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Fabrication of coral-like Cu2O nanoelectrode for solar hydrogen generation
Journal of Power Sources 242 (2013) 541e547
Contents lists available at SciVerse ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Fabrication of coral-like Cu2O nanoelectrode for solar hydrogen generation Yu-Kuei Hsu a, *, Chun-Hao Yu a, Ying-Chu Chen b, Yan-Gu Lin c a
Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan c Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA b
h i g h l i g h t s Coral-like Cu2O were synthesized through thermal transformation of Cu(OH)2. High PEC activity of Cu2O nanocorals was achieved the conversion efficiency of 1.47%. IPCE data revealed the band gap of 2.08 eV in the Cu2O nanocorals.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2013 Received in revised form 19 April 2013 Accepted 23 May 2013 Available online 7 June 2013
The direct-grown and coral-like p-type Cu2O nanostructural film on copper foil is successfully fabricated via a facile and cost-effective template route through transformation of Cu(OH)2 nanowires for photoelectrochemical (PEC) hydrogen generation. The dense Cu2O nanocoral electrode thermally transfers from Cu(OH)2 nanowires by means of the dehydrate and deoxidization processes under nitrogen atmosphere. MotteSchottky plot shows the flat-band potential of the coral-like Cu2O nanostructural film to be 0.1 V and a hole concentration of 8.2 1019 cm3. Direct band gap of 2.08 eV in Cu2O film is determined via incident photon-to-electron conversion efficiency measurement. Significantly, this Cu2O nanocoral photocathode exhibits remarkable photocurrent of 1.3 mA cm2 at a potential of 0.6 V vs Ag/AgCl, corresponding to the solar conversion efficiency of 1.47%. These results demonstrate the great potential of Cu2O nanocoral film in solar hydrogen applications. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Cuprous oxide Nanostructures Photoelectrochemical Hydrogen generation
1. Introduction Since the fossil fuels is limited resource in the earth and is rapidly running out within several decades. Based on this concern of urgency in energy requirement, technologies for the generation of new types of energy must be developed. Among clean energy production methods, the solar-driven splitting of water into hydrogen and oxygen is a favorable means of converting solar energy into a “solar fuel”, because both water and sunlight are vastly abundant [1,2]. The route of photoelectrochemical (PEC) water decomposition under solar illumination to production of hydrogen is much simpler and more environmentally friendly than the catalytic reforming of hydrocarbon fuels [3e6]. In principle, the properties of the semiconductor materials that are used as photoelectrodes determine the solar conversion efficiency and the
* Corresponding author. Tel.: þ886 3 863 4196; fax: þ886 3 863 4180. E-mail address: [email protected] (Y.-K. Hsu).
stability of PEC water decomposition. Most of materials that are being developed for use in photoelectrodes, such as TiO2, GaN, WO3 and ZnO, have been modified to shift their energy band gap by substitutional element doping to extend their photoresponse to visible light [7e9]. But the inherently unavoidable trap centers, which created by dopant, decrease their photoresponse significantly, and the limitation of solubility of dopants also hampers the shrinkage of band gap in photoactive materials. Hence, cuprous oxide (Cu2O) with direct band gap of 2.1 eV exhibits great potential applications in solar energy conversion. Beside the p-type photoactive materials, copper is abundant and the materials can be processed by industrially proven, low-cost methods [10,11]. Recently, the highly active photocathode of Cu2O thin film, which can achieve the high photocurrents of 7.6 mA cm2 at a potential of 0 V and remained active for 1 h of testing was proposed [10]. These results demonstrate the great potential of Cu2O for PEC water decomposition. Actually, the PEC activity can be improved by the use of nanometer-sized materials, which are particularly relevant on account of their high surface-to-volume ratio and short diffusion
0378-7753/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.05.107
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length for carrier transport compared with their bulk counterparts [11]. Up to now, various Cu2O nanostructures, including nanowires, nanotubes, nanocubes, nanospheres, and soon, have been successfully fabricated with a variety of techniques, such as hydrothermal method, electrochemical deposition, and chemical bath method [12e15]. But, the studies targeted at the nanostructural Cu2O for PEC water decomposition are still few lately. In this work, the phase transformation route for the direct-grown Cu2O nanocoral film on copper foil as photocathode is proposed by means of the thermal reduction of Cu(OH)2 nanowires as template under the nitrogen atmosphere. The optimal PEC activities of Cu2O nanocoral photocathode are systematically analyzed. Moreover, these corallike Cu2O nanowire films not only function as photocathode for PEC hydrogen generation, but also offer a new opportunity to develop electronic and photoelectronic devices based on 3D hierarchical nanomaterials.
respectively. The photoelectrochemical behavior of the electrodes was measured in 0.5 M Na2SO4 solution by CHI 627D potentiostat/ galvanostat. A conventional three-electrode system consisting of the Cu2O nanocoral array as the working electrode, square platinum sheet as the auxiliary electrode, and an Ag/AgCl reference electrode in 3 M KCl solution were implemented. All potentials reported in this article were regarding Ag/AgCl (3 M KCl, 0.207 V vs. SHE). A 150 W Xe lamp light source with an AM 1.5 filter was used, and the intensity of the illumination at the sample position was determined to be 100 mW cm2. In addition, a 150 W Xe lamp equipped a monochromator was used as the excitation light source to obtain monochromatic light for incident photon-to-electron conversion efficiency measurement. The incident light was irradiated onto Cu2O nanocoral electrodes from the front face through the quartz window and the electrolyte unless noted otherwise. 3. Results and discussion
2. Experimental 3.1. Structural and composition characterizations The Cu2O nanostructural arrays on copper foil were fabricated via thermally transferring the Cu(OH)2 nanowires as template, which were synthesized through chemical oxidation process, as shown in Scheme 1. First, in order to prepare the Cu(OH)2 nanowires, an aqueous solution was prepared in a 100 mL glass bottle; a 2.5 M NaOH and 0.125 M (NH4)2S2O8 solution was mixed and the total solution volume was kept at 50 mL. A piece of copper foil, which had been ultrasonically cleaned in acetone and subsequently in de-ionized water, was immersed in the solution. In 10 min, a light-blue film covered the copper foil surface and the copper foil was extracted from the solution, rinsed with water and ethanol, and dried in air. Then the samples were thermally treated at various temperatures of 200, 300, 400, 500, and 600 C, respectively in nitrogen atmosphere for 1 h. The wire-like electrode was prepared by bonding a copper wire onto the edge of approximately 1 1 cm2. The bonding was accomplished by using silver paste, cured for 20 min. at 80 C. The bonding pad was covered with epoxide to expose only the copper oxide surface to the test solutions. The morphology of hierarchical Cu2O nanocoral arrays were examined by scanning electron microscopy (SEM, JEM-4000EX), and the structure of the samples were analyzed using X-ray diffractometer (XRD, Bruker D8 Advance diffractometer) with Cu Ka radiation (l ¼ 0.1506 nm). The chemical states of the elements were determined by X-ray photoelectron spectroscopy (XPS, PerkinElmer model PHI 1600) and Auger spectroscopy (VG Scientific, Microlab 350). Photoluminescence (PL) and Raman spectra were measured using a LabRAM HR 550 system equipped with a HeeCd laser (325 nm, 30 mW) and a He-Ne laser (632.8 nm, 5 mW),
Fig. 1a shows the wire-like nanostructures of Cu(OH)2 on a copper substrate. The dense and straight nanowire structures cover a large area of the copper substrate uniformly and compactly, while the nanowires have a length of over 8 mm and a diameter of 70e120 nm. Under the thermal treatment at various temperatures of 200, 300, 400, 500, and 600 C in nitrogen atmosphere, the morphologic evolution of those samples are displayed in Fig. 1bef, respectively. Obviously, all the nanowires are deformed by thermal treatment of 200, 300 and 400 C in the form of curl shape, but the length and diameter of nanowires still remain. At the temperature of 500 C, the apparent change in the shape of nanowires to corallike nanostructures can be observed. Above the temperature of 600 C, the significant decrease in the density of coral-like nanostructures may be ascribed the deoxidization process of materials in nitrogen atmosphere. The crystal phase of the samples with chemical oxidation and subsequently thermal reduction were determined using powder XRD, as shown in Fig. 2a. The XRD pattern of copper foil is also displayed for comparison. After chemical oxidation, the XRD pattern of the light-blue sample shows that all of the peaks marked with circles can be readily indexed to orthorhombic-phase Cu(OH)2 (JCPDS card No. 72-0140), except those marked with an asterisk of the copper substrate. As annealing temperature above 200 C, the diffraction peaks marked with triangle indexed to cubic-phase Cu2O (JCPDS card No. 78-2076) appear along with those marked with an asterisk of the copper substrate, but orthorhombic-phase Cu(OH)2 and crystalline CuO phase cannot be observed. This reveals
Scheme 1. Schematic diagram of synthesis of Cu2O nanocorals on copper foil.
Y.-K. Hsu et al. / Journal of Power Sources 242 (2013) 541e547
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Fig. 1. (a) FESEM images of Cu(OH)2 nanowires; SEM images of thermally treated Cu(OH)2 nanowires at temperature of (b) 200 C, (c) 300 C, (d) 400 C, (e) 500 C, and (f) 600 C under nitrogen atmosphere.
that the transformation of Cu(OH)2 to Cu2O through thermal reduction in nitrogen atmosphere was accomplished. This variation in structures is consistent with the finding of SEM, and also confirms the formation of curl-like Cu2O nanowires owing to the dehydration and deoxidization processes of Cu(OH)2. With increasing the temperature of thermal treatment, the diffraction peaks from Cu2O become stronger; meanwhile the intensity of Cu (220) peak suddenly appear at temperature of 300 and 400 C. Subsequently, the diffraction peak of Cu (200) peak replaced that of (220) peak in the sample with annealing temperature of 500 C, but the diffraction peaks of Cu2O still dominate and become stronger in the whole structure. It is worthwhile to notice that the formation of pure Cu2O phase can be always accomplished by co-existence of metallic Cu phase under thermal reduction, because of the unavoidable and rapid deoxidization process of Cu2O. Even though the metallic Cu was formed, the crystalline property of Cu2O nanostructures could be significantly improved at annealing temperature of 500 C. Herein, the presence of Cu may be responsible for the formation of coral-like Cu2O nanostructure. As the annealing temperature above 600 C, the higher temperature drive the serious decomposition of Cu2O according to rapid deoxidization
process, and then led to low density of Cu2O nanocorals, which also consisted with the finding of SEM. Furthermore, in order to probe the crystalline property of nanostructures, the diffraction peak of (111) of Cu2O were analyzed by comparing with the FWHM and peak intensity, as shown in Fig. 2b. Obviously, the FWHM value and the intensity of (111) peak changed with increase of annealing temperature. It is worthwhile to notice that the sample at annealing temperature of 500 C illustrated the smallest FWHM value and the highest XRD intensity. These reflect that the sample at annealing temperature of 500 C displayed the best crystalline quality in all samples according to Scherrer’s equation. The structural transformation of the Cu2O nanostructures and the effect of thermal treatment were also studied by Raman scattering spectroscopy. Fig. 2c shows the Raman spectra of the Cu2O samples at annealing temperature of 200, 300, 400, 500, and 600 C. The Raman spectrums of all samples show the characteristic phonon frequencies of the crystalline Cu2O, except the phonon modes of CuO phase at 273 and 327 cm1. However, the observation of CuO phase only in Raman spectra indicated their crystallization within short-range order, and confirmed the structural transformation through dehydration of Cu(OH)2 and subsequent
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CuO contrarily disappeared. This result reflected that the best crystallization of Cu2O nanostructures can be achieved at annealing temperature of 500 C, and this finding is also consistent with the results of XRD. From the XPS measurements, the Cu 2p peaks were resolved as shown in Fig. 3a. At temperature of 200 C, the Cu 2p peaks of
Fig. 2. (a) X-ray diffraction patterns, and (c) Raman spectra of Cu2O nanocorals at various annealing temperatures. (b) Fitting results of FWHM and intensity of Cu2O (111) peak as function as annealed temperature.
