Significance of the Nanograin Size on the H 2 S-Sensing Ability of CuO-SnO 2 Composite Nanofibers

CuO-SnO 2 composite nanofibers with various nanograin sizes were synthesized for investigating their sensing properties with respect to H 2 S gas. The nanograin size in the CuO-SnO 2 composite nanofibers was controlled by changing the thermal treatment duration under isothermal conditions.Thenanograin sizewas found to be critical for the sensing ability of the composite nanofibers. The CuO-SnO 2 composite nanofibers comprised of small-sized nanograins were more sensitive to H 2 S than those with larger-sized nanograins. The superior sensing properties of the CuO-SnO 2 composite nanofibers with the smaller nanograins were attributed to the formation of the larger number of p-CuO-n-SnO 2 junctions and their transformation tometallic-CuS-n-SnO 2 contacts upon exposure to H 2 S gas. The results suggest that smaller nanograins are conducive to obtaining superior H 2 S-sensing properties in CuO-SnO 2 composite nanofibers.


Introduction
The air quality in industrial sectors situated in urban areas, such as power plants, is of major concern.Gases, such as CO, H 2 S, and NO  , have been identified as highly toxic air pollutants.Among them, H 2 S is highly flammable and hazardous to human health even at very low concentrations (<10 ppm) [1].
Semiconductor metal oxides, such as SnO 2 , ZnO, NiO, CuO, TiO 2 , and WO 3 , have potential applications for the detection of hazardous gaseous species.Among them, ptype CuO is highly sensitive and selective towards H 2 S [2].p-type CuO transforms to metallic-CuS in the presence of H 2 S and recovers its p-type CuO characteristics in air.This property of CuO has been used successfully by means of p-CuO-n-semiconductor heterostructures [2][3][4][5][6][7][8].Among them, the CuO-SnO 2 composite materials are recognized as the best material system for the detection of H 2 S gas [4,6].
A range of CuO-SnO 2 composite materials systems, such as CuO-SnO 2 composite thin films [9], CuO-functionalized SnO 2 nanowires [6], and CuO-SnO 2 composite nanofibers [10], were used successfully for the detection of H 2 S gas.In particular, composite nanofibers are more promising because they have the potential to manipulate the number of p-n junctions between CuO and SnO 2 nanograins.According to earlier studies [11,12], the number of p-n junctions greatly influences the sensing ability of CuO-SnO 2 composite nanofibers.The maximum number of p-n junctions was essential for obtaining superior H 2 S-sensing properties.On the other hand, nanograins of different sizes have also been found to affect the sensing ability of metal oxide nanofibers [13][14][15][16][17][18][19].The size of the nanograins can be changed by heating at different temperatures or different time intervals under isothermal conditions.
In this study, CuO-SnO 2 composite nanofibers were synthesized and their gas sensing ability was investigated particularly in terms of the nanograin size.For this, the CuO-SnO 2 composite nanofibers were heat treated at different temperatures under isothermal conditions.The results suggest that the nanograin size has a significant effect on the H 2 Ssensing ability of CuO-SnO 2 composite nanofibers, demonstrating the optimization of the nanograin size is one of key parameters to obtain the best H 2 S sensing performance.

Experimental
The CuO-SnO 2 composite nanofibers were synthesized by an electrospinning process.The composition of the prepared 2 Journal of Sensors electrospinning solution is as follows: xCuO-(1 − x)SnO 2 , where x = 0.5.The precursor materials, tin (II) chloride dehydrate (SnCl 2 ⋅2H 2 O, Sigma-Aldrich Corp.), copper chloride dihydrate (CuCl 2 ⋅2H 2 O, Junsei Chemical Co.Ltd.), and solvents N, N-dimethylformamide anhydrous (DMF, Sigma-Aldrich Corp.), ethanol (Sigma-Aldrich Corp.), and polyvinyl acetate (PVAc, Sigma-Aldrich Corp.) polymer were used to synthesize the composite nanofibers.The following gives a brief summary of the procedure for the synthesis of the CuO-SnO 2 composite nanofibers.First, a mixed DMF and ethanol solution was prepared and the PVAc polymer was added to enhance the viscosity of the prepared solution.Proper amounts of the SnCl 2 and CuCl 2 precursor materials were added to the prepared solution and stirred at room temperature for 6 h.The prepared electrospinning solution was poured into a glass syringe equipped with a 21-gauge stainless steel needle with an inner diameter of 0.51 mm.The flow rate, applied voltage, and needle to collector distance were 0.05 mL/h, 15 kV, and 20 cm, respectively.The as-spun fibers were deposited over SiO 2 (250 nm) grown Si substrates.The as-spun fibers were then calcined at 600 ∘ C at a heating rate of 10 ∘ C/min in air for different time intervals from 0.5 to 48 h.The heat treatment led to the decomposition of the polymers and precursors in the as-spun fibers and transforms them to the required oxide phase.Finally, the individual nanofibers assembled with nanosized grains were prepared.
Thermogravimetric-differential thermal analysis (TGA-DTA, STA 409 PC, Netzsch) of the prepared as-spun nanofibers was performed to determine the appropriate calcination temperature.The microstructure and phase of the prepared nanofibers were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4200) and X-ray diffraction (XRD, Philips X'pert MRD diffractometer).
For the sensing measurements, a bilayer electrode consisting of a 50/200 nm thick Ti/Au layer was deposited by sputtering with an interdigital electrode mask.Figure 1(a) shows a schematic diagram of the fabrication process of the CuO-SnO 2 composite nanofiber sensors.The gas sensing performance of the CuO-ZnO composite nanofibers was tested for H 2 S. The sensing measurements were performed using a gas sensing system.The H 2 S gas concentrations were controlled by maintaining the mixing ratio of the dry air-balanced target gas and dry air through accurate mass flow controllers.The configuration and design of the sensing system are reported elsewhere [11,12].The optimal measurement temperature was found to be 300 ∘ C after a series of preliminary experiments reported in the previous work [11].The gas response, , was evaluated from the ratio,   /  , where   is the resistance in the absence of H 2 S and   is the resistance measured in the presence of H 2 S.

Results and Discussion
Figures 1(b) and 1(c) show FE-SEM images of the as-spun nanofibers, which were composed of mixture of PVAc, copper chloride, and tin chloride with solvents.The as-spun nanofibers were distributed uniformly over the SiO 2 grown Si substrates.TGA of the as-spun nanofibers was carried out to determine the temperature at which the solvent and organic components decomposed completely.As shown in Figure 1(e), marginal weight loss occurred up to 200 ∘ C, which was attributed to the evaporation of a small amount of solvent in the as-spun nanofibers.On the other hand, the sharp weight loss at above 200 ∘ C was assigned to the degradation of the polymer chains in PVAc.The decomposition of the PVAc polymer continued up to 600 ∘ C. No further weight loss was observed above 600 ∘ C.This suggests that the calcination temperature of 600 ∘ C is needed to remove the polymer content and transform the copper chloride and tin chloride to their corresponding CuO and SnO 2 phases, respectively.TGA confirmed that the optimal calcination temperature was 600 ∘ C. Figure 1(d) shows a typical FE-SEM image of the CuO-SnO 2 composite nanofibers obtained after calcination at 600 ∘ C. The composite nanofibers were distributed randomly and uniformly over the substrate.
Figures 2(a), 2(b), and 2(c) show high magnification FE-SEM images of CuO-SnO 2 nanofibers obtained after calcination at 600 ∘ C for 0.5, 12, and 48 h, respectively.They clearly showed that all the nanofibers were composed of nanosized grains and their size changed as a function of the calcination time under the isothermal condition.The insets show the corresponding low-magnification FE-SEM images, revealing the overall features of the nanofibers.Figure 2(d) shows the summarized nanograin size as a function of the calcination time under the isothermal condition.As is evident, the nanograin size increases with increasing calcination time.The grain growth is a thermally activated process, occurring at the expense of smaller nanograins.By this grain growth phenomenon, a longer calcination time led to the generation of larger nanograins.Earlier investigations [12][13][14] suggest that the activation energy required for the growth of nanograins is about one order of magnitude smaller than the bulk counterparts and the growth mechanism mainly involves lattice and/or surface diffusion.
Figure 3 shows the XRD pattern of the CuO-SnO 2 composite nanofibers calcined at 600 ∘ C for 48 h.The peaks match well with the SnO 2 and CuO phases (JCPDS: 880287 for SnO 2 and JCPDS: 895899 for CuO), demonstrating the creation of CuO-SnO 2 composite material.
To examine the effects of the nanograin size on the H 2 S-sensing ability of the CuO-SnO 2 composite nanofibers, the fabricated sensors were tested for 1 and 10 ppm H 2 S at 300 ∘ C. Figure 4 presents the typical resistance curves of the CuO-SnO 2 nanofibers sensors with various nanograin sizes.All sensors clearly detect H 2 S gas.The resistance of the sensors decreases upon the supply of H 2 S and increases upon its stoppage.The response of the sensors decreases sharply with increasing nanograin size (see Figure 5(a)).The CuO-SnO 2 composite nanofibers with the smallest nanograins were the most sensitive among all the sensors.In addition, the response of the CuO-SnO 2 composite nanofibers for 10 ppm H 2 S is summarized as a function of the grain size in Figure 5(b).This suggests that the composite nanofibers should be prepared with smaller nanograins; that is, the response improves with decreasing nanograin size.It is of note that the sensors fabricated in this study were stable for longer than six months and showed a negligible variance with respect to samples.In the current work, the CuO-SnO 2 composite nanofibers with the nanograin size 11 nm showed the response ∼25799, but the same composite nanofibers with the nanograin size 29 nm showed the response ∼4399 for 10 ppm of H 2 S. With the same composition but different grain sizes, there is very much difference in H 2 S response, indicating the grain size is one of key parameters.Previously, the effect of composition in CuO-SnO 2 nanofibers was thoroughly investigated by some of the authors [12].It has been found that the response was the highest at 0.5CuO-0.5SnO 2 composition.In this regard, the composition is also an important parameter.In addition, the surface structure can also influence the sensing properties

Journal of Sensors
CuO-SnO 2 composite nanofiber This leads to the destruction of the resistive p-CuO-n-SnO 2 junctions to metallic-CuS-n-SnO 2 contact and facilitates the flow of electrons from metallic-CuS to n-SnO 2 .The whole phenomenon leads to a dramatic decrease in the resistance of the composite nanofibers.Upon the stoppage of the H 2 S supply, the air molecules again adsorb on the metallic-CuS and transform it back to a p-CuO phase by the following reaction: CuS(s) + 3/2O 2 (g) → CuO(s) + SO 2 (g).In this way, it acquires its original p-n band configuration.Figure 6 shows a schematic diagram of the sensing mechanism.According to Figure 5(b), the response of the CuO-SnO 2 composite nanofibers decreases sharply with increasing nanograin size.Based on the above discussion, the composite nanofibers with small nanograins consist of a large number of p-n junctions, which results in a larger change in resistance during the adsorption and desorption of H 2 S molecules.This shows that composite nanofibers comprised of small-sizes nanograins are needed to obtain superior sensing properties.

Conclusion
This study examined the effects of the nanograin size on the sensing abilities of CuO-SnO 2 composite nanofibers towards H 2 S gas.The CuO-SnO 2 composite nanofibers were prepared by a sol-gel based electrospinning process.The nanograin size in CuO-SnO 2 composite nanofibers was changed by calcination for different time intervals under the isothermal condition.The CuO-SnO 2 composite nanofibers with smallsized nanograins were highly sensitive towards H 2 S gas compared to those comprised of larger nanograins.The stronger response of the CuO-SnO 2 composite nanofibers was explained based on the number of p-n junctions between the p-CuO and n-SnO 2 nanograins and higher resistance modulation during the transformation of p-CuO-n-SnO 2 to metallic-CuS-n-SnO 2 in the presence of H 2 S and vice versa.The results show that optimization of the nanograin size is a vital parameter for achieving superior sensing properties and can be controlled by changing the calcination temperature and time.

Figure 1 :
Figure 1: (a) Schematic diagram of the synthesis of CuO-SnO 2 composite nanofibers.Typical FE-SEM images of (b), (c) as-spun and (d) CuO-SnO 2 composite nanofibers synthesized by an electrospinning method.(d) Thermal profile obtained from TGA for the as-spun fibers.

Figure 2 :Figure 3 :
Figure 2: FE-SEM images of CuO-SnO 2 composite nanofibers calcined at 600 ∘ C for (a) 0.5, (b) 12, and (c) 48 h.(d) Summary of the grain size as a function of the calcination time.

Figure 4 :Figure 5 :
Figure 4: (a) Dynamic gas response curves of the CuO-SnO 2 composite nanofibers with nanograins of different sizes.

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[11]matic diagrams of the H 2 S-sensing mechanism operated in the CuO-SnO 2 composite nanofibers.andfurtherinvestigationsarerequired to clarify this.The response behavior of the CuO-SnO 2 composite nanofibers as a function of the nanograin size can be understood based on the formation of a number of p-n junctions among the neighboring CuO and SnO 2 nanograins.Previous transmission electron microscopy investigations of CuO-SnO 2 composite nanofibers confirmed that the individual CuO-SnO 2 composite nanofiber was comprised of CuO and SnO 2 nanograins[11].Figure5(c) summarizes the initial resistance of the CuO-SnO 2 composite nanofibers with respect to their grain size.The resistance of composite nanofibers decreases with increasing nanograin size.This suggests that the composite nanofibers with small-sized nanograins are likely to contain a large number of p-n junctions.Initially, the transfer of holes and electrons occurs between the CuO and SnO 2 nanograins and establishes a state of equilibrium.This results in band bending at the conduction band edge of the CuO and SnO 2 and forms a highly depleted region at the interface, which restricts the flow of electrons.The charge carriers will encounter a series of resistive p-n junctions in the composite nanofibers and increase the overall resistance.The CuO-SnO 2 composite nanofibers with smaller nanograins will have a larger number of p-n junctions, resulting in a larger resistance compared to the nanofibers composed of larger nanograins.
[2]se resistive p-n junctions contribute towards resistance modulation in the composite nanofibers during the absorption and desorption of gas molecules.The sensing mechanism that operates in the CuO-SnO 2 composite nanofibers is described as follows.When CuO-SnO 2 composite nanofibers are exposed to H 2 S, the p-CuO is likely to transform to a metallic-CuS phase.The transformation of p-CuO to metallic-CuS is well described in the literature[2].The transformation occurs according to the following reaction between CuO and H 2 S molecules: CuO(s) + H 2 S(g) → CuS(s) + H 2 O(g).