Enhanced Gas-Sensing Properties of the Hierarchical TiO₂ Hollow Microspheres with Exposed High-Energy {001} Crystal Facets.

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Enhanced gas sensing properties of the hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets Yong Yang, Yan Liang, Guozhong Wang, Liangliang Liu, C.L. Yuan, Ting Yu, Qinliang Li, Fanyan Zeng, and Gang Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08372 • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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ACS Applied Materials & Interfaces

Enhanced gas sensing properties of the hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets Yong Yang,∗,† Yan Liang, ‡ Guozhong Wang, § Liangliang Liu, ¶ Cailei Yuan, † Ting Yu, † Qinliang Li, † Fanyan Zeng, † and Gang Gu∗,† †Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, P.R. China ‡Department of Science Education, Jiangxi University of Technology, Nanchang 330098, Jiangxi, P.R. China §Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China ¶School of Physics and Technology, Wuhan University, Wuhan 430072, P.R. China

∗

Correspondence e-mail: [email protected].

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ABSTRACT: Anatase hierarchical TiO2 with innovative designs (hollow microspheres with exposed high-energy {001} crystal facets, hollow microspheres without {001} crystal facets, and solid microspheres without {001} crystal facets) were synthesized via a one-pot hydrothermal method and characterized. Based on these materials, gas sensors were fabricated and used for gas sensing tests. It was found that the sensor based on hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets exhibited an enhanced acetone sensing properties compared to the sensors based on the other two materials, due to the exposing of high-energy {001} crystal facets and special hierarchical hollow structure. First-principle calculations were performed to illustrate the sensing mechanism, which suggested that the adsorption process of acetone molecule on TiO2 surface was spontaneous, and the adsorption on high-energy {001} crystal facets would be more stable than that on the normally exposed {101} crystal facets. Further characterization indicated that {001} surface was highly reactive for the adsorption of active oxygen species, which was also responsible for the enhanced sensing performance. The present studies revealed the crystal facets-dependent gas sensing properties of TiO2, and provided a new insight into improving the gas sensing performance by designing hierarchical hollow structure with special crystal facets exposing. KEYWORDS: hierarchical, TiO2, hollow microspheres, crystal facet, gas sensing

1 INTRODUCTION Gas sensors have drawn wide attention during the past years for security and environmental applications.1,2 As an important semiconductor gas sensing materials, TiO2 shows the unique advantages of non-toxicity, biocompatibility, low cost and excellent stability compared with other metal oxides,3 such as SnO2, ZnO, In2O3, Fe2O3 and WO3. For example, it was reported


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that gas sensing performance of the commonly used SnO2 would significantly decrease in wet ambient, while TiO2 was substantially stable.4 Thus, tremendous interest has been focused on the studies of TiO2 nanomaterial in the applications of gas sensor.3 Over the past few years, TiO2 nanomaterials with various nanomorphologies, including nanoparticles,5 nanorods,6 nanowires,7 nanobelts8 and three-dimensional hierarchical structures9 have been studied with the aim of improving the gas sensing performance. Among them, three-dimensional hierarchical structured TiO2 assembled by nano-sized building blocks (including nanoparticles, nanorods, nanobelts and so on) showed enhanced gas sensing performance owing to its large specific surface area and abundant accessible pores properties,3,9 which could facilitate the effective adsorption and diffusion of the target gas molecules on the entire material surface. Besides, compared to monomorphological nano-sized structure with high tendency to form agglomerates, three-dimensional hierarchical structure are relative stable due to their larger dimensions and high dispersion, promoting the gas sensing stability.9 Study also showed that a much less hindrance was found in electron transport process for the hierarchical structure, leading to higher sensor sensitivity.3 All in all, three-dimensional hierarchical structured materials are considered to be good candidates used in gas sensors. However, the application of three-dimensional hierarchical structured TiO2 in the construction of gas sensors is still lacking. In addition to nanomorphologies, the exposed crystal facet is also a pivotal factor influencing the sensing properties. In principle, sensing process of semiconductors materials is based on the physical or chemical reactions between target gases and semiconductor surface, which results in the electrical conductance change. As the surface properties (such as dangling bonds, electronic structures and surface defects) of the different crystal facets differs greatly from each other, which will lead to an obvious difference in gas sensing properties.10 Previous works have


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reported the crystal facets-dependent gas sensing properties of SnO2,10,11 ZnO,12 Fe2O313 and WO3.14 For example, study of Han et al suggested that {221} and {332} crystal facets of tetragonal rutile SnO2 showed enhanced sensing sensitivity compared to that of {110} facets,10,11 thanks to the higher surface energy accompanied with higher reactivity of {221} and {332} crystal facets. It was also reported that WO3 nanorods with (002) crystal facets exposing showed much better acetone sensing activity compared to those with exposed (100) facets.14 These studies unmistakably suggested that the gas sensing properties of semiconductor metal oxides is largely related to the nature instincts of exposed surface.10-14 Thus, control of crystal facets is significant for the designing of gas sensors with better performance. However, as far as we know, there is no experimental study regarding crystal facets-dependent gas sensing properties of TiO2. On the other hand, since the pathbreaking work by Yang et al for the synthesis of anatase phase TiO2 with high-energy {001} crystal facets exposing,15 study of TiO2 nanomaterials with tailored high-energy crystal facets, including {001}, {100}, {110} and {111} crystal facets raised an upsurge. However, a great deal of attention has been focused on their control synthesis and applications in photocatalysis,16-18 the application of TiO2 high-energy crystal facets in the field of gas sensing is still in its infancy and needs to be explored. TiO2 high-energy crystal facets which have higher surface energy and reactivity are expected to show enhanced gas sensing sensitivity. Moreover, it is possible to achieve better selectivity by controlling of certain highenergy crystal facets.10 Here, in this work, based on the hierarchical TiO2 hollow microspheres with exposed highenergy {001} crystal facets, effect of hierarchical structure and exposed crystal facets on the gas sensing properties of TiO2 were examined. The gas sensing mechanism was also discussed in detail based on XPS characterization and first-principle calculations. Compared to SnO2, ZnO


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and so on,10-14 TiO2 has the intrinsic advantages of non-toxicity, low-cost, biocompatibility and excellent stability.3,9 Besides the exposing of high-energy facets,10-14 it was shown that the construction of three-dimensional hierarchical hollow structures could further enhance both the gas sensing responses and stability. The present study revealed the crystal facets-dependent gas sensing properties of TiO2, and provided a new insight into improving the gas sensing performance by designing hierarchical hollow structure with special crystal facets exposing. The results not only expanded the study of TiO2 sensing material, but also greatly enriched the gas sensing mechanism of metal oxide semiconductor.

2 EXPERIMENTAL SECTION 2.1 Sample preparation TiO2 samples with different nanomorphologies were prepared by a hydrothermal method, achieved by using of different structure-induced agents similar to our previous works.19,20 Commercially available titanium sulfate (Ti(SO4)2, CP), urea (CO(NH2)2, AR) and ammonium fluoride (NH4F, AR) were used. In a typical process, Ti(SO4)2 (3 mmol), urea and NH4F with different mole ratio (denoted as R) were added to 60 mL deionized water. After stirring for 2 h, the obtained mixture was transferred into a Teflon-lined autoclave (100 mL), which was then heated at 180 °C for 10 h. After cooling, the precipitates were collected, washed with deionized water and ethanol, and afterwards dried at 70 °C for 10 h. Three samples obtained with R value of 12 mmol: 0 mmol, 12 mmol: 1.5 mmol, and 0 mmol: 9 mmol were labeled as TS, THS and THS001, respectively. For comparison, nano-sized anatase TiO2 without hierarchical structure was synthesized when 12 mmol NaHCO3 was used to replace urea and NH4F.


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2.2 Characterization The phases of the samples were characterized by X-ray diffraction analysis (XRD, Philips X’pert PRO). The morphologies were revealed by field emission scanning electron microscope (FESEM, Sirion 200 FEI) and transmission electron microscopy (TEM, JEOL-2010, 200 kV). The specific surface areas and pore structures measurement were conducted by nitrogen adsorption-desorption experiment (BELSORP-miniⅡ). 2.3 Measurement of gas sensing properties The sensor fabrication processes were similar to the literature.21,22 First, sensing paste was obtained by mixing the as-prepared TiO2 powders with deionized water (weight ratio 2:1). Second, the obtained paste was coated on a ceramic tube with two gold electrodes and platinum wires, then Ni-Cr heating wire was inserted into the tube, which was used as a resistor to control the working temperature. At last, an intelligent test system controlled by LabVIEW software was used to measure sensing properties of the sensors, which was similar to the literature.21,22 All the sensors were aged for 24 h before test. The response (S) was the ratio of resistances in air and gas (Ra/Rg), the response/recovery time were defined as the time taking by the resistance changes to 90% 21,22. More details of gas sensing measurement could be found in previous works.21,22 2.4 Calculation Methods 2.4.1 Calculation detail First-principle calculations were used to study the adsorption behaviour of a CH3COCH3 molecule on a stoichiometric anatase TiO2 surface. The calculations were performed using Vienna ab initio simulation package (VASP) code.23-25 GGA-PBE exchange-correlation functional wave coupled with projector augmented wave (PAW) pseudopotentials were applied to study the adsorption processes, and a 2×2×1 Monkhorst-Pack K-point mesh was used in the


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calculation.26-28 The kinetic energy cutoff was 400 eV, and the convergence criteria for the electronic and ionic relaxation were 10-4 eV and 0.05 eV/Å. The adsorption energy of a CH3COCH3 molecule (Ead) was calculated as follow: Ead = Eslab – Esurf – ECH3COCH3 Where Eslab was the system energy of a CH3COCH3 molecule on the anatase surface, Esurf was the energy of the surface without any adsorbed molecule, ECH3COCH3 was the energy of a CH3COCH3 molecule in the gas phase.

Figure 1. Optimized atomic configurations of (a) anatase (101) surface and (b) anatase (001) surface. Ti atoms are represented by the larger gray spheres and the small red ones are O atoms. 2.4.2 Configuration of the computational model The used lattice parameters were a = 3.830 Å and c = 9.613 Å, which was the same with our previous calculations.29,30 A 3×2 (101) surface supercell and a 3×3 (001) surface (there are 108 atoms in each supercell, see Figure 1) were used to study the adsorption process of a CH3COCH3 molecule in the neighboring supercells because of the periodic boundary condition. The used vacuum layer was 13 Å on the top of the surface. In all calculations, for the anatase (101) surface, the third bottom (TiO2) layer was fixed and the other atoms in the supercell were free to relax, while for the anatase (001) surface, the forth bottom (TiO2) layer was fixed and the other atoms were free to relax, which were similar to our previous calculations.29,30


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3 RESULTS AND DISCUSSIONS 3.1 Morphological and structural characteristics

Figure 2. XRD patterns of the different samples. All the three samples were identified by XRD to be pure anatase phase of TiO2 (JPCDS no. 21-1272) (see Figure 2), no secondary phases were observed. The difference in relative broadening of the XRD peaks of the different samples was related to the difference in crystallite sizes.5,9 By the Scherrer formula,9 mean crystallite sizes of the THS001, THS and TS were determined to be 41, 13 and 9 nm, respectively.

Figure 3. (a) and (b) FESEM images of THS001, inset in (a) is the corresponding enlarged image, inset in (b) is the schematic model of TiO2 single crystal exposing {001} facets; (c) TEM image of THS001; (d) FESEM and (e) TEM image of THS, inset in (d) is the enlarged FESEM image; (f) FESEM images of TS, inset is the enlarged FESEM image.


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The morphologies of the as-prepared TiO2 are shown in Figure 3. THS001 consisted of massive hollow microspherical structure with a diameter of ~1 µm was observed (see Figure 3a), a broken microsphere clearly displayed the hollow structure with a rough surface (see the inset in Figure 3a). Enlarged FESEM image (see Figure 3b) showed that the external surface of the hollow microsphere was composed of randomly aggregated single crystal with a truncated octahedron morphology, suggesting the exposing of anatase high-energy {001} crystal facets according to the shape symmetry and previous reports.16,20 The percentage of high-energy {001} crystal facets in THS001 estimated by the surface area of each facet from FESEM images and calculation of average aspect ratio (B/A) was ~35%.31,32 The size of the truncated octahedron single crystal was about 50 nm, in agreement with the XRD analysis. TEM image in Figure 3c further confirmed the hollow structure of THS001. THS with a hollow microspherical structure similar to that of THS001 was also shown (see Figure 3d and 3e), but it was composed of small anatase nanoparticles without the exposing of high-energy {001} crystal facets. For further comparison, solid TiO2 microspheres composed of small anatase nanoparticles were also prepared, as shown in Figure 3f.

Figure 4. High-resolution TEM (HRTEM) images of (a) THS001, (b) THS and (c) TS, inset is the corresponding enlarged TEM image of the edge of a single microsphere. To further confirm crystalline characteristics of the as-prepared TiO2, HRTEM images and the enlarged TEM image were recorded (see Figure 4). HRTEM image of THS001 recorded


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perpendicular to anatase TiO2 {001} crystal facets (the region marked by a rectangular box in the inset of Figure 4a) clearly showed (200) atomic planes with an interplanar distance of 0.19 nm (see Figure 4a), suggesting the existence of high-energy {001} crystal facets.16,20 The enlarged TEM images of the edge of a single microsphere confirmed that THS and TS were composed of many small nanoparticles around 10 nm (see the inset in Figure 4b and 4c). From the HRTEM images (see Figure 4b and 4c), the crystal fringe of anatase TiO2 (101) facets with an interplanar distance of 0.35 nm could be seen, confirming that THS and TS were both well-crystallized anatase phase TiO2.19

Figure 5. (a) Nitrogen adsorption-desorption isotherms and (b) corresponding pore size distribution curves of the different samples. As the gas sensing properties is very sensitive to the specific surface area and pore structure of the sensing material, in order to determine those parameters, nitrogen adsorption-desorption measurement was performed (see Figure 5a). The nitrogen isotherms for all the samples could be ascribed to type IV according to Brunauer-Deming-Deming-Teller classification,19,20 indicating the existence of abundant mesoporous structures,19,20 which have been proved to be beneficial to the gas sensing performance.21 By the BET equation, the calculated specific surface areas of THS001, THS and TS were 26, 124 and 190 m 2g-1, respectively, which was directly related to the crystallite sizes. THS and TS with the similar particle dimensions showed a narrow pore size


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distribution centred at ~ 4 nm (see Figure 5b). In contrast, an increased pore size of 4-12 nm was observed for THS001, probably related to its increased particle size. 3.2 Gas sensing properties

Figure 6. (a) Response of the different TiO2 sensors to 100 ppm acetone versus working temperature; (b) Sensing properties of THS001-based sensor to different acetone concentrations versus time at 320 °C; (c) Response of the different TiO2 sensors to 10-5000 ppm of acetone; (d) Selectivity of the different TiO2 sensors on exposure to 100 ppm of various gases. Considering the structural properties of the as-prepared TiO2 materials, they were used for the detection of gases. Acetone, as an industrial solvent, was chosen as the target gas.33 Figure 6a shows response of the three sensors to 100 ppm acetone versus working temperatures from 200 to 440 °C. It was found that all the response curves exhibited a “parabola” shape as the working temperatures increased, and notably, the maximum response values of THS001-based sensor was 6.9, which was about 3.8 and 1.8 times greater than that of the sensors based on TS and THS, respectively. The response value of THS001-based sensor was also much larger than that of the sensors based on TiO2 nanofibers,22 monodispersed TiO2 spherical colloids34 and so on (summarized in Table 1). For a more reasonable comparison, we also synthesized the nano-sized


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TiO2 without hierarchical structure in a similar reaction conditions (see EXPERIMENTAL SECTION). The XRD and FESEM characterization suggested that the nano-sized TiO2 were of pure anatase phase of TiO2 (JPCDS No. 21-1272) with an average crystallite size of ~20 nm (see Figure S1). Typically, the response values of the sensor based on the nano-sized TiO2 were very low (< 1.3) towards 100 ppm acetone at the working temperature ranging from 200 to 440 °C, which were remarkably decreased compared to that of TS, THS and THS001. The sensing properties of the THS001-based sensor on different concentrations of acetone were also studied (see Figure 6b). It was found that the response increased gradually as the acetone concentrations increased, and an obvious response was found even to acetone concentration as low as 10 ppm. Besides, the THS001-based sensor showed a fast response and recovery time (< 10 s) towards different acetone concentration, which is important for the real-time detection. Figure 6c shows that the response of the three sensors increased with acetone concentration at the optimal working temperature, and the THS001-based sensor showed the greatest increase in response. The response curves were nearly linear at low acetone concentrations (10-200 ppm) and increased slowly at high acetone concentration, due to the more or less saturation of the sensors.21,22 In order to demonstrate the selectivity, responses of the sensors towards different gases including ethanol, H2, NH3, H2S were measured (see Figure 6d). The results indicated that the THS001-based sensor exhibited a much greater response to acetone compared to other gases, implying the possibility of selective detection of acetone. In contrast, the selectivity of other two sensors to acetone was much poorer. The reproducibility of THS001-based sensor to 100 ppm acetone at 320 °C was further investigated (as shown in the Figure S2). It was found that there is no clear change in response and recovery during 6 successive sensing tests, indicating the excellent reproducibility and stability of the THS001-based sensor.


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Table 1. Gas responses to acetone in this work and those reported in the literature. Sensing material

Concentration (ppm)

Temperature (°C)

Response (Ra / Rg)

This work

100

320

6.9

TiO2 spherical colloids 34

1000

350

2.3

Ag nanoparticles modified TiO2 35

100

350

3.9

TiO2 nanofibers22

50

320

2.4

Ag loaded mesoporous titania35

400

400

3.1

Flower-like ZnO36

100

320

2.2

Flower-like α-Fe2O337

50

280

2.6

3.3 Gas sensing mechanism

Figure 7. Schematic diagram of the sensing mechanism of the THS001-based sensor (Left). Schematic model of a truncated octahedron TiO2 single crystal exposing {001} facets (Right). It has been widely accepted that gas sensing properties of semiconducting oxide depends on the electron transfer between gas molecules and oxide surface.6 In ambient air, oxygen molecules are adsorbed on the n-type TiO2 surface to create anionic oxygen adsorbates (O2- or O-), resulting in the formation of electron depletion region and decrease of sensor conductivity.6,8 When TiO2 is exposed to acetone gas, acetone as a reducing gas will react with the surface anionic oxygen


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adsorbates, and electrons are injected into TiO2. The injection of electrons will reduce the electron depletion region and increase the concentration of free carrier, leading to the increase of sensor conductivity.6,8 In the present study, THS001 and THS showed an enhanced gas sensing properties compared to TS, which could be ascribed to the better permeation of hollow structure that facilitated the reaction of acetone molecules on the whole TiO2 body (see the left in Figure 7).38 In contrast, solid microspherical structure of TS might be unfavourable for the adsorption and diffusion of gas molecules into the inside,38 especially at the temperature of several hundred degrees, inducing the actually available surface area reduced greatly, which could explain its relative low gas sensing properties. It should be noticed that THS001 with the lowest specific surface area showed the best gas sensing properties. Considering the similar preparation conditions and nanomorphologies of the three TiO2 samples, the presence of high-energy {001} crystal facets in THS001 contributed significantly to the enhanced gas sensing properties. The observed phenomena were similar to the previous reports about SnO2,10 in which the difference in sensing activity was mainly due to the nature of exposed crystal facets rather than the specific surface area.

Figure 8. Optimized adsorption geometries of a CH3COCH3 molecule on the anatase TiO2 (a) (101) surface (Type A) and (b) (001) surface (Type B). The C atoms are yellow, and the O atom of CH3COCH3 molecule is blue, and the H atoms are cyan. The unit length is Å.


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Table 2. The adsorption energies of a CH3COCH3 molecule. Configuration Adsorption Energy (eV)

Type A

Type B

-0.479

-2.876

As the gas sensing response of a sensor is evaluated by the changes in resistance, which is largely determined by the gas adsorption process.6 To further understand the exact role of highenergy {001} crystal facets on the enhanced acetone sensing properties, we respectively calculated the adsorption of a CH3COCH3 molecule on the anatase (101) and (001) surface, and considered all kinds of adsorption geometries of the CH3COCH3 molecule on each surface. The most stable adsorption configurations on the (101) and (001) surface are shown in Figure 8, the adsorption energies are also listed in Table 2. Here, the values of the adsorption energies for both the two types were negative, which meant the adsorption processes were exothermic and spontaneous. Besides, CH3COCH3 molecule adsorbed on (001) surface would be more stable than that on (101) surface, since the adsorption energy of the CH3COCH3 molecule on the (001) surface was much lower.29,30 In other words, high-energy {001} crystal facets in THS001 with more dangling bonds were more advantageous to the adsorption of acetone molecules in contrast to that of the normally exposed {101} crystal facets, which confirmed that the exposure of highenergy {001} crystal facets was the main reason for the enhanced acetone sensing properties of THS001. Besides gas adsorption, it has been reported that surface adsorbed active oxygen species (O-, O2-, O2- and so on) are also important in determining the gas sensing properties of metal oxide semiconductors, which are directly related to the surface structure of crystals or crystal facets.10-14 XPS characterization was performed to study the surface adsorbed active oxygen species of THS001 and THS (see Figure S3). It was found both the two spectra showed a similar peak at 529.7 and 529.8 eV, corresponding to the lattice oxygen (O2-) in TiO2.13,39,40


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Besides, it was worth noting that the THS001 also showed another two obvious peaks at 530.2 and 531.5 eV, which could be attributed to surface-adsorbed oxygen species and oxygen vacancies, while THS only showed a very weak peak at 531.5 eV.13 This means that there are more active oxygen species on the THS001 surfaces. Since there are more unsaturated coordination atoms and dangling bonds on the high-energy {001} crystal facets compared to that of {101} facets, making the surface highly reactive for the adsorption of active oxygen species,18,41,42 which is responsible for the enhanced sensing performance of THS001. It should be mentioned that the present TiO2 sensors showed low response to other gases besides acetone (see Figure 6d), which might be ascribed to the low adsorption activities of TiO2 surface towards these gases. For example, density functional theory calculations have suggested that H2 interacted weakly with the anatase TiO2 (001) and (101) surface.29 At last, it is worth noting that the difference in selectivity of the three sensors towards acetone may also be attributed to the difference in the exposed crystal facets. In brief, exposing of high-energy {001} crystal facets provides a feasible way to enhance both sensitivity and selectivity of the TiO2 gas sensors. In fact, crystal facets-dependent gas sensing properties of TiO2 has not received particular attention. Deep understanding of the crystal facets-dependent gas sensing properties of TiO2 is still in urgent need, and more work should be done for the better designing of semiconductor gas sensors. For example, gas sensing properties of other anatase TiO2 high-energy crystal facets such as {100}, {110} and {111} needs to be explored.


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4 CONCLUSIONS In conclusion, anatase hierarchical TiO2 hollow microspheres with exposed high-energy {001} crystal facets have been prepared and characterized. The gas sensing properties was studied and discussed. It was found that the exposing of high-energy {001} crystal facets and special hierarchical hollow structure could both contribute to the enhanced gas sensing properties. Results of theoretical calculations suggested that high-energy {001} crystal facets with more dangling bonds were more advantageous to the adsorption of acetone molecules in contrast to that of the normally exposed {101} crystal facets, which confirmed that the exposing of highenergy {001} crystal facets was the main reason for the enhanced acetone sensing properties. The present results revealed the crystal facets-dependent gas sensing properties of TiO2, and provided a new insight into improving the gas sensing performance by designing hierarchical hollow structure with special crystal facets exposing. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Yong Yang); [email protected] (Gang Gu). Jiangxi Key Laboratory of Nanomaterials and Sensors, Jiangxi Key Laboratory of Photoelectronics and Telecommunication, School of Physics, Communication and Electronics, Jiangxi Normal University, Nanchang 330022, Jiangxi, P.R. China, Fax: (86) 0791-88120370; Tel: (86) 079188120370. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT This work was supported by Natural Science Foundation of Jiangxi province of China (Grant No. 20151BAB216008), Chinese Recruitment Program of Global Experts, Jiangxi Department of Science and Technology Project ( 20133ACE50006), Jiangxi Department of Education Project (KJLD14020), and Natural Science Foundation of China (Grant No. 61561026, 1272255, 51461019, 51072199 and 21177132).

REFERENCES 1 Gardon, M.; Guilemany, J. M. A Review on Fabrication, Sensing Mechanisms and Performance of Metal Oxide Gas Sensors. J. Mater. Sci. : Mater. Electron. 2013, 24, 1410-1421. 2 Stetter, J. R.; Li, J. Amperometric Gas Sensors-A Review. Chem. Rev. 2008, 108, 352366. 3 Bai, J.; Zhou, B. X. Titanium Dioxide Nanomaterials for Sensor Applications. Chem. Rev. 2014, 114, 10131-10176. 4 Jun, Y. K.; Kim, H. S.; Lee, J. H.; Hong, S. H. CO Sensing Performance in Micro-arc Oxidized TiO2 Films for Air Quality Control. Sens. Actuator, B 2006, 120, 69-73. 5 Arafat, M. M.; Haseeb, A. S. M. A.; Akbar, S. A. A Selective Ultrahigh Responding High Temperature Ethanol Sensor Using TiO2 Nanoparticles. Sensors 2014, 14, 13613-13627. 6 Huang, L.; Liu, T. M.; Zhang, H. J.; Guo, W. W.; Zeng, W. Hydrothermal Synthesis of Different TiO2 Nanostructures: Structure, Growth and Gas Sensor Properties. J. Mater. Sci. : Mater. Electron. 2012, 23, 2024-2029.


18

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7 Sennik, E.; Soysal, U.; Ozturk, Z. Z. Pd Loaded Spider-Web TiO2 Nanowires: Fabrication, Characterization and Gas Sensing Properties. Sens. Actuators, B 2014, 199, 424-432. 8 Hu, P. Q.; Du, G. J.; Zhou, W. J.; Cui, J. J.; Lin, J. J.; Liu, H.; Liu, D.; Wang, J. Y.; Chen, S. W. Enhancement of Ethanol Vapor Sensing of TiO2 Nanobelts by Surface Engineering. ACS Appl. Mater. Interface 2010, 2, 3263-3269. 9 Wang, C. X.; Yin, L. W.; Zhang, L. Y.; Qi, Y. X.; Lun, N.; Liu, N. N. Large Scale Synthesis and Gas-Sensing Properties of Anatase TiO2 Three-Dimensional Hierarchical Nanostructures. Langmuir 2010, 26, 12841-12848. 10 Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3, 154-165. 11 Wang, C. X.; Cai, D. P.; Liu, B.; Li, H.; Wang, D. D.; Liu, Y.; Wang, L. L.; Wang, Y. R.; Li, Q. H.; Wang, T. H. Ethanol-Sensing Performance of Tin Dioxide Octahedral Nanocrystals with Exposed High-energy {111} and {332} Facets. J. Mater. Chem. A 2014, 2, 10623-10628. 12 Wang, Z. H.; Xue, J.; Han, D. M.; Gu, F. B. Controllable Defect Redistribution of ZnO Nanopyramids with Exposed {101 ̅1} Facets for Enhanced Gas Sensing Performance. ACS Appl. Mater. Interfaces 2015, 7, 308-317. 13 Liu, X. H.; Zhang, J.; Wu, S. H.; Yang, D. J.; Liu, P. R.; Zhang, H. M.; Wang, S. R.; Yao, X. D.; Zhu, G. S.; Zhao, H. J. Single Crystal Alpha-Fe2O3 with Exposed {104} Facets for High Performance Gas Sensor Applications. RSC Adv. 2012, 2, 6178-6184.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

14 Jia, Q. Q.; Ji, H. M.; Wang, D. H.; Bai, X.; Sun, X. H.; Jin, Z. G. Exposed Facets Induced Enhanced Acetone Selective Sensing Property of Nanostructured Tungsten Oxide. J. Mater. Chem. A 2014, 2, 13602-13611. 15 Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638-646. 16 Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H. M. Synthesis of Anatase TiO2 Rods with Dominant Reactive {010} Facets for the Photoreduction of CO2 to CH4 and Use in Dye-sensitized Solar Cells. Chem. Commun. 2011, 50, 2133-2137. 17 Xu, H.; Reunchan, P.; Ouyang, S. X.; Tong, H.; Umezawa, N.; Kako, T.; Ye, J. H. Anatase TiO2 Single Crystals Exposed with High-Reactive {111} Facets Toward Efficient H2 Evolution. Chem. Mater. 2013, 25, 405-411. 18 Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T.; Mohamed, A. R. Highly Reactive {001} Facets of TiO2-based Composites: Synthesis, Formation Mechanism and Characterization. Nanoscale 2014, 6, 1946-2008. 19 Yang, Y.; Wang, G. Z.; Deng, Q.; Ng, D. H. L.; Zhao, H. J. Microwave-assisted Fabrication of Nanoparticulate TiO2 Microspheres for Synergistic Photocatalytic Removal of Cr(VI) and Methyl Orange. ACS Appl. Mater. Interfaces 2014, 6, 3008-3015. 20 Yang, Y.; Wang, G. Z.; Deng, Q.; Wang, H. M.; Zhang, Y. X.; Ng, D. H. L.; Zhao, H. J. Enhanced Photocatalytic Activity of Hierarchical Structure TiO2 Hollow Spheres with Reactive (001) Facets for the Removal of Toxic Heavy Metal Cr(VI). RSC Adv. 2014, 4, 34577-34583.


20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 Lou, Z.; Li, F.; Deng, J. A.; Wang, L. L.; Zhang, T. Branch-like Hierarchical Heterostructure (α-Fe2O3/TiO2): A Novel Sensing Material for Trimethylamine Gas Sensor. ACS Appl. Mater. Interfaces 2013, 5, 12310-12316. 22 Deng, J. A.; Wang, L. L.; Lou, Z.; Zhang, T. Design of CuO-TiO2 Heterostructure Nanofibers and Their Sensing Performance J. Mater. Chem. A 2014, 2, 9030-9034. 23 Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561. 24 Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the LiquidMetalamorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251-14269. 25 Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15-50. 26 Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. 27 Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758-1775. 28 Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Open Shell Transition Metals. Phys. Rev. B 1993, 48, 13115-13118. 29 Liu, L. L.; Liu, Q.; Zheng, Y. P.; Wang, Z.; Pan, C. X.; Xiao, W. O2 Adsorption and Dissociation on a Hydrogenated Anatase (101) Surface. J. Phys. Chem. C 2014, 118, 3471-3482. 30 Liu, L. L.; Wang, Z.; Pan, C. X.; Xiao, W.; Cho, K. Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface. ChemPhysChem 2013, 14, 996-1002.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

31 Liu, S. W.; Yu, J. G.; Jaroniec, M. Tunable Photocatalytic Selectivity of Hollow TiO2 Microspheres Composed of Anatase Polyhedra with Exposed {001} Facets. J. Am. Chem. Soc. 2010, 132, 11914-11916. 32 Pan, J. H.; Han, G.; ;Zhou, R.; Zhao, X. S. Hierarchical N-doped TiO2 Hollow Microspheres Consisting of Nanothorns with Exposed Anatase {101} Facets. Chem. Commun. 2011, 47, 6942-6944. 33 Sun, X. H.; Ji, H. M.; Li, X. L.; Cai, S.; Zheng, C. M. Mesoporous In2O3 with Enhanced Acetone Gas-Sensing Property. Mater. Lett. 2014, 120, 287-291. 34 Cheng, X. L.; Xu, Y. M.; Gao, S. Zhao, H.; Huo, L. H. Ag Nanoparticles Modified TiO2 Spherical Heterostructures with Enhanced Gas-Sensing Performance. Sens. Actuators, B 2011, 155, 716-721. 35 Bahadur, N.; Jain, K.; Pasricha, R.; Govind; Chand, S. Selective Gas Sensing Response from Different Loading of Ag in Sol-Gel Mesoporous Titania Powders. Sens. Actuators, B 2011, 159, 112-120. 36 Lou, Z.; Deng, J. N.; Wang, L. L.; Wang, L. J.; Fei, T.; Zhang, T. Toluene and Ethanol Sensing Performances of Pristine and PdO-decorated Flower-like ZnO Structures. Sens. Actuators, B 2013, 176, 323-329. 37 Wang, L. L.; Fei, T.; Lou, Z.; Zhang, T. Three-Dimensional Hierarchical Flowerlike alpha-Fe2O3 Nanostructures: Synthesis and Ethanol-Sensing Properties. ACS Appl. Mater. Interfaces 2011, 3, 4689-4694. 38 Wang, L. L.; Dou, H. M.; Lou, Z.; Zhang, T. Encapsuled Nanoreactors (Au@SnO2): A New Sensing Material for Chemical Sensors. Nanoscale 2013, 5, 2686-2691.


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Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39 Lu, W. W.; Gao, S. Y.; Wang, J. J. One-Pot Synthesis of Ag/ZnO Self-Assembled 3D Hollow Microspheres with Enhanced Photocatalytic Performance. J. Phys. Chem. C 2008, 112, 16792-16800 40 Wu, N.; Wei, H. H.; Zhang, L. Z. Efficient Removal of Heavy Metal Ions with Biopolymer Template Synthesized Mesoporous Titania Beads of Hundreds of Micrometers Size. Environ. Sci. Technol. 2012, 46, 419-425. 41 Liu, G.; Yang, H. G.; Pan, J.; Yang, Y. Q.; Lu, G. Q; Cheng H. M. Titanium Dioxide Crystals with Tailored Facets. Chem. Rev. 2014, 114, 9559-9612. 42 Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515-582.


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SYNOPSIS TOC graphic


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Highly exposed surface area of {001} facets dominated BiOBr nanosheets with enhanced visible light photocatalytic activity.

Enhanced photocatalytic CO₂-reduction activity of anatase TiO₂ by coexposed {001} and {101} facets.

Au photosensitized TiO2 ultrathin nanosheets with {001} exposed facets.

graphene nanosheets for enhanced visible-light photocatalytic activity.

CuO single crystal with exposed {001} facets--a highly efficient material for gas sensing and Li-ion battery applications.

Self-assemble ZnMn2O4 hierarchical hollow microspheres into self-supporting architecture for enhanced biosensing performance.

Highly efficient low-temperature plasma-assisted modification of TiO2 nanosheets with exposed {001} facets for enhanced visible-light photocatalytic activity.

Highly exposed {001} facets of titanium dioxide modified with reduced graphene oxide for dopamine sensing.

Synthesis of a highly efficient BiOCl single-crystal nanodisk photocatalyst with exposing {001} facets.

Anatase TiO2 Nanoparticles with Exposed {001} Facets for Efficient Dye-Sensitized Solar Cells.

Efficient gaseous toluene photoconversion on graphene-titanium dioxide nanocomposites with dominate exposed {001} facets.

Layer-by-layer self-assembly of TiO2 hierarchical nanosheets with exposed {001} facets as an effective bifunctional layer for dye-sensitized solar cells.

Electrochemical degradation of refractory pollutants using TiO2 single crystals exposed by high-energy {001} facets.

Anatase TiO₂ films with dominant {001} facets fabricated by direct-current reactive magnetron sputtering at room temperature: oxygen defects and enhanced visible-light photocatalytic behaviors.

Hierarchical TiO₂-SnO₂-graphene aerogels for enhanced lithium storage.

A Chelation Strategy for In-situ Constructing Surface Oxygen Vacancy on {001} Facets Exposed BiOBr Nanosheets.

Hydrothermal etching fabrication of TiO2@graphene hollow structures: mutually independent exposed {001} and {101} facets nanocrystals and its synergistic photocaltalytic effects.

Hybrids of Two-Dimensional Ti3C2 and TiO2 Exposing {001} Facets toward Enhanced Photocatalytic Activity.

Facile synthesis of hierarchical porous TiO(2) ceramics with enhanced photocatalytic performance for micropolluted pesticide degradation.

BiOBr heterojunctions via facets coupling for enhanced photocatalytic activity.

Facile synthesis of hierarchical porous γ-Al2O3 hollow microspheres for water treatment.

Green synthesis of anatase TiO(2) nanocrystals with diverse shapes and their exposed facets-dependent photoredox activity.

Controllable defect redistribution of ZnO nanopyramids with exposed {101̅1} facets for enhanced gas sensing performance.

Synthesis of anatase TiO2 with exposed (100) facets and enhanced electrorheological activity.