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One-step synthesis of flower-like Cu2O photoelectric materials by hydrothermal method
Solar Energy 188 (2019) 265–270
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Data Article
One-step synthesis of flower-like Cu2O photoelectric materials by hydrothermal method ⁎
T
⁎
Binxia Yuana, , Xiaobo Liua, Honghong Fub, Jianfeng Liua, , Qunzhi Zhua, Maoliang Wua a b
College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Institute of Technology, Shanghai 201418, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu2O Flower-like Photoelectric materials Hydrothermal method
In the paper, the flower-like Cu2O had been obtained in the distilled water under very low reaction temperature. This hydrothermal method provided a homogenous environment for the nucleation and growth of the Cu2O. The reaction temperature, time, solvent, and additives would affect the purity and morphology of samples. When the reaction temperature was greater than 90 °C, elementary copper appeared in the as-obtained sample. With the reaction temperature and time increase, the morphologies also had been changed. The changing process of these morphologies had also been discussed. When ethanol amine replaced some or all of the aqueous solution, the resulting product is pure copper. Meantime, the effect of copper sources and additives on purity and morphology of products were investigated. Finally, the optical properties and PEC properties of the flower-like Cu2O were studied. This paper provided a very simple and low cost method to synthesis of flower-like Cu2O, which would be of great potential for the synthesis of other metal chalcogenides.
1. Introduction Cu2O is considered as significantly attractive in both cost and availability comparing with other compounds. Cu2O has a suitable band gap energy with about 2.0 eV, abundance, and nontoxicity (Paracchino et al., 2012; Golden et al., 1996; Shanid and Khadar, 2008). The hole mobility (100 cm2/Vs) of Cu2O is much higher than that of CuO crystals (0.1 cm2/Vs) (Dolai et al., 2017; Lee et al., 2011). Cu2O is applied as ptype semiconductor in solar cell, charge transporting interlayer in organic polymer and perovskite based solar cells, cathode material in lithium batteries, and etc (Perng et al., 2017; Chatterjee and Pal, 2016; Li et al., 2011; Minami et al., 2014). Cu2O with different morphologies had been explored for charge storage capability (Ameri et al., 2017). These applications required simple fabrication protocol with low energy consuming. Several researchers had reported the synthesis of Cu2O crystals using variety of techniques including electrochemical route (Ameri et al., 2017; Abdelfatah et al., 2015), magnetron sputtering (Dolai et al., 2017), hydrothermal synthesis (Lan et al., 2011; Kumar et al., 2016; Gupta et al., 2018); microplasma synthesis (Du and Xiao, 2014), and also by direct heating of Cu foil (Jiang et al., 2002). All these methods give rise to variation in crystal quality and hence the optoelectronic property of the material. Some of the methods use toxic materials like hydrazine (Xu et al., 2006; Wang et al., 2002) and NaBH4 (Zhang et al., ⁎
2006) as reducing agents for synthesis of Cu2O. In order to improve the practicability and popularization of synthetic route, all of the raw materials in this paper were non-toxic and cheap. In the paper, we reported a hydrothermal process in water solvent for the synthesis of Cu2O crystals without expensive single-source precursors and complex craft. The reaction temperature and solvent played a critical role in the formation of pure Cu2O crystals. When the reaction temperature was greater than 90 °C, elementary copper appeared in the as-obtained sample. The morphologies of the samples had also changed as the temperature changed. When the partial or total solution of water was replaced with ethanol amine, XRD showed that the products all were elementary Cu in all the reaction conditions. Meantime, the copper sources and additives also had great influence on the morphology of samples. The formation mechanism of these morphologies had also been studied. Accordingly, the absorption spectrum was measured in order to study the optical properties of flower-like Cu2O. Finally, the Instantaneous I-t curve of flower-like Cu2O was used for investigating the photoelectrochemical cell (PEC) properties.
Corresponding authors. E-mail address: [email protected] (B. Yuan).
https://doi.org/10.1016/j.solener.2019.06.014 Received 20 March 2019; Received in revised form 3 June 2019; Accepted 5 June 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 188 (2019) 265–270
B. Yuan, et al.
2. Experimental 2.1. Chemicals All the reactions and operations were carried out in open-air. All chemicals were used directly without further purification. Copper acetate (Cu(CH3COO)2, chemical reagent), copper nitrate (CuNO3·3H2O, analytical reagent), copper sulfate (CuSO4·5H2O, chemical reagent) and glucose (C6H12O6, analytical reagent) were purchased from Shanghai Chemical Reagent (SCR). Hexadecyl trimethyl ammonium bromide (CTAB), Polyvinyl Pyrrolidone (PVP), and ethanol amine were purchased from Aldrich. 2.2. Synthesis of the pyrite Cu2O In a typical procedure, 2 mmol Cu(CH3COO)2, and 2 mmol glucose were added to a given amount of distilled water and were dispersed to form a blue emulsion by vigorous stirring. Then, the resulting mixture was transferred into a Telfon-lined stainless autoclave (60 ml). The autoclave was sealed and maintained at the given temperature for 12 h, then cooled to room temperature naturally. After that the resulting red solid products were collected by centrifugation, washed with alcohol to remove the excess surfactants, and finally dried naturally as well as utilized for further characterization.
Fig. 1. XRD pattern of the obtained sample.
Pn3m (JPCD NO. 05-0667). The lattice constants of Cu2O were a = b = c = 4.270 Å. The results were consistent with the value given in the standard card. No impurities peaks were observed, which indicated that the samples were pure. The sharp and narrow peaks illuminated the high crystallization of the samples. The size and morphology of as-obtained Cu2O products were illustrated by SEM and TEM image, as shown in Fig. 2. The low-magnification FESEM image (Fig. 2a) demonstrated that the samples were composed of lots of flower-like structure. The average size of a flower branch was about 2 μm by 10 μm. The high-magnification picture (Fig. 2b) indicated that the surface of samples was smooth. Further insight into the morphology was conducted by TEM. The same morphology of the flower-like microspheres was confirmed by TEM image, as shown in Fig. 2c. The electron diffraction pattern (Fig. 2d) taken from the edge of flower-like microsphere showed the structure was well-crystallized. The chemical composition of the as-obtained samples was characterized by EDS analysis (Fig. 2e). Peaks of the elements Cu and O were observed and the molar ratio was about 2:0.89. Thus, the samples were Cu2O in further evidence. The signals of the C element originated from the conductive adhesive during the measurement. It was worthwhile to mention that the reaction temperatures had strong influences on the morphology and purity of Cu2O samples. Fig. 3 showed the XRD patterns of different products prepared at different temperature for 12 h. When the reaction temperature was under 90 °C, the XRD pattern showed that the red powder can be indexed as pure Cu2O crystals. When the reaction temperature was 110 °C, the characteristic peaks of Cu (5.4 wt%) were detected. With the reaction temperature increase, the strength of Cu diffraction peaks also went up. The weight of cubic Cu accounted for 16.6% at 150 °C. When the reaction temperature was increased to 200 °C, the main product was proved to be Cu (93.5 wt%) with minor Cu2O crystals (6.5 wt%). Thus, as-obtained Cu2O crystals were reduced to Cu by glucose with increasing of reaction temperature. The SEM images of as-obtained products prepared at different temperature were shown in Fig. 4. When the reaction temperature was 50 °C, the morphology of sample (Fig. 4a) was flower-like structure. When the reaction temperature was increased to 90 °C, unregularly stick-like structure with minor flower-like structure can be observed. It was probably that the reactive system had sufficient thermal energy to destroy the connecting key between flower-like samples at relatively high temperatures. Meantime, the solution was close to the boiling point of water in the autoclave, the unstable reaction process lead to the product with unregularly structure. As the reaction temperature rose to 110 °C, the Cu particles (marked with one yellow circle) were detected from Fig. 4c. When the reaction temperature was 150 °C, more Cu particles and the dendritic with some holes (marked with red circle) were observed. We thought that the hole was caused by Cu2O reduction
2.3. Preparation working electrode of PEC 10 mg Cu2O powders and a certain amount of film former were added into 5 ml ethanol, which was sonicated for 15 min. The obtained homogeneous suspension was dropped onto a piece of FTO conductive glass. After drying in air, the Cu2O film coated FTO electrode was further handled at 200 °C for 3 h in a muffle furnace, and then cooled to room temperature naturally. 2.4. Characterization X-ray diffraction (XRD) patterns were recorded with a Rigaku D/ max 2250 V diffractometer operating with Cu Kα radiation. The operation voltage and current were set as 40 kV and 100 mA, respectively, and the samples were powder without further treatment. Field emission scanning electron microscope (FESEM) images were acquired using S4800 operated at an acceleration voltage of 10 kV, and the samples were prepared by sonicating the products in absolute ethanol, then evaporating one drop of suspension on conductive adhesive. Energydispersive spectrum (EDS) was taken on S-4800 field emission scanning electron microscope. High resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV, and the samples were prepared by dipping an amorphous carbon–copper grid in dilute solution of samples dispersed in chloroform. The absorption properties were acquired by SHIMADZU UV-3000 plus spectrophotometer. The measurement samples were powder. The PEC measurements was conducted in a typical three-electrode electrochemical cell by CHI660E electrochemical workstation in 0.5 M Na2SO4, where 2 cm2 Pt plate was used as the counter electrode, Ag/AgCl in saturated KCl was used as the reference electrode, and the prepared Cu2O films with an area of 2 × 2 cm2 were used as the working electrode. Full band spectrum was used as the illumination source with 300 W xenon lamp. 3. Results and discussion 3.1. Structures and morphologies XRD was used to verify the phase structures of final products. Fig. 1 showed the XRD pattern of the typical sample. All the diffraction peaks observed in this pattern can be indexed as Cu2O with the space group 266
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Fig. 2. (a, b) SEM images, (c) TEM images, and (d) the corresponding electron diffraction patterns, (e) EDS pattern of Cu2O samples.
To further investigate the conversion process of the Cu2O morphology, SEM analysis was conducted to characterize the samples at different reaction time, as shown in Fig. 5. These samples were proved to be Cu2O crystals by XRD analysis. When the reaction time was 2 h, a lot of uniform flower-like crystals were observed from low-magnification SEM image (Fig. 5a). Further magnification of SEM image (Fig. 5b), most of branch at flower-like structure was rough and looked like pieces stacked structure. The morphology of sample obtained at 6 h was similar with the sample obtained at 2 h. When the reaction time was increased to 12 h, most of samples were still flower-like with smooth branch. As the reaction time was extended to 24 h, the connecting key between flower-likes was destroyed, some samples changed from flower to dendritic structure (Fig. 5e). And the shape of the dendrite also was converted from conical cones to cuboids (marked with one yellow circle). Interestingly, when the reaction time was further extended to 48 h, the cuboids changed to octahedral pyramid (marked with one red circle) (Fig. 4f). In order to investigate the influence of solvent on the phase structure and morphology of the products obtained at from 50 °C to 200 °C, XRD and SEM analysis were used to characterize the samples. When the ethanol amine substituted part or all of the water as the solvent, the samples were all element Cu by XRD analysis (Fig. 6a). All the diffraction peaks observed in this pattern can be indexed as Cu with lattice constants a = 3.615 Å (JPCD No. 04-0836). SEM image (Fig. 6a) demonstrated that the samples were composed of lots of spheroidal particles with an average diameter of 2 μm. Meanwhile, we also found that the morphologies of the Cu2O samples changed with various copper sources, as shown in Fig. 7. XRD patterns showed that the acquired crystals were pure Cu2O crystal. If CuSO4 was taken as copper source while keeping other conditions unchanged, the as-prepared samples were almost composed of flower-like. Compared with the flower-like microsphere obtained by Cu(Ac)2, these flowers seemed like the pyramid structure of stacked stones. If using Cu (NO3)2 as copper source, SEM image indicated that flower-like with carambola structure can be observed. When the copper acetate was 2 mmol, the effects of PVP and CTAB additives on samples morphologies were investigated, as shown in
Fig. 3. Evolution of the XRD patterns from Cu2O (●) to Cu (◆) at 12 h with the reaction temperature increase.
into copper by glucose at high reaction temperature. When the temperature reached to 200 °C, the production was composed of globular, cubic, flocculent, and etc. If the temperature continued to rise in the experiment, the solution would leak out during the reaction. Obviously, the pressure in the autoclave should be reach to the maximum value at 200 °C. Thus, the morphologies of as-obtained sample were various and clutter.
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Fig. 4. SEM images at the different reaction temperatures: (a) 50 °C; (b) 90 °C; (c) 110 °C; (d) 150 °C; (e) 200 °C.
Fig. 5. SEM images with the different reaction temperatures: (a, b) 2 h; (c) 6 h; (d) 12 h; (e) 24 h; (f) 48 h.
crystalline morphology.
Fig. 8. If PVP was added into solution while keeping other conditions unchanged, the as-obtained sample was confirmed as pure Cu2O crystal by XRD measurement. Form Fig. 8a, the flower-like with carambola structure can be observed, but the branches were roughly and there were holes in the branches to be observed. When the weight of CTAB was 500 mg, the product were blue flocculent and the red precipitate was no found. When the weight of CTAB was reduced to 100 mg, the asobtained samples were the red precipitate of Cu2O by XRD analysis. And the morphology of Cu2O (Fig. 8b) had also a great change. Most of the sample with laminar structure gathered together, and a small number of tiny particles were distributed in the above. As the weight of CTAB was 40 mg, the product was composed of globular with diameter of 6 μm and irregular particles. Thus, the concentration of CTAB affected the crystallization process of cuprous oxide, and changed its
3.2. Optical properties Fig. 9 showed the UV–vis-NIR absorption spectrum of flower-like Cu2O. It included the four absorption peaks at 243 nm, 315 nm, 424 nm, and 549 nm. On the basis of tangent graphic of (α hv)2vs. hv , we estimated the band gap of the flower-like Cu2O to be 1.88 eV. On the other hand, the band gap with absorption peak could be calculated by the formula (Eg = 1240/λmax, λmax is the first exciton peak), which was 2.26 eV. From the absorption curve, it could be found that the absorption performance of the ultraviolet and visible light was much higher than that of the near infrared region, which meant the sample was more suitable for the fabrication of photovoltaic solar cells. 268
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Fig. 6. XRD and SEM of the products obtained by ethanol amine as solvent.
Fig. 7. SEM images of the Cu2O products obtained with different copper sources: (a) Cu(Ac)2, (b) CuSO4, and (b) Cu(NO3)2.
Fig. 8. SEM images with the different additives (a) PVP; (b) n (Cu2+): w (CTAB) = 2 mmol: 100 mg; (c) n (Cu2+): w (CTAB) = 2 mmol: 40 mg.
Fig. 10. Instantaneous I-t curve of flower Cu2O at 0.5 V vs Ag/AgCl. Fig. 9. UV–vis-NIR spectroscopic characterization of the flower Cu2O.
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3.3. PEC properties
characterization of high-photoactivity electrodeposited Cu2O solar absorber by photoelectrochemistry and ultrafast spectroscopy. J. Phys. Chem. C 116, 7341–7350. Golden, T.D., Shumsky, M.G., Zhou, Y., Vander Werf, R.A., Van Leeuwen, R.A., Switzer, J.A., 1996. Electrochemical deposition of copper(I) oxide films. Chem. Mater. 8, 2499–2504. Shanid, N., Khadar, M.A., 2008. Evolution of nanostructure, phase transition and band gap tailoring in oxidized Cu thin films. Thin Solid Films 516, 6245–6252. Dolai, S., Dey, R., Das, S., Hussain, S., Bhar, R., Pal, A.K., 2017. Cupric oxide (CuO) thin films prepared by reactive d.c. magnetron sputtering technique for photovoltaic application. J. Alloy. Comp. 724, 456–464. Lee, Y.S., Winkler, M.T., Siah, S.C., Brandt, R., Buonassisi, T., 2011. Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering. Appl. Phys. Lett. 98, 192115. Perng, D.C., Hong, M.H., Chen, K.H., Chen, K.H., 2017. Enhancement of short-circuit current density in Cu2O/ZnO heterojunction solar cells. J. Alloy. Comp. 695, 549–554. Chatterjee, S., Pal, A.J., 2016. Introducing Cu2O thin films as a hole-transport layer in efficient planar perovskite solar cell structures. J. Phys. Chem. C 120, 1428–1437. Li, S.-K., Guo, X., Wang, Y., Huang, F.-Z., Shen, Y.-H., Wang, X.-M., Xie, A.-J., 2011. Rapid synthesis of flower-like Cu2O architectures in ionic liquids by the assistance of microwave irradiation with high photochemical activity. Dalton Trans. 40, 6745–6750. Minami, T., Miyata, T., Nishi, Y., 2014. Cu2O-based heterojunction solar cells with an Aldoped ZnO/oxide semiconductor thermally oxidized Cu2O sheet structure. Sol. Energy 2014 (105), 206–217. Ameri, B., Davarani, S.S.H., Roshani, R., Moazami, H.R., Tadjarodi, A., 2017. A flexible mechanochemical route for the synthesis of copper oxide nanorods/nanoparticles/ nanowires for supercapacitor applications: the effect of morphology on the charge storage ability. J. Alloy. Comp. 695, 114–123. Abdelfatah, M., Ledig, J., El-Shaer, A., Wagner, A., Sharafeev, A., Lemmens, P., Mosaad, M.M., Waag, A., Bakin, A., 2015. Fabrication and characterization of flexible solar cell from electrodeposited Cu2O thin film on plastic substrate. Sol. Energy 122, 1193–1198. Lan, X., Zhang, J., Gao, H., Wang, T., 2011. Morphology-controlled hydrothermal synthesis and growth mechanism of microcrystal Cu2O. CrystEngComm 13, 633–636. Kumar, R., Rai, P., Sharma, A., 2016. Facile synthesis of Cu2O microstructures and their morphology dependent electrochemical supercapacitor properties. RSC Adv. 6, 3815–3822. Gupta, D., Mehera, S.R., Illyaskutty, N., Alex, Z.C., 2018. Facile synthesis of Cu2O and CuO nanoparticles and study of their structural, optical and electronic properties. J. Alloys Comp. 743, 737–745. Du, C.M., Xiao, M.D., 2014. Cu2O nanoparticles synthesis by microplasma. Sci. Rep. 4, 7339. Jiang, X., Herricks, T., Xia, Y., 2002. CuO nanowires can be synthesized by heating copper substrates in air. Nano Lett. 2, 1333–1338. Xu, H., Wang, W., Zhu, W., 2006. Shape evolution and size-controllable synthesis of Cu2O octahedra and their morphology-dependent photocatalytic properties. J. Phys. Chem. B 110, 13829–13834. Wang, W.Z., Wang, G.H., Wang, X.S., Zhan, Y.J., Liu, Y.K., Zheng, C.L., 2002. Synthesis and characterization of Cu2O nanowires by a novel reduction route. Adv. Mater. 14, 67–69. Zhang, J., Liu, J., Peng, Q., Wang, X., Li, Y., 2006. Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors. Chem. Mater. 18, 867–871.
The samples were measured as the working electrode under on/off of 20 s at 0.5 V vs Ag/AgCl with illumination of full band spectrum 300 W incident intensity. The photocurrent of the flower-like Cu2O samples was about 0.0039 mA/cm2, as shown in Fig. 10. The photocurrent can keep up to 92% after 5 cycles under on/off switching irradiation, meaning the samples had high light stability and reproducible. The ratio of photocurrent to dark current was about 1.5 for the flower-like Cu2O samples with relatively low photosensitive properties. 4. Conclusion In summary, we had described the synthesis of pure phase and flower-like Cu2O via a simple hydrothermal method using water as solvent. When the reaction temperature was less than 90 °C, the samples were pure Cu2O crystal. With the reaction temperature increase, elemental copper appeared in the sample. The initial flower-like transformed into dendritic Cu2O, and then dendritic with holes and small particles Cu. When the reaction temperature was set at 50 °C, the branch of flower-like changed from pieces stacked, smooth conical cones, cuboids to octahedral pyramid as reaction time lengthened. When the ethanol amine replaced part or all of the water as the solvent, the samples were all spheroidal Cu particles. When the Cu(Ac)2 was instead by CuSO4 or Cu(NO3)2 as copper source, the as-prepared samples were almost composed of flower-like with pyramid structure of stacked stones and carambola structure, respectively. The additives CTAB had a strong effect on the morphologies. Finally, the four absorption peaks at 243 nm, 315 nm, 424 nm, and 549 nm were observed in the absorption spectrum, which showed the flower-like Cu2O with higher absorption performance in visible light range. The photocurrent of the flower-like Cu2O samples was about 0.0039 mA/cm2 with high reproducible. Acknowledgments This work was supported by the financial supports from the Fundamental Research Funds for National Nature Science Foundation of China (51576119), Young Eastern Scholar (QD 2016052). References Paracchino, A., Brauer, J.C., Moser, J.E., Thimsen, E., Graetzel, M., 2012. Synthesis and
270
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Data Article
One-step synthesis of flower-like Cu2O photoelectric materials by hydrothermal method ⁎
T
⁎
Binxia Yuana, , Xiaobo Liua, Honghong Fub, Jianfeng Liua, , Qunzhi Zhua, Maoliang Wua a b
College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Institute of Technology, Shanghai 201418, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu2O Flower-like Photoelectric materials Hydrothermal method
In the paper, the flower-like Cu2O had been obtained in the distilled water under very low reaction temperature. This hydrothermal method provided a homogenous environment for the nucleation and growth of the Cu2O. The reaction temperature, time, solvent, and additives would affect the purity and morphology of samples. When the reaction temperature was greater than 90 °C, elementary copper appeared in the as-obtained sample. With the reaction temperature and time increase, the morphologies also had been changed. The changing process of these morphologies had also been discussed. When ethanol amine replaced some or all of the aqueous solution, the resulting product is pure copper. Meantime, the effect of copper sources and additives on purity and morphology of products were investigated. Finally, the optical properties and PEC properties of the flower-like Cu2O were studied. This paper provided a very simple and low cost method to synthesis of flower-like Cu2O, which would be of great potential for the synthesis of other metal chalcogenides.
1. Introduction Cu2O is considered as significantly attractive in both cost and availability comparing with other compounds. Cu2O has a suitable band gap energy with about 2.0 eV, abundance, and nontoxicity (Paracchino et al., 2012; Golden et al., 1996; Shanid and Khadar, 2008). The hole mobility (100 cm2/Vs) of Cu2O is much higher than that of CuO crystals (0.1 cm2/Vs) (Dolai et al., 2017; Lee et al., 2011). Cu2O is applied as ptype semiconductor in solar cell, charge transporting interlayer in organic polymer and perovskite based solar cells, cathode material in lithium batteries, and etc (Perng et al., 2017; Chatterjee and Pal, 2016; Li et al., 2011; Minami et al., 2014). Cu2O with different morphologies had been explored for charge storage capability (Ameri et al., 2017). These applications required simple fabrication protocol with low energy consuming. Several researchers had reported the synthesis of Cu2O crystals using variety of techniques including electrochemical route (Ameri et al., 2017; Abdelfatah et al., 2015), magnetron sputtering (Dolai et al., 2017), hydrothermal synthesis (Lan et al., 2011; Kumar et al., 2016; Gupta et al., 2018); microplasma synthesis (Du and Xiao, 2014), and also by direct heating of Cu foil (Jiang et al., 2002). All these methods give rise to variation in crystal quality and hence the optoelectronic property of the material. Some of the methods use toxic materials like hydrazine (Xu et al., 2006; Wang et al., 2002) and NaBH4 (Zhang et al., ⁎
2006) as reducing agents for synthesis of Cu2O. In order to improve the practicability and popularization of synthetic route, all of the raw materials in this paper were non-toxic and cheap. In the paper, we reported a hydrothermal process in water solvent for the synthesis of Cu2O crystals without expensive single-source precursors and complex craft. The reaction temperature and solvent played a critical role in the formation of pure Cu2O crystals. When the reaction temperature was greater than 90 °C, elementary copper appeared in the as-obtained sample. The morphologies of the samples had also changed as the temperature changed. When the partial or total solution of water was replaced with ethanol amine, XRD showed that the products all were elementary Cu in all the reaction conditions. Meantime, the copper sources and additives also had great influence on the morphology of samples. The formation mechanism of these morphologies had also been studied. Accordingly, the absorption spectrum was measured in order to study the optical properties of flower-like Cu2O. Finally, the Instantaneous I-t curve of flower-like Cu2O was used for investigating the photoelectrochemical cell (PEC) properties.
Corresponding authors. E-mail address: [email protected] (B. Yuan).
https://doi.org/10.1016/j.solener.2019.06.014 Received 20 March 2019; Received in revised form 3 June 2019; Accepted 5 June 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 188 (2019) 265–270
B. Yuan, et al.
2. Experimental 2.1. Chemicals All the reactions and operations were carried out in open-air. All chemicals were used directly without further purification. Copper acetate (Cu(CH3COO)2, chemical reagent), copper nitrate (CuNO3·3H2O, analytical reagent), copper sulfate (CuSO4·5H2O, chemical reagent) and glucose (C6H12O6, analytical reagent) were purchased from Shanghai Chemical Reagent (SCR). Hexadecyl trimethyl ammonium bromide (CTAB), Polyvinyl Pyrrolidone (PVP), and ethanol amine were purchased from Aldrich. 2.2. Synthesis of the pyrite Cu2O In a typical procedure, 2 mmol Cu(CH3COO)2, and 2 mmol glucose were added to a given amount of distilled water and were dispersed to form a blue emulsion by vigorous stirring. Then, the resulting mixture was transferred into a Telfon-lined stainless autoclave (60 ml). The autoclave was sealed and maintained at the given temperature for 12 h, then cooled to room temperature naturally. After that the resulting red solid products were collected by centrifugation, washed with alcohol to remove the excess surfactants, and finally dried naturally as well as utilized for further characterization.
Fig. 1. XRD pattern of the obtained sample.
Pn3m (JPCD NO. 05-0667). The lattice constants of Cu2O were a = b = c = 4.270 Å. The results were consistent with the value given in the standard card. No impurities peaks were observed, which indicated that the samples were pure. The sharp and narrow peaks illuminated the high crystallization of the samples. The size and morphology of as-obtained Cu2O products were illustrated by SEM and TEM image, as shown in Fig. 2. The low-magnification FESEM image (Fig. 2a) demonstrated that the samples were composed of lots of flower-like structure. The average size of a flower branch was about 2 μm by 10 μm. The high-magnification picture (Fig. 2b) indicated that the surface of samples was smooth. Further insight into the morphology was conducted by TEM. The same morphology of the flower-like microspheres was confirmed by TEM image, as shown in Fig. 2c. The electron diffraction pattern (Fig. 2d) taken from the edge of flower-like microsphere showed the structure was well-crystallized. The chemical composition of the as-obtained samples was characterized by EDS analysis (Fig. 2e). Peaks of the elements Cu and O were observed and the molar ratio was about 2:0.89. Thus, the samples were Cu2O in further evidence. The signals of the C element originated from the conductive adhesive during the measurement. It was worthwhile to mention that the reaction temperatures had strong influences on the morphology and purity of Cu2O samples. Fig. 3 showed the XRD patterns of different products prepared at different temperature for 12 h. When the reaction temperature was under 90 °C, the XRD pattern showed that the red powder can be indexed as pure Cu2O crystals. When the reaction temperature was 110 °C, the characteristic peaks of Cu (5.4 wt%) were detected. With the reaction temperature increase, the strength of Cu diffraction peaks also went up. The weight of cubic Cu accounted for 16.6% at 150 °C. When the reaction temperature was increased to 200 °C, the main product was proved to be Cu (93.5 wt%) with minor Cu2O crystals (6.5 wt%). Thus, as-obtained Cu2O crystals were reduced to Cu by glucose with increasing of reaction temperature. The SEM images of as-obtained products prepared at different temperature were shown in Fig. 4. When the reaction temperature was 50 °C, the morphology of sample (Fig. 4a) was flower-like structure. When the reaction temperature was increased to 90 °C, unregularly stick-like structure with minor flower-like structure can be observed. It was probably that the reactive system had sufficient thermal energy to destroy the connecting key between flower-like samples at relatively high temperatures. Meantime, the solution was close to the boiling point of water in the autoclave, the unstable reaction process lead to the product with unregularly structure. As the reaction temperature rose to 110 °C, the Cu particles (marked with one yellow circle) were detected from Fig. 4c. When the reaction temperature was 150 °C, more Cu particles and the dendritic with some holes (marked with red circle) were observed. We thought that the hole was caused by Cu2O reduction
2.3. Preparation working electrode of PEC 10 mg Cu2O powders and a certain amount of film former were added into 5 ml ethanol, which was sonicated for 15 min. The obtained homogeneous suspension was dropped onto a piece of FTO conductive glass. After drying in air, the Cu2O film coated FTO electrode was further handled at 200 °C for 3 h in a muffle furnace, and then cooled to room temperature naturally. 2.4. Characterization X-ray diffraction (XRD) patterns were recorded with a Rigaku D/ max 2250 V diffractometer operating with Cu Kα radiation. The operation voltage and current were set as 40 kV and 100 mA, respectively, and the samples were powder without further treatment. Field emission scanning electron microscope (FESEM) images were acquired using S4800 operated at an acceleration voltage of 10 kV, and the samples were prepared by sonicating the products in absolute ethanol, then evaporating one drop of suspension on conductive adhesive. Energydispersive spectrum (EDS) was taken on S-4800 field emission scanning electron microscope. High resolution transmission electron microscope (HRTEM) images were acquired using a JEOL JEM-2100F operated at an acceleration voltage of 200 kV, and the samples were prepared by dipping an amorphous carbon–copper grid in dilute solution of samples dispersed in chloroform. The absorption properties were acquired by SHIMADZU UV-3000 plus spectrophotometer. The measurement samples were powder. The PEC measurements was conducted in a typical three-electrode electrochemical cell by CHI660E electrochemical workstation in 0.5 M Na2SO4, where 2 cm2 Pt plate was used as the counter electrode, Ag/AgCl in saturated KCl was used as the reference electrode, and the prepared Cu2O films with an area of 2 × 2 cm2 were used as the working electrode. Full band spectrum was used as the illumination source with 300 W xenon lamp. 3. Results and discussion 3.1. Structures and morphologies XRD was used to verify the phase structures of final products. Fig. 1 showed the XRD pattern of the typical sample. All the diffraction peaks observed in this pattern can be indexed as Cu2O with the space group 266
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Fig. 2. (a, b) SEM images, (c) TEM images, and (d) the corresponding electron diffraction patterns, (e) EDS pattern of Cu2O samples.
To further investigate the conversion process of the Cu2O morphology, SEM analysis was conducted to characterize the samples at different reaction time, as shown in Fig. 5. These samples were proved to be Cu2O crystals by XRD analysis. When the reaction time was 2 h, a lot of uniform flower-like crystals were observed from low-magnification SEM image (Fig. 5a). Further magnification of SEM image (Fig. 5b), most of branch at flower-like structure was rough and looked like pieces stacked structure. The morphology of sample obtained at 6 h was similar with the sample obtained at 2 h. When the reaction time was increased to 12 h, most of samples were still flower-like with smooth branch. As the reaction time was extended to 24 h, the connecting key between flower-likes was destroyed, some samples changed from flower to dendritic structure (Fig. 5e). And the shape of the dendrite also was converted from conical cones to cuboids (marked with one yellow circle). Interestingly, when the reaction time was further extended to 48 h, the cuboids changed to octahedral pyramid (marked with one red circle) (Fig. 4f). In order to investigate the influence of solvent on the phase structure and morphology of the products obtained at from 50 °C to 200 °C, XRD and SEM analysis were used to characterize the samples. When the ethanol amine substituted part or all of the water as the solvent, the samples were all element Cu by XRD analysis (Fig. 6a). All the diffraction peaks observed in this pattern can be indexed as Cu with lattice constants a = 3.615 Å (JPCD No. 04-0836). SEM image (Fig. 6a) demonstrated that the samples were composed of lots of spheroidal particles with an average diameter of 2 μm. Meanwhile, we also found that the morphologies of the Cu2O samples changed with various copper sources, as shown in Fig. 7. XRD patterns showed that the acquired crystals were pure Cu2O crystal. If CuSO4 was taken as copper source while keeping other conditions unchanged, the as-prepared samples were almost composed of flower-like. Compared with the flower-like microsphere obtained by Cu(Ac)2, these flowers seemed like the pyramid structure of stacked stones. If using Cu (NO3)2 as copper source, SEM image indicated that flower-like with carambola structure can be observed. When the copper acetate was 2 mmol, the effects of PVP and CTAB additives on samples morphologies were investigated, as shown in
Fig. 3. Evolution of the XRD patterns from Cu2O (●) to Cu (◆) at 12 h with the reaction temperature increase.
into copper by glucose at high reaction temperature. When the temperature reached to 200 °C, the production was composed of globular, cubic, flocculent, and etc. If the temperature continued to rise in the experiment, the solution would leak out during the reaction. Obviously, the pressure in the autoclave should be reach to the maximum value at 200 °C. Thus, the morphologies of as-obtained sample were various and clutter.
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Fig. 4. SEM images at the different reaction temperatures: (a) 50 °C; (b) 90 °C; (c) 110 °C; (d) 150 °C; (e) 200 °C.
Fig. 5. SEM images with the different reaction temperatures: (a, b) 2 h; (c) 6 h; (d) 12 h; (e) 24 h; (f) 48 h.
crystalline morphology.
Fig. 8. If PVP was added into solution while keeping other conditions unchanged, the as-obtained sample was confirmed as pure Cu2O crystal by XRD measurement. Form Fig. 8a, the flower-like with carambola structure can be observed, but the branches were roughly and there were holes in the branches to be observed. When the weight of CTAB was 500 mg, the product were blue flocculent and the red precipitate was no found. When the weight of CTAB was reduced to 100 mg, the asobtained samples were the red precipitate of Cu2O by XRD analysis. And the morphology of Cu2O (Fig. 8b) had also a great change. Most of the sample with laminar structure gathered together, and a small number of tiny particles were distributed in the above. As the weight of CTAB was 40 mg, the product was composed of globular with diameter of 6 μm and irregular particles. Thus, the concentration of CTAB affected the crystallization process of cuprous oxide, and changed its
3.2. Optical properties Fig. 9 showed the UV–vis-NIR absorption spectrum of flower-like Cu2O. It included the four absorption peaks at 243 nm, 315 nm, 424 nm, and 549 nm. On the basis of tangent graphic of (α hv)2vs. hv , we estimated the band gap of the flower-like Cu2O to be 1.88 eV. On the other hand, the band gap with absorption peak could be calculated by the formula (Eg = 1240/λmax, λmax is the first exciton peak), which was 2.26 eV. From the absorption curve, it could be found that the absorption performance of the ultraviolet and visible light was much higher than that of the near infrared region, which meant the sample was more suitable for the fabrication of photovoltaic solar cells. 268
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Fig. 6. XRD and SEM of the products obtained by ethanol amine as solvent.
Fig. 7. SEM images of the Cu2O products obtained with different copper sources: (a) Cu(Ac)2, (b) CuSO4, and (b) Cu(NO3)2.
Fig. 8. SEM images with the different additives (a) PVP; (b) n (Cu2+): w (CTAB) = 2 mmol: 100 mg; (c) n (Cu2+): w (CTAB) = 2 mmol: 40 mg.
Fig. 10. Instantaneous I-t curve of flower Cu2O at 0.5 V vs Ag/AgCl. Fig. 9. UV–vis-NIR spectroscopic characterization of the flower Cu2O.
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3.3. PEC properties
characterization of high-photoactivity electrodeposited Cu2O solar absorber by photoelectrochemistry and ultrafast spectroscopy. J. Phys. Chem. C 116, 7341–7350. Golden, T.D., Shumsky, M.G., Zhou, Y., Vander Werf, R.A., Van Leeuwen, R.A., Switzer, J.A., 1996. Electrochemical deposition of copper(I) oxide films. Chem. Mater. 8, 2499–2504. Shanid, N., Khadar, M.A., 2008. Evolution of nanostructure, phase transition and band gap tailoring in oxidized Cu thin films. Thin Solid Films 516, 6245–6252. Dolai, S., Dey, R., Das, S., Hussain, S., Bhar, R., Pal, A.K., 2017. Cupric oxide (CuO) thin films prepared by reactive d.c. magnetron sputtering technique for photovoltaic application. J. Alloy. Comp. 724, 456–464. Lee, Y.S., Winkler, M.T., Siah, S.C., Brandt, R., Buonassisi, T., 2011. Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering. Appl. Phys. Lett. 98, 192115. Perng, D.C., Hong, M.H., Chen, K.H., Chen, K.H., 2017. Enhancement of short-circuit current density in Cu2O/ZnO heterojunction solar cells. J. Alloy. Comp. 695, 549–554. Chatterjee, S., Pal, A.J., 2016. Introducing Cu2O thin films as a hole-transport layer in efficient planar perovskite solar cell structures. J. Phys. Chem. C 120, 1428–1437. Li, S.-K., Guo, X., Wang, Y., Huang, F.-Z., Shen, Y.-H., Wang, X.-M., Xie, A.-J., 2011. Rapid synthesis of flower-like Cu2O architectures in ionic liquids by the assistance of microwave irradiation with high photochemical activity. Dalton Trans. 40, 6745–6750. Minami, T., Miyata, T., Nishi, Y., 2014. Cu2O-based heterojunction solar cells with an Aldoped ZnO/oxide semiconductor thermally oxidized Cu2O sheet structure. Sol. Energy 2014 (105), 206–217. Ameri, B., Davarani, S.S.H., Roshani, R., Moazami, H.R., Tadjarodi, A., 2017. A flexible mechanochemical route for the synthesis of copper oxide nanorods/nanoparticles/ nanowires for supercapacitor applications: the effect of morphology on the charge storage ability. J. Alloy. Comp. 695, 114–123. Abdelfatah, M., Ledig, J., El-Shaer, A., Wagner, A., Sharafeev, A., Lemmens, P., Mosaad, M.M., Waag, A., Bakin, A., 2015. Fabrication and characterization of flexible solar cell from electrodeposited Cu2O thin film on plastic substrate. Sol. Energy 122, 1193–1198. Lan, X., Zhang, J., Gao, H., Wang, T., 2011. Morphology-controlled hydrothermal synthesis and growth mechanism of microcrystal Cu2O. CrystEngComm 13, 633–636. Kumar, R., Rai, P., Sharma, A., 2016. Facile synthesis of Cu2O microstructures and their morphology dependent electrochemical supercapacitor properties. RSC Adv. 6, 3815–3822. Gupta, D., Mehera, S.R., Illyaskutty, N., Alex, Z.C., 2018. Facile synthesis of Cu2O and CuO nanoparticles and study of their structural, optical and electronic properties. J. Alloys Comp. 743, 737–745. Du, C.M., Xiao, M.D., 2014. Cu2O nanoparticles synthesis by microplasma. Sci. Rep. 4, 7339. Jiang, X., Herricks, T., Xia, Y., 2002. CuO nanowires can be synthesized by heating copper substrates in air. Nano Lett. 2, 1333–1338. Xu, H., Wang, W., Zhu, W., 2006. Shape evolution and size-controllable synthesis of Cu2O octahedra and their morphology-dependent photocatalytic properties. J. Phys. Chem. B 110, 13829–13834. Wang, W.Z., Wang, G.H., Wang, X.S., Zhan, Y.J., Liu, Y.K., Zheng, C.L., 2002. Synthesis and characterization of Cu2O nanowires by a novel reduction route. Adv. Mater. 14, 67–69. Zhang, J., Liu, J., Peng, Q., Wang, X., Li, Y., 2006. Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors. Chem. Mater. 18, 867–871.
The samples were measured as the working electrode under on/off of 20 s at 0.5 V vs Ag/AgCl with illumination of full band spectrum 300 W incident intensity. The photocurrent of the flower-like Cu2O samples was about 0.0039 mA/cm2, as shown in Fig. 10. The photocurrent can keep up to 92% after 5 cycles under on/off switching irradiation, meaning the samples had high light stability and reproducible. The ratio of photocurrent to dark current was about 1.5 for the flower-like Cu2O samples with relatively low photosensitive properties. 4. Conclusion In summary, we had described the synthesis of pure phase and flower-like Cu2O via a simple hydrothermal method using water as solvent. When the reaction temperature was less than 90 °C, the samples were pure Cu2O crystal. With the reaction temperature increase, elemental copper appeared in the sample. The initial flower-like transformed into dendritic Cu2O, and then dendritic with holes and small particles Cu. When the reaction temperature was set at 50 °C, the branch of flower-like changed from pieces stacked, smooth conical cones, cuboids to octahedral pyramid as reaction time lengthened. When the ethanol amine replaced part or all of the water as the solvent, the samples were all spheroidal Cu particles. When the Cu(Ac)2 was instead by CuSO4 or Cu(NO3)2 as copper source, the as-prepared samples were almost composed of flower-like with pyramid structure of stacked stones and carambola structure, respectively. The additives CTAB had a strong effect on the morphologies. Finally, the four absorption peaks at 243 nm, 315 nm, 424 nm, and 549 nm were observed in the absorption spectrum, which showed the flower-like Cu2O with higher absorption performance in visible light range. The photocurrent of the flower-like Cu2O samples was about 0.0039 mA/cm2 with high reproducible. Acknowledgments This work was supported by the financial supports from the Fundamental Research Funds for National Nature Science Foundation of China (51576119), Young Eastern Scholar (QD 2016052). References Paracchino, A., Brauer, J.C., Moser, J.E., Thimsen, E., Graetzel, M., 2012. Synthesis and
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