Accepted Manuscript Title: Highly selective detection of NH3 and H2 S using the pristine CuO and mesoporous In2 O3 @CuO multijunctions nanofibers at room temperature Authors: Jiao Zhou, Muhammad Ikram, Afrasiab Ur Rehman, Jing Wang, Yiming Zhao, Kan Kan, WeiJun Zhang, Fazal Raziq, Li Li, Keying Shi PII: DOI: Reference:
S0925-4005(17)31637-4 http://dx.doi.org/10.1016/j.snb.2017.08.200 SNB 23067
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4-5-2017 22-7-2017 29-8-2017
Please cite this article as: Jiao Zhou, Muhammad Ikram, Afrasiab Ur Rehman, Jing Wang, Yiming Zhao, Kan Kan, WeiJun Zhang, Fazal Raziq, Li Li, Keying Shi, Highly selective detection of NH3 and H2S using the pristine CuO and mesoporous In2O3@CuO multijunctions nanofibers at room temperature, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.08.200 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly selective detection of NH3 and H2S using the pristine CuO and mesoporous In2O3@CuO multijunctions nanofibers at room temperature Jiao Zhoua, Muhammad Ikrama, Afrasiab Ur Rehmana, Jing Wangd, Yiming Zhaoa, Kan Kanc,d, WeiJun Zhangd, Fazal Raziqa, Li Li*a,b, Keying Shi*a
a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
School of Chemistry and Material Science, Heilongjiang University, Harbin, 150080, P. R. China. b
Key Laboratory of Chemical Engineering Process＆Technology for High-efficiency
Conversion, Heilongjiang University, Harbin, 150080, P. R. China. c
Daqing Branch, Heilongjiang Academy of Sciences, Daqing 163319, P. R. China.
d
Institute of Advanced Technology, Heilongjiang Academy of Science, Harbin,
150080, P. R. China. * Corresponding author E-mail:
[email protected]; 1993036@ hlju.edu.cn Fax: +86 4518667 3647; Tel: +86 451 8660 9141
Graphical Abstract
The multijunctions pristine CuO and In2O3@CuO nanofibers have been synthesized using an electrospinning process. The pristine CuO had many p-p homojunctions and lots of chemisorbed oxygen, which displayed excellent gas sensing property towards H2S at room temperature (RT). The mesoporous In2O3@CuO NFs contained many heterojunctions, homojunctions, defects and pores, displayed excellent sensing performance to NH3 at RT. Therefore, 1D the pristine CuO and In2O3@CuO NFs sensors were promising candidates of practical detectors to H2S and NH3 at RT.
Highlights:
The multijunctions were created within the 1D pristine CuO and In2O3@CuO nanofibers by using an electrospinning and thermal treatment.
The pristine CuO nanofibers displayed excellent gas sensing property towards H2S at room temperature.
The detection limit and selectivity of the In2O3@CuO sensor for NH3 have been significantly improved after adding In2O3.
Abstract: To further develop sensing materials for the detection of reducing gases like NH3 and H2S and improve their response efficiency and selectivity, mesoporous In2O3@CuO composite multijunctions nanofibers (ICCNs) was synthesized by an electrospinning approach with a subsequent thermal treatment. Comparison was made with the one-dimensional (1D) pristine CuO multijunctions nanofibers (NFs). It was found that the CuO NFs have many p-p homojunctions and lots of chemisorbed oxygen, allowing for excellent gas sensing behaviors towards H2S at room temperature (RT). Interestingly, the ICCNs’ sensors favor to detect NH3 gas. Amongst them, ICCN-5 (short for the sensor composite where the molar ratio of Cu: In is 100: 5) behaves very fast response, excellent selectivity and good stability (within 30 days) toward 10 ppm NH3 at RT. The significantly enhanced sensing property of ICCNs to NH3 could be attributed to the synergistic effect of In2O3 promoter, multijunctions and unique mesoporous structure of NFs. Our studies demonstrated that 1D-CuO and ICCN-5 sensors were promising candidates of practical detectors to H2S and NH3 at RT. Keywords: electrospinning; nanofibers; multijunctions; selectivity; NH3/H2S gas
sensing 1. Introduction Poisonous gases like H2S and NH3 are capable of irritating the respiratory system of human skin and eyes, which could lead to vomiting, headaches, pneumonedema and even death [1-3]. So, the demand for a fast and high-sensitive poisonous gas monitors was highly desirable across many industrial, health and security areas. Metal oxide semiconductors (MOS) are one of the most promising gas sensing materials to different toxic gases, owing to their low cost, short responding time, stable, and nonhazardous nature [4-6]. Among a large number of MOS gas sensing materials investigated, cupric oxide (CuO) shows outstanding gas sensing properties. It is well known that CuO is a p-type semiconductor with a narrow band gap of ~1.2 eV, which displays high catalytic activity, good electrochemical performance, low-cost and excellent sensing response to some reducing and oxidizing gases [7-11]. Nowadays, many studies have been reported on the gas sensing characteristics of CuO nanomaterials towards NH3 and H2S. Singh et al. [12] reported that the CuO film sensor exhibited the maximum response of 92% for 50 ppm NH3 at RT. The sensing properties of CuO microstructures studied by Arrak and co-workers [13] displayed the response of 21.9% to 1055 ppm NH3 at 350 oC. The p-type CuO (111) nano cuboidsbased H2S gas sensor [3] was reported to give a maximum response of ~93% for 2 ppm H2S at 200 oC. And the Cu-doped SnO2 porous film [14] showed a good response of 1.25 with respond times of 234 s when being tested with the H2S concentrations as low as 10 ppb. Although these sensors somehow possessed
improved properties of gas sensor, high operating temperature and long response time still limit their applications as NH3 and H2S sensors. So, it is essential to investigating low-cost and ambient operation gas sensors with excellent selective and fast response. In our previous studies, the hierarchical CuO microspheres and Cu/CuxO nanoarchitectures were synthesized by a hydrothermal method using surfactants for NOx detection at RT [15, 16]. However, their selective property was very low to reducing gases such NH3 and H2S. It has been established that the gas sensing selectivity of the MOS nanofibers’ sensing materials could be improved in two ways. 1) Noble metals (Au, Pd, and so on)-decorated 1D nanomaterials were expected to improve the selectivity of gas sensing [17]. Nevertheless, the optimal performance was not achieved due to a complicated preparation process. Meanwhile, it was difficultly commercialized because of its high cost in practical applications. 2) Reducing gases can catalytically react with oxygen adsorbates, which allows the free electrons back to the sensor surface and thus is detected by the change in the sensor conductivity [18]. But, the surface of CuO, from the structural perspective, possesses a small coverage of chemisorbed oxygen at RT. Indium oxide (In2O3) as a remarkable n-type additive could provide high oxygen vacancies/defects taken as shallow donors [19], which help to provide electrons to O2 in air, and easily form oxygen adsorbates at RT. Furthermore, the nanofibers composition control in electrospinning process was relatively easy to present the design of a selective gas sensor. Therefore, to further develop this versatile approach to 1D CuO nanomaterials and achieve high response and good selectivity gas sensor towards reducing gases NH3 and H2S, 1D
CuO semiconducting sensors were fabricated by adding In2O3 as dopants to control the selectivity. This synthetic idea was displayed in Scheme 1. In this paper, uniform 1D CuO multijunctions NFs and their In2O3-modified derivatives have been successfully synthesized by a simple, one-step electrospinning approach. The interactions between domethyl formamide (DMF) and metal ions (Cu2+ or In3+) acted as a cross linking point or a bridge among the polyvinyl pyrrolidone (PVP) chains. After that, the 1D CuO and mesoporous In2O3@CuO multijunctions NFs were formed by annealing at 600 oC. The multijunctions CuO NFs had many p-p homojunctions and lots of chemisorbed oxygen, which brought about excellent gas sensing property towards H2S at RT. Nevertheless, the ICCN-5 sensor (the molar ratio of Cu: In at 100: 5), containing many heterojunctions, homojunctions, pores and defects, presented excellent sensing performance to NH3 at RT. More importantly, the selective detection to NH3 was optimized but minor sensitivity to H2S at RT. The simple synthetic method would make the preparation of semiconductor sensors more effective in the commercial gas-sensing applications.
2. Experimental 2.1 Sample preparation. The precursor solution was prepared by dissolving 2.0 g PVP (MW = 1 300 000, Aldrich) in 20 mL of ethanol and DMF. Then, 0.5 g Cu(CH3COO)2·H2O was added to the precursor solution. Different amounts of In(NO3)3·4.5H2O (0.029 g, 0.047 g and 0.067 g) were added in the precursor solution to obtain the molar ratios of Cu: In at 100:
3; 100: 5; 100: 7, respectively. The mixed solution was stirred vigorously at RT for 6 h. After that, the transparent solution was transferred into a 10 mL syringe with a capillary tip (0.8 mm in diameter) for spinning. A high voltage of 18 kV was applied at the spinneret by a direct-current power supply (DFS-001, Beijing KAIWEIXIN Technology Ltd, China). The solution was pushed out of the spinneret with a feeding rate of 0.025 mL·min-1, the In2O3@CuO as-spun nanofibers (NFs) were synthesized by electrospinning using the spinning equipment as shown in Supporting Information Fig. S1, and In2O3@CuO as-spun NFs were collected using aluminium foil at RT. After spinning, treated at 600 °C for 4 h in air with a heating rate of 3 oC·min−1. Then, the obtained ICCNs with different molar ratios of Cu: In at 100: 3, 100: 5, 100:7 were marked as ICCN-3, ICCN-5 and ICCN-7, respectively. For comparison, the pristine CuO without In2O3 was synthesized as well. 2.2 Material characterizations The crystalline structures of the samples was identified by X-ray powder diffraction (XRD, D/max-IIIB-40 KV, Japan, Cu Kα radiation, λ = 1.5406 Å). Raman spectra were recorded on a Jobin Yvon HR 800 micro-Raman spectrometer at RT. The morphologies and structures were characterized by scanning electron microscope (SEM, HITACHIS-4800) and transmission electron microscopy (TEM, JEOL-2100), respectively. The Brunauer-Emmett-Teller (BET) surface area of the samples was measured by N2 adsorption-desorption (TriStar II3020). Thermo gravimetricdifferential scanning calorimetry (TG-DSC) analysis was performed by using a TASDTQ600. X-Ray photoelectron spectra (XPS) were recorded with an AXIS ULPRA
DLD (Shimadzu Corporation) system equipped to analyze the decomposition temperature of the samples. 2.3 Preparation of film sensors 1 mg ICCNs was evenly dispersed in 1.0 mL ethanol to obtain a suspension. Then, 0.05 mL suspension was dropped onto the inter-digital Au electrode (7×5×0.254 mm, 99.6 %) and then dried at 60 oC for 5 h to obtain ICCNs film sensors. Each Au electrode contained 100 fingers which interleaved each other, and the distance between two fingers was 20 µm. The parameters and SEM image of a crossed finger gold electrode was shown in Supporting Information of Fig. S2. 2.4 Gas sensing tests The sensor was installed into a test chamber with an inlet and an outlet. The electrical resistance measurements of the sensor were carried out at RT and the relative humidity (RH) was controlled by an air moistener to achieve the desired humidity (24%). Then, the NH3/H2S gas was injected into the chamber using a micro syringe and then sensor response was measured. The NH3/H2S gas concentration from high to low was 100, 50, 30, 10, 5, 3, 1, 0.5 and 0.3 ppm, respectively. The sensor response (R) was defined as the ratio (Rg-Ra)/Ra, where Ra and Rg were the resistances measured in air and NH3/H2S gas atmosphere, respectively. 3. Results and discussion 3.1 Structural and morphological characteristics
Thermal analysis of the as-spun nanofibers was conducted using TG/DSC, which was performed to determine the decomposition temperature and a suitable annealing temperature of the as-spun nanofibers. TG/DSC characterizations for electrospinning precursors could be seen in Fig.1. For pure PVP, the process of weight losses was divided into three stages. The first weight loss (ca. 5%) below 140 oC was mainly attributed to the removal of the absorbed water. The second stage of weight losses (ca. 5%) was caused by the cracking of five-membered rings or the decomposition of PVP side chains (140~400 o
C). The final one (ca. 72%) from 400 oC to 650 oC was attributed to the
decomposition of PVP main chains which decomposed into gases (CO2, H2O and NO2) with giving off great heat [20, 21]. For the pristine CuO and ICCN-5 as-spun nanofibers (Fig. S3), the process of weight losses also included three stages. The first stage is below 100 oC (ca.9% in Fig.1b, or ca.13% in Fig.1c), mainly being attributed to the evaporation of solvents (ethanol and DMF). Similarly, the second stage with ca. 34% or 31% weight losses in the range of 100-350 oC may be due to the decomposition of PVP side chains. Subsequently, the last stage of weight losses ca. 48% or 49% from 350 oC to 450 oC was ascribed to the decomposition of PVP main chains and the inorganic salts (Cu(CH3COOH)2 or In(NO3)2) converted into metal oxides [22, 23], corresponding to an exothermic peak at about 394 (or 405) oC. This demonstrated the strong interaction between PVP and Cu(CH3COOH)2 or In(NO3)2), resulting in a decreased temperature of PVP decomposition. As the temperature went
up to 500 oC, the weight of the whole system kept unchanged. This suggested that the annealing temperature selected at 600 oC was reasonable. Additionally, four exothermic peaks in pristine CuO and ICCN-5 as-spun NFs were observed from 200 oC to 450 oC in Fig.1 (b, c), being at 218 (220), 280 (276), 314 (318) and 394 (405) oC. The exothermic peaks of In2O3@CuO as-spun fibers shifted to higher temperature compared with pristine CuO, which was mainly due to the introduction of In2O3 [24]. The CuO nanoparticles (Cu2+ ionic radius with 73 pm) doped by In3+ (ionic radius with 80 pm) tended to aggregate and might affect the diameter of the material during thermal treatment. Although In2O3@CuO composites were doped by low In2O3 (the molar ratios of Cu: In with 100: 3, 100: 5, 100: 7 respectively), here the In3+ had an obvious effect on the separation to prevent CuO aggregation and formation of large size of CuO particles [25]. To study the structural features of samples, XRD diffraction patterns of samples were characterized in Fig. 2a. It was illustrated that the diffraction main peaks at 2θ values of 32.2°, 35.4°, 38.3°, 49.3° and 61.7° were assigned to CuO (110), (-111), (111), (-202) and (-113) planes with d spacing of 2.78, 2.53, 2.35, 1.85 and 1.50 Ǻ (JCPDS card No. 89-2531), respectively. This indicated the formation of the monoclinic of CuO. And one can see that the weak peak around 30.59° was (222) facet of the In2O3 (JCPDS card NO. 71-2194). Obviously, the In2O3 peak was weaker than those of CuO. This case may be the presence of small quantities of In2O3. There was no peak shift and no new phases of samples with an increasing amount of In2O3 for the ICCNs (the ion radius of Cu2+ and In3+ are 73 pm and 80 pm, during
calcination, if some lattice structure of Cu2+ ions was substituted by In3+ ions without obvious swelling) [25]. Additionally, the XRD measurement of ICCNs was obviously observed that with In2O3 content increasing, the intensity of the CuO peaks gradually decreased. The size of the CuO particles was influenced by the presence of In2O3 in the nanofibers. This had been proved by SEM images (Fig. S4 (a, b)). Fig. 2b exhibited Raman spectra of the pristine CuO and ICCN-5. The Raman spectra revealed three main phonon modes at 282, 325 and 613 cm-1 (pristine CuO), corresponding to the Ag, Bg1 and Bg2 symmetries, respectively [26-27]. These vibrations were believed to originate from the stretching of Cu-O. The Raman spectral vibrations of ICCN-5 at 123, 282, 327, 610 and 1093 cm-1 indicated the laser radiation in Cu-In-O activation. Here, the two-phonon scattering (327 cm-1 (Bg1) and 610 cm-1 (2Bg2)) of CuO was added to verify the Cu-In-O, through the corresponding vibration at 1093 cm-1. Similarly, the prevailing In2O3 characteristics in the ICCN-5 sample were also inferred through the peak centered at 123 cm-1, corresponding to the In-O vibration and the basic structural units derived from the InO6 [27]. The N2 adsorption-desorption isotherms and the BET pore-size distribution of pristine CuO and ICCN-5 were measured to determine the specific surface areas of the samples and the corresponding results were presented in Fig. 2(c, d). It could be seen that the adsorption-desorption isotherms of the samples showed type IV curves, and the BET specific surface areas of pristine CuO and ICCN-5 were approximately 27.6 and 48.7 m2·g-1. Owing to In2O3 doped into CuO NFs, the BET surface areas of ICCN-5 slightly increased compared with pristine CuO. The isotherms of ICCN-5
also indicated a large quantity of adsorbed N2, implying an existence of large fraction of mesoporous in the sample. In addition, the illustration of Fig. 2d showed the pore size distribution curve. According to the pore size distribution curve, the pristine CuO and ICCN-5 contained a large number of mesopores and the size of dominated pores was mainly between 1.9 and 18.2 (22.9) nm. The mesoporous structure may be due to the accumulation of In2O3 and CuO nanoparticles (NPs). Morphology of the samples was examined by SEM and TEM. From Fig. S4, one could see that the pristine CuO and ICCN-5 presented 1D multijunctions structure. Furthermore, TEM/HRTEM images of CuO NFs and CuO-CuO homojunctions in CuO NFs were shown in Fig. 3. It could be clearly seen that the pristine CuO NFs were composed of many CuO NPs (Fig. 3d) that modified the surface of big single crystal CuO particles (Fig. 3e) whose size was 80~100 nm. And Fig. 3a presented a representative TEM image of the pristine CuO. The lattice fringes of the pristine CuO with d spacing of 0.235, 0.253 and 0.278 nm corresponded to the (111), (-111) and (110) planes of CuO, respectively (as shown in Fig. 3(b-f)). Some of small CuO NPs were highly dispersed and anchored on single crystal CuO particles and CuO NFs. Moreover, many defects and homojunctions existed in the interfaces between CuO NPs and big single crystal CuO particles. Fig. 4a presented a representative TEM image of the ICCN-5, which clearly displayed lots of mesopores with the approximate size of several nanometers among the nanoparticles due to the removal and decomposition of PVP. It could greatly improve the specific surface areas of the ICCN-5 (48.7 m2·g-1) [28]. It could be seen
that the ICCN-5 possessed more mesoporous structures (Fig. 4e). This special nanoporous structure was possibly advantageous to the gas sensing properties [25]. To further investigate the structure of the ICCN-5, HRTEM images had been obtained. The interplanar spacing of ICCN-5 was measured to be 0.185, 0.235, 0.253 and 0.278 nm, which corresponded to the d spacing of the (-202), (111), (-111) and (110) planes of the monoclinic-phase CuO, respectively. And the interplanar spacing of ICCN-5 was measured to be 0.292 (or 0.295) nm, showing the d spacing of the (222) plane of cubic In2O3. The HRTEM image showed that the CuO NPs were uniformly distributed around the In2O3 (Fig. 4(c, d)). Connection with In2O3 formed a lot of p-p homojunctions and p-n heterojunctions (see in Fig. 4(b-d)), which might be produced by the oxygen vacancies/defects (proved by Fig. 5 and Table S1). The polycrystalline diffraction rings from the outside to inside corresponded to the (-113), (-202), (111), (-111), (110) planes of CuO, and (222) plane of In2O3 (Fig. S5), which agreed with aforementioned XRD results. Moreover, HRTEM images of ICCN-3 and ICCN-7 in which (110), (-111), (-202) and (111) planes of CuO, (222) planes of In2O3 could be observed in Fig. S6.
3.2 Formation mechanism of the In2O3@CuO composites NFs (ICCNs). The formation mechanism of the ICCNs was proposed in Scheme 2. The precursor solution was prepared by dissolving PVP (MW = 1,300,000) powder in 20 mL ethanol and DMF. Then Cu(CH3COO)2·H2O was added to the precursor solution to obtain a transparent precursor solution. For PVP-DMF solution, the carbonyl group of the PVP and the formamide of DMF might form a hydrogen bond. As a result,
DMF molecules could be used as cross-linking points among the entangled PVP chains. When being added into the mixed solution of ethanol and DMF, Cu(CH3COO)2·H2O was hydrolyzed to produce Cu2+, which was first preferentially solvated by DMF solvent molecules, and then electrostatically attracted with the carbonyl groups in PVP chains (Scheme 2a) [29, 30]. After that, a small amount of In(NO3)3 was sequentially added to mixed solution. In3+ ions and NO3- ions were also interacting with the active sites (-C=O) of PVP-DMF solution (Scheme 2a) [31]. After spinning, the long nanofibers were obtained (Fig. S2) [32], and the porous In2O3@CuO NFs were formed in the absence of PVP by annealing at 600 ℃ for 4 h in air with a heating rate of 3 oC/min. The In2O3 and CuO NPs were connected each other (Scheme 2b). 3.3 XPS analysis of In2O3@CuO composites NFs In general, the chemical adsorption oxygen and oxygen defect/vacancy were the most important factors that affect the gas-sensing performance, which commonly acted as electron donors. In order to further investigate the response feasibility to gas detection, the pristine CuO and ICCNs were further studied by X-ray photo-electron spectroscopy (XPS) analysis. Fig. S7 showed the Cu2p XPS spectrum, where two peaks at the binding energies of 932.6/932.9 and 952.7/952.6 eV corresponded to Cu 2p3/2 and Cu 2p1/2 of pristine CuO/ICCN-5 [33], respectively. And the value around 19.8 eV indicated the presence of the spin-orbit characteristics of Cu2+ ions [34, 35]. As for O1s in Fig. 5 (a-d), the O1s spectra of four samples were divided into three peaks (Oa, Ob, Oc) by using Gaussian fitting. The Oa peak at 529.1(~529.8) eV was attributed to lattice
oxygen, the Ob peak at about 530.0(~530.7) was generally due to oxide defects/vacancy states and the Oc peak at 531.3(~531.9) eV was ascribed to chemisorbed oxygen. The relative quantitative analysis of each peak was listed in Table S1. It could be seen that the calculated oxygen defect/vacancy (Ob peaks area) of the four samples were about 11.4%, 22.6%, 45.4% and 28.3%, respectively. Obviously, the peak area of oxygen defect/vacancy of the ICCN-5 sample was the largest. In addition, the peak area ratio (%) of adsorbed oxygen (O2-) of the pristine CuO NFs was the largest, and the peak area of adsorbed oxygen (O2-) of the ICCN-5 was more than those of the ICCN-3 and ICCN-7 samples. Hence, both the pristine CuO NFs and ICCN-5 possess lots of chemisorbed oxygen, pores and oxygen defects/vacancy, which made these samples easily adsorb gas like NH3 and/or H2S and then improved the sensing performance. 3.4 Gas sensing performance For comparison, sensors based on as-synthesized pristine CuO and ICCNs were fabricated. And the base resistance of ICCNs sensing NH3 decreased as the Cu: In ratio (proved by MS and EIS, Fig. S8 and Table S2) increased. Fig. 6a and 6b showed the response and response time of pristine CuO and ICCNs sensors to different concentrations of NH3 at RT. The response of all samples exhibited a decrease with the reduction of NH3 concentrations. Notably, the ICCN-5 sensor could reach 0.3 ppm in the minimum detection concentration, yielding the highest responsiveness and faster response in the wide range of NH3 concentrations. As shown in Table S3, the responses of the four sensors (the pure CuO, ICCN-3, ICCN-5 and ICCN-7) to 100 ppm NH3 were 0.242, 0.77, 1.57 and 0.84, respectively. The ICCN-5 sensor behaves
1.9 times higher response than other three sensors (Fig. S9 and Table S3). Most importantly, the ICCN-5 sensor had the shortest response time (2s) to 100 ppm NH3. Moreover, the response time of the ICCN-5 sensor always retained within 8 s for all of the NH3 concentrations ranging from 0.3 to 100 ppm. Comparison of ICCN-5 sensor for ammonia sensing properties with reported results in literatures was summarized in Table S4. Obviously, the ICCN-5 sensor displayed better performance than previously reported ones did. As shown in Fig. 7a, a stable response of the sensing signal was observed around 1.57. The ICCN-5 sensor exhibited a reversible response signals to NH3 gas both in the adsorption and desorption process, even being repeated for eight successive cycles to 100 ppm NH3. In addition, the sensing stability of the ICCN-5 to 10 ppm NH3 at RT was shown in Fig. 7b; the response and response time kept unchanged for almost 30 days at RT, indicating that ICCN-5 sensor had a good stability. Fig. 7c was the most favorable linearity relationship over the whole range of 0.3-100 ppm NH3. It revealed that the calibration curve of ICCN-5 sensor have the prominent linearity relationship plotted by logS against logC, and R2 could reach 0.987. The selectivity of gas sensor was an important aspect to evaluate gas-sensing properties in practice applications. So, the response to some gas such as H2S, H2, CO and CH4 at RT was also measured. Fig. 7d illustrated that the ICCN-5 sample exhibited higher selectivity for NH3 and the response toward NH3 could get to 1.57, H2S to 0.59 and H2 to 0.09, while no response to CH4 and CO. And the ICCN-5 sensor had a faster response time (only 2 s) to 100 ppm NH3 than the other gases. In this work, In2O3 played a
synergistic role in improving the selectivity of the sensor as observed. The p-n heterojunction formed between n-type In2O3 and p-type CuO could possibly enhance the sensitivity of NH3 with the unique lone-pair electrons of nitrogen [2]. That was the reason why the sensor had a fast and highly selective response to NH3. Also, that NH3 possessed a low dissociation energy [3] was another reason to explain the increase in selectivity of gas sensitivity. The pristine CuO usually showed some good response to H2S. For further determination that In2O3 as an additive control the selectivity of the In2O3@CuO semiconducting sensors, the H2S gas sensing properties of the 1D pristine CuO and ICCN-5 samples were also investigated.
Fig. 8 exhibited the gas-sensing performance of the two sensors for H2S at RT. It could be seen the pristine CuO sensor had a higher gas sensing response and faster response time than the ICCN-5 toward the same concentration of H2S. For the pristine CuO and ICCN-5 sensor, a comparison of response and response time were summarized in Table S5-1. When the concentration of H2S was 100 ppm, the response time was only 4.3 s, and the highest sensor response could reach to 2.23, which was 3.8 times higher than that of ICCN-5. Fig. 8(b, c) given the response results of the pristine CuO NFs sensor exposed to different H2S concentrations at RT (The sensor curves and response of the ICCNs were also investigated as shown in Fig. S10 and Table S5-2). It could be clearly seen that the gas response of the sensors based on pristine CuO was considerably better than that of the ICCN-5 sensor. In addition, a comparison for the H2S sensor responses in present work and previous
reports were summarized in Table S6. It could be concluded that the pristine CuO sensor had a higher gas sensing response and faster response time for H2S at RT. From above analyses, it could be speculated that sensitization effect of In2O3 NPs on the surface of CuO NFs greatly enhanced gas selectivity performance by means of promoting the interaction between the surface of sensing materials and gas molecules. It is also worth mentioning that the response of the pristine CuO sample to H2S was higher than that of ICCN-5 at RT. Thus, these two types of sensors could be chosen to detect toxic gases (H2S, NH3) appropriately. 3.5 Gas sensing mechanism discussions Building on the materials’ structure and composition as well as characterizations mentioned above, the sensing mechanisms that mesoporous In2O3@CuO composite NFs detect NH3 and the pristine CuO NFs monitor H2S were proposed. The ICCN-5 possessed a large specific surface, abundant mesopores, and lots of oxygen defects/vacancy and chemisorbed oxygen, which could adsorb NH3 gas easily. The HRTEM image (Scheme 3(a)) showed that the CuO NPs was connected with In2O3 NPs, forming a lot of junctions, such as CuO-In2O3 heterojunctions, In2O3-In2O3, CuO-CuO homojunctions (Scheme 3 (a-b)) and constructing mesopores in the ICCN5 sample. The NH3 sensing properties of ICCN-5 were greatly enhanced due to the addition of n-type semiconductor In2O3, that created multijunctions. It has been known that the energy band gap (Eg) of In2O3 is 3.2 eV [25] and that of CuO 1.4 eV [36]. When In2O3 NPs was incorporated into CuO NFs, electrons would transfer from
CB of CuO to that of In2O3, while the holes were to immigrate in the opposite direction from VB of In2O3 to that of CuO. In this manner, the energy band bends to hole-accumulation layer (HAL) and the Fermi level (Ef) becomes uniform. This process leaded to the formation of p-n heterojunctions in NFs, which was responsible for the enhancement of gas sensing [36]. Regarding the ICCN-5 sensor exposing to air (Scheme 3 (b)), the NFs surfaces readily adsorb the oxygen molecules that would capture electrons from the NFs’ CB and form the adsorbed oxygen (O2- ) at RT (see Eqs (1) and (2)) [37]. Since a p-type semiconductor has hole carriers, the oxygen adsorption (O2-) produces an increase of hole density in the valence band. Consequently, the HAL was formed because of the loss of electrons as shown in Scheme 3 (b). However, once the materials were exposed to reducing gas of NH3, NH3 would contact with O2- and react to form NO. The resulting gases were absorbed onto the surface of the ICCN-5 and released electrons back to the surface of the sample (Eqs (3)) [38]. O2(gas) O2(ads)
(1)
O2(ads) e O 2 (ads)
(2)
4NH3(ads) 5O2 (ads) 4NO 6H 2O 5e (3) At this stage, those released electrons would return to the conduction band of the materials, which finally resulted in a thin HAL (a decrease in the number of holes carriers in ICCN-5 and an increase in resistance Rg [19] of the sensor). That is, the
response increases according to the defined (Rg-Ra)/Ra of response. Thereby, the ICCN-5 obviously caused a high response. Furthermore, the HRTEM image (Scheme 3(c)) showed that the pristine CuO NFs samples were composed of big p-type CuO particles and small p-type CuO NPs on the surface. The samples had many p-p homojunctions and lots of chemisorbed oxygen, which could adsorb and react with H2S gas. When the pristine CuO sensors were detecting H2S (Scheme 3 (d)), CuO reacted with H2S and then transformed CuS which was a metallic material with a good electrical conductivity [3, 14]. See reaction as follows:
H 2S(g) Cu O(s) CuS 6H 2O(g)
(4)
In this process, CuS would increase the connectivity between adjacent CuO NPs and reduce NFs’ resistance. The CuO NFs favored to adsorb H2S gas molecules, thus facilitating the transformation of CuS. The In2O3@CuO NFs (ICCN-5) possessed many p-n heterojunctions. When the ICCN-5 sensor was exposed to H2S, the CuO part could also react with H2S to produce CuS [3]. However, the resistance of In2O3@CuO (ICCN-5) upon exposure to H2S had a downward trend (see in Fig. S8c and Table S2). The response of the ICCN-5 sensor was lower than that of the pristine CuO due to the disruption of the resistive, nanoscale p-n (CuO-In2O3) junction structure could be formed by sulfurization of CuO into highly conducting CuS phase, and CuS phase acted as the sensing mechanism upon exposure to H2S [1]. This suggested that the formation of a conductive CuS was responsible for H2S detection.
After the H2S gas was replaced with dry air, the response would recovery and initiate immediately. On the basis of a schematic illustration of the recovery pathway as shown in Scheme 3(d), the CuS would transform back into CuO in air according to Eqs (5) below:
2CuS(s) 3O2 (s) 2CuO 6SO2 (g) (5) Formation of CuS on the CuO has been verified by the XPS results. First, the pristine CuO was placed in a gas chamber at RT for H2S, and then resulting materials (Sensed materials) were measured by XPS. The pristine CuO without contact with the H2S gas (Raw materials) was also characterized for comparison. Fig. 9 showed the two Cu2p peaks (2p3/2 = 933.0 eV, and 2p1/2 = 952.8 eV) for raw CuO materials [39, 40]. Comparatively, a significant negative shift of Cu 2p3/2 (or Cu 2p1/2) peaks was found for the H2S sensed materials. This could be attributed to the mixture of CuO and CuS, respectively [41, 42]. Furthermore, peaks centered at 161.8 and 163.0 eV (Fig. 9b) were attributed to S 2p3/2 and S 2p1/2 characters, respectively. The current results clearly confirmed the formation of CuS [14, 42, 43]. When refreshed by air once again, the XPS spectrum was found to change back, indicating the transformation from CuS to CuO [14]. As expected, the pristine CuO towards H2S displayed an increase in sensitivity. The main reason resides in the conversion ability from CuO to CuS upon exposure to H2S. Due to the higher metallic character of CuS, an increase in the free charge carrier concentration was recorded. This leaded to a decrease in the overall electrical
resistance of the material. Since the reaction underwent at RT, only a thin layer of copper sulfide was formed on the surface of materials. In lattice structure, some Cu2+ ions were substituted by In3+ ions, and the reaction of the In2O3@CuO composites with H2S was not easy especially at that mild condition like RT. In Fig. S11, we could see that S2p peaks of the composites (ICCN-5) in XPS spectra was weaker than those of the pristine CuO, i.e., suggesting less amount of CuS in former NFs’ materials (Table S7 and Table S8). This could be attributed to the adsorption of less H2S gas. Additionally, the disruption of the resistive nanoscale p-n (CuO-In2O3) junction structure by the sulfurization of CuO into highly conducting CuS, thus facilitating that the response sensitivity was quite low (0.59, See in Table S5-1). Moreover, when the CuO sensor was exposed to NH3, the resistance of the resulting sample also reduced. The measured response sensitivity was much lower (0.242) than that toward H2S. Due to being absorbed onto the CuO nanostructures at RT, water molecules reacted with NH3 to form NH4+ [14, 44]. Therefore, a small amount of NH3 gases were adsorbed by the CuO samples. It has been established that the In2O3@CuO composites possessed a larger specific surface, abundant mesopores, and lots of oxygen defects/vacancy and chemisorbed oxygen. This structural feature firstly helped to adsorb NH3 gas easily and rapidly. And then, the reaction of NH3 and O2- was thus facilitated to form NO. At last, these produced gases were absorbed on the surface of the In2O3@CuO composites. Moreover, the reaction forming NO was very fast. It just took 2 s to accomplish the reaction for the 100 ppm NH3 gas at RT. 4. Conclusions
The pristine CuO and In2O3@CuO multijunctions NFs were synthesized through an electrospinning process. Subsequent heating was used to remove PVP. The multijunctions were created within the 1D composites NFs, being formed on the CuOIn2O3, CuO-CuO and In2O3-In2O3 interfaces. Notably, the response of the ICCN-5 sensor is 1.57 to 100 ppm NH3, and its response time always retains within 8 s for all NH3 concentrations in the range of 0.3-100 ppm; the detection limit and selectivity of this sensor for NH3 is significantly improved upon adding In2O3. Differently, the pristine CuO NFs sensor displays an excellent gas sensing property toward H2S; the response time was only 4.3 s when the H2S concentration is 100 ppm, and the highest sensor response could reach 2.23. The improved sensing properties were attributed to the synergism of a higher specific surface, mesopores, lots of oxygen defects, chemisorbed oxygen and formation of the p-n heterojunctions or p-p homojunctions. In brief, the 1D CuO and In2O3@CuO NFs could become promising materials to fabricate excellent selectivity and high sensing performance on reducing gases (like H2S and HN3) at RT. Our method would be prospectively applied to prepare the pristine CuO and In2O3@CuO NFs sensors due to its merits of facile process and low cost. Acknowledgments This work was supported by the Program for Innovative Research Team in Chinese Universities (IRT1237); the National Natural Science Foundation of China (No.2167010747;21671060);International Cooperation in Science and Technology Projects of China (2014DFR40480);Applied Technology Research and Development Program Foreign Co operation Project of Heilongjiang Province (WB15C101).
References
1. I.S. Hwang, J.K. Choi, S.J. Kim, K.Y. Dong, J.H. Kwon, B.K. Ju, J.H. Lee, Enhanced H2S sensing characteristics of SnO2 nanowires functionalized with CuO, Sens. Actuators B 142 (2009) 105-110. 2. L. Tabrizi, H. Chiniforoshan, High-performance room temperature gas sensor based on gold (III) pincer complex with high sensitivity for NH3, Sens. Actuators B 245 (2017) 815-820. 3. A. Stanoiu, C.E. Simiona, J.M. Calderon-Moreno, P. Osiceanu, M. Floreac, V.S. Teodorescu, S. Somacescu, Sensors based on mesoporous SnO2-CuWO4 with high selective sensitivity to H2S at low operating temperature, J. Hazard. Mater. 331 (2017) 150-160. 4. J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: Overview, Sens. Actuators B 140 (2009) 319-336. 5. J.C. Chen, J.Q. Xu, SnO2-based R134a gas sensor: Sensing materials preparation, gas response and sensing mechanism, Sens. Actuators B 157 (2011) 494-499. 6. M. Sturaro, E.D. Gaspera, N. Michieli, C. Cantalini, S.M. Emamjomeh, M. Guglielmi, A. Martucci, Degenerately doped metal oxide nanocrystals as plasmonic and chemoresistive gas sensors, ACS Appl. Mater. Interfaces 8 (2016) 30440-30448. 7. A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review, Ceram. Int. 42 (2016) 15119-15141. 8. H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide
semiconductors: Overview, Sens. Actuators B 192 (2014) 607-627. 9. Z. Li, X. Zhou, J. Shi, X. Zou, X. Huang, E. Haroon, H. Mel, Fast response ammonia sensor based on porous thin film of polyaniline/sulfonated nickel phthalocyanine composites, Sens. Actuators B 226 (2016) 553-562. 10. Y.L. Tang, Z.J. Li, J.Y. Ma, Y.J. Guo, Y.Q. Fu, X.T. Zu, Ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices, Sens. Actuators B 201 (2014) 114-121. 11. S. Bhuvaneshwari, N. Gopalakrishnan, Enhanced ammonia sensing characteristics of Cr doped CuO nanoboats, J. Alloys Compd. 654 (2016) 202-208. 12. I. Singh, R.K. Bedi, Influence of pH on the synthesis and characterization of CuO powder for thick film room-temperature NH3 gas sensor, J. Mater. Sci. 46 (2011) 5568-5580. 13. K. Arrak, T. Titipun, T. Somchai, Characterization and gas sensing properties of CuO synthesized by DC directly applying voltage, Appl. Surf. Sci. 313 (2014) 640646. 14. Z. Li, N. Wang, Z. Lin, J. Wang, W. Liu, K. Sun, Y.Q. Fu, Z. Wang, Roomtemperature high-performance H2S sensor based on porous CuO nanosheets prepared by hydrothermal method, ACS Appl. Mater. Interfaces 8 (2016) 2096220968. 15. L. Yang, L. Li, Y. Yang, G. Zhang, L.H. Gong, L.Q. Jing, H.G. Fu, K.Y. Shi, Facile synthesis of Cu/CuxO nanoarchitectures with adjustable phase composition for effective NOx gas sensor at room temperature, Mater. Res. Bull. 48 (2013) 3657-
3665. 16. W.Z. Song, H.Y. Wu, J.C. Wang, Y.F. Lin, J.B. Song, Y. Xie, L. Li, K.Y. Shi, Facile synthesis of hierarchical CuO microspheres and their gas sensing properties for NOx at room temperature, Aust. J. Chem. 68 (2015) 1569-1576. 17. J.K. Choi, I.S. Hwang, S.J. Kim, J.S. Park, S.S. Park, U. Jeong, Y.C. Kang, J.H. Lee, Design of selective gas sensors using electrospun Pd-doped SnO2 hollow nanofibers, Sens. Actuators, B 150 (2010) 191-199. 18. M.I. Baraton, L. Merhari, Advances in air quality monitoring via nanotechnology, J. Nanopart. Res. 6 (2004) 107-117. 19. F. Gu, R. Nie, D. Han, Z. Wang, In2O3-graphene nanocomposite based gas sensor for selective detection of NO2 at room temperature, Sens. Actuators, B 219 (2015) 94-99. 20. G. Liu, S. Liu, Q. Lu, H. Sun, F. Xu, G. Zhao, Synthesis of monoclinic BiVO4 microribbons by sol-gel combined with electrospinning process and photocatalytic degradation performances, J. Sol-Gel Sci. Technol. 70 (2014) 24-32. 21. Z. Liu, Q. Lu, C. Wang, J. Liu, G. Liu, Preparation of bamboo-shaped BiVO4 nanofibers by electrospinning method and the enhanced visible-light photocatalytic activity, J. Alloys Compd. 651 (2015) 29-33. 22. J. Cao, B. Luo, H. Lin, B. Xu, S. Chen, Thermodecomposition synthesis of WO3/H2WO4 heterostructures with enhanced visible light photocatalytic properties, Appl. Catal. B: Environ. 111-112 (2012) 288-296. 23. S. Sun, D. Mao, J. Yu, Z. Yang, G. Lua, Z. Ma, Low-temperature CO oxidation on
CuO/CeO2 catalysts: the significant effect of copper precursor and calcination temperature, Catal. Sci. Technol. 5 (2015) 3166-3181. 24. J.K. Sharmaa, P. Srivastavaa, G. Singha, M.S. Akhtar, S. Ameenc, Catalytic thermal decomposition of ammonium perchlorate and combustion of composite solid propellants over green synthesized CuO nanoparticles, Thermochim. Acta 614 (2015) 110-115. 25. S. Xu, J. Gao, L.L. Wang, K. Kan, Y. Xie, P.K. Shen, L. Li, K.Y. Shi, Role of the heterojunctions in In2O3-composite SnO2 nanorod sensors and their remarkable gas-sensing performance for NOx at room temperature, Nanoscale 7 (2015) 1464314651. 26. M.E. Mazhar, G. Faglia, E. Comini, D. Zappa, C. Baratto, G. Sberveglieri, Kelvin probe as an effective tool to develop sensitive p-type CuO gas sensors, Sens. Actuators B 222 (2016) 1257-1263. 27. G.M. Kumar, P. Ilanchezhiyan, A.M. Kumar, T.S. Shabi, S.T. Selvan, S. Suresh, S.U.
Yuldasheva,
T.W.
Kang,
Chemically-derived
CuO/In2O3-based
nanocomposite for diode applications, CrystEngComm 17 (2015) 5932-5939. 28. C. Jiang, S. Xu, G. Zhang, L. Li, Y. Yang, K.Y. Shi, Facile synthesis of CaO-SnO2 nanocrystalline composite rods by electrospinning method with enhanced gas sensitive performance at room temperature, CrystEngComm 15 (2013) 2482-2489. 29. Z. Feng, Z. Wang, C. Gao, J. Shen, Template polymerization to fabricate hydrogenbonded poly(acrylic acid)/poly(vinylpyrrolidone) hollow microcapsules with a pHmediated swelling deswelling property, Chem. Mater. 19 (2007) 4648-4657.
30. C.W. Hao, Y. Zhao, Y. Zhou, L.J. Zhou, Y.Z. Xu, D.J. Wang, D.F. Xu, Interactions between metal chlorides and poly(vinyl pyrrolidone) in concentrated solutions and solid-state films, J. Polym. Sci., Part B: Polym. Phys. 45 (2007) 1589-1598. 31. L.L. Wang, J. Gao, B.F. Wu, K. Kan, S. Xu, Y. Xie, L. Li, K.Y. Shi, Designed synthesis of In2O3 beads@TiO2-In2O3 composite nanofibers for high performance NO2 sensor at room temperature, ACS Appl. Mater. Interfaces 7 (2015) 2715227159. 32. C. Hao, Y. Zhao, X. Dong, Y. Zhou, Y. Xu, D. Wang, G. Lai, J. Jiang, Anomalous rheological behavior of poly (1-vinyl-2-pyrrolidone) and CuCl2 in solution and their interactions in solid composites, PolymInt 58 (2009) 906-911. 33. A. Shanmugavani, R.K. Selvan, Improved electrochemical performances of CuCo2O4/CuO nanocomposites for asymmetric supercapacitors, Electrochim. Acta 188 (2016) 852-862. 34. A. Amri, X. Duanb, C.Y. Yinc, Z. Jianga, M.M. Rahmana, T. Pryor, Solar absorptance of copper-cobalt oxide thin film coatings with nano-size, grain-like morphology: optimization and synchrotron radiation XPS studies, Appl. Surf. Sci. 275 (2013) 127-135 35. Z.Y. Tian, H. Vieker, P.M. Kouotoub, A. Beye, In situ characterization of Cu-Co oxides for catalytic application, In situ characterization of Cu-Co oxides for catalytic application, Faraday Discuss. 177 (2015) 249-262. 36. S. Bai, W. Guo, J. Sun, J. Li, Y. Tian, A. Chena, R. Luo, D. Li, Synthesis of SnO2CuO heterojunction using electrospinning and application in detecting of CO, Sens.
Actuators B 226 (2016) 96-103. 37. L.L. Wang, A.U. Rehman, H.Y. Wu, B.F. Wu, L. Li, K.Y. Shi, Submicrochains composed of massage ball-like WO3@CuWO4 composites for high-efficiency CO gas sensing applications at room temperature, RSC Adv. 6 (2016) 69999-70007. 38. N. Kumar, A.K. Srivastava, R. Nath, B.K. Gupta, G.D. Varma, Probing the highly efficient room temperature ammonia gas sensing properties of a luminescent ZnO nanowire array prepared via an AAO-assisted template route, Dalton Trans. 43 (2014) 5713-5720. 39. A. Amri, X.F. Duan, C.Y. Yin, Z.T. Jiang, M. Rahman, T. Pryor, Solar absorptance of copper-cobalt oxide thin film coatings with nano-size, grain-like morphology: optimization and synchrotron radiation XPS studies, Appl. Surf. Sci. 275 (2013) 127-135. 40. J. Zhou, J.W. Zhang, A.U. Rehman, K. Kan, L. Li, K.Y. Shi, Synthesis, characterization, and ammonia gas sensing properties of Co3O4@CuO nanochains, J. Mater. Sci. 52 (2017) 3757-3770. 41. R.A. Zarate, F. Hevia, S. Fuentes, V.M. Fuenzalida, A. Zuniga, Novel route to synthesize CuO nanoplatelets, J. Solid State Chem. 180 (2007) 1464-1469. 42. P. Liu, Y. Huang, J. Yan, Y.W. Yang, Y. Zhao, Construction of CuS nanoflakes vertically aligned on magnetically decorated graphene and their enhanced microwave absorption properties, ACS Appl. Mater. Interfaces 8 (2016) 55365546. 43. A.D. Amare, G.T. Andebet, H.M. Chen, A.B. Taame, C.J. Pan, W.N. Su, B.J.
Hwang, A highly stable CuS and CuS-Pt modified Cu2O/CuO heterostructure as an efficient photocathode for the hydrogen evolution reaction, J. Mater. Chem. A 4 (2016) 2205-2216.
44. Y.L. Tang, Z.J. Li, X.T. Zu, J.Y. Ma, L. Wang, J. Yang, B. Du, Q.K. Yu, Roomtemperature NH3 gas sensors based on Ag-doped α-Fe2O3/SiO2 composite films with sub-ppm detection ability, J. Hazard. Mater. 2989 (2015) 154-161.
Author Biographies Jiao Zhou has been pursuing her master's degree from Heilongjiang University, China, since 2014. Her research interests are mainly directed towards semiconductor oxide materials and their application as gas sensors. Muhammad Ikram has been pursuing his Ph.D. from Heilongjiang University, China, since 2016. His research interests are mainly directed towards semiconductor oxide materials and heavy metal ions detection. Afrasiab Ur Rehman has been pursuing his Ph.D. from Heilongjiang University, China, since 2015. His research interests are mainly directed towards semiconductor oxide materials and heavy metal ions detection. Jing Wang is researcher of the Institute of Advanced Technology, Heilongjiang Academy of Science. His research interests are mainly directed towards semiconductor oxide materials and their application as gas sensors. Yiming Zhao has been pursuing his master's degree from Heilongjiang University, China, since 2015. His research interests are mainly directed towards semiconductor oxide materials and their application as gas sensors. Kan Kan received her Ph.D. from Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Material Science, Heilongjiang University, China, in 2015. Her research interests are mainly directed towards semiconductor oxide materials and their application as gas sensors.
WeiJun Zhang received his bachelor's degree from Qingdao Institute of Chemical Industry, China, in 1986. He is president of the Institute of Advanced Technology, Heilongjiang Academy of Science. His research interests are mainly directed towards semiconductor oxide materials and their application as gas sensors. Fazal Raziq has been pursuing his Ph.D. from Heilongjiang University, China, since 2014. His research interests are mainly directed towards photocataylsis on metal free g-C3N4 for CO2 reduction, water splitting and pollutants degradation. Li Li received her Ph.D. from Harbin Institute of Technology, China, in 2004. She is a professor of School of Chemistry and Materials Science, Heilongjiang University. Her research interests are mainly directed towards the aspects of functional inorganic material chemistry and organic-inorganic catalytic materials. Keying Shi obtained her Ph.D. from Harbin Institute of Technology, China, in 2001. She is a professor of School of Chemistry and Materials Science, Heilongjiang University. Her research interests are mainly directed towards the development of nano-materials and organic-inorganic hybrid materials and their application as gas sensors.
Scheme captions
Scheme 3 (a), (c) HRTEM image of multijunctions ICCN-5 and the pristine CuO; (b), (d) Mechanism of the ICCN-5 and the pristine CuO sensors upon exposure to air and NH3 or H2S
Figure captions Fig.1 TG-DSC curves of (a) PVP, the pristine CuO and ICCN-5 (b, c) as-spun fibers with a temperature ramp of 8 °C/min under air
Fig. 2 (a) XRD diffraction patterns of the pristine CuO and ICCNs; (b) Raman spectra and (c, d) N2 adsorption-desorption isotherms and pore-size distribution curves (the inset) of pristine CuO and ICCN-5 Fig. 3 (a1, a) TEM image of the pristine CuO multijunctions NFs; (b-f) HRTEM images of (a) Fig. 4 (a) TEM image and enlarged TEM images of the ICCN-5; (b-d) HRTEM images of (a) Fig. 5 (a-d) O1s XPS spectra of the pristine CuO multijunctions NFs, ICCN-3, ICCNC-5 and ICCN-7, respectively Fig. 6 The results of the gas response of pristine CuO and ICCNs thin film sensors to NH3 at RT. (a) Gas response and (b) response time for samples; (c) Dynamical response transient of ICCN-5; (d) Response and response time of ICCN-5 to 0.3-100 ppm NH3 Fig. 7 (a) The reproducibility of the ICCN-5 sensor on successive exposure (8 cycles) to 100 ppm NH3; (b) The response of the ICCN-5 for 30 days with 100 ppm NH3 at RT; (c) The calibration curve of ICCN-5 to 0.3-100 ppm NH3; (d) The selectivity for various gases Fig. 8 The results of the gas response of pristine CuO and ICCN-5 thin film sensors to H2S at RT. (a) Gas response and (b) Response time for samples; (c) Dynamical response transient of pristine CuO; (d) Response and response time of pristine CuO to 1-100 ppm H2S
Fig. 9 (a) Cu2p and (b) S2p XPS spectra of the CuO samples before and after exposure to H2S
Fig. 1 TG-DSC curves of (a) PVP, the pristine CuO and ICCN-5 (b, c) as-spun fibers with a temperature ramp of 8 °C/min under air
Fig. 2 (a) XRD diffraction patterns of the pristine CuO and ICCNs; (b) Raman spectra and (c, d) N2 adsorption-desorption isotherms and pore-size distribution curves (the inset) of pristine CuO and ICCN-5
Fig. 3 (a1, a) TEM image of the pristine CuO multijunctions NFs; (b-f) HRTEM images of (a)
Fig. 4 (a) TEM image and enlarged TEM images of the ICCN-5; (b-d) HRTEM images of (a)
Fig. 5 (a-d) O1s XPS spectra of the pristine CuO multijunctions NFs, ICCN-3, ICCNC-5 and ICCN-7, respectively
Fig. 6 The results of the gas response of pristine CuO and ICCNs thin film sensors to NH3 at RT. (a) Gas response and (b) response time for samples; (c) Dynamical response transient of ICCN-5; (d) Response and response time of ICCN-5 to 0.3-100 ppm NH3
Fig. 7 (a) The reproducibility of the ICCN-5 sensor on successive exposure (8 cycles) to 100 ppm NH3; (b) The response of the ICCN-5 for 30 days with 100 ppm NH3 at RT; (c) The calibration curve of ICCN-5 to 0.3-100 ppm NH3; (d) The selectivity for various gases
Fig. 8 The results of the gas response of pristine CuO and ICCN-5 thin film sensors to H2S at RT.
(a) Gas response and (b) Response time for samples; (c) Dynamical response transient of pristine CuO; (d) Response and response time of pristine CuO to 1-100 ppm H2S
Fig. 9 (a) Cu2p and (b) S2p XPS spectra of the CuO samples before and after exposure to H2S
Scheme 1 The designed synthesis of the pristine CuO and porous In2O3@ CuO multijunctions NFs.
Scheme 2 Illustrated of the formation process of the In2O3@CuO multijunctions NFs.
