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Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4
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Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4 Ying Liu, Yu-Xiang Yu, Wei-De Zhang* School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China
article info
abstract
Article history:
Novel photocatalysts, which consist of two visible light responsive semiconductors
Received 7 November 2013
including graphite-like carbon nitride (g-C3N4) and Fe2O3, were successfully synthesized via
Received in revised form
electrodeposition followed by chemical vapor deposition. The morphology of the g-C3N4/
29 March 2014
Fe2O3 can be tuned from regular nanosheets to porous cross-linked nanostructures.
Accepted 31 March 2014
Remarkably, the optimum activity of the g-C3N4/Fe2O3 is almost 70 times higher than that
Available online 27 April 2014
of individual Fe2O3 for photoelectrochemical water splitting. The enhancement of photoelectrochemical activity could be assigned to the morphology change of the photocatalysts
Keywords:
and the effective separation and transfer of photogenerated electrons and holes originated
Graphite-like carbon nitride
from the intimately contacted interfaces. The g-C3N4/Fe2O3 composites could be developed
Fe2O3
as high performance photocatalysts for water splitting and other optoelectric devices.
Nanocomposites
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Charge transfer Photoelectrochemical property
Introduction Following light-induced water-splitting experiment with a TiO2 photoanode by Fujishima and his colleagues in 1972 [1], worldwide studies have been focused on the conversion of solar energy to hydrogen energy. In such a process, the key is to develop stable and low-cost photocatalysts which are able to absorb the majority of solar photons. Compared to other semiconductors, Fe2O3 has many potential advantages for photoelectrochemical splitting of water to produce hydrogen. However, the pure hematite is a low conductive material with short diffusion length of holes around the electrolyte interface, which limits its application as a
photoanode. The traditional approach to overcome these drawbacks is either to dope heteroatom or to control the morphology of Fe2O3 photocatalyst. Efforts to generate nanostructured hematite through morphological control, including the formation of nanoplatelets [2], nanotubes [3], and nanospheres [4], resulted in improved performance of Fe2O3. These structural strategies either improve the light collection or minimize the distance that holes travel to the solideliquid interface. Recently, polymeric graphite-like carbon nitride (g-C3N4) has been introduced as a metal-free photocatalyst for solardriven application. g-C3N4 has small direct bandgap due to the sp2 hybridization of carbon and nitrogen forming the pconjugated graphitic planes [5,6]. Thus, g-C3N4 (ca. 2.7 eV)
* Corresponding author. Tel./fax: þ86 20 8711 4099. E-mail addresses: [email protected], [email protected] (W.-D. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2014.03.248 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Fig. 1 e SEM images of (A) Fe2O3, (B) cross-section of Fe2O3, (C) and (D) g-C3N4/Fe2O3.
has sufficient absorption and shows photocatalytic activity for water to be reduced to H2 or oxidized to O2 under visible light irradiation. Wang et al. [7] firstly reported that g-C3N4 was functionally achieved as a visible light driven metal-free photocatalyst for H2 production. However, the photocatalytic efficiency of g-C3N4 is limited by the high recombination rate of photogenerated electrons and holes. In order to enhance the photoactivity of g-C3N4, such efforts have been made, including preparing porous structure [8], doping or coupling of g-C3N4 with metals [9,10], semiconductive oxides [11], graphene [12e14], and being activated by organic dyes [15]. In particular, constructing heterojunctions by combining g-C3N4 with other semiconductors provides a feasible method to enhance photocatalytic performance. Our group has previously reported gC3N4-modified TiO2 nanorod arrays with high visible light photoelectrochemical activity [16]. It is expected that the combination of g-C3N4 with Fe2O3 would be ideal in inhibiting the recombination of photogenerated electronehole pairs and enhancing the photoelectrical activity compared to individual g-C3N4 or Fe2O3. Recently, Qiu and his colleagues firstly reported the fabrication of magnetic Fe2O3/g-C3N4 photocatalyst and its enhanced photodegradation rate toward organic dye under visible light [17]. However, in most previous studies, g-C3N4 was used as a supporting material and the composites were constructed by decorating the nanoparticles onto the surface of g-C3N4. Herein, we present the novel morphology-controllable g-C3N4-coated Fe2O3 nanostructure photocatalysts. The
g-C3N4/Fe2O3 photocatalysts were prepared by electrodeposition followed by chemical vapor deposition. The morphology of the composites changed with the different filling degrees of the precursor during the annealing process. The morphology of the g-C3N4/Fe2O3 was gradually tuned from regular nanosheets to porous cross-linked nanostructures. Meanwhile, the photoelectrochemical activity of the optimized g-C3N4/Fe2O3 composite was 70 times higher than that of pure Fe2O3. The effect of morphology, the synergy effect between Fe2O3 and gC3N4, and the possible mechanism of the photoelectrochemical activity enhancement via hybridization were systematically investigated.
Experimental section Chemicals Conductive fluorine-doped tin oxide (FTO)-coated glass (25 U cm2) was purchased from Nippon Sheet, Japan. Deionized water (Resistivity > 18.4 MU cm) was produced using a pure water system (GWA-UN, Beijing, China). Melamine (C3H6N6) was obtained from Kermel. (NH4)2Fe(SO4)2 was purchased from Guangzhou Chemical Reagent Factory. All chemicals were used without further purification.
Synthesis of g-C3N4/Fe2O3 The Fe2O3 nanosheet arrays were prepared by electrodeposition. Briefly, the films of Fe(OH)3 were obtained in aqueous
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Fig. 2 e (A) TEM and (B) HRTEM images of g-C3N4/Fe2O3. (C) Elemental mapping of a single g-C3N4/Fe2O3 nanorod.
solution containing 0.010 M (NH4)2Fe(SO4)2 and 0.040 M pyridine. All electrodeposition processes were carried out in a configured glass cell at 70 C at a potential of 0.644 V vs. Ag/ AgCl. The deposition process lasted for 1 h. The Fe2O3/FTO was obtained by heating the as-prepared Fe(OH)3/FTO under the protection of N2 at 550 C for 2 h. The preparation of g-C3N4/Fe2O3 is as follows: 3 g of melamine (C3H6N6) powder was put in a ceramic crucible and the as-prepared Fe(OH)3 arrays on FTO were topedown placed above the melamine. The crucible was heated to 390 C in a muffle furnace for 1 h with a heating rate of 5 C min1; then heated at 550 C for another 2 h. After the chemical vapor deposition process, g-C3N4 was successfully deposited onto the Fe2O3. The reaction can be tuned simply by increasing or decreasing the filling degree of melamine in the ceramic crucible.
