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Preparation of submicron-sized cuprous oxide crystallites by electrodeposition with polyethylene glycol as additive
Journal of Crystal Growth 354 (2012) 193–197
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Preparation of submicron-sized cuprous oxide crystallites by electrodeposition with polyethylene glycol as additive Zhen Zhang, Wenbin Hu, Cheng Zhong, Yida Deng, Lei Liu, Yating Wu n State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
abstract
Article history: Received 8 April 2012 Received in revised form 24 May 2012 Accepted 9 June 2012 Communicated by T. Nishinaga Available online 17 June 2012
Submicron-sized cuprous oxide crystallites was prepared by electrodeposition, and polyethylene glycol (PEG) was introduced to improve the dispersity of crystallites. Transmission electron microscopy (TEM) and scanning electronic microscopy (SEM) were applied to characterize its crystal structure and morphology. Based on the analysis, it was found that PEG can affect the growth mode of cuprous oxide crystallites, which improves of the dispersity of the crystallites. The growth of cuprous oxide crystallites without PEG presents obvious aggregation dominant, while diffusion becomes the dominant mechanism when PEG is introduced into the electrolyte. The photo transmission character of cuprous oxide crystallites was measured by UV–vis spectrophotometer, and the calculated band gap energy of which is approximately 2.17 eV. The photoelectrochemical property of the crystallites was also characterized with the current–potential responses to a linear potentiodynamic scan under chopped illumination, and it is found that the photocurrent of the cuprous oxide crystallites electrodeposited with PEG is obviously larger than the one without PEG. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystallite A2. Electrochemical growth B1. Inorganic compounds B1. Oxides B2. Semiconducting materials
1. Introduction As a promising p-type semiconductor, cuprous oxide (Cu2O) has many advantages such as non-toxicity, simple availability, low-cost and chemical stability. Cu2O crystallites has been widely applied in photoelectric field due to its wide band gap [1–5]. In previous reports, chemical synthesis by liquid phase method is the main route to prepare Cu2O crystallites [6–12]. However, Cu2O crystallites prepared by this method is not proper in the application of photoelectric conversion. On the contrary, electrodeposition is a relatively simple, environmentally friendly, and low-cost way to deposit Cu2O crystallites on almost all kinds of conductive substrate, including the conducting glass. Some attempts have been made to prepare Cu2O crystalline film and crystallites by electrodeposition [13–15]. Wan et al. [13] regarded the pH value as a crucial factor in the electrodeposition process, by adjusting pH value of electrodeposition electrolyte, they prepared Cu2O crystallites with different dendrite structures. Anionic surfactants (SDS) was applied by Siegfried and Choi [14] to control the growth of Cu2O crystallites, and the SDS molecular was assumed to adsorb on certain planes of Cu2O crystallites and then affect the preferred orientation of crystallites. Pulsed electrodeposition in Na-citrate electrolyte was applied by Gu et al. [15] to improved the nucleation rate, thus flower-like Cu2O
n
Corresponding author. Tel.: þ86 21 3420 2981. E-mail address: [email protected] (Y. Wu).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.06.020
particles were prepared. However, monodisperse submicronsized Cu2O crystallites hasn’t received enough attention, and polymeric nonionic surfactants have rarely been applied to control the growth of Cu2O crystallites. In addition, it has been already proved that the performance of Cu2O crystallites can be significantly affected by the grain size and specific surface area [14,16–20], so preparation of submicron-sized Cu2O crystallites with good dispersity is very worthwhile. In this study, monodispersed submicron-sized Cu2O crystallites was attempted to electrodeposite on indium tin oxide (ITO) glass, and a typical nonionic surfactant (polyethylene glycol 6000) was added to improve the dispersity. The effect mechanism of polyethylene glycol (PEG) on the Cu2O crystallites growth is emphatically discussed. The band gap energy and the photoelectric conversion property of submicron-sized Cu2O crystallites were finally characterized.
2. Experimental All chemical agents such as copper nitrate (Cu(NO3)2 3H2O), PEG 6000, lactic acid, sodium hydroxide (NaOH) were analytical pure. As the substrate, ITO glass was cut as 3 cm 1 cm pieces and ultrasonically cleaned in deionized water and acetone for at least 20 min sequentially, then dried in nitrogen flow. The electrolyte was made up of 0.05 mol/L Cu(NO3)2 and PEG 6000 of different concentrations (1 g/L, 2 g/L and 5 g/L). The electrodeposition was performed in alkaline condition (pH¼10,
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by adding sodium hydroxide solution), and lactic acid was added to inhibit the precipitation of Cu(OH)2. The three-electrode system included a CHI-660D electrochemical workstation, a platinum counter electrode and a saturated calomel reference electrode, the ITO was working electrode (the exposed area was 1 cm2). Electrodeposition was carried out under galvanostatic (0.57 mA/cm2) and thermostatic (60 1C) condition for 60 s. The morphology of the submicron-sized Cu2O crystallites was observed with FEI Sirion 200 scanning electronic microscopy (SEM, 5 kV). The high resolution images and lattice structure were characterized by JEM-2100F FETEM transmission electron microscopy (TEM, 200 kV) provided by National Engineering Research Center for Nanotechnology (NERCN). To prepare the TEM sample, Cu2O crystallites was scraped from ITO substrate and dispersed in ethanol. The conductivity of deposition solution was measured with DDSJ-308A conductivity meter. The UV–vis transmission spectrum of Cu2O crystallites was measured by EV300 UV–vis spectrophotometer with blank ITO glass as reference. The current–potential responses curve was measured under the illumination of a XQ750W Xenon lamp (10 W/cm2) with 0.5 M/L Na2SO4 as the electrolyte.
The growth mechanism of electrodeposited Cu2O crystallites is also investigated. It has been demonstrated that there is a confluence of diffusion [22,23] and aggregation [24,25] in the process of crystallites’ growth, and these two mechanisms usually act in a competitive way [8]. In Fig. 1(A), the aggregated crystallites is obviously more than dispersed crystallites, so aggregation is more dominant when there is no PEG in deposition electrolyte. If PEG is introduced into the electrolyte, the PEG network can form a covering layer on the surface of Cu2O crystallites, so the grains can hardly aggregate. On the other hand, the Cu2 þ ions can
3. Results and discussion Fig. 1(A) exhibits the SEM images of submicron-sized Cu2O crystallites with the size of 100–300 nm. Obviously, the Cu2O crystallites electrodeposited without PEG presents poor dispersity and is prone to aggregating. On the contrary, the dispersity of crystallites is dramatically improved (Fig. 1(B)–(D)) when PEG was introduced into the electrolyte. Meanwhile, the size of crystallites also increases (300–600 nm). So PEG is supposed to play a key role in controlling the growth of Cu2O crystallites, which is schematically illustrated in Fig. 2. The PEG molecule usually represents amphiphilic character in aqueous solution: the oxygen-bridge atom (–O–) is hydrophilic, and the methylene (– CH2–) is hydrophobic [21] (Fig. 2(A)). Therefore the oxygenbridge can adsorb on the surface of ITO substrate as well as the Cu2O crystallites, so that PEG network can both reduce the nucleation points on substrate, and isolate the initial crystallites from aggregating (Fig. 2(B)). Resultantly, the dispersity of Cu2O crystallites is greatly improved.
Fig. 2. Schematic diagram of the growth of electrodeposited submicron-sized Cu2O crystallites with PEG as additive.
Fig. 1. SEM images of submicron-sized Cu2O crystallites electrodeposited on ITO substrate with (B)¼ 1 g/L, (C)¼ 2 g/L, (D) ¼ 5 g/L and without (A) PEG as additive.
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diffuse through PEG network and be reduced on the substrate, so the Cu2O crystallites can be kept isolated state in the process of growth (schematically shown in Fig. 2(B)). TEM was applied to take high resolution image in order to characterize the crystalline structure, which is shown in Fig. 3. In Fig. 3(B) and (E), the lattice fringe and diffraction spot clearly indicate that the Cu2O crystallites is monocrystalline. The lattice spaces of Fig. 3(C) and (D) are ˚ corresponding to the (110) and (111) planes of 3.03 A˚ and 2.56 A, Cu2O, respectively. If the aggregation is still the dominant mechanism in the process of crystal growth, the crystallites will present polycrystalline. Conversely, monocrystalline crystallites will be achieved. According to the results of TEM, it can be concluded that diffusion is the dominant mechanism in the electrodeposition of Cu2O crystallites with PEG. To prolong the deposition duration to 300 s (Fig. 4(A)), there was scarcely any increment of nucleation point, and the Cu2O crystallites still remained isolated, the size of which was about 700–1200 nm, so diffusion kept dominant. When the duration of electrodeposition was 600 s (Fig. 4(B)), crystallites begins to aggregate, and Cu2O film emerged. This indicates that at the later stage of crystalline growth, namely, film growth stage, PEG network can no longer impede the aggregation of crystallites.
