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Preparation of zinc and cerium or both doped Cu2O photoelectric material via hydrothermal method
Solar Energy 196 (2020) 74–79
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Preparation of zinc and cerium or both doped Cu2O photoelectric material via hydrothermal method
T
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Binxia Yuan , Xiaobo Liu, Xiaodong Cai, Xinyi Fang, Jianfeng Liu, Maoliang Wu, Qunzhi Zhu College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal method Doping Cuprous oxide Optical property Growth Mechanism
In the paper, the zinc and cerium or both doped cuprous oxide (Cu2O) crystals were successfully synthesized under low temperature by a simple hydrothermal process. The surface morphologies, structure properties, and optical properties of Cu2O crystals were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-visible absorption spectroscopy. The morphology of pure Cu2O crystals, Zn:Cu2O crystals, Ce:Cu2O crystals and Zn/Ce:Cu2O crystals evolved to irregularly cubic structure, exfoliated octahedron, were regular octahedron, and like-spherical aggregate, respectively. The doping of Cu2O crystals not only affected the surface morphologies but also significantly influenced the optoelectronic properties. Compared with undoped cuprous oxide, the doped samples had larger optical band gaps. Finally, the aggregation and diffusion were the most important factors for the morphology evolution of Cu2O crystal.
1. Introduction Semiconductor materials have widely utilization in areas such as optics, electricity, and microelectronics (Osorio-Rivera et al., 2018). Cuprous oxide (Cu2O) is one of the earliest p-type semiconductor materials to be utilized. It shows extremely promise of a variety of applications in the manufacture of modern electronic devices and solar cells, etc, due to its advantages which include stable structure, low cost, nontoxicity, high absorption coefficient and excellent electrical properties (Ye et al., 2016; Zhu and Panzer, 2015; Yuan et al., 2019). Element doping was a substantial method of material modification. Metal and non-metallic elements were increasingly doped with Cu2O, including Na (Minami et al., 2015; Chen et al., 2018), K (Chen et al., 2018), Li (Chen et al., 2018; Kim et al., 2016); Mg (Resende et al., 2016), Fe (Sieberer, 2007), Co (Sieberer, 2007; Tsur and Riess, 1995), Ni (Kikuchi et al., 2006), Zn (Ye et al., 2016; Zhu and Panzer, 2015; Hu et al., 2016), Ce (Wang et al., 2016; Yong et al., 2017; Liu, 2013), In (Cai et al., 2015; Cai et al., 2017); Mn (Sieberer, 2007), Ag (Upadhyay et al., 2014) and group-IVA elements (Ishizuka and Akimoto, 2004) as well as F (Ye et al., 2017), Cl (Bouderbala et al., 2018; Han et al., 2010; Han et al., 2009), Br (Han et al., 2012), I (Tsai et al., 2015), N (Han et al., 2018; Ishizuka et al., 2002; Kim et al., 2018; Li et al., 2011; Lu et al., 2005; Malerba et al., 2012; Sberna et al., 2016; Su et al., 2018), etc. Rare earth metals as dopants were also good candidates in optoelectronic material field. Zn and Ce were special attention due to Zn-
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doped Cu2O with optimized thermoelectric properties and Ce-doped with improved the photocatalytic activity of composite materials and enhanced the light-absorption properties. It was universally known that copper reacted readily with oxygen to form three different phases of oxides: (a) CuO (tenorite); (b) Cu2O (cuprite); and (c) Cu4O3 (paramelaconite) (Murali and Aryasomayajula, 2018). Among three kinds oxides of copper, copper atoms were present in two valence states: Cu(I) and Cu(II). The valence states of element had a directly or indirectly influence on the structure, morphology and optical band gap of crystals. Therefore, it was desirable to observe an alteration in the light absorption performance of zinc and cerium or both doped Cu2O crystals. In this context, the present paper reported the synthetically craft of hydrothermal process that was a low-temperature technique for elemental doping and Cu2O crystal fabrication. The pure, Zn-doped, Cedoped and Zn/Ce co-doped with Cu2O crystals were obtained by hydrothermal process in the similarly experimental condition. It was found that the morphology was changed with the variety of doping element. Moreover, the band gap of doped cuprous oxide did not obviously make it widen or narrowed, and remained within the normal range of optical band gap of cuprous oxide from 1.8 eV to 2.2 eV. Eventually, the growth mechanism of doped Cu2O crystal had been stated its in-depth process.
