Facile synthesis of Cu₂O nanocages and gas sensing performance towards gasoline

Abstract

Facile synthesis of uniform cubic cuprous oxide (Cu₂O) nanocages has been realized via an acidic etching method without any surfactant at 35 °C. The edge length of the Cu₂O nanocages was 80–90 nm, and the thickness of the walls was about 5 nm. Moreover, the wall thickness of these nanocages could be adjusted by changing the reaction temperature. The formation mechanism of the Cu₂O nanocages was investigated. Ascorbic acid played an important role in this experiment. Owing to its reducing action, Cu(OH)₂ was first reduced to solid Cu₂O nanocubes in an aqueous solution. Almost immediately these nanocubes were etched into hollow ones through the acidic etching effect. Eventually, Cu₂O nanocages which had thinner walls and more complete structures than those previously mentioned were produced in the presence of hydrazine hydrate (N₂H₄·H₂O). The as-prepared Cu₂O nanocages have superior gas sensing performance toward gasoline comparing with the solid Cu₂O nanocubes.

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1. Introduction

As a typical p-type semiconductor with a band gap of 2.17 eV, Cu₂O has been proven to be a promising functional material, and widely used in many fields, such as photocatalysis,¹ Li-ion batteries,² gas detection,^3,4 organic reaction catalysis,^5,6 and non-enzymatic biosensors.⁷ In the past decade, synthesis of Cu₂O micro- and nanocrystals has attracted great interest. Up to now, a variety of Cu₂O nanostructures such as cubes,⁸ octahedra,⁹ spheres,¹⁰ nanocages,¹¹ nanowires,^12,13 and rhombic dodecahedra¹ have been reported. Among these morphologies, nanostructures with hollow interiors have aroused widespread attention because of their special qualities, including low density, high specific surface area, good permeability, and excellent optical properties. For example, Zhang et al. reported that hollow Cu₂O microspheres with multilayered and porous shells showed higher sensitivity in the ethanol sensor than solid Cu₂O nanocubes.³ Feng and co-workers detected the performance of the prepared Cu₂O nanomaterials on photocatalytic degradation of organic pollutants, and demonstrated that hollow octahedral Cu₂O possessed higher photocatalytic activity than the solid one.¹⁴

Up to date, various synthesis methods have been applied to generate hollow Cu₂O nanostructures. Single-crystalline octahedral Cu₂O nanocages,¹⁵ Cu₂O polyhedral nanocages,¹⁶ hollow Cu₂O nanospheres,¹⁷ Cu₂O microspheres with multilayered and porous shells,³ have been successfully prepared by different research groups. These hollow nanostructures exhibited superior physical and chemical properties to their solid analogues, indicating the advantages of the hollow nanostructures. However, the above synthetic processes require relatively long time, high temperature, extra surfactants or catalysts and complex process, and it is still a meaning work to develop facile, time-saving and template-free methods for the synthesis of hollow Cu₂O nanostructures.

In this study, cubic Cu₂O nanocages are synthesized by a facile method. In aqueous solution, hollow Cu₂O nanocubes with thick walls were quickly formed by reduction and acid etching role of ascorbic acid (AA) without any surfactant, and then well-defined Cu₂O nanocages with thin walls were obtained after adding N₂H₄·H₂O into the above solution. Formation mechanism of the Cu₂O nanocages was analyzed based on the experimental results. In addition, it is demonstrated that Cu₂O nanocages have superior performance as gas sensor to the solid Cu₂O nanocubes.

