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Hierarchical NiO Cube/Nitrogen-Doped Reduced Graphene Oxide Composite with Enhanced H2S Sensing Properties at Low Temperature Ming Yang,†,‡ Xianfa Zhang,† Xiaoli Cheng,† Yingming Xu,*,† Shan Gao,† Hui Zhao,† and Lihua Huo*,† †

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China ‡ School of Pharmacy, Jiamusi University, Jiamusi 154007, China S Supporting Information *

ABSTRACT: A novel hierarchical NiO cube (hc-NiO)/nitrogen-doped reduced graphene oxide (N-rGO) composite is synthesized via a facile hydrothermal method and a postcalcination treatment without any templates and surfactants added. The NiO cubes assembled by abundant nanoparticles in situ grow on the surface of N-rGO layers. The combination of hc-NiO and N-rGO results in enhanced sensing properties with the contributions of the N-rGO providing high specific surface area and more efficient active sites for the adsorption of H2S molecules and the hierarchically structured NiO cubes providing high sensitivity and distinctive selectivity to H2S gas. At the optimal operating temperature of 92 °C, the hc-NiO/N-rGO composite based sensor shows not only high response to H2S in a range of 0.1−100 ppm but also excellent selectivity for H2S against the other seven gases. The gaseous product, produced from the contact of H2S with the hc-NiO/N-rGO composite at 92 °C, is measured by GC-MS technique. The change of the surface composition and the chemical state of the hc-NiO/N-rGO composite before and after exposure to H2S are investigated by XPS. The possible sensing mechanism of the hc-NiO/N-rGO composite is similar to that of semiconductor oxides. The H2S molecules that absorbed on the sensor surface transform to SO2 by reacting with the adsorbed oxygen anions. Meanwhile, the electrons restricted by the surface-adsorbed oxygen return to the bulk and neutralize the holes, producing a change in resistance. KEYWORDS: hierarchical structure, NiO, nitrogen-doped reduced graphene oxide, H2S gas sensor, low temperature, sensing mechanism

1. INTRODUCTION Hydrogen sulfide (H2S) is a malodorous toxic and highly corrosive gas. When exposed to 2 ppm of H2S gas, asthmatic individuals show bronchial constriction. When the concentration rises to 5−10 ppm, the blood lactate concentration increases, and the skeletal muscle citrate synthase activity and the oxygen uptake decrease. With further increasing of the H2S concentration, many tragic consequences might be caused, such as olfactory paralysis, respiratory distress, and even death if the H2S concentration is over 700 ppm.1,2 In terms of the security criterion set by the American Conference of Government Industrial Hygienists, the threshold concentration of H2S is 10 ppm in atmosphere. For the purpose of protecting human health, a highly sensitive, selective, and reliable H2S sensor is in great demand. As a typical gas-sensing material, metal oxide semiconductors (e.g., ZnO,3 In2O3,4 NiO,5 Fe2O3,6 WO3,7 SnO2,8 MoO39) have been widely applied in H2S gas sensors. Among them, NiO is one of the most promising materials, and especially the hierarchically nanostructured NiO sensing materials are viewed as a research hotspot for H2S gas sensor application.10 To the best of our knowledge, the hierarchical nanostructures can © 2017 American Chemical Society

prevent the aggregation of individual nanoparticles and possess porous structures, which can enlarge the specific surface areas, increase active sensing sites, and promote the transfer of electrons.11,12 Moreover, the strategy of combining metal oxides and graphene-based materials for improving their respective gas sensing properties has been proven workable in recent years. The composites consisting of metal oxides and graphene-based materials exhibit not only the respective characteristics of each component but also the extra new properties coming from the synergy between them.13,14 Therefore, researchers made a lot of studies on the composites of metal oxides with graphene-based materials to enhance H2S gas sensing performance. For instance, Fe2O3/graphene,15 MoO3/rGO,16−18 WO3/graphene-based material,19−21 SnO2/graphene-based material,22−25 and Cu2O/graphene26 have been synthesized successfully and showed improved H2S gas sensing performance compared with their single pristine component. Furthermore, Received: April 8, 2017 Accepted: July 13, 2017 Published: July 13, 2017 26293

DOI: 10.1021/acsami.7b04969 ACS Appl. Mater. Interfaces 2017, 9, 26293−26303

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ACS Applied Materials & Interfaces

powder as raw material.35 In detail, 3.0 g of natural graphite powder (325 mesh) was dispersed in 70 mL of concentrated sulfuric acid under stirring for 8 h. Then, 9.0 g of KMnO4 was slowly added to the above suspension in an ice bath to ensure the temperature below 20 °C. The reaction solution was kept stirring at 40 °C in water bath for 1 h. Successively, the mixture solution was diluted with 150 mL of water and stirred for another 0.5 h at 95 °C. After the solution cooled, it was further diluted with 500 mL of deionized water and added dropwise to 15 mL of H2O2 (30%). The mixture was centrifuged and washed with 10% HCl solution for many times. The product was dispersed in 600 mL of deionized water and further purified by dialysis. Finally, it was dried at 60 °C for 24 h to obtain GO. Preparation of the hc-NiO/N-rGO Composite. The hc-NiO/NrGO composite was prepared via a hydrothermal method and postcalcination treatment. Specific steps were as follows: GO (20 mg) was dispersed in deionized water (30 mL) and ultrasonic exfoliation for 1 h to obtain a stabilized aqueous dispersion. Then, 0.25 g of Ni(Ac)2·4H2O was added and stirred for 20 min. Successively, 1 g of urea was introduced slowly and kept stirring vigorously for another 20 min. After that, the obtained mixture solution was placed in a 50 mL Teflon-lined autoclave and maintained at 180 °C for 12 h. The asprepared precursor was collected by centrifugation and washed with deionized water and ethanol for several times. Finally, the product of hc-NiO/N-rGO composite was obtained after calcined at 300 °C under N2 flow for 3 h. For comparison, hc-NiO and N-rGO were also prepared by a similar process only without GO or Ni(Ac)2·4H2O addition. Characterization. The crystal phase and crystallinity of the products were analyzed by X-ray powder diffraction (XRD) using a Bruker D8-Advance diffractometer with Cu Kα radiation (1.5046 Å) in the 2θ range between 5° and 80°. The Raman spectrum was recorded on a Renishaw 1000 Micro-Raman spectrometer (457.9 nm excitation). The microscopic morphology of the products was observed by a field emission scanning electron microscope (FESEM, Hitachi S-4800). The high-resolution transmission electron microscopy (HR-TEM) and selected area electronic diffraction (SAED) patterns were obtained by a JEOL-JEM-2010 TEM. The specific surface area and pore size distribution were determined by N2 adsorption−desorption at 77 K using a TriStar II 3020 system. The surface chemistry of the products was measured by an X-ray photoelectron spectrometer (XPS, ULTRA AXIS DLD). The binding energy values were adjusted by referencing to C 1s (284.6 eV). Thermogravimetric analysis (TGA) was implemented by a PERKIN ELMER 6300 thermogravimetric analyzer. The gas chromatogram and mass spectrum of the gaseous products which were obtained after the hc-NiO/N-rGO exposed to H2S were measured by GC-MS (AGILENT, 6890−5973N) at a N2 flow rate of 1 mL min−1. Sensor Fabrication and Measurement. The sensor used for sensing measurement was fabricated by coating sensing material on a hollow Al2O3 tube (4 mm in length and 0.8 mm in internal diameter) with two parallel gold electrodes spaced 1 mm. Each Au electrode was attached with two Pt wires to connect the measurement system. The as-prepared sensing materials were ground with terpineol (25 wt % in the total weight) to obtain a uniform paste and brush-coated on the surface of the Al2O3 tube. Then it was dried at 60 °C and calcined at 200 °C for 2 h in air to obtain a thick sensing film. After that, a Ni−Cr wire, as the heater of sensor, was inserted through the tube for adjusting the working temperature. The Ni−Cr wire and Pt wires were soldered onto the base, and the whole sensor was connected to a digital gas-sensitive measurement system. The sensor was aged at 280 °C for 24 h before the gas-sensing performance test. The sensor fabrication and measurement were performed according to ref 36. The gas responses of sensors were measured by a digital gassensitive measurement system (model JF02F, Kunming Guiyan Jinfeng Technology Co. Ltd., China). The static testing method was adopted for surveying the gas-sensitive performance. The certain testing gas concentration was obtained by injecting the corresponding volume of target gas into a 10 L testing chamber which was vacuumed by a pump beforehand and then balancing the pressure of the inner and outer of chamber with fresh air. The schematic diagram of the gas-

substitutional doping of graphene with nonmetal hetero atoms (such as nitrogen, boron, etc.) offers significant advantages over pristine graphene in gas sensing performance because the substituted atoms can provide additional reactive sites for gas adsorption.27−30 It is therefore very interesting to consider the combination of nonmetal hetero-atom-doped graphene and metal oxides, in order to produce novel gas sensors. However, by surveying the literature, there is only one article that just reported on using TiO2/nitrogen-doped reduced graphene oxide composites31 as sensing material for isopropanol, ethanol, and acetone gas detection. There remains a lot of work to explore the sensing properties of metal oxides/nonmetal hetero-atom-doped graphene composites. With this background, we designed a facile route to synthesize the hierarchical NiO cube (hc-NiO)/nitrogendoped reduced graphene oxide (N-rGO) composite via the hydrothermal method and postcalcination treatment (Scheme 1). Without the addition of any templates and surfactants, the Scheme 1. Schematic Preparation Diagram of the hc-NiO/NrGO Composite Material

slow hydrolysis of urea was used to adjust the pH value of the reaction solution and provided a nitrogen source for doping of rGO.32,33 Meanwhile, the hydrolysate CO2 reacted with Ni2+ which anchored on GO sheets34 to form basic nickel carbonate under the appropriate pH conditions. As a desired precursor for the formation of hierarchically structured NiO cubes, the basic nickel carbonate hexagonal prisms in situ grew on N-rGO sheets and decomposed into NiO, H2O, and CO2 at 300 °C under N2 flow. The release of gaseous CO2, during the decomposition process, was beneficial to the formation of hierarchical structure. The as-prepared hc-NiO/N-rGO composite based sensor exhibited high sensitivity, selectivity, and repeatability toward H2S gas at a low optimal operating temperature of 92 °C, and the sensitivity followed a good linear relationship with H2S concentration within the scope of 0.1−1 ppm and 1−100 ppm, respectively.

2. EXPERIMENTAL SECTION Chemical Reagent and Materials. The starting materials were nickel acetate (Ni(Ac)2·4H2O, 98%), urea ((NH2)2CO, 99%), natural graphite flake (325 mesh), sulfuric acid (H2SO4, 95−98%), potassium permanganate (KMnO4), hydrochloric acid (HCl, 36.5−38%), and H2O2 (30 wt %). All of them were AR grade reagents and used without further purification. All chemical reagents involved in the experiments were bought from Tianjin Chemical Reagent Co. Ltd. (China). Preparation of Graphene Oxide. Graphene oxide (GO) was prepared by an improved Hummer’s method using natural graphite 26294


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Figure 1. XRD patterns of the as-prepared precursors (a) and the products calcined at 300 °C for 3 h under N2 flow (b). sensitive test device is displayed in Figure S1. The concentration of the testing gas can be calculated by the formula as follows: C = Vin × 105 ppm, where C is the concentration of testing gas whose unit is ppm and Vin is the volume of injected gas whose unit is L. The operating temperature of the sensor was controlled by heating voltage. The temperature and the relative humidity during the measurement were 25 ± 1 °C and 45 ± 5 RH%, respectively. The response and recovery characteristics could be determined when the sensor was inserted in/ removed from the testing gas chamber. The sensor responses to different relative humidity were measured at 92 °C, and the certain relative humidity level was achieved by using saturated aqueous solutions of LiCl (11 RH%), CH3COOK (23 RH%), MgCl2 (33 RH %), K2CO3 (43 RH%), Mg(NO3)2 (54 RH%), NaCl (75 RH%), KCl (85 RH%), and KNO3 (94 RH%). Each saturated salt solution (about 1 L) was stored in a sealed 5 L testing chamber for 48 h so that the air above could reach the desired humidity. The gas response (S) of the sensors to target gas is defined as S = Rg/Ra, where Rg and Ra are the resistances of sensors in target gases and air, respectively. The response time of sensors is defined as 100 s, starting with the sensor exposed to the target gas. The recovery time is defined as the time needed to recover 63% of the whole resistance change. The selectivity of the hcNiO/N-rGO-based sensor to H2S in this paper is expressed quantitatively by the selective sensing coefficient (KH2S/x) which is defined as SH2S/Sx, where SH2S and Sx are the responses in H2S gas and in the other seven interfering gases, respectively.

the XRD pattern of the precursor, the N-rGO obtained from the calcination process shows the same diffraction peak positions and enhanced intensities, indicating that there is no new phase generation but an increased crystallinity. After the precursor of hc-NiO is calcined, the XRD pattern changes obviously with the appearance of five new diffraction peaks at 37.2°, 43.3°, 62.8°, 75.4°, and 79.4°, which can be attributed to the diffraction of the (111), (200), (220), (311), and (222) crystal planes of the cubic NiO phase (JCPDS file no. 47-1049, space group Fm3̅m). This result reveals that the basic nickel carbonate can convert to NiO at 300 °C in N2; in the meantime, CO2 and H2O are released. For the XRD pattern of the hc-NiO/N-rGO composite, both the characteristic diffraction peaks of hc-NiO and N-rGO are observed, and the positions and the relative intensities of these characteristic diffraction peaks remain unchanged corresponding to their pristine ones, without any impure peaks. It indicates that the structural properties of hc-NiO and N-rGO do not change after the composition process, and the hc-NiO/N-rGO composite has been synthesized successfully. Incidentally, when the sintering temperature increased to 400 °C, NiO is reduced to elemental Ni by carbon under N2 flow, and the product converts to the Ni/N-rGO composite (see Figure S2). Raman spectra of hc-NiO, N-rGO, and the hc-NiO/N-rGO composite are comparably shown in Figure 2. Three Raman peaks located at about 539, 678, and 1070 cm−1 are observed in the hc-NiO Raman spectrum. The first peak at 539 cm−1 can be attributed to the first-order longitudinal optical phonon mode of NiO, and the latter two peaks at 678 and 1070 cm−1

3. RESULTS AND DISCUSSION Structure and Morphology of Products. Figure 1a reveals the X-ray diffraction patterns of GO and the as-prepared precursors of products. In the XRD pattern of GO (Figure 1a4), there is a feature diffraction peak located at 2θ = 11.2° corresponding to an interplanar spacing (d-space) of 0.79 nm, which comes from the (001) crystal plane.37,38 After the hydrothermal and doping process, this feature diffraction peak of GO completely disappears, while two new weak and broad diffraction peaks appear at 25.1° and 43.5° which correspond to the (002) and (100) planes of the precursor of N-rGO (Figure 1a-3), indicating the reduction of GO.39,40 The precursor of hcNiO (Figure 1a-1) exhibits the identical XRD pattern of the Ni(HCO3)2 phase (JCPDS file no. 15-0782, space group P4̅3m), which should be corrected to the basic nickel carbonate Ni12(CO3)8(OH)8·(5−7)H2O phase suggested by Rincke et al. recently.41 Additionally, the precursor of hc-NiO/N-rGO (Figure 1a-2) exhibits similar diffraction peaks to the precursor of hc-NiO because the two weak diffraction peaks of N-rGO are covered by basic nickel carbonate Ni12(CO3)8(OH)8·(5− 7)H2O. The XRD patterns of the products that calcined at 300 °C in N2 for 3 h are shown in Figure 1b. Compared with

Figure 2. Raman spectra of hc-NiO, N-rGO, and the hc-NiO/N-rGO composite. 26295


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pattern (inset of Figure 3e) recorded from the hc-NiO/N-rGO composite exhibits concentric diffraction rings, which can be indexed as (111), (200), and (220) planes of the cubic NiO from inside to outside. It indicates that the hc-NiO/N-rGO composite is a polycrystalline structure. These results further confirm the successful formation of hc-NiO/N-rGO composite, which are in accordance with the results of XRD and Raman analyses. The specific surface area and the pore construction of hcNiO, N-rGO, and hc-NiO/N-rGO are comparably appraised by nitrogen adsorption−desorption measurement (Figure 4a) and BJH pore size distribution analysis (Figure 4b). The BET specific surface areas of hc-NiO, N-rGO, and hc-NiO/N-rGO are about 119.7, 247.7, and 196.1 m2 g−1, respectively. This result reveals that the hc-NiO/N-rGO composite possesses larger specific surface area and stronger adsorption ability than pure hc-NiO. As can be seen from Figure 4a, the hc-NiO/NrGO composite demonstrates a type IV isotherm with an H3 hysteresis loop. From the shape of the desorption isotherm, the hc-NiO/N-rGO composite possesses dual desorption characteristics of hc-NiO and N-rGO, indicating that the hc-NiO/NrGO composite may have both pore structures of hc-NiO and N-rGO. This speculation is confirmed by the pore size distribution curves (Figure 4b). The hc-NiO/N-rGO composite has not only the micropores and mesopores coming from the N-rGO and the close packing of NiO small particles but also the macropores originating from the stacking of NiO cubes. Beyond that, the hc-NiO/N-rGO composite also contains some additional mesopores and macropores that come from the interspaces between the N-rGO and the hierarchical NiO cubes. To analyze the chemical bond configuration and the surface composition of samples, XPS measurement was carried out. The XPS full survey spectra of GO, N-rGO, and hc-NiO/NrGO are shown in Figure 5a. It can be observed that there are two distinct peaks in GO at 284.6 and 532.2 eV corresponding to C 1s and O 1s, respectively. After the hydrothermal-doping process with urea and the following calcination, the intensity of the O 1s peak reduces significantly, indicating that most of the oxygen-containing functional groups are eliminated in N-rGO, and it is worth noting that a new peak at 399.8 eV emerges in N-rGO, which is attributed to the binding energy of N 1s, suggesting that the N atom is doped into rGO. To further clarify the aforementioned results, we compare the C 1s fine spectra of GO and N-rGO and show them in Figure 5b. For GO, three peaks centered at 284.5, 286.7, and 289.0 eV are obtained after fitting, corresponding to the C−C, C−O, and O−CO, respectively. In comparison with the C 1s fine spectrum of GO, the peak of C−C bonding in that of the NrGO remains unchanged, and the oxygen-containing groups’ component (C−O and O−CO) diminishes greatly. Notably, two new peaks at 285.6 and 287.7 eV appear in N-rGO, which can be attributed to N-sp2 C and N-sp3 C bonding, respectively.46−49 In addition, the N 1s spectrum of N-rGO (Figure 5c) can be fitted into three peaks at 398.4, 399.9, and 401.9 eV, assigned to the pyridinic N, pyrrolic N, and graphitic N, which are the common three forms of N atoms doped into rGO.45 The full survey spectrum of hc-NiO/N-rGO (Figure 5a) demonstrates the existence of Ni, C, N, and O elements in the composite. There is no peak corresponding to the unexpected element, indicating the high purity of the asprepared composite. Comparing with N-rGO, a stronger O 1s peak is also observed in the full survey spectrum of hc-NiO/N-

correspond to second-order transverse optical and longitudinal optical phonon modes of NiO, respectively.42,43 The N-rGO sample exhibits two obvious peaks at 1353 and 1580 cm−1 attributed to the D and G band, respectively, which are the common features of carbon materials in Raman spectra. The D band can be assigned to the breathing mode of κ-point phonons of A1g symmetry, and the G band can be ascribed to first-order scattering of the in-plane E2g vibration mode of sp2 carbon domains.44,45 The Raman spectrum of the hc-NiO/NrGO composite shows the typical peaks of hc-NiO and contains D and G bands of N-rGO, which confirms the successful synthesis of the hc-NiO/N-rGO composite. This conclusion is consistent with the results of XRD measurement. The morphology and microstructure of the precursor and the calcined product of hc-NiO/N-rGO were characterized by SEM and TEM. Figure 3a displays the morphology of the precursor,

Figure 3. SEM images of the precursor (a) and the calcined product (b) of the hc-NiO/N-rGO composite, TEM (c and d) and HRTEM images (e) and SAED pattern (f) of the hc-NiO/N-rGO composite.

in which the relatively uniform hexagonal prisms with smooth surface and average size of 200 nm in-situ grow on the surface of N-rGO layers. After calcination, basic nickel carbonate hexagonal prisms transform into hierarchically structured NiO cubes, and the morphology of N-rGO remains unchanged, as shown in Figure 3b. For comparison, the SEM images of hcNiO and N-rGO are provided in Figure S3. In order to further investigate the microstructure of the hc-NiO/N-rGO composite, TEM is carried out. From the TEM image of hc-NiO/NrGO (Figure 3c and d), it can be seen that the hierarchical NiO cubes are composed of NiO nanoparticles of 3−5 nm in diameter, which implies excellent porosity of the prepared hcNiO/N-rGO composite. The HRTEM image (Figure 3e) of the hc-NiO/N-rGO composite displays clear lattice fringes with spacing of 0.208 and 0.241 nm, attributed to the (200) and (111) crystal planes of cubic NiO, respectively. The SAED 26296


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Figure 4. Nitrogen adsorption−desorption isotherms (a) and pore diameter distribution curves (b) of hc-NiO, N-rGO, and the hc-NiO/N-rGO composite.

Figure 5. XPS characterizations of GO, N-rGO, and hc-NiO/N-rGO samples. (a) Full survey spectra of GO, N-rGO, and hc-NiO/N-rGO, (b) C 1s fine spectra of GO and N-rGO, (c) N 1s fine spectrum of N-rGO, and (d) Ni 2p fine spectrum of the hc-NiO/N-rGO composite.

a satellite peak of Ni 2p3/2 at 855.6 and 860.8 eV, respectively, as well as a satellite peak of Ni 2p1/2 at 879.4 eV are observed, which further demonstrate the presence of Ni2+. The mass concentrations of Ni, O, C, and N in the composite are 56.87, 17.06, 22.87, and 3.20 wt %, respectively, deriving from the XPS

rGO, which arises from the newly generated hc-NiO. In the Ni 2p spectrum of the hc-NiO/N-rGO composite (Figure 5d), two major peaks present at binding energy values of 853.7 and 872.6 eV, corresponding to the Ni 2p3/2 and Ni 2p1/2 spin− orbit peaks of the NiO phase.50 Moreover, a shoulder peak and 26297


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Figure 6. (a) Responses of N-rGO, hc-NiO, mixture, and hc-NiO/N-rGO-based sensors toward 50 ppm of H2S at different working temperatures. (b) Response and recovery curve of the hc-NiO/N-rGO-based sensor toward 0.1−100 ppm of H2S at 92 °C. (c) The relationship between the responses of the hc-NiO/N-rGO-based sensor and H2S concentration at 92 °C.

Table 1. H2S Sensing Responses of hc-NiO/N-rGO in This Work Compared with Other Metal Oxides/Graphene-Based Materials Reported in the Literature sensing material

response/H2S concentration

working temperature

detection limit

reference

hc-NiO/N-rGO

31.95/50 ppm 24.96/50 ppm chemiluminescence measurements 59/40 ppm 44.7/40 ppm 4120/50 ppm 65.6/5 ppm 19.66/5 ppm 168.58/40 ppm 130/50 ppm 78/10 ppm 34/5 ppm 34/50 ppm 36%/100 ppb

92 °C 50 °C 130 °C 110 °C 110 °C 160 °C 300 °C 300 °C 330 °C 260 °C 100 °C 200 °C 22 °C room temperature

100 ppb  10 ppm 5 ppm 5 ppm  100 ppb 100 ppb 10 ppb 1 ppm 500 ppb 1 ppm 43 ppb 5 ppb

this work this work 12 13 14 15 16 17 18 19 20 21 22 23

Fe2O3/graphene nanosheets MoO3/rGO rGO-MoO3 hybrid MoO3-rGO WO3 nanofibers/graphene WO3 hemitubes/graphene rGO/hexagonal WO3 nanosheets SnO2 nanorods/graphene SnO2@rGO SnO2 nanofibers/rGO SnO2 quantum wire/rGO Cu2O/functionalized graphene

6a). As can be seen in Figure 6a, the N-rGO-based sensor has low sensitivity to H2S gas in the whole range of measuring temperature. For the hc-NiO-based sensor, the highest response (S = 8.42) to 50 ppm of H2S appears at 133 °C. However, the hc-NiO/N-rGO-based sensor exhibits a different change trend from the hc-NiO-based sensor. The gas response of hc-NiO/N-rGO-based sensor increases monotonously with increasing temperature from 25 °C and reaches the maximum value (S = 31.95) at 92 °C, and then it falls dramatically with the measuring temperature further increasing. So, the best operating temperature examined is 92 °C for the hc-NiO/NrGO-based sensor. Since the pure hc-NiO shows the highest response at 133 °C, the dramatic falling of response for the hcNiO/N-rGO composite at 133 °C may be mainly due to the

measurement. Consequently, it is reasonable to conclude that the hc-NiO/N-rGO composite is obtained. To further determine the carbon and nitrogen content in the hc-NiO/NrGO composite, the TGA curve is carried out under air flow from room temperature to 800 °C (see Figure S4). The weight loss after 300 °C can be attributed to the release of carbon and nitrogen components, so their total content is approximately 24.17 wt % in the hc-NiO/N-rGO composite. This result is basically consistent with that of XPS analysis. Gas-Sensing Properties. As we all know, the gas-sensing property of the sensor is closely related to the working temperature. Thus, the responses of the sensors based on NrGO, hc-NiO, and hc-NiO/N-rGO toward 50 ppm of H2S were investigated first at different operating temperatures (Figure 26298


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Figure 7. Responses of hc-NiO/N-rGO-based sensor toward: (a) 10 ppm of different gases, (b) 10 ppm of H2S gas for 5 times of continuous measurements, (c) different relative humidity, and (d) 50 ppm of H2S gas during three months at 92 °C.

Figure 6c. With the increase of H2S concentration, the responses of the sensor increase and present good linear relationships with the concentration in the ranges of 0.1−1 ppm (R2 = 0.992) and 1−100 ppm (R2 = 0.993). Thus, the sensor based on the as-synthesized hc-NiO/N-rGO composite can be used for monitoring H2S gas in ppm and subppm levels. To evaluate the selectivity of the hc-NiO/N-rGO composite to H2S, the responses of the sensor to 10 ppm of different gases, including ethanol (C2H5OH), carbinol (CH3OH), formaldehyde (HCHO), triethylamine ((C2H5)3N), benzene (C6H6), ammonia (NH3), acetone (C3H6O), and hydrogen sulfide (H2S), were investigated at 92 °C (Figure 7a). Obviously, the hc-NiO/N-rGO-based sensor shows the highest response (S = 10.39) to H2S among the eight gases, and the responses to the other testing gases are, in descending order, C2H5OH (2.03), (C2H5)3N (1.76), CH3OH (1.67), HCHO (1.34), C6H6 (1.28), C3H6O (1.25), and NH3 (1.21). The selective sensing coefficients (KH2S/x) to the other seven gases are 5.12, 5.90, 6.22, 7.75, 8.12, 8.31, and 8.59, respectively. It indicates that the sensor based on the hc-NiO/N-rGO composite has not only high response but also excellent selectivity to H2S at the optimal operating temperature of 92 °C. Figure 7b shows the repeatability of hc-NiO/N-rGO-based sensor toward 10 ppm of H2S. The responses of five consecutive measurements are 10.12, 10.53, 10.60, 10.41, and 10.29; the average relative deviation is 1.5%, demonstrating a good repeatability of this sensor to H2S. The recovery time of the sensor to 10 ppm of H2S is about 36 s. In order to investigate the influence of environment humidity on the sensor, the responses of hc-NiO/N-rGO-based sensor to different relative humidity (RH) were tested at 92 °C, as shown in Figure 7c. The responses are less than 1.4 in the whole measurement range of 11%−94% RH, which indicates that the

decrease of the adsorption ability of the N-rGO component toward H2S at this temperature. Consequently, the hc-NiO/NrGO composite displays not only approximately 3.8 times higher sensitivity but also lower optimal operating temperature toward H2S gas than those of the hc-NiO sample. In addition, we also prepared the physical mixture of hc-NiO (76 wt %) and N-rGO (24 wt %) for comparison, as shown in Figure 6a. The response of the mixture to 50 ppm of H2S is significantly lower than that of the hc-NiO/N-rGO composite at 92 °C. This result implies that the junction of hc-NiO and N-rGO plays an important role in H2S sensing. The real-time gas response−recovery characteristic curves of the hc-NiO/N-rGO-based sensor toward various concentrations of H2S gas are measured at its optimal operating temperature of 92 °C (Figure 6b). As illustrated in Figure 6b, the hc-NiO/N-rGO-based sensor shows evident resistance changes and reversible response signals to 0.1−100 ppm of H2S gas. The sensor shows high response (S = 54.06) and short recovery time (12 s) in 100 ppm of H2S (see Figure S5). With the concentration of H2S decreasing, the response of the sensor decreases, and the recovery time is prolonged. When the concentration of H2S decreases down to 0.1 ppm, the response reduces to 1.6, and the recovery time increases to 197 s. The comparison of the responses to H2S gas between the hc-NiO/ N-rGO composite and other metal oxides/graphene-based materials reported in the literature is shown in Table 1. In contrast to most of the metal oxides/graphene-based materials, the hc-NiO/N-rGO composite in this work has relatively lower operating temperature and detection limited to H2S gas. It is worth mentioning that the hc-NiO/N-rGO-based sensor can also give a satisfactory response to 50 ppm of H2S gas at 50 °C in this work but needs a long recovery time, as shown in Figure S6. The correlation between H2S concentration and response of the hc-NiO/N-rGO-based sensor at 92 °C is shown in 26299


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Figure 8. Gas chromatogram (a) and mass spectrum (b) of the gaseous product that was produced from the contact of H2S with the hc-NiO/N-rGO composite at 92 °C for 30 min.

Figure 9. (a) Full XPS survey spectra, (b) S 2p fine spectrum, and (c) O 1s fine spectra of the hc-NiO/N-rGO composite before and after exposure to H2S at 92 °C.

Second, is there a change in the surface composition or the chemical state of the hc-NiO/N-rGO composite after exposure to H2S? Herein, we answer these questions by GC-MS and XPS techniques. Figure 8 shows the gas chromatogram and mass spectrum of the gaseous product that was produced from the contact of H2S with the hc-NiO/N-rGO composite at 92 °C for 30 min. Three eluting peaks at the retention time of 1.42, 1.45, and 1.48 min, respectively, are observed from the gas chromatogram (Figure 8a). For the eluting compound at 1.48 min, the corresponding mass spectrum (Figure 8b) shows the intense molecular ion peak at m/z = 64, which could be speculated to the SO2 gas molecule. So, it is rational to think that H2S undergoes an oxidation reaction and converts to SO2 when the hc-NiO/N-rGO composite is exposed to H2S. This conclusion is also confirmed by the results of XPS. Figure 9a shows the comparison of the XPS survey spectra of the hcNiO/N-rGO composite before and after exposure to H2S at 92 °C. Two new peaks at 167.9 and 232.6 eV emerge after

environment humidity has little impact on the sensor. For examining the long-term stability, the responses of hc-NiO/NrGO-based sensor in 50 ppm of H2S gas were measured once every 3 days at its optimal operating temperature during three months (Figure 7d). The response remains basically its original value to 50 ppm of H2S with a little decline of less than 4% after 90 days. Taking into consideration multiple sensing properties including the high gas response, the low operating temperature, the excellent selectivity, the satisfactory repeatability, the slight humidity disturbance, and the good long-term stability, the hcNiO/N-rGO composite is a nice sensing candidate for the detection of H2S gas. Gas Sensing Mechanism of the hc-NiO/N-rGO Composite to H2S. To infer the gas sensing mechanism of the hc-NiO/N-rGO composite, two things should be clarified. First, if the H2S gas can react with something in the hc-NiO/NrGO composite during the sensing process, leading to the significant change in resistance, what is the reaction product? 26300


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rGO. The abundant micropores, mesopores, and macropores, which come from the N-rGO, the close packing of NiO nanoparticles, the stacking of NiO cubes, and the wrapping of N-rGO on NiO the cube surface, together provide the porosity of the composite. The N-rGO supplies not only the large specific surface area but also the more efficient active sites for the H2S molecules to be adsorbed. The hierarchically structured NiO cubes dominate the sensing processes, providing high sensitivity and distinctive selectivity to H2S gas. Finally, the combination of hc-NiO and N-rGO results in enhanced sensing properties toward H2S gas.

exposure to H2S, corresponding to the binding energy of S 2s and S 2p, respectively. The S 2p XPS fine spectrum is shown in Figure 9b, and two peaks located at 161.8 and 168.2 eV are observed, which could be attributed to S 2− and S 4+ components. It implies the existence of H2S and SO2 on the surface of the hc-NiO/N-rGO composite after exposure to H2S. In addition, both of the O 1s fine spectra (Figure 9c) that before and after sensing could be deconvoluted into two peaks at 531.3/531.2 and 529.4/529.3 eV can be assigned to the surface-adsorbed oxygen and lattice oxygen, respectively. Interestingly, the percentage of adsorbed oxygen drops from 57.7% to 48.6% after the hc-NiO/N-rGO composite is exposed to H2S, suggesting that the surface adsorption oxygen is involved in the redox reaction with H2S. Therefore, the conceivable sensing mechanism of the hc-NiO/N-rGO-based sensor toward H2S could be deduced as below: (a) the hcNiO/N-rGO composite exhibits the characteristics of a p-type semiconductor, whose resistance increases dramatically when exposed to reducing H2S gas. (b) The sensing mechanism of the hc-NiO/N-rGO composite is similar to that for semiconductor oxides. In air, abundant oxygen molecules are adsorbed on the surface of the hc-NiO/N-rGO composite and capture electrons to form adsorbed oxygen anions (eqs 1 and 2). (c) H2S gas molecules adsorb onto the surface of the sensor (eq 3) and react with the adsorbed oxygen anions (eq 4). Simultaneously, the electrons captured by adsorbed oxygen are released to neutralize bulk holes (eq 5), resulting in a decrease in the effective carrier concentration of holes, hence giving an electrical response. The schematic diagram of the possible H2S sensing process on the surface of the hc-NiO/N-rGO-based sensor is shown in Figure 10.

O2(gas) ↔ O2(ads) −

O2(ads) + e ↔

(1)

O−2(ads)

(2)

H 2S(gas) ↔ H 2S(ads) 2H 2S(ads) +

3O−2(ads)

e− + h+ ↔ Null

4. CONCLUSIONS In summary, we designed a simple route to synthesize a novel hc-NiO/N-rGO composite which consists of hierarchical NiO cubes and nitrogen-doped reduced graphene oxide. The stacking of nanoparticles to form NiO cubes and the wrapping of N-rGO on the NiO cube surface afford the porosity and the high specific surface area of the composite. The sensor based on the hc-NiO/N-rGO composite exhibits high response in detecting 0.1−100 ppm of H2S at the low operating temperature of 92 °C, and the responses present good linear relationships with the H2S concentration in ranges of 0.1−1 ppm (R2 = 0.992) and 1−100 ppm (R2 = 0.993). In addition, the sensor also shows distinctive selectivity to H2S against the gases of ethanol, carbinol, formaldehyde, triethylamine, benzene, ammonia, and acetone. The ambient humidity has little influence on this sensor at 92 °C. Considering the gaseous product and the change of the surface-adsorbed oxygen content, the H2S-sensing mechanism can be described as a redox reaction between H2S molecules and the adsorbed oxygen anion, taking place on the surface of the hc-NiO/NrGO composite. The enhanced H2S sensing performance should be credited to the unique porous structure and the more efficient active adsorption sites. The hc-NiO/N-rGO composite takes advantage of the two materials to result in a high performance material for ultrasensitive H2S detection. It suggests the possibility of other metal oxides and nonmetal hetero-atom-doped graphene combinations for improved sensing applications.

(3)

↔ 2SO2(ads) + 2H 2O + 3e

−

(4)

■

(5)

The enhanced H2S sensing properties of the hc-NiO/N-rGO composite could be attributed to the unique structure of the composite and the synergistic effect between hc-NiO and N-

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04969. Schematic diagram of gas-sensitive test device; XRD patterns of the products calcined at 400 °C for 3 h under N2 flow; SEM images of N-rGO and hc-NiO; TGA curve of the hc-NiO/N-rGO composite under air flow; response−recovery curve of the hc-NiO/N-rGO-based sensor toward 100 ppm of H2S at 92 °C and toward 50 ppm of H2S at 50 °C (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Y. M. Xu) E-mail: [email protected]. *(L. H. Huo) E-mail: [email protected]. ORCID

Shan Gao: 0000-0001-6370-4994 Notes

Figure 10. Schematic diagram of H2S sensing process on the hc-NiO/ N-rGO-based sensor surface.

The authors declare no competing financial interest. 26301


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (61271126, 21547012, and 21305033), International Science & Technology Cooperation Program of China (2016YFE0115100), Program for Science and Technology Project of Heilongjiang province (B2015008), Heilongjiang Educational Department (2013TD002), Youth Foundation of Harbin (2015RQQXJ047), and Heilongjiang Postdoctoral Scientific Developmental fund (LBH-Q15118, LBH-Q16198).

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