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Effects of annealing temperature on GO–Cu2O composite films grown by electrochemical deposition for PEC photoelectrode
Current Applied Physics 15 (2015) 473e478
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effects of annealing temperature on GOeCu2O composite films grown by electrochemical deposition for PEC photoelectrode Tae Gyoum Kim a, Hyukhyun Ryu a, *, Won-Jae Lee b, Jang-Hee Yoon c a
Department of Nano Science and Engineering, High Safety Vehicle Core Technology, Research Center, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea b Department of Materials and Components Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea c High Technology Components & Materials Research Center, Korea Basic Science Institute, 1275 Jisa-dong, Gangseo-Gu, Busan 618-230, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2014 Received in revised form 16 January 2015 Accepted 20 January 2015 Available online 3 February 2015
In this work, graphene oxideecuprous oxide (GOeCu2O) composite films were grown on fluorine-doped tin oxide substrates by electrochemical deposition. We investigated the effects of the annealing temperature on the morphological, structural, optical and photoelectrochemical (PEC) properties of GO eCu2O composite films. As a result, our work shows that while GOeCu2O composite films exhibit the highest XRD (111) peak intensity at 300 C sample, the highest photocurrent density value obtained was 4.75 mA/cm2 at 200 C sample (using 0.17 V versus a reversible hydrogen electrode (RHE)). In addition, a reduction reaction at 300 C sample was observed using XPS analysis from the shift in the O1s peak in addition to a weaker O1s peak intensity. © 2015 Elsevier B.V. All rights reserved.
Keywords: GOeCu2O composite films Electrochemical deposition (ECD) Photoelectrochemical (PEC) Annealing X-ray photoelectron spectroscopy (XPS)
1. Introduction Photoelectrochemical (PEC) cells were first invented by Honda and Fujishima in 1972 [1]. Since then, the research surrounding PEC cells has been focused on finding suitable photoelectrode semiconductor materials, such as ZnO, Fe2O3, TiO2, WO3, CuO, and Cu2O [2e7]. A PEC cell is based on a semiconductor/electrolyte junction in which the minority charges (electrons and holes for a p-type and a n-type semiconductor, respectively) generated by light absorption in the semiconductor are driven into the electrolyte by the electric field at the junction, where they can drive a redox reaction, such as the reduction of Hþ to H2 for a p-type semiconductor [8]. Among the types of photoelectrode materials, cuprous oxide (Cu2O) is an attractive choice because its conduction band and valence band edge positions include the redox level of water splitting. Consequently, H2 generation can then occur at a relatively low overpotential, which makes Cu2O a promising material for water splitting [9]. In addition, Cu2O is a typical p-type material due to the presence of copper vacancies and to being a direct band-gap semiconductor (Eg ¼ 1.8e2.3 eV) [10]. Moreover, Cu2O is also an
* Corresponding author. E-mail address: [email protected] (H. Ryu). http://dx.doi.org/10.1016/j.cap.2015.01.023 1567-1739/© 2015 Elsevier B.V. All rights reserved.
abundant resource, is nontoxic and has a low production cost [11]. Many process methods for Cu2O growth have been developed, including the use of chemical vapor deposition (CVD) [12], atomic layer deposition (ALD) [13], RF-sputtering [14], hydrothermal synthesis [15], thermal oxidation [16] and electrochemical deposition (ECD) [17]. Among these methods, the ECD method has been most commonly used due to its low processing temperature, low cost and simplicity [18e23]. Cu2O could be electrodeposited by reduction of an alkaline aqueous solution of cupric lactate according to the reaction, 2Cu2 þ þ 2e þ 2OH / Cu2O þ H2O [24,25]. Although many studies using Cu2O have attempted to increase PEC performance, as of yet, no significant change in performance has been achieved. This poor performance can possibly be attributed to the low electrical conductivity and photo-corrosion of Cu2O [26,27]. Graphene has received significant attention since its discovery in 2004 [28] due to its outstanding mechanical, thermal, optical and electrical properties [29,30]. However, graphene has many problems. Graphene monolayers can be agglomerated in an aqueous solution due to van der walls forces between them [31]. In addition, graphene do not contain functional groups, such as hydroxyl groups (OH) and carboxyl groups (COOH), which can hinder the formation of composites with different materials [32]. Accordingly, it is difficult to form graphene composites with other materials due
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to the absence of functional groups. Consequently, many researchers have attempted to identify an alternate material, such as reduced graphene oxide (rGO) or graphene oxide (GO) [33,34]. The surface properties of GO can be adjusted by chemical modification, which offers tremendous opportunity regarding the development of functionalized graphene-based materials [35]. However, rGO needs high temperature annealing treatment unlike GO [36]. Due to the functional groups of GO, many researchers have studied GOcomposite materials with semiconductors, such as ZnO, Fe2O3 and
TiO2, for improving the PEC performance [37e39]. In this study, we synthesized GOeCu2O composite films using GO which has good electrical properties and easily forms composite films with different semiconductor materials using functional groups. And, we investigate the effects of the annealing temperature on the morphological, structural, optical and PEC properties of GOeCu2O composite films. The GOeCu2O composite film samples with various annealing temperatures were analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction
Fig. 1. Top view SEM images of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures: (a) pure-Cu2O, (b) 100 C, (c) 200 C, (d) 300 C, (e) 400 C and (f) 500 C.
Fig. 2. Cross-sectional view SEM images of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures: (a) pure-Cu2O, (b) 100 C, (c) 200 C, (d) 300 C, (e) 400 C and (f) 500 C.
