In2O3 Mesocages with Double-Shell Architectures

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Octahedral-Like CuO/In2O3 Mesocages with Double-shell Architectures: Rational Preparation and Application in Hydrogen Sulfide Detection Xiaowei Li, Changlu Shao, Dong-Xiao Lu, Geyu Lu, Xinghua Li, and Yichun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15488 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Octahedral-Like CuO/In2O3 Mesocages with Double-Shell Architectures: Rational Preparation and Application in Hydrogen Sulfide Detection

Xiaowei Li,a Changlu Shao,a*Dongxiao Lu, b Geyu Lu,b* Xinghua Lia and Yichun Liua

a

Center for Advanced Optoelectronic Functional Materials Research, and Key

Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education, 5268 Renmin Street, Changchun 130024, China b

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science

and Engineering, Jilin University, Changchun 130012, China

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ABSTRACT This contribution describes a facile strategy for constructing octahedral-like CuO/In2O3 mesocages with double-shell architectures. The synthetic method included first preparation of unifrom Cu2O as an ideal self-sacrificial template and then decoration by a In2O3 outer layer through room-temperature Cu2O-engaged redox etching

reaction

combined

with

subsequent

annealing

process.

Various

characterization techniques manifested that In2O3 nanoparticles were uniformly grown on the surface of CuO mesocages, resulting in a well-defined double–shelled heterostructure. When evaluated as a novel sensing material for hydrogen sulfide (H2S) detection, the resultant octahedral-like CuO/In2O3 heterostructures exhibited obviously enhanced sensing response, lower operating temperature as well as faster response/recover speed during the dynamic measurement compared to the pristine CuO particles, which is likely related to the high-level of adsorbed oxygen concentration, resistance modulation effect and unique microstructure of as-prepared CuO/In2O3 heterostructure.

Keywords: CuO/In2O3, octahedral-like, heterostructure, gas sensor, H2S


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1. INTRODUCTION In virtue of unique characteristics of high sensitivity, low power consumption, as well as reliable response, chemiresistive gas sensors stand out from numerous well-established sensing strategies and have been regarded as a promising candidate for detecting various toxic and explosive gases for versatile applications.1-3 It is generally known that the gas sensing properties are tightly dependent on the chemical composition and microstructure of sensing materials.4-5 In this regard, numerous metal oxide semiconductors with diverse nanostructures have been extensively studied for gas sensing over the past few decades. Among them, p-type oxide semiconductors, including NiO,6-7 Co3O4,8 CuO,9-10 Cr2O311 or Mn3O4,12 are believed to hold great promise in gas-sensing application because of their intriguing catalytic effect that stems from the multiple oxidation states of metal element. However, up to now, only a small proportion (lower than 10%) of gas sensors are based on p-type oxide semiconductors, probably because p-type oxide semiconductors suffer from an inherently lower response to the same tested gases compared to n-type oxide semiconductors.13-14 According to the calculation deduced by M. Hübner, the response of p-type oxide semiconductors is about the square root of n-type ones when the two different type gas sensors are in identical circumstance.15 Therefore, motivated by the driving force of improving the sensing performance of traditional p-type semiconductors, several strategies including structure modulation, surface functionalization and construction of heterojunctions, have been explored and employed.16-18 Among these various methods, constructing heterostructured oxide semiconductors with p-p and p-n junctions have 3 ACS Paragon Plus Environment


stimulated a steadily ascending research interest, as heterostructured semiconductors are hopeful to gain much enhanced sensing properties by synergistic effect and/or resistance modulation effect.19-21