Fig. 4. SEM images of Cu2O crystallites electrodeposited on ITO substrate with 1 g/L PEG for (A) 300 s, and (B) 600 s.
The concentration of PEG can also influence the dispersity of Cu2O crystallites. As shown in Fig. 1(B)–(D), when the concentration of PEG is 1 g/L, the dispersity of Cu2O crystallites is not very good, and the dispersity of Fig. 1(D) is better than Fig. 1(C), therefore the isolating effect of PEG network increases with the concentration. But if the concentration of PEG exceeds 5 g/L, the conductivity of deposition solution dramatically decreases (the relationship of the conductivity of deposition solution and the concentration of PEG is illustrated in Fig. 5), and makes the electrodeposition process hard to proceed. Moreover, it has been proved that as the concentration of surfactant increased, more Cu2 þ ions were adsorbed by surfactant molecule, so the deposition potential can shift negatively [25]. The result of cyclic voltammetry method shown in Fig. 6 confirms this theory. The deposition potential of Cu2O crystallites without PEG is about 0.56 V, which are 0.62 V, 0.62 V and 0.66 V when the concentrations of PEG are 1 g/L, 2 g/L and 5 g/L, respectively. The highest deposition current emerges when PEG is 5 g/L, and it is well known that the nucleation rate is proportional to the current density, this may be the reason that the crystallites is denser in Fig. 1(D). The photoelectrochemical property of the electrodeposited submicron-sized Cu2O crystallites was also characterized. The optical transmission spectrum of Cu2O crystallites electrodeposited with PEG is exhibited in Fig. 7(A), from which the optical band gap energy, Eg, can be determined. Eg was calculated according to the formula [26]:
ahu ¼ AðhuEg Þn=2 Fig. 3. (A) TEM image of the Cu2O crystallite, (B), (C) and (D) are the high resolution transmission electron microscope (HRTEM) images of corresponding Cu2O crystallite, (E) is the diffraction spot by nano electron beam.
where a is the absorption coefficient, h is the Planck constant, n is the absorption frequency, A and n are constants that depend on the nature of the transition. By calculation, the Eg of Cu2O
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Fig. 5. Relationship between the concentration of PEG and the conductivity of deposition solution.
Fig. 6. Cyclic voltammetry spectrum of electrodeposited Cu2O crystallites on ITO with PEG of different concentrations.
crystallites is approximately 2.17 eV. The crystallite size effect is speculated to be the main reason of high band gap energy [19]. Fig. 8 shows the current–potential responses of the Cu2O crystallites electrodeposited with and without PEG to a linear potentiodynamic scan (5 mV/s) under chopped illumination of Xenon lamp. According to Fig. 8, the photocurrent of the ‘‘with PEG’’ sample is obviously larger than the ‘‘without PEG’’ one. The difference is the result of the dispersity of Cu2O crystallites. On one hand, the larger specific surface area makes more photons to be absorbed. On the other hand, the illumination can be reflected among the dispersed crystallites, so the absorption efficiency is dramatically improved. Hence, the photoelectric application of dispersed submicron-sized Cu2O crystallites is more promising than ordinary Cu2O thin film.
4. Conclusion Submicron-sized cuprous oxide crystallites was prepared by electrodeposition technique with PEG as additives. The dispersity of Cu2O crystallites can be significantly improved by adding PEG into the deposition electrolyte, which is ascribed to the isolation effect of PEG network. The growth mechanism of Cu2O crystallites with and without PEG in electrolyte is different; diffusion and aggregation are proved to be the dominant modes, respectively. The result of UV–vis transmission spectrum of Cu2O crystallites indicates that the band gap width of crystallites is approximately 2.17 eV. The photoelectric conversion property of the Cu2O crystallites electrodeposited with PEG is obviously better than the one without PEG because of the better dispersity. So the
Z. Zhang et al. / Journal of Crystal Growth 354 (2012) 193–197
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Fig. 7. UV–vis absorption spectra of the electrodeposited submicron-sized spherical Cu2O crystallites with 1 g/L PEG (A), and the relationship between (ahn)2 and hn (B).