Corresponding author. E-mail address: [email protected] (B. Yuan).
https://doi.org/10.1016/j.solener.2019.11.093 Received 7 September 2019; Received in revised form 15 November 2019; Accepted 26 November 2019 Available online 12 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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2. Experiments 2.1. Material All reagents were analytical grade and were purchased from the Aladdin Chemical Reagent website. Particles were prepared by a hydrothermal method using Zinc acetate dihydrate (C4H6O4Zn·2H2O, analytical reagent, 99.0%), cerium nitrate hexahydrate (Ce (NO3)3·6H2O, analytical reagent, 99.95%), copper(II) acetate monohydrate (Cu(CH3COO)2·H2O, analytical reagent, 99%) and glucose (C6H12O6, analytical reagent, 98%) as reagents. Zinc acetate dihydrate and cerium nitrate hexahydrate were used as dopant source; deionized water was used as solvent. 2.2. Hydrothermal process All the samples were prepared by hydrothermal method which realized the zinc, the cerium mono-doped and co-doped cuprous oxide powders. A concrete example of the synthetic protocol for doped Cu2O microcrystals was as follows: 3 mmol Cu(CH3COO)2·H2O and 6 mmol C6H12O6 were dissolved in 50 mL of deionized water and stirred well at room temperature. Subsequently, 50 mL of a mixed aqueous solution of the above solution was added to the three-necked flask and kept in a Magnetic Heating Agitator at a temperature of 80 °C for 1 h. The mixed solution gradually converted from light blue to brick red during this process. After completion of the reaction, the brick-red precipitates were obtained by centrifugation, cleaned repeatedly with anhydrous ethanol, and were finally dried naturally at room temperature. The pure phase cuprous oxide powders were obtained. In accordance with the same process above, three groups of the transparent pale blue solution was prepared by mixing 3 mmol Cu (CH3COO)2·H2O and 6 mmol C6H12O6 powder in 50 mL deionized water. 0.3 mmol C4H6O4Zn·2H2O, 0.3 mmol Ce(NO3)3·6H2O, 0.3 mmol C4H6O4Zn·2H2O and 0.3 mmol Ce(NO3)3·6H2O was added into preparing Zn-doped, Ce-doped and Zn/Ce co-doped cuprous oxide, respectively.
Fig. 1. XRD patterns of crystals of (a) the pure Cu2O, (b) the Zn:Cu2O, (c) the Ce:Cu2O, (d) the Zn/Ce:Cu2O.
substitution of Cu2+by Zn2+ was owing to their comparable ionic radii which valued of 0.46 Å and 0.40 Å, respectively (Zhu and Panzer, 2015; Nesa et al., 2017). Likewise, it may be possible due to the low content of cerium in the sample, which was well dissolved in the Cu2O crystal lattice. It should be noted that no peaks other impurity phases were present in the diffraction spectra of the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals, which further suggested that Cu lattice sites were substituted of zinc or cerium. In other words, no new phase was formed anything but cuprous oxide.
3.2. Surface morphology Surface profile directly affecting the properties of materials was an important parameter. Therefore, to observe the shape evolution of doped Cu2O microcrystals, the morphologies of the pure Cu2O crystals, the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals were showed in Fig. 2. All the powders exhibited clearly micron-scale grains that were typical of Cu2O crystals grown under low temperature. The SEM image of the pure Cu2O crystals in the low-magnification demonstrated that the sample was irregularly cubic structure, as shown in Fig. 2a. The high-magnification picture (Fig. 2b) showed that the surface of crystals became relatively uniform with a collection of closely spaced particles, and whose surfaces were homogeneous and free of cracks. Fig. 2c and d showed the Zn/Ce:Cu2O crystals that its average size was 4–5 μm. The like-spherical aggregates were clearly observed. The appearance of sample was transformed from the original cubic structure to the like-spherical aggregates, as it can be seen, which displayed that the dopant changed the original morphology of Cu2O. From Fig. 2, the magnification of SEM images (Fig. 2e and 2g) on the left was at 7.0 k, and that of the right (Fig. 2f and h) was at 20.0 k. Fig. 2e indicated that the surface of Zn:Cu2O crystals exposed the presence of particle agglomerates, while it is noticed that some of crystals of cubic cuprous oxide had evolved into an exfoliated octahedron (Fig. 2f). As such, the slight Zn-doping could trigger a large lattice distortion, and would in turn substantially affect the PEC properties (Hu et al., 2016). Similar behavior has also been reported in literature. The picture of as-obtained Ce:Cu2O products had grown a regular octahedron, as shown in Fig. 2g and h. It was noticed that Ce:Cu2O crystals was slightly higher than pure phase Cu2O crystals on the surface roughness. The reason was that the stress in the crystal existed for Cedoped Cu2O. In order to further explore the doping of elements, EDS