Copper (II) oxide, a typical p-type semiconductor with many acceptor levels, has been widely investigated as an ideal sensing material in selectively detecting H2S gas owing to its intriguing chemical affinity with H2S as well as thermal stability under elevated temperature.15, 22-23 However, in general, single CuO usually exhibits low and inert sensing response due to its intrinsic p-type characteristics, which seriously restricts its further application in high-performance gas sensor. Indium oxide, featured as a n-type sensing material with excellent electrical conductivity, has been demonstrated to be sensitive to various toxic gases including CO,24 NH3,25 O3,26 NOx,27 H2S28 and many volatile organic compounds (VOC) gases.29-32 Nevertheless, the drawbacks of such broad sensitive characteristic arise simultaneously, as it makes In2O3 lose the selectivity to a certain gas. On account of the possible synergistic effect of heterostructure, it is reasonable to anticipate that the combination of CuO and In2O3 would be one of the most effective strategies to balance the defective sensing performance of these two individual components. While the question is that the lattice constants of CuO and In2O3 do not match each other, which makes traditional methods hard to prepare CuO/In2O3 composites with well-defined configurations and uniformly dispersed heterojunctions. Recently, in sharp contrast, Cu2O-templated growth of oxide semiconductor heterostructures has received high expectation because of the easily shape-controlled synthesis as well as in situ formation under 4 ACS Paragon Plus Environment

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mild conditions.33-34 However, to the best of our knowledge, such intriguing method has not been employed for rationally preparing CuO/In2O3 heterostructure for gas sensing.

Herein, in this contribution, uniform Cu2O truncated octahedrons were first designed through one-step liquid-phase reaction, which provides a good self-sacrificial template for In2O3 in-situ growth. Subsequently, CuO/In2O3 heterostructures consisting of inner CuO mesocages and outer In2O3 layers were constructed by combining Cu2O-template-engaged redox etching under ambient condition with subsequent thermal treatment. Furthermore, as a proof-of-concept, gas sensors based on the as-obtained CuO/In2O3 composites were fabricated and their gas sensing properties were examined. As expected, the results suggest that CuO/In2O3 composites manifest much enhanced sensing properties in the aspects of sensitivity as well as response/recover speed compared to the bare CuO counterparts, which evidently demonstrated the potential of CuO/In2O3 composites in gas sensing.

2. EXPERIMENTAL PROCEDURE 2.1 Starting materials

Poly(vinyl pyrrolidone) (PVP), copper(II) sulfate pentahydrate (CuSO4•5H2O), indium chloride tetrahydrate (InCl3•4H2O) and dextrose monohydrate (C6H12O6•H2O) in this experiment were purchased from Sinopharm Chemical Reagent Co. Ltd. Sodium carbonate (Na2CO3), trisodium citrate dihydrate (Na3C6H5O7•2H2O) and sodium chloride (NaCl) were obtained from Beijing Chemical Works. All of the chemical reagents were directly used as received without further purification. 5 ACS Paragon Plus Environment


2.2 Preparation of Cu2O and CuO truncated octahedrons

Octahedral-like Cu2O was synthesized by referring to previous report with modification,23 the synthesis processes of which were mainly based on the redox reactions between copper-citrate complex and glucose molecules. In a typical procedure, 60 mL of aqueous solution composed of 4.5 g of PVP, 0.51 g of CuSO4•5H2O, 0.653 g of Na3C6H5O7•2H2O, 0.382 g of Na2CO3 and 0.831 g of C6H12O6•H2O was first prepared by vigorous stirring for 30 min at room temperature. Then, the resulting light blue solution was aged at 80 °C by thermostatic water bath for 2 h. After the reaction, the brick-red Cu2O precipitates were repeatedly washed and collected with the assistance of centrifugation before being dried at 60 °C. Furthermore, the octahedral-like CuO particles were obtained by following calcining process at 550 °C in air.

2.3 Preparation of CuO/In2O3 mesocages

For the preparation of CuO/In2O3 heterostructures, 50 mg of octahedral Cu2O particles were first ultrasonicated uniformly in 20 mL of absolute ethanol. Then, 0.5 mL of NaCl aqueous solution and 10 mL of InCl3•4H2O ethanol solution were dropwise added into the above solution, respectively. After simply stirring for 10 min at room temperature, the resulting products were separated by centrifugation and washed with deionized water and absolute ethanol several times. After vacuum drying, the precipitates were sintered at 550 °C for 2 h to obtain the final CuO/In2O3 mesocages. According to the above procedures, three CuO/In2O3 heterostructures 6 ACS Paragon Plus Environment

