Fabrication of Novel 2 D NiO Nanosheet Branched on 1 D-ZnO Nanorod Arrays for Gas Sensor Application

Fabrication of 3D structures composed of 1D n-type ZnO nanorods (NRs) and 2D p-type NiO nanosheets (NSs) by a low-cost, lowtemperature, and large-area scalable hydrothermal process and its use in highly sensitive NO 2 gas sensors were studied. The p-n heterojunctions formed byNiO-ZnO interfaces as well as large area two-dimensional NiONSs themselves increased the adsorption of NO 2 . Moreover, the charge transfer between NiO and ZnO enhanced the responsivity and sensitivity of NO 2 sensing even at a concentration of 1 ppm. The 30-min NiO NS growth on ZnO NRs in the hybrid sensor showed the highest sensitivity due to the formation of optimum p-n heterojunctions between ZnO NRs and NiO NSs for gas adsorption and carrier transport. Low responsivity toward reducing gases was also observed.


Introduction
Among various metal oxide semiconductors, zinc oxide (ZnO) has been used extensively due to its beneficial electrical properties such as wide band gap (3.37 eV) and large binding energy (60 meV).When hybridized with other materials, ZnO showed unique properties that can be used effectively for many types of electronic devices including gas sensors [1,2], solar cells [3,4], and UV sensors [5,6].Moreover, it can be easily grown by hydrothermal methods in the form of nanostructures such as nanorods and nanowires that can exhibit enhanced properties due to increased surface area to volume ratios.Due to its excellent resistance to various chemicals, nickel oxide (NiO) has been widely explored for gas sensors [7,8], supercapacitors [9,10], electrochromic devices [11,12], and lithium ion batteries [13,14].A wide range of NiO nanostructures, such as thin films [15,16], nanotubes [17,18], and flake-like structures [19], can be formed by sputtering methods [20,21], sol-gel processes [22,23], and hydrothermal synthesis [24,25].Recent studies also demonstrated that the hybrid structures composed of NiO nanotubes and ZnO shells exhibited highly improved hydrogen sulfide (H 2 S) sensing properties by the formation of heterointerfaces [26].In our previous work we showed that optimized hybrid structures of NiO and ZnO can be effectively used as optical sensors due to improved charge transfer and decreased recombination of excitons.
In this paper, the 3D hybrid structures composed of ntype 1D ZnO nanorods (NRs) and p-type 2D NiO nanosheets (NSs) with different NiO growth times were fabricated using low-cost and easy hydrothermal methods (Figure 7).These were used to detect toxic gas, such as nitrogen oxide (NO 2 ), which has a characteristic sharp, biting odor and is regarded as a prominent air pollutant.Due to the charge transfer between the two nanostructures as well as increased adsorption sites formed by large-area 2D NiO NSs and electron-depleted p-n junctions, the NiO NS/ZnO NR hybrid structures exhibited improved NO 2 sensing properties.At optimized conditions, the hybrid structures showed a 2800fold higher sensitivity than pure ZnO NRs and pure NiO NSs, even at very low NO 2 concentrations (1 ppm).They also exhibited good selectivity toward NO 2 when other gases, including H 2 , NH 3 , and H 2 S, were tested and compared.

Experimental
The hybrid structure was fabricated step by step as described in our previous study [27].
Figure 1: The MST-5000 gas sensor system (MST-5000, MS-Tech) and the chamber equipped with a heating plate and probes (inset), gas flow was controlled precisely by mass flow controllers, and nitrogen was used as a carrier gas.
2.1.Growth of ZnO NRs.The primary step is growing vertically aligned ZnO NRs on the sensing electrode via a hydrothermal process.In brief, diethanolamine (HN(CH 2 CH 2 OH) 2 , Sigma-Aldrich) was added slowly into the mixed solution, which was composed of zinc acetate dehydrate (Zn(CH 3 COO) 2 ⋅2H 2 O 98%, Sigma-Aldrich) and 2-methoxyethanol (CH 3 OC 2 H 5 OH, Sigma-Aldrich), under continuous magnetic stirring at 70 ∘ C for 2 h.Then, the solution was spin-coated onto the sensing electrode patterned SiO 2 /Si wafer and annealed in air at 400 ∘ C for 4 h.ZnO NRs were grown by exposing the prepared seed layer to the mixed aqueous solution of zinc nitrate hexahydrate 0.02 M (Zn(NO 3 ) 2 ⋅6H 2 O, Sigma-Aldrich) and hexamethylenetetramine 0.02 M (C 6 H 12 N 4 , Sigma-Aldrich) at 90 ∘ C for 5 h.ZnO NRs were rinsed with deionized water several times and dried in a vacuum oven before the next steps.

Growth of NiO NSs.
To grow NiO NSs on the ZnO NR surface, the Ni seed solution was first prepared by mixing nickel acetate tetrahydrate (Ni(OCOCH 3 ) 2 ⋅4H 2 O, Sigma-Aldrich) with 2-methoxyethanol (CH 3 OCH 2 CH 2 OH, Sigma-Aldrich) and diethanolamine (HN(CH 2 CH 2 OH) 2 , Sigma-Aldrich) under continuous magnetic stirring at 70 ∘ C for 2 h.Then, the mixed solution was spin-coated onto the ZnO NRs and annealed in air at 400 ∘ C for 4 h to form a Ni seed layer.Next, the ZnO NRs covered with the Ni seed layer were directly exposed to a mixed aqueous solution of nickel nitrate hexahydrate 0.02 M (Ni(NO 3 ) 2 ⋅6H 2 O 98%, Sigma-Aldrich) and hexamethylenetetramine 0.02 M (C 6 H 12 N 4 , Sigma-Aldrich) at 90 ∘ C to grow NiO NSs.Finally, NiO NSs/ZnO NRs were rinsed and annealed in air at 400 ∘ C for 4 h.

Instrumental Analysis. The structures of NiO NSs/ZnO
NRs, NiO NSs, and ZnO NRs were characterized by Xray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) after thin film growth on the SiO 2 /Si wafer.Gas sensing properties were measured using a MST-5000 chamber (MS-Tech, Figure 1) at 200 ∘ C. Gas flow was precisely controlled using a mass flow controller (GMC 1200, ATOVAC) and nitrogen (N 2 ) was used as a carrier gas.A semiconductor parameter analyzer (Hewlett-Packard-4155A) was used to measure the resistance of sensing devices.

Results and Discussion
The morphology change of NiO NSs on the ZnO NRs at different NiO growth times was investigated and is shown in Figure 2. We can only infer the growth of NiO NSs by the increased roughness of ZnO NRs or some fibrillike structures between ZnO NRs at short growth times (Figures 2(a In order to verify the crystal structure and phase purity of the samples, XRD analysis was used, and these results are shown in Figure 3. NiO NSs exhibit three characteristic peaks at 37.30 ∘ , 43.28 ∘ , and 65.23 ∘ , which, respectively, correspond to the (111), (200), and (220) planes of bunsenite structure NiO [28].ZnO NR shows peaks at 31.87 ∘ , 34.54 ∘ , 56.81 ∘ , and 67.23 ∘ , which, respectively, correspond to the (100), ( 002), (110), and (201) planes of hexagonal structure of ZnO [29].The intensity ratio of I Ni(220) /I Zn(002) was increased from 0.2538 to 4.4134 as the NiO growth time increased from 5 min to 60 min, which indicates continuous growth of NiO NSs on the ZnO NRs.
The atomic composition and TEM image were shown in Figure 4.Both the Zn and Ni atoms were observed in the energy dispersive X-ray (EDX) analysis and ZnO NR covered with NiO NS was also clearly observed in the TEM image.
The - characteristics of ZnO NRs, NiO NSs, and NiO NSs/ZnO NRs were measured to investigate the electrical resistance of each sample.The electrodes used in this study were prepared following our previous study [27].As shown in Figure 5, after decorating the NiO NSs with ZnO NRs, the conductance was decreased due to the formation of local electron depletion layers at the heterojunctions between ptype NiO and n-type ZnO.The electrical resistivity of ZnO nanorods calculated using the thickness of the ZnO seed layer, as shown in the inset of Figure 4, was approximately 4.14 × 10 −5 Ω⋅cm, which was higher than that of a previous report because hydrothermal synthesis was used in this study [28].
NO 2 responsivity (RS, (  −   )/  (%)) and sensitivity (RS/gas concentration (ppm)) of the NiO NSs/ZnO NRs and NiO NSs alone were measured at low levels of NO 2 , where   and   are the resistances of the sensing layer measured in the atmosphere of only N 2 and NO 2 /N 2 gas mixture, respectively.As shown in Figure 6, the NiO NS/ZnO NR hybrid structure exhibited around a 6-fold higher responsivity and 9-fold higher sensitivity than those of NiO NSs, which has a large enough adsorption area.The sensitivity obtained in this study of 3.4026 ppm −1 from the NiO NS (30 min growth time)/ZnO NR sample is three orders (∼2800-fold) higher than that of pure ZnO NRs and even one order (∼20-fold) higher than that of 0-D CuO nanoparticles decorated with 1D ZnO NR  that were reported previously [29].The fast response time of NiO NS/ZnO NR than that of NiO NS was observed, which can be due to the high amount of p-n junction sites that can effectively adsorb NO 2 molecules.It is also interesting to note that the NO 2 sensitivity increased initially as the NiO NS growth time increased until 30 min then the NO 2 sensitivity decreased gradually, which indicates the presence of an optimum ratio of NiO NSs and ZnO NRs.In turn, excess growth of NiO NSs will cover the whole ZnO NR surface and thus synergetic effects between NiO NSs and ZnO NRs cannot be expected.The improved NO 2 response and sensitivity of the NiO NS/ZnO NR hybrid structure can be explained as follows.First, increased surface area after NiO NS growth on ZnO provided a larger NO 2 adsorption area.In addition, the numerous electron-depletion layers that formed at the p-NiO NS and n-ZnO heterojunction region attracted the gases more than bare ZnO NRs or NiO NSs alone, which results in more NO 2 adsorption even at low concentration [30].Another important fact is the charge transfer between ptype NiO NSs and n-type ZnO NRs [28].When NO 2 gas is exposed to the NiO NSs, the NO 2 first reacts with the adsorbed O − ions on NiO as NO 2 has higher electron affinity than that of the preadsorbed oxygen [31]; this results in the generation of holes on NiO surfaces: NO 2 (ads) → NO 2 − (ads Then, the transfer of holes from p-type NiO to n-type ZnO (loss of electrons of n-type ZnO) leads to a resistance increase of the ZnO seed layer that carries charges between two electrodes.Without NiO-ZnO heterojunctions (NiO   alone), there will be hole accumulation on the NiO surface, which will cause low responsivity due to the suppressed further adsorption of NO 2 .On the other hand, in the presence of p-n heterojunctions, hole transfer from NiO NSs to ZnO NRs will prevent the accumulation of holes in NiO NSs, thus maintaining the adsorption of NO 2 .This will enhance both the responsivity and sensitivity of sensing layers.
The responsivity of fabricated NiO NS/ZnO NR gas sensors toward reducing gases, such as H 2 S, H 2 , and NH 3 , was also investigated.As shown in Figure 6, the responsivity for reducing gases was lower than that for NO 2 gas, which can be due to the low catalytic effect during the adsorption of reducing gases [32].As described in the following reaction, when reducing gases are adsorbed onto the metal oxide surface they are oxidized through the reaction with preadsorbed oxygen ions.Due to the low dissociative chemisorption of hydrogen by the NiO, there is less electron generation, which results in the lower responsivity of NiO NS/ZnO NR sensors for reducing gases than that for NO 2 .The surface modification by Pt, which has excellent H 2 adsorption and catalytic dissociation ability, can enhance the responsivity of NiO-based sensors toward reducing gases [33]: The highest responsivity at 30 min of NiO NS growth indicates the similar synergetic effect of charge transfer between NiO NSs and ZnO NRs as the NO 2 gas case.The negative responsivity of the hybrid sensor for reducing gases is due to the electron generated by the oxidation of reducing gases.Electrons transferred from the p-type NiO NSs to ntype ZnO might decrease the resistance of n-type ZnO, which acts as channel materials for carrier transport in the sensing devices.

Conclusions
The 3D structures composed of 1D ZnO NRs and 2D NiO NSs were fabricated by an easy and cost-effective hydrothermal synthesis method, and these were then used for NO 2 sensors.Due to the increased surface areas, formation of electron depletion layers at ZnO-NiO heterojunctions, and effective carrier transport between two nanostructured semiconductors, the NiO NS/ZnO NR sensors exhibited highly improved sensitivity toward NO 2 gases over pure NiO NS and ZnO NR sensors.It was also observed that at a 30 min NiO NS growth on ZnO NRs, hybrid sensors exhibited maximum NO 2 sensitivity due to the formation of optimized 1D-2D p-n heterojunction hybrid structures.Due to the low dissociative chemisorption of H 2 on NiO, hybrid sensors exhibited low response toward reducing gases, which results in improved selectivity.We think that this type of 3D structures can be effectively used in many gas-sensing applications due to its simple fabrication process and high sensing performance.
) and 2(b)).Instead, the 3D networks of NiO NSs between ZnO NRs are clearly observed after longer growth times, which implies that various morphologies of NiO NS/ZnO NR hybrid structures that have different Ni/Zn ratios can be realized by changing the NiO growth time.

IFigure 3 :
Figure 3: XRD spectra of NiO NSs grown on ZnO NRs at various NiO NS growth times from 5 min to 60 min.

Figure 4 :
Figure 4: (a) EDX spectrum of NiO NSs/ZnO NRs and (b) TEM image of ZnO NR covered with NiO NS.

Figure 5 :
Figure 5: - curves of ZnO NRs and NiO NSs/ZnO NRs.The inset shows a cross-sectional SEM image of ZnO seed layer.

Figure 6 :Figure 7 :
Figure 6: (a) Responsivity of pure NiO NSs and NiO NSs/ZnO NRs at 30 min NiO growth and (b) the sensitivity of NiO NSs/ZnO NRs at various NiO growth times toward various NO 2 gas concentrations.