Characterization The crystal phase of the products was characterized by X-ray diffraction (XRD) on an X’Pert MPD Pro X-ray diffractometer, using Cu Ka radiation (l ¼ 0.15418 nm). Field emission scanning electron microscopy (FESEM) images were recorded on LEO 1530Vp. Transmission electron microscopy (TEM) images
were taken on FEI Tecnai G20 which was operated at 200 kV. X-ray photoelectron spectroscopy with Al Ka X-rays radiation (Kratos, Axis Ultra) was used to investigate the surface properties. All the binding energy values were calibrated by using the C 1s level at 284.8 eV as an internal standard. Fourier transform infrared spectra (FT-IR) were measured using Spectrum One infrared spectrometer with KBr as the reference. UVevis diffuse reflectance spectroscopy was recorded with a Hitachi U-3010 spectrophotometer. The photoluminescence spectra were measured with a fluorescence spectrophotometer (Hitachi F-4500) using a Xe lamp as an excitation source with optical filters. Impedance-potential measurements were carried out on Chenhua electrochemical workstation (CHI660c, Shanghai, China) with a frequency of 1 kHz in 0.50 M KCl containing equimolar [Fe(CN)6]3/4 (0.010/0.010 M) solution. Photoelectrochemical measurement was performed with an electrochemical workstation (Ingsens-1030, Guangzhou, China) in 1.0 M NaOH solution (pH ¼ 13.6) with a Pt plate as a counter electrode and Ag/AgCl as a reference electrode. Substrates were immersed in solution and illuminated through a quartz window on the Fe2O3 side as a photoanode. The light source was a 300 W Xe lamp (PLS-SXE 300/300UV) equipped with an ultraviolet cutoff filter. The integrated visible light intensity was about
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Fig. 3 e XRD patterns of Fe(OH)3, Fe2O3, g-C3N4/Fe2O3 (* and # indicate as FTO substrate and Fe2O3, respectively). Inset: (a)e(d) correspond to the optical color of samples in Fig. S1 (A) and Fig. 1(A), (C) and (D), respectively.
150 mW cm2. A 500 W Xe lamp (CHF-XM-500, Changtuo Technology Co., Ltd.) with a monochromator (monochromator 300, Changtuo Technology Co., Ltd.) was employed to study wavelength-dependent photocurrent. The light intensity was calibrated with a radiometer (PM120VA, Thorlabs).
Results and discussion Characterization of the g-C3N4/Fe2O3 samples The morphology and microstructure of the samples were revealed by SEM and TEM. As shown in Fig. 1A, dense nanoplatelets are perpendicularly grown on the FTO substrate. The nanoplatelets are smooth, with a thickness of ca. 30 nm and width of 600 nm. The thickness of the films is about 1 mm which can be seen from the cross-section in Fig. 1B. After the chemical vapor deposition process, g-C3N4 is successfully coated onto the surface of Fe2O3 nanoplatelets. The morphology of the Fe2O3 nanoplatelets is gradually transformed to porous cross-linked nanostructures from the original nanoplatelets (Fig. 1C and D). The morphology change can be attributed to the influence of ammonia gas generated during the process in which melamine polycondenses to carbon nitride (C3H6N6 / C3N4 þ 2NH3). The generated ammonia could react with the Fe2O3 to form Fe at high temperature in an enclosed space (Eq. (1)). Meanwhile, the NH3 can be oxidized by oxygen with the catalysis of Fe2O3 to form nitrogen oxides, which further oxidize Fe to Fe2O3 (Eqs. (4) and (5)). The control experiment was carried out by annealing Fe(OH)3 arrays under NH3 atmosphere. The morphology of the Fe2O3 gradually changes to nanoparticles (Fig. S1 B in Supplementary Materials). Thus, the initiator melamine plays dual roles: a source to form g-C3N4 and a generator of NH3 which changes the morphology of the Fe2O3.
Fig. 4 e XPS spectra of (A) C 1s, and (B) N 1s.
2NH3 þ Fe2 O3 /N2 þ 2Fe þ 3H2 O Fe2 O3
(1)
4NH3 þ 5O2 / 4NO þ 6H2 O
(2)
2NO þ O2 /2NO2
(3)
2NO þ 2Fe/N2 þ 2FeO
(4)
NO2 þ 2FeO/NO þ Fe2 O3
(5)
The morphology of the g-C3N4/Fe2O3 was further explored by TEM and HRTEM, which are shown in Fig. 2A and B. The lattice structure of Fe2O3 is orderly and the outer boundary of the sample is distinctly different from the Fe2O3 (Fig. 2B). The fringe with d ¼ 0.270 nm matches with the (104) crystallographic plane of Fe2O3 and the thickness of the gC3N4 layer coated on the Fe2O3 is about 2.5 nm. The HRTEM analysis suggests that the g-C3N4 was uniformly dispersed on the surface of Fe2O3 and the heterojunction indeed formed between g-C3N4 and Fe2O3. The elemental mapping of a single g-C3N4/Fe2O3 nanostructure (Fig. 2C) shows that the Fe2O3 porous nanostructure is completely covered by gC 3N 4.
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XRD is used to investigate the phase structure of the samples, and the typical diffraction patterns are shown in Fig. 3. The Fe(OH)3 precursor is completely changed to Fe2O3 after annealing. The peaks in the XRD pattern of Fe2O3 can be indexed to the rhombohedral phase (JCPDS 33-0664). No significant peaks of g-C3N4 can be detected in the g-C3N4/Fe2O3 sample, which can be explained by the small amount and high dispersion of g-C3N4. Meanwhile, the color of the samples changes from brown to light tan with g-C3N4 loading (inset of Fig. 3), which implies that the g-C3N4 is successfully introduced onto the Fe2O3. The formation of g-C3N4 was further confirmed by XPS spectrum. XPS was carried out to determine the chemical composition of the g-C3N4/Fe2O3 and the valence states of the elements. The binding energy was calibrated by C 1s at 284.8 eV. Fig. 4A presents the XPS spectrum of C 1s which can be divided into three peaks with binding energy of 284.8, 286.5 and 288.4 eV, corresponding to the CeC (additional carbon), C]N, and CeN. Fig. 4B is the magnified spectra of N 1s with the binding energy at 399.1, 400.1 and 402.0 eV, respectively. The peak at binding energy of 399.1 eV is attributed to the sp2-hybridized nitrogen (C]NeC) and the two peaks of 400.1 and 402.0 eV can be assigned to tertiary nitrogen (N(C)3) and amino functional group with a hydrogen atom (CeNeH).
Optical property of the g-C3N4/Fe2O3 samples The optical properties of the Fe2O3, g-C3N4, and g-C3N4/Fe2O3 were investigated by UVevis DRS spectroscopy, as illustrated in Fig. 5. The g-C3N4 absorbs from the UV to 460 nm, which is consistent with the bandgap energy of g-C3N4 (2.7 eV). The Fe2O3 exhibits a broad absorption in 350e750 nm region with the onset wavelength at around 620 nm. The visible light response of g-C3N4/Fe2O3 is changed by incorporating g-C3N4 with Fe2O3, which causes possible charge transfer between gC3N4 and Fe2O3. Photoluminescence (PL) is widely used for investigation of migration, transfer and recombination of the photogenerated electronehole pairs. The proposed mechanism for the enhanced visible light photoelectrochemical activity of the
Fig. 5 e UVevis absorption spectra of Fe2O3, g-C3N4/Fe2O3 and g-C3N4.
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Fig. 6 e Fluorescence spectra of g-C3N4, Fe2O3 and g-C3N4/ Fe2O3.
Fe2O3 covered by g-C3N4 was further confirmed by PL spectroscopy. Fig. 6 shows the PL spectra of the g-C3N4, Fe2O3, and g-C3N4/Fe2O3 samples excited by UV light at 350 nm. The gC3N4 exhibits strong photoluminescence that is centered at about 460 nm, which can be attributed to the bandeband PL signal with the energy of light equal to the bandgap energy of g-C3N4. The bandeband PL signal is related with excitonic PL, which results from the n / p* electronic transition involving lone pairs of N atom in g-C3N4 [16,18]. The Fe2O3 exhibits weak fluorescence intensity under excitation with wavelength of 365 nm. The PL intensity of the g-C3N4/Fe2O3 is significantly quenched compared to that of the g-C3N4, which presents an effective inhibition of recombination of photogenerated electrons and holes. The efficient transfer of electrons indicates that coupling g-C3N4 with Fe2O3 is very effective in separation of electronehole pairs, which leads to the enhanced photoactivity of the photocatalyst.
Fig. 7 e EIS Nyquist plots of Fe2O3 and g-C3N4/Fe2O3 electrodes in 0.10 M KCl solution containing equimolar [Fe(CN)6]3L/4L (0.010/0.010 M) in the dark and under illumination.
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Electrochemical and photoelectrochemical studies The interface charge separation efficiency was investigated by impedance spectroscopy (EIS) since the separation efficiency of electrons and holes is crucial for the photocatalytic activity. Fig. 7 shows the EIS Nyquist plots of Fe2O3 and g-C3N4/Fe2O3 electrodes with or without visible light irradiation. The impedance can be considered to be predominantly determined by the charge transfer resistance and diffusioncontrolled mass transfer. The radius of the arc on the EIS spectra reflects the reaction rate occurring at the surface of the electrode [19e21]. Under illumination, the impedance decreases considerably due to the generation of abundant photogenerated charge carriers. The arc radius of the g-C3N4/ Fe2O3 is smaller than that of Fe2O3 either under illumination or in the dark, suggesting that the introduction of g-C3N4 was beneficial to the separation of electrons and holes, thus resulting in a faster interfacial charge transfer which is favorable for enhancing photoelectrical activity. MotteSchottky analysis is based on the assumption that the capacitance of the space charge layer is much smaller than that of the Helmholtz layer. MotteSchottky plots are conducted and illustrated in Fig. 8. According to the figure, the flat band potential of Fe2O3 and g-C3N4/Fe2O3 electrodes in the dark are 0.2 and 0.5 V, respectively. Under irradiation, the
Fig. 8 e MotteSchottky plots of (A) Fe2O3 and (B) g-C3N4/ Fe2O3 in 0.10 M NaOH with the frequency of 1 kHz.
flat band potential of the g-C3N4/Fe2O3 negatively shifts to 0.6 V. The negative shift of the flat band potential presents that holes on the surface are depleted [22,23]. The higher photoactivity of the g-C3N4/Fe2O3 can be attributed to the excellent photocatalytic property of porous structure and efficient charge transport property of the vertically oriented channels of the Fe2O3. The majority carrier donor concentrations (Nd) calculated from the MotteSchottky analysis are 3.5 1020, 3.8 1020 cm3 for the Fe2O3 and 9.0 1021, 1.2 1022 cm3 for the g-C3N4/Fe2O3 in the dark and under irradiation, respectively. Fig. 9A shows the IeV characteristics of the Fe2O3 and gC3N4/Fe2O3 in 1.0 M NaOH electrolyte. The IeV response for both electrodes is similar and shows no evidence of enhanced photoelectrochemical response in the dark. Even though Fe2O3 can absorb visible light, the poor electron mobility results in a negligible photocurrent. On the contrary, the g-C3N4/ Fe2O3 electrode shows a significantly enhanced photocurrent density of ca. 0.8 mA cm2 at þ0.23 V. A remarkable shift in the onset potential from 0.2 V for Fe2O3 to 0.3 V for g-C3N4/ Fe2O3 is observed, which indicates the effective hole scavenging that liberates more electrons and then results in more electronegative of the anode potential [24,25]. The flat band
Fig. 9 e (A) Photocurrentepotential curves of Fe2O3 and g-C3N4/Fe2O3 electrodes in the dark and under visible light irradiation in 1.0 M NaOH; (B) Iet curves of Fe2O3 and g-C3N4/Fe2O3 electrodes at the applied potential of 0.23 V vs. Ag/AgCl in 1.0 M NaOH solution.
