Electrochemical Deposition of Semiconductor Oxides on Reduced

Jun 17, 2010 - Deepti Krishnan , Franklin Kim , Jiayan Luo , Rodolfo Cruz-Silva , Laura J. Cote , Hee Dong Jang , Jiaxing ... Bruno F. Machado , Phili...
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Electrochemical Deposition of Semiconductor Oxides on Reduced Graphene Oxide-Based Flexible, Transparent, and Conductive Electrodes Shixin Wu,† Zongyou Yin,† Qiyuan He,† Xiao Huang,† Xiaozhu Zhou,† and Hua Zhang*,†,‡ School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore, and Center for Biomimetic Sensor Science, Nanyang Technological UniVersity, 50 Nanyang DriVe, Singapore 637553, Singapore ReceiVed: April 24, 2010; ReVised Manuscript ReceiVed: June 4, 2010

Flexible, transparent, and conductive electrodes are prepared by reduction of the graphene oxide (GO) films which were spin-coated on the polyethylene terephthalate (PET) substrates. On the reduced graphene oxide (rGO) films, ZnO nanorods, as well as p-type and n-type Cu2O films with good crystallinity have been electrochemically deposited and then characterized. Meanwhile, the effect of pH value of the deposition bath on the morphology, structure, and semiconducting property of the electrochemical deposited Cu2O has been studied. Our results provide a possible way to replace the indium tin oxide (ITO) and fluorine tin oxide (FTO) electrodes with rGO films in the electrochemical synthesis, and make it promising to synthesize semiconductor oxides on rGO films for future flexible photovoltaic applications. Introduction Graphene, a new two-dimensional material, comprises a single layer of sp2-hybridized carbon atoms. It has shown various unique properties, such as 0 eV band gap semiconducting,1 zero effective mass,2 and remarkable ambipolar electric field effect with the high charge carrier mobility at ambient conditions.3,4 The commonly used methods for preparation of graphene-like materials include the micromechanical cleavage of graphite,3 chemical vapor deposition (CVD) of graphene on metal substrates,5 epitaxial growth of graphene on insulating substrates,6 synthesis of graphene from organic precursors,7 chemical reduction of exfoliated graphene oxide (GO),8–10 and electrochemical reduction of GO11 to obtain the reduced graphene oxide (rGO). Among the aforementioned methods, the chemical reduction method is attractive because it can produce a large amount of rGO, which can be used for preparation of largearea conductive films with relatively easily controlled and lowcost experimental processes, such as drop casting,12 vacuum filtration,8,13 and spin-coating.14,15 Research related to the conductive rGO films9b,14–17 is attracting increasing interest since the rGO film might replace the indium tin oxide (ITO) and fluorine tin oxide (FTO) as window electrodes in photovoltaic14,18–20 and light emitting devices.21 The reasons for reducing the use of ITO and FTO include the instability of ITO and FTO in acids or bases, the diffusion of indium ion into the coating layer, the limited amount of indium in the earth,18–20 etc. Recently, the electrochemical method has shown much importance in the synthesis of organoceramic films,22 polymer films,23 and semiconductors,24 since it is a low-temperature, lowcost, and facile technique with good control of the deposited composition.22–24 It has been widely used to synthesize ZnO,25–27 Cu2O,28–30 TiO2,31,32 and NiO.33,34 * To whom correspondence should be addressed. E-mail: hzhang@ ntu.edu.sg and [email protected]. Website: http://www.ntu.edu.sg/ home/hzhang/. † School of Materials Science and Engineering. ‡ Center for Biomimetic Sensor Science.

ZnO is an important semiconductor material with a wide band gap of 3.37 eV.35 Because of its exceptional electrical, optical, and chemical properties, ZnO has great potential in various applications, such as optoelectronics,36,37 solar cells,14,38,39 field emission,40,41 gas sensors,42 etc. ZnO nanorods have been successfully prepared by the hydrothermal method,43 chemical vapor deposition,44 and pulsed laser deposition.35 The use of the electrochemical method to prepare ZnO nanorods is promising because of its low cost, low reaction temperature, and facility for large-scale production.45 It is also capable of controlling the dimension of as-synthesized ZnO nanorods with different deposition conditions.26 Cu2O is an inexpensive, nontoxic, semiconductor material with high solar absorbance and a narrow band gap of 1.9-2.2 eV.46 It is typically known as a p-type semiconducting material,47 which is widely used as a solar energy harvesting material in photovoltaic devices48,49 and as a catalyst.50 Cu2O can be produced by several methods, such as the solution-phase method,51 anodic oxidation,47 and thermal oxidation.52 Besides the aforementioned methods, the electrochemical method is also well suitable for synthesis of Cu2O. The p-type and n-type doping for Cu2O can be easily controlled by only changing the pH value of the deposition bath.53 Herein, we report the electrochemical deposition of ZnO nanorods, and p-type and n-type Cu2O films on rGO films, which were first coated on the flexible substrates of polyethylene terephthalate (PET), referred to as rGO-PET. To the best of our knowledge, it is the first time the electrochemical deposition of metal oxides on rGOPET has been reported. The flexibility of the rGO film makes it suitable for flexible electronic device applications, such as solar cells, touch panels, and flexible displays. In addition, our work demonstrates the potential of rGO films to replace ITO and FTO in the future. Results and Discussion The rGO-PET electrode was prepared by reduction of the spin-coated GO film with a thickness of ca. 11 nm on the 3-aminopropyltriethoxysilane (APTES)-modified PET substrate (see the photograph of rGO-PET substrate in Figure S1A in

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Figure 1. (A) Raman spectrum and (B) UV-vis transmittance spectrum of rGO film on PET substrate.

Figure 2. (A) SEM images and (B) XRD pattern of electrochemically deposited ZnO nanorods on rGO-PET (XRD peaks labeled with ∆ are from PET). (C) TEM image of a ZnO nanorod. Inset: SAED pattern. (D) HRTEM image at the edge of the ZnO nanorod in panel C.

the Supporting Information, SI). The AFM image shows that the rGO-PET surface is quite rough (rms )2.6 nm in 3 × 3 µm2, Figure S1B in the SI). The Raman spectrum of rGO-PET (Figure 1A) contains the typical D, G and 2D bands of rGO, at 1342, 1571 and 2696 cm-1, corresponding to the disorderinduced mode, E2g optical mode and double resonant model, respectively.54,55 The higher intensity of the D band than that of the G band confirms that rGO was obtained after the reduction of GO.55 The other Raman peaks are attributed to the PET substrate.56 The UV-vis spectrum shows a transmittance of ∼47% for the obtained rGO-PET substrate (Figure 1B). The measured resistance of the rGO film with a thickness of about 11 nm on PET was ca. 15 kΩ/sq. Thus-obtained rGO-PET substrates are used in our experiments for electrochemical deposition of oxides. The electrochemical reactions happening in an oxygensaturated solution bath to deposit ZnO rods include the reduction of O2 and precipitation of ZnO at the cathode,43 i.e., O2 + 2H2O

+ 4e- f 4OH- and Zn2+ + 2OH- f Zn(OH)2 ) ZnO + H2O. The morphology of the obtained ZnO rods resulted from the higher generation rate of OH- than the diffusion rate of Zn2+ at the cathode.26,45 Hence, O2 plays a significant role in the formation of ZnO rods. The O2 flow rate and the distance for O2 to reach the rGO film should be carefully controlled during the deposition process. At a low potential (e.g., -1 V), the obtained ZnO on rGOPET did not show the good rod morphology (Figure S2 in the SI). When the deposition potential was changed to -1.9 V, the electrochemically deposited ZnO nanorods on rGO-PET exhibited good crystalline structures (Figure 2A). The inset in Figure 2A shows the enlarged SEM image of ZnO nanorods. Most of the nanorod tips exhibit the well-defined hexagonal structures. Compared to the XRD pattern of rGO-PET (Figure S3 in the SI, note that the peak of rGO is difficult to observe due to its weak signal and the overlap of peak position at 26° with PET57,58), the XRD pattern of ZnO nanorods deposited on rGO-

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PET (Figure 2B) reveals the wurtzite structure of ZnO, which corresponds to the space group of P63mc.59 The relatively high intensity of the ZnO (002) diffraction peak indicates the anisotropic growth of ZnO nanorods along their c-axes of the [001] direction and perpendicular to the rGO-PET substrate. The vertical growth can also be observed from the SEM images (Figure 2A). Figure 2C shows the TEM image of a ZnO nanorod. The selected area electron diffraction (SAED) pattern clearly shows the resolved diffraction spots of the [210] zone axis (inset in Figure 2C), revealing the single crystalline characteristic and wurtzite structure of the ZnO nanorod. In a high-resolution TEM (HRTEM) image (Figure 2D), the lattice spacing of 0.26 nm matches well with the interplanar distance of (002) crystal planes of ZnO. On the other hand, the electrochemical deposition of Cu2O contains two steps, i.e., the reduction of Cu2+ ions and the formation of Cu2O, which can be expressed by the following equations:47–49,53

Cu2+ + e- f Cu+ and 2Cu+ + 2OH- f Cu2O + H2O In our experiments, the successful deposition of Cu2O with high density and good crystallinity on the rGO-PET electrode has been achieved. In addition, the pH effect on Cu2O morphologies, structures, and properties has been discussed in detail. Figure 3A and B show the SEM images of Cu2O deposited at pH 9. To characterize the semiconducting behavior of the deposited Cu2O, the Mott-Schottky (MS) plot was measured at the interface between Cu2O and the liquid electrolyte of Na2HPO4, Figure 3C. It exhibits a negative slope, suggesting Cu2O grown in the basic solution is p-type semiconducting (referred to as p-type Cu2O), which is believed to be due to the copper vacancies present in the Cu2O lattice.60 Through the calculations based on the following MS theory53

Wu et al.

1 2 kT )( V - VFB 2 2 e Csc eεε0A ND/A

(

)

((: “+” represents n-type and “-” represnets p-type), where Csc is the capacitance of the space charge region, ε is the dielectric constant of the semiconductor, εo is the permittivity of the free space, A is the working electrode area, ND/A is the carrier (donor or acceptor) concentration, V is the applied potential, and VFB is the flatband potential. The acceptor concentration was calculated to be 1.40 × 1020 cm-3 (assuming 6.3 as the dielectric constant of Cu2O53). The p-type Cu2O exhibits a pyramidal shape (Figure 3A,B). The (111) planes at the slope side of the crystal suggest the slower growth rate of p-type Cu2O along the [111] direction.48 Figure 3D shows the XRD pattern of p-type Cu2O film deposited on rGO-PET, which confirms the derivation of Cu2O rather than CuO or Cu.47 The higher intensity of the (200) diffraction peak in p-type Cu2O (compared to the standard XRD data ICPDS 5-667) may be attributed to the deposition current density. It has been reported that Cu2O grew with a much stronger [100] orientation, when it was deposited at a current density of 5 × 10-5 A/cm2.61 Note that the deposition current density in the electrolyte of pH 9 for p-type Cu2O in our experiment was ∼7 × 10-5 A/cm2, similar to that used in the previous report (5 × 10-5 A/cm2).61 Figure 3E shows the TEM image of a p-type Cu2O crystal, which was scratched from the rGO film in order to prepare the sample for TEM. The SAED pattern (inset in Figure 3E) shows the bright individual diffraction dots, which confirm the single crystallinity of the deposited p-type Cu2O on rGO-PET. Figure 3F gives the HRTEM image of a portion of the crystal, where the lattice spacings of 0.14 and 0.12 nm correspond to the interplanar distances of (2j20) and (3j1j1) planes in the Cu2O lattice, respectively. When the pH value of deposition solution was changed to pH 7, the deposited Cu2O showed morphology (Figure 4A,B),

Figure 3. (A, B) SEM images of electrochemically deposited Cu2O on rGO-PET at pH 9. (C) Mott-Schottky plot of as-obtained p-type Cu2O on rGO-PET. (D) XRD pattern of p-type Cu2O on rGO-PET (peaks labeled with ∆ are from PET). (E) TEM image of a p-type Cu2O crystal. Inset: SAED pattern. (F) HRTEM image at the edge of p-type Cu2O in panel E.

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Figure 4. (A, B) SEM images of electrochemically deposited Cu2O on rGO-PET at pH 7. (C) Mott-Schottky plot of as-obtained n-type Cu2O on rGO-PET. (D) XRD pattern of n-type Cu2O on rGO-PET (XRD peaks labeled with ∆ are from PET). (E) TEM image of an n-type Cu2O crystal. Inset: SAED pattern. (F) HRTEM image at the edge of n-type Cu2O in panel E.

which is quite different compared to the p-type Cu2O. Figure 4C shows the MS plot of Cu2O deposited at pH 7, which displays a linear relationship between the (Csc)-2 and applied potential with a positive slope, indicating the n-type semiconducting Cu2O deposited in the neutral solution (referred to as n-type Cu2O). The donor concentration was calculated at about 3.50 × 1019 cm-3. This shows that the doping concentration of n-type Cu2O is lower than that of p-type Cu2O. The XRD pattern confirmed the obtained Cu2O film (Figure 4D). Figure 4E shows the TEM image of an n-type Cu2O sheet scratched from the rGO film. The SAED pattern (inset in Figure 4E) from the thin sheet exhibits the resolved diffraction spots, which prove the good crystallinity of deposited n-type Cu2O on rGO-PET. The HRTEM image (Figure 4F) at the inner portion of Cu2O sheets shows the lattice spacing of crystal planes is about 0.21 nm, which corresponds to the interplanar distance between (020) planes. Conclusion We have reported the successful electrochemical deposition of highly crystalline ZnO nanorods and doping-type controlled Cu2O films on the flexible, transparent, and conductive rGOPET substrates. Comprehensive understanding on the structures of the deposited materials has been demonstrated. The pHinduced difference in morphology and doping behavior of electrochemically deposited Cu2O has been characterized, analyzed, and discussed in detail. We believe this work paves the way to synthesize functional materials on flexible rGO-PET substrates with a promising potential for solar cells, which is under investigation. Experimental Section Preparation of rGO-PET Electrodes. The preparation of GO in methanol solution is described elsewhere.9,14 The polyethylene terephthalate (PET) film (3M, USA) was first modified with 3-aminopropyltriethoxysilane (APTES, SigmaAldrich).62 Briefly, after PET was immersed into an aqueous

solution of 3% APTES for 30 min, followed by thorough rinsing with DI water, the APTES self-assembled monolayers (SAMs) were formed on the PET surface. Then the GO in methanol solution (0.5 mg/mL) was spin-coated on the APTES-modified PET substrate at 4000 rpm to obtain ca. 11 nm GO thin film on PET. The rGO-PET electrode was obtained by reduction of the GO film on PET in hydrazine vapor at 60 °C overnight. Electrochemical Deposition of ZnO Nanorods on rGOPET Electrodes. The electrochemical experiments were performed based on the previous reports25,26 in an electrochemical workstation (CHI600C, CH Instrument Inc., USA). Briefly, the electrochemical deposition of ZnO on rGO-PET was carried out in a conventional three-electrode electrochemical cell. The rGO-PET electrode, a Pt mesh, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The electrolyte was an oxygen-saturated aqueous solution containing ZnCl2 and supporting electrolyte of KCl (0.1 M). The electrochemical synthesis of ZnO nanorods includes two steps, i.e., the buffer layer growth by a galvanostatic deposition (constant current of 1 × 10-5 A, charge density of ∼3.6 × 10-1 C/cm2) at room temperature, and then the rod growth by a potentiostatic process (potential of -1.9 V, charge density of 2 C/cm2) at 80 °C. The ZnCl2 concentration was 5 × 10-3 and 1 × 10-3 M for the buffer layer growth and rod growth, respectively. Electrochemical Deposition of Cu2O on rGO-PET Electrodes. The electrochemical deposition of Cu2O was carried out by using the reported methods.47,53 Briefly, Cu2O was electrochemically deposited on rGO-PET in a conventional threeelectrode electrochemical cell. The rGO-PET electrode, a Pt mesh, and a saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. The electrolyte contained 0.4 M CuSO4 and 3 M lactic acid with the electrolyte pH adjusted by addition of 4 M NaOH. To deposit the p-type and n-type Cu2O, the pH of the solution was carefully adjusted to 9.0 ( 0.2 and 7.0 ( 0.2, respectively. Cu2O was

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deposited by a potentiostatic process (potential of -0.4 V, charge density of 2 C/cm2) in the prepared electrolyte at 60 °C. Characterization. The AFM image was obtained by the Dimension 3100 (Veeco, CA, USA) with a Si tip (resonance frequency: 320 kHz; spring constant: 42 N/m) in the tapping mode under ambient conditions with a scanning rate of 1 Hz and scanning line of 512. Raman spectra were measured with a WITec CRM200 confocal Raman microscopy system with the excitation line of 488 nm and an air cooling charge coupled device (CCD) as the detector (WITec Instruments Corp, Germany). The Raman band of a silicon wafer at 520 cm-1 was used as the reference to calibrate the spectrometer. The UV-vis transmittance spectrum was recorded on a UV1800 spectrophotometer (Shimadzu, Asia Pacific Pte Ltd.) at a wavelength of 1000 nm. The sheet resistance of rGO-PET was measured by the manual four-point resistivity probing equipment (Signatone, Gilroy, CA). X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (Rigaku D/max 2250 V), using Cu Ka radiation (λ ) 1.5406 Å). The accelerating voltage and the applied current were 40 mV and 40 mA, respectively. Scanning electron microscopy (SEM) was performed on a JEOL JSM-6340F field-emission scanning electron microanalyzer at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were collected on a JEOL JEM2010 transmission electron microscope with an accelerating voltage of 200 kV. The TEM samples were prepared as follows. After the deposited materials on the rGO-PET substrates were scratched with a needle, they were sonicated in water. A 2 µL sample of thus solution was dropped onto the copper grid. After the solution was dried, the sample was immediately used for TEM measurements. The Mott-Schottky (MS) plot was recorded by the electrochemical workstation (CHI600C, CH Instrument Inc., USA), using the ac impedance method. The measurement was conducted in a conventional three-electrode cell, using a rGO-PET electrode with the deposited materials, a standard saturated Ag/ AgCl, and a Pt mesh as the working, reference, and counter electrodes, respectively. To measure the MS plot of Cu2O (area of 0.4 cm2), the amplitude of the ac potential was set at 5 mV, the frequency was 1 kHz, and the electrolyte contained 0.1 M Na2HPO4 with solution pH adjusted to 10 ( 0.2 by 4 M NaOH. Supporting Information Available: Figure S1, photograph and AFM image of rGO film on PET substrate, Figure S2, SEM image of electrochemically deposited ZnO on rGO-PET at -1 V, and Figure S3, XRD pattern of rGO-PET substrate. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (4) Geim, A. K. Science 2009, 324, 1530. (5) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457, 706. (6) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191. (7) Yang, X.; Dou, X.; Rouhanipour, A.; Zhi, L.; Ra¨der, H. J.; Mu¨llen, K. J. Am. Chem. Soc. 2008, 130, 4216.

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