2. Experimental section

2.1. Synthesis of Cu₂O nanocages

All the chemical reagents were of analytical grade and used without further purification. In a typical synthesis, under constant and vigorous stirring, 50 mL of deionized water, 3 mL of CuCl₂ solution (0.05 mol dm⁻³), and 0.33 mL of NaOH solution (2 mol dm⁻³) were added into a round-bottomed glass flask at 35 °C. Then, ascorbic acid (AA) solution (0.108 g of ascorbic acid was dissolved in 1 mL deionized water) was quickly added into the above solution. The blue solution turned yellow in a few seconds, indicating that the Cu₂O nanoparticles appeared. At this time, the pH of the solution was about 5.0. After reacting for 5 min, 2 mL of N₂H₄·H₂O (85%) was dropped into the mixture, and then the mixture was stirred for another 30 min. The color of the mixture became brown slowly. The precipitate was collected through centrifugation and washed two times with distilled water and once with absolute ethanol. The solid products were obtained after drying under vacuum at 60 °C for 4 h.

2.2. Characterization

The X-ray diffraction (XRD) patterns were collected using a Rigaku D/MAX-2200PC diffractometer with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscope (TEM, JEOL JEM-1230), field emission scanning electron microscopy (FESEM, JEOL JSM 6700F) and high-resolution transmission electron microscopic (HR-TEM, JEOL JEM-2100, and JEM-2010) with an accelerating voltage of 200 kV were used to characterize the morphology and microstructure of the products.

2.3. Gas sensing performance

The fabrication process of the gas sensor is as follows. Cu₂O nanocages were uniformly dispersed in ethanol to form a suspension liquid, and then the suspension liquid was evenly coated on the surface of the ceramic tube. After the ceramic tube was treated by heating at 180 °C for 10 h, the four platinum wire connectors of the ceramic tube and two ends of the heating wire were welded as shown in Fig. S1.† The prepared component was aged for 2 h under air atmosphere using the Ni–Cr alloy heater with the resistance of 33 Ω and electron voltage of 2.5 V. Gas sensing properties of sensor were detected on a navigation 4000 series-intelligent system (Beijing ZhongKe Micro-Nano Co. Ltd) using the static gas mixing method at a fixed voltage of 5 V. The detected gas was selected as 93# gasoline. As a comparison, the sensing property of the sensor fabricated by the Cu₂O nanocubes was also detected.

3. Results and discussion

3.1. Morphologies and structure

The X-ray diffraction (XRD) pattern of the product in Fig. 1a shows that all the diffraction peaks can be indexed to the pure cubic phase Cu₂O (space group Pn3m, a = 0.427 nm, JCPDS file no. 05-0667), and no other impurity peaks were detected. The TEM image of the as-prepared Cu₂O shows the cubic morphology of the particles with the uniform size of about 80–90 nm (Fig. 1b). From the contrast difference between the edge and the interior of the particles it can be simply identified the hollow structures of the nanoparticles. HRTEM image shows the average thickness of the walls of ca. 5 nm (Fig. 1d). The SEM image of the product (Fig. 1c) shows that there are some broken nanocages, further indicating their hollow structure. From Fig. 1c, it also can be observed that the walls of the nanocages are composed of small nanoparticles with size less than 5 nm, and the corresponding HRTEM images (Fig. 1e and S2, ESI†) also reveal this point. Fig. 1f shows the lattice fringe with d spacing of 2.39 Å, corresponding to (111) lattice plane of cubic Cu₂O. The selected-area electron diffraction (SAED) pattern confirms that the nanocages are formed through an oriented arrangement of the Cu₂O nanoparticles with the exposed planes of {110}.

Fig. 1 (a) XRD pattern, (b) TEM, (c) SEM and (d–f) HRTEM images of the as-prepared Cu₂O nanocages formed at 35 °C for 35 min. The insets of (e) and (f) respectively show the corresponding TEM image and SAED pattern.

3.2. Formation process of the Cu₂O nanocages

Synthesis of hollow Cu₂O nanostructure can be realized by various methods, such as template synthesis,^18,19 acid etching,^11,20 oxidative etching,^16a and so on. To shed light on the formation of Cu₂O nanocages in this study, time dependent experiment was carried out. When AA as reducing agent was added into the flask, the color of the solution changed from blue to yellow in several seconds, indicating that Cu(OH)₂ was quickly reduced to Cu₂O nanoparticles (Fig. S3 and S4, ESI†). In order to obtain the corresponding TEM image, 2–3 drops of the above reaction liquid was dropped onto copper grid directly when the reaction time came to 10 s. The TEM image in Fig. 2a shows that the product is solid nanocubes composed of small nanoparticles at this time. After a reaction time of 30 s, the size of the solid cubes slightly increased with the surface changing smooth (Fig. 2b), indicating that the surface part of the cubes has a higher crystallinity than their interiors. Prolonging reaction time to 5 min, the solid nanostructures give place to the hollow ones with thick walls and the walls of these hollow Cu₂O cubes have a certain degree of rupture. The transformation of the structure was fast and amazing. In this study, considering the lack of surfactant, the acidity of the solution, and the rapid formation of hollow structure, we ruled out the possibility of template synthesis and oxidative etching, and inferred that the formation of hollow Cu₂O nanocubes was attributed to the selective etching of the solid Cu₂O nanocubes under the acidic condition.

Fig. 2 TEM images of Cu₂O nanoparticles obtained at 35 °C for 10 s (a), 30 s (b) and for 5 min (c).

In order to prove the acid etching formation mechanism of the hollow structure, the verification experiment was carried out. Firstly, a certain amount of NaOH was dropped into the reaction mixture quickly to adjust the solution pH to 9 when the reaction time came to 10 s. Here, NaOH was used to raise pH value of the solution, and then terminated the acid etching process. The resulting product was centrifuged and washed several times with deionized water. Corresponding TEM image in Fig. 3a shows that the product was solid nanocubes. Then, at room temperature, the prepared solid Cu₂O nanocubes were added into 50 mL deionized water under stirring, and then a certain amount of HCl was added into the dispersion to adjust the pH of the solution to 5.5. This pH value was consistent with that of the solution after adding AA in the typical experiment. The suspension was pipetted from the above mixture each 30 min and dropped onto copper grid directly for TEM observations (Fig. 3b–d). The TEM images confirmed that the original solid structure was gradually etched into hollow one as the extension of reaction time. That is to say, the interior nanoparticles of solid Cu₂O were selectively etched at proper acidity in this system. Therefore, it is concluded that the formation of hollow structure is caused by acidic etching. In addition, it can be observed that the surface of solid Cu₂O with loose structure in Fig. 2a is rough. With the reaction time extending, the surface of solid Cu₂O in Fig. 2b becomes smooth and has a fine crystalline. Once a small area of solid Cu₂O cubes' surface is etched under the suitable acidity, its interior nanoparticles will be etched with the outer shells being preserved (Fig. 2c), owing to the different crystalline states between the inside and outside of Cu₂O cubes. To further confirm the crystallinity difference between the inner part and the shell of the solid cubes, we compared the XRD patterns of the samples shown in Fig. 3a and d (Fig. S5, ESI†). It can be seen that the reflections of nanocages exhibit higher intensity than those of the solid cubes, which revealed the higher crystallinity of the shell than the inner part of the solid cubes. Thus, the crystallinity difference between surface and interior of the solid cubes leads to the selective acidic etching.

Fig. 3 (a) TEM image of Cu₂O nanoparticles obtained by adding NaOH to the mixture to terminate the acid etching process. (b–d) TEM images of Cu₂O nanoparticles obtained after etching by HCl for 30 min (b), 60 min (c), and 90 min (d).