References
Fig. 8. Current–potential responses curve of the Cu2O crystallites electrodeposited with 5 g/L and without PEG to a linear potentiodynamic scan under chopped illumination of Xenon lamp. The scan rate is 5 mV/s, from 0.2 V to 0.6 V.
photoelectric application of dispersed submicron-sized Cu2O crystallites is more promising than ordinary Cu2O thin film. Acknowledgments This work was supported by the Shanghai Nature Science Foundation of China (No. 11ZR1418000). Instrumental Analysis Center of Shanghai Jiaotong University, as well as the National Engineering Research Center for Nanotechnology (NERCN) are specially acknowledged for the SEM and TEM characterizations.
[1] S. Bordiga, C. Paze, G. Berlier, et al., Catalysis Today 70 (2001) 91. [2] E.W. Bohannan, M.G. Shumsky, J.A. Switzer, Chemistry of Materials 11 (1999) 2289. [3] J.A. Switzer, B.M. Maune, E.R. Raub, et al., Journal of Physical Chemistry B 103 (1999) 395. [4] P.E. de Jongh, D. Vanmaelkelbergh, J.J. Kelly, Chemical Communications (1999) 1069. [5] M. Hara, H. Hasei, M. Yashima, et al., Applied Catalysis A 190 (2000) 35. [6] I. Prakash, P. Muralidharan, N. Nallamuthu, et al., Materials Research Bulletin 42 (2007) 1619. [7] L. Huang, F. Peng, H. Yu, et al., Solid State Sciences 11 (2009) 129. [8] Y.J. Dong, Y.D. Li, C. Wang, et al., Journal of Colloid and Interface Science 243 (2001) 85. [9] J.L. Long, J.G. Dong, X.X. Wang, et al., Journal of Colloid and Interface Science 333 (2009) 791. [10] J.Y. Ho, M.H. Huang, Journal of Physical Chemistry 113 (2009) 14159. [11] X.J. Zhang, G.F. Wang, A.X. Gu, et al., Solid State Communications 148 (2008) 525. [12] X.J. Zhang, G.F. Wang, H.B. Wu, et al., Materials Letters 62 (2008) 4363. [13] L.J. Wan, Z.Q. Wang, Z.S. Yang, et al., Journal of Crystal Growth 312 (2010) 3085. [14] M.J. Siegfried, K.S. Choi, Advanced Materials 16 (2004) 1743. [15] Y.E. Gu, X. Su, Y.L. Du, et al., Applied Surface Science 256 (2010) 5862. [16] Y.W. Tang, Z.G. Chen, Z.J. Jia, et al., Materials Letters 59 (2005) 434. [17] J. Nian, C. Hu, H. Teng, International Journal of Hydrogen Energy 33 (2008) 2897. [18] T. Mahalingam, J.S.P. Chitra, S. Rajendran, et al., Journal of Crystal Growth 216 (2000) 304. [19] S. Joseph, P.V. Kamath, Journal of the Electrochemical Society 156 (2009) E143. [20] F. Sun, Y.P. Guo, W.B. Song, et al., Journal of Crystal Growth 304 (2007) 425. [21] V.K. Lamer, R.H. Dinegar, Journal of the American Chemical Society 72 (1950) 4847. [22] V.K. Lamer, Industrial and Engineering Chemistry Research 44 (1952) 1270. [23] M. Ocana, E. Matijevic, Journal of Materials Research 5 (1990) 1083. [24] M. Ocana, R. Rodriguez-Clemente, C.J. Serna, Advanced Materials 7 (1995) 212. [25] Z.F. Liu, Templating Assembly of ZnO Ordered Porous Films and Their Electrode Performance, 2006. [26] A.E. Rakhshani, Solid State Electronics 29 (1986) 7.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Preparation of submicron-sized cuprous oxide crystallites by electrodeposition with polyethylene glycol as additive Zhen Zhang, Wenbin Hu, Cheng Zhong, Yida Deng, Lei Liu, Yating Wu n State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
abstract
Article history: Received 8 April 2012 Received in revised form 24 May 2012 Accepted 9 June 2012 Communicated by T. Nishinaga Available online 17 June 2012
Submicron-sized cuprous oxide crystallites was prepared by electrodeposition, and polyethylene glycol (PEG) was introduced to improve the dispersity of crystallites. Transmission electron microscopy (TEM) and scanning electronic microscopy (SEM) were applied to characterize its crystal structure and morphology. Based on the analysis, it was found that PEG can affect the growth mode of cuprous oxide crystallites, which improves of the dispersity of the crystallites. The growth of cuprous oxide crystallites without PEG presents obvious aggregation dominant, while diffusion becomes the dominant mechanism when PEG is introduced into the electrolyte. The photo transmission character of cuprous oxide crystallites was measured by UV–vis spectrophotometer, and the calculated band gap energy of which is approximately 2.17 eV. The photoelectrochemical property of the crystallites was also characterized with the current–potential responses to a linear potentiodynamic scan under chopped illumination, and it is found that the photocurrent of the cuprous oxide crystallites electrodeposited with PEG is obviously larger than the one without PEG. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Crystallite A2. Electrochemical growth B1. Inorganic compounds B1. Oxides B2. Semiconducting materials