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(XRD), X-ray photoelectron spectroscopy (XPS), UVevisible spectrophotometry and a potentiostat/galvanostat. 2. Experimental details We grew GOeCu2O composite films on fluorine-doped tin oxide (FTO) substrates using electrochemical deposition. A threeelectrode system was used for the oxide growth, which included an FTO/glass substrate (sheet resistance: 7U/,, 2.5 2.0 cm2) as the working electrode, an Ag/AgCl (Sat. 1 M KCl) reference electrode and a Pt sheet as the counter electrode. The electrochemical equipment consisted of a potentiostat/galvanostat (Model PL-9, Physio Lab, South Korea). The following experimental conditions used in this study were from our previous study [21]. FTO/glass substrates were cleaned ultrasonically in acetone and methanol for 10 min each. The substrates were then rinsed in deionized water and dried using a filtered air gun. The electrolyte used in the GOeCu2O composite films contained 0.1 M copper (ІІ) sulfate (CuSO4) (SigmaeALDRICH, purity 99%), which was used as the Cuþ precursor, and 3 M lactic acid (Fluka), which was used as the oxygen source. In the composite material, GO (Angstron Materials, Inc., Model: N002 e PS) was used to construct a 1-wt% GOeCu2O film. Sodium hydroxide (NaOH, SigmaeALDRICH, purity 98%) was used to adjust the pH value of the solution to 11. The growth temperature was maintained at 65 C, and the electrochemical deposition was conducted at e 0.4 V (versus Ag/AgCl) for 600 s. After electrochemical deposition of the GOeCu2O composite films, the annealing process was performed using RTP (rapid thermal process) at various annealing temperatures for 30 min in vacuum: 100, 200, 300, 400 and 500 C. For comparison to the 1-wt % GOeCu2O composite films, a 0-wt% sample (no GO) was prepared at 200 C for 30 min in vacuum. (Hereafter, 0-wt% 200 C, 1-wt% 100 C, 1-wt% 200 C, 1-wt% 300 C, 1-wt% 400 C and 1-wt% 500 C samples will be referred to as pure-Cu2O, 100 C sample, 200 C sample, 300 C sample, 400 C sample and 500 C sample, respectively.) The three-electrode system was used for measuring the PEC performance of the GOeCu2O film photoelectrode, which consisted of a GOeCu2O substrate as the working electrode, a graphite rod (Alfa Aesar) as the counter electrode and an SCE (saturated calomel electrode, Sat. 3.3 M KCl) reference electrode. A 1-M KOH (potassium hydroxide, SigmaeALDRICH, purity 85%) solution was used for photocurrent measurements. The grown GOeCu2O composite electrodes were set to face a 300-W xenon arc lamp (light intensity: 100 mW/cm2) containing a UV filter (filtered AM 1.5). Using the linear sweep voltammetry mode (LSV) of the Cu2O electrodes, the potential was scanned from 0 to 0.80 V (versus reversible hydrogen electrode (RHE)) with a scan rate of 10 mV/s in a 1-M KOH solution while being irradiated with a chopped xenon lamp light. The morphology was characterized using a field emission scanning electron microscope (FE-SEM, Quanta 200 FEG). The structural properties were analyzed using X-ray diffraction (XRD) with Cu Ka radiation and X-ray photoelectron spectroscopy (XPS). The optical properties were measured by a UVevisible spectrophotometer (SCINCO, S-3100).
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As shown in Fig. 1(e), this agglomeration could be caused by the sintering effect [40]. There is an obvious grain shape difference and grain size difference when Fig. 1(a)e(c) is compared to Fig. 1(d)e(f). Fig. 2 shows the cross sectional SEM images of the GOeCu2O composite films. The film thicknesses were roughly 1.94, 2.04, 2.02, 2.00, 1.61 and 1.44 mm for the pure-Cu2O, 100 C, 200 C, 300 C, 400 C and 500 C samples, respectively. The film thicknesses were similar from the pure-Cu2O sample to the 300 C sample. However, the thicknesses of the 400 C and 500 C samples sharply decreased due to the effect of agglomeration by sintering. The XRD spectra of pure-Cu2O and GOeCu2O composite films with various annealing temperatures are shown in Fig. 3(a). The polycrystalline Cu2O structures of the pure-Cu2O, 100 C and 200 C samples were determined using XRD measurement; the peaks of (111), (200) and (310) were 36.441, 42.329 and 69.622 , respectively (JCPDS Card No.01 e 078 e 2076). The samples were pure Cu2O thin-films, and no changes in their crystal structures were observed after annealing at 100 C and 200 C. In addition, the (111) peaks were much stronger than the (200) and (310) peaks. This result was attributed to the pH value during the electrochemical deposition process. In general, it is known that the (111) directional growth mainly occurs at pH 11 in electrochemical deposition [41]. However, the 300 C sample showed XRD Cu2O peaks for (111),
3. Results and discussion Fig. 1 shows FE-SEM images of pure-Cu2O and GOeCu2O composite films after annealing at various temperatures. Four-sided pyramidal-shaped grains were observed for the pure-Cu2O, 100 C and 200 C samples, as shown in Fig. 1(a)e(c). Cubic particle shapes were observed in the 300 C and 400 C samples. Agglomerations of cubic particles were observed in the 500 C sample.
Fig. 3. (a) XRD spectra (* for substrate, C for Cu2O (111), B for Cu2O (200), A for Cu2O (310), △ for Cu (111), , for Cu (002) and : for Cu (022)) and (b) (111) peak intensity of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures.