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were prepared and referred to as CuO/In2O3-1, CuO/In2O3-2 and CuO/In2O3-3 by using different concentrations (0.02 M, 0.04 M and 0.08 M) of InCl3•4H2O solution as additive. 2.4 Gas sensing Measurement

Sensor devices were built by the same procedures as stated in our previous work35-36 and the detail structure of the gas sensor was schematically shown in Figure S1. Gas sensing measurements were carried out on an automated dynamical gas sensing system. The concentration of target gases were precisely controlled with the assistance of mass flow controllers and the flow rates for both dry air and tested gases (balanced with dry nitrogen) were fixed at 1000 sccm. The resistance of sensing material was continuously recorded by digital multimeter and the sensing response (S) was defined as the percentage of relative variation in resistance (∆R/R0*100%), where ∆R represents the variation of resistance before and after contacting with target gases, while R0 represents the resistance of gas sensor in initial state. The response/recovery time was defined as the required time for the resistance to reach 90 % of total resistance variation relative to the former equilibrium value upon exposure to/removal from target gases.

3. RESULTS AND DISCUSSION 3.1 Structural and Morphological Characteristics

Figure 1 presents the typical X-ray diffraction (XRD) and scanning electron microscopy (SEM) images of as-synthesized Cu2O particles. As can be seen, all of



diffraction peaks in XRD pattern (Figure 1a) can be well indexed to cubic phase of Cu2O (PDF file No. 77-0199). The relative strong intensity of (111) peak suggested that massive exposed planes of Cu2O octahedrons belong to {111} facets, which are believed to possess high surface reactivity.37 From the panoramic SEM image (Figure 1b), it can be observed that the resultant Cu2O particles exhibit a uniform octahedral morphology with average size of approximately 1 µm. The enlarged FESEM images (Figure 1c) indicated that these octahedral-like Cu2O had a smooth surface and excellent monodispersity. Notably, as indicated by the white dashed line in Figure 1d, every corner of Cu2O octahedrons was flat, indicating that the truncated octahedral Cu2O particles were successfully prepared in our experiment.

Figure 1. (a) X-ray diffraction pattern of as-obtained Cu2O particles; (b-d) FESEM images of octahedral-like Cu2O particles with different magnification.


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Inspired by the current interest of constructing promising gas sensing materials, the above pristine octahedral-like Cu2O was employed as sacrificial template to further synthesize CuO/In2O3 heterostructures, a designed procedure is schematically illustrated in Figure 2. According to the well-designed route, pristine Cu2O octahedrons were first prepared by reducing copper-citrate complex with glucose at 80 °C, then the CuO/In2O3 mesocages were synthesized by Cu2O-template-engaged redox etching in solution and self-transformation process during the following annealing process. In the initial stage, In3+ and Cl- ions introduced to the solution will gather on the surface of bare Cu2O templates. Driven by the coordination etching reactions between InCl3 and Cu2O as described by the following equations, the In2O3 outer shell was formed in-situ around the pristine Cu2O.

2InCl + 3Cu O + xH O ↔ In O ∙ xH O + 6CuCl (1)

CuCl + (x − 1)Cl ↔ CuCl (2)

Notably, this coordinating etching reaction was performed at room temperature for just 10 min by using InCl3•4H2O as etching reagent. Therefore, the thermal energy is not enough for crystal formation and thus In2O3 outer shell is expectedly amorphous in nature. With the assistance of subsequent annealing process, the CuO/In2O3 heterostructures were finally obtained, accompany with the In2O3 crystal formation and phase transformation of Cu2O to CuO. Meanwhile, the interior void structure can be formed by thermal oxidation coupled with the famous Kirkendall effect. During the annealing process, pristine Cu2O particles on the surface was firstly oxidized into 9 ACS Paragon Plus Environment


Figure 2. Schematic illustration of the formation of CuO/In2O3 heterostructure: (I) immobilizing In2O3 nanoparticles on the surface of bare Cu2O templates; (II) fabrication of CuO/In2O3 mesocages via self-transformation process.