2.3. Characterization The crystal structure of the samples was characterized with X-ray diffraction (XRD, Rigaku: Ultima IV, Cu Kα radiation). The shape, surface morphology and energy-dispersive spectroscopy (EDS) line scan of the compounds were acquired using Field emission scanning electron microscope (FESEM, Hitachi: S4800 and JEOL:7800F) at an acceleration voltage of 5 kV. UV/visible (UV/vis) spectra was measured and the absorption spectra was recorded using a Shimadzu UV-3600Plus spectrophotometer. The range of wavelength was 200–2000 nm. In test, the powder was extruded in an oblate cylindrical metal sample cell equipped with a glass bottom. 3. Results and discussion 3.1. Crystal structure Fig. 1 showed the XRD patterns of the undoped, and the zinc, the cerium mono-doped and co-doped cuprous oxide crystals. The samples were detected at the 2θ angles between 10° and 90°. The XRD patterns shown in Fig. 1 revealed that all of the crystals, including the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals as well as the pure Cu2O crystals, displayed three peaks assigned to cubic Cu2O (cuprite, JCPDS No. 05-0667). Three major peaks were appeared at 36.418°, 42.297° and 61.344° related to the crystal orientation of (1 1 1), (2 0 0) and (2 2 0), respectively. From Fig. 1, only Cu2O diffraction peaks were observed, while no peak corresponding to Zn impurity did not been seen. It may be possible due to the substitution of Cu2+ ions in the lattice structure of Cu2O by Zn2+ ions. The reason of 75
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Fig. 2. SEM images of (a) and (b) the pure Cu2O crystals, (c) and (d) the Zn/Ce:Cu2O crystals, (e) and (f) the Zn:Cu2O crystals, (g) and (h) the Ce:Cu2O crystals.
molar ratio of copper acetate to glucose was increased in the experiment without changing the reaction temperature and time. Cuprous oxide was synthesized by hydrothermal method when the molar ratio of Cu(CH3COO)2·H2O and C6H12O6 was 0.5:1, 1:2, 3:6, and 4:8. The synthesized powders exhibited that the color of all as-obtained ones was red except the ratio of 1:2 was dark-black. And then all compound powders were identified with XRD. The dark-black powder was Cu3O4 crystals, as revealed in Fig. 4b. It was well known that paramelaconite (Cu3O4) present tetragonal (space group I41/amd) (Murali and Aryasomayajula, 2018) and that were p-type semiconductor. It was
spectroscopies were used to characterize the composition of elements from the products of chemical reaction. The EDS results shown in Fig. 3 revealed the existence of zinc, cerium and zinc/cerium in the synthesis compounds. The EDS image in pure Cu2O crystals (Fig. 3a) had only Cu and O elements. At the same time, the element doped Cu2O was also analyzed. The Zn and Ce were observed in Fig. 3c and b, respectively. While the four elements for Cu, O, Zn and Ce were showed in the Zn/ Ce:Cu2O crystals (Fig. 3d). It more fully demonstrates that Cu2O doped with Zn and Ce was successfully prepared. Taking the effect of solution concentration account into doping, the 76
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Fig. 3. EDS patterns of (a) the pure Cu2O crystals (b) the Zn:Cu2O crystals (c) the Ce:Cu2O crystals (d) the Zn/Ce:Cu2O crystals.
slowly for the Cu3O4 crystals. The absorption properties mainly depended upon some of the factors, such as particle size, oxygen deficiency, lattice strain, thickness, and etc (Yathisha and Arthoba Nayaka, 2018). To further understand the influence of doping, the optical band gap of samples was calculated from Tauc plots (Fig. 6), and the relation between the absorption coefficient α and the optical band gap Eg is calculated as follows:
suspected the concentration of cuprous oxide and glucose was increased, resulting in the conversion of cubic Cu2O to Cu3O4. Simultaneously, the electron micrograph photo of Cu3O4 crystals (Fig. 2b) was observed distinctly flocculent spherical aggregates and irregularly angular aggregates. The surface of sample formed fine particles with different sizes and shapes.
3.3. Optical properties
(α hυ)m = A (hυ − Eg ) The absorbance spectra of the UV–Vis spectrophotometer of the doped Cu2O crystals were shown in Fig. 5. The samples were measured the range of wavelength from 200 to 800 nm. All the samples had the similar trend of optics in the visible wavelength range. It included the four absorption peaks at 244 nm, 316 nm, 419 nm, and 510 nm. The absorbance decreased sharply with the increase of wavelength for undoped and doped Cu2O crystals in the visible region, while one dropped
where “hυ” is the incident photon energy, “Eg” is the optical bandgap energy, “α” is the absorption coefficient, “A” is a constant and m is a constant determined by the nature of the semiconductor electronic transition, m equals to 2 for direct transition, because Cu2O is a dipole transition allowed by direct bandgap semiconductors. It could be concluded from the formula that the (αhυ)2 and hυ are linear relations, and
Fig. 4. (a) XRD pattern of polyhedral crystals of Cu3O4 (b) SEM of Cu3O4. 77
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in the formation of cuprous oxide morphology. After grain nucleation, the growth of small crystal nuclei could diffuse and aggregate in simultaneous, as shown in Fig. 2. It reflected the advantages of hydrothermal preparation of powders, including integrated crystal, small crystallite size and uniform distribution, and relatively low degrees of agglomeration. It was worth mentioning that reaction temperature had great influences on the crystallinity and morphology evolution of Cu2O crystals. When the temperature was low, the diffusion rate of atoms was slow, and the aggregation mechanism became the main way of grain growth. Small crystal nuclei gathered together, and the direction of their junction was disordered, growing into isotropic spherical polycrystals. When the temperature was high, the atomic diffusion speed was fast, and the diffusion mechanism acted a major role in grain growth. On the contrary, the crystallinity of grains was higher in high temperature, and the crystal growth was easy to more and more orderly. Thus, single crystal with good crystallinity was obtained. In order to judge whether the crystal was a single crystal structure or a polycrystalline structure. The structure type of the crystal was confirmed by X-ray diffraction. The curve (Fig. 1) was unimodal and the peak was high and sharp, which indicated the doped cuprous oxide was a single crystal. While the curve in the same peak segment that was showed in Fig. 4b existed bimodal at a certain range, which demonstrated that paramelaconite was polycrystalline. Generally, Cu4O3 crystals coexisted with CuO crystals and Cu2O crystals, making it difficult to synthesize a pure phase product, because it was a metastable material. Since paramelaconite contained Cu(I) atoms and Cu(II) atoms, it had the corresponding photoelectric characteristics with both CuO crystals and Cu2O crystals. Thus, it was a new and potential type of semiconductor material.
Fig. 5. The curve of UV-Vis spectra of all the samples.
4. Conclusions This paper was devoted to the elaboration of zinc and cerium or both doped Cu2O crystals by a hydrothermal process with simple and efficient. The influences of different concentration and doping elements on the morphology and properties were verified. The molar ratio of Cu (CH3COO)2·H2O and C6H12O6 was 0.5:1, 1:2, 3:6, and 4:8. When the molar ratio was 1:2, the as-obtained sample was Cu4O3 crystals by XRD analysis; other samples were proved to be the Cu2O crystals. The asobtained Cu2O samples had different morphologies. The morphology of pure Cu2O crystals, Zn:Cu2O crystals, Ce:Cu2O crystals, and Zn/ Ce:Cu2O crystals evolved to irregularly cubic structure, exfoliated octahedron, regular octahedron, and like-spherical aggregate, respectively. In addition, the four absorption peaks at 244 nm, 316 nm, 419 nm, and 510 nm were observed in the absorption spectrum. The absorption value of Zn/Ce:Cu2O crystals was higher than that of other samples, which indicated it had higher absorption performance in visible light range. Finally, the main factors affecting crystal growth were aggregation and diffusion mechanisms. The reaction temperature had also a great influence on the morphology evolution of Cu2O crystal.
Fig. 6. Variation of (αhν)2 with hν for all the powders.
the optical band gap could be obtained by first drawing the plot of (αhυ)2 versus hυ and second extrapolating the linear region to intercept the hυ axis, as shown in Fig. 6. The optical band gap of the samples of the undoped, the zinc, the cerium mono-doped, and co-doped cuprous oxide crystals was found to be 1.94 eV, 1.95 eV, 1.96 eV and 1.97 eV, respectively. Because the amount of doping was small, the band gap changed little. The inset was magnified image of them. The optical band gaps of all the samples were in the range of the direct bandgap of Cu2O (1.8–2.2 eV (Siah et al., 2012), which were in good agreement with the literature (Bai et al., 2012). However, the original optical band gap of pure Cu2O was found to be smaller than the doped samples, possibly due to the results of substitutional and interstitial doping. The red line represented the Eg for Cu3O4 crystals which were obtained from (αhν)2 versus hν plots. The optical band gap of Cu3O4 crystals in the literature was reported to 1.34 eV or 2.47 eV (Murali and Aryasomayajula, 2018). It could be inferred from Fig. 6 that its bandgap was 1.34 eV.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the financial supports from the Fundamental Research Funds for National Nature Science Foundation of China (No.: 51576119), Young Eastern Scholar (QD 2016052).