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(200) and (310) at 36.441, 42.329 and 69.622 , respectively (JCPDS Card No.01 e 078 e 2076), and XRD Cu peaks for (111) at 43.418 (JCPDS Card No. 98 e 062 e 7117), indicating a mixed phase of the polycrystalline Cu2O. This result demonstrates the coexistence of Cu2O and Cu phases. The Cu originates from the Cu2O through reduction reaction: partial phase change from Cu2O to Cu occurs via reduction in the 300 C sample. The 400 C and 500 C samples showed pure Cu peaks for (111), (002) and (022) at 43.418 , 50.569 and 74.318 , respectively (JCPDS Card No. 98 e 062 e 7117), indicating complete conversion of Cu2O to Cu by reduction reaction at the high annealing temperatures. The sample color was changed from deep red (Cu2O) to a mixture of black and deep yellow color (Cu). As shown in Fig. 3(a), all samples exhibit a preferred (111) orientation growth except for the 400 C and 500 C samples. Fig. 3(b) shows the (111) XRD peak intensities at various annealing temperatures. The (111) peak intensity steadily increased until 300 C. In general, it is known that Cu2O samples with higher (111) XRD peak intensities exhibit favorable PEC properties [42]. The XPS measurement results for GOeCu2O composite films with various annealing temperatures are shown in Fig. 4. Cu, O and C were detected from all samples, as shown in Fig. 4(a). The Cu peaks were slightly shifted (not shown here). The detailed XPS spectra for the O1s and C1s peaks are shown in Fig. 4(b) and (c), respectively. For the O1s peak, which represents the lattice oxygen O2 of Cu2O, a 160-meV red shift from 530.52 (100 C sample) to 530.35 eV (200 C sample) and a 170-meV blue shift from 530.35 (200 C sample) to 530.52 eV (300 C sample) were observed. In addition, the O1s peak intensity of the 300 C sample was much weaker than the intensities of the other samples. The peak shift and weaker O1s peak intensity of the 300 C sample were attributed to a reduction reaction from Cu2O to Cu during annealing. C1s peaks were observed at the same position of 284.6 eV for all samples, as
shown in Fig. 4(c). While a C1s peak shift was not observed, the intensity of the C1s peak did change. The C1s peak intensity of the 300 C sample was much stronger than those for the 100 C and 200 C samples. The atomic ratio values of Cu/O/C in these samples (100 C, 200 C and 300 C samples) have been calculated from the XPS spectra and are shown in Fig. 4(d). The increase in the copper/ oxygen atomic ratio from the reduction reaction clearly occurs when the annealing temperature increases from 200 C to 300 C, which is shown in Fig. 4 and is confirmed by the XRD results shown in Fig. 3. In addition, the large grain shape and size differences shown in the SEM images of Fig. 1(a)e(c) and Fig. 1(d)e(f) may also be explained by the existence of the Cu phase. Fig. 5(a) shows the transmittance data of pure-Cu2O and GOeCu2O composite films at various annealing temperatures. Transmittance decreased when the annealing temperatures increased. Fig. 5(b) shows (ahn)2 versus hn plots of pure-Cu2O and GOeCu2O composite films at various annealing temperatures. All samples have an absorption coefficient that obeys the following relation for photon energies: (ahn)2 ¼ A(hn Eg), where A is a constant, a is the absorption coefficient (per centimeter) and hn (electron volt) is the energy of excitation. The absorption coefficient a can be obtained by a ¼ In (T)/d, where T is the transmittance and d is the thickness of film, respectively. Moreover, the effect of the thickness on the glancing estimate of Eg was negligible. Therefore, we assumed the absorption coefficient a ~ In (T) existed in the fundamental absorption region, and a better linearity was observed from the (ahn)2 versus hn plots. Finally, the band gap energy of the samples could be deduced from the slopes of the plot [43]. The optical energy band gap values were calculated to be 2.20, 2.18, 1.94 and 1.60 eV for the pure-Cu2O, 100 C, 200 C and 300 C samples, respectively. The energy band gap of Cu2O is generally known to have a range of 1.8e2.2 eV. The pure-Cu2O, 100 C and
Fig. 4. XPS data of GOeCu2O composite films grown using various annealing temperatures: (a) XPS spectra, (b) O1s peak spectra, (c) C1s peak spectra and (d) atomic ratio for Cu/O/ C.
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Fig. 5. (a) Transmittance and (b) (ahn)2 versus hn plot of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures.
200 C samples have optical energy band gap values of 2.20, 2.18 and 1.94 eV, respectively, which are similar to the reported results [44]. However, the 300 C sample has a very low value, 1.60 eV, which could possibly be a result of the mixed Cu2O and Cu phases. In addition, the optical energy band gap values for 400 C and 500 C samples were not available due to the Cu phase. The PEC properties of GOeCu2O composite films with various annealing temperatures are shown in Fig. 6. We observed the open circuit potential at 0.68 V (versus RHE) for all samples except for 400 C and 500 C samples, indicating that the photo activities are first observed at 0.68 V by a light source. As shown in Fig. 6(a), the generated photocurrents were observed using a negative value at an applied reverse bias, which means that the samples were grown as p-type GOeCu2O composite films. However, we confirmed that the electron hole pairs (EHP) were not generated under 0.17 V (versus RHE). This result is apparent from the photo corrosion of the working electrodes [13]. The photocurrents were not observed for 400 C and 500 C samples due to the Cu phases. Fig. 6(b) shows the photocurrent density and dark current density from Fig. 6(a) (at 0.17 V versus RHE). The photocurrent density values were 0.34, 4.75 and 2.89 mA/cm2 for the 100 C, 200 C and 300 C samples, respectively. There were many studies for
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Fig. 6. PEC performance of GOeCu2O composite films grown using various annealing temperatures: (a) linear sweep voltammetry mode and (b) photocurrent density and dark current density (at 0.17 V versus RHE).