When the reaction time reached to 5 min, N₂H₄·H₂O was added to the mixture. The color of the solution gradually turned from yellow to tan (Fig. S4, ESI†). The TEM image in Fig. 1b showed that the structure of the eventually formed Cu₂O nanocages was perfect and complete, and the thickness of the walls was much thinner than that shown in Fig. 2c. To make clear the role of N₂H₄·H₂O, time-dependent experiments have been conducted. When the total reaction time was prolonged to 5 h, the TEM image (Fig. 4a) showed that the wall thickness of Cu₂O nanocages became thinner and many small irregular nanoparticles appeared. When the reaction time reached to 9 h, the nanocages almost disappeared. However, the number of small nanoparticles which aggregated into membrane structure increased (Fig. 4b). The XRD pattern in Fig. 4c showed that diffraction peaks of Cu appeared when the reaction time was prolonged to 4 h, and their intensity gradually increased as the reaction time prolonging. Thus it is reasonable to believe that the small nanoparticles shown in Fig. 4a and b were Cu nanoparticles which were generated through the reduction of Cu(I) by N₂H₄·H₂O. This reduction is slow due to the low Cu(I) concentration and the formed Cu nanoparticles were mainly located on the Cu₂O surface at the initial stage. This reduction further promoted the dissolution of Cu₂O, and the wall thickness of Cu₂O nanocages became thinner and the volume of the cavity became bigger. In the typical experiment, the formed Cu nanoparticles were small and less, so they were oxidized to Cu₂O during the centrifugation and washing processes, which can be inferred from the color change from tan to dark yellow (Fig. S4, ESI†). With the reaction time prolonging, the amount of Cu nanoparticles increased, and the excessive dissolution of Cu₂O led to the collapse of the hollow structure. Then the Cu nanoparticles aggregated, which showed relatively higher stability and were difficult to be oxidized to Cu₂O. So Cu diffraction peaks can be detected by XRD.

Fig. 4 TEM images of the product synthesized at 35 °C for 5 h (a), 9 h (b). (c) XRD patterns of the product prepared at different reaction time.

According to above observations and analyses, the formation process of Cu₂O nanocages is summarized. At first, Cu(OH)₂ was transformed to Cu₂O nanoparticles with the addition of AA through a dissolution-recrystallization process. Because AA was added quickly, a large amount of Cu₂O nanoparticles formed rapidly, which aggregated to Cu₂O nanocubes to reduce the surface energy. With the proceeding of the reaction, the reagents' concentration gradually decreased and the rapid homogeneous nucleation of Cu₂O could not occur, and the crystal growth subsequently occurred on the aggregates' surface to form a fine shell. With the reaction the pH value of the solution gradually decreased to meet the requirement of the Cu₂O dissolution, then acid etching started. Due to the lower crystallinity of the interior of the Cu₂O nanocubes, once a part of the shell was dissolved the selective etching preferred to occur in the interior, and the hollow structure with highly crystalline thick wall was formed. Then, with the addition of N₂H₄·H₂O, the chemical equilibrium was broken once more. The Cu(I) in solution was deoxidized to metal Cu by N₂H₄·H₂O, which promoted the continuous dissolution of Cu₂O to form hollow structure with thin wall. To reduce the surface energy, the formation of Cu mainly occurred through the heterogeneous nucleation mode on the Cu₂O hollow cubes' surface. At last, during the centrifugation and washing processes the surface Cu nanoparticles were oxidized to Cu₂O, and Cu₂O nanocages with thin wall were obtained. The formation mechanism is in well agreement with the color change of the system (Fig. S4, ESI†).

In order to further verify the above inferred formation process of Cu₂O nanocages, the following experiments were carried out. At first, the role of AA was studied. When the amount of AA changed from 0.108 g to 0.054 g, the final nanoparticles were solid Cu₂O nanocubes rather than Cu₂O nanocages (Fig. 5). Decreasing the amount of AA led to the raise of the pH of the solution, then the acidity of the solution could not meet the requirement of acidic etching, and solid Cu₂O nanocubes rather than the hollow ones were formed.

Fig. 5 TEM image of Cu₂O nanocubes formed by decreasing the amount of AA to 0.054 g.

The amount of NaOH also has important influence on the morphology of the product. In this study, the role of NaOH could be summarized into two aspects. On the one hand, NaOH can react with Cu²⁺ to generate stable Cu(OH)₂ precipitation, which decreases the concentration of free Cu²⁺ and the redox reaction speed between AA and Cu²⁺. On the other hand, as is well known, the reducing ability of AA dramatically depends on the pH of the solution.²¹ NaOH can affect the reduction ability of AA by adjusting the pH value. At the same time, it should be noted that AA itself also has the ability to regulate the pH value of the solution. Thus, in order to obtain Cu₂O nanocages, the amount of NaOH must be appropriate when the amount of AA is certain. That is to say, adjusting the amounts of AA and NaOH simultaneously can still fabricate Cu₂O nanocages. Here, when the amount of AA was decreased to 0.054 g and the volume of NaOH was 0.2 mL, the Cu₂O nanocages with average size of 90 nm were prepared (Fig. 6a and b). When the amount of NaOH was changed to 0.4 mL with other conditions remain unchanged, the final product contained a large amount of solid Cu₂O nanocubes (Fig. 6c). This is because increasing the amount of NaOH led to a higher pH value of the solution, which was not beneficial to the acid etching process. These results further confirmed the acidic etching formation mechanism of the Cu₂O nanocages.

Fig. 6 (a and b) TEM and SEM images of Cu₂O nanocages formed by adjusting the amount of AA and NaOH. (c) TEM image of Cu₂O nanoparticles prepared by using 0.4 mL NaOH.

3.3. Gas sensing properties

Fig. 7 shows the typical response curves of Cu₂O sensors toward different concentrations of gasoline. It can be seen from Fig. 7 that the resistance of the sensors changed obviously as the transformation of atmosphere. When gasoline was injected into the testing system, the resistance of the sensor decreased rapidly. The resistance returned to the initial value quickly when the sensor was replaced in air atmosphere, indicating a fast response and recovery speed. By comparing Fig. 7a and b it can be found that Cu₂O nanocages toward different concentrations of gasoline had higher sensitivity (S = R_air/R_gas) than the solid ones (Fig. S6, ESI†). Previous researches have proved that changes in resistance of gas sensor is mainly caused by adsorption/desorption process of target gas molecules and reactions on the surface.^14,22,23 What is more, only the surface of sensing materials can be accessed by gas molecules. So the sensitivity of the materials largely depends on their surface accessibility. Cu₂O nanocages have large specific surface area and holes on the walls, whose inside and outside surface can participate in the adsorption/desorption process and provide more active sites than solid Cu₂O nanocubes. In addition, the hollow structures lead to a better accessibility of the gas molecules to the particles' surface, so Cu₂O nanocages exhibit higher sensitivity than their solid analogues.

Fig. 7 Dynamic responses to different concentration of gasoline of Cu₂O sensors with (a) Cu₂O nanocages showed in Fig. 1b, (b) solid Cu₂O nanocubes shown in Fig. S6.†

4. Conclusions

In conclusion, uniform Cu₂O nanocages with thin walls were successfully prepared without any surfactant by separately using AA and N₂H₄·H₂O as reducing agents. The generation of the hollow structure was caused by acidic etching. Time-dependent experiments confirmed that N₂H₄·H₂O could reduce part of Cu₂O on inside and outside surface of the walls to Cu. Owing to special hollow structure and surface state, Cu₂O nanocages showed higher gas sensing sensitivity toward gasoline than the solid Cu₂O nanocubes. This work provides a facile route to Cu₂O nanocages and may be applied in the preparation of other hollow nanostructures.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant 21271118), and the Taishan Scholars Climbing Program of Shandong Province.

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Footnote

† Electronic supplementary information (ESI) available: TEM image of the 5 min product, color change of the solution with the reaction time, fabrication of gas sensor, synthesis of the solid Cu₂O cubes. See DOI: 10.1039/c5ra07638b

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