Preadsorption of O2 on the Exposed (001) Facets of ZnO

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

Article

Pre-Adsorption of O2 on the Exposed (001) Facets of ZnO Nanostructures for Enhanced Sensing of Gaseous Acetone Chaochao Li, Hegen Zhou, Shichao Yang, Liyuan Wei, Zhizhong Han, Yongfan Zhang, and Haibo Pan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00942 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9 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

ACS Applied Nano Materials

An oxygen pre-adsorption and gas sensing selectivity for high exposed (001) facet nest-like ZnO nanomaterials are illustrated by the gas-response measurement and DFT computation.

ACS Paragon Plus Environment

ACS Applied Nano Materials 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

Pre-Adsorption of O2 on the Exposed (001) Facets of ZnO Nanostructures for Enhanced Sensing of Gaseous Acetone Chaochao Lia, Hegen Zhoub, Shichao Yanga, Liyuan Weia, Zhizhong Hanc, Yongfan Zhanga, Haibo Pana, c, d* College of Chemistry, Qishan Campus, Fuzhou University, Fuzhou, Fujian 350116, China College of Chemical and Biological Engineering, Yichun University, Yichun, Jiangxi 336000, China c Fujian Key Lab of Medical Instrument & Pharmaceutical Technology, Fuzhou University, Fuzhou, Fujian 350002, China d School of Pharmacy, Fujian Medical University, Fuzhou, Fujian 350108, China a

b

KEYWORDS: Interfaces, Nanostructures, Adsorption, ZnO, Gas sensor ABSTRACT: The O2 pre-adsorption properties prior to the application for nanomaterials has rarely attracted attention, however, it greatly affects the surface nature between gas and nanomaterials. Here, a hierarchically ZnO nest-like architectures (ZnO NAs) with nanosheets was synthesized by a facile hydrothermal method without structure-directing agents and templates. The percentage of exposed (001) facet for ZnO NAs is ca. 95 % according to its micromorphology. A gas sensor fabricated by ZnO NAs exhibits high sensitivity, low detection limit, fast response and good selectivity to acetone at the low working temperature (105 °C). The distinct gas-sensing properties of ZnO NAs are mainly attributed to the specific surface area (63.46 m2/g), and high active (001) facet for the nanosheets. Note that a pre-adsorption of O2 from air on ZnO NAs and the gas reaction mechanism are put forward based on the pre-adsorbed behavior and target gas response. Moreover, by the aid of first principles on the analysis of its surface adsorption energy and adsorption structure at (001) facet of ZnO NAs, it is identified that an oxygen pre-adsorption step on the facet occurs once it contact to air due to a lowest surface adsorption energy (-3.149 eV) for oxygen molecule. After the O2 pre-adsorption onto the surface, acetone is with the lowest surface adsorption energy of -0.687 eV, assigned to a chemical adsorption compared with the other gases. It is benefited to the acetone adsorption on the (001) facet for ZnO NAs, as well as following electron transfer and gas response. The sensitivity and selectivity for gas sensor based on ZnO NAs are well certified by both gas-resistance response and computational simulation.

1. INTRODUCTION ZnO, wurtzite structure, C46v-P63mc, as a functional n-type semiconductor gas sensor has been used to detect C2H5OH,1,2,3 CH3COOH,4 CH3COCH3,5,6,7 CO,8 NH3,9 H2,10 H2S11 and NO212 owing to its large specific surface area and exposed active facets. Note that the chemical reactions occur mainly at active sites for gas sensing materials, indicating that the larger specific surface area and more active sites, and better gas response. Thus, the designing morphology for ZnO with high specific surface area and active exposed facet will effectively improve the gas-sensing properties of materials, where the order of gas sensing of ZnO crystal planes is (0001) > (10 1 1) > (10 1 1) and (000 1 ).13 Thus, the synthesis for high exposed polar (001) or (0001) facet of ZnO is the most challenge for its gas nature. The high percentage of exposed active facets can be obtained by a designable controlling approach for ZnO nanostructure,14,15,16 which is beneficial for its gas sensing behavior. Two-dimensional nanosheets are with the maximum active sites, and a stack-free nanosheet is also with the maximum exposed area by its both faces. It is assumed that nanomaterials used as a gas sensor with a three-dimensional (3D) nest structure is ideal morphology for gas response.

Although the nest-like ZnO structures have been reported, these nest structures were synthesized by all using structuredirecting agents and templates.1,2,6,13 The latter would cause negative residuals on the surface of ZnO, inducing adverse effect for its gas response. Acetone as an important gas in the chemical industry has great harm to our bodies. For example, acetone is a kind of effective biomarkers in detecting type I diabetes. The acetone concentration in breathe gas for diabetic patients is two times than that of healthy people.17 As for a gas-sensing materials used as the gas sensor, semiconductor oxide has great potential for detecting of acetone, such as ZnO,5 Fe3O4,18 and WO3.19,20 Recently, Jia et al.21 synthetized hierarchical ZnO acetone sensor for hazardous odor markers, where the working temperature was operated at 230 ºC. DFT (Density Functional Theory) based on the ab initio theory is one of the effective means to deep research physical and chemical properties of materials surface. It is mainly used to explain the surface characteristics and reaction mechanism which are difficult to be characterized and analyzed by up-todate experimental approach.22 Mo et al.23 in 2019 published a paper about a pure DFT study on the gas-sensitive