improving PEC photocurrent density values [27,45]. Paracchino et al. reported that the photocurrent density was improved by using Pt/TiO2/AZO/Cu2O photocathode grown by electrochemical deposition and ALD (atomic layer deposition) method. They improved PEC photocurrent density to 6.5 mA/cm2 [27]. And, Zhang et al. reported that the photocurrent density value was improved to 3.95 mA/cm2 by using carbon coated Cu2O NWs/Cu mesh photocathode grown by electrochemical deposition method [45]. In our study, the photocurrent density value of the 100 C sample was very weak because the thermal energy was very low; thus, the remaining hydroxyl (OH) groups were not frequently oxidized to oxygen (O2) in this sample. The 300 C sample was expected to have the highest PEC photocurrent density because it showed the highest (111) XRD peak intensity. However, our results show that the photocurrent density value of the 300 C sample was lower than that of the 200 C sample, which can be attributed to the mixture of the Cu2O and Cu phases in the 300 C sample. The Cu metal does not absorb light; however, it does scatter light. In addition, the dark current density values were 0.12, 0.21 and 1.73 mA/cm2 for the 100 C, 200 C and 300 C samples, respectively. The highest dark current density was observed
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with 1.73 mA/cm2 at 300 C, which was also attributed to the mixture of the Cu2O and Cu phases. The highest photocurrent density value in this study was observed with 4.75 mA/cm2 for the 200 C sample. 4. Conclusions In this work, we investigated the effect of annealing temperature on GOeCu2O composite films using various annealing temperatures. As a result, GOeCu2O composite films with the highest XRD (111) peak intensity, were obtained at 300 C. However, GOeCu2O composite films with the highest PEC photocurrent density were obtained with 4.75 mA/cm2 at 200 C sample. In this study, we also observed the co-existence of the Cu2O and Cu phases after reduction over 300 C samples via SEM, XRD, and XPS, which could explain the decrease in the PEC photocurrent density. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110012847). References [1] L. Chen, S. Shet, H. Tang, H. Wang, T. Deutsch, Y. Yan, J. Turner, M. Al-Jassim, Electrochemical deposition of copper oxide nanowires for photoelectrochemical applications, J. Mater. Chem. 20 (2010) 6962e6967. [2] X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Langmuir 29 (2013) 3097e3105. [3] E. Thimsen, F. Le Formal, M. Gr€ atzel, S.C. Warren, Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water splitting, Nano Lett. 11 (2010) 35e43. [4] I.S. Cho, Z. Chen, A.J. Forman, D.R. Kim, P.M. Rao, T.F. Jaramillo, X. Zheng, Branched TiO2 nanorods for photoelectrochemical hydrogen production, Nano Lett. 11 (2011) 4978e4984. [5] J. Su, X. Feng, J.D. Sloppy, L. Guo, C.A. Grimes, Vertically aligned WO3 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis and photoelectrochemical properties, Nano Lett. 11 (2010) 203e208. [6] S. Liu, J. Tian, L. Wang, Y. Luo, X. Sun, One-pot synthesis of CuO nanoflowerdecorated reduced graphene oxide and its application to photocatalytic degradation of dyes, Catal. Sci. Technol. 2 (2012) 339e344. [7] M.A. Mahmoud, W. Qian, M.A. El-Sayed, Following charge separation on the nanoscale in Cu2OeAu nanoframe hollow nanoparticles, Nano Lett. 11 (2011) 3285e3289. [8] K. Maeda, Photocatalytic water splitting using semiconductor particles: history and recent developments, J. Photochem. Photobiol. C: Photochem. Rev. 12 (2011) 237e268. [9] J. Kondo, Cu2O as a photocatalyst for overall water splitting under visible light irradiation, Chem. Commun. (1998) 357e358. [10] Y. Liu, Y. Liu, R. Mu, H. Yang, C. Shao, J. Zhang, Y. Lu, D. Shen, X. Fan, The structural and optical properties of Cu2O films electrodeposited on different substrates, Semicond. Sci. Technol. 20 (2005) 44. [11] P. de Jongh, J. Kelly, Cu2O: a catalyst for the photochemical decomposition of water? Chem. Commun. (1999) 1069e1070. [12] T. Maruyama, Copper oxide thin films prepared by chemical vapor deposition from copper dipivaloylmethanate, Sol. Energy Mater. Sol. Cells 56 (1998) 85e92. €tzel, E. Thimsen, Highly active oxide [13] A. Paracchino, V. Laporte, K. Sivula, M. Gra photocathode for photoelectrochemical water reduction, Nat. Mater. 10 (2011) 456e461. [14] S. Ishizuka, K. Suzuki, Y. Okamoto, M. Yanagita, T. Sakurai, K. Akimoto, N. Fujiwara, H. Kobayashi, K. Matsubara, S. Niki, Polycrystalline n-ZnO/p-Cu2O heterojunctions grown by RF-magnetron sputtering, Phys. Stat. Solidi. C. 1 (2004) 1067e1070. [15] H. Yu, J. Yu, S. Liu, S. Mann, Template-free hydrothermal synthesis of CuO/ Cu2O composite hollow microspheres, Chem. Mater. 19 (2007) 4327e4334. [16] A. Musa, T. Akomolafe, M. Carter, Production of cuprous oxide, a solar cell material, by thermal oxidation and a study of its physical and electrical properties, Sol. Energy Mater. Sol. Cells 51 (1998) 305e316. [17] R.P. Wijesundera, M. Hidaka, K. Koga, M. Sakai, W. Siripala, J.Y. Choi, E.S. Nark, Effects of annealing on the properties and structure of electrodeposited semiconducting CueO thin films, Phys. Stat. Sol. (b) 244 (2007) 4629e4642.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effects of annealing temperature on GOeCu2O composite films grown by electrochemical deposition for PEC photoelectrode Tae Gyoum Kim a, Hyukhyun Ryu a, *, Won-Jae Lee b, Jang-Hee Yoon c a
Department of Nano Science and Engineering, High Safety Vehicle Core Technology, Research Center, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea b Department of Materials and Components Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea c High Technology Components & Materials Research Center, Korea Basic Science Institute, 1275 Jisa-dong, Gangseo-Gu, Busan 618-230, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2014 Received in revised form 16 January 2015 Accepted 20 January 2015 Available online 3 February 2015
In this work, graphene oxideecuprous oxide (GOeCu2O) composite films were grown on fluorine-doped tin oxide substrates by electrochemical deposition. We investigated the effects of the annealing temperature on the morphological, structural, optical and photoelectrochemical (PEC) properties of GO eCu2O composite films. As a result, our work shows that while GOeCu2O composite films exhibit the highest XRD (111) peak intensity at 300 C sample, the highest photocurrent density value obtained was 4.75 mA/cm2 at 200 C sample (using 0.17 V versus a reversible hydrogen electrode (RHE)). In addition, a reduction reaction at 300 C sample was observed using XPS analysis from the shift in the O1s peak in addition to a weaker O1s peak intensity. © 2015 Elsevier B.V. All rights reserved.
