanning rate of 0.4o min-1. To perform the X-ray absorption spectroscopy (XAS) analysis, the powdered samples were first ground with agate mortar and pestle for 20 min to obtain highly dispersed powders, then we use a 400-mesh sieve to separate particle sizes. The resulting powders were uniformly painted on the plastic radiation-resistant scotch tape (Kapton® tape, the duct tape of the synchrotron) with length in 10 cm and width in 1 cm. To achieve the compact particle distribution on the scotch tape without the pinholes, after shaking off the large particles, the sample tape was cut into 10 small pieces, which were stacked layer by layer on the another scotch tape and covered with a piece of scotch tape. Ge K-edge and Zn K-edge X-ray absorption spectra (XAS) were carried out on the 1W1B beam line of the Beijing synchrotron radiation
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facility, China, operated at ~200 mA and ~ 2.5 GeV. Ge, GeO2, Zn2GeO4, Zn, and ZnO powders were used as reference samples and all samples were measured in the transmission mode. The transmission electron microscopy images were acquired by JEOL-2100 highresolution transmission electron microscopy (TEM, JEOL, Japan) operating at 200 kV. The UVvis diffuse reflectance spectra were recorded with a UV-vis spectrophotometer (UV-2550, Shimadzu, Japan) at room temperature and transformed into the absorption spectra according to the Kubelka–Munk relationship. The X-ray photoelectron spectroscopy (XPS) data were collected using an X-ray photoelectron spectrometer PHI5000 Versa Probe (ULVAC-PHI, Japan) with a nominal energy resolution of 0.6 eV. The catalyst powders were pressured into a thin disk and XPS measurements were performed using an ultrahigh vacuum chamber with a base pressure below 6.7×10-10 mbar. XPS spectra were acquired using a photon beam of 1486.6 eV from an Al anode X-ray source. Photo beam perpendicularly irradiated the surface of measured sample with an analysis depth of about 6 nm. Data were collected at room temperature. The binding energy was determined by reference to the C 1s line at 284.6 eV. Elemental content (Cx) was determined by formula of
Cx =
I x Sx ∑ I i Si
, where I is the XPS peak area and S is the
i
sensitivity factor. The electron paramagnetic resonance (EPR) spectra were obtained using a Bruker (model EMX-10/12 X-band, Bruker, Germany) electron paramagnetic resonance spectrometer at room temperature (25 oC). The settings used were: center field of 3480.0 G; microwave frequency of 9.2-9.8 GHz; power of 19.97 mW. The quantitative EPR measurement was performed by using 50 mg of samples. The g factor is calculated by g = hυ/βH, where h is Planck’s constant, υ is the microwave frequency, β is the Bohr magneton and H is the magnetic field at which resonance occurs. Oxygen vacancy concentration was obtained by referencing to the spin numbers of lone electrons of the natural coal. The natural coal is a stable and highly
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EPR-sensitive material. Here, we use the natural coal (100 mg, particle size 5µm, Center of Modern Analysis, Nanjing University) as standard sample to obtain the conversion coefficient between EPR peak area and mole amount of spin electron. Using the conversion coefficient, concentration of oxygen vacancies trapped one electron in the α-Zn-Ge-O with different irradiation times was calculated based on their EPR peak areas. The room-temperature photoluminescence (PL) spectra were obtained by using a picosecond Ti:Sapphire laser (Mira HP, from Coherent) as the excitation source. The excitation wavelength is 485 nm. The PL was collected vertically from the surface by a 50x microscope objective and sent through a 0.5 m spectrometer to a CCD camera. The sample PL could be alternatively sent to an APD in the timecorrelated single-photon counting system with a time resolution of ~100 ps. The yield of carbon was determined with a Vario-Micro elemental analyzer (EA, Elementar, Germany). Raman scattering was recorded in the 1250-1850 cm-1 range, using a Jobin-YvonT6400 spectrometer (HORIBA JobinYvon, France) (argon ion laser with λexc= 514.5 nm, power density:
