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Enhancing the Sensing Properties of TiO2 Nanosheets with Exposed {001} Facets by a Hydrogenation and Sensing Mechanism Ye Wang,† Junfang Liu,† Miao Wang,† Cuijin Pei,† Bin Liu,† Yukun Yuan,† Shengzhong Liu,‡ and Heqing Yang*,† †

Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Laboratory for Advanced Energy Technology; Key Laboratory of Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ABSTRACT: Hydrogenation is successfully employed to improve sensing performances of the gas sensors based on TiO2 nanosheets with exposed {001} facets for the first time. The hydrogenated TiO2 nanosheets show a significantly higher response toward ethanol, acetone, triethylamine, or formaldehyde than the samples without hydrogenation, and the response further increases with an increase of the hydrogenation temperature. The excellent sensing performances are ascribed to an increase of the density of unsaturated Ti5c atoms on the {001} surface resulting from the hydrogenation process. The unsaturated Ti5c atoms are considered to serve as sensing reaction active sites. They can generate noncontributing (free) electrons and adsorb oxygen molecules, and the detailed sensing mechanism is described at atomic and molecule level. The hydrogenated strategy may be employed to enhance the sensing performances of other metal oxide sensors and catalytic reaction activities of catalyst. The concept of the surface unsaturated metal atoms serving as sensing reaction active sites not only deepens the understanding of the sensing reaction and catalytic reaction mechanism but also provides new insights into the design of advanced gas sensing materials, catalysts, and photoelectronic devices. as forming TiO2−WO3 nanocomposites.37 Recently, TiO2 nanostructured materials with exposed {001} facets also exhibited enhanced sensing properties.38,39 However, the mechanism involved in the enhanced sensing performance is not yet known. The fundamental mechanism of the metal-oxide like TiO2 sensing materials is based on their resistance changes upon exposure to air or target gases.40 The change in resistance of the sensors is mainly connected with the oxidation−reduction reaction of the chemisorbed oxygen with the target gas molecules occurring on the surface of metal oxide sensing materials.41 Therefore, the sensing performance of metal oxides may be related to the amount of the adsorbed oxygen on the surface. The amount of the adsorbed oxygen may be increased by removing the surface O−H groups of metal oxide sensing materials through hydrogenation. However, such a strategy to enhance sensing properties has never been developed to date, and very little is known about the sensing reaction active sites. Herein, taking TiO2 nanosheets with exposed {001} facets as an example, we report on the enhanced gas-sensing properties of the hydrogenated TiO2 nanosheets. The as-obtained hydrogenated TiO2 nanosheets show superior sensing performance, compared to TiO2 nanosheets without hydrogenation as a

1. INTRODUCTION In recent years, semiconducting nanocrystals with controlled shapes and facets have attracted significant research interest because various crystal facets have different surface atomic structures and exhibit different physical and chemical properties.1−4 Titanium dioxide (TiO2) is an important n-type semiconductor with a wide band gap (∼3.2 eV).5 The TiO2 nanostructured materials with various morphologies and crystal facets have been studied widely due to their unique optoelectronic property, high stability, and low toxicity as well as promising applications for photocatalysis,6−11 photoelectrocatalysis,12,13 dye-sensitized solar cells,14,15 perovskite solar cells,16,17 photonic crystals,18 photochromic devices,19 lithium ion batteries,20,21 supercapacitors,22,23 and capture of tumor cells.24 On the other hand, TiO2 is also a promising gas-sensing material, which has been well-studied because of its high sensitivity, fast response, low cost, and long-term stability.25 The nanotubes,26 nanorods, nanobelts,27 nanofibers,28 nanosponge arrays,29 and hierarchical structures30,31 of TiO2 have been used to fabricate gas sensors for detection of inflammable and toxic gases. The morphology of TiO2 nanostructured materials was found to significantly affect their gas-sensing properties. Moreover, the sensing properties of TiO 2 nanostructures could be improved by loading Ag,32 Pd,33 NiO,34 and PbS nanoparticles35 and CuO nanocubes36 as well © XXXX American Chemical Society

Received: October 26, 2016

A

DOI: 10.1021/acs.inorgchem.6b02603 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry benchmarking material. A new concept of unsaturated Ti5C atoms on the (001) surface serving as active sites for the sensing reaction is proposed to explain the enhanced gas sensing properties by hydrogenation.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The TiO2 nanosheets were prepared by the hydrothermal method.6 In a typical synthesis procedure, 3 mL of hydrofluoric acid (HF) solution (with a concentration of 40 wt %) was added into a Teflon-lined autoclave with a capacity of 30 mL, and 10 mL of tetrabutyl titanate [Ti(OC4H9)4] was slowly added into the hydrofluoric acid solution under stirring. The mixed solution was stirred continuously for 30 min, and the Teflon-lined stainless steel autoclave was heated at 200 °C for 24 h. After solvothermal reaction, the resulting white precipitates were collected by centrifugation and washed with deionized water and ethanol three times. The as-prepared samples were dried at 60 °C for 5 h and heated at 600 °C for 2 h. Then, the samples were cooled to room temperature. To obtain the hydrogenated samples, the as-prepared TiO2 nanosheets were heated in a horizontal furnace at 400−600 °C for 24 h under a H2 gas flow (2 L/h). 2.2. Sample Characterization. Morphological observations and crystal structure analyses were carried out on a SU-8000 field emission scanning electron microscope (Hitachi, Japan), a DX-2700 X-ray diffractometer (Haoyuan, China), and a Tecnal G2 F20 field emission transmission electron microscope (JEOL, Japan). The sample surface chemical composition was analyzed with an Axis ultra X-ray photoelectron spectrometer (Kratos, Japan) and a Tensor 27 infrared spectrometer (Bruker, Germany). 2.3. Measurement of Gas-Sensing Properties. The sensing performances of the gas sensors based on the as-prepared TiO2 nanosheets and hydrogenated samples were measured using a WS30A system (Weisheng Instruments Co, China). First, 5 mg of TiO2 sample was mixed with 2 drops of terpineol solvent to form a sensing paste. Second, the paste was coated on a ceramic tube with a pair of gold electrodes that were connected by four platinum wires. Finally, a Ni−Cr wire was put through the tube, which was used as a heater element to control the working temperature by tuning the heating voltage. To improve the sensor’s stability and repeatability, all of the as-prepared TiO2 sensors were aged at 350 °C for 7 days in air before testing. The sensor response was defined as the ratio of resistances in air and in test gas at a working temperature of about 350 °C (Ra/Rg).

Figure 1. (a) XRD pattern and (b) FESEM and (c) FETEM images of the as-synthesized TiO2 nanosheets. (d) HRTEM image from the box in part c and the inset is the FFT pattern from part d.

Figure 2. (a) XRD patterns of the samples hydrogenated at different temperatures for 24 h. (b) FESEM image of the sample hydrogenated at 600 °C for 24 h.

3. RESULTS AND DISCUSSION The products were synthesized through a simple solvothermal method using tetrabutyl titanate Ti(OBu)4 as the source of titanium and hydrofluoric acid solution as the solvent.6 Figure 1a shows a powder X-ray diffraction (XRD) pattern of the assynthesized products. Peaks at 2θ = 25.3, 37.8, 48.1, 53.9, 55.1, 62.7, 68.8, and 75.0° in the XRD pattern are assigned to (101), (004), (200), (105), (211), (204), (116), and (215) diffraction peaks of the pure anatase TiO2, respectively. Additionally, the higher intensity ratios of (004) to other diffraction peaks were observed in comparison with those in the corresponding standard pattern of anatase TiO2, indicating a strong [001] preferred orientation. The texture coefficient of (004) plane TC(004) is given by42 TC(004) =

I(004) ⎧ 1 ⎨ I0(004) ⎩ n

∑

−1 I(hkl) ⎫ ⎬ I0(hkl) ⎭

Figure 3. Response and recovery curves of the sensors based on the as-synthesized TiO2 nanosheets and the hydrogenated samples to different concentrations of (a) ethanol, (b) acetone, (c) triethylamine, and (d) formaldehyde at 350 °C.

(1)

where I(hkl) are the measured intensities of the (hkl) reflection, I0(hkl) are the powder diffraction intensities of the anatase TiO2 according to the JCPDS card No. 21-1272, and n is the number of diffraction peaks used in the calculation. For powderlike samples with random crystallographic orientations,

the texture coefficient of any (hkl) reflection should be 1. The TC(004) value of the TiO2 nanosheets is measured to be 2.37, B

DOI: 10.1021/acs.inorgchem.6b02603 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Resistances of the sensors based on the as-synthesized TiO2 nanosheets and the hydrogenated samples in air and different concentrations of ethanol at 350 °C.

Table 2. Resistances of Four Kinds of TiO2 Sensors in Air (Ra) and in Saturated Ethanol (Rsg) and the Saturated Concentration of Ethanol

Figure 4. Response curves of the sensors based on the as-synthesized TiO2 nanosheets and the hydrogenated samples toward different concentrations of (a) ethanol, (b) acetone, (c) triethylamine, and (d) formaldehyde at 350 °C.

an obvious strong indication of [001] preferred orientation. Parts b and c of Figure 1 show field emission scanning electron microscopy (FESEM) and field emission transmission electron microscopy (FETEM) images for the as-synthesized samples, respectively, which indicate that the samples consist of a large quantity of square and rectangular sheetlike nanostructures. Figure 1d shows the high-resolution TEM (HRTEM) image, and the inset shows the fast Fourier transform (FFT) pattern from part d. The (200) and (020) atomic planes with a lattice distance of 0.19 nm and an interfacial angle of 90° are observed in the HRTEM image. The FFT pattern can be indexed as the [001] zone axis of anatase TiO2. The results reveal that the anatase TiO2 nanosheets are single crystalline, grow along the [100] and [010] directions, and are enclosed by {001} top and bottom surfaces. The hydrogenated TiO2 samples were obtained by heating the as-synthesized TiO2 nanosheets at 400, 500, and 600 °C for 24 h in a H2 atomsphere, which were labeled as TiO2−H-400, TiO2−H-500, and TiO2−H-600, respectively. Parts a and b of

TiO2 sample

Ra (MΩ)

Rsg (MΩ)

saturated concentration of ethanol (ppm)

TiO2 TiO2−H-400 TiO2−H-500 TiO2−H-600

28.97 39.72 51.05 75.47

11.34 10.17 7.56 2.81

1300 3100 4300 7300

Figure 2 show XRD patterns of the samples hydrogenated at 400, 500, and 600 °C and a SEM image of the sample hydrogenated at 600 °C, respectively. It is clear that the hydrogenated samples still consisted of anatase TiO2 sheetlike nanostructures. The phase structure and geometric morphology of the samples cannot be changed by the hydrogenation. The transient response characteristics of sensors based on hydrogenated and non-hydrogenated TiO2 nanosheets toward various concentrations of ethanol, triethylamine, acetone, and formaldehyde were investigated at 350 °C, and the results are displayed in Figure 3. As ethanol, acetone, triethylamine, or formaldehyde was injected, the electric resistances of four types of TiO2 sensors decreased sharply. After the test gas was released, the electric resistances increased quickly and recovered to their respective initial resistance value. The

Table 1. Responses of the TiO2, TiO2−H-600, and Other Metal Oxide Sensors to Ethanol, Acetone, Triethylamine, and Formaldehyde sensing material V2O5 nanobelt2 ZnO nanorod2 WO3 nanoigloos V2O5 hollow spheres SnO2 flowerlike architectures α-Fe2O3 nanostrings SnO2 nanoparticles TiO2

TiO2−H-600

VOC vapor

concentration (ppm)

response (Ra/Rg)

ethanol ethanol ethanol acetone acetone triethylamine formaldehyde formaldehyde ethanol acetone triethylamine formaldehyde ethanol acetone triethylamine formaldehyde

100 100 50 50 100 100 100 100 100 100 100 400 100 100 100 400

1.5 1.82 1.44 1.16 1.5 2.5 1.3 1.2 1.29 1.21 1.26 1.02 2.5 1.74 2.55 1.41

C

ref 43 44 45 46 47 48 49 this work

this work

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Figure 7. IR spectra of the four types of TiO2 samples.

vapors were summarized in Table 1.43−49 It is very clear that the TiO2−H-600 based sensors show a higher response than the TiO2, V2O5, ZnO, WO3, SnO2, and Fe2O3 nanostructured materials toward the four kinds of VOC vapors. In order to understand why the hydrogenation would increase the response of TiO2 sensors, the resistances of sensors based on four types of TiO2 sensors in air and in different concentrations of ethanol were measured, and the results are shown in Figure 5 and Table 2. It can be seen that the resistances of the hydrogenated TiO2 samples are higher than that of sample without hydrogenation in air and the resistance of the TiO2−H-600 based sensor is the greatest. As ethanol is injected into the testing chamber, the resistances of the four types of TiO2 sensors decrease, and they further decrease with increasing concentration of ethanol, finally reaching a constant value. For TiO2, TiO2−H-400, TiO2−H500, and TiO2−H-600, the saturated concentrations of ethanol are 1300, 3100, 4300, and 7300 ppm, respectively. In the saturated ethanol vapor, electric resistances of the hydrogenated TiO 2 sensors are lower than that of sample without hydrogenation, and the resistance of TiO2−H-600 is the smallest (Table 2). In order to investigate the surface structures, X-ray photoelectron spectroscopy (XPS) spectra of the hydrogenated and non-hydrogenated TiO2 samples were measured, and the results are shown in Figure 6. Figure 6a shows the wide-scan survey spectra of four types of TiO2 samples. Figure 6b displays Ti 2p spectra for TiO2, TiO2−H-400, TiO2−H-500, and TiO2− H-600, in which the binding energy of Ti 2p3/2 and Ti 2p1/2 is 458.8 and 464.5 eV, respectively. The O 1s peaks from four kinds of TiO2 samples are shown in Figure 6c−f. Apparently, each O 1s XPS peak can be decomposed into two Gaussian components, respectively. The components centered at about 529.4 and 531.4 eV correspond to O2− ions in the TiO2 lattice (OL), chemisorbed oxygen species (OC), and −OH groups,35,50 respectively. The relative percentages of OL, OC, and −OH components from TiO2, TiO2−H-400, TiO2−H-500, and TiO2−H-600 are listed in Table 3. As shown in Table 3, the relative percentages of the OC and OH components increase with increasing hydrogenation temperature. Additionally, infrared (IR) analysis was used, and IR spectra of the four kinds of TiO2 samples are shown in Figure 7. The characteristic peak about 3480 cm−1 is attributed to the asymmetrical stretching vibration of O−H groups. It is clear that the intensity of the O−H peak of the hydrogenated TiO2 samples is notably lower than that of the sample without hydrogenation, and it further decreases with an increase in the hydrogenation temperature, which indicates that the amount of O−H groups

Figure 6. XPS spectra of the four types of TiO2 nanosheets: (a) the survey spectra, (b) Ti 2p XPS spectra, and (c−f) O 1s XPS spectra of TiO2, TiO2−H-400, TiO2−H-500, and TiO2−H-600, respectively.

Table 3. Relative Percentages of the OL and OC Components from the Four Types of TiO2 Nanosheets TiO2 sample TiO2

TiO2−H-400

TiO2−H-500

TiO2−H-600

binding energy (eV) relative percentage (%) binding energy (eV) relative percentage (%) binding energy (eV) relative percentage (%) binding energy (eV) relative percentage (%)

OL (Ti−O)

Oc (chemisorbed) and −OH

529.7

531.3

85.81

14.19

529.1

531.2

73.35

26.65

529.6

531.4

70.94

29.06

529.3

531.3

70.47

29.53

change in resistance of these sensors is a typical sensing behavior of the n-type semiconductor sensors. The sensing response curves of the four types of TiO2 sensors to different concentrations of ethanol, acetone, triethylamine, and formaldehyde are shown in Figure 4a−d. It can be seen that, for four kinds of volatile organic compound (VOC) vapors, the response of the hydrogenated TiO2 sensor is higher than that of the sample without hydrogenation, and the response of the TiO2−H-600 based sensor is the greatest. The responses of the TiO2 and TiO2−H-600 as well as V2O5, ZnO, WO3, SnO2, and Fe2O3 nanostructured materials to the four kinds of VOC D

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Figure 8. (a) Schematic diagram of the atomic structure of the anatase TiO2(001) surface. O2c and Ti5c denote surface 2-fold coordinated O atoms and 5-fold coordinated Ti atoms, respectively. Reproduced from ref 53 with permission. Copyright 2008 Nature. (b, c) Schematic diagram of the hydrogenation reaction of TiO2 nanosheets with exposed {001} facets. (d−f) Sensing reaction mechanism on the anatase TiO2(001) surface. The black e− and red e− are captured electrons by O2 and free electrons, respectively.

of the unsaturated Ti5C atoms can increase the amount of the adsorbed oxygen. Therefore, in comparison with TiO2 nanosheets without hydrogenation, the hydrogenated TiO2 nanosheets have much less free electrons, showing lower conductivity. When the TiO2 nanosheet sensors are exposed to ethanol, triethylamine, acetone, or formaldehyde vapor, the adsorbed oxygen can be reduced by the VOC molecules, and the electrons captured by O2 molecules are released into the TiO2 to form free electrons. With an increase in the number of free electrons, the conductivity of TiO2 gas sensors increases, as shown in Figure 8e. In the saturated VOC vapor, all of the adsorbed oxygen molecules are removed, and thus the conductivity is the biggest (Figure 8f). Moreover, the unsaturated Ti5C atoms can generate noncontributing (free) electrons.56,57 An increase in the density of the unsaturated Ti5C atoms can induce an increase in the amount of free electrons in TiO2, and thus, the hydrogenated TiO2 samples have smaller electric resistances than samples without hydrogenation in the saturated ethanol vapor. The hydrogenated TiO2 nanosheets have a higher density of unsaturated Ti5C atom active sites, and show a higher response toward the VOC vapors, in comparison with samples without hydrogenation.

were reduced on the sample surface. Therefore, the relative percentage of OC components of the hydrogenated TiO2 samples is higher than that from samples without hydrogenation, and it further increases with an increase in the hydrogenation temperature. It is reasonable to conclude that the enhanced gas-sensing properties of the hydrogenated TiO2 with exposed {001} facets may arise from the decrease in the amount of O−H groups and the increase in the relative percentages of the OC components, as we expected. It is well-known that bulk anatase TiO2 is composed of 6coordinated Ti and 3-coordinated oxygen species.51,52 According to refs 53 and 54, the structure of an ideal anatase TiO2{001} surface is shown in Figure 8a. It can be found that the TiO2(001) facet is exposed with 5-fold coordinated Ti atoms (Ti5C) and 2-fold and 3-fold coordinated O atoms (O2c, O3c). However, IR spectra in Figure 7 indicate that there are a certain amount of O−H groups on the surface of the assynthesized TiO2 nanosheets with exposed {001} facets (Figure 8b). After the surface O−H groups were removed by hydrogenation, more unsaturated Ti5C atoms were produced at the (001) surface (Figure 8c) and the gas-sensing property of TiO2 nanosheets thus is improved remarkably (Figures 3 and 4). Therefore, we conclude that the unsaturated Ti5C atoms may serve as active sites for the sensing reaction. As is known, TiO2 is an n-type metal oxide semiconductor, in which electrons are majority carriers.55 In air, the unsaturated Ti5C atoms can adsorb oxygen molecules and the adsorbed oxygen molecules can capture free electrons. The number of free electrons deceases, and thus, the TiO2 nanosheet sensors show a high resistance state (Figure 8d). The increase in the density

4. CONCLUSION In summary, the hydrogenated TiO2 nanosheets with exposed {001} facets demonstrate enhanced gas-sensing performance, compared with samples without hydrogenation. The unsaturated Ti5C atoms on the (001) facet are found to serve as active sites for the sensing reaction, and the sensing mechanism is described in detail at an atomic and molecule level. The density E

DOI: 10.1021/acs.inorgchem.6b02603 Inorg. Chem. XXXX, XXX, XXX−XXX

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of the unsaturated Ti5C atom active sites is increased by the hydrogenation, and thus, the hydrogenated TiO2 samples show enhanced performance. The hydrogenation strategy may be exploited for improving the sensing properties of other metal oxide sensors and the catalytic activity of catalysts. In addition, the concept of the surface unsaturated metal atoms serving as reaction active sites may be promising and intriguing for understanding the sensing and catalytic reaction mechanisms and designing advanced sensing materials, catalysts, and photoelectronic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-29-81530702. ORCID

Heqing Yang: 0000-0002-6535-9378 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21073116 and 21501116) and the Fundamental Research Funds for the Central Universities (Grant GK 201601003).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02603 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02603 Inorg. Chem. XXXX, XXX, XXX−XXX