Page 2 of 9

Page 3 of 9 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

ACS Applied Nano Materials characteristics of acetone on the surface of metal oxides (BeO, MgO, ZnO), where the acetone was adsorbed to the metal oxide nanoparticles with forming weak chemical bonds according to the calculation without oxygen pre-adsorption. With similar approach, Impeng et al.24 in 2018 studied on the gas-sensitive characteristics of CO on MnN4-graphene (MnN4GP) facet. It was found that among thirteen gases, NO, NO2, O2, CO, and SO2 adsorbed on MnN4-GP with strong chemisorption energies, while the rest of the gases are assigned to physisorption interactions with MnN4-GP. Moreover, the strong chemisorption of the five gases, drastically changed in the DOS of MnN4-GP and pronounced charge transfer between the adsorbed gas and MnN4-GP. Tehrani et al.25 in 2019 used a combination of experiments and calculations to discover that nitrogen (and sulfur) defects play a key role in enhancing the adsorption capacity of H2S and CO2 molecules. Specifically, an overlook problem for gas sensor is the pre-adsorption nature in gas response, i.e., the oxygen pre-adsorption in air to sensing surface prior to contact target gas, and the former greatly affects its post target-gas response. However, the oxygen pre-adsorption in their works is mostly neglected. Many papers have mentioned the adsorption of target gases on the surface of nanomaterials, but have few mentioned the pre-adsorption and identified with experimental method. Herein, we synthesized nest-like architectures (NAs) ZnO with exposed (001) facet by a facile temple-free hydrothermal method and then sintering at 300 ºC (0.5 h). Then the crystallite structures, morphologies and electricity properties of ZnO NAs were investigated. So, the pre-adsorption phenomenon on the surface of ZnO in this work was focused on the gas-response resistance test under low oxygen pressure or infrabar as simulated vacuum situation, uncovering the preadsorption affect under normal atmospheric condition. In addition, the changes of energy band and adsorption energy after O2 pre-adsorption and post-adsorption of target gas on ZnO (001) exposed facet were calculated via DFT method, clarifying the relationship between adsorption characteristics and the impedance of gas-sensing materials. Then the gas sensitive reaction mechanism is proposed. Finally, we proved the pre-adsorption behavior of O2 in ZnO surface and the selectivity of ZnO to acetone. 2. EXPERIMENTAL PROCEDURES 2.1 Preparation of ZnO NAs and ZnO nanoparticles (ZnO NPs) Large-scale synthesis of ZnO NAs were obtained by simple solution process at 80 ºC with zinc carbonate hydroxide hydrate (Zn4CO3(OH) 6·H2O) as precursor. All chemicals were analytical grade reagents and used as received without further purification. In a typical reaction process, 0.05 M ZnCl2 and 1 M CO(NH2)2 were dissolved in deionized water under continuous stirring. The pH of the solution was adjusted to pH=5 by the addition of HCl (2 wt %), then resultant solutions was placed in a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 80 ºC for 24 h. The white products were repeatedly washed with deionized water and acetone, and then dried at 80 ºC for 12 h. Finally, the white products were annealed at 300 ºC for 0.5 h to obtain final ZnO NAs. For comparison, the ZnO NPs were obtained by mixing 0.4 M NaOH and 0.1M ZnCl2, and resultant solutions were placed

in a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 80 ºC for 24 h followed by centrifuged and drying at 80 ºC, then annealed at 300 ºC for 0.5 h similar as post process of ZnO NAs. 2.2 Materials characterization Functional groups for ZnO and precursor were measured by Fourier transform infrared spectrometer (FT-IR Spectrum 2000, PerkinElmer, USA, KBr pellets). Crystalline structures of ZnO were analyzed by powder X-ray diffraction (XRD, Mini FlexTM II, Japan). Thermal analysis (TG/DSC, STA449C, Netzsch, Germany) was performed at a heating rate of 5 ºC/min under a air atmosphere. Surface morphologies of the as-prepared samples were characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi S4800, Japan) and transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, 200kV, FEI, USA). Specific surface areas were measured via the Brunauer–Emmett–Teller (BET) method using a N2 adsorption at 50 ºC after treating the samples at 150 ºC and 10−4 Pa for 2 h using a Tristar-3000 apparatus (BET, NovaWin2, Quanta-chrome, USA). The surface properties of samples were characterized by X-ray photoelectron spectroscopy (XPS) in a PHI Quantum 2000 Scanning ESCA Microprobe system (PHI. Corp., USA). All binding energies were referenced to the C 1s peak at 284.58 eV of the surface adventitious carbon. 2.3 Gas response measurements The gas sensor was fabricated by coating aqueous slurry of the as-prepared ZnO materials onto an alumina tube, which is positioned with a pair of Ni electrodes and four Pt wires on both ends of the tube.26 The working temperature on the surface of gas sensor was monitored by a FLIR IR (Infrared Radiation) camera player (FLIR company, Germany). Gas sensing tests were performed on a commercial Gas Sensing Measurement System (JFO2, Jinfeng Tech. Co. Ltd., Kunming, China). The sensors were aged for 10 days to improve the stability. Target gas was introduced into the testing chamber by a microsyringe. The sensor sensitivity is defined as the ratio S = Ra/Rg, where Ra and Rg are the electrical resistances of the sensor in air and in test gas, respectively.27 Then the response time (tre) was defined as the time needed to reach 90 % of the resistance change value when the gas sensor contacted the detected gas, while recovery time was defined yet as the time need to reach 90 % of the resistance change value when the gas sensor was separated from detected gas. 2.4 Computational details All the density functional theory (DFT) calculations were performed by employing the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional28 within the frame of Vienna Ab-initio Simulation Package (VASP),29,30 where a 400 eV cutoff energy was used for the plane-wave basis set and dipole correction was included for slab model. The projector-augmented plane wave (PAW)31 was used to describe the electron-ion interactions, 5 × 5 × 1 Gammacentered k-points was used for slab calculation. The convergence for the total energy and force was set to 1 × 10-5 eV and 0.01 eV/Å, respectively. A periodic slab (3 × 3 supercell) with 72 atoms for ZnO (001) facets was built, where the slab includes eight atomic layers with the bottom four layers fixed and the top four layers