Keywords: GOeCu2O composite films Electrochemical deposition (ECD) Photoelectrochemical (PEC) Annealing X-ray photoelectron spectroscopy (XPS)
1. Introduction Photoelectrochemical (PEC) cells were first invented by Honda and Fujishima in 1972 [1]. Since then, the research surrounding PEC cells has been focused on finding suitable photoelectrode semiconductor materials, such as ZnO, Fe2O3, TiO2, WO3, CuO, and Cu2O [2e7]. A PEC cell is based on a semiconductor/electrolyte junction in which the minority charges (electrons and holes for a p-type and a n-type semiconductor, respectively) generated by light absorption in the semiconductor are driven into the electrolyte by the electric field at the junction, where they can drive a redox reaction, such as the reduction of Hþ to H2 for a p-type semiconductor [8]. Among the types of photoelectrode materials, cuprous oxide (Cu2O) is an attractive choice because its conduction band and valence band edge positions include the redox level of water splitting. Consequently, H2 generation can then occur at a relatively low overpotential, which makes Cu2O a promising material for water splitting [9]. In addition, Cu2O is a typical p-type material due to the presence of copper vacancies and to being a direct band-gap semiconductor (Eg ¼ 1.8e2.3 eV) [10]. Moreover, Cu2O is also an
* Corresponding author. E-mail address: [email protected] (H. Ryu). http://dx.doi.org/10.1016/j.cap.2015.01.023 1567-1739/© 2015 Elsevier B.V. All rights reserved.
abundant resource, is nontoxic and has a low production cost [11]. Many process methods for Cu2O growth have been developed, including the use of chemical vapor deposition (CVD) [12], atomic layer deposition (ALD) [13], RF-sputtering [14], hydrothermal synthesis [15], thermal oxidation [16] and electrochemical deposition (ECD) [17]. Among these methods, the ECD method has been most commonly used due to its low processing temperature, low cost and simplicity [18e23]. Cu2O could be electrodeposited by reduction of an alkaline aqueous solution of cupric lactate according to the reaction, 2Cu2 þ þ 2e þ 2OH / Cu2O þ H2O [24,25]. Although many studies using Cu2O have attempted to increase PEC performance, as of yet, no significant change in performance has been achieved. This poor performance can possibly be attributed to the low electrical conductivity and photo-corrosion of Cu2O [26,27]. Graphene has received significant attention since its discovery in 2004 [28] due to its outstanding mechanical, thermal, optical and electrical properties [29,30]. However, graphene has many problems. Graphene monolayers can be agglomerated in an aqueous solution due to van der walls forces between them [31]. In addition, graphene do not contain functional groups, such as hydroxyl groups (OH) and carboxyl groups (COOH), which can hinder the formation of composites with different materials [32]. Accordingly, it is difficult to form graphene composites with other materials due
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to the absence of functional groups. Consequently, many researchers have attempted to identify an alternate material, such as reduced graphene oxide (rGO) or graphene oxide (GO) [33,34]. The surface properties of GO can be adjusted by chemical modification, which offers tremendous opportunity regarding the development of functionalized graphene-based materials [35]. However, rGO needs high temperature annealing treatment unlike GO [36]. Due to the functional groups of GO, many researchers have studied GOcomposite materials with semiconductors, such as ZnO, Fe2O3 and
TiO2, for improving the PEC performance [37e39]. In this study, we synthesized GOeCu2O composite films using GO which has good electrical properties and easily forms composite films with different semiconductor materials using functional groups. And, we investigate the effects of the annealing temperature on the morphological, structural, optical and PEC properties of GOeCu2O composite films. The GOeCu2O composite film samples with various annealing temperatures were analyzed using field emission scanning electron microscopy (FE-SEM), X-ray diffraction
Fig. 1. Top view SEM images of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures: (a) pure-Cu2O, (b) 100 C, (c) 200 C, (d) 300 C, (e) 400 C and (f) 500 C.
Fig. 2. Cross-sectional view SEM images of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures: (a) pure-Cu2O, (b) 100 C, (c) 200 C, (d) 300 C, (e) 400 C and (f) 500 C.
