Detection of Oxygen Vacancies in Oxides by Defect-Dependent

Article pubs.acs.org/ac

Detection of Oxygen Vacancies in Oxides by Defect-Dependent Cataluminescence Lijuan Zhang, Si Wang, and Chao Lu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Oxygen vacancies can control a number of distinct properties of oxides. However, rapid and simple detection of oxygen vacancies is a great challenge owing to their elusive species and highly diluted contents. In this work, we have discovered that cataluminescence (CTL) intensity in diethyl ether oxidation reaction on the surface of TiO2 nanoparticles is proportional to the content of oxygen vacancies. The oxygen vacancy-dependent diethyl ether CTL is attributed to the fact that abundant chemisorbed O2 in oxygen vacancies could facilitate its contact reaction with chemisorbed diethyl ether molecules, resulting in an obvious improvement of CTL intensity. Therefore, diethyl ether CTL can be employed as a simple probe for oxygen vacancies in TiO2 nanoparticles. Its feasibility is validated by detecting the CTL intensity of diethyl ether on the surface of TiO2 with variable oxygen vacancies by metal ion-doped TiO2 nanoparticles (Cu, Fe, Co, and Cr) and hydrogen-treated TiO2 nanoparticles at different temperatures. The content of oxygen vacancies by the present CTL probe is in good agreement with that obtained by conventional X-ray photoelectron spectroscopy (XPS) technique. The superior properties of the developed CTL probe over already-developed methods include fast response, easy operation, low cost, long-term stability, and simple configuration. We believe that the oxygen vacancy-sensitive CTL probe has a great potential in distinguishing oxygen vacancies in oxides.

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respect to simplicity for probing oxygen vacancies could potentially be encountered. Cataluminescence (CTL) has attracted considerable interest as it offers unique insights during heterogeneous catalytic oxidation of gas molecules on the surface of a solid catalyst in an atmosphere containing oxygen.21,22 Over the past few decades, scientists are always concentrating their efforts on new CTL strategies to improve selectivity and sensitivity of CTL by tuning solid catalysts or array technique.23−25 However, they do not attach high importance contribution of oxygen to the CTL signal amplification.26 Researchers have demonstrated that both gaseous diethyl ether and O2 molecules in CTL systems initially diffuse from outer gas phase, and then they are chemisorbed on the surface of solid catalyst. The chemisorbed O2 can capture a free electron to form superoxide oxygen species (O2−). Finally, the contact reaction of O2− with adsorbed gaseous molecules leads to excited-state species, which subsequently emits light by radiating photon when they return to ground-state molecules.24,27 Therefore, it is concluded that abundant oxygen vacancies on oxides can make catalyst chemisorb more oxygen, causing an increase in the chemical rates of CTL emission.28,29 Inspired by oxygen-sensitive CTL emissions, it is reasonable to

owadays, it has been demonstrated that a number of distinct properties of oxides are controlled not so much by their geometric/electronic structure but by defects in the structure.1−3 There is a considerable effort to better characterize the chemical, electrical, and optical properties of defects in oxides so as to manipulate the concentration of defects.4−7 Among all the defects, oxygen vacancies are supposed to be the prevalent point defects in oxides.8,9 Theoretical calculations and experimental characterizations revealed that oxygen vacancies in oxides (e.g., TiO2) can not only serve as photoinduced charge traps but also lead to an enhancement of photocatalytic activity.10−13 Therefore, it is centrally important for determining the concentration of oxygen vacancies in oxides for tuning the properties of oxides in a desired manner. However, oxygen vacancies are often elusive species, highly diluted, and thus difficult to detect.14 The existence of oxygen vacancies could be proved based on the change of a spectroscopic response (e.g., Raman spectroscopy) as a function of the number of oxygen vacancies present in the sample. 15 In addition, some sophisticated techniques as well as density functional theory calculations have been performed to detect oxygen vacancies,16−20 such as X-ray photoelectron spectroscopy (XPS),16,17 electron spin resonance,18 and atomically resolved transmission electron microscopy.19,20 The already-developed methods have contributed much to improving the discrimination ability of oxygen vacancies, but it requires professionals to manipulate these sophisticated equipment. Therefore, limitations with © 2015 American Chemical Society

Received: April 18, 2015 Accepted: June 24, 2015 Published: June 24, 2015 7313

DOI: 10.1021/acs.analchem.5b02267 Anal. Chem. 2015, 87, 7313−7320

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Figure 1. CTL configuration system for oxygen vacancy identification through diethyl ether CTL reaction on the surface of TiO2.

to form a thin gel layer. After aging at 100 °C for 12 h, the gel was ground to powder and then calcined at 550 °C for 2 h with a temperature-programmed method. The concentrations of Cu doped into TiO2 were 0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt %, respectively. Similarly, Co, Cr, and Fe ion-doped TiO2 samples were prepared according to the above procedure in the presence of the corresponding metal ion salt precursors to give a doping content of 1.0 wt %. Preparation Hydrogen-Treated TiO2 Nanoparticles. A thermal reduction process was performed to prepare the hydrogen-treated TiO2 nanoparticles.31 The as-synthesized TiO2 gel powder was first calcined in air at 500 °C for 2 h and then calcined in hydrogen atmosphere for an additional 1 h at various temperatures (e.g., 250 °C, 350 °C, and 400 °C) in a tube furnace. Flow rate of hydrogen gas was set at 30 mL min−1, and an increasing rate was 10 °C min−1. The obtained taupe powders were denoted as H-250, H-350, and H-400, respectively. The TiO2 nanoparticles calcined in air at 500 °C for 2 h without hydrogen-treated were denoted as Ti-500. Apparatus and Characterization. The crystal phase, composition, and crystallinity of TiO2 and Cu/Fe/Co/Crdoped TiO2 were recorded with a Bruck (Germany) D8 ADVANCE X-ray diffractometer (XRD) equipped with graphite-monochromatized Cu/Kα radiation (λ = 1.54178 Å). The samples as unoriented powders were step-scanned in steps of 0.02° (2θ) in the range of 20−80°. UV−visible diffused reflectance spectra were obtained in the range 220−800 nm for dry-pressed disk samples using a Shimadzu UV-3600 spectrometer (Tokyo, Japan). BaSO4 was used as a reference. The morphology of samples was studied using high resolution transmission electron microscopy (HRTEM) (JEOL JEM3010) with an accelerating voltage of 300 kV. Specific surface areas were obtained using the Brunauer−Emmett−Teller (BET) method from N2 adsorption−desorption isotherms at 77 K, which were measured using Autosorb-IQ-MP nitrogen adsorption apparatus (Quantachrome, USA). XPS measurements were performed on a photoelectron spectrometer (VG ESCALAB MKII, Thermo ESCALAB 250, USA) at 2 × 10−9 Pa using Al Kα X-ray as excitation source. Inductively coupled plasma optical emission spectrometry (ICP−OES, Thermo iCAP 6300 ICP−OES) was used to obtain the actual contents

explore a new and rapid strategy for probing oxygen vacancies in oxides by means of the corresponding CTL performances. In this work, it is found that CTL intensity in diethyl ether oxidation reaction on the surface of TiO2 nanoparticles is proportional to the content of oxygen vacancies. The oxygen vacancy-sensitive CTL is ascribed to the fact that abundant oxygen vacancies in TiO2 nanoparticles could adsorb more O2, facilitating a contact reaction of O2− with chemisorbed diethyl ether molecules (Figure 1). Such an intriguing discovery demonstrates diethyl ether CTL could be employed as a simple and stable strategy for sensing oxygen vacancies on the surface of metal ion-doped TiO2 nanoparticles (Cu, Fe, Co, and Cr) and hydrogen-treated TiO2 nanoparticles at different temperatures. A good matching between the proposed CTL probe and XPS technique was obtained. It seems reasonable to anticipate that the CTL technique may become an attractive alternative to traditional methods for determining oxygen vacancies in oxides with low cost, simple configuration, fast response, long-term stability, and easy operation.

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EXPERIMENTAL SECTION

Chemicals and Materials. All reagents were of analytical grade and used without further purification. Tetrabutyl titanate (Ti(OBu)4) was purchased from Alfa Aesar (Ward Hill, MA). Diethyl ether, ethanol, acetic acid, Cu(NO3)2·3H2O, Co(NO3)2·6H2O, Cr(NO3)3, and Fe(NO3)3 were supplied by Beijing Chemical Reagent Company (Beijing, China). High performance liquid chromatography (HPLC) grade acetonitrile was from Merck KGaA (Darmstadt, Germany). All solutions were freshly prepared with deionized water (18.2 MU cm, Milli Q, Millipore, Barnstead, CA, USA). Preparation of TiO2 Nanoparticles and Metal IonDoped TiO2 Nanoparticles. A series of Cu-doped TiO2 powder within the range of 0−2.5 wt % were prepared by a simple sol−gel method according to literature procedures with little modification.30 Briefly, 300 μL of acetic acid and 5.7 mL of Ti(OBu)4 were slowly added into 16.6 mL of ethanol successively under vigorously stirring to form transparent sol at room temperature. A certain amount of Cu(NO3)2·3H2O dissolved in 5.0 mL of ethanol was then added dropwise with stirring for 12 h. The as-prepared sols were cast into a Petri dish 7314

DOI: 10.1021/acs.analchem.5b02267 Anal. Chem. 2015, 87, 7313−7320

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Figure 2. (A) XRD patterns and (B) UV−vis diffuse reflectance spectra of TiO2 nanoparticles with different Cu loadings. (C) HRTEM images of TiO2 and 1.5 wt % Cu-doped TiO2 nanoparticles. (D) Relative CTL signals of 100 mM diethyl ether on the surface of TiO2 and Cu-doped TiO2 with different doping contents. Air flow rate, 250 mL min−1; working temperature, 170 °C.

The mixed gas was pumped into the reaction cell. A BPCL ultraweak chemiluminescence analyzer was used to monitor the CTL signals with a work voltage of 1000 V and data integration time 1 s per spectrum. Finally, the CTL signals were imported to the computer for data analysis. In Situ FT-IR. In situ FT-IR measurements were carried out to investigate reaction intermediates of a diethyl ether CTL reaction. All catalyst samples for in situ FT-IR were compressed into self-supporting pellets prior to measurement. These samples were placed in the thermostatic reactor mounted in the cell compartment of an FT-IR spectrometer. The background records were collected at room temperature and 170 °C by a temperature-programmed process, respectively. Then, the sample was completely adsorbed by exposing the sample into air flow containing diethyl ether for about 1 h, followed by desorption with air for about 30 min at room temperature until the intensity of the IR spectrum was stable. Next, the temperature was raised from room temperature to 170 °C at a heating rate of 10 °C min−1 and then remained at 170 °C for 30 min. The sample was then scanned continuously to get IR signals. All IR spectra were measured over the range of 4000−650 cm−1 at 8 cm−1 resolution and the accumulation of 32 scans. GC/MS Measurements. GC/MS was employed to identify the CTL reaction products of diethyl ether on the surface of TiO2 and Cu-doped TiO2 nanoparticles. In the GC/MS experiments, 10 mL of HPLC grade acetonitrile was used to collected the resultant gases during the CTL reaction through repeated injection of 100 mM diethyl ether into the CTL system for about 4 h. Note that the carrier gas flow rate should be kept as low as possible in order to prevent the overflow of reaction products accompanying with air bubbles.

of dopants in TiO2 samples. A biophysics chemiluminescence (BPCL) luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) was used to detect CTL intensity. A Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan) was used to obtain CTL spectra at a slit of 20 nm and a scanning rate of 3000 nm min−1. The excitation lamp was off. In situ Fourier Transform Infrared Spectroscopy (FT-IR) experiments were carried out on a Nicolet 380 system (Thermo, USA) containing a controlled environment chamber equipped with CaF2 windows. The gas chromatography-mass spectrometer (GC/MS) experiments were performed on a Thermo Trace 1300-ISQ GC/MS system (Thermo, USA) equipped with a TR-5MS column (length = 30 m, inner diameter = 0.25 mm, film thickness = 0.25 μm). Volatile organic gases were delivered by an air pump (Beijing Zhongxing Huili Co. Ltd., Beijing, China). The heater controller (Shenzhen Hengtai Electric Equipment Factory, China) was used to provide heater for the ceramic rod. The TiO2 was calcined in air atmosphere in KXL-1100X muffle (Hefei Kejing Materials Technology Co. Ltd., Anhui, China). OTF-1200X tube furnace (Hefei Kejing Materials Technology Co. Ltd., Anhui, China) was used to obtain hydrogen-treated TiO2 nanoparticles. CTL Measurements. A catalyst layer about 0.2 mm was formed after sintering 0.2 g of synthesized catalyst powder onto the cylindrical ceramic heater (inner diameter 5.0 mm, length 10.5 cm) and then putting it into a quartz tube (diameter 1.0 cm, length 10 cm). 25.0 μL of 100 mM diethyl ether solution was injected into a gasification room and then reached the reaction cell delivered by the steady air carrier stream from an air pump. Carrier gas containing different O2 concentrations was obtained by adjusting the volume flow rates of O2 and N2. 7315

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Figure 3. XPS spectra of O 1s on the surface of (A) TiO2, (B) 1.5 wt %, (C) 2.0 wt %, and (D) 2.5 wt % Cu-doped TiO2 nanoparticles, respectively.

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RESULTS AND DISCUSSION Oxygen Vacancy-Dependent Diethyl Ether Cataluminescence. TiO2 and a series of Cu-doped TiO2 nanoparticles were synthesized to investigate the influences of their oxygen vacancies on the CTL intensity of diethyl ether. XRD patterns of TiO2 nanoparticles with different Cu loadings showed that both the crystallite size and d110 lattice spacing decreased with an increase in Cu loadings (Figure 2A), indicating that Cu ions were successfully doped into TiO2 lattice.32 In UV−vis diffuse reflectance spectra, for Cu-doped TiO2, there showed an extension of the absorption edge to the visible light region in comparison with the pure TiO2 (Figure 2B), which may be assigned to Cu2+ dopant in these samples.33 XPS data showed that weak peaks related to Cu shifted toward the lower binding energy (Figures S1−S3), while the Ti 2p binding energy of Cudoped TiO2 increased compared with that of pure TiO2 (Figure S4),34 further demonstrating that Cu2+ was successfully incorporated into a TiO2 lattice to form O−Cu−O bonding. The Cu contents of TiO2 nanoparticles with different Cu loadings were quantified by ICP−OES, which were almost consistent with the added values (Table S1). Note that the crystallite sizes of all the synthesized samples calculated from the Scherrer equation were around 16 nm, which were approximate with HRTEM results (Figure 2C). The synthesized TiO2 and a series of Cu-doped TiO2 nanoparticles were coated onto a cylindrical ceramic heater for CTL measurements by injecting 100 mM diethyl ether into the proposed flow system as shown in Figure S5. It could be found that the CTL intensity of diethyl ether increased gradually with an increase in Cu doping content up to 1.5 wt %; however, the CTL intensity decreased when the Cu doping content was higher than 1.5 wt % (Figure 2D). As we know, the surface area of catalyst always has a strong impact on catalytic properties,35 but there seems no such correlation in the present

system based on the results of BET measurements (see Table S1). We assume that the obvious improvement of CTL intensity of diethyl ether on the surface of Cu-doped TiO2 may be attributed to an increase in oxygen vacancies. In order to verify this possibility, XPS spectra of TiO2 with the variable doping contents of Cu (0, 1.5, 2.0, and 2.5 wt %) were detected (Figure 3). In general, there were three O 1s peaks appearing after deconvolution, which were attributed to lattice oxygen (OL, 530.2 eV), surface hydroxyl oxygen (O−OH, 532.1 eV), and adsorbed oxygen (OS, 533.3 eV), respectively.34 In the present system, in comparison to the pure TiO2, the content of adsorbed oxygen increased remarkably at the surface of 1.0 and 1.5 wt % Cu-doped TiO2 nanoparticles. Accordingly, based on the reported formula,34 it was calculated that oxygen vacancies of 0, 1.0, and 1.5 wt % Cu-doped TiO2 nanoparticles increased from 19.8% to 28.4%, 30.0%. However, the oxygen vacancies decreased to 27.6% and 25.8% for 2.0 and 2.5 wt % Cu-doped TiO2 nanoparticles, respectively (Table S2). Therefore, it is concluded that the variation trend of CTL intensity is consistent with the content change of oxygen vacancies in TiO2 nanoparticles. Note that when the doping content of Cu was higher than 1.5 wt %, there appeared two peaks at 2θ of 35.6° and 38.2° related to CuO as shown in Figure 2A.36 In addition, there was an obvious absorption in the range of 600−800 nm, which was assigned to CuO in octahedral symmetry (Figure 2B).37 Therefore, we concluded that the bulk of TiO2 cannot accommodate more Cu doping when the amount of Cu dopant is too high. The excess CuO will appear on the surface of TiO2, leading to a decreased coverage of the surface oxygen vacancies and a turning point for both measured CTL signals and oxygen vacancy concentration. Therefore, we can get the maximum oxygen vacancy concentration from the CTL 7316

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Figure 4. (A) Effect of oxygen concentration on the CTL signals of diethyl ether on the surface of TiO2 nanoparticles. Air flow rate, 250 mL min−1; working temperature, 170 °C; diethyl ether concentration, 100 mM. (B) In situ FT-IR spectra of diethyl ether oxide reaction on the surface of TiO2 nanoparticles at different reaction temperatures and time. (C) GC/MS chromatogram from the catalytic oxidation of diethyl ether on the surface of TiO2 nanoparticles. (D) CTL spectrum of diethyl ether on the surface of TiO2 nanoparticles. Air flow rate, 250 mL min−1; working temperature, 170 °C; diethyl ether concentration, 200 mM.

nm (Figure 4D), meaning that the emitter of the present system was the electronic excited state CH3CHO*.26 On the basis of the above discussion, the possible mechanism of oxygen vacancy-dependent diethyl ether CTL was summarized as follows: abundant oxygen vacancies in TiO2 nanoparticles facilitated to adsorb more O2, which could form lots of O2−.24 A contact reaction of O2− with chemisorbed diethyl ether molecules led to excited-state CH3CHO molecules, and then it fell to ground state with emission. Finally, the CH3CHO molecules could further be oxidized by O2− to form CO2 (Figure 1). Note that the same CTL reaction process of diethyl ether on the surface of Cu-doped TiO2 nanoparticles was obtained (Figure S6). CTL Probe for Rapid Detection of Oxygen Vacancies in TiO2. There are many types of techniques to introduce oxygen vacancies into TiO2 nanoparticles mainly including doping with metal ions, hydrogen thermal treatment, and high energy particle bombard.40−43 Therefore, in order to evaluate the feasibility of the proposed CTL method for sensing oxygen vacancies, variable oxygen vacancies in TiO2 were prepared by metal ion-doped TiO2 nanoparticles and hydrogen-treated TiO2 nanoparticles at different temperatures. It is widely known that the doping of cations with valence lower than Ti4+ could effectively enhance the catalytic activity through the introduction of oxygen vacancies.44 On the other

emissions, which makes the greatest contribution to catalytic reactions. Mechanism of Oxygen Vacancy-Dependent Diethyl Ether Cataluminescence. First, we investigated the influence of the chemisorbed O2 on the surface of TiO2 nanoparticles on CTL intensity. Figure 4A showed a direct proportion between CTL intensity and oxygen concentration in the carrier gas from 0% to 50%. These results indicated that CTL intensity depended on the content of the chemisorbed O2. The intermediates of the diethyl ether CTL reaction on the surface of TiO2 nanoparticles were investigated by in situ FT-IR technique.38 Figure 4B indicated that there were an obvious increase in sharp features at 2330, 2356, and 1735 cm−1 when the temperature was increased from 25 to 170 °C. The first two peaks were attributed to the presence of CO2 in gas phase. The band at 1735 cm−1 was assigned to the CO stretch in CH3CHO.39 In addition, the products of the proposed system were further verified by GC/MS. Figure 4C showed that there was only one peak at 1.72 min in the GC chromatogram. In the full scan mass spectra, the main fragment ions for the peaks were m/z 44, 29, and 15, which were assigned to CH3CHO. Finally, the CTL spectrum of diethyl ether on the surface of TiO2 nanoparticles showed only one emission band in the range of 400−500 nm with a maximum emission wavelength at ∼440 7317

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Figure 5. (A) Relative CTL intensity of 100 mM diethyl ether and (B) the content of oxygen vacancies in metal ion-doped TiO2 nanoparticles (Cu, Fe, Co, and Cr). (C) Relative CTL intensity of 100 mM diethyl ether and (D) the content of oxygen vacancies in TiO2 without hydrogen-treated (Ti-500) and hydrogen-treated TiO2 at 250 °C, 350 °C, and 400 °C (H-250, H-350, and H-400). Air flow rate, 250 mL min−1; working temperature, 170 °C.

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CONCLUSIONS In summary, a fast and effective CTL probe was first introduced to provide a reliable platform for detecting oxygen vacancies in oxides by the direct correlation between CTL intensity of diethyl ether oxidation reaction and the content of oxygen vacancies in TiO2 nanoparticles. Our proposed strategy was confirmed by detecting the CTL intensity on the surface of TiO2 with variable oxygen vacancies achieved by transition metal ion-doped TiO2 and hydrogen-treated TiO2 at different temperatures. A corresponding relationship between the proposed CTL probe and XPS technique was obtained. This new approach owned a series of advantages such as simple, rapid, stable, and easy-operation. In addition, this work supplied a better understanding of the relationship between oxygen vacancy defects in TiO2 nanoparticles and CTL characteristics. We anticipate that the well-designed CTL probe would open promising prospects for the application of CTL to probe oxygen vacancies in other oxides.

hand, different metal ions can be introduced into TiO2 to tune the content of oxygen vacancies owing to the inherent nature (e.g., ion radius, electronic orbit, and valence) of different metal ions.45,46 Here, the other three metal ions (Fe, Co, and Cr)doped TiO2 nanoparticles were prepared using the same procedure as Cu-doped TiO2 nanoparticles (Figure S7). The as-prepared four kinds of metal ion-doped TiO2 catalysts were applied to the CTL measurements. The results showed that the CTL intensity can be ranked as Cu-doped TiO2 > Co-doped TiO2 > Fe-doped TiO2 > Cr-doped TiO2 (Figure 5A), which was consistent with the content of oxygen vacancies in these metal ion-doped TiO2 (Figure 5B, Figure S8, and Table S3). In addition, hydrogen-treated TiO2 at different temperatures was considered to be an efficient technique to improve TiO2 catalytic activity through the change of oxygen vacancy.47−50 In this work, pure TiO2 without hydrogen-treated (i.e., calcined at 500 °C under air atmosphere, Ti-500) and a series of hydrogentreated TiO2 nanoparticles at 250 °C, 350 °C, and 400 °C (denoted as H-250, H-350, and H-400, and their characteristics were illustrated in Figure S9) were prepared for CTL measurements. Interestingly, the CTL intensity can be ranked as H-400 > H-350 > H-250 >Ti-500 (Figure 5C), which were in accordance with the variation trend of oxygen vacancy (Figure 5D, Figure S10, and Table S4). These interesting results further demonstrate that the diethyl ether CTL could be an attractive alternative to traditional methods for determining oxygen vacancies. The dynamic response analysis of the present CTL system was investigated by injecting 10 mM diethyl ether into the present system. It can be found the CTL intensity rapidly increased from the baseline to the maximum value within one second after sample injection, indicating the rapid diethyl ether CTL reaction (Figure S11). In addition, the operational reproducibility of the proposed CTL probe was studied by continually injecting 100 mM diethyl ether into the system 30 times (Figure S12). The relative standard deviation (RSD) was 3.17%.

