Cation-Exchange Synthesis of Cu2Se Nanobelts and Thermal

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Cation-exchange synthesis of Cu2Se nanobelts and thermal conversion to porous CuO nanobelts with highly selective sensing toward H2S Yao Su, Gang Li, Zheng Guo, Yong-Yu Li, Yi-Xiang Li, Xing-Jiu Huang, and Jinhuai Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00106 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Nano Materials

Cation-exchange synthesis of Cu2Se nanobelts and thermal conversion to porous CuO nanobelts with highly selective sensing toward H2 S Yao Su,†,‡,ǁ Gang Li,†,‡,ǁ Zheng Guo,†,‡,* Yong-Yu Li,†,‡ Yi-Xiang Li,†,‡ Xing-Jiu Huang†,‡,* Jin-Huai Liu,†,‡

†

Key Laboratory of Environmental Optics and Technology, Institute of Intelligent

Machines, Chinese Academy of Sciences, Hefei 230031, PR China ‡

Department of Chemistry, University of Science and Technology of China, Hefei

230026, PR China

*Correspondence should be addressed to Z. Guo and X. J. Huang. E-mail: [email protected] (Z. Guo), [email protected] (X. J. Huang) Tel.: +86-551-65591167; fax: +86-551-65592420. ǁY. Su and G. Li contributed equally to this work.

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Abstract Cu2Se nanobelts have been developed via a facile cation-exchange approach at room temperature, employing ZnSe·0.5N2H4 hybrid nanobelts as the templated precursors. Detailed Characterizations demonstrate that the morphologies of the templated precursors are well preserved in the cation-exchange reaction, due to the spatial confinement effect from the coated layer of poly(vinylpyrrolidone) (PVP) surfactant. Simultaneously, Cu2+ cations diffusing through the coated layer of PVP are in situ reduced to be Cu+ cations by the ligands of N2H4, thereby forming Cu2Se nanobelts with the complete replacement of Zn2+ cations in the templated precursors. After thermal oxidation in air, the obtained Cu2Se nanobelts are further converted into porous CuO nanobelts. Considering that this special morphology processes a large active surface area and is favorable for gas diffusion, gas-sensing properties of porous CuO nanobelts have been explored. The results indicate that porous CuO nanobelts exhibit highly selective sensing toward H2S with a low detection limit less than 10 ppb. Moreover, they also present a good sensing reproducibility. Finally, their sensing mechanism toward H2S has been discussed.

Keywords: cation-exchange; copper selenide; porous; nanobelt; copper oxide


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Introduction Copper based nanomaterials have been receiving great attention due to their unique properties. Especially for copper selenide (Cu2Se) and copper oxide (CuO), they as typical p-type semiconductors have potential applications in the fields of thermoelectric conversion, catalysis, therapy, sensors, and so on.1-8 Generally, the structure and morphology of nanomaterials can make a significant impact on their properties. Based on this view, various nanostructures of the abovementioned two materials have also been developed. For copper selenide, their reported nanostructures are dominated by nanocrystals, nanowires, and nanoflakes, and so forth.9-12 To the best of our investigation, their belt/ribbon-like nanostructures with a rectangular cross section have not been involved till now. Concerning to copper oxide, their belt/ribbon-like nanostructures have been only demonstrated in a few reports.13-15 However, their porous nanobelts have been rarely developed to fabricate nanosensors.16-17 Actually belt-like nanomaterials, firstly reported by Wang et al., have aroused great interest because of their promising building-block function for nanoelectronics,

optoelectronics

and

nanosensors.18-20

Endowed

belt-like

nanomaterials with porous structure, their intrinsic characteristics especially for gas sensing performances are further enhanced, which is attributed to higher active surface-to-volume ratios and more channels for gas diffusion in contrast to solid nanobelts.21-22 Accordingly, to develop new synthetic strategies of porous copper selenide and copper oxide nanobelts will not only enrich current copper-based nanostructures, but also further widen their potential applications.