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potential (EFB) is assumed to be equivalent to the onset potential of anodic photocurrent [26e29]. The flat band potential is more negative than the photocurrent onset potential in our study, which suggests a high recombination rate and kinetic hindrance on the surface of hematite that benefits from a surface catalyst [30]. The transient photocurrent response of the Fe2O3 and gC3N4/Fe2O3 electrodes was measured in 1.0 M NaOH electrolyte and the results are shown in Fig. 9B. The photocurrent is widely regarded as the most reliable evidence to demonstrate the charge separation of a photocatalyst. The electrodes are prompt in generating reproducible response to oneoff cycles. The value of photocurrent represents the charge collection efficiency. The photocurrent spike upon light irradiation in the photoelectrochemical measurement can be attributed to the charge recombination on the surface of photocatalysts [31]. The pure Fe2O3 shows very low photocurrent density of 0.010 mA cm2, which can be attributed to the low electron mobility and fast recombination of photogenerated electrons and holes by pyridine-assisted preparation method. However, the photocurrent of the g-C3N4/Fe2O3 was enhanced by 70 times (saturated at 0.78 mA cm2) under visible light irradiation compared to pure Fe2O3, which represents higher separation efficiency of e and hþ at the interface between two semiconductors. This is beneficial to the photocatalytic reaction. The significant enhancement of photocurrent is originated from the change of morphology and the improvement of photoinduced charge separation. To quantitatively investigate the photoactivity of the photocatalysts as a function of wavelength, the incident photon to electron conversion efficiency (IPCE) measurements were conducted on the Fe2O3 and g-C3N4/Fe2O3 electrodes at different potentials. The IPCE of the samples was calculated as: IPCE ¼ 1240I/lJlight, where I is the measured photocurrent density, l is the wavelength of the incident light, and Jlight is the measured irradiance at the measurement wavelength. Fig. 10 shows the IPCE curves of the Fe2O3 and g-C3N4/Fe2O3 electrodes at different applied potentials (0 V, 0.2 V, and 0.4 V
Fig. 10 e IPCE for the Fe2O3 and g-C3N4/Fe2O3 electrodes at different applied potential (0 V, 0.2 V, 0.4 V) in 1.0 M NaOH, collected at the incident wavelength range from 300 to 800 nm. Inset is the magnified IPCE profiles of the Fe2O3 electrode.
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vs. Ag/AgCl) in 1.0 M NaOH solution. The g-C3N4/Fe2O3 electrode shows a 36-fold improvement over the Fe2O3 electrode at 420 nm at an applied potential of 0 V, which can be ascribed to the change in sample morphology and the formation of heterojunction between Fe2O3 and g-C3N4. The maximum IPCE that was achieved on the optimized g-C3N4/Fe2O3 is 1.5% at 420 nm and 0.4 V, which is nearly 70 times higher than that of the pristine Fe2O3.
Mechanism of enhancement photoelectrochemical activity The high photoelectrochemical performance can be ascribed to electric field assisted charge transfer at the heterojunction interfaces. The driving force of charge transfer owes to the matched energy levels between g-C3N4 and Fe2O3. The appropriate band structure alignment of the semiconductors could create space charge accumulation/depletion at the interfaces that promotes the separation of photoinduced electrons and holes. The possible mechanism of separation and transportation of electronehole pairs at the g-C3N4/Fe2O3 photocatalyst interface under visible light is proposed and illustrated in Scheme 1. The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) potentials of g-C3N4 are 1.27 and 1.57 eV, respectively, as was previously reported [32,33]. Both g-C3N4 and Fe2O3 can absorb visible light to produce charge carriers. The charge transfer between g-C3N4 and Fe2O3 heterojunction causes an accumulation of electrons on the CB of Fe2O3 via the well-developed interface. Therefore, the Fe2O3 acts as an electron trap to facilitate the separation of photogenerated electronehole pairs, and it also promotes the interfacial electron transfer process. The separation of electronehole
Scheme 1 e Schematic for electronehole separation and transportation at g-C3N4/Fe2O3 photocatalyst interface. Ec is the contact electric field, Eb is the potential barrier in the interfacial depletion layer, E1 and E2 are the internal electric fields induced by the redistribution of the spatial charges in Fe2O3 and g-C3N4, respectively.
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pairs is also driven by the internally rebuilt field which reduces the recombination of electronehole pairs and leads to the accumulation of electrons and holes on the surface of Fe2O3 and g-C3N4, respectively. Thus, the heterojunction makes the charge separation more efficient and meanwhile reduces the recombination of photogenerated electrons and holes. Furthermore, the high separation efficiency of the g-C3N4/ Fe2O3 also results from the morphologically controlled structure. The porous cross-linked nanostructure favors the transport of electrons and holes across Fe2O3 nanostructure driven by strong dipolar field arising from charged surface domains [33e35]. The charged surface domains of Fe2O3 would drive the photoexcited electrons or holes from the inner to the outer layers.
Conclusion Novel g-C3N4/Fe2O3 composite was prepared via electrodeposition and chemical vapor deposition process. The morphology of the g-C3N4/Fe2O3 can be transferred from regular nanosheets to porous cross-linked nanostructures. The high photoelectrochemical activity of the g-C3N4/Fe2O3 indicates the efficient separation of the photogenerated charge carriers and the enhancement of water splitting under visible light. The synergy effect of the heterojunction was explained based on PL spectra and photoelectrochemical measurements. The g-C3N4/Fe2O3/FTO exhibits excellent photoelectrochemical performance and is very promising in applications in fields such as solar cells, photocatalysis, and hydrogen generation. g-C3N4 is regarded as a very prospective candidate for development of highly active photocatalyst. Our study highlights that the design of heterojunctions with controllable morphology by hybridization of g-C3N4 with other semiconductors could be a new approach to prepare novel photocatalysts with high performance.
Acknowledgments The authors thank the National Natural Science Foundation of China (No. 21273080 and 21003051) for financial support.
Appendix A. Supplementary data Supplementary data related to this article can be found at doi: 10.1016/j.ijhydene.2014.03.248.