relaxed during the calculation. The vacuum gap thickness was set to be 12 Å to avoid interactions between the adjacent slabs. The adsorption energy E(ads) is defined as the follow equation: E(ads) = E(slab + gas) – E(slab) – E(gas) ··················(1) Here, E(slab + gas) is the total energy of the slab with adsorbates, E(slab) is the energy of the slab, and E(gas) is the energy of adsorbates in gas phase. 3. RESULTS AND DISCUSSION 3.1 Structural analysis of ZnO NAs The thermal stability of the precursor was examined by TGDSC analysis. The test was under air at the flow rate of 5 ºC/min from 30 ºC to 400 ºC. TG curve (Figure S1) exhibits a total weight loss of 26.19 % attributed to the decomposition of carbonate and hydroxide in the precursor, as described in the following equation: Zn4(CO)3(OH)6·H2O → 4ZnO + 4H2O↑ (16.3 mass loss %) + 3CO2↑ (10.00 mass loss %) ····································(2) DSC curve (Figure S1) displays a board endothermic peak at around 249 ºC, which is attributed to the decomposition of carbonate and hydroxide in the precursor. The results indicate that the calcination temperature above 249 ºC was required to obtain pure ZnO from its precursor. In our work, the calcination temperature was selected at 300 ºC.

cm−1 is assigned to the O−H stretching from H2O bonds.5 The peaks between 1500 - 800 cm-1 have been greatly weakened for ZnO NAs, indicating the dehydration of the precursor. The intensive bands in the range of 330-600 cm-1 are attributed to the vibrational modes of Zn−O.33

Figure 2. XRD patterns of (a) ZnO NAs and (b) the precursor. The inset: hexagonal cell of ZnO.

Figure 1. FTIR spectra of (a) ZnO NAs and (b) the precursor.

To examine the composition of the precursor and ZnO NAs, FTIR was performed in the range of 4000 - 400 cm-1 (Figure 1). The peaks at 709, 834, 1390, and 1505 cm-1 in the precursor (curve b) are corresponding to the bending and vibration modes of CO32-,32 and they are weakened for ZnO NAs (curve a), indicating the decomposition of carbonate in the precursor. In addition, the broad peak at 3340 cm-1 in curve b is related to O-H stretching vibrations of the large amount of OH- group, corresponding to the absorbed water molecules in the precursor. Although the wide band in curve a at 3440 cm-1 is wakened for ZnO NAs, implying the dehydration of the precursor after calcination (300 ºC, 0.5 h), there are still an amount of OH- groups. And the characteristic peak at 1620


Page 4 of 9

Page 5 of 9 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

ACS Applied Nano Materials Figure 3. SEM (a-c) and TEM (d) of ZnO NAs, HRTEM (e) of red box in TEM (d), showing interplanar spacing, viewed along the [001] direction, and SEM (f) of ZnO NPs. Inset in (e): SAED pattern of ZnO NAs.

To further examine the crystallinity and crystal phases of ZnO NAs, XRD was performed in the range of 2θ (5~ 80º). From Figure 2, all peaks of the precursor can be indexed to Zn4CO3(OH)6·H2O (JCPDS card No. 11-0287). All of the peaks can be indexed to hexagonal ZnO with a wurtzite structure (JCPDS card No. 36-1451). The peak at 31.77º, 34.42º, and 36.25º are corresponding to the (100), (002), and (101) facet, respectively. In addition, no other diffraction peaks were detected, indicating that no impurity exists in the as-prepared samples. 3.2 Morphological analysis of ZnO NAs and ZnO NPs The morphology of ZnO NAs was characterized by TEM and FE-SEM (Figure 3). From Figure 3 (a), ZnO NAs shows a 3D nest-like architecture with size of 8.0 - 18.0 μm, where a core (~ 1 μm) in the middle. The magnified image (Figure 3 (b) and (c)) shows that ZnO nanosheets are with very smooth surfaces and their edge thicknesses are around 60 nm. Abundant interval between two nanosheets with around 90 nm appears, inducing two exposed surfaces for each ZnO nanosheet. The percentage of exposed (001) facets for ZnO NAs is ca. 95%, where 100 nanosheets were chosen to estimate the average exposed area according to their geometrical dimensions (Figure 3 (a) - (d)). Figure 3 (d) shows a typical TEM image of ZnO NAs. The high-resolution HRTEM image of nanosheets displays resolved fringes with separations of 0.16 nm and 0.28 nm, which corresponds to the (110) and (100) lattice spacing of a hexagonal ZnO crystal (Figure 3 (e)), respectively. Thus the exposed facets for ZnO NAs are certain with (001) facet, which is beneficial for absorbing gas molecules and surface chemical reactions in gas-sensing process. Highly flat surfaces also facilitate the simulation with DFT approach. And the corresponding selected-area electron diffraction (SEAD) (inset in Figure 3 (e)) also confirmed that the d-pacing is ascribed to (110) and (100) facets which is agreement with the XRD results above. Figure 3 (f) also shows that the particle morphology presents in ZnO NPs. 3.3 Surface properties of ZnO NAs and ZnO NPs To identify the porous structure and pores size distribution of ZnO NAs, the Brunauer-Emmett-Teller (BET) measurement was performed. According to the IUPAC classifications, the isotherm is Ⅴ type, H3 hysteresis loop.34 As shown in Figure S2, the loop observed at higher relative pressures (P/P0 = 0.7 - 1), indicating that it assigned to the feature of slit pores. This is a perfect fit with the SEM image (Figure 3c). The specific surface area of ZnO NAs (63.46 m2/g) (Figure S2 (a)) is larger than that of ZnO NPs (19.97 m2/g) (Figure S2 (b)). High specific surface area for ZnO NAs enhances the percentage of surface atoms and active sites, which can contribute to the enhancement of the oxygen adsorption in accord with discussion in Section 3.1. From the pore diameter distribution curves (inset in Figure S2), it is found that the macropore sizes for ZnO NAs are not uniform, and a peak is at around 90 nm, corresponding to the results of SEM (Figure 3 (c)), i.e., the interval between two nanosheets. The regular flat nanosheets are good for our simulation and calculation (Section 3.4).

To further analysis the surface properties of as-prepared ZnO NAs, X-ray photoelectron spectroscopy (XPS) was performed on ZnO NAs and ZnO NPs for comparison. Figure 4 illustrates the electron binding energies of Zn 2p and O 1s for the ZnO NAs and ZnO NPs. Figure 4 (a) displays the Zn 2p spectra of ZnO NAs and ZnO NPs, and we can see that there are two characteristic peaks at about 1045 and 1021 eV, assigning to Zn 2p1/2 and Zn 2p3/2, respectively. For ZnO NAs, the peak has a little negative shift with 0.15 eV, from 1021.10 to 1020.95 eV for Zn 2p3/2. The negative shift of Zn 2p should be contributed to the increase of electron density around Zn atom.35 On the surface of ZnO NAs, both Zn and O atoms are not able to reach tetra-coordination, leading to the present of more dangling bonds (Zn–) which readily absorb gas molecules on (001) facet. The atomic composition ratios of Zn : O on the surfaces is 1.09 : 1 for ZnO NPs and 1.19 : 1 for ZnO NAs, respectively. Therefore, there are more oxygen vacancies on the latter for the adsorption of gas molecules on ZnO NAs. From the O1s fitting spectra of ZnO NAs and ZnO NPs (Figure 4 (b)), it indicates that the O1s for oxygen atom is composed of two peaks. Note that the peak at around 530 eV for ZnO NAs is attributed to the lattice oxygen of ZnO, and the peak at 531.50 eV is attributed to O-. Compared with ZnO NPs, two O1s binding energies of ZnO NAs are with obvious positive shift of 0.14 eV from 529.93 to 530.07 eV, and 0.16 eV from 531.29 to 531.45 eV, respectively. Also the percentage of O- increased from 21.65 % for ZnO NPs to 29.21 % for ZnO NAs, which is beneficial for adsorbing more acetone molecules, detecting gas described as below. 3.4 Pre-adsorption O2 based on DFT Calculation of ZnO NAs (001) To prove the existence of pre-adsorbed O2 on the surface of ZnO NAs, the resistances of the gas sensor were measured in air and infrabar at 300 ppm O2 (0.003 atm) (Figure 5 (a)). It is clearly demonstrated that the resistance acutely and alternately changes under air atmosphere and low oxygen pressure, the gas sensing process is assumed as described at Equation 3. Wherein, O2 (ads) would obtain electrons leading to a increase

Figure 4. (a) Zn 2p spectra of ZnO NAs and ZnO NP in XPS spectra, (b) O 1s fit spectra of ZnO NAs and ZnO NPs.



Figure 5. (a) Response in air and infrabar (300 ppm O2) of ZnO gas sensor at 105 ºC and 40% RH. The clean facet (b) of ZnO (001) with computational model, and (c) O2 adsorption on ZnO (001) facet. Zn and O atoms are shown as gray and red color, respectively. Charge density difference of (d) O2/ZnO interfaces, where blue and yellow represent charge loss and charge accumulation, respectively. The isosurface value is set to 0.03 e/Å3.

of resistance. To explain the experimental results, the DFT method was used to simulate the adsorption behavior of O2 molecules on ZnO (001) facet. The parameter settings are listed in Section 2.4 and the simulation results are obtained in Table 1. Adsorption energy data is one of the most intuitive manifestation as to adsorption behavior. Generally, it is considered as physical adsorption that the adsorption energy is lower than -0.2 eV.36 On the contrary, it is assigned to chemical adsorption. The adsorption energy of O2 at ZnO (001) facet is -3.419 eV, which is significantly lower than those of other gases (Table 1). Therefore, ZnO (001) facet preferentially adsorbs O2 molecules once met in the air, this is a strong chemical adsorption compared to the other gas molecules in the table (Figure 5 (c)), where the stable bridged bonds are speculated between O2 and zinc ions on the surface of ZnO (001) facet. In order to further investigate the electron distribution on the surface of ZnO before and after preadsorption, the differential charge density was calculated (Figure 5 (d)). The adsorption behavior of O2 obviously changed the distribution of electrons on the surface of ZnO, and more electrons were obtained from O2 and the surface of ZnO. In the other words, once ZnO is exposed to air, O2 in air is rapidly pre-adsorbed to the surface of ZnO. This is consistent with our proposed pre-adsorption mechanism and resistance varies under atmosphere (Figure 5). This detail for pre-adsorption certified by both experiment and computational simulation has not been reported yet.

According to Figure 6 (a), the response of ZnO NAs with concentration of acetone was analyzed. The range from 101000 ppm has well linear relation with a limitation of 10 ppm (S/N≥ 3). The relation of Figure 6 (b) is corresponding to log(S-1) - log(concentration) plot, where the response (S = Ra/Rg) of MOS (Metal oxide semiconductor) gas sensors is usually empirically represented as log(S - 1) = b log(C) + log a (a and b are the constants and C is the concentration of the test gas).35 And the key factor b has an ideal value of either 0.5 or 1, which is derived from the surface interaction between chemisorbed oxygen and reductive gas for n-type semiconductor.5 It can be seen that the linear relationship: y = 0.5392 lg(x) - 0.6053 (Figure 6 (b) A line) for ZnO NAs, and b is close to 0.5. Compared with y = 0.6381 lg(x) - 1.679 (Figure 6 (b) B line) for ZnO NPs sensor, ZnO NAs sensor is with outstanding gas sensing properties. Because b is close to 0.5, indicating that the reaction medium is O2-.38 Moreover, the response of ZnO NAs has a good linear relationship (R2 = 0.9982) with the acetone concentration in logarithmic forms, suggesting that ZnO NAs work as an excellent sensing materials for the detecting of practical acetone.

Table 1. The calculated adsorption energies of various gases on ZnO (001) facet. Gas types

O2

CH3COCH3

CH4

CO

C6H6

CHCl3

H2

Clean facet (eV) -3.149

-0.388

0.0004

-0.405

-0.093

-0.029 -0.095

After O2 preadsorption (eV)

-0.687

-0.012

-0.145

-0.340

-0.035 -0.074

3.5 Gas sensing properties of ZnO NAs and ZnO NPs The gas sensing performances of ZnO NAs sensors were examined in different conditions. Firstly, we performed ZnO NAs sensors to various acetone concentrations at different operating temperature (85 - 115 ºC) (Figure S3). It is obvious that the response of ZnO sensors is the highest at 105 ºC, which is greatly lower than the other ZnO gas sensors to acetone reported over 200 ºC.6,37 Then different gas concentrations were tested in the concentration ranging from 10 to 10000 ppm at 105 ºC. Figure 6 (a) displays the correlation between the average resistances of ZnO NAs sensor to acetone, where the resistances of ZnO NAs step down with the concentration of acetone. Note that there are sharp falls between 100 to 200 ppm and 1000 to 2000 ppm due to various injecting volume.

Figure 6. (a) Dynamic response curves and expected fit curves of gas sensor based on ZnO NAs to acetone with concentrations, (b) Response versus acetone concentration of ZnO NAs (A) and ZnO NPs (B) with log(S - 1) vs log(concentration), (c) Responserecovery curve of gas sensor to 1000 ppm acetone, and. (d) The cross-response to acetone and the other gases.


Page 6 of 9

Page 7 of 9 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

ACS Applied Nano Materials Scheme 1. O2 adsorption and conversion.

Scheme 2. The working mechanism of the CH3COCH3 sensor. Figure 7. (a) Acetone adsorption on ZnO (001) facet, and (b) acetone adsorption after O2 pre-adsorption on ZnO (001) facet with computational model. Zn and O atoms are shown in gray and red, respectively. Charge density difference of (c) acetone/ZnO and (d) O2-acetone/ZnO interfaces, where blue and yellow represent charge loss and charge accumulation, respectively. The isosurface value is set to 0.03 e/Å3.

The response-recovery behavior of ZnO NAs sensor (Figure 6 (c)) shows a typical response-and-recovery curve to acetone at concentration of 1000 ppm under 105 °C. The response time (tre) for 1000 ppm acetone is less than 8 s and recovery time is 50 s, indicating that ZnO NAs sensor is with quick response. This cycle nature shows the high reproducibility of the sensor (Figure 6 (c)). Over 180 days, the response for ZnO NAs sensor decreases by 5%, also displaying the good stability. In addition, other gases were tested at 105 °C in 1000 ppm for comparison, including CH4, CHCl3, C6H6, H2, and CO. As shown in Figure 6 (d), the ZnO NAs has higher response to CH3COCH3 than to other gases, indicating excellent selectivity. Based on the results as above, the gas sensing mechanism is proposed. ZnO NAs sensor with oxygen vacancies (VO) firstly contact the ambient oxygen, and oxygen molecules are absorbed onto the high active (001) facets, forming absorbed oxygen sites (VO·) (Equation 3). Then adsorbed oxygen, O2 (ads), would obtain electrons from ZnO NAs and change into O2- (Equation 4) (Scheme 1) as determined above (Figure 6(b) A) , leading to the increasing of resistance for ZnO NAs. Then acetone molecules would react with O2- and be oxidized into carbon dioxide and water (Scheme 2). Furthermore, the previous electrons transferred to adsorbed oxygen would be released back to ZnO (Equation 5). As a result, the resistance will decrease, i.e., gas-sensing process: VO + O2(g) → O2(ads) + VO ·································(3) O2(ads) + 4e→ 2O2(reaction) ····························(4) CH3COCH3(g) + 4O2- (reaction) → 3CO2(g) + 3H2O(g) + 8e······························································(5)

To further verify these experimental results, the DFT method was used to probe the adsorption behavior of different gas molecules on ZnO (001) facets after O2 pre-adsorption. The parameter settings are described in Table 1 that O2 was pre-adsorbed on ZnO (001) facet, then, it found that the adsorption energy of acetone decreased significantly. Acetone is preferentially adsorbed. In contrast to other gases, only C6H6 adsorption can be reduce and less than -0.2 eV. As known, C6H6 is difficult to be oxidized. This explains the selectivity of ZnO (001) facets. The situation of acetone adsorption was simulated, as shown in Figure 7. In the presence of O2, acetone molecules form a relatively stable structure on the surface of ZnO (001) facet. If oxygen on carbonyl group bonds with zinc ions exposed on ZnO surface, O2(ads) and hydrogen on acetone has electrostatic attraction (Figure 7 (b)), conducing to the stability of the whole structure. E(bind) should be the binding energy of O2 and acetone. The calculated result is 0.299 eV (-28.74 KJ/mol), being equivalent to a hydrogen bonding energy. The surface of ZnO bonded with O2 and acetone has obvious electron aggregation (Figure 7(d)). The electron cloud overlaps obviously in the bonding region, which will facilitate the transfer of charges and thus increase the gas sensitivity of ZnO. The adsorption energies of other test gases (Table 1) is too low to facilitate electron transfer, Therefore, the gas sensing responses are low (Table 1). This also reasonably proves the pre-adsorption of O2 on ZnO NAs. The source of a decrease of adsorption energy is also explained, and it is beneficial to the gas-sensitive catalytic reaction. In addition, Figure S4 shows the total density of states (TDOS) and partial density of states (PDOS) for O2, acetone and acetone+O2 pre-adsorption on ZnO (001) facet, respectively. The fermi level in the Figure S4 has been set at the zero energy point, and the energy corresponding to the original fermi level is indicated in the Figure S4. At the same



time, the electrons near the Fermi energy level are mainly supplied by ZnO and O2, rather than acetone. In Figure S4, the Fermi energy level (-1.0187eV) after O2 pre-adsorption increases successively compared with free pre-adsorption (1.4757eV), which means that electrons can more easily jump from valence band conduction band, benefitting to the whole gas sensing reaction. 4. CONCLUSIONS In summary, nest-like ZnO nanosheets with average thickness of 60 nm was synthesized by simple hydrothermal methods without structure-directing agents and templates. The structures, morphologies and conductive property as a gas sensor of ZnO NAs were investigated. Results show that the diameter of ZnO NAs is around 8.0 - 18.0 μm, the highest active and exposed facet of ZnO NAs is (001) facet, where the specific surface area is 63.46 m2/g and the percentage of exposed (001) facet is up to ca. 95 %. The gas sensor based on ZnO NAs exhibit high response to acetone, a low detection limit (10 ppm), fast response (tre = 8 s) and good selectivity at low temperature (105 °C). This is due to its high specific surface area, high percentage of oxygen vacancies, great conductivity and high activity at (001) facet. Moreover, on the basis of experiment, we put forward the reaction mechanism and confirmed the pre-adsorption of O2 by DFT calculation. So, with these advantages, ZnO NAs acetone sensor has potential use in practical gas detecting based on its long stability and high selectivity.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TG-DTA curves of ZnO NAs precursor; Typical N2 adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of ZnO NAs (a) and ZnO NPs (b); Response curves of gas sensor based on ZnO NAs to acetone at different operating temperatures; TDOS and PDOS of ZnO (001) facet.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Pan). ORCID Haibo Pan: 0000-0003-1273-0433

Notes The authors declare no competing financial interest. Chaochao Li and Hegen Zhao are both as co-first authors.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from Project for International S & T Cooperation of China (2012DFM30040), National Science Foundation of China (NSFC)

(21201035, 61201397, J1103303(J2013-004)), and Department of Science and Technology (2012J01204).

Page 8 of 9 Fujian

REFERENCES 1.Zhang, J.; Wang, S. R.; Xu, M. J.; Wang, Y.; Zhu, B. L.; Zhang, S. M.; Huang, W. P.; Wu, S. H., Hierarchically porous ZnO architectures for gas sensor application. Crystal Growth & Design 2009, 9 (8), 3532-3537. 2.Qin, N.; Wang, X. H.; Xiang, Q.; Xu, J. Q., A biomimetic nest-like ZnO: Controllable synthesis and enhanced ethanol response. Sensors and Actuators B: Chemical 2014, 191, 770-778. 3.Xue, Z.; Cheng, Z.; Xu, J.; Xiang, Q.; Wang, X.; Xu, J., Controllable Evolution of Dual Defect Zni and VO Associate-Rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection. ACS applied materials & interfaces 2017, 9 (47), 41559-41567. 4.Dimitrov, O.; Nesheva, D.; Blaskov, V.; Stambolova, I.; Vassilev, S.; Levi, Z.; Tonchev, V., Gas sensitive ZnO thin films with desired (002) or (100) orientation obtained by ultrasonic spray pyrolysis. Materials Chemistry and Physics 2014, 148 (3), 712-719. 5.Rao, J.; Yu, A.; Shao, C.; Zhou, X., Construction of hollow and mesoporous ZnO microsphere: a facile synthesis and sensing property. ACS applied materials & interfaces 2012, 4 (10), 5346-5352. 6.Li, J.; Fan, H. Q.; Jia, X. H., Multilayered ZnO nanosheets with 3D porous architectures: Synthesis and gas sensing application. The Journal of Physical Chemistry C 2010, 114, 14684-14691. 7.Li, G.; Cheng, Z.; Xiang, Q.; Yan, L.; Wang, X.; Xu, J., Bimetal PdAu decorated SnO2 nanosheets based gas sensor with temperature-dependent dual selectivity for detecting formaldehyde and acetone. Sensors and Actuators B: Chemical 2019, 283, 590-601. 8.Zeng, Y.; Qiao, L.; Bing, Y.; Wen, M.; Zou, B.; Zheng, W.; Zhang, T.; Zou, G., Development of microstructure CO sensor based on hierarchically porous ZnO nanosheet thin films. Sensors and Actuators B: Chemical 2012, 173, 897-902. 9.Wagh, M. S.; Jain, G. H.; Patil, D. R.; Patil, S. A.; Patil, L. A., Modified zinc oxide thick film resistors as NH3 gas sensor. Sensors and Actuators B: Chemical 2006, 115 (1), 128-133. 10.Hassan, J. J.; Mahdi, M. A.; Chin, C. W.; Abu-Hassan, H.; Hassan, Z., A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays. Sensors and Actuators B: Chemical 2013, 176, 360-367. 11.Kim, J.; Yong, K., Mechanism study of ZnO nanorod-bundle sensors for H2S gas sensing. The Journal of Physical Chemistry C 2011, 115 (15), 7218-7224. 12.Rai, P.; Kwak, W. K.; Yu, Y. T., Solvothermal synthesis of ZnO nanostructures and their morphology-dependent gas-sensing properties. ACS applied materials & interfaces 2013, 5 (8), 3026-3032. 13.Han, X. G.; He, H. Z.; Kuang, Q.; Zhou, X.; Zhang, X. H.; Xu, T.; Xie, Z. X.; Zheng, L. S., Controlling morphologies and tuning the related properties of Nano/Microstructured ZnO crystallites. The Journal of Physical Chemistry C 2009, 113, 584-589. 14.Liu, J.; Hu, Z.-Y.; Peng, Y.; Huang, H.-W.; Li, Y.; Wu, M.; Ke, X.-X.; Tendeloo, G. V.; Su, B.-L., 2D ZnO mesoporous single-crystal nanosheets with exposed {0001} polar facets for the depollution of cationic dye molecules by highly selective adsorption and photocatalytic decomposition. Applied Catalysis B: Environmental 2016, 181, 138-145. 15.Chen, H.; Wei, Z.; Yan, K.; Bai, Y.; Zhu, Z.; Zhang, T.; Yang, S., Epitaxial growth of ZnO Nanodisks with large exposed polar facets on nanowire arrays for promoting photoelectrochemical water splitting. Small 2014, 10 (22), 4760-4769. 16.Chetia, T. R.; Ansari, M. S.; Qureshi, M., Ethyl Cellulose and Cetrimonium Bromide Assisted Synthesis of Mesoporous, Hexagon Shaped ZnO Nanodisks with Exposed +/-{0001} Polar Facets for Enhanced Photovoltaic Performance in Quantum Dot Sensitized Solar Cells. ACS applied materials & interfaces 2015, 7 (24), 13266-13279. 17.Marco, R.; Antonio, T.; Sotiris E., P., Si:WO3 sensors for highly selective detection of acetone for easy diagnosis of diabetes by breath analysis. Analytical Chemistry 2010, 82, 3581-3587. 18.Zhai, Q. F.; Du, B.; Feng, R.; Xu, W. Y.; Wei, Q., A highly sensitive gas sensor based on Pd-doped Fe3O4nanoparticles for volatile organic compounds detection. Analytical Methods 2014, 6 (3), 886-892. 19.Xiao, T.; Wang, X. Y.; Zhao, Z. H.; Li, L.; Zhang, L.; Yao, H. C.; Wang, J. S.; Li, Z. J., Highly sensitive and selective acetone sensor based on C-doped WO3 for potential diagnosis of diabetes mellitus. Sensors and Actuators B: Chemical 2014, 199, 210-219.


Page 9 of 9 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

ACS Applied Nano Materials 20.Liu, S. Y.; Zhang, F.; Li, H.; Chen, T.; Wang, Y. D., Acetone detection properties of single crystalline tungsten oxide plates synthesized by hydrothermal method using cetyltrimethyl ammonium bromide supermolecular template. Sensors and Actuators B: Chemical 2012, 162 (1), 259-268. 21.Jia, Q.; Ji, H.; Zhang, Y.; Chen, Y.; Sun, X.; Jin, Z., Rapid and selective detection of acetone using hierarchical ZnO gas sensor for hazardous odor markers application. Journal of Hazardous Materials 2014, 276, 262-270. 22.Xu, J.; Xue, Z.; Qin, N.; Cheng, Z.; Xiang, Q., The crystal facetdependent gas sensing properties of ZnO nanosheets: Experimental and computational study. Sensors and Actuators B: Chemical 2017, 242, 148157. 23.Mo, Y. M.; Li, H. Y.; Zhou, K.; Ma, X. C.; Guo, Y.; Wang, S. B.; Li, L. Q., Acetone adsorption to (BeO)12, (MgO)12 and (ZnO)12 nanoparticles and their graphene composites: A density functional theory (DFT) study. Applied Surface Science 2019, 469, 962-973. 24.Impeng, S.; Junkaew, A.; Maitarad, P.; Kungwan, N.; Zhang, D.; Shi, L. Y.; Namuangruk, S., A MnN4 moiety embedded graphene as a magnetic gas sensor for CO detection: A first principle study. Applied Surface Science 2018, 473, 820-827. 25.Tehrani, N. H. M. H.; Alivand, M. S.; Maklavany, D. M.; Rashidi, A.; Samipoorgiri, M.; Seif, A.; Yousefian, Z., Novel asphaltene-derived nanoporous carbon with N-S-rich micro-mesoporous structure for superior gas adsorption: Experimental and DFT study. Chemical Engineering Journal 2019, 358, 1126-1138. 26.Ying, M. H.; Hou, J. M.; Xie, W. Q.; Xu, Y. J.; Shen, S. F.; Pan, H. B.; Du, M., Synthesis, semiconductor characteristics and gas-sensing selectivity for cerium-doped neodymium vanadate nanorods. Sensors and Actuators B: Chemical 2018, 260, 125-133. 27.Han, Z. Z.; Wang, J. J.; Liao, L.; Pan, H. B.; Shen, S. F.; Chen, J. Z., Phosphorus doped TiO2 as oxygen sensor with low operating temperature and sensing mechanism. Applied Surface Science 2013, 273, 349-356. 28.John P. Perdew; Kieron Burke; Ernzerhof, M., Generalized gradient approximation made simple. Physical Review Letters 1996, 77 (18), 38653868.

29.Kresse, G., Ab initio molecular dynamics for liquid metals. Journal of Non-Crystalline Solids 1995, 192-193, 222-229. 30.Kresse, G., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Physical Review B 1996, 54 (16), 11169-11186. 31.Blöchl, P. E., Projector augmented-wave method. Physical Review B 1994, 50 (24), 17953-17979. 32.Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Chen, S.; Ding, W. P.; Xu, N. P., Microspheric organization of multilayered ZnO nanosheets with hierarchically porous structures. The Journal of Physical Chemistry C 2008, 112, 11722-11728. 33.Qin, N.; Xiang, Q.; Zhao, H. B.; Zhang, J. C.; Xu, J. Q., Evolution of ZnO microstructures from hexagonal disk to prismoid, prism and pyramid and their crystal facet-dependent gas sensing properties. CrystEngComm 2014, 16 (30), 7062-7073. 34.Zhang, L. X.; Zhao, J. H.; Lu, H. Q.; Li, L.; Zheng, J. F.; Li, H.; Zhu, Z. P., Facile synthesis and ultrahigh ethanol response of hierarchically porous ZnO nanosheets. Sensors and Actuators B: Chemical 2012, 161 (1), 209-215. 35.Hong, Y.; Tian, C. G.; Jiang, B. J.; Wu, A. P.; Zhang, Q.; Tian, G. H.; Fu, H. G., Facile synthesis of sheet-like ZnO assembly composed of small ZnO particles for highly efficient photocatalysis. Journal of Materials Chemistry A 2013, 1 (18), 5700-5708. 36.Yang, C.; Li, Y. R.; Li, J. S., Ab initiototal energy study of ZnO adsorption on a sapphire (0001) surface. Physical Review B 2004, 70 (4), 045413. 37.Kim, J. W.; Porte, Y.; Ko, K. Y.; Kim, H.; Myoung, J. M., Micropatternable Double-Faced ZnO Nanoflowers for Flexible Gas Sensor. ACS applied materials & interfaces 2017, 9 (38), 32876-32886. 38.Hongsith, N.; Wongrat, E.; Kerdcharoen, T.; Choopun, S., Sensor response formula for sensor based on ZnO nanostructures. Sensors and Actuators B: Chemical 2010, 144 (1), 67-72.


Preadsorption of O2 on the Exposed (001) Facets of ZnO

Recommend Documents