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(XRD), X-ray photoelectron spectroscopy (XPS), UVevisible spectrophotometry and a potentiostat/galvanostat. 2. Experimental details We grew GOeCu2O composite films on fluorine-doped tin oxide (FTO) substrates using electrochemical deposition. A threeelectrode system was used for the oxide growth, which included an FTO/glass substrate (sheet resistance: 7U/,, 2.5 2.0 cm2) as the working electrode, an Ag/AgCl (Sat. 1 M KCl) reference electrode and a Pt sheet as the counter electrode. The electrochemical equipment consisted of a potentiostat/galvanostat (Model PL-9, Physio Lab, South Korea). The following experimental conditions used in this study were from our previous study [21]. FTO/glass substrates were cleaned ultrasonically in acetone and methanol for 10 min each. The substrates were then rinsed in deionized water and dried using a filtered air gun. The electrolyte used in the GOeCu2O composite films contained 0.1 M copper (ІІ) sulfate (CuSO4) (SigmaeALDRICH, purity 99%), which was used as the Cuþ precursor, and 3 M lactic acid (Fluka), which was used as the oxygen source. In the composite material, GO (Angstron Materials, Inc., Model: N002 e PS) was used to construct a 1-wt% GOeCu2O film. Sodium hydroxide (NaOH, SigmaeALDRICH, purity 98%) was used to adjust the pH value of the solution to 11. The growth temperature was maintained at 65 C, and the electrochemical deposition was conducted at e 0.4 V (versus Ag/AgCl) for 600 s. After electrochemical deposition of the GOeCu2O composite films, the annealing process was performed using RTP (rapid thermal process) at various annealing temperatures for 30 min in vacuum: 100, 200, 300, 400 and 500 C. For comparison to the 1-wt % GOeCu2O composite films, a 0-wt% sample (no GO) was prepared at 200 C for 30 min in vacuum. (Hereafter, 0-wt% 200 C, 1-wt% 100 C, 1-wt% 200 C, 1-wt% 300 C, 1-wt% 400 C and 1-wt% 500 C samples will be referred to as pure-Cu2O, 100 C sample, 200 C sample, 300 C sample, 400 C sample and 500 C sample, respectively.) The three-electrode system was used for measuring the PEC performance of the GOeCu2O film photoelectrode, which consisted of a GOeCu2O substrate as the working electrode, a graphite rod (Alfa Aesar) as the counter electrode and an SCE (saturated calomel electrode, Sat. 3.3 M KCl) reference electrode. A 1-M KOH (potassium hydroxide, SigmaeALDRICH, purity 85%) solution was used for photocurrent measurements. The grown GOeCu2O composite electrodes were set to face a 300-W xenon arc lamp (light intensity: 100 mW/cm2) containing a UV filter (filtered AM 1.5). Using the linear sweep voltammetry mode (LSV) of the Cu2O electrodes, the potential was scanned from 0 to 0.80 V (versus reversible hydrogen electrode (RHE)) with a scan rate of 10 mV/s in a 1-M KOH solution while being irradiated with a chopped xenon lamp light. The morphology was characterized using a field emission scanning electron microscope (FE-SEM, Quanta 200 FEG). The structural properties were analyzed using X-ray diffraction (XRD) with Cu Ka radiation and X-ray photoelectron spectroscopy (XPS). The optical properties were measured by a UVevisible spectrophotometer (SCINCO, S-3100).
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As shown in Fig. 1(e), this agglomeration could be caused by the sintering effect [40]. There is an obvious grain shape difference and grain size difference when Fig. 1(a)e(c) is compared to Fig. 1(d)e(f). Fig. 2 shows the cross sectional SEM images of the GOeCu2O composite films. The film thicknesses were roughly 1.94, 2.04, 2.02, 2.00, 1.61 and 1.44 mm for the pure-Cu2O, 100 C, 200 C, 300 C, 400 C and 500 C samples, respectively. The film thicknesses were similar from the pure-Cu2O sample to the 300 C sample. However, the thicknesses of the 400 C and 500 C samples sharply decreased due to the effect of agglomeration by sintering. The XRD spectra of pure-Cu2O and GOeCu2O composite films with various annealing temperatures are shown in Fig. 3(a). The polycrystalline Cu2O structures of the pure-Cu2O, 100 C and 200 C samples were determined using XRD measurement; the peaks of (111), (200) and (310) were 36.441, 42.329 and 69.622 , respectively (JCPDS Card No.01 e 078 e 2076). The samples were pure Cu2O thin-films, and no changes in their crystal structures were observed after annealing at 100 C and 200 C. In addition, the (111) peaks were much stronger than the (200) and (310) peaks. This result was attributed to the pH value during the electrochemical deposition process. In general, it is known that the (111) directional growth mainly occurs at pH 11 in electrochemical deposition [41]. However, the 300 C sample showed XRD Cu2O peaks for (111),
3. Results and discussion Fig. 1 shows FE-SEM images of pure-Cu2O and GOeCu2O composite films after annealing at various temperatures. Four-sided pyramidal-shaped grains were observed for the pure-Cu2O, 100 C and 200 C samples, as shown in Fig. 1(a)e(c). Cubic particle shapes were observed in the 300 C and 400 C samples. Agglomerations of cubic particles were observed in the 500 C sample.
Fig. 3. (a) XRD spectra (* for substrate, C for Cu2O (111), B for Cu2O (200), A for Cu2O (310), △ for Cu (111), , for Cu (002) and : for Cu (022)) and (b) (111) peak intensity of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures.
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(200) and (310) at 36.441, 42.329 and 69.622 , respectively (JCPDS Card No.01 e 078 e 2076), and XRD Cu peaks for (111) at 43.418 (JCPDS Card No. 98 e 062 e 7117), indicating a mixed phase of the polycrystalline Cu2O. This result demonstrates the coexistence of Cu2O and Cu phases. The Cu originates from the Cu2O through reduction reaction: partial phase change from Cu2O to Cu occurs via reduction in the 300 C sample. The 400 C and 500 C samples showed pure Cu peaks for (111), (002) and (022) at 43.418 , 50.569 and 74.318 , respectively (JCPDS Card No. 98 e 062 e 7117), indicating complete conversion of Cu2O to Cu by reduction reaction at the high annealing temperatures. The sample color was changed from deep red (Cu2O) to a mixture of black and deep yellow color (Cu). As shown in Fig. 3(a), all samples exhibit a preferred (111) orientation growth except for the 400 C and 500 C samples. Fig. 3(b) shows the (111) XRD peak intensities at various annealing temperatures. The (111) peak intensity steadily increased until 300 C. In general, it is known that Cu2O samples with higher (111) XRD peak intensities exhibit favorable PEC properties [42]. The XPS measurement results for GOeCu2O composite films with various annealing temperatures are shown in Fig. 4. Cu, O and C were detected from all samples, as shown in Fig. 4(a). The Cu peaks were slightly shifted (not shown here). The detailed XPS spectra for the O1s and C1s peaks are shown in Fig. 4(b) and (c), respectively. For the O1s peak, which represents the lattice oxygen O2 of Cu2O, a 160-meV red shift from 530.52 (100 C sample) to 530.35 eV (200 C sample) and a 170-meV blue shift from 530.35 (200 C sample) to 530.52 eV (300 C sample) were observed. In addition, the O1s peak intensity of the 300 C sample was much weaker than the intensities of the other samples. The peak shift and weaker O1s peak intensity of the 300 C sample were attributed to a reduction reaction from Cu2O to Cu during annealing. C1s peaks were observed at the same position of 284.6 eV for all samples, as
shown in Fig. 4(c). While a C1s peak shift was not observed, the intensity of the C1s peak did change. The C1s peak intensity of the 300 C sample was much stronger than those for the 100 C and 200 C samples. The atomic ratio values of Cu/O/C in these samples (100 C, 200 C and 300 C samples) have been calculated from the XPS spectra and are shown in Fig. 4(d). The increase in the copper/ oxygen atomic ratio from the reduction reaction clearly occurs when the annealing temperature increases from 200 C to 300 C, which is shown in Fig. 4 and is confirmed by the XRD results shown in Fig. 3. In addition, the large grain shape and size differences shown in the SEM images of Fig. 1(a)e(c) and Fig. 1(d)e(f) may also be explained by the existence of the Cu phase. Fig. 5(a) shows the transmittance data of pure-Cu2O and GOeCu2O composite films at various annealing temperatures. Transmittance decreased when the annealing temperatures increased. Fig. 5(b) shows (ahn)2 versus hn plots of pure-Cu2O and GOeCu2O composite films at various annealing temperatures. All samples have an absorption coefficient that obeys the following relation for photon energies: (ahn)2 ¼ A(hn Eg), where A is a constant, a is the absorption coefficient (per centimeter) and hn (electron volt) is the energy of excitation. The absorption coefficient a can be obtained by a ¼ In (T)/d, where T is the transmittance and d is the thickness of film, respectively. Moreover, the effect of the thickness on the glancing estimate of Eg was negligible. Therefore, we assumed the absorption coefficient a ~ In (T) existed in the fundamental absorption region, and a better linearity was observed from the (ahn)2 versus hn plots. Finally, the band gap energy of the samples could be deduced from the slopes of the plot [43]. The optical energy band gap values were calculated to be 2.20, 2.18, 1.94 and 1.60 eV for the pure-Cu2O, 100 C, 200 C and 300 C samples, respectively. The energy band gap of Cu2O is generally known to have a range of 1.8e2.2 eV. The pure-Cu2O, 100 C and
Fig. 4. XPS data of GOeCu2O composite films grown using various annealing temperatures: (a) XPS spectra, (b) O1s peak spectra, (c) C1s peak spectra and (d) atomic ratio for Cu/O/ C.
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Fig. 5. (a) Transmittance and (b) (ahn)2 versus hn plot of pure-Cu2O and GOeCu2O composite films grown using various annealing temperatures.
200 C samples have optical energy band gap values of 2.20, 2.18 and 1.94 eV, respectively, which are similar to the reported results [44]. However, the 300 C sample has a very low value, 1.60 eV, which could possibly be a result of the mixed Cu2O and Cu phases. In addition, the optical energy band gap values for 400 C and 500 C samples were not available due to the Cu phase. The PEC properties of GOeCu2O composite films with various annealing temperatures are shown in Fig. 6. We observed the open circuit potential at 0.68 V (versus RHE) for all samples except for 400 C and 500 C samples, indicating that the photo activities are first observed at 0.68 V by a light source. As shown in Fig. 6(a), the generated photocurrents were observed using a negative value at an applied reverse bias, which means that the samples were grown as p-type GOeCu2O composite films. However, we confirmed that the electron hole pairs (EHP) were not generated under 0.17 V (versus RHE). This result is apparent from the photo corrosion of the working electrodes [13]. The photocurrents were not observed for 400 C and 500 C samples due to the Cu phases. Fig. 6(b) shows the photocurrent density and dark current density from Fig. 6(a) (at 0.17 V versus RHE). The photocurrent density values were 0.34, 4.75 and 2.89 mA/cm2 for the 100 C, 200 C and 300 C samples, respectively. There were many studies for
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Fig. 6. PEC performance of GOeCu2O composite films grown using various annealing temperatures: (a) linear sweep voltammetry mode and (b) photocurrent density and dark current density (at 0.17 V versus RHE).