3.4. Growth mechanism of doped Cu2O crystal The early studies had shown that molecules or ions in hydrothermal solution were selectively adsorbed on different crystal surfaces which could change the growth rate of different crystal directions, resulting in that particles grew into different morphologies. According to the experimental results, the growth mechanism played an indispensable role
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Preparation of zinc and cerium or both doped Cu2O photoelectric material via hydrothermal method
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Binxia Yuan , Xiaobo Liu, Xiaodong Cai, Xinyi Fang, Jianfeng Liu, Maoliang Wu, Qunzhi Zhu College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal method Doping Cuprous oxide Optical property Growth Mechanism
In the paper, the zinc and cerium or both doped cuprous oxide (Cu2O) crystals were successfully synthesized under low temperature by a simple hydrothermal process. The surface morphologies, structure properties, and optical properties of Cu2O crystals were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and UV-visible absorption spectroscopy. The morphology of pure Cu2O crystals, Zn:Cu2O crystals, Ce:Cu2O crystals and Zn/Ce:Cu2O crystals evolved to irregularly cubic structure, exfoliated octahedron, were regular octahedron, and like-spherical aggregate, respectively. The doping of Cu2O crystals not only affected the surface morphologies but also significantly influenced the optoelectronic properties. Compared with undoped cuprous oxide, the doped samples had larger optical band gaps. Finally, the aggregation and diffusion were the most important factors for the morphology evolution of Cu2O crystal.
1. Introduction Semiconductor materials have widely utilization in areas such as optics, electricity, and microelectronics (Osorio-Rivera et al., 2018). Cuprous oxide (Cu2O) is one of the earliest p-type semiconductor materials to be utilized. It shows extremely promise of a variety of applications in the manufacture of modern electronic devices and solar cells, etc, due to its advantages which include stable structure, low cost, nontoxicity, high absorption coefficient and excellent electrical properties (Ye et al., 2016; Zhu and Panzer, 2015; Yuan et al., 2019). Element doping was a substantial method of material modification. Metal and non-metallic elements were increasingly doped with Cu2O, including Na (Minami et al., 2015; Chen et al., 2018), K (Chen et al., 2018), Li (Chen et al., 2018; Kim et al., 2016); Mg (Resende et al., 2016), Fe (Sieberer, 2007), Co (Sieberer, 2007; Tsur and Riess, 1995), Ni (Kikuchi et al., 2006), Zn (Ye et al., 2016; Zhu and Panzer, 2015; Hu et al., 2016), Ce (Wang et al., 2016; Yong et al., 2017; Liu, 2013), In (Cai et al., 2015; Cai et al., 2017); Mn (Sieberer, 2007), Ag (Upadhyay et al., 2014) and group-IVA elements (Ishizuka and Akimoto, 2004) as well as F (Ye et al., 2017), Cl (Bouderbala et al., 2018; Han et al., 2010; Han et al., 2009), Br (Han et al., 2012), I (Tsai et al., 2015), N (Han et al., 2018; Ishizuka et al., 2002; Kim et al., 2018; Li et al., 2011; Lu et al., 2005; Malerba et al., 2012; Sberna et al., 2016; Su et al., 2018), etc. Rare earth metals as dopants were also good candidates in optoelectronic material field. Zn and Ce were special attention due to Zn-
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doped Cu2O with optimized thermoelectric properties and Ce-doped with improved the photocatalytic activity of composite materials and enhanced the light-absorption properties. It was universally known that copper reacted readily with oxygen to form three different phases of oxides: (a) CuO (tenorite); (b) Cu2O (cuprite); and (c) Cu4O3 (paramelaconite) (Murali and Aryasomayajula, 2018). Among three kinds oxides of copper, copper atoms were present in two valence states: Cu(I) and Cu(II). The valence states of element had a directly or indirectly influence on the structure, morphology and optical band gap of crystals. Therefore, it was desirable to observe an alteration in the light absorption performance of zinc and cerium or both doped Cu2O crystals. In this context, the present paper reported the synthetically craft of hydrothermal process that was a low-temperature technique for elemental doping and Cu2O crystal fabrication. The pure, Zn-doped, Cedoped and Zn/Ce co-doped with Cu2O crystals were obtained by hydrothermal process in the similarly experimental condition. It was found that the morphology was changed with the variety of doping element. Moreover, the band gap of doped cuprous oxide did not obviously make it widen or narrowed, and remained within the normal range of optical band gap of cuprous oxide from 1.8 eV to 2.2 eV. Eventually, the growth mechanism of doped Cu2O crystal had been stated its in-depth process.
Corresponding author. E-mail address: [email protected] (B. Yuan).
https://doi.org/10.1016/j.solener.2019.11.093 Received 7 September 2019; Received in revised form 15 November 2019; Accepted 26 November 2019 Available online 12 December 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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2. Experiments 2.1. Material All reagents were analytical grade and were purchased from the Aladdin Chemical Reagent website. Particles were prepared by a hydrothermal method using Zinc acetate dihydrate (C4H6O4Zn·2H2O, analytical reagent, 99.0%), cerium nitrate hexahydrate (Ce (NO3)3·6H2O, analytical reagent, 99.95%), copper(II) acetate monohydrate (Cu(CH3COO)2·H2O, analytical reagent, 99%) and glucose (C6H12O6, analytical reagent, 98%) as reagents. Zinc acetate dihydrate and cerium nitrate hexahydrate were used as dopant source; deionized water was used as solvent. 2.2. Hydrothermal process All the samples were prepared by hydrothermal method which realized the zinc, the cerium mono-doped and co-doped cuprous oxide powders. A concrete example of the synthetic protocol for doped Cu2O microcrystals was as follows: 3 mmol Cu(CH3COO)2·H2O and 6 mmol C6H12O6 were dissolved in 50 mL of deionized water and stirred well at room temperature. Subsequently, 50 mL of a mixed aqueous solution of the above solution was added to the three-necked flask and kept in a Magnetic Heating Agitator at a temperature of 80 °C for 1 h. The mixed solution gradually converted from light blue to brick red during this process. After completion of the reaction, the brick-red precipitates were obtained by centrifugation, cleaned repeatedly with anhydrous ethanol, and were finally dried naturally at room temperature. The pure phase cuprous oxide powders were obtained. In accordance with the same process above, three groups of the transparent pale blue solution was prepared by mixing 3 mmol Cu (CH3COO)2·H2O and 6 mmol C6H12O6 powder in 50 mL deionized water. 0.3 mmol C4H6O4Zn·2H2O, 0.3 mmol Ce(NO3)3·6H2O, 0.3 mmol C4H6O4Zn·2H2O and 0.3 mmol Ce(NO3)3·6H2O was added into preparing Zn-doped, Ce-doped and Zn/Ce co-doped cuprous oxide, respectively.
Fig. 1. XRD patterns of crystals of (a) the pure Cu2O, (b) the Zn:Cu2O, (c) the Ce:Cu2O, (d) the Zn/Ce:Cu2O.
substitution of Cu2+by Zn2+ was owing to their comparable ionic radii which valued of 0.46 Å and 0.40 Å, respectively (Zhu and Panzer, 2015; Nesa et al., 2017). Likewise, it may be possible due to the low content of cerium in the sample, which was well dissolved in the Cu2O crystal lattice. It should be noted that no peaks other impurity phases were present in the diffraction spectra of the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals, which further suggested that Cu lattice sites were substituted of zinc or cerium. In other words, no new phase was formed anything but cuprous oxide.
3.2. Surface morphology Surface profile directly affecting the properties of materials was an important parameter. Therefore, to observe the shape evolution of doped Cu2O microcrystals, the morphologies of the pure Cu2O crystals, the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals were showed in Fig. 2. All the powders exhibited clearly micron-scale grains that were typical of Cu2O crystals grown under low temperature. The SEM image of the pure Cu2O crystals in the low-magnification demonstrated that the sample was irregularly cubic structure, as shown in Fig. 2a. The high-magnification picture (Fig. 2b) showed that the surface of crystals became relatively uniform with a collection of closely spaced particles, and whose surfaces were homogeneous and free of cracks. Fig. 2c and d showed the Zn/Ce:Cu2O crystals that its average size was 4–5 μm. The like-spherical aggregates were clearly observed. The appearance of sample was transformed from the original cubic structure to the like-spherical aggregates, as it can be seen, which displayed that the dopant changed the original morphology of Cu2O. From Fig. 2, the magnification of SEM images (Fig. 2e and 2g) on the left was at 7.0 k, and that of the right (Fig. 2f and h) was at 20.0 k. Fig. 2e indicated that the surface of Zn:Cu2O crystals exposed the presence of particle agglomerates, while it is noticed that some of crystals of cubic cuprous oxide had evolved into an exfoliated octahedron (Fig. 2f). As such, the slight Zn-doping could trigger a large lattice distortion, and would in turn substantially affect the PEC properties (Hu et al., 2016). Similar behavior has also been reported in literature. The picture of as-obtained Ce:Cu2O products had grown a regular octahedron, as shown in Fig. 2g and h. It was noticed that Ce:Cu2O crystals was slightly higher than pure phase Cu2O crystals on the surface roughness. The reason was that the stress in the crystal existed for Cedoped Cu2O. In order to further explore the doping of elements, EDS
