Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23495−23502
www.acsami.org
Oxygen-Defective Ultrathin BiVO4 Nanosheets for Enhanced Gas Sensing Dong Yao,†,‡,§ Chunwei Dong,† Qiming Bing,§ Yi Liu,*,† Fengdong Qu,*,# Minghui Yang,*,# Bingbing Liu,*,‡ Bai Yang,† and Hao Zhang*,† State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry and ‡State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China § Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China # Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Downloaded via BUFFALO STATE on July 17, 2019 at 13:37:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: BiVO4 nanomaterials are potentially applicable in gas sensing, but the sensing performance is limited by the less active sites on the BiVO4 surface. In this work, we propose a strategy to improve the gas-sensing performance of BiVO4 by forming ultrathin nanosheets and introducing oxygen vacancies, which increase the surface active sites. Two-dimensional (2D) BiVO4 nanosheets with oxygen vacancies are prepared through a colloidal method with the assistance of nitric acid. Gas sensors based on the oxygen-defective 2D ultrathin BiVO4 nanosheets exhibit an enhanced sensing response, which is 3.4 times higher than those of the sensors based on oxygen-abundant BiVO4 nanosheets. The density functional theory calculation is employed to uncover the promoting effects of oxygen vacancies on enhancing the O2 adsorption capability of BiVO4 nanosheets. This work is not only expected to build a wide range of 2D metal oxide semiconductors with a high gas-sensing performance but also gives an insight into the mechanism of the enhanced response induced by the oxygen vacancies, which will be a guideline for further designing high-performance sensing materials. KEYWORDS: colloidal synthesis, BiVO4, 2D nanosheets, oxygen vacancy, gas sensing
■
INTRODUCTION Metal oxide semiconductor-based gas sensors have been widely applied in Internet of Things (IoT) and Industrial Internet of Things (IIoT) for their advantages such as its simple structure, relatively high response to various gases, low cost, and low power consumption.1−9 However, further improvements in sensing performances regarding the response, selectivity, and sensing mechanism are needed.10,11 The key is to design and prepare high-performance sensing materials with a high response, a well selectivity, and a fast response/recovery speed.12−15 In particular, systematically modulating some parameters of sensing materials to achieve high performance is highly desirable and remains challenging. Great efforts have been devoted to increase the response via material designing, such as doping impurity ions, constructing heterostructures, adjusting morphologies and compositions, and so forth.16−22 However, issues regarding about achieving high sensing characteristics are still unsatisfactorily solved, including the easy tendency of aggregation and the low amount of active sites. Two-dimensional (2D) structures are highly interesting for building nanosensors because of their unique 2D geometry, large surface area, facile facet engineering, enriched active sites, and enormous potential to derive 3D architectures.23−25 Besides, 2D nanosheets with the thickness of atomic level © 2019 American Chemical Society
can provide the possibilities to engineer the electronic structures of the material beyond the bulk realm, owing to the quantum size effect, which is vital for tuning the sensing property, including the selectivity, sensitivity, stability, and speed of response/recovery.26,27 To date, numerous chemical and physical methods have been developed for fabricating metal oxides with 2D sensing nanostructures, such as exfoliation, vapor deposition, and extraction.28,29 Although these approaches are rational, considering the simplicity and possibility of mass production, exploring facile synthesis methods for 2D metal oxide nanomaterials, especially the capability of surface modification, is still necessary to further extend the scope of their sensing application. Defect engineering is emerging as an effective strategy for modulating the properties of functional metal oxides, which has been widely utilized to tune physicochemical characteristics and applied in various fields, such as catalysis, energy, and especially sensors.30−34 It has been demonstrated that the introduction of oxygen vacancies into metal oxides could improve their gas-sensing performance.35−42 However, the effective utilization of oxygen vacancies is still elusive and the Received: March 30, 2019 Accepted: June 7, 2019 Published: June 7, 2019 23495
DOI: 10.1021/acsami.9b05626 ACS Appl. Mater. Interfaces 2019, 11, 23495−23502
Research Article
ACS Applied Materials & Interfaces mechanism remains unclear. Therefore, more efforts are required to engineer oxygen vacancies and explain the mechanism of the improvement by introducing oxygen vacancies, which may provide a design principle for preparing high-performance gas-sensing materials. With the aforementioned issues in mind, herein, we demonstrate the improvement of the gas-sensing performance of BiVO4 by forming oxygen-defective ultrathin nanosheets. The oxygen vacancies throughout the ultrathin BiVO 4 nanosheets supply numerous active sites for the gas sensing reaction, resulting in a response of over 3.4 times higher than that of perfect BiVO4 nanosheets. In addition, the density functional theory (DFT) calculation indicates that the oxygen vacancies can significantly enhance the O2 adsorption capability of BiVO4 nanosheets and lower the oxidizing ability of surface-absorbed O 2 molecules, thus enabling the implementation for the improved gas-sensing performance. Although BiVO4 nanosheets have been studied for water splitting, they are still fresh for gas sensing. This study not only provides an effective method to build a series of oxygendefective 2D metal oxide materials but also sheds light on the promoting effects of defect engineering on enhancing their sensing performance, which will further promote the development of gas sensing.
■ ■
Figure 1. Representative (a) TEM image, (b) HRTEM image, (c) inverse FFT image of the areas inside the dashed squares in (b), (d− g) HRTEM-EDS elemental mapping, (h) XRD patterns, and (i) AFM image of the ultrathin BiVO4 nanosheets. The inset of (b) is the corresponding SAED image. The inset of (c) is the FFT image of the areas inside the dashed squares in (b). The inset of (i) is the corresponding height profiles. The scale bars in (a), (b), (c), and (d) are 500, 5, 2, and 200 nm, respectively.
EXPERIMENTAL SECTION
Experimental details can be found in the Supporting Information.
RESULTS AND DISCUSSION Oxygen-Defective Ultrathin BiVO4 Nanosheets. The preparation of oxygen-defective ultrathin BiVO4 nanosheets was accomplished according to the two-phase method previously developed by our group.43 Typically, a mixture of Bi(NO3)3·5H2O, oleylamine (OLA), oleyl acid (OA), and octadecene (ODE) was degassed and then heated in a threenecked flask. Afterwards, a pre-prepared hot NH4VO3/HNO3 aqueous solution was injected into the three-necked flask. The reaction was maintained for a desired period and terminated by removing the heating source (see the Supporting Information for detailed experimental descriptions). Note that the amount of HNO3 is critical not only for dissolving NH4VO3 but also for the formation of the 2D ultrathin structure and for the introduction of oxygen vacancies. Hydroxyl groups in HNO3 could reduce the surface energy of the BiVO4 (010) facet terminated by oxygen atoms, promote the growth of the 2D structure, and create highly acidic environment for the formation of oxygen vacancies.43 Comparing the products prepared with a series amount of HNO3, the current 2 mL value is the optimum (Figure 1a and Figure S1). As shown in Figure 1a, the transmission electron microscopy (TEM) image indicates that the as-prepared 2D BiVO4 nanosheets are quasi-square-sheet in shape with an average lateral size of 500−1300 nm, which is consistent with the large view results from the scanning electron microscopy (SEM) images (Figure S2). The Moiré patterns formed in the stacked areas of the BiVO4 nanosheets can be significantly observed, indicating the extremely low thickness of the nanosheets.44 High-resolution TEM (HRTEM) observation exhibits apparent silk-like horizontal-vertical lattice fringes, revealing the well-crystallized structure (Figure 1b). The slight contrast of the HRTEM image is attributed to the ultralow thickness of the BiVO4 nanosheets. Selected-area fast Fourier transform (FFT) and inverse FFT are performed on the dashed-square
area in Figure 1b to show the distinct interplanar d-spacings (Figure 1c and inset). The interplanar d-spacings of 0.255 and 0.260 nm are consistent with the (002) and (200) crystallographic facets of monoclinic BiVO4, respectively, and a dihedral angle of 90° coincides with the viewpoint along the [010] zone axis. The phase structure is also verified by selected area electron diffraction (SAED) and the X-ray diffraction (XRD) pattern with an intensive (040) peak, further indicating that the monoclinic BiVO4 nanosheets are oriented along the [010] zone axis (Figure 1b(inset),h).43 The composition of the BiVO4 nanosheets is revealed by energy-dispersive X-ray spectroscopy (EDS), indicating a Bi/V/O atomic ratio of 1.0:1.1:2.6 (Figure S3). The Bi/V molar ratio is close to the stoichiometric ratio of 1:1, while the O content is of discrepancy, implying oxygen deficiency in the as-prepared BiVO4 nanosheets. The elemental distribution of the BiVO4 nanosheets is characterized by the elemental mapping in Figure 1d−g, revealing the homogenously distributed Bi, V, and O. Furthermore, atomic force microscopy (AFM) is performed to validate the thickness of the BiVO4 nanosheets (Figure 1i). The height profile demonstrates that the as-prepared BiVO4 materials are ultrathin nanosheets with a thickness of 3.0 nm, which is less than the three-unit-cell monoclinic packing thickness along the [010] zone axis. To investigate the valence state of the constituents Bi, V, and O in the as-prepared 2D nanosheets, X-ray photoelectron spectroscopy (XPS) is employed (Figure 2). The Bi 4f spectrum in Figure 2b exhibits two peaks at 159.2 and 164.5 V with a peak splitting of 5.3 eV, indicating Bi3+. The V 2p peaks at 516.4 and 524.0 eV with a peak splitting of 7.6 in Figure 2c 23496
DOI: 10.1021/acsami.9b05626 ACS Appl. Mater. Interfaces 2019, 11, 23495−23502
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Total, (b) Bi 4f, (c) V 2p, and (d) O 1 s XPS spectra of the ultrathin BiVO4 nanosheets. The relative percentage values of the OL, OV, and OC are calculated to be 47.7, 30, and 22.3%, respectively.
represent V5+.43,45 Since both the XPS peaks of Bi 4f and V 2p are broad and asymmetric, we cannot arbitrarily rule out the significant existence of Bi2+ and V4+. The asymmetric O 1s XPS spectrum in Figure 1d is fitted into three components with peak centered at about 529.6, 530.7, and 532.6 eV, and the three peaks are corresponding to lattice oxygen (OL), deficient oxygen (OV), and chemisorbed oxygen (OC), respectively.45−48 Quantitative analysis shows that the proportions of the OL, OV, and OC are 47.7, 30, and 22.3%, respectively, which demonstrate that a large amount of oxygen vacancies exist in the as-prepared ultrathin BiVO4 nanosheets.47,48 To further verify the existence of the oxygen vacancies, thermal treatment is performed on the ultrathin BiVO4 nanosheets by virtue of the passivation effects of O2. Figure S4 presents the XPS of the ultrathin BiVO4 nanosheets after annealing at 500 °C in air. Obviously, after the thermal treatment, the relative proportion of OV is largely decreased to 13.2%, indicating the elimination of oxygen vacancies (Figure S4d). Therewith, the relative proportion of OC drops sharply from 22.3 to 2.4%, signifying the small amount of chemisorbed O2 without oxygen vacancies.43,45 The Bi 4f and V 2p spectra in Figure S4b,c show narrower and more symmetrical peaks than those in Figure 2, which is ascribed to the decrement of the Bi2+ and V4+ contents with the elimination of oxygen vacancies. After the annealing treatment, the composition of the 2D BiVO4 nanosheets is characterized by EDS in Figure S5. The improvement of the O content is attributed to the introduction of O in air into the lattice of the BiVO4 nanosheets, which coincides with the elimination of oxygen vacancies. In addition, electron paramagnetic resonance (EPR) spectra are performed on the pristine 2D BiVO4 nanosheets and samples after the annealing treatment to validate the existence of oxygen vacancies. As shown in Figure 3, the pristine 2D BiVO4 nanosheets exhibits a symmetrical resonance line centered at the position with a g value of 2.003, corresponding to the existence of oxygen vacancy.45 In monoclinic BiVO4,
Figure 3. Room-temperature EPR spectra of the ultrathin BiVO4 nanosheets before (orange line) and after (blue line) annealing at 500 °C.
neither high valent ions, such as Bi3+ and V5+, nor low valent ions, such as Bi+, V3+, and O2−, contribute to the EPR signal due to the lack of unpaired electrons. The double-charged positive oxygen vacancy (Vo2+) would not contribute to the EPR signal either because there is no diamagnetic electron. Here, the distinct EPR signal should originate from the singlecharged negative oxygen vacancy (Vo−) with a trapped free electron provided by Bi2+ and/or V4+.43,45 After annealing, the dramatical decrement of the EPR signal implies that the introduction of oxygen from the atmosphere eliminates the oxygen vacancies of the 2D BiVO4 nanosheets. The 2D BiVO4 nanosheets before and after annealing treatment can be regard as 2D nanosheets with and without oxygen vacancies. Gas-Sensing Property. As a proof-of-concept, we demonstrate the application of the ultrathin BiVO4 nanosheets for chemiresistive gas sensing. Taking the fact that the operating temperature has strong effects on the sensing performance, we first check the optimum operating temperature of the BiVO4-based sensors by valuating major parameters, including the response and response time.49,50 As shown in Figure 4a, the response of sensors based on ultrathin 23497
DOI: 10.1021/acsami.9b05626 ACS Appl. Mater. Interfaces 2019, 11, 23495−23502
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Response of the sensors based on the BiVO4 nanosheets before and after annealing. (b) Response and response time of the sensors based on the BiVO4 nanosheets before and after annealing as a function of operating temperature. (c) Cross-sensitivities for the sensor based on the BiVO4 nanosheets before annealing to various gases at 300 °C.
facet of BiVO4 and the adsorption energy of the gas molecules to the surface of BiVO4. The optimum temperatures of the sensor to ethanol, acetone, and xylene are compared, experimentally. As shown in Figure S7, the optimum operating temperature for ethanol, acetone, and xylene detection is 220, 300, and 320 °C, respectively, further indicating the different activities of the gas/vapor molecules on the BiVO4 (010) surface.50,52 Besides, compared to the annealed BiVO4 nanosheet sensor, the oxygen-defective BiVO4 nanosheet sensor exhibits an enhanced response to all the gas/vapors. The above measurements highlight the advantages of constructing oxygen vacancies to improve the gas-sensing performance of BiVO4-based sensors. As shown in Figure S8, the reproducibility investigation of oxygen-defective BiVO4 nanosheet sensor is conducted under 5 cycles of dynamic transient measurements to 100 ppm acetone at 300 °C. The initial resistance of the oxygendefective BiVO4 nanosheet sensor keeps unchanged, and the resistance in 100 ppm acetone is in uniform in each trial, indicating promising response and recovery characteristics as well as good cyclability. The sensor response to 100 ppm acetone is recorded over a period of ∼30 days to identify the long-term stability. It can be observed that the response keeps quite steady with a relative standard deviation of about 13.8% (Figure S9). These results verify that the gas sensor based on the BiVO4 nanosheets with oxygen vacancies possess good stability regarding to reproducibility and long-term stability. Figure S10 shows the humidity dependence of oxygendefective BiVO4 nanosheet sensor responses to 100 ppm acetone. The response slightly decreases with the increment of humidity, which is caused by the adsorption of OH− on the oxygen adsorption site so that the adsorption of the O2 molecules is disturbed to some extent.53 The effect of the humidity on the responses is also evaluated by calculating the coefficient of variation, which is defined as the ratio of the standard deviation to the average value of the response with different humidity values. As shown in Figure S10, the coefficient of variation is only 12.9%, when the relative humidity is changed from 11 to 95%, further implying the good anti-interference performance of the oxygen-defective BiVO4 nanosheet sensor.10 Gas-Sensing Mechanism. To better understand the influence of the oxygen vacancies on the gas-sensing performance of the BiVO4 nanosheets, DFT calculations of the electronic properties of the BiVO4 (010) surface without
BiVO4 nanosheets with and without oxygen vacancies to 100 ppm acetone both exhibit an “increase-max-decrease” trend. The response of the sensor based on the BiVO4 nanosheets with oxygen vacancies reaches a maximum value of 7.1 at 300 °C, which is 3.4 times higher than a value of 2.1 for the sensor based on the BiVO4 nanosheets without oxygen vacancies. Besides, the response times of both the BiVO4-based sensors decreases largely with increasing the operating temperature and subsequently comes into saturation. According to the above-mentioned parameters, the optimum operating temperature of the BiVO4-based sensor is determined to be 300 °C. EPR spectrum is also performed on the BiVO4 nanosheets after annealing at 300 °C for 30 min, which demonstrates that the BiVO4 nanosheets are stable enough to maintain the abundant oxygen vacancies on their surface at the optimum operating temperature for acetone sensing (Figure S6). Figure 4b exhibits the response of the sensors based on the BiVO4 sheets with and without oxygen vacancies as a function of acetone concentrations. The curves show a linear relationship at low concentrations but come into saturation when the concentrations increase. These curves can be well fitted with the equation: S = A / (1 + [B / C]), where S represents the response, C represents the concentration, and A and B are the parameters of the curve. It can be explained by the surface coverage of adsorbed molecules following the Langmuir isotherm.40,41 Typically, at the low concentration (