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ASSOCIATED CONTENT

* Supporting Information S

XPS survey spectrum of synthesized TiO2 and 1.5 wt % Cudoped TiO2 nanoparticles, Cu 2p spectrum of 1.5 wt % Cudoped TiO2 nanoparticles, XPS spectra of Ti 2p of TiO2 and 1.5 wt % Cu-doped TiO2 nanoparticles, schematic diagram of the CTL configuration system, effect of oxygen concentration on the CTL signals of diethyl ether on the surface of 1.5 wt % Cu-doped TiO2 nanoparticles, CTL spectrum of diethyl ether on the surface of 1.5 wt % Cu-doped TiO2 nanoparticles, in situ FT-IR spectra of diethyl ether oxide reaction on the surface of 1.5 wt % Cu-doped TiO2 nanoparticles, GC/MS chromatograms of diethyl ether CTL reaction on the surface of 1.5 wt % Cu-doped TiO2 nanoparticles, XRD and UV−vis diffuse reflectance spectra of TiO2 and 1.0 wt % Fe, Cr, Co iondoped TiO2 nanoparticles, XPS spectra of O 1s on the surface of different metal ion-doped TiO2 nanoparticles, XRD and UVvis diffuse reflectance spectra of Ti-500 and hydrogen treated 7318

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TiO2 nanoparticles with different temperatures, XPS spectra of O 1s on the surface of Ti-500 and hydrogen treated TiO2 nanoparticles with different temperatures, CTL response profiles of 10 mM diethyl ether on the surface of TiO2 nanoparticles, relative CTL intensity of the repeated injections of 100 mM diethyl ether vapor on the surface of TiO2 nanoparticles, average crystallite sizes, d101 lattice spacing, actual values of Cu doping content and surface areas for Cudoped TiO2 samples, XPS results of different chemical states of O and Ti elements at the surface of pure TiO2 and Cu-doped TiO2 nanoparticles, XPS results of different chemical states of O and Ti elements at the surface of 1.0 wt % transition metal ion-doped TiO2 nanoparticles, XPS results of different chemical states of O and Ti elements at the surface of Ti-500 and hydrogen treated TiO2 nanoparticles with different temperatures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.analchem.5b02267.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 86 10 64411957. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Foundation of China (21375006), the 973 Program (2011CBA00503), and the Fundamental Research Funds for the Central Universities (JD1311). We also thank Prof. Xue Duan, Beijing University of Chemical Technology, for his valuable discussions.

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DOI: 10.1021/acs.analchem.5b02267 Anal. Chem. 2015, 87, 7313−7320

Article

Analytical Chemistry (50) Liu, H.; Ma, H. T.; Li, X. Z.; Li, W. Z.; Wu, M.; Bao, X. H. Chemosphere 2003, 50, 39−46.

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DOI: 10.1021/acs.analchem.5b02267 Anal. Chem. 2015, 87, 7313−7320