deoxidization of CuO. The strong peak at 218 cm1 originated from the second-order Raman-allowed mode of the Cu2O crystals. The peak at 148 cm1 may be attributed to Raman scattering from phonons of symmetry G15. In addition, the weak peaks at 308 and 515 cm1 correspond to the second-order overtone mode 2G15 (1) and the Raman-allowed mode, respectively. The peak at 416 cm1 is assigned to four-phonon mode 3G12 þ G25. The peak at 635 cm1 is attributed to the infrared-allowed mode [16]. With increasing the temperature of thermal treatment, the peak of 218 cm1 from Cu2O became stronger, and the phonon modes of
Fig. 3. (a) Cu 2p, (b) O 1s XPS spectra, and (c) Cu LMM of Cu2O nanocorals at various annealing temperatures.
Y.-K. Hsu et al. / Journal of Power Sources 242 (2013) 541e547
annealed sample at 934.4 eV and 954.2 eV corresponding to Cu 2p3/ and 2p1/2, respectively, reveal an oxidation state of þ2; meanwhile, the peaks from low-energy side of 932.2 eV and 952.1 eV correspond to an oxidation state of þ1 and/or 0 [17]. The occurrence of a satellite feature on the higher binding energy (943.2 eV) side of the Cu 2p main peak indicates the presence of CuO, because cupric oxide (Cu2þ) has hole states in the Cu3d band (Cu 3d9 configuration). Whereas the 3d band of cuprous oxide (Cuþ) is filled (Cu 3d1 ), and the 4s band is unoccupied, thus no satellites are expected. Therefore, as annealing temperature further increase, main peaks of Cu 2p3/2 and 2p1/2 shift to low-energy side of 932.2 eV and 952.1 eV; meanwhile the intensity of high energy peaks and satellite decrease dramatically with increase annealing temperatures. This result can confirm the thermal reduction of oxide species with Cu2þ to Cu2O/metallic Cu, which agrees with the findings of XRD and Raman. The observed amount of CuO is very tiny due to the slightly oxidized surface of Cu2O under their exposure to the atmosphere. All of these results bear out that the thermal reduction method can effectively and easily transfer Cu(OH)2 to Cu2O. In addition, the O 1s binding energy region displays in Fig. 3b. There are four oxygen components namely CueO, CueOeCu, oxygen on surface (O-surf), and water molecule at 529.1, 532, 531.5, and 535 eV, respectively. However, O-surf presents the component of metallic Cu, because metallic Cu could attract more oxygen on surface [18]. At annealing temperature of 200 C, the peak from CueOeCu dominates whole spectrum, reflecting main composition of Cu2O in sample. Besides, the peaks from CueO and water molecule were also detected, indicating the existence of CuO and hydrate nature. With increasing the temperature of thermal treatment, the peaks of CueO and water molecule decrease and even disappear, but the peak from CueOeCu remains. Significantly, the appearance of new peak from O-surf refers to the formation of metallic Cu in the annealed samples as the temperature of thermal treatment beyond 300 C. Furthermore, Cu LMM spectra of all samples are shown in Fig. 3c. At annealed temperature of 200 C, the sample presents the two peaks of 916.4 eV and 917.9 eV, corresponding to the state of Cu1þ and Cu2þ, respectively. When the annealed temperatures increase, the peak of Cu2þ disappears, and meanwhile, the peak of 918.8 eV, which is assigned to the state of Cu , appear [19]. These results could support the finding of XRD. Besides, the result of PL spectrum, which can provide some clues of physical properties of electronic transitions under corresponding excitation, such as defect-induced energy states, is shown in Fig. 4. 2
Fig. 4. Photoluminescence of Cu2O nanocorals at the annealing temperatures of 500 C.
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The excitation wavelength of 325 nm is used as the excitation source with photon of 3.8 eV, which was higher than the band gap of Cu2O and beyond the conduction band. The predominate peak at emitting energy around 1.95 eV is produced by the near band edge emission, and the emission at 1.35 eV is due to the transition from excitons bounded by copper vacancies (VCu, b-state) to VCu level (so-called b-luminescence) [20]. Herein, the formation of VCu may be ascribed to the separated phase of metallic Cu in Cu2O nanostructures according to the XRD finding. 3.2. PEC characterizations For solar water splitting, Cu2O possesses favorable energy band positions with the conduction band lying more negative of the hydrogen evolution potential and the valence band lying just positive of the oxygen evolution potential [10]. Under light illumination, the photo-generated electrons in Cu2O process hydrogen evolution; meanwhile, the photo-generated holes will be transferred to counter electrode to perform oxygen evolution, as shown in Scheme 2. To investigate the PEC characteristics of coral-like Cu2O nanostructure film as a photocathode for the solar generation of hydrogen by the decomposition water, a three-electrode electrochemical setup was adopted. Here, the chopped light was periodically illuminated on the sample at a rate of 2 s exposure following every 2 s unilluminated, and the current was recorded while the voltage was changed from 0 to 0.6 V with the scan rate of 10 mV s1. For all samples, a clear increase in the magnitude of the cathodic photocurrent was seen under light exposure, as shown in Fig. 5. These photo-induced cathodic currents result from the reduction of proton involving the photo-generation of electrons, revealing the p-type nature of coral-like Cu2O hierarchical photoelectrode. At annealing temperatures of 200, 300, and 400 C, a weak photocurrent and large dark-current can be ascribed to the poor crystallization of Cu2O phase and amorphous phase of CuO caused by pseudocapacitive nature, respectively [21]. As the thermal treatment increase at 500 C, the residual CuO could be thermally converted to Cu2O phase according to the finding from Raman results; meanwhile, the (111) diffraction peak of the narrowest FWHM from XRD results confirms the improvement of crystallization of Cu2O. Based on these findings, the strongest crystallization Cu2O nanocorals and complete thermal reduction of CuO in the samples at annealing temperature of 500 C obviously
Scheme 2. Schematic diagram of PEC water splitting of Cu2O photoelectrode.
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Fig. 5. Photocurrent-voltage responses of Cu2O nanocorals at various annealing temperatures of (a) 300 C, (b) 400 C, (c) 500 C, and (d) 600 C; inset of (a), 200 C.
enhance the photocurrent and diminish the dark current at the same time. Significantly, it is noteworthy that the maximum photocurrent for Cu2O nanocoral electrode at annealing temperature of 500 C can achieve the value of 1.3 mA cm2 at a potential of 0.6 V, corresponding to the solar conversion efficiency of 1.47% [22]. This high PEC activity, which exceeds recent reported values for Cu2O nanospheres and dendritic Cu2O film, may result from the excellent crystallization of Cu2O, complete thermal reduction of CuO, and the direct-grown nanostructures with low interfacial loss [17,23]. As the temperature exceed 600 C, the partly decomposed Cu2O nanocorals results in the decreased photocurrent. Furthermore, the photocurrent action spectra (Fig. 6) plot the incident photon-to- electron conversion efficiency (IPCE) as a function of excitation wavelength at a potential of 0.4 V. Significantly, the photoresponse of Cu2O shows the onset wavelength for the photocurrent generation at around 620 nm in the electrolyte. Furthermore, the gradual increase in the IPCE, h, with increasing photon energy is related to band gap energy by the following equation [6]:
hhn ¼ AðL þ WÞ hn Eg
m
Electrochemical impedance spectroscopy measurements were made on coral-like Cu2O photocathode in the dark. Capacitances at the semiconductoreelectrolyte interface (SEI) with the use of an equivalent circuit are described by a MotteSchottky plot, in order to estimate the flat-band position for Cu2O. Fig. 7 shows a Motte Schottky plot of data analyzed by using the CNLS fitting method based on an equivalent circuit, as shown in the inset. However, the
(1)
where Eg is the band gap energy while the exponent m equals 1/2 for a direct and 2 for an indirect electronic transition. W is the width of space charge layer. L is the diffusion length. The inset of Fig. 6 shows that Eq. (1) with m ¼ 1/2 is closely followed in a range of photon energies close to the absorption threshold indicating that the optical transition near the band gap is direct. From the intercepts of the straight lines with the abscissa, the band gap energy for Cu2O nanocoral derived as 2.08 eV. This result agrees with the literature report.
Fig. 6. IPCE as a function of excitation wavelength at a potential of 0.4 V from Cu2O nanocoral at temperature of 500 C; inset, variation of the square root of IPCE (h) times hv with photon energy for Cu2O nanocorals.
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structure and composition at various annealing temperatures, the results of XRD, Raman, and XPS were confirmed the transformation of orthorhombic-phase Cu(OH)2 to cubic-phase Cu2O. Furthermore, MotteSchottky analysis reveals the flat-band potential of 0.1 V vs. Ag/AgCl and hole concentration of 8.2 1019 cm3 in Cu2O nanocorals. The band gap energy, estimated at around 2.08 eV, of these coral-like Cu2O films is suited for efficient utilization of solar light. From the PEC measurements, the high active photocathode of Cu2O nanocorals at annealing temperature of 500 C can achieve the photocurrent of 1.3 mA cm2 at a potential of 0.6 V, corresponding to the solar conversion efficiency of 1.47%. Significantly, our study suggests that these direct-grown Cu2O nanocorals are not only promising for hydrogen generation, but also offer a new opportunity to develop electronic and photoelectronic devices based on 3D hierarchical nanomaterials. Acknowledgments The authors would like to thank the National Dong Hwa University and the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 101-2221-E-259-011.
Fig. 7. MotteSchottky plot of Cu2O nanocorals; inset, equivalent circuit.
MotteSchottky equation relates the capacitance of the semiconductor to the carrier concentration (Nd) and the other constants such as the fundamental charge constant (e), dielectric constant (ε of Cu2O is 7.6), vacuum permittivity (εo), temperature (T), Boltzmann constant (kB), and the flat-band potential (VFB) [24]:
1 ¼ C2
2 eεε0 Nd
k T V VFB B e
(2)
The slope of the linear part of the curve in the MotteSchottky plot is negative, indicating a p-type semiconductor. Furthermore, plotting 1/C2 versus V allows the estimation of the flat-band and surface carrier concentration, with the flat-band being the xintercept and the carrier concentration calculated from the slope. The value of flat-band locates at 0.1 V vs. Ag/AgCl, which corresponds to valence band. Estimation of the acceptor concentrations shows that in the Cu2O the concentration is 8.2 1019 cm3. The high value of Na can be attributed to a higher density of the Cu vacancies in oxide films, which resulted from the reduction of metallic Cu phase, in agreement with the previous reports [24,25]. 4. Conclusion In summary, the direct-grown p-type Cu2O film with nanocorals on copper foil is successfully synthesized via a facile and costeffective template route through transformation of Cu(OH)2 nanowires. During the thermal reduction, the Cu(OH)2 nanowires transfer to Cu2O nanocorals based on rapid dehydrate and deoxidization processes under nitrogen atmosphere. As regards the evolution of
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Contents lists available at SciVerse ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Fabrication of coral-like Cu2O nanoelectrode for solar hydrogen generation Yu-Kuei Hsu a, *, Chun-Hao Yu a, Ying-Chu Chen b, Yan-Gu Lin c a
Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan c Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA b
h i g h l i g h t s Coral-like Cu2O were synthesized through thermal transformation of Cu(OH)2. High PEC activity of Cu2O nanocorals was achieved the conversion efficiency of 1.47%. IPCE data revealed the band gap of 2.08 eV in the Cu2O nanocorals.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2013 Received in revised form 19 April 2013 Accepted 23 May 2013 Available online 7 June 2013
The direct-grown and coral-like p-type Cu2O nanostructural film on copper foil is successfully fabricated via a facile and cost-effective template route through transformation of Cu(OH)2 nanowires for photoelectrochemical (PEC) hydrogen generation. The dense Cu2O nanocoral electrode thermally transfers from Cu(OH)2 nanowires by means of the dehydrate and deoxidization processes under nitrogen atmosphere. MotteSchottky plot shows the flat-band potential of the coral-like Cu2O nanostructural film to be 0.1 V and a hole concentration of 8.2 1019 cm3. Direct band gap of 2.08 eV in Cu2O film is determined via incident photon-to-electron conversion efficiency measurement. Significantly, this Cu2O nanocoral photocathode exhibits remarkable photocurrent of 1.3 mA cm2 at a potential of 0.6 V vs Ag/AgCl, corresponding to the solar conversion efficiency of 1.47%. These results demonstrate the great potential of Cu2O nanocoral film in solar hydrogen applications. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Cuprous oxide Nanostructures Photoelectrochemical Hydrogen generation
1. Introduction Since the fossil fuels is limited resource in the earth and is rapidly running out within several decades. Based on this concern of urgency in energy requirement, technologies for the generation of new types of energy must be developed. Among clean energy production methods, the solar-driven splitting of water into hydrogen and oxygen is a favorable means of converting solar energy into a “solar fuel”, because both water and sunlight are vastly abundant [1,2]. The route of photoelectrochemical (PEC) water decomposition under solar illumination to production of hydrogen is much simpler and more environmentally friendly than the catalytic reforming of hydrocarbon fuels [3e6]. In principle, the properties of the semiconductor materials that are used as photoelectrodes determine the solar conversion efficiency and the
* Corresponding author. Tel.: þ886 3 863 4196; fax: þ886 3 863 4180. E-mail address: [email protected] (Y.-K. Hsu).
stability of PEC water decomposition. Most of materials that are being developed for use in photoelectrodes, such as TiO2, GaN, WO3 and ZnO, have been modified to shift their energy band gap by substitutional element doping to extend their photoresponse to visible light [7e9]. But the inherently unavoidable trap centers, which created by dopant, decrease their photoresponse significantly, and the limitation of solubility of dopants also hampers the shrinkage of band gap in photoactive materials. Hence, cuprous oxide (Cu2O) with direct band gap of 2.1 eV exhibits great potential applications in solar energy conversion. Beside the p-type photoactive materials, copper is abundant and the materials can be processed by industrially proven, low-cost methods [10,11]. Recently, the highly active photocathode of Cu2O thin film, which can achieve the high photocurrents of 7.6 mA cm2 at a potential of 0 V and remained active for 1 h of testing was proposed [10]. These results demonstrate the great potential of Cu2O for PEC water decomposition. Actually, the PEC activity can be improved by the use of nanometer-sized materials, which are particularly relevant on account of their high surface-to-volume ratio and short diffusion
0378-7753/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.05.107
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length for carrier transport compared with their bulk counterparts [11]. Up to now, various Cu2O nanostructures, including nanowires, nanotubes, nanocubes, nanospheres, and soon, have been successfully fabricated with a variety of techniques, such as hydrothermal method, electrochemical deposition, and chemical bath method [12e15]. But, the studies targeted at the nanostructural Cu2O for PEC water decomposition are still few lately. In this work, the phase transformation route for the direct-grown Cu2O nanocoral film on copper foil as photocathode is proposed by means of the thermal reduction of Cu(OH)2 nanowires as template under the nitrogen atmosphere. The optimal PEC activities of Cu2O nanocoral photocathode are systematically analyzed. Moreover, these corallike Cu2O nanowire films not only function as photocathode for PEC hydrogen generation, but also offer a new opportunity to develop electronic and photoelectronic devices based on 3D hierarchical nanomaterials.
respectively. The photoelectrochemical behavior of the electrodes was measured in 0.5 M Na2SO4 solution by CHI 627D potentiostat/ galvanostat. A conventional three-electrode system consisting of the Cu2O nanocoral array as the working electrode, square platinum sheet as the auxiliary electrode, and an Ag/AgCl reference electrode in 3 M KCl solution were implemented. All potentials reported in this article were regarding Ag/AgCl (3 M KCl, 0.207 V vs. SHE). A 150 W Xe lamp light source with an AM 1.5 filter was used, and the intensity of the illumination at the sample position was determined to be 100 mW cm2. In addition, a 150 W Xe lamp equipped a monochromator was used as the excitation light source to obtain monochromatic light for incident photon-to-electron conversion efficiency measurement. The incident light was irradiated onto Cu2O nanocoral electrodes from the front face through the quartz window and the electrolyte unless noted otherwise. 3. Results and discussion
2. Experimental 3.1. Structural and composition characterizations The Cu2O nanostructural arrays on copper foil were fabricated via thermally transferring the Cu(OH)2 nanowires as template, which were synthesized through chemical oxidation process, as shown in Scheme 1. First, in order to prepare the Cu(OH)2 nanowires, an aqueous solution was prepared in a 100 mL glass bottle; a 2.5 M NaOH and 0.125 M (NH4)2S2O8 solution was mixed and the total solution volume was kept at 50 mL. A piece of copper foil, which had been ultrasonically cleaned in acetone and subsequently in de-ionized water, was immersed in the solution. In 10 min, a light-blue film covered the copper foil surface and the copper foil was extracted from the solution, rinsed with water and ethanol, and dried in air. Then the samples were thermally treated at various temperatures of 200, 300, 400, 500, and 600 C, respectively in nitrogen atmosphere for 1 h. The wire-like electrode was prepared by bonding a copper wire onto the edge of approximately 1 1 cm2. The bonding was accomplished by using silver paste, cured for 20 min. at 80 C. The bonding pad was covered with epoxide to expose only the copper oxide surface to the test solutions. The morphology of hierarchical Cu2O nanocoral arrays were examined by scanning electron microscopy (SEM, JEM-4000EX), and the structure of the samples were analyzed using X-ray diffractometer (XRD, Bruker D8 Advance diffractometer) with Cu Ka radiation (l ¼ 0.1506 nm). The chemical states of the elements were determined by X-ray photoelectron spectroscopy (XPS, PerkinElmer model PHI 1600) and Auger spectroscopy (VG Scientific, Microlab 350). Photoluminescence (PL) and Raman spectra were measured using a LabRAM HR 550 system equipped with a HeeCd laser (325 nm, 30 mW) and a He-Ne laser (632.8 nm, 5 mW),
Fig. 1a shows the wire-like nanostructures of Cu(OH)2 on a copper substrate. The dense and straight nanowire structures cover a large area of the copper substrate uniformly and compactly, while the nanowires have a length of over 8 mm and a diameter of 70e120 nm. Under the thermal treatment at various temperatures of 200, 300, 400, 500, and 600 C in nitrogen atmosphere, the morphologic evolution of those samples are displayed in Fig. 1bef, respectively. Obviously, all the nanowires are deformed by thermal treatment of 200, 300 and 400 C in the form of curl shape, but the length and diameter of nanowires still remain. At the temperature of 500 C, the apparent change in the shape of nanowires to corallike nanostructures can be observed. Above the temperature of 600 C, the significant decrease in the density of coral-like nanostructures may be ascribed the deoxidization process of materials in nitrogen atmosphere. The crystal phase of the samples with chemical oxidation and subsequently thermal reduction were determined using powder XRD, as shown in Fig. 2a. The XRD pattern of copper foil is also displayed for comparison. After chemical oxidation, the XRD pattern of the light-blue sample shows that all of the peaks marked with circles can be readily indexed to orthorhombic-phase Cu(OH)2 (JCPDS card No. 72-0140), except those marked with an asterisk of the copper substrate. As annealing temperature above 200 C, the diffraction peaks marked with triangle indexed to cubic-phase Cu2O (JCPDS card No. 78-2076) appear along with those marked with an asterisk of the copper substrate, but orthorhombic-phase Cu(OH)2 and crystalline CuO phase cannot be observed. This reveals
Scheme 1. Schematic diagram of synthesis of Cu2O nanocorals on copper foil.
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Fig. 1. (a) FESEM images of Cu(OH)2 nanowires; SEM images of thermally treated Cu(OH)2 nanowires at temperature of (b) 200 C, (c) 300 C, (d) 400 C, (e) 500 C, and (f) 600 C under nitrogen atmosphere.
that the transformation of Cu(OH)2 to Cu2O through thermal reduction in nitrogen atmosphere was accomplished. This variation in structures is consistent with the finding of SEM, and also confirms the formation of curl-like Cu2O nanowires owing to the dehydration and deoxidization processes of Cu(OH)2. With increasing the temperature of thermal treatment, the diffraction peaks from Cu2O become stronger; meanwhile the intensity of Cu (220) peak suddenly appear at temperature of 300 and 400 C. Subsequently, the diffraction peak of Cu (200) peak replaced that of (220) peak in the sample with annealing temperature of 500 C, but the diffraction peaks of Cu2O still dominate and become stronger in the whole structure. It is worthwhile to notice that the formation of pure Cu2O phase can be always accomplished by co-existence of metallic Cu phase under thermal reduction, because of the unavoidable and rapid deoxidization process of Cu2O. Even though the metallic Cu was formed, the crystalline property of Cu2O nanostructures could be significantly improved at annealing temperature of 500 C. Herein, the presence of Cu may be responsible for the formation of coral-like Cu2O nanostructure. As the annealing temperature above 600 C, the higher temperature drive the serious decomposition of Cu2O according to rapid deoxidization
process, and then led to low density of Cu2O nanocorals, which also consisted with the finding of SEM. Furthermore, in order to probe the crystalline property of nanostructures, the diffraction peak of (111) of Cu2O were analyzed by comparing with the FWHM and peak intensity, as shown in Fig. 2b. Obviously, the FWHM value and the intensity of (111) peak changed with increase of annealing temperature. It is worthwhile to notice that the sample at annealing temperature of 500 C illustrated the smallest FWHM value and the highest XRD intensity. These reflect that the sample at annealing temperature of 500 C displayed the best crystalline quality in all samples according to Scherrer’s equation. The structural transformation of the Cu2O nanostructures and the effect of thermal treatment were also studied by Raman scattering spectroscopy. Fig. 2c shows the Raman spectra of the Cu2O samples at annealing temperature of 200, 300, 400, 500, and 600 C. The Raman spectrums of all samples show the characteristic phonon frequencies of the crystalline Cu2O, except the phonon modes of CuO phase at 273 and 327 cm1. However, the observation of CuO phase only in Raman spectra indicated their crystallization within short-range order, and confirmed the structural transformation through dehydration of Cu(OH)2 and subsequent
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CuO contrarily disappeared. This result reflected that the best crystallization of Cu2O nanostructures can be achieved at annealing temperature of 500 C, and this finding is also consistent with the results of XRD. From the XPS measurements, the Cu 2p peaks were resolved as shown in Fig. 3a. At temperature of 200 C, the Cu 2p peaks of
Fig. 2. (a) X-ray diffraction patterns, and (c) Raman spectra of Cu2O nanocorals at various annealing temperatures. (b) Fitting results of FWHM and intensity of Cu2O (111) peak as function as annealed temperature.
deoxidization of CuO. The strong peak at 218 cm1 originated from the second-order Raman-allowed mode of the Cu2O crystals. The peak at 148 cm1 may be attributed to Raman scattering from phonons of symmetry G15. In addition, the weak peaks at 308 and 515 cm1 correspond to the second-order overtone mode 2G15 (1) and the Raman-allowed mode, respectively. The peak at 416 cm1 is assigned to four-phonon mode 3G12 þ G25. The peak at 635 cm1 is attributed to the infrared-allowed mode [16]. With increasing the temperature of thermal treatment, the peak of 218 cm1 from Cu2O became stronger, and the phonon modes of
Fig. 3. (a) Cu 2p, (b) O 1s XPS spectra, and (c) Cu LMM of Cu2O nanocorals at various annealing temperatures.
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annealed sample at 934.4 eV and 954.2 eV corresponding to Cu 2p3/ and 2p1/2, respectively, reveal an oxidation state of þ2; meanwhile, the peaks from low-energy side of 932.2 eV and 952.1 eV correspond to an oxidation state of þ1 and/or 0 [17]. The occurrence of a satellite feature on the higher binding energy (943.2 eV) side of the Cu 2p main peak indicates the presence of CuO, because cupric oxide (Cu2þ) has hole states in the Cu3d band (Cu 3d9 configuration). Whereas the 3d band of cuprous oxide (Cuþ) is filled (Cu 3d1 ), and the 4s band is unoccupied, thus no satellites are expected. Therefore, as annealing temperature further increase, main peaks of Cu 2p3/2 and 2p1/2 shift to low-energy side of 932.2 eV and 952.1 eV; meanwhile the intensity of high energy peaks and satellite decrease dramatically with increase annealing temperatures. This result can confirm the thermal reduction of oxide species with Cu2þ to Cu2O/metallic Cu, which agrees with the findings of XRD and Raman. The observed amount of CuO is very tiny due to the slightly oxidized surface of Cu2O under their exposure to the atmosphere. All of these results bear out that the thermal reduction method can effectively and easily transfer Cu(OH)2 to Cu2O. In addition, the O 1s binding energy region displays in Fig. 3b. There are four oxygen components namely CueO, CueOeCu, oxygen on surface (O-surf), and water molecule at 529.1, 532, 531.5, and 535 eV, respectively. However, O-surf presents the component of metallic Cu, because metallic Cu could attract more oxygen on surface [18]. At annealing temperature of 200 C, the peak from CueOeCu dominates whole spectrum, reflecting main composition of Cu2O in sample. Besides, the peaks from CueO and water molecule were also detected, indicating the existence of CuO and hydrate nature. With increasing the temperature of thermal treatment, the peaks of CueO and water molecule decrease and even disappear, but the peak from CueOeCu remains. Significantly, the appearance of new peak from O-surf refers to the formation of metallic Cu in the annealed samples as the temperature of thermal treatment beyond 300 C. Furthermore, Cu LMM spectra of all samples are shown in Fig. 3c. At annealed temperature of 200 C, the sample presents the two peaks of 916.4 eV and 917.9 eV, corresponding to the state of Cu1þ and Cu2þ, respectively. When the annealed temperatures increase, the peak of Cu2þ disappears, and meanwhile, the peak of 918.8 eV, which is assigned to the state of Cu , appear [19]. These results could support the finding of XRD. Besides, the result of PL spectrum, which can provide some clues of physical properties of electronic transitions under corresponding excitation, such as defect-induced energy states, is shown in Fig. 4. 2
Fig. 4. Photoluminescence of Cu2O nanocorals at the annealing temperatures of 500 C.
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The excitation wavelength of 325 nm is used as the excitation source with photon of 3.8 eV, which was higher than the band gap of Cu2O and beyond the conduction band. The predominate peak at emitting energy around 1.95 eV is produced by the near band edge emission, and the emission at 1.35 eV is due to the transition from excitons bounded by copper vacancies (VCu, b-state) to VCu level (so-called b-luminescence) [20]. Herein, the formation of VCu may be ascribed to the separated phase of metallic Cu in Cu2O nanostructures according to the XRD finding. 3.2. PEC characterizations For solar water splitting, Cu2O possesses favorable energy band positions with the conduction band lying more negative of the hydrogen evolution potential and the valence band lying just positive of the oxygen evolution potential [10]. Under light illumination, the photo-generated electrons in Cu2O process hydrogen evolution; meanwhile, the photo-generated holes will be transferred to counter electrode to perform oxygen evolution, as shown in Scheme 2. To investigate the PEC characteristics of coral-like Cu2O nanostructure film as a photocathode for the solar generation of hydrogen by the decomposition water, a three-electrode electrochemical setup was adopted. Here, the chopped light was periodically illuminated on the sample at a rate of 2 s exposure following every 2 s unilluminated, and the current was recorded while the voltage was changed from 0 to 0.6 V with the scan rate of 10 mV s1. For all samples, a clear increase in the magnitude of the cathodic photocurrent was seen under light exposure, as shown in Fig. 5. These photo-induced cathodic currents result from the reduction of proton involving the photo-generation of electrons, revealing the p-type nature of coral-like Cu2O hierarchical photoelectrode. At annealing temperatures of 200, 300, and 400 C, a weak photocurrent and large dark-current can be ascribed to the poor crystallization of Cu2O phase and amorphous phase of CuO caused by pseudocapacitive nature, respectively [21]. As the thermal treatment increase at 500 C, the residual CuO could be thermally converted to Cu2O phase according to the finding from Raman results; meanwhile, the (111) diffraction peak of the narrowest FWHM from XRD results confirms the improvement of crystallization of Cu2O. Based on these findings, the strongest crystallization Cu2O nanocorals and complete thermal reduction of CuO in the samples at annealing temperature of 500 C obviously
Scheme 2. Schematic diagram of PEC water splitting of Cu2O photoelectrode.
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Fig. 5. Photocurrent-voltage responses of Cu2O nanocorals at various annealing temperatures of (a) 300 C, (b) 400 C, (c) 500 C, and (d) 600 C; inset of (a), 200 C.
enhance the photocurrent and diminish the dark current at the same time. Significantly, it is noteworthy that the maximum photocurrent for Cu2O nanocoral electrode at annealing temperature of 500 C can achieve the value of 1.3 mA cm2 at a potential of 0.6 V, corresponding to the solar conversion efficiency of 1.47% [22]. This high PEC activity, which exceeds recent reported values for Cu2O nanospheres and dendritic Cu2O film, may result from the excellent crystallization of Cu2O, complete thermal reduction of CuO, and the direct-grown nanostructures with low interfacial loss [17,23]. As the temperature exceed 600 C, the partly decomposed Cu2O nanocorals results in the decreased photocurrent. Furthermore, the photocurrent action spectra (Fig. 6) plot the incident photon-to- electron conversion efficiency (IPCE) as a function of excitation wavelength at a potential of 0.4 V. Significantly, the photoresponse of Cu2O shows the onset wavelength for the photocurrent generation at around 620 nm in the electrolyte. Furthermore, the gradual increase in the IPCE, h, with increasing photon energy is related to band gap energy by the following equation [6]:
hhn ¼ AðL þ WÞ hn Eg
m
Electrochemical impedance spectroscopy measurements were made on coral-like Cu2O photocathode in the dark. Capacitances at the semiconductoreelectrolyte interface (SEI) with the use of an equivalent circuit are described by a MotteSchottky plot, in order to estimate the flat-band position for Cu2O. Fig. 7 shows a Motte Schottky plot of data analyzed by using the CNLS fitting method based on an equivalent circuit, as shown in the inset. However, the
(1)
where Eg is the band gap energy while the exponent m equals 1/2 for a direct and 2 for an indirect electronic transition. W is the width of space charge layer. L is the diffusion length. The inset of Fig. 6 shows that Eq. (1) with m ¼ 1/2 is closely followed in a range of photon energies close to the absorption threshold indicating that the optical transition near the band gap is direct. From the intercepts of the straight lines with the abscissa, the band gap energy for Cu2O nanocoral derived as 2.08 eV. This result agrees with the literature report.
Fig. 6. IPCE as a function of excitation wavelength at a potential of 0.4 V from Cu2O nanocoral at temperature of 500 C; inset, variation of the square root of IPCE (h) times hv with photon energy for Cu2O nanocorals.
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structure and composition at various annealing temperatures, the results of XRD, Raman, and XPS were confirmed the transformation of orthorhombic-phase Cu(OH)2 to cubic-phase Cu2O. Furthermore, MotteSchottky analysis reveals the flat-band potential of 0.1 V vs. Ag/AgCl and hole concentration of 8.2 1019 cm3 in Cu2O nanocorals. The band gap energy, estimated at around 2.08 eV, of these coral-like Cu2O films is suited for efficient utilization of solar light. From the PEC measurements, the high active photocathode of Cu2O nanocorals at annealing temperature of 500 C can achieve the photocurrent of 1.3 mA cm2 at a potential of 0.6 V, corresponding to the solar conversion efficiency of 1.47%. Significantly, our study suggests that these direct-grown Cu2O nanocorals are not only promising for hydrogen generation, but also offer a new opportunity to develop electronic and photoelectronic devices based on 3D hierarchical nanomaterials. Acknowledgments The authors would like to thank the National Dong Hwa University and the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 101-2221-E-259-011.
Fig. 7. MotteSchottky plot of Cu2O nanocorals; inset, equivalent circuit.
MotteSchottky equation relates the capacitance of the semiconductor to the carrier concentration (Nd) and the other constants such as the fundamental charge constant (e), dielectric constant (ε of Cu2O is 7.6), vacuum permittivity (εo), temperature (T), Boltzmann constant (kB), and the flat-band potential (VFB) [24]:
1 ¼ C2
2 eεε0 Nd
k T V VFB B e
(2)
The slope of the linear part of the curve in the MotteSchottky plot is negative, indicating a p-type semiconductor. Furthermore, plotting 1/C2 versus V allows the estimation of the flat-band and surface carrier concentration, with the flat-band being the xintercept and the carrier concentration calculated from the slope. The value of flat-band locates at 0.1 V vs. Ag/AgCl, which corresponds to valence band. Estimation of the acceptor concentrations shows that in the Cu2O the concentration is 8.2 1019 cm3. The high value of Na can be attributed to a higher density of the Cu vacancies in oxide films, which resulted from the reduction of metallic Cu phase, in agreement with the previous reports [24,25]. 4. Conclusion In summary, the direct-grown p-type Cu2O film with nanocorals on copper foil is successfully synthesized via a facile and costeffective template route through transformation of Cu(OH)2 nanowires. During the thermal reduction, the Cu(OH)2 nanowires transfer to Cu2O nanocorals based on rapid dehydrate and deoxidization processes under nitrogen atmosphere. As regards the evolution of
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