Since the cation-exchange reaction was successfully adopted to fabricate nanocrystals by Alivisatos et al., it has been regarded as an accessible and promising synthetic approach to prepare nanomaterials, especially for nanostructured chalcogenides.23-25 Till now, the interconversion among their nanocrystals has been widely demonstrated via the cation-exchange reaction.26-27 An obvious characteristic of this approach is to preserve the morphology or phase structure of precursors. From this view, recently this synthetic approach has been further extended to prepare regular Ag2Se nanobelts through employing ZnSe nanobelts as the templated precursors to exchange with Ag+.28 As an alternative approach to preserve the precursor morphology, the post thermal treatment has also been widely reported. Accompanied with the thermal decomposition and oxidation of precursors, their initial morphologies are often endowed with porous structures. Especially for metal oxide, their porous nanostructures have been widely developed such as porous ZnO nanobelts and nanoplates, porous CdO nanowires, and porous In2O3 nanobelts.21-22, 29-31 Enlightened by the characteristics of the abovementioned approaches, herein a facile cation-exchange strategy has been first developed to synthesize Cu2Se nanobelts, which can be further converted into porous CuO nanobelts via a thermal oxidation. Figure 1 describes a schematic illumination of the cation-exchange synthesis of Cu2Se nanobelts and their thermal conversion to porous CuO nanobelts. First, Cu2Se nanobelts are obtained through employing ZnSe·0.5N2H4 nanobelts as precursors combined with a facile cation-exchange reaction at room temperature. Due to the spatial confinement effect from the coated layer of PVP, the obtained Cu2Se nanobelts


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well preserve belt-like morphologies of precursors. Via a thermal oxidization, as-prepared Cu2Se nanobelts are further converted into porous CuO nanobelts. Then the gas-sensing behaviors of porous CuO nanobelts have been carefully investigated. They exhibit highly selective and reproducibility responses toward H2S with a low detection limit. As expected, this strategy can be extended to prepare other porous semiconductor metal oxide nanobelts and develop high-performance gas nanosensors.

Experimental Section Chemicals and reagents. All chemicals and reagents were analytical grad and used as received without further purification from Shanghai Chemical Reagent Co., Ltd. (China). Milli-Q water with a resistivity of greater than 18.0 MΩ·cm was used in the preparation of aqueous solutions. Cation-exchange synthesis of Cu2Se nanobelts and thermal conversion to porous CuO nanobelts. First ZnSe·0.5N2H4 nanobelts were synthesized, according to our previous report.28 After centrifugation and washing with distilled water to completely remove residual reactants, they were dispersed into distilled water (10 mL) again. As-prepared solution was then added in an aqueous solution (20 mL) containing 0.6 g of poly(vinylpyrrolidone) (PVP, K30) surfactants. Under magnetic stirring for 1 h, a homogenous solution was formed. Afterwards, a solution (10 mL) containing 0.6 mmol of Cu(NO3)2 was drop by drop added into the above solution under vigorous stirring at room temperature. Immediately, the white solution turned gray and gradually black. Continuously stirring for 12 h, the achieved sample was centrifugated and washed with distilled water and ethanol for several times, respectively. Finally,



Cu2Se nanobelts were obtained after completely removing the redundant of Zn2+, Cu2+ and PVP. To prepare porous CuO nanobelts, a thermal oxidation approach was employed. Typically, the obtained Cu2Se nanobelts were calcined in air at 350 °C (with a low rising rate of 1 °C/min from room temperature) for 90 min. After cooling down to room temperature, porous CuO nanobelts were achieved. Fabrication and measurements of gas-sensing devices. Similar with previous reports, alumina ceramic tubes (1 mm in length and 4 mm in diameter) were employed as substrates to fabricate gas-sensing devices.32 Two interdigital Au electrodes on their outer surface were used for measuring the electronic signals of the fabricated gas-sensing devices. A Ni–Cr resistor about 30 Ω in their inner was employed as a heater to provide their working temperature. To construct uniform sensing film, an assembled technique was performed. First as-prepared Cu2Se nanobelts were assembled to form a compact and ordered film according to our previous reports.21 Via a typical dipping and pulling approach for several times, the alumina ceramic tubes were coated with a uniform and compact film consisted of assembled Cu2Se nanobelts. Followed with the abovementioned thermal conversion approach, it was further transformed into a uniform porous CuO nanobelts sensing film. To evaluate the gas-sensing performance of porous CuO nanobelts, a Keithley 6487 picoameter/voltage sourcemeter was used as both voltage source and current reader. All gas sensing measurements were performed in the dry air by our previously


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reported setup and program.32 The relative response was defined as S = Rg/Ra, where, Ra is the resistance of the sensing film in dry air and Rg is that in dry air mixed with the detected gases. Characterization of as-prepared samples. The morphologies and microstructures of as-prepared samples were characterized by a scanning electron microscope (SEM, ZEISS AURIGA) equipped with the energy-dispersive X-ray spectroscopy (EDX), and their transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses were performed using a JEOL-2010 transmission electron microscope. X-ray diffraction (XRD) patterns of all samples were taken on a Philips X’pert diffractometer (X’Pert Pro MPD) with Cu Kα radiation (1.5418 Å). X-ray photoelectron spectroscopy (XPS) measurements were examined with a VG Scientific MKII spectrometer using an Mg Kα X-ray source (1253.6 eV, 120 W) at a constant analyzer. Infrared (IR) spectra were obtained with a Nicolet Nexus-670 FT-IR spectrometer. Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) were measured at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility. The specific surface area was analyzed on a Coulter Omnisorp 100CX Brunauer–Emmett-Teller (BET) by nitrogen adsorption.

Results and Discussion Based on the described experimental details, typical synthesis of Cu2Se nanobelts have been first developed via a facile cation-exchange approach at room temperature. As described in Figure S1 (Supporting Information), with an addition of 0.6 g of PVP



surfactant, the initial solution containing ZnSe·0.5N2H4 precursor nanobelts are homogeneous and white under magnetic stirring. Once adding Cu2+ cations, it immediately turns brown. With the increase of the adding amount of Cu2+ cations, it gradually becomes black, meaning that a new composition is formed. Moreover, the precursor morphologies have been well preserved after the exchange reaction with Cu2+ cations. This result can be inferred from SEM images of as-prepared ZnSe·0.5N2H4 nanobelts and the achieved sample, as presented in Figure 2a and b, respectively. Based on high-magnification SEM image presented in the inset of Figure 2b, their surfaces seem smooth. TEM image in Figure 2c further confirms that they present regular belt-like structure. From high-magnification TEM image in Figure 2d, the contrast is not uniform along its individual nanobelt. To obtain more information about its structure, a HRTEM image is performed and shown in the right-bottom inset of Figure 2d. Mesoporous structure and random direction of crystal lattice are clearly presented, implying that the achieved nanobelts are polycrystalline. This result is well consistent with its SAED pattern, as shown in the left-top inset of Figure 2d. The interplanar spacing marked in its HRTEM image is about 0.33 nm, which is indexed to its (111) lattice planes of cubic Cu2Se. As shown in Figure S2 (Supporting Information), the obtained nanobelt is easily broken and decomposed after illumination of electron beam for a while. It suggests that the achieved nanobelts are unstable under the illumination of electron beam. Figure 2e exhibits FT-IR spectra of the precursor nanobelts of ZnSe·0.5N2H4 before and after the cation-exchange reaction. For ZnSe·0.5N2H4 nanobelts, there are two sharp peaks


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ascribed to the N-H stretching vibration band (centered at about 3204 and 3113 cm-1, respectively) and other peaks of the NH2 scissor, wag and twist bands (centered at about 1583, 1384 and 1150 cm-1, respectively), which is well consistent with that reported by Li, et al.33 Absorption at 567 cm-1 was maybe caused by the stretching band from the ligands N2H4 coordinated with Zn2+ cations. However, these peaks are disappeared after completely exchanging with Cu2+ cations, indicating that the ligands of N2H4 are released or decomposed from the nanobelts. This phenomenon is similar with that of inorganic-organic hybrid ZnSe-Amine nanoflakes exchanged with Cd2+ cations.34 XRD patterns given in the Figure 2f show that all diffraction peaks of the obtained nanobelts are well indexed into Cu2Se (JCPDS NO. 88-2043) without observing any other diffraction peaks. In addition, the elemental composition is further analyzed by EDX spectrum, as shown in Figure S3 (Supporting Information). Apart from the peak of Si arising from the Si substrate, the strong peaks of Cu and Se are emerged without observing the peaks of Zn, indicating that Zn2+ cations of precursor nanobelts are completely replaced and also released from the nanobelts. Notably, the PVP surfactant is critical to keep up the initial morphologies of the templated precursors during the cation-exchange reaction. From SEM image shown in Figure 3a, it can be seen that irregular nanostructures are obtained without PVP. Adding 0.2 g PVP, the regular belt-like morphology of precursors has been fundamentally preserved, as presented in Figure 3b. Continuously increasing the amount of PVP to 0.4 g and 0.6 g, the same morphologies have been obtained, which can be observed from SEM images shown in Figure 3c and d, respectively. Apart



from their morphologies, the compositions and phases of the obtained nanobelts also depend on the PVP surfactant. This result can be concluded from XRD patterns in Figure 3e. Without PVP, the corresponding XRD pattern can be fundamentally indexed into Cu3Se2 except Cu2Se, implying that the obtained sample is a complex multiphase. However, with the adding amount of PVP increased from 0.2 to 0.6 g, it can be observed that the diffraction peaks of Cu3Se2 phase are gradually weakened and finally disappeared. Simultaneously, the diffraction peaks corresponding to a single phase of the cubic structure of Cu2Se are kept and strengthened. Undoubtedly, the copper valence state for Cu2Se nanobelts don’t match with Cu2+ cations, which is employed to exchange with Zn2+ cations of the precursor nanobelts. To identify the elemental valence state and purity of the obtained nanobelts, X-ray photoelectron spectroscopy measurements were performed. From the survey spectra of the precursor nanobelts and the obtained Cu2Se nanobelts shown in Figure 4a, it demonstrates the emergence of Cu element and the disappearance of Zn element after the cation-exchange reaction. This result further confirms that the cation-exchange reaction is complete. As displayed in Figure 4b, the Cu 2p core level spectrum has two peaks (Cu 2p3/2 at 932.3 eV and Cu 2p1/2 at 952.2 eV) that they are symmetric, narrow, and free of satellite peaks, which excludes the existence of Cu2+.35-36 Further combined with its XRD patterns without observing any diffraction peaks of Cu in Figure 2f, it can be concluded that the valence of Cu elements is pure +1. Therefore, it is well consistent with the valence of Cu element of the obtained Cu2Se nanobelts. To obtain more information about their crystal structures, EXAFS measurements at


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Cu K-edge are performed and shown in Figure S4a (Supporting Information). In Figure S4b (Supporting Information), k-weighted EXAFS curves with their fitted curves are displayed for the achieved samples with PVP (0.6 g). Their corresponding Fourier transforms and their fitted curves of EXAFS date with uncorrected phase shift are depicted in Figure S4c (Supporting Information). The fitting of EXAFS data is conducted by the software ARTEMIS to get information about coordination numbers and bond distance. Clearly all fitted curves match well with the initial data. Based on the fitting data, the nearest neighbor distance of Cu-Se bond is about 2.38 ± 0.004 Å, and the corresponding coordination numbers are 3.5 ± 0.2. These results are fundamentally consistent with previous reports about crystal structures of Cu2Se.37 To illuminate the above phenomena, a possible cation-exchange mechanism has been provided in Figure 5. Without PVP, ZnSe·0.5N2H4 precursor nanobelts directly encounter with Cu2+. According to previous reports, it is a thermodynamic and dynamic favorability for the cation exchange between Zn2+ cations of the precursor nanobelts and Cu2+ cations.25, 38 Accordingly, Zn2+ cations is rapidly exchanged with Cu2+ cations and released from the precursor nanobelts. Simultaneously the initial coordination interaction between Zn2+ cations and the ligands of N2H4 is broken. Then it further breaks the morphological structure of the precursor nanobelts, leading to form irregular nanostructures. However, adding an amount of PVP, the precursor nanobelts are coated with a layer of PVP. Cu2+ cations should be diffused through its layer and exchanged with Zn2+ cations. Although the coordination interaction between Zn2+ cations and the ligands of N2H4 is still broken, the cation-exchange reaction and



the formation of copper selenide are spatially confined due to the coated layer of PVP. Attributed to this spatial confinement effect, the achieved samples well preserve the initial belt-like morphology of the precursors. Apart from their morphologies, the compositions of the achieved samples are also different at different amounts of the PVP surfactant, as presented in Figure 3e. As is well-known, hydrazine (N2H4) is a reducing molecule.39 Accordingly, encountering with the ligands of hydrazine, Cu2+ cations are easily reduced to Cu+ cations. Without PVP, the cation-exchange reaction is analogous to an open environment for the ZnSe·0.5N2H4 precursor nanobelts. Accompanied with the coordination interaction between Zn2+ cations and the ligands of N2H4 broken, parts of N2H4 molecules are immediately released into the solution. Therefore, Cu2+ cations, exchanged with Zn2+ cations of the precursor nanobelts, are partly reduced to Cu+ cations, naturally forming a multi-composition copper selenides containing Cu2Se. When the precursor nanobelts are coated with a PVP layer, the diffusion of Cu2+ cations is slow down and the release of hydrazine molecules is blocked to a certain extent. Therefore, more Cu2+ cations replacing Zn2+ cations are reduced to Cu+ cations, causing that the main composition of the achieved sample is Cu2Se. With the increase of PVP amount, it means that the thickness of PVP layer increases. Then the release of hydrazine molecules will be completely blocked and confined into the spatial room coated by the PVP layer. The diffusing and exchanging Cu2+ cations are completely reduced to Cu+ cations by sufficient hydrazine molecules, leading to form the purified Cu2Se nanobelts. Based on the above analysis,


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undoubtedly the morphology and composition of the achieved sample greatly depend on the PVP surfactant. Inspired by the thermal conversion of the ZnSe·0.5N2H4 precursor nanobelts to porous ZnO nanobelts, as-prepared Cu2Se nanobelts have also been thermally treated in air.21 After thermal oxidation at 350 °C (with a low rising rate of 1 °C/min from room temperature) for 90 min, the as-prepared Cu2Se nanobelts have been transformed into porous CuO nanobelts, which can be observed from SEM image shown in Figure 6a. Undoubtedly the belt-like morphology has also been fundamentally preserved. From high-magnification SEM image in Figure 6b, it can be seen that the dense and uniform nanopores are distributed along the nanobelts. This result can be further demonstrated from TEM image of individual nanobelt shown in Figure 6c. From HRTEM image and SAED pattern in its insets, it indicates that the obtained porous CuO nanobelts are polycrystalline, which is consisted of numerous nanopores and nanocrystals. The interplanar spacing marked in its HRTEM image is about 0.233 nm, which is indexed to its (111) lattice planes. Elemental mapping patterns of individual nanobelt show that the Cu and O elements are both uniformly distributed and well matches with its morphology, as presented in Figure 6d and e, respectively. Additionally, the content of Se element cannot be observed from EDX spectrum in Figure S5a (Supporting Information), indicating that Cu2Se nanobelts have been completely oxidized. To determine the BET specific surface areas of the porous CuO nanobelts, the N2 adsorption-desorption isotherms have been performed, as shown in Figure S5b (Supporting Information). The as-prepared porous CuO



nanobelts exhibit a high BET surface area of 13.19 m2/g, which originate from the existence of numerous nanocrystals and nanopores. In Figure 6f, all diffraction peaks are well indexed to a monoclinic CuO phase (JCPDS no. 80-1916) without observing any other diffraction peaks. Furthermore, the clear and sharp diffraction peaks reveal that porous CuO nanobelts possess an excellent quality. From XPS survey spectrum of the obtained CuO nanobelts shown in Figure S5c (Supporting Information), clearly it demonstrates the disappearance of Se element and the enhancement of O element compared with that of Cu2Se nanobelts. Moreover, Figure S5d (Supporting Information) shows that the Cu2p peaks broadened and underwent splitting from the initial symmetric, narrow, and devoid of satellite peaks corresponding to Cu2Se nanobelts, forming pronounced satellite peaks. These results further illuminate that Cu2Se nanobelts are completely transformed into CuO nanobelts after thermal oxidation. Considering as-prepared CuO with the porous and belt-like nanostructure, their gas sensing properties have been further explored. The whole fabrication process of their sensing devices is similar with that of gas-sensing devices of porous ZnO nanobelts.21 First, the Cu2Se nanobelts are assembled and transferred to the substrate of device, forming a compact and uniform Cu2Se nanobelt film. Following with the abovementioned thermal oxidation, the sensing devices consisted of porous CuO nanobelts are obtained. Figure 7a presents the real-time response curves of the fabricated device toward H2S (1 ppm) at different working temperatures. The relationship between the relative response and working temperature is shown in


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Figure 7b. With the working temperature increased from 125 to 175 °C, the relative response gradually increases. Higher than 175 °C, it decreases. Concerning to the response and recovery time, it greatly turns short with the incensement of working temperature. This result is mainly ascribed that the adsorption and desorption rates of the detected gas molecules increase with the incensement of working temperature on their surface. Evidently it reaches the highest relative response about 7.6 toward 1 ppm H2S with a short response (~3 s) and recovery time (~21 s) when the working temperature is at 175 °C. Theoretical calculations, reported by Pullithadathil, et al., demonstrated that the relative response of CuO nanostructures toward H2S was directly related with adsorption kinetics of the O2-/O- and H2S species and their activation energy.40 Following this view, it can be thought that the optimal response of porous

CuO

nanobelts

toward

H2S

is

mainly

attributed

to

optimum

adsorption/desorption kinetics of gas species and activation energy at 175 °C. In Figure 7c, the real-time response curve toward different concentrations of H2S is displayed at the optimal working temperature of 175 °C. Clearly the relative response increases with the incensement of H2S concentrations. Notably, when the concentration of H2S is down to 10 ppb, an obvious response can be still observed. It means that the fabricated nanodevice exhibits a lower detection limit than 10 ppb. As listed in Table S1 (Supporting Information), as-prepared porous CuO nanobelts presented a better sensing performance toward H2S in contrast to previous reports. These results are mainly attributed to their porous belt-like structures, which are favorable for the sufficient diffusion of the gases detected among the sensing film.



More importantly, the formed porous structures with numerous surface defects present a higher activity in contrast to others. Figure 7d exhibits the relationship between the relative response and different concentrations of H2S. The relative response seems to linearly increase when the concentration of H2S is below 1 ppm. However, higher than 1 ppm, its relative response gradually inclines to be saturated. Based on the six-cycle response curve toward 250 ppb H2S in Figure 8a, it can be concluded that the fabricated sensing devices are of good reproducibility. Furthermore, the sensing device fabricated with porous CuO nanobelts also presents a high selective sensing toward H2S. To demonstrate this point, some inorganic and organic gases with 50 ppm are employed such as NH3, acetone, methylbenzene, formaldehyde, chlorobenzene, ethanol, diethylether, isopropanol, and propanol. As shown in Figure 8b, their relative responses are all much lower than that of 1 ppm H2S. Due to their high relative response, good reproducibility, and highly selective sensing toward H2S at ppm and subppm levels, the fabricated sensors can be potentially applied for the early diagnosis of lung disease through monitoring H2S of its biomarker.41-42 To clarify the above sensing performance of porous CuO nanobelts, a possible mechanism has been provided in Figure 9. In many previous reports, the sensing behavior of CuO toward H2S is usually regarded as that H2S molecules directly react with CuO to form Cu2-xS on their surface.43-44 Owing to the lower resistivity of Cu2-xS, it causes that the resistance of its sensing film decreases. However, for as-prepared porous CuO nanobelts, the resistance of their sensing film increases after encountering with H2S gas molecules, which is similar with its response to reductive


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gas molecules.40 As is well-known, the conductivity of CuO, as a typical p-type semiconductor metal oxide, mainly depends on the concentration of holes. When porous CuO nanobelts are exposed in air, numerous oxygen species (O–, O2–, and O2–, etc) are easily formed on their surface owing to porous structure with large surface area and numerous defects with high reactive activity, resulting in the incensement of the hole density and the enhancement of the conductivity of porous CuO nanobelts. Therefore, encountering with low concentration of H2S gas molecules, the surface reaction is dominated by their oxidation with oxygen species, not forming Cu2-xS. Then the reduced oxygen species are desorbed from the surface of porous CuO nanobelts. A large quantity of electrons are then released back to the bulk of porous CuO nanobelts and recombined with holes, which reduces the hole-accumulation layer width and decreases the conductivity of porous CuO nanobelts. Moreover, the as-prepared CuO nanobelts are porous structure. On the one hand, it is favorable for the diffusion of H2S molecules, generating a short response/recovery time. On the other hand, porous structure offers CuO nanobelts with high active surface-to-volume ratios, greatly enhancing the sensitivity toward H2S with a low detection limit.

Conclusions In summary, Cu2Se nanobelts have been successfully prepared via a facile cation-exchange reaction at room temperature, employing ZnSe·0.5N2H4 hybrid nanobelts as the templated precursors. Due to the spatial confinement effect from the coated layer of PVP, the initial precursor morphology has been well preserved. Simultaneously, Cu2+ cations diffused through the PVP layer are in situ reduced to



Cu+ cations by the ligands of N2H4, leading to from single-phase Cu2Se nanobelts, not CuSe nanobelts. Based on further thermal oxidation, as-prepared Cu2Se nanobelts can be converted into porous CuO nanobelts. Arising from porous and ultrathin belt-like structure, they exhibit high sensing performance toward H2S with a good reproducibility and a low detection limit less than 10 ppb. Furthermore, they also present high selectivity toward H2S among all investigated gases. As expected, the developed two-step approach can be extended to prepare other porous semiconductor metal oxide nanobelts for the fabrication of high-performance gas nanosensors.

Associated Content Supporting Information available: Optical images of the solution containing ZnSe·0.5N2H4 precursor nanobelts under the cation-exchange reaction with Cu2+ (Figure S1); SEM images of Cu2Se nanobelts before and after exposing the electron beam (Figure S2); EDX spectrum of Cu2Se nanobelts (Figure S3); Cu K-edge XANES spectra of Cu2Se nanobelts (Figure S4); EDX spectrum, nitrogen adsorption–desorption isotherms, and XPS survey spectrum of porous CuO nanobelts (Figure S5); Sensing performances of various CuO based sensors toward H2S (Table S1).

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 61474122, 61774159, and 61573334), the CASHIPS Director’s Fund (Grant


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No.YZJJ201701) and the CAS Interdisciplinary Innovation Team of the Chinese Academy of Sciences.



Figure 1. Schematic illumination of cation-exchange synthesis of Cu2Se nanobelts from ZnSe·0.5N2H4 precursor nanobelts and their thermal conversion to porous CuO nanobelts.


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Figure 2. (a) SEM image of ZnSe·0.5N2H4 precursor nanobelts, (b) SEM image of as-prepared Cu2Se nanobelts with the inset corresponding to its high-magnification SEM image, (c) low-magnification TEM image of as-prepared Cu2Se nanobelts, (d) high-magnification TEM image of single Cu2Se nanobelt with the insets corresponding to its HRTEM image and SAED pattern, (e) IR spectrums of precursor nanobelts and as-prepared Cu2Se nanobelts, and (f) XRD pattern of as-prepared Cu2Se nanobelts.



Figure 3. SEM images of the obtained samples under different adding amounts of PVP: (a) without PVP, (b) 0.2 g PVP, (c) 0.4 g PVP, and (d) 0.6 g PVP, respectively, and (e) their corresponding XRD patterns.


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Figure 4. (a) XPS spectra of as-prepared ZnSe·0.5N2H4 and Cu2Se nanobelts, (b) XPS spectrum of Cu 2p for Cu2Se nanobelts.



Figure 5. Possible cation-exchange mechanism with or without PVP. (a) Effect of PVP to manipulate the morphology of the achieved sample, (b) effect of PVP to manipulate the composition of the achieved sample.


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Figure 6. (a) and (b) SEM images of the obtained porous CuO nanobelts, (c) TEM image of single porous CuO nanobelt with the insets corresponding to its HRTEM image and SAED pattern, (d) and (e) Cu and O elemental mapping patterns recorded from single nanobelt, respectively, and (f) XRD pattern of as-prepared porous CuO nanobelts.



Figure 7. (a) Response curves of as-prepared porous CuO nanobelts toward 1 ppm of H2S at different working temperatures, (b) The relationship between the relative response and working temperature, (c) Real-time responses curves toward different concentrations of H2S at the optimal working temperature of 175 ˚C, and (d) The relationship between the relative response and different concentrations of H2S at the working temperature of 175 ˚C.


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Figure 8. (a) Six-cycled response curves of porous CuO nanobelts toward 250 ppb of H2S at 175 ˚C, and (b) the relative responses of porous CuO nanobelts toward 1 ppm of H2S and 50 ppm of other gases (NH3, acetone, methylbenzene, formaldehyde, chlorobenzene, ethanol, diethylether, isopropanol, and propanol) at 175 ˚C.



Figure 9. Schematic illustration of possible sensing mechanism for porous CuO nanobelts toward H2S.


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Cation-Exchange Synthesis of Cu2Se Nanobelts and Thermal

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