improving PEC photocurrent density values [27,45]. Paracchino et al. reported that the photocurrent density was improved by using Pt/TiO2/AZO/Cu2O photocathode grown by electrochemical deposition and ALD (atomic layer deposition) method. They improved PEC photocurrent density to 6.5 mA/cm2 [27]. And, Zhang et al. reported that the photocurrent density value was improved to 3.95 mA/cm2 by using carbon coated Cu2O NWs/Cu mesh photocathode grown by electrochemical deposition method [45]. In our study, the photocurrent density value of the 100 C sample was very weak because the thermal energy was very low; thus, the remaining hydroxyl (OH) groups were not frequently oxidized to oxygen (O2) in this sample. The 300 C sample was expected to have the highest PEC photocurrent density because it showed the highest (111) XRD peak intensity. However, our results show that the photocurrent density value of the 300 C sample was lower than that of the 200 C sample, which can be attributed to the mixture of the Cu2O and Cu phases in the 300 C sample. The Cu metal does not absorb light; however, it does scatter light. In addition, the dark current density values were 0.12, 0.21 and 1.73 mA/cm2 for the 100 C, 200 C and 300 C samples, respectively. The highest dark current density was observed
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with 1.73 mA/cm2 at 300 C, which was also attributed to the mixture of the Cu2O and Cu phases. The highest photocurrent density value in this study was observed with 4.75 mA/cm2 for the 200 C sample. 4. Conclusions In this work, we investigated the effect of annealing temperature on GOeCu2O composite films using various annealing temperatures. As a result, GOeCu2O composite films with the highest XRD (111) peak intensity, were obtained at 300 C. However, GOeCu2O composite films with the highest PEC photocurrent density were obtained with 4.75 mA/cm2 at 200 C sample. In this study, we also observed the co-existence of the Cu2O and Cu phases after reduction over 300 C samples via SEM, XRD, and XPS, which could explain the decrease in the PEC photocurrent density. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110012847). References [1] L. Chen, S. Shet, H. Tang, H. Wang, T. Deutsch, Y. Yan, J. Turner, M. Al-Jassim, Electrochemical deposition of copper oxide nanowires for photoelectrochemical applications, J. Mater. Chem. 20 (2010) 6962e6967. [2] X. Bai, L. Wang, R. Zong, Y. Lv, Y. Sun, Y. Zhu, Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization, Langmuir 29 (2013) 3097e3105. [3] E. Thimsen, F. Le Formal, M. Gr€ atzel, S.C. Warren, Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water splitting, Nano Lett. 11 (2010) 35e43. [4] I.S. Cho, Z. Chen, A.J. Forman, D.R. Kim, P.M. Rao, T.F. Jaramillo, X. Zheng, Branched TiO2 nanorods for photoelectrochemical hydrogen production, Nano Lett. 11 (2011) 4978e4984. [5] J. Su, X. Feng, J.D. Sloppy, L. Guo, C.A. Grimes, Vertically aligned WO3 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis and photoelectrochemical properties, Nano Lett. 11 (2010) 203e208. [6] S. Liu, J. Tian, L. Wang, Y. Luo, X. Sun, One-pot synthesis of CuO nanoflowerdecorated reduced graphene oxide and its application to photocatalytic degradation of dyes, Catal. Sci. Technol. 2 (2012) 339e344. [7] M.A. Mahmoud, W. Qian, M.A. El-Sayed, Following charge separation on the nanoscale in Cu2OeAu nanoframe hollow nanoparticles, Nano Lett. 11 (2011) 3285e3289. [8] K. Maeda, Photocatalytic water splitting using semiconductor particles: history and recent developments, J. Photochem. Photobiol. C: Photochem. Rev. 12 (2011) 237e268. [9] J. Kondo, Cu2O as a photocatalyst for overall water splitting under visible light irradiation, Chem. Commun. (1998) 357e358. [10] Y. Liu, Y. Liu, R. Mu, H. Yang, C. Shao, J. Zhang, Y. Lu, D. Shen, X. Fan, The structural and optical properties of Cu2O films electrodeposited on different substrates, Semicond. Sci. Technol. 20 (2005) 44. [11] P. de Jongh, J. Kelly, Cu2O: a catalyst for the photochemical decomposition of water? Chem. Commun. (1999) 1069e1070. [12] T. Maruyama, Copper oxide thin films prepared by chemical vapor deposition from copper dipivaloylmethanate, Sol. Energy Mater. Sol. Cells 56 (1998) 85e92. €tzel, E. Thimsen, Highly active oxide [13] A. Paracchino, V. Laporte, K. Sivula, M. Gra photocathode for photoelectrochemical water reduction, Nat. Mater. 10 (2011) 456e461. [14] S. Ishizuka, K. Suzuki, Y. Okamoto, M. Yanagita, T. Sakurai, K. Akimoto, N. Fujiwara, H. Kobayashi, K. Matsubara, S. Niki, Polycrystalline n-ZnO/p-Cu2O heterojunctions grown by RF-magnetron sputtering, Phys. Stat. Solidi. C. 1 (2004) 1067e1070. [15] H. Yu, J. Yu, S. Liu, S. Mann, Template-free hydrothermal synthesis of CuO/ Cu2O composite hollow microspheres, Chem. Mater. 19 (2007) 4327e4334. [16] A. Musa, T. Akomolafe, M. Carter, Production of cuprous oxide, a solar cell material, by thermal oxidation and a study of its physical and electrical properties, Sol. Energy Mater. Sol. Cells 51 (1998) 305e316. [17] R.P. Wijesundera, M. Hidaka, K. Koga, M. Sakai, W. Siripala, J.Y. Choi, E.S. Nark, Effects of annealing on the properties and structure of electrodeposited semiconducting CueO thin films, Phys. Stat. Sol. (b) 244 (2007) 4629e4642.
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