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[22] Mohapatra SK, John SE, Banerjee S, Misra M. Water photooxidation by smooth and ultrathin a-Fe2O3 nanotube arrays. Chem Mater 2009;21:3048e55. [23] Rangaraju RR, Panday A, Raja KS, Misra M. Nanostructured anodic iron oxide film as photoanode for water oxidation. J Phys D Appl Phys 2009;42:135303. [24] Mao A, Shin K, Kim JK, Wang DH, Han GY, Park JH. Controlled synthesis of vertically aligned hematite on conducting substrate for photoelectrochemical cells: nanorods versus nanotubes. ACS Appl Mater Interfaces 2011;3:1852e8. [25] Zheng JY, Kang MJ, Song G, Son SI, Suh SP, Kim CW, et al. Morphology evolution of dendritic Fe wire array by electrodeposition, and photoelectrochemical properties of a-Fe2O3 dendritic wire array. Cryst Eng Comm 2012;14:6957e61. [26] Jaramillo TF, Baeck SH, Kleiman-Shwarsctein A, Choi KS, Stucky GD, McFarland EW. Automated electrochemical synthesis and photoelectrochemical characterization of Zn1xCoxO thin films for solar hydrogen production. J Comb Chem 2005;7:264e71. [27] John SE, Mohapatra SK, Misra M. Double-wall anodic titania nanotube arrays for water photooxidation. Langmuir 2009;25:8240e7. [28] Yoon KH, Shin CW, Kang DH. Photoelectrochemical conversion in a WO3 coated p-Si photoelectrode: effect of annealing temperature. J Appl Phys 1997;81:7024e9.
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Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4 Ying Liu, Yu-Xiang Yu, Wei-De Zhang* School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China
article info
abstract
Article history:
Novel photocatalysts, which consist of two visible light responsive semiconductors
Received 7 November 2013
including graphite-like carbon nitride (g-C3N4) and Fe2O3, were successfully synthesized via
Received in revised form
electrodeposition followed by chemical vapor deposition. The morphology of the g-C3N4/
29 March 2014
Fe2O3 can be tuned from regular nanosheets to porous cross-linked nanostructures.
Accepted 31 March 2014
Remarkably, the optimum activity of the g-C3N4/Fe2O3 is almost 70 times higher than that
Available online 27 April 2014
of individual Fe2O3 for photoelectrochemical water splitting. The enhancement of photoelectrochemical activity could be assigned to the morphology change of the photocatalysts
Keywords:
and the effective separation and transfer of photogenerated electrons and holes originated
Graphite-like carbon nitride
from the intimately contacted interfaces. The g-C3N4/Fe2O3 composites could be developed
Fe2O3
as high performance photocatalysts for water splitting and other optoelectric devices.
Nanocomposites
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Charge transfer Photoelectrochemical property
Introduction Following light-induced water-splitting experiment with a TiO2 photoanode by Fujishima and his colleagues in 1972 [1], worldwide studies have been focused on the conversion of solar energy to hydrogen energy. In such a process, the key is to develop stable and low-cost photocatalysts which are able to absorb the majority of solar photons. Compared to other semiconductors, Fe2O3 has many potential advantages for photoelectrochemical splitting of water to produce hydrogen. However, the pure hematite is a low conductive material with short diffusion length of holes around the electrolyte interface, which limits its application as a
photoanode. The traditional approach to overcome these drawbacks is either to dope heteroatom or to control the morphology of Fe2O3 photocatalyst. Efforts to generate nanostructured hematite through morphological control, including the formation of nanoplatelets [2], nanotubes [3], and nanospheres [4], resulted in improved performance of Fe2O3. These structural strategies either improve the light collection or minimize the distance that holes travel to the solideliquid interface. Recently, polymeric graphite-like carbon nitride (g-C3N4) has been introduced as a metal-free photocatalyst for solardriven application. g-C3N4 has small direct bandgap due to the sp2 hybridization of carbon and nitrogen forming the pconjugated graphitic planes [5,6]. Thus, g-C3N4 (ca. 2.7 eV)
* Corresponding author. Tel./fax: þ86 20 8711 4099. E-mail addresses: [email protected], [email protected] (W.-D. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2014.03.248 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Fig. 1 e SEM images of (A) Fe2O3, (B) cross-section of Fe2O3, (C) and (D) g-C3N4/Fe2O3.
has sufficient absorption and shows photocatalytic activity for water to be reduced to H2 or oxidized to O2 under visible light irradiation. Wang et al. [7] firstly reported that g-C3N4 was functionally achieved as a visible light driven metal-free photocatalyst for H2 production. However, the photocatalytic efficiency of g-C3N4 is limited by the high recombination rate of photogenerated electrons and holes. In order to enhance the photoactivity of g-C3N4, such efforts have been made, including preparing porous structure [8], doping or coupling of g-C3N4 with metals [9,10], semiconductive oxides [11], graphene [12e14], and being activated by organic dyes [15]. In particular, constructing heterojunctions by combining g-C3N4 with other semiconductors provides a feasible method to enhance photocatalytic performance. Our group has previously reported gC3N4-modified TiO2 nanorod arrays with high visible light photoelectrochemical activity [16]. It is expected that the combination of g-C3N4 with Fe2O3 would be ideal in inhibiting the recombination of photogenerated electronehole pairs and enhancing the photoelectrical activity compared to individual g-C3N4 or Fe2O3. Recently, Qiu and his colleagues firstly reported the fabrication of magnetic Fe2O3/g-C3N4 photocatalyst and its enhanced photodegradation rate toward organic dye under visible light [17]. However, in most previous studies, g-C3N4 was used as a supporting material and the composites were constructed by decorating the nanoparticles onto the surface of g-C3N4. Herein, we present the novel morphology-controllable g-C3N4-coated Fe2O3 nanostructure photocatalysts. The
g-C3N4/Fe2O3 photocatalysts were prepared by electrodeposition followed by chemical vapor deposition. The morphology of the composites changed with the different filling degrees of the precursor during the annealing process. The morphology of the g-C3N4/Fe2O3 was gradually tuned from regular nanosheets to porous cross-linked nanostructures. Meanwhile, the photoelectrochemical activity of the optimized g-C3N4/Fe2O3 composite was 70 times higher than that of pure Fe2O3. The effect of morphology, the synergy effect between Fe2O3 and gC3N4, and the possible mechanism of the photoelectrochemical activity enhancement via hybridization were systematically investigated.
Experimental section Chemicals Conductive fluorine-doped tin oxide (FTO)-coated glass (25 U cm2) was purchased from Nippon Sheet, Japan. Deionized water (Resistivity > 18.4 MU cm) was produced using a pure water system (GWA-UN, Beijing, China). Melamine (C3H6N6) was obtained from Kermel. (NH4)2Fe(SO4)2 was purchased from Guangzhou Chemical Reagent Factory. All chemicals were used without further purification.
Synthesis of g-C3N4/Fe2O3 The Fe2O3 nanosheet arrays were prepared by electrodeposition. Briefly, the films of Fe(OH)3 were obtained in aqueous
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Fig. 2 e (A) TEM and (B) HRTEM images of g-C3N4/Fe2O3. (C) Elemental mapping of a single g-C3N4/Fe2O3 nanorod.
solution containing 0.010 M (NH4)2Fe(SO4)2 and 0.040 M pyridine. All electrodeposition processes were carried out in a configured glass cell at 70 C at a potential of 0.644 V vs. Ag/ AgCl. The deposition process lasted for 1 h. The Fe2O3/FTO was obtained by heating the as-prepared Fe(OH)3/FTO under the protection of N2 at 550 C for 2 h. The preparation of g-C3N4/Fe2O3 is as follows: 3 g of melamine (C3H6N6) powder was put in a ceramic crucible and the as-prepared Fe(OH)3 arrays on FTO were topedown placed above the melamine. The crucible was heated to 390 C in a muffle furnace for 1 h with a heating rate of 5 C min1; then heated at 550 C for another 2 h. After the chemical vapor deposition process, g-C3N4 was successfully deposited onto the Fe2O3. The reaction can be tuned simply by increasing or decreasing the filling degree of melamine in the ceramic crucible.
Characterization The crystal phase of the products was characterized by X-ray diffraction (XRD) on an X’Pert MPD Pro X-ray diffractometer, using Cu Ka radiation (l ¼ 0.15418 nm). Field emission scanning electron microscopy (FESEM) images were recorded on LEO 1530Vp. Transmission electron microscopy (TEM) images
were taken on FEI Tecnai G20 which was operated at 200 kV. X-ray photoelectron spectroscopy with Al Ka X-rays radiation (Kratos, Axis Ultra) was used to investigate the surface properties. All the binding energy values were calibrated by using the C 1s level at 284.8 eV as an internal standard. Fourier transform infrared spectra (FT-IR) were measured using Spectrum One infrared spectrometer with KBr as the reference. UVevis diffuse reflectance spectroscopy was recorded with a Hitachi U-3010 spectrophotometer. The photoluminescence spectra were measured with a fluorescence spectrophotometer (Hitachi F-4500) using a Xe lamp as an excitation source with optical filters. Impedance-potential measurements were carried out on Chenhua electrochemical workstation (CHI660c, Shanghai, China) with a frequency of 1 kHz in 0.50 M KCl containing equimolar [Fe(CN)6]3/4 (0.010/0.010 M) solution. Photoelectrochemical measurement was performed with an electrochemical workstation (Ingsens-1030, Guangzhou, China) in 1.0 M NaOH solution (pH ¼ 13.6) with a Pt plate as a counter electrode and Ag/AgCl as a reference electrode. Substrates were immersed in solution and illuminated through a quartz window on the Fe2O3 side as a photoanode. The light source was a 300 W Xe lamp (PLS-SXE 300/300UV) equipped with an ultraviolet cutoff filter. The integrated visible light intensity was about
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Fig. 3 e XRD patterns of Fe(OH)3, Fe2O3, g-C3N4/Fe2O3 (* and # indicate as FTO substrate and Fe2O3, respectively). Inset: (a)e(d) correspond to the optical color of samples in Fig. S1 (A) and Fig. 1(A), (C) and (D), respectively.
150 mW cm2. A 500 W Xe lamp (CHF-XM-500, Changtuo Technology Co., Ltd.) with a monochromator (monochromator 300, Changtuo Technology Co., Ltd.) was employed to study wavelength-dependent photocurrent. The light intensity was calibrated with a radiometer (PM120VA, Thorlabs).
Results and discussion Characterization of the g-C3N4/Fe2O3 samples The morphology and microstructure of the samples were revealed by SEM and TEM. As shown in Fig. 1A, dense nanoplatelets are perpendicularly grown on the FTO substrate. The nanoplatelets are smooth, with a thickness of ca. 30 nm and width of 600 nm. The thickness of the films is about 1 mm which can be seen from the cross-section in Fig. 1B. After the chemical vapor deposition process, g-C3N4 is successfully coated onto the surface of Fe2O3 nanoplatelets. The morphology of the Fe2O3 nanoplatelets is gradually transformed to porous cross-linked nanostructures from the original nanoplatelets (Fig. 1C and D). The morphology change can be attributed to the influence of ammonia gas generated during the process in which melamine polycondenses to carbon nitride (C3H6N6 / C3N4 þ 2NH3). The generated ammonia could react with the Fe2O3 to form Fe at high temperature in an enclosed space (Eq. (1)). Meanwhile, the NH3 can be oxidized by oxygen with the catalysis of Fe2O3 to form nitrogen oxides, which further oxidize Fe to Fe2O3 (Eqs. (4) and (5)). The control experiment was carried out by annealing Fe(OH)3 arrays under NH3 atmosphere. The morphology of the Fe2O3 gradually changes to nanoparticles (Fig. S1 B in Supplementary Materials). Thus, the initiator melamine plays dual roles: a source to form g-C3N4 and a generator of NH3 which changes the morphology of the Fe2O3.
Fig. 4 e XPS spectra of (A) C 1s, and (B) N 1s.
2NH3 þ Fe2 O3 /N2 þ 2Fe þ 3H2 O Fe2 O3
(1)
4NH3 þ 5O2 / 4NO þ 6H2 O
(2)
2NO þ O2 /2NO2
(3)
2NO þ 2Fe/N2 þ 2FeO
(4)
NO2 þ 2FeO/NO þ Fe2 O3
(5)
The morphology of the g-C3N4/Fe2O3 was further explored by TEM and HRTEM, which are shown in Fig. 2A and B. The lattice structure of Fe2O3 is orderly and the outer boundary of the sample is distinctly different from the Fe2O3 (Fig. 2B). The fringe with d ¼ 0.270 nm matches with the (104) crystallographic plane of Fe2O3 and the thickness of the gC3N4 layer coated on the Fe2O3 is about 2.5 nm. The HRTEM analysis suggests that the g-C3N4 was uniformly dispersed on the surface of Fe2O3 and the heterojunction indeed formed between g-C3N4 and Fe2O3. The elemental mapping of a single g-C3N4/Fe2O3 nanostructure (Fig. 2C) shows that the Fe2O3 porous nanostructure is completely covered by gC 3N 4.
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XRD is used to investigate the phase structure of the samples, and the typical diffraction patterns are shown in Fig. 3. The Fe(OH)3 precursor is completely changed to Fe2O3 after annealing. The peaks in the XRD pattern of Fe2O3 can be indexed to the rhombohedral phase (JCPDS 33-0664). No significant peaks of g-C3N4 can be detected in the g-C3N4/Fe2O3 sample, which can be explained by the small amount and high dispersion of g-C3N4. Meanwhile, the color of the samples changes from brown to light tan with g-C3N4 loading (inset of Fig. 3), which implies that the g-C3N4 is successfully introduced onto the Fe2O3. The formation of g-C3N4 was further confirmed by XPS spectrum. XPS was carried out to determine the chemical composition of the g-C3N4/Fe2O3 and the valence states of the elements. The binding energy was calibrated by C 1s at 284.8 eV. Fig. 4A presents the XPS spectrum of C 1s which can be divided into three peaks with binding energy of 284.8, 286.5 and 288.4 eV, corresponding to the CeC (additional carbon), C]N, and CeN. Fig. 4B is the magnified spectra of N 1s with the binding energy at 399.1, 400.1 and 402.0 eV, respectively. The peak at binding energy of 399.1 eV is attributed to the sp2-hybridized nitrogen (C]NeC) and the two peaks of 400.1 and 402.0 eV can be assigned to tertiary nitrogen (N(C)3) and amino functional group with a hydrogen atom (CeNeH).
Optical property of the g-C3N4/Fe2O3 samples The optical properties of the Fe2O3, g-C3N4, and g-C3N4/Fe2O3 were investigated by UVevis DRS spectroscopy, as illustrated in Fig. 5. The g-C3N4 absorbs from the UV to 460 nm, which is consistent with the bandgap energy of g-C3N4 (2.7 eV). The Fe2O3 exhibits a broad absorption in 350e750 nm region with the onset wavelength at around 620 nm. The visible light response of g-C3N4/Fe2O3 is changed by incorporating g-C3N4 with Fe2O3, which causes possible charge transfer between gC3N4 and Fe2O3. Photoluminescence (PL) is widely used for investigation of migration, transfer and recombination of the photogenerated electronehole pairs. The proposed mechanism for the enhanced visible light photoelectrochemical activity of the
Fig. 5 e UVevis absorption spectra of Fe2O3, g-C3N4/Fe2O3 and g-C3N4.
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Fig. 6 e Fluorescence spectra of g-C3N4, Fe2O3 and g-C3N4/ Fe2O3.
Fe2O3 covered by g-C3N4 was further confirmed by PL spectroscopy. Fig. 6 shows the PL spectra of the g-C3N4, Fe2O3, and g-C3N4/Fe2O3 samples excited by UV light at 350 nm. The gC3N4 exhibits strong photoluminescence that is centered at about 460 nm, which can be attributed to the bandeband PL signal with the energy of light equal to the bandgap energy of g-C3N4. The bandeband PL signal is related with excitonic PL, which results from the n / p* electronic transition involving lone pairs of N atom in g-C3N4 [16,18]. The Fe2O3 exhibits weak fluorescence intensity under excitation with wavelength of 365 nm. The PL intensity of the g-C3N4/Fe2O3 is significantly quenched compared to that of the g-C3N4, which presents an effective inhibition of recombination of photogenerated electrons and holes. The efficient transfer of electrons indicates that coupling g-C3N4 with Fe2O3 is very effective in separation of electronehole pairs, which leads to the enhanced photoactivity of the photocatalyst.
Fig. 7 e EIS Nyquist plots of Fe2O3 and g-C3N4/Fe2O3 electrodes in 0.10 M KCl solution containing equimolar [Fe(CN)6]3L/4L (0.010/0.010 M) in the dark and under illumination.
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Electrochemical and photoelectrochemical studies The interface charge separation efficiency was investigated by impedance spectroscopy (EIS) since the separation efficiency of electrons and holes is crucial for the photocatalytic activity. Fig. 7 shows the EIS Nyquist plots of Fe2O3 and g-C3N4/Fe2O3 electrodes with or without visible light irradiation. The impedance can be considered to be predominantly determined by the charge transfer resistance and diffusioncontrolled mass transfer. The radius of the arc on the EIS spectra reflects the reaction rate occurring at the surface of the electrode [19e21]. Under illumination, the impedance decreases considerably due to the generation of abundant photogenerated charge carriers. The arc radius of the g-C3N4/ Fe2O3 is smaller than that of Fe2O3 either under illumination or in the dark, suggesting that the introduction of g-C3N4 was beneficial to the separation of electrons and holes, thus resulting in a faster interfacial charge transfer which is favorable for enhancing photoelectrical activity. MotteSchottky analysis is based on the assumption that the capacitance of the space charge layer is much smaller than that of the Helmholtz layer. MotteSchottky plots are conducted and illustrated in Fig. 8. According to the figure, the flat band potential of Fe2O3 and g-C3N4/Fe2O3 electrodes in the dark are 0.2 and 0.5 V, respectively. Under irradiation, the
Fig. 8 e MotteSchottky plots of (A) Fe2O3 and (B) g-C3N4/ Fe2O3 in 0.10 M NaOH with the frequency of 1 kHz.
flat band potential of the g-C3N4/Fe2O3 negatively shifts to 0.6 V. The negative shift of the flat band potential presents that holes on the surface are depleted [22,23]. The higher photoactivity of the g-C3N4/Fe2O3 can be attributed to the excellent photocatalytic property of porous structure and efficient charge transport property of the vertically oriented channels of the Fe2O3. The majority carrier donor concentrations (Nd) calculated from the MotteSchottky analysis are 3.5 1020, 3.8 1020 cm3 for the Fe2O3 and 9.0 1021, 1.2 1022 cm3 for the g-C3N4/Fe2O3 in the dark and under irradiation, respectively. Fig. 9A shows the IeV characteristics of the Fe2O3 and gC3N4/Fe2O3 in 1.0 M NaOH electrolyte. The IeV response for both electrodes is similar and shows no evidence of enhanced photoelectrochemical response in the dark. Even though Fe2O3 can absorb visible light, the poor electron mobility results in a negligible photocurrent. On the contrary, the g-C3N4/ Fe2O3 electrode shows a significantly enhanced photocurrent density of ca. 0.8 mA cm2 at þ0.23 V. A remarkable shift in the onset potential from 0.2 V for Fe2O3 to 0.3 V for g-C3N4/ Fe2O3 is observed, which indicates the effective hole scavenging that liberates more electrons and then results in more electronegative of the anode potential [24,25]. The flat band
Fig. 9 e (A) Photocurrentepotential curves of Fe2O3 and g-C3N4/Fe2O3 electrodes in the dark and under visible light irradiation in 1.0 M NaOH; (B) Iet curves of Fe2O3 and g-C3N4/Fe2O3 electrodes at the applied potential of 0.23 V vs. Ag/AgCl in 1.0 M NaOH solution.