CuO in the presence of ambient oxygen molecular, and the diffusion couple composed of interior Cu2O core and exterior CuO shell was therefore formed simultaneously. Due to Cu2O shows a higher diffusion rate than CuO,38 the outward mass transportation of Cu2O will be faster than the inward mass transportation of CuO at the interface, which finally leads to the formation of hollow framework.

Figure 3 shows the corresponding FESEM images of as-obtained CuO and CuO/In2O3 samples. As can be seen, after simple thermal treatment in air, Cu2O particles were transformed into thermodynamically stable CuO phase whilst preserving the highly well-regulated octahedral morphology with dense and clean surface (Figure 3a). When 0.02 M of indium chloride was introduced as indium source, the obtained CuO/In2O3 heterostructures exhibit a porous surface (Figure 3b), which might due to the immobilization of In2O3 nanoparticles as well as ion etching in the indium chloride solution. As the concentration of indium chloride increased to 10 ACS Paragon Plus Environment

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Figure 3. (a) Low- and high-magnification FESEM images of bare CuO obtained by directly thermal treatment of Cu2O particles at 550 °C in air; (b-d) Low- and high-magnification FESEM images of CuO/In2O3 composites prepared by adding different concentrations (0.02 M, 0.04 M and 0.08 M) of InCl3•4H2O source.

0.04 M, in addition to the obvious change in surface morphology, some partially broken particles (shown in Figure 3c) clearly unveil a hollow interior, which will be further confirmed by the TEM analysis in the later section. Additionally, the enlarged SEM image clearly shows that many In2O3 nanoparticles are randomly attached on the 11 ACS Paragon Plus Environment


surface of the composites, implying the formation of double-shelled architectures. With a further increase of the concentration of indium chloride to 0.08 M, large number of pristine Cu2O particles on the surface would transform to soluble [CuClx]1-x due to the strong etching effect. Consequently, the excessive consumption of Cu2O leads to the formation of CuO/In2O3 heterostructures with cracked shells (Figure 3d).

Moreover, we compared the CuO/In2O3 nanocomposite of current work with those of previous reports in terms of dimension, morphology, assembling way, interior structure, and preparation method, as summarized in Table S1. Notably, numerous methods such as hydrothermal, electrospinning, sputtering, and template have been employed to prepare CuO/In2O3 heterostructures with diverse morphologies and dimensions. Setting aside the rigid experimental conditions and tedious synthesis procedures of these methods above, most of CuO/In2O3 products were constructed through simple random stacking of CuO and In2O3 nanocrystals. In stark contrast, we present a rare coordinating etching method for constructing octahedral-like CuO/In2O3 mesocages with well-defined double-shelled architectures in liquid-phase conditions. Considering that the present method is totally based on facile chemical etching reaction, we believe this method can be applicable to synthesize other CuO/In2O3 heterostructures with different morphologies and sizes.

The crystalline phase and purity of as-obtained CuO and CuO/In2O3 was examined and the corresponding XRD patterns are shown in Figure 4. It is obvious 12 ACS Paragon Plus Environment

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that all of diffraction peaks of bare CuO octahedrons are matched well with those in standard JCPDS card (no. 89-2529), indicating that the obtained CuO powder ought to be monoclinic phase with high purity. For CuO/In2O3 heterostructures, except most of diffraction peaks being assigned to above monoclinic CuO, the residual recorded peaks are readily indexed to standard cubic In2O3 (JCPDS card no. 06-416). In addition, as expected, the higher concentration of indium chloride is introduced during the synthesis process, the higher intensity of In2O3 peaks will appear (emphasized by the cyan rectangles), which implies that the In2O3 components in the composites are gradually increased as expected.

Figure 4. XRD patterns of pristine CuO and CuO/In2O3 heterostructures.