1. Introduction As a promising p-type semiconductor, cuprous oxide (Cu2O) has many advantages such as non-toxicity, simple availability, low-cost and chemical stability. Cu2O crystallites has been widely applied in photoelectric field due to its wide band gap [1–5]. In previous reports, chemical synthesis by liquid phase method is the main route to prepare Cu2O crystallites [6–12]. However, Cu2O crystallites prepared by this method is not proper in the application of photoelectric conversion. On the contrary, electrodeposition is a relatively simple, environmentally friendly, and low-cost way to deposit Cu2O crystallites on almost all kinds of conductive substrate, including the conducting glass. Some attempts have been made to prepare Cu2O crystalline film and crystallites by electrodeposition [13–15]. Wan et al. [13] regarded the pH value as a crucial factor in the electrodeposition process, by adjusting pH value of electrodeposition electrolyte, they prepared Cu2O crystallites with different dendrite structures. Anionic surfactants (SDS) was applied by Siegfried and Choi [14] to control the growth of Cu2O crystallites, and the SDS molecular was assumed to adsorb on certain planes of Cu2O crystallites and then affect the preferred orientation of crystallites. Pulsed electrodeposition in Na-citrate electrolyte was applied by Gu et al. [15] to improved the nucleation rate, thus flower-like Cu2O
n
Corresponding author. Tel.: þ86 21 3420 2981. E-mail address: [email protected] (Y. Wu).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.06.020
particles were prepared. However, monodisperse submicronsized Cu2O crystallites hasn’t received enough attention, and polymeric nonionic surfactants have rarely been applied to control the growth of Cu2O crystallites. In addition, it has been already proved that the performance of Cu2O crystallites can be significantly affected by the grain size and specific surface area [14,16–20], so preparation of submicron-sized Cu2O crystallites with good dispersity is very worthwhile. In this study, monodispersed submicron-sized Cu2O crystallites was attempted to electrodeposite on indium tin oxide (ITO) glass, and a typical nonionic surfactant (polyethylene glycol 6000) was added to improve the dispersity. The effect mechanism of polyethylene glycol (PEG) on the Cu2O crystallites growth is emphatically discussed. The band gap energy and the photoelectric conversion property of submicron-sized Cu2O crystallites were finally characterized.
2. Experimental All chemical agents such as copper nitrate (Cu(NO3)2 3H2O), PEG 6000, lactic acid, sodium hydroxide (NaOH) were analytical pure. As the substrate, ITO glass was cut as 3 cm 1 cm pieces and ultrasonically cleaned in deionized water and acetone for at least 20 min sequentially, then dried in nitrogen flow. The electrolyte was made up of 0.05 mol/L Cu(NO3)2 and PEG 6000 of different concentrations (1 g/L, 2 g/L and 5 g/L). The electrodeposition was performed in alkaline condition (pH¼10,
194
Z. Zhang et al. / Journal of Crystal Growth 354 (2012) 193–197
by adding sodium hydroxide solution), and lactic acid was added to inhibit the precipitation of Cu(OH)2. The three-electrode system included a CHI-660D electrochemical workstation, a platinum counter electrode and a saturated calomel reference electrode, the ITO was working electrode (the exposed area was 1 cm2). Electrodeposition was carried out under galvanostatic (0.57 mA/cm2) and thermostatic (60 1C) condition for 60 s. The morphology of the submicron-sized Cu2O crystallites was observed with FEI Sirion 200 scanning electronic microscopy (SEM, 5 kV). The high resolution images and lattice structure were characterized by JEM-2100F FETEM transmission electron microscopy (TEM, 200 kV) provided by National Engineering Research Center for Nanotechnology (NERCN). To prepare the TEM sample, Cu2O crystallites was scraped from ITO substrate and dispersed in ethanol. The conductivity of deposition solution was measured with DDSJ-308A conductivity meter. The UV–vis transmission spectrum of Cu2O crystallites was measured by EV300 UV–vis spectrophotometer with blank ITO glass as reference. The current–potential responses curve was measured under the illumination of a XQ750W Xenon lamp (10 W/cm2) with 0.5 M/L Na2SO4 as the electrolyte.