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potential (EFB) is assumed to be equivalent to the onset potential of anodic photocurrent [26e29]. The flat band potential is more negative than the photocurrent onset potential in our study, which suggests a high recombination rate and kinetic hindrance on the surface of hematite that benefits from a surface catalyst [30]. The transient photocurrent response of the Fe2O3 and gC3N4/Fe2O3 electrodes was measured in 1.0 M NaOH electrolyte and the results are shown in Fig. 9B. The photocurrent is widely regarded as the most reliable evidence to demonstrate the charge separation of a photocatalyst. The electrodes are prompt in generating reproducible response to oneoff cycles. The value of photocurrent represents the charge collection efficiency. The photocurrent spike upon light irradiation in the photoelectrochemical measurement can be attributed to the charge recombination on the surface of photocatalysts [31]. The pure Fe2O3 shows very low photocurrent density of 0.010 mA cm2, which can be attributed to the low electron mobility and fast recombination of photogenerated electrons and holes by pyridine-assisted preparation method. However, the photocurrent of the g-C3N4/Fe2O3 was enhanced by 70 times (saturated at 0.78 mA cm2) under visible light irradiation compared to pure Fe2O3, which represents higher separation efficiency of e and hþ at the interface between two semiconductors. This is beneficial to the photocatalytic reaction. The significant enhancement of photocurrent is originated from the change of morphology and the improvement of photoinduced charge separation. To quantitatively investigate the photoactivity of the photocatalysts as a function of wavelength, the incident photon to electron conversion efficiency (IPCE) measurements were conducted on the Fe2O3 and g-C3N4/Fe2O3 electrodes at different potentials. The IPCE of the samples was calculated as: IPCE ¼ 1240I/lJlight, where I is the measured photocurrent density, l is the wavelength of the incident light, and Jlight is the measured irradiance at the measurement wavelength. Fig. 10 shows the IPCE curves of the Fe2O3 and g-C3N4/Fe2O3 electrodes at different applied potentials (0 V, 0.2 V, and 0.4 V
Fig. 10 e IPCE for the Fe2O3 and g-C3N4/Fe2O3 electrodes at different applied potential (0 V, 0.2 V, 0.4 V) in 1.0 M NaOH, collected at the incident wavelength range from 300 to 800 nm. Inset is the magnified IPCE profiles of the Fe2O3 electrode.
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vs. Ag/AgCl) in 1.0 M NaOH solution. The g-C3N4/Fe2O3 electrode shows a 36-fold improvement over the Fe2O3 electrode at 420 nm at an applied potential of 0 V, which can be ascribed to the change in sample morphology and the formation of heterojunction between Fe2O3 and g-C3N4. The maximum IPCE that was achieved on the optimized g-C3N4/Fe2O3 is 1.5% at 420 nm and 0.4 V, which is nearly 70 times higher than that of the pristine Fe2O3.
Mechanism of enhancement photoelectrochemical activity The high photoelectrochemical performance can be ascribed to electric field assisted charge transfer at the heterojunction interfaces. The driving force of charge transfer owes to the matched energy levels between g-C3N4 and Fe2O3. The appropriate band structure alignment of the semiconductors could create space charge accumulation/depletion at the interfaces that promotes the separation of photoinduced electrons and holes. The possible mechanism of separation and transportation of electronehole pairs at the g-C3N4/Fe2O3 photocatalyst interface under visible light is proposed and illustrated in Scheme 1. The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) potentials of g-C3N4 are 1.27 and 1.57 eV, respectively, as was previously reported [32,33]. Both g-C3N4 and Fe2O3 can absorb visible light to produce charge carriers. The charge transfer between g-C3N4 and Fe2O3 heterojunction causes an accumulation of electrons on the CB of Fe2O3 via the well-developed interface. Therefore, the Fe2O3 acts as an electron trap to facilitate the separation of photogenerated electronehole pairs, and it also promotes the interfacial electron transfer process. The separation of electronehole
Scheme 1 e Schematic for electronehole separation and transportation at g-C3N4/Fe2O3 photocatalyst interface. Ec is the contact electric field, Eb is the potential barrier in the interfacial depletion layer, E1 and E2 are the internal electric fields induced by the redistribution of the spatial charges in Fe2O3 and g-C3N4, respectively.
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pairs is also driven by the internally rebuilt field which reduces the recombination of electronehole pairs and leads to the accumulation of electrons and holes on the surface of Fe2O3 and g-C3N4, respectively. Thus, the heterojunction makes the charge separation more efficient and meanwhile reduces the recombination of photogenerated electrons and holes. Furthermore, the high separation efficiency of the g-C3N4/ Fe2O3 also results from the morphologically controlled structure. The porous cross-linked nanostructure favors the transport of electrons and holes across Fe2O3 nanostructure driven by strong dipolar field arising from charged surface domains [33e35]. The charged surface domains of Fe2O3 would drive the photoexcited electrons or holes from the inner to the outer layers.
Conclusion Novel g-C3N4/Fe2O3 composite was prepared via electrodeposition and chemical vapor deposition process. The morphology of the g-C3N4/Fe2O3 can be transferred from regular nanosheets to porous cross-linked nanostructures. The high photoelectrochemical activity of the g-C3N4/Fe2O3 indicates the efficient separation of the photogenerated charge carriers and the enhancement of water splitting under visible light. The synergy effect of the heterojunction was explained based on PL spectra and photoelectrochemical measurements. The g-C3N4/Fe2O3/FTO exhibits excellent photoelectrochemical performance and is very promising in applications in fields such as solar cells, photocatalysis, and hydrogen generation. g-C3N4 is regarded as a very prospective candidate for development of highly active photocatalyst. Our study highlights that the design of heterojunctions with controllable morphology by hybridization of g-C3N4 with other semiconductors could be a new approach to prepare novel photocatalysts with high performance.
Acknowledgments The authors thank the National Natural Science Foundation of China (No. 21273080 and 21003051) for financial support.
Appendix A. Supplementary data Supplementary data related to this article can be found at doi: 10.1016/j.ijhydene.2014.03.248.
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