The detailed interior structure and geometrical morphology of as-synthesized CuO/In2O3-2 are further characterized by TEM (Figure 5a), from which the hollow nature of octahedral CuO/In2O3 can be easily discerned. Furthermore, the claimed double-shelled architecture can be clearly verified by the different contrast between 13 ACS Paragon Plus Environment


Figure 5. (a) Typical TEM image of double-shelled CuO/In2O3-2 composites; (b-d) HRTEM images taking from the edge of CuO/In2O3; (e-h) STEM image and corresponding elemental mapping of individual octahedral-like CuO/In2O3-2 mesocage.

the relative pale outer fringe and the dark inner fringe. Figure 5b presents the high-resolution TEM image that was taken from the surface of the outer shell. The magnified HRTEM images shown in Figure 5(c and d) present clear fringes with inter-plane spacing of about 0.29 nm, which can be well assigned to the distance between the (222) crystal planes of cubic In2O3, demonstrating the nanoparticles located on the fringe of as-prepared composites belong to indium oxide crystal. Figure 5(e-h) displays the scanning transmission electron microscopy (STEM) and corresponding X-ray energy dispersion spectrum (EDS) images of an individual 14 ACS Paragon Plus Environment

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CuO/In2O3 particle. It is obvious that three main elements including copper, indium and oxygen can be clearly identified by their uniform distribution along with the octahedral framework, which further indicates that the In2O3 nanoparticles were homogeneously deposited on the surface of CuO octahedrons.

To shed light on the chemical composition and electronic states of the elements existing in the pure CuO and CuO/In2O3 composites, X-ray photoelectron spectroscopy (XPS) was characterized. The survey spectra shown in Figure 6a manifest the coexistence of Cu 2p, O 1s and C 1s in both samples, while the C 1s might be derived from the adventitious carbon-based contaminant and the binding energy of whole spectrum was corrected by referencing the C 1s peak to 284.6 eV.39-40 In comparison to the pure CuO sample, two new In 3d peaks appear in the CuO/In2O3-2 sample, as shown in Figure 6a. Figure 6b presents the high-resolution spectrum of In 3d, from which the In (III) oxidation state can be clearly identified by the two main characteristic peaks corresponding to In 3d3/2 and In 3d5/2 with binding energies at ~453.6 eV and ~445.9 eV.41 The Cu 2p spectra in Figure 6c present two peaks corresponding to Cu 2p1/2 and Cu 2p3/2 with two shake-up satellites at higher binding energy sides, which are characteristics of Cu2+ in its oxide form.42-43 Besides, it is worth noting that Cu 2p peaks of CuO/In2O3 composites are strongly attenuated compared with those of pure CuO particles, which probably because pristine CuO were partially covered by the outer In2O3 particles, thus leading to most of X-ray cannot reach the surface of CuO. In the O 1s spectra (Figure 6d), three major fitting peaks centered in the range of 530.1 to 533.0 eV could be assigned to lattice oxygen 15 ACS Paragon Plus Environment


(OL), oxygen at the oxygen-deficient site (OV) within the matrix of crystal and surface adsorption oxygen (OS) from low to high binding energy in turn, respectively.42, 44 For pristine octahedral CuO, the relative percentage of OL, OV and OS was calculated to be about 57.7%, 34.3% and 8.0%, while for CuO/In2O3 composites case, their relative percentage was approximately 16.0%, 36.9% and 47.1%, respectively. Obviously, these results manifest that the proportion of adsorption oxygen component significantly increased after the modification of In2O3 nanoparticles on pristine CuO.

Figure 6. (a) XPS survey results of pure CuO and CuO/In2O3-2 and their (b) In 3d; (c) Cu 2p; and (d) O 1s high-resolution spectra. 16 ACS Paragon Plus Environment

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3.2 Gas sensing characteristics

It is well acknowledged that the sensing performance of oxide semiconductor is strongly subject to operating temperature, which has been testified to have considerable influence on adsorption/desorption processes of gases, as well as the surface chemical reactions between the absorbed oxygen and target gases.45-46 Therefore, the temperature-dependent sensing responses to 10 ppm H2S were first