2.3. Characterization The crystal structure of the samples was characterized with X-ray diffraction (XRD, Rigaku: Ultima IV, Cu Kα radiation). The shape, surface morphology and energy-dispersive spectroscopy (EDS) line scan of the compounds were acquired using Field emission scanning electron microscope (FESEM, Hitachi: S4800 and JEOL:7800F) at an acceleration voltage of 5 kV. UV/visible (UV/vis) spectra was measured and the absorption spectra was recorded using a Shimadzu UV-3600Plus spectrophotometer. The range of wavelength was 200–2000 nm. In test, the powder was extruded in an oblate cylindrical metal sample cell equipped with a glass bottom. 3. Results and discussion 3.1. Crystal structure Fig. 1 showed the XRD patterns of the undoped, and the zinc, the cerium mono-doped and co-doped cuprous oxide crystals. The samples were detected at the 2θ angles between 10° and 90°. The XRD patterns shown in Fig. 1 revealed that all of the crystals, including the Zn:Cu2O crystals, the Ce:Cu2O crystals and the Zn/Ce:Cu2O crystals as well as the pure Cu2O crystals, displayed three peaks assigned to cubic Cu2O (cuprite, JCPDS No. 05-0667). Three major peaks were appeared at 36.418°, 42.297° and 61.344° related to the crystal orientation of (1 1 1), (2 0 0) and (2 2 0), respectively. From Fig. 1, only Cu2O diffraction peaks were observed, while no peak corresponding to Zn impurity did not been seen. It may be possible due to the substitution of Cu2+ ions in the lattice structure of Cu2O by Zn2+ ions. The reason of 75
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Fig. 2. SEM images of (a) and (b) the pure Cu2O crystals, (c) and (d) the Zn/Ce:Cu2O crystals, (e) and (f) the Zn:Cu2O crystals, (g) and (h) the Ce:Cu2O crystals.
molar ratio of copper acetate to glucose was increased in the experiment without changing the reaction temperature and time. Cuprous oxide was synthesized by hydrothermal method when the molar ratio of Cu(CH3COO)2·H2O and C6H12O6 was 0.5:1, 1:2, 3:6, and 4:8. The synthesized powders exhibited that the color of all as-obtained ones was red except the ratio of 1:2 was dark-black. And then all compound powders were identified with XRD. The dark-black powder was Cu3O4 crystals, as revealed in Fig. 4b. It was well known that paramelaconite (Cu3O4) present tetragonal (space group I41/amd) (Murali and Aryasomayajula, 2018) and that were p-type semiconductor. It was
spectroscopies were used to characterize the composition of elements from the products of chemical reaction. The EDS results shown in Fig. 3 revealed the existence of zinc, cerium and zinc/cerium in the synthesis compounds. The EDS image in pure Cu2O crystals (Fig. 3a) had only Cu and O elements. At the same time, the element doped Cu2O was also analyzed. The Zn and Ce were observed in Fig. 3c and b, respectively. While the four elements for Cu, O, Zn and Ce were showed in the Zn/ Ce:Cu2O crystals (Fig. 3d). It more fully demonstrates that Cu2O doped with Zn and Ce was successfully prepared. Taking the effect of solution concentration account into doping, the 76
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Fig. 3. EDS patterns of (a) the pure Cu2O crystals (b) the Zn:Cu2O crystals (c) the Ce:Cu2O crystals (d) the Zn/Ce:Cu2O crystals.
slowly for the Cu3O4 crystals. The absorption properties mainly depended upon some of the factors, such as particle size, oxygen deficiency, lattice strain, thickness, and etc (Yathisha and Arthoba Nayaka, 2018). To further understand the influence of doping, the optical band gap of samples was calculated from Tauc plots (Fig. 6), and the relation between the absorption coefficient α and the optical band gap Eg is calculated as follows:
suspected the concentration of cuprous oxide and glucose was increased, resulting in the conversion of cubic Cu2O to Cu3O4. Simultaneously, the electron micrograph photo of Cu3O4 crystals (Fig. 2b) was observed distinctly flocculent spherical aggregates and irregularly angular aggregates. The surface of sample formed fine particles with different sizes and shapes.