The growth mechanism of electrodeposited Cu2O crystallites is also investigated. It has been demonstrated that there is a confluence of diffusion [22,23] and aggregation [24,25] in the process of crystallites’ growth, and these two mechanisms usually act in a competitive way [8]. In Fig. 1(A), the aggregated crystallites is obviously more than dispersed crystallites, so aggregation is more dominant when there is no PEG in deposition electrolyte. If PEG is introduced into the electrolyte, the PEG network can form a covering layer on the surface of Cu2O crystallites, so the grains can hardly aggregate. On the other hand, the Cu2 þ ions can
3. Results and discussion Fig. 1(A) exhibits the SEM images of submicron-sized Cu2O crystallites with the size of 100–300 nm. Obviously, the Cu2O crystallites electrodeposited without PEG presents poor dispersity and is prone to aggregating. On the contrary, the dispersity of crystallites is dramatically improved (Fig. 1(B)–(D)) when PEG was introduced into the electrolyte. Meanwhile, the size of crystallites also increases (300–600 nm). So PEG is supposed to play a key role in controlling the growth of Cu2O crystallites, which is schematically illustrated in Fig. 2. The PEG molecule usually represents amphiphilic character in aqueous solution: the oxygen-bridge atom (–O–) is hydrophilic, and the methylene (– CH2–) is hydrophobic [21] (Fig. 2(A)). Therefore the oxygenbridge can adsorb on the surface of ITO substrate as well as the Cu2O crystallites, so that PEG network can both reduce the nucleation points on substrate, and isolate the initial crystallites from aggregating (Fig. 2(B)). Resultantly, the dispersity of Cu2O crystallites is greatly improved.
Fig. 2. Schematic diagram of the growth of electrodeposited submicron-sized Cu2O crystallites with PEG as additive.
Fig. 1. SEM images of submicron-sized Cu2O crystallites electrodeposited on ITO substrate with (B)¼ 1 g/L, (C)¼ 2 g/L, (D) ¼ 5 g/L and without (A) PEG as additive.
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diffuse through PEG network and be reduced on the substrate, so the Cu2O crystallites can be kept isolated state in the process of growth (schematically shown in Fig. 2(B)). TEM was applied to take high resolution image in order to characterize the crystalline structure, which is shown in Fig. 3. In Fig. 3(B) and (E), the lattice fringe and diffraction spot clearly indicate that the Cu2O crystallites is monocrystalline. The lattice spaces of Fig. 3(C) and (D) are ˚ corresponding to the (110) and (111) planes of 3.03 A˚ and 2.56 A, Cu2O, respectively. If the aggregation is still the dominant mechanism in the process of crystal growth, the crystallites will present polycrystalline. Conversely, monocrystalline crystallites will be achieved. According to the results of TEM, it can be concluded that diffusion is the dominant mechanism in the electrodeposition of Cu2O crystallites with PEG. To prolong the deposition duration to 300 s (Fig. 4(A)), there was scarcely any increment of nucleation point, and the Cu2O crystallites still remained isolated, the size of which was about 700–1200 nm, so diffusion kept dominant. When the duration of electrodeposition was 600 s (Fig. 4(B)), crystallites begins to aggregate, and Cu2O film emerged. This indicates that at the later stage of crystalline growth, namely, film growth stage, PEG network can no longer impede the aggregation of crystallites.
Fig. 4. SEM images of Cu2O crystallites electrodeposited on ITO substrate with 1 g/L PEG for (A) 300 s, and (B) 600 s.
The concentration of PEG can also influence the dispersity of Cu2O crystallites. As shown in Fig. 1(B)–(D), when the concentration of PEG is 1 g/L, the dispersity of Cu2O crystallites is not very good, and the dispersity of Fig. 1(D) is better than Fig. 1(C), therefore the isolating effect of PEG network increases with the concentration. But if the concentration of PEG exceeds 5 g/L, the conductivity of deposition solution dramatically decreases (the relationship of the conductivity of deposition solution and the concentration of PEG is illustrated in Fig. 5), and makes the electrodeposition process hard to proceed. Moreover, it has been proved that as the concentration of surfactant increased, more Cu2 þ ions were adsorbed by surfactant molecule, so the deposition potential can shift negatively [25]. The result of cyclic voltammetry method shown in Fig. 6 confirms this theory. The deposition potential of Cu2O crystallites without PEG is about 0.56 V, which are 0.62 V, 0.62 V and 0.66 V when the concentrations of PEG are 1 g/L, 2 g/L and 5 g/L, respectively. The highest deposition current emerges when PEG is 5 g/L, and it is well known that the nucleation rate is proportional to the current density, this may be the reason that the crystallites is denser in Fig. 1(D). The photoelectrochemical property of the electrodeposited submicron-sized Cu2O crystallites was also characterized. The optical transmission spectrum of Cu2O crystallites electrodeposited with PEG is exhibited in Fig. 7(A), from which the optical band gap energy, Eg, can be determined. Eg was calculated according to the formula [26]:
ahu ¼ AðhuEg Þn=2 Fig. 3. (A) TEM image of the Cu2O crystallite, (B), (C) and (D) are the high resolution transmission electron microscope (HRTEM) images of corresponding Cu2O crystallite, (E) is the diffraction spot by nano electron beam.
where a is the absorption coefficient, h is the Planck constant, n is the absorption frequency, A and n are constants that depend on the nature of the transition. By calculation, the Eg of Cu2O
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Fig. 5. Relationship between the concentration of PEG and the conductivity of deposition solution.
Fig. 6. Cyclic voltammetry spectrum of electrodeposited Cu2O crystallites on ITO with PEG of different concentrations.
crystallites is approximately 2.17 eV. The crystallite size effect is speculated to be the main reason of high band gap energy [19]. Fig. 8 shows the current–potential responses of the Cu2O crystallites electrodeposited with and without PEG to a linear potentiodynamic scan (5 mV/s) under chopped illumination of Xenon lamp. According to Fig. 8, the photocurrent of the ‘‘with PEG’’ sample is obviously larger than the ‘‘without PEG’’ one. The difference is the result of the dispersity of Cu2O crystallites. On one hand, the larger specific surface area makes more photons to be absorbed. On the other hand, the illumination can be reflected among the dispersed crystallites, so the absorption efficiency is dramatically improved. Hence, the photoelectric application of dispersed submicron-sized Cu2O crystallites is more promising than ordinary Cu2O thin film.
4. Conclusion Submicron-sized cuprous oxide crystallites was prepared by electrodeposition technique with PEG as additives. The dispersity of Cu2O crystallites can be significantly improved by adding PEG into the deposition electrolyte, which is ascribed to the isolation effect of PEG network. The growth mechanism of Cu2O crystallites with and without PEG in electrolyte is different; diffusion and aggregation are proved to be the dominant modes, respectively. The result of UV–vis transmission spectrum of Cu2O crystallites indicates that the band gap width of crystallites is approximately 2.17 eV. The photoelectric conversion property of the Cu2O crystallites electrodeposited with PEG is obviously better than the one without PEG because of the better dispersity. So the
Z. Zhang et al. / Journal of Crystal Growth 354 (2012) 193–197
197
Fig. 7. UV–vis absorption spectra of the electrodeposited submicron-sized spherical Cu2O crystallites with 1 g/L PEG (A), and the relationship between (ahn)2 and hn (B).
References
Fig. 8. Current–potential responses curve of the Cu2O crystallites electrodeposited with 5 g/L and without PEG to a linear potentiodynamic scan under chopped illumination of Xenon lamp. The scan rate is 5 mV/s, from 0.2 V to 0.6 V.
photoelectric application of dispersed submicron-sized Cu2O crystallites is more promising than ordinary Cu2O thin film. Acknowledgments This work was supported by the Shanghai Nature Science Foundation of China (No. 11ZR1418000). Instrumental Analysis Center of Shanghai Jiaotong University, as well as the National Engineering Research Center for Nanotechnology (NERCN) are specially acknowledged for the SEM and TEM characterizations.
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