Figure 7. (a) Temperature-dependent sensor responses toward 10 ppm H2S gas; (b) Sensing transient of CuO/In2O3-2 heterostructure to 10 ppm H2S at different temperature. 17 ACS Paragon Plus Environment


investigated in the temperature range of 150-275 °C (Figure 7a). Obviously, for the sensors based on pure CuO samples, the response reached maximum value at 200 °C. By contrast, it is clear that CuO/In2O3-based sensors exhibit higher maximum response and their optimum operating temperature significantly reduced in the meantime, which evidently manifests that gas-sensing performance of bare CuO can be effectively improved by the modification of In2O3 particles on the surface. However, because the H2S molecules are not active enough to overcome the barrier and react with the surface-absorbed oxygen species at low temperature, the gas sensing and reaction processes become too sluggish as the temperature is lower than 200 °C (the detailed response and recover times are shown in Figure S2). Therefore, from the perspective of practical application, we choose 200 °C as the optimum operating temperature for CuO/In2O3-based sensors by sacrificing some sensing response in return for faster response and recover behaviour. At 200 °C, the sensor based on CuO/In2O3-2 exhibits the highest response among three CuO/In2O3 composites, which is about 2 times higher than that of bare CuO. Figure 7b and Figure S3 present five-cycle dynamic test curves of CuO/In2O3-2 and bare CuO, and each single cycle was measured for 450 s, from which the good stability as well as their temperature-dependent sensing behaviour can be further confirmed. Moreover, it is clear that the resistances of the two kinds of sensors increased dramatically upon being exposed to H2S gas. Generally speaking, once the gas sensors based on oxide semiconductors are exposed to reducing gases (e.g., H2S), the ionized oxygen species (i.e., O2−, O−, and O2−) on the surface of sensing material will react with the reducing 18 ACS Paragon Plus Environment

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gases. Consequently, the electrons trapped in the ionized oxygen species will release back into the sensing materials through surface redox reaction. For n-type oxide semiconductors, the resistance will decrease due to the injection of extra electrons in this process. On the contrary, because the majority carrier of p-type oxide semiconductors is holes, the injection of electrons will reduce the concentration of holes, which in turn leading to the increase of electrical resistance. Therefore, it can be concluded that the present two kinds of sensors both exhibit typical p-type behaviour.

Figure 8. (a) Dynamic response curves of the sensors when orderly exposed to different concentration of H2S gas at 200 °C, inset depicts the responses of the two sensors as a function of H2S concentration; (b) Response of the sensors based on CuO/In2O3-2 composites and pure CuO to 10 ppm various gases (S: Hydrogen Sulfide, E: ethanol, A: acetone, M: methanol, F: formaldehyde, N: NH3, X: xylene).

Figure 8a depicts the dynamic response curves of the sensors based on as-synthesized bare CuO and CuO/In2O3-2 composites towards toxic H2S gas with the 19 ACS Paragon Plus Environment


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concentration ranging from 1-70 ppm. As can be seen, the two sensors both exhibit steadily ascending responses along with the increasing of H2S concentration, and the H2S-sensing

performances

are

significantly

enhanced

for

CuO/In2O3-2

heterostructures in comparison to its bare CuO counterpart under different concentration. Besides, it is obvious that the responses of the sensors fabricated by CuO/In2O3-2 heterostructures exhibit a faster increasing rate and better linearity vs. H2S concentrations in the full range (inset of Figure 8a). Those results manifest that the CuO/In2O3 composites have a broad detection range towards H2S toxic gas.

Since the selectivity of sensing material is directly associate with the recognition ability of gas sensor to certain gas, the selectivity of CuO and CuO/In2O3-2 heterostructures were further examined by comparing their response towards H2S and to other common interfering gases, including ethanol, acetone, methanol, formaldehyde, NH3, and xylene (displayed in Figure 8b). Each of test gases was measured at 200 °C with a concentration of 10 ppm. Apparently, from the conspicuous distinction of sensing responses towards H2S and other interfering gases, it can be seen that the sensor based on CuO/In2O3-2 heterostructure exhibits better selectivity in comparison to bare CuO, which makes CuO/In2O3-2 an excellent material for selectively detecting H2S gas. By contrast, the sensor based on pristine p-type CuO exhibits poor responses to all tested gases due to its intrinsic p-type characteristics mentioned above. Consequently, the little differences between gas-sensing responses might be the possible reason for the low selectivity toward H2S gas for pure CuO. However, in virtue of the unique strong chemical affinity between 20 ACS Paragon Plus Environment

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H2S and CuO,15,

23

the selective recognition towards H2S can be significantly

amplified in the presence of other gases through improving the intrinsic sensing properties of pure CuO. Therefore, we speculate the higher gas sensing selectivity of CuO/In2O3 observed here might be related to the enhanced response after the decoration with In2O3 nanoparticles on the surface of CuO. Even so, deep analysis is still needed to get a full understanding about the underlying mechanism, because the detailed mechanism about the selectivity may be much more complicated and the selectivity of gas sensor can be influenced by working temperature, the amount of gas adsorption, activation energy of different gas molecule, and chemical reaction on the surface of semiconductor.47-49

Figure 9. (a) Reproducibility and temporal stability test of the two kinds of sensors based on CuO/In2O3-2 and pristine CuO particles; (b) Long-term stability of the sensors under continuous test for 30 days.

To get insight into the stability of sensors, consecutive tests towards 10 ppm H2S were carried out at the operating temperature of 200 °C. As can be seen, ten reversible 21 ACS Paragon Plus Environment


cycles of measurement shown in Figure 9a evidently demonstrated the as-obtained two sensors exhibit robust response and recovery characteristics and excellent repeatability. Furthermore, long-term stability was recorded by continuously monitoring the response value of both sensors every few days, as shown in Figure 9b. It is clear that the responses of the two sensors nearly fluctuated around the initial value and exhibited no obvious variations for over 1 month. These observations further verified the splendid stability of the sensors.

3.3 Sensing mechanism

As demonstrated above, the sensors in the present work exhibited typical p-type gas sensing characteristics, which implies the sensing performance of sensors is mainly decided by p-type CuO skeleton materials. According to the well-established surface controlled model for oxide semiconductor gas sensor, the adsorption of negatively charged oxygen would induce the formation of a wide hole accumulation layer on the surface region of pristine CuO.50 Once the sensors were exposed to reducing gases (H2S), the electrons trapped in the negatively charged oxygen would release back into CuO through the surface redox reaction, and thus resulting in a remarkable shrinkage of hole accumulation layer and obvious increase of resistance (Schematically shown in Figure 10a).51-53 Therefore, it is concluded that the resistance of sensor increase in proportion to the concentration of reducing gas as well as adsorbed oxygen species. Based on the XPS observation in Figure 6d and Figure S4, the relative percentage of surface adsorbed oxygen was estimated to be 8.0 %, 33.5 %, 22 ACS Paragon Plus Environment

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47.1 % and 53.6 % for pristine CuO and CuO/In2O3-1, 2, 3 samples (summarized in Table S2). Therefore, we speculate that the enhanced surface adsorption oxygen observed here might be one of main reason for the increased sensing response of CuO/In2O3 composites. Furthermore, as demonstrated by many literatures, CuO is a narrow band gap p-type semiconductor (about 1.7 eV) with an electron affinity of about 4.96 eV, while In2O3 is a typical n-type semiconductor, the band gap and electron affinity of which is about 2.8 and 3.88 eV(schematically shown in Figure 10c) 54

. Because of the presence of acceptor and donor levels, Fermi level (EF) lies just

above valence band (EV) for p-type CuO and just below conduction band (EC) for n-type In2O3. Therefore, when the n-type In2O3 nanoparticles were uniformly decorated on the surface of p-type CuO, a typical p-n heterojunction formed at the interface. The electrons will transfer from n-type In2O3 to p-type CuO, while the holes will move in the opposite direction until the uniform Fermi levels of CuO and In2O3 was obtained (Figure 10d). Consequently, the carrier depletion layer was gradually produced during this process, which causes the dramatic reduction of holes in CuO (schematically shown in Figure 10b). Accordingly, the lower background hole concentration will make the sensing materials more sensitive to the subsequent injection of electrons and lead to higher chemiresistive variation if the amount of electron given by the gas sensing reaction were constant upon exposure to H2S gas. Meanwhile, Figure S5 presents the nitrogen adsorption–desorption isotherms of as-obtained bare CuO and CuO/In2O3-2 samples. Compared with the pristine CuO, the CuO/In2O3-2 heterostructure not only possesses higher specific surface area, but 23 ACS Paragon Plus Environment


also exhibits higher adsorption capacity under different relative pressure. Therefore, it can be concluded that the well-defined interior voids as well as the rough surface endow octahedral CuO/In2O3 mesocages much more active sites, which can significantly promote the surface reaction and contribute to a higher response. On the basis of the above discussion, the observed enhanced response of CuO/In2O3 composites can be understood as the combined effect of high-level of adsorbed oxygen, the resistance modulation effect of heterostructure and the unique mesocages structure and rough surface of CuO/In2O3 heterostructure.

Figure 10. The schematic of carrier transportation and gas sensing mechanism for (a) bare CuO and (b) CuO/In2O3 heterostructure; (c) Energy band diagram of individual CuO and In2O3 before contact; (d) Energy level diagram of CuO/In2O3 heterostructure.

4. CONCLUSIONS In conclusion, a facile and controllable method to synthesize octahedral-like 24 ACS Paragon Plus Environment

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CuO/In2O3

heterostructures

with

double-shelled

architecture

is

presented.

Homogeneous pristine self-sacrificial templates, octahedral-like Cu2O, were first prepared by one-step liquid-phase reaction. Then a uniform In2O3 shell layer composed of many nanoparticles was successfully decorated on the outer surface of CuO via room-temperature redox etching reaction and subsequent annealing process. In virtue of the modification of In2O3 nanoparticles, the resultant CuO/In2O3 composites exhibit higher response, lower operating temperature and faster response/recover speed during the dynamic measurement. The response of the optimized CuO/In2O3-based sensor exhibits about two times higher than that of bare CuO. And the improvement of sensing performances may arise from the combined effect of three factors: ( ⅰ ) the higher proportion of surface adsorbed oxygen compared with its pristine CuO counterpart could promote the surface reaction upon exposure to target gases; (ⅱ) owing to the carrier transfer between p-type CuO and n-type In2O3, the thinner hole accumulation layer will make the CuO/In2O3 composites more sensitive to the subsequent injection of electrons; (ⅲ) the rough surface of octahedral CuO/In2O3 endows it much more active sites, which can significantly enhance the utilization of sensing materials and contribute to a higher response finally.

ASSOCIATED CONTENT

Supporting Information



Characterization, Structure schematic of a typical gas sensor, Response and recover times of the sensors based on pure CuO and CuO/In2O3-2 heterostructure at different temperature, Sensing transient of bare CuO to 10 ppm H2S at different temperature, High-resolution O 1s spectra of CuO/In2O3-1 and CuO/In2O3-3 composite, Nitrogen adsorption–desorption isotherms of as-obtained bare CuO and CuO/In2O3-2 composite, Comparison of microstructures and synthesis methods of various CuO/In2O3 heterostructures, Relative percentages of OL, OV and OS estimated by the fitted results of O 1s spectra for bare CuO and CuO/In2O3 composites. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION

Corresponding authors *E-mail: [email protected] (Changlu Shao) Fax: +86 431 85098803. Tel: +86 431 85098803. *E-mail: [email protected] (Geyu Lu) Fax: +86 431 85167808. Tel: +86 431 85167808. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 26 ACS Paragon Plus Environment

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This work is supported by the China Postdoctoral Science Foundation (No. 2017M610188), the National Natural Science Foundation of China (No. 51572045, 51272041, 61201107, 11604044, and 91233204), the National Basic Research Program of China (973 Program) (No. 2012CB933703), the 111 Project (No. B13013), the Natural Science Foundation of Jilin Province of China (20160101313JC), the Fundamental Research Funds for the Central Universities (No.2412017QD007, 2412017FZ009).

ABBREVIATIONS

VOC, volatile organic compounds XRD, X-ray diffraction SEM, scanning electron microscopy FESEM, field emission scanning electron microscopy TEM, transmission electron microscopy HRTEM, high resolution transmission electron microscopy EDS, energy dispersive X-ray spectrometer STEM, scanning transmission electron microscopy XPS, X-ray photoelectron spectroscopy

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In2O3 Mesocages with Double-Shell Architectures

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