3.3. Optical properties
(α hυ)m = A (hυ − Eg ) The absorbance spectra of the UV–Vis spectrophotometer of the doped Cu2O crystals were shown in Fig. 5. The samples were measured the range of wavelength from 200 to 800 nm. All the samples had the similar trend of optics in the visible wavelength range. It included the four absorption peaks at 244 nm, 316 nm, 419 nm, and 510 nm. The absorbance decreased sharply with the increase of wavelength for undoped and doped Cu2O crystals in the visible region, while one dropped
where “hυ” is the incident photon energy, “Eg” is the optical bandgap energy, “α” is the absorption coefficient, “A” is a constant and m is a constant determined by the nature of the semiconductor electronic transition, m equals to 2 for direct transition, because Cu2O is a dipole transition allowed by direct bandgap semiconductors. It could be concluded from the formula that the (αhυ)2 and hυ are linear relations, and
Fig. 4. (a) XRD pattern of polyhedral crystals of Cu3O4 (b) SEM of Cu3O4. 77
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in the formation of cuprous oxide morphology. After grain nucleation, the growth of small crystal nuclei could diffuse and aggregate in simultaneous, as shown in Fig. 2. It reflected the advantages of hydrothermal preparation of powders, including integrated crystal, small crystallite size and uniform distribution, and relatively low degrees of agglomeration. It was worth mentioning that reaction temperature had great influences on the crystallinity and morphology evolution of Cu2O crystals. When the temperature was low, the diffusion rate of atoms was slow, and the aggregation mechanism became the main way of grain growth. Small crystal nuclei gathered together, and the direction of their junction was disordered, growing into isotropic spherical polycrystals. When the temperature was high, the atomic diffusion speed was fast, and the diffusion mechanism acted a major role in grain growth. On the contrary, the crystallinity of grains was higher in high temperature, and the crystal growth was easy to more and more orderly. Thus, single crystal with good crystallinity was obtained. In order to judge whether the crystal was a single crystal structure or a polycrystalline structure. The structure type of the crystal was confirmed by X-ray diffraction. The curve (Fig. 1) was unimodal and the peak was high and sharp, which indicated the doped cuprous oxide was a single crystal. While the curve in the same peak segment that was showed in Fig. 4b existed bimodal at a certain range, which demonstrated that paramelaconite was polycrystalline. Generally, Cu4O3 crystals coexisted with CuO crystals and Cu2O crystals, making it difficult to synthesize a pure phase product, because it was a metastable material. Since paramelaconite contained Cu(I) atoms and Cu(II) atoms, it had the corresponding photoelectric characteristics with both CuO crystals and Cu2O crystals. Thus, it was a new and potential type of semiconductor material.
Fig. 5. The curve of UV-Vis spectra of all the samples.
4. Conclusions This paper was devoted to the elaboration of zinc and cerium or both doped Cu2O crystals by a hydrothermal process with simple and efficient. The influences of different concentration and doping elements on the morphology and properties were verified. The molar ratio of Cu (CH3COO)2·H2O and C6H12O6 was 0.5:1, 1:2, 3:6, and 4:8. When the molar ratio was 1:2, the as-obtained sample was Cu4O3 crystals by XRD analysis; other samples were proved to be the Cu2O crystals. The asobtained Cu2O samples had different morphologies. The morphology of pure Cu2O crystals, Zn:Cu2O crystals, Ce:Cu2O crystals, and Zn/ Ce:Cu2O crystals evolved to irregularly cubic structure, exfoliated octahedron, regular octahedron, and like-spherical aggregate, respectively. In addition, the four absorption peaks at 244 nm, 316 nm, 419 nm, and 510 nm were observed in the absorption spectrum. The absorption value of Zn/Ce:Cu2O crystals was higher than that of other samples, which indicated it had higher absorption performance in visible light range. Finally, the main factors affecting crystal growth were aggregation and diffusion mechanisms. The reaction temperature had also a great influence on the morphology evolution of Cu2O crystal.
Fig. 6. Variation of (αhν)2 with hν for all the powders.
the optical band gap could be obtained by first drawing the plot of (αhυ)2 versus hυ and second extrapolating the linear region to intercept the hυ axis, as shown in Fig. 6. The optical band gap of the samples of the undoped, the zinc, the cerium mono-doped, and co-doped cuprous oxide crystals was found to be 1.94 eV, 1.95 eV, 1.96 eV and 1.97 eV, respectively. Because the amount of doping was small, the band gap changed little. The inset was magnified image of them. The optical band gaps of all the samples were in the range of the direct bandgap of Cu2O (1.8–2.2 eV (Siah et al., 2012), which were in good agreement with the literature (Bai et al., 2012). However, the original optical band gap of pure Cu2O was found to be smaller than the doped samples, possibly due to the results of substitutional and interstitial doping. The red line represented the Eg for Cu3O4 crystals which were obtained from (αhν)2 versus hν plots. The optical band gap of Cu3O4 crystals in the literature was reported to 1.34 eV or 2.47 eV (Murali and Aryasomayajula, 2018). It could be inferred from Fig. 6 that its bandgap was 1.34 eV.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the financial supports from the Fundamental Research Funds for National Nature Science Foundation of China (No.: 51576119), Young Eastern Scholar (QD 2016052).
3.4. Growth mechanism of doped Cu2O crystal The early studies had shown that molecules or ions in hydrothermal solution were selectively adsorbed on different crystal surfaces which could change the growth rate of different crystal directions, resulting in that particles grew into different morphologies. According to the experimental results, the growth mechanism played an indispensable role
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