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Freestanding atomically-thin cuprous oxide sheets for improved visible-light photoelectrochemical water splitting
Nano Energy (2014) 8, 205–213
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journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Freestanding atomically-thin cuprous oxide sheets for improved visible-light photoelectrochemical water splitting Shan Gaoa,1, Yongfu Suna,1, Fengcai Leia, Jiawei Liua, Liang Lianga, Tanwei Lia, Bicai Pana, Jingfang Zhoub, Yi Xiea,n a
Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026, PR China b Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received 20 March 2014; received in revised form 27 May 2014; accepted 28 May 2014 Available online 11 June 2014
KEYWORDS
Abstract
Cuprous oxide; Non-layered; Atomically-thin; Visible-light; Photoelectrochemical water splitting
Atomically-thin sheet is a new class of two-dimensional materials that bring a wide range of extraordinary properties. However, the lack of intrinsic growth anisotropy makes the synthesis of atomically-thin non-layered materials a great challenge. Here, a versatile rapid-heating strategy is proposed and applied, in which the lamellar hybrid intermediate transforms into the clean and freestanding atomically-thin non-layered Cu2O sheet with 4 atomic thicknesses at 300 1C in less than 8 min. Taking advantage of efficient visible-light harvesting, improved carrier density, and fast interfacial charge transfer as well as facile electrochemical reactions, the ultrathin Cu2O sheets-based photoelectrode yields a photocurrent density up to 3.98 mA cm 2, 36 times higher than that of bulk counterpart. Also, the photoelectrode reaches a visible-lightconversion-efficiency of 26.2% that is superior to most existing reports. The rapid-heating strategy guarantees the full exfoliation of lamellar hybrid intermediate into ultrathin Cu2O sheets with the removal of organic component, opening a new way to produce atomically-thin non-layered materials and holding great promise in the applications of visible-light photoelectrochemical water splitting. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. E-mail address: [email protected] (Y. Xie). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2014.05.017 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Photoelectrochemical (PEC) water splitting using semiconductor materials could provide a promising and environmentally benign pathway to produce clean and renewable hydrogen energy [1–3]. Among the abundant semiconductors, cuprous oxide (Cu2O) has recently attracted extensive attention as a means of solar-to-hydrogen conversion [4–6]. In addition to possessing low cost and environmental friendliness, the fascination also comes from its peculiar conduction band lying 0.7 V negative of the hydrogen evolution potential [4,7], which theoretically points out its feasibility to drive the water splitting into hydrogen. In addition, the high theoretical photocurrent of 14.7 mA cm 2 and light-to-hydrogen conversion efficiency of 18.7% suggest its huge potential applications in solar hydrogen production [8]. Also, the relatively small direct band gap of 2.2 eV endows Cu2O with efficient adsorption of visible light, which accounts for 43% of the incoming solar energy [9]. Moreover, the large absorption coefficient of 3.7 106 cm 1 facilitates efficient light absorption, while the high carrier mobility of 4105 cm2 V 1 s 1 favors for fast separation of photoexcited electron–hole pairs [10,11]. Furthermore, Cu2O can effectively scavenge the photogenerated electrons through adsorbing molecular oxygen and hence constrain the recombination of electron–hole pairs, thus definitely improve the photoconversion efficiency [12]. Despite these excellent advantages of Cu2O, the relatively low photoconversion efficiency, especially in the visible light range, unfortunately hinders the promising applicability. Therefore, further breakthroughs in the design and synthesis of novel Cu2O structures with high visible light water splitting efficiency, hold the key to the development of PEC water splitting. Recently, our studies have shown that the atomicallythick two-dimensional sheets provide the ideal architecture for promoting the solar hydrogen production [2,13]. The peculiar geometric structure can afford the atomic thickness for fast carrier transport from interior to surface, the two-dimensional (2D) conducting channels for quick separation of electron–hole pairs as well as the large surface area for efficient light harvesting and facile surface reactions. Also, the 2D configuration endows them with a much better grain boundary connectivity and allows for intimate contact with the substrate and high interfacial contact area with the electrolyte, thus facilitating fast interfacial charge transfer and electrochemical reactions as well as low corrosion rates. Enlightened by the above discussions, controllable synthesis of the atomically-thick Cu2O sheets is of great importance. As is well-known, controllable exfoliation of anisotropic layered bulk materials have been
S. Gao et al. regarded as the most effective pathway to obtain twodimensional sheets with atomic thickness, owing to the strong in-plane bonds and weak van der Waals interaction between layers [14,15]. However, for the non-layered compounds such as cubic Cu2O, controlling the synthesis of their atomically-thick two-dimensional sheets is extremely challenging on account of the difficulty in bondcleavage and the lack of intrinsic driving force for anisotropic growth. Despite the recent progress in fabricating two-dimensional CdSe and PbS sheets with few atomic thickness [16,17], the hard removal of surface organic surfactant would inevitably obscure their properties. Herein, we describe our efforts to develop a high-yield and scalable fast-heating strategy for fabricating clean and freestanding atomically-thick sheets with non-layered structures, taking the cubic Cu2O as an example (Figure S1) [18]. As shown in Scheme 1, the successful fabrication of the atomically-thin Cu2O sheets takes advantage of an intermediate precursor of lamellar Cu2O–oleate complex microplates (Figures S2 and S3). Notably, the oleate ions play a fundamental role in the formation of this intermediate: the oleate ions initially interact with Cu + to form Cu–oleate complexes via an electrostatic interaction. After hydrothermal treatment at 100 1C for 24 h, the Cu–oleate complexes assemble into lamellar Cu2O–oleate complex intermediate microplates, which are fully characterized in Figures S2 and S3. And then, upon heating at 300 1C for a short time of 8 min in air, the quick release of enormous H2O and CO2 gases contributes to weaken the interaction between the adjacent inorganic layers, and hence fully exfoliate the lamellar Cu2O–oleate complex into clean and freestanding ultrathin Cu2O sheets with 4 atomic thicknesses (Figure 1 and Figures S4 and S5). Of note, this developed fast-heating strategy is simple, high-yield and can be easily scaled up for large-scale synthesis of the atomically-thin non-layered materials.
Experimental section Preparation of lamellar Cu2O–oleate complex intermediate The lamellar Cu2O–oleate complex intermediate was synthesized by a hydrothermal method. In a typical procedure, 99 mg CuCl, 914 mg sodium oleate, 831 mg hexamethylenetetramine (HMT) and 100 mL N2H4 H2O were added into 40 mL distilled water orderly. After vigorous stirring for 10 min, the mixed solution was transferred into 50 mL Teflon cup and heated in a sealed autoclave at 100 1C for 24 h. Upon cooling to room temperature naturally, the final
Scheme 1 Schematic rapid-heating strategy for fabricating clean and freestanding atomically-thin Cu2O sheets, taking advantage of the lamellar Cu2O–oleate complex intermediate.
Freestanding atomically-thin cuprous oxide sheets
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Figure 1 Characterizations for the ultrathin Cu2O sheets with 4 atomic thicknesses: (a) TEM image and the corresponding colloidal ethanol dispersion displaying the Tyndall effect; (b) AFM image, inset yellow line in b corresponds to the height profile in (c); (d) HRTEM and (e) the corresponding fast Fourier transform image; (f) the ideal crystal structure of 4-atomically-thin Cu2O slab along the [01-1] direction.
product was collected by centrifuging the mixture, washed with distilled water and absolute ethanol for several times, and then dried in vacuum at 60 1C overnight for further analysis.
indium tin oxide (ITO)-coated glass at 500 rpm for 30 s with a desktop spin coater (KW-4A). Subsequently, the ITOcoated glasses were put into a 60 1C oven for 30 min in order to evaporate the solvent and then the electrode were obtained for further measurements.
Preparation of atomically-thin Cu2O sheets In a typical procedure, 50 mg lamellar Cu2O–oleate complex intermediate was put in a quartz boat and then heated at 300 1C for 8 min in air, then the sample was allowed to cool down to room temperature naturally. The Cu2O colloidal solution was achieved by dispersed 20 mg calcined product into 10 mL absolute ethanol by ultrasonic for 40 min. The supernatant was removed from the Cu2O colloidal solution which has been put aside for two days. The final product was collected by centrifuging the supernatant at 12,000 rpm for 10 min, and then dried in vacuum overnight for further characterization.
Preparation of bulk Cu2O Bulk Cu2O was synthesized according to a modified method of Zhang et al. [19]. 2 mmol Cu(NO3)2 3H2O was dissolved in the mixed solvent of 5 mL water and 10 mL ethanol under stirring for 30 min, 1 mmol stearic acid were added into the above solution. After sonicating in water for 15 min, the mixture was put into 50 mL Teflon cup and heated in a sealed stainless steel autoclave at 180 1C for 12 h. The product was washed with water and ethanol several times, and then dried in vacuum at 60 1C for further characterization.
Preparation of Cu2O electrodes The two Cu2O electrodes were prepared by uniformly spincoated the ethanol dispersion of ultrathin Cu2O sheets and bulk counterpart (5 mL, 1.0 mg/mL) on the 1.5 cm 2 cm
Characterization The crystallographic information of the as-prepared samples were characterized by using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ=1.54178 Å). Transmission electron microscopy (TEM) images and highresolution TEM image were carried out on a JEOL-2010 TEM at an acceleration voltage of 200 kV. The field emission scanning electron microscopy (FE-SEM) images were performed by means of FEI Sirion-200 SEM. Atomic force microscopy (AFM) study in the present work was measured on a Veeco DI Nano-scope MultiMode V system. X-ray photoelectron spectra (XPS) of the samples were taken on an ESCALAB MKII with Mg Kα (hυ=1253.6 eV) as the excitation source. All binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.8 eV arising from adventitious hydrocarbon. Thermal gravimetric analysis (TGA) of the as-synthesized samples was detected by a Shimadzu TA-50 thermal analyzer at a heating rate of 5 1C min 1 from room temperature to 500 1C with a flowing nitrogen gas stream at 100 mL/min. Room-temperature UV/vis diffuse reflectance spectroscopy (DRS) and UV/vis absorption spectra were recorded on a Perkin Elmer Lambda 950 UV/vis–NIR spectrophotometer. The Fourier transform infrared (FT-IR) spectra were acquired on a NICOLET FT-IR spectrometer in a KBr tablets, scanning from 4000 to 400 cm 1 at room temperature. Photoelectrochemical measurement To investigate the photoelectrochemical performance of the atomically-thin Cu2O sheets as a photocathode for the solar
208 hydrogen generation by water splitting, conventional threeelectrode configuration was adopted. A Platinum wire and a Ag/AgCl reference electrode were used as counter electrode and reference electrode, respectively. 0.5 M Na2SO4 solution (pH = 6.6) was used as the electrolyte. The Na2SO4 electrolyte was purged with nitrogen for 30 min prior to the measurement. The photoelectrochemical performance of the Cu2O photocathodes was measured on an electrochemical station (Zahner IM6ex, Germany) at room temperature with irradiation of a 300 W Xe lamp (PLS-SXE300/ 300UV, Trusttech Co., Ltd. Beijing) with a 420 nm cutoff filter. The intensity of light source used in the measurement is 100 mW cm 2. The linear sweep voltammetry was conducted from 1.0 to 0.6 V vs. Ag/AgCl with a scan rate of 10 mV/s. Mott–Schottky plots were estimated at a frequency of 1 kHz in the dark under the applied potentials from 1.4 to 0.4 V vs. Ag/AgCl. Incident photon conversion to charge carrier conversion efficiency (IPCE) spectra were recorded on a QE/IPCE setup (Newport, USA) by monitoring the photocurrent at different incident wavelengths. And all the kit components were in control of Oriel Tracq Basic V5.0 software automatically. The light from a 300 W Xe lamp was focused through a monochromator (74125 Oriel Cornerstone 260 1/4 m) onto the cell, and the monochromator was incremented through the spectral range (300–450 nm) to generate a photocurrent action spectrum with a sampling interval of 10 nm and a current sampling time of 2 s, where the light intensity and the photocurrent generated were measured with a 2931-C dual channel power/current meter and a 71675 calibrated UV silicon photodetector. The IPCE was calculated from the following equation: IPCE ¼ hcI=λJlight where h is the Planck's constant, c is the speed of light, I is the measured photocurrent density at a specific wavelength, λ is the incident light wavelength and Jlight is the recorded irradiance intensity at a specific wavelength. The electrochemical impedance spectroscopy (EIS) was performed on an electrochemical station (Zahner IM6ex, Germany) in 0.5 M Na2SO4 electrolyte. The Nyquist plots were measured at a bias potential of 0.9 V vs. Ag/AgCl with a frequency of 100 kHz–100 mHz under irradiation of a 300 W Xe lamp. The measured spectra were fitted by using the ZSimpWin program (from Echem software). Moreover, dye absorption experiments were carried out to obtain the actual surface area of the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart, according to the previous report [20]. By studying the adsorption of the soluble dye—methylene blue on the fabricated Cu2O-based film photoelectrode, the actual surface area of the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart were determined to be 68 cm2 and 39 cm2, respectively. Although the actual surface area difference between the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart was tremendous, it is not as large as expected. This could be ascribed to the following two reasons: (1) more space in the loosely packed bulk Cu2O-based film photoelectrode helps to adsorb more dye molecules (Figure S6b), thereby leading to their relatively larger actual surface area and (2) the atomic thickness endowed the atomically-thin Cu2O sheets with remarkably more grain boundaries, which inevitably leaded to their interface overlap in the
S. Gao et al. fabricated film photoelectrode. Thus, the ultrathin Cu2O sheets-based photoelectrode could not expose 100% surface area to the electrolyte. Calculation method The first-principles density functional theory (DFT) calculations were performed using a plane wave basis set with the projector augmented plane-wave (PAW) method [21–23]. The exchange–correlation interaction is described within the generalized gradient approximation (GGA) in the form of PW91 [24,25]. The energy cutoff is set to 400 eV, and the atomic positions are allowed to relax until the energy and force are less than 10 4 eV and 10 2 eV/Å, respectively. The [01-1]-oriented cubic Cu2O quadruple layer are simulated by periodically repeating the four atomic layers along the [01-1] direction of the unit cell. Each Cu2O quadruple layer model is separated by a vacuum region of 15 Å. A 2 2 two-dimensional unit cell was used to mimic the Cu2O quadruple layer with four atomic thicknesses. The compared calculations of bulk counterpart were performed within a supercell constructed from a standard unit cell of Cu2O lattice.
Results and discussion Owing to the fact that it is currently infeasible to perform Xray diffraction (XRD) characterization on an individual ultrathin sheet, the XRD measurement is performed on the collected powder sample accumulated by the as-obtained freestanding ultrathin sheets. The corresponding XRD peaks in Figure S4a could be indexed to cubic Cu2O, corresponding to JCPDS no. 78-2076. In addition, their X-ray photoelectron spectroscopy in Figure S5a–c demonstrated the formation of pure Cu2O, further verified by the corresponding IR spectrum in Figure S3a [26,27]. This suggested the absence of oleate on the surface of the as-obtained Cu2O sample. As shown in Figure 1a and Figure S4b, the nearly transparent feature and the clear Tyndall effect indicated the ultrathin thickness of the as-obtained Cu2O sheets, further verified by their atomic force microscopy (AFM) image in Figure 1b and c. Also, compared to the bulk counterpart, the Raman bands of the ultrathin Cu2O sheets shifted towards lower wavenumbers, which could be ascribed to the phonon confinement effect and reasonably inferred their ultrathin thickness [2,28]. Moreover, the high-resolution TEM image in Figure 1d illustrated that the ultrathin Cu2O sheets possessed a [01-1] orientation, confirmed by their corresponding FFT image in Figure 1e. Their AFM image and the corresponding height profile in Figure 1b and c showed a uniform thickness of 0.62 nm, corresponding to the 0.60 nm thickness of 4 atomically-thick Cu2O slab along the [01-1] direction. Notably, the surface energy of cubic Cu2O typically follows the sequence (111)o(100)o(110) [19], indicating that the high-energy (110) facet could exhibit the highest reactivity. Considering the (110) and (01-1) facets are the equivalent facets, the (01-1) facet could also possess the large surface energy with a high activity. As such, upon the formation of Cu2O–oleate complex, the oleate ions tended to preferentially adsorb on the surface of (01-1) facet and hence impede the crystal growth along the [01-1] direction [19], thus finally resulting in the [01-1] orientation of the
Freestanding atomically-thin cuprous oxide sheets atomically-thick Cu2O sheets. Therefore, all the abovementioned results verified the formation of clean and freestanding ultrathin Cu2O sheets with four atomic thicknesses. It is noticeable that the atomically-thin nanosheets are usually accompanied by obvious structural distortion [2,13], which hence leads to the increase of density of states (DOS) at the edge of valence band or conduction band. The increased DOS near the Fermi level could lead to increased carrier mobility as well as decreased band gap. As such, for the ultrathin Cu2O sheets with four atomic thicknesses, it is reasonable that they also possess increased surface structural distortion compared with the bulk counterpart, which hence leads to increased DOS at the edge of valence band or conduction band. To verify this viewpoint, densityfunctional (DFT) calculations were performed to study the electronic structure of the atomically-thin Cu2O sheets and bulk counterpart. As shown in Figure 2a, the calculated DOS clearly revealed that the atomically-thin Cu2O sheets possessed greatly increased DOS at the edge of valence band and also exhibited extended conduction band edge with respect to bulk counterpart, which was further confirmed by their calculated charge density contour in Figure 2b and c. This implied that the ultrathin Cu2O sheets with 4 atomic thicknesses favored for higher carrier mobility
209 [2] and narrower band gap, thus ensuring promoted PEC efficiency. Taking advantage of atomic thickness, large surface area and obviously increased DOS, the ultrathin Cu2O sheets with 4 atomic thicknesses would bring on remarkably improved PEC properties. Thus, the photoelectrode of the ultrathin Cu2O sheets was prepared by spin coating the corresponding ethanol dispersions for testing their water splitting performances (Figure S6a). As depicted in Figure 3c and Figure S6a, the large specific surface area and relatively smooth surface of the ultrathin Cu2O sheets-based photoelectrode facilitated the greatly increased visible light harvesting and intimate contact with the ITO substrate and electrolyte, while obvious light transmittance and reflection usually occurred in the loosely packed bulk particles (Figure S6b) [20]. As shown in Figure 3a, both the photocathodes comprising of the ultrathin Cu2O sheets and bulk counterpart possessed negligible dark currents at the applied potentials between 1.0 and 0.4 V vs. Ag/AgCl. Upon the irradiation of 300 W Xe lamp with a 420 nm cutoff filter (λ4420 nm), the photocathode of the ultrathin Cu2O sheets displayed pronounced photocurrent density up to 3.98 mA cm 2 at 1.0 V vs. Ag/AgCl, remarkably higher than that of the bulk counterpart (0.11 mA cm 2), indicating their
Figure 2 (a) Calculated density of states (DOS) for the ultrathin Cu2O sheets with 4 atomic thicknesses and bulk counterpart, in which the shadow part gives the increased DOS at the valence band edge of the atomically-thin Cu2O sheets. (b and c) The charge densities for the valence band maximum of the ultrathin Cu2O sheets and their bulk counterpart.
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Figure 3 Photoelectrochemical properties for the ultrathin Cu2O sheets and bulk counterpart: (a) photocurrent vs. Ag/AgCl curves at 300 W Xe lamp irradiation (λ4420 nm). (b) Incident photon-to-current conversion efficiency. (c) UV/vis diffuse reflectance spectra. (d) Electrochemical impedance spectra. (e) Mott–Schottky plots. (f) I–t curves at 0.9 V vs. Ag/AgCl at irradiation of a 300 W Xe lamp (λ4420 nm; I=photocurrent density and t =time).
superior photoactivities. Figure 3b displays the incidentphoton-to-current-conversion efficiency (IPCE) data of the Cu2O photocathodes, the photocathode of the ultrathin Cu2O sheets showed an onset IPCE at around 680 nm, which corresponded to a band gap of about 1.82 eV and also fairly consisted with their absorption edge of 1.92 eV (Figure 3c). It is interesting that the band gap of the ultrathin Cu2O sheets is much smaller than 2.17 eV for the bulk counterpart (Figure 3c), indicating the ultrathin Cu2O sheets could
efficiently harvest visible light. As stated above, the narrowed band gap of the ultrathin Cu2O sheets could be ascribed to their increased surface structural distortion, which leads to the increased DOS at the valence band edge as well as the extended conduction band edge (Figure 2a). In fact, the structural distortion in semiconductors has also been demonstrated to narrow its band gap and hence enhance the visible and infrared absorption [29,30]. Importantly, an IPCE up to 26.2% was achieved for the
Freestanding atomically-thin cuprous oxide sheets photocathode of the ultrathin Cu2O sheets at 500 nm, obviously higher than the 3.3% value for the bulk counterpart, implied that the photogenerated carriers could realize efficient separation and transport in the ultrathin Cu2O sheets-based photocathode. To further understand the promoted PEC performances of the ultrathin Cu2O sheets-based photocathode, their electrochemical impedance spectroscopy was performed in dark conditions at a bias potential of 0.9 V vs. Ag/AgCl. As displayed in Figure 3d, the interfacial charge transfer resistance for the ultrathin Cu2O sheets-based photoelectrode was eminently lower than that of the bulk counterpart, suggesting that their large surface and [01-1] orientation allowed for better grain boundary connectivity and intimate contact with the ITO substrate [2,31], which could also be verified by the SEM image of the ultrathin Cu2O sheets-based photoelectrode in Figure S6a. In this case, the 4 atomic thicknesses endowed the ultrathin Cu2O sheets with faster carrier transport from inside to surface and hence decreased the recombination chance of electron– hole pairs [2,13,32], thus increasing the efficiency of visible-light water splitting and improving the stability of the photocathode. To further investigate the carrier transport in the ultrathin Cu2O sheets-based photoelectrode, the capacitance measurements were employed to estimate the flat band potential (Vfb) and carrier density (ND), which can be obtained according to the following Mott–Schottky Equation [20]: 2 kT Csc 2 ¼ V V fb e0 e0 ε0 εr ND A2 where CSC is the capacitance of semiconductor space charge layer, ND is the carrier density, Vfb is the flat band potential, e0 is the electron's charge, ε0 is the vacuum permittivity, εr is the dielectric constant of the semiconductor (εr of Cu2O is 7.6), A is the actual surface area, V the applied potential, T is the absolute temperature and k is the Boltzmann constant. ND could be calculated by the following equation: " #1 2 dð1=C2 Þ ND ¼ dV e0 ε0 εr A2 The slopes in Figure 3e and Figure S7 were negative, indicating that both the ultrathin Cu2O sheets and bulk counterpart were p-type semiconductors [33]. The flat-band potential for the ultrathin Cu2O sheets-based photoelectrode was determined to be 0.93 V vs. Ag/AgCl, which was more positive than the 1.13 V value for the bulk counterpart, reflecting the greatly decreased carrier recombination [34]. The positive Vfb shift could be attributed to the adequately increased hole density that led their energy band downward [35]. The ND for the ultrathin Cu2O sheets was calculated to be 4.4 1014 cm 3, roughly 4 times higher than the 1.1 1014 cm 3 for the bulk material. The much increased ND would help to enhance the charge transport in the ultrathin Cu2O sheets and at the interface between the ITO and the ultrathin Cu2O sheets [2]. Notably, the photocorrosion of the photoelectrode could be reduced by the increase of carrier mobility [8]. Therefore, it is reasonable that the photoelectrode of the ultrathin Cu2O sheets with four atomic thicknesses would also possess enhanced carriers mobility and improved photostability.
211 To investigate the stability of the photoelectrode, the photocurrent stability measurements were carried out at 0.9 V vs. Ag/AgCl under 300 W Xe lamp irradiation with irradiation intensity of 100 mW cm 2 (λ4420 nm). As shown in Figure 3f, despite the ultrathin Cu2O sheetsbased photocathode behaved relatively large photocurrent density variations in the first 20 min, its photocurrent possessed a stable value of 1.26 mA cm 2 and exhibited negligible variations in the following 10 h. By contrast, as depicted in Figure S8, the bulk counterpart showed serious I–t fluctuations, demonstrating the high photostability of the ultrathin Cu2O sheets-based photoelectrode. Meanwhile, gas chromatography has also been used to measure the H2 and O2 content during the photoelectrochemical water splitting. As shown in Figure S9, the H2 evolved rate was roughly two times larger than the O2 evolved rate, consisting with the principle of the water splitting into H2 and O2. Also, both the experimental H2 and O2 evolved rates were fairly consisting with the corresponding theoretical H 2 and O2 evolved rates, indicating that there was no obvious photo-corrosion occurred on the ultrathin Cu2O sheets-based photoelectrode. To further clarify the stability of the ultrathin Cu2O nanosheets-based photoelectrode, XPS spectra and XRD pattern were conducted to examine the composition of the samples before and after stability test (10 h). As shown in Figure S10, there were no noticeable difference in the XPS spectra and XRD patterns of the ultrathin Cu2O nanosheets before and after stability test, indicating that the Cu2O nanosheets were neither reduced nor oxidized after photoelectrochemical measurements. Therefore, all the above-mentioned results unambiguously demonstrated that the photoelectrode consisting of the ultrathin Cu2O sheets is stable in 0.5 M Na2SO4 solution (pH = 6.6) under illumination.
Conclusion In conclusion, clean and freestanding ultrathin Cu2O sheets with 4 atomic thicknesses were first artificially synthesized via a high-yield and scalable rapid-heating strategy at 300 1C in less than 8 min, taking advantage of the lamellar Cu2O–oleate complex intermediate. Benefiting from efficient visible-light harvesting, improved carrier density, and fast interfacial charge transfer as well as facile electrochemical reactions, the atomically-thin Cu2O sheets-based photoelectrode exhibited an IPCE up to 26.2% at 500 nm, strikingly higher than 3.3% for the bulk counterpart. Such a visible-light conversion efficiency is also superior to most existing reports, strongly highlighting their charming photoactivity. Moreover, the photocathode comprising of the atomically-thin Cu2O sheets displayed a photocurrent of 3.98 mA cm 2, which is over 36 times higher than that of the bulk counterpart. This work develops a versatile fast-heating strategy to fully exfoliate the lamellar hybrid intermediate into atomically-thin sheets with the removal of organic component, opening new avenues for fabricating atomicallythin non-layered materials and hence bringing a series of unprecedented performances.
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Acknowledgments This work was financially supported by National Natural Science Foundation of China (21331005, 11079004, 90922016, 21201157, 11321503), Chinese Academy of Science (XDB01020300), Program for New Century Excellent Talents in University (NCET-13-0546), Fundamental Research Funds for the Central Universities (WK2310000022) and Australian Research Council (ARC) Discovery Project (Project ID: DE120101788).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.05.017.
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[26] Y.Y. Xu, X.L. Jiao, D.R. Chen, J. Phys. Chem. C 112 (2008) 16769. [27] Q. Hua, K. Chen, S.J. Chang, Y.S. Ma, W.X. Huang, J. Phys. Chem. C 115 (2011) 20618. [28] H.Y.H. Chan, C.G. Takoudis, M.J. Weaver, J. Phys. Chem. B 103 (1999) 357. [29] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C.L. Bianchi, R. Psaro, V.D. Santo, J. Am. Chem. Soc. 134 (2012) 7600. [30] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746. [31] Y.F. Sun, C.Z. Wu, R. Long, Y. Cui, S.D. Zhang, Y. Xie, Chem. Commun. (2009) 4542. [32] Y.F. Sun, Y. Xie, C.Z. Wu, S.D. Zhang, S.S. Jiang, Nano Res. 9 (2010) 620. [33] I.S. Cho, Z.B. Chen, A.J. Forman, D.R. Kim, P.M. Rao, T. F. Jaramillo, X.L. Zheng, Nano Lett. 11 (2011) 4978. [34] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energy Environ. Sci. 4 (2011) 1781. [35] G.M. Wang, H.Y. Wang, Y.C. Ling, Y.C. Tang, X.Y. Yang, R. C. Fitzmorris, C.C. Wang, J.Z. Zhang, Y. Li, Nano Lett. 11 (2011) 3026. Shan Gao received his BS degree in chemistry from West Anhui University, China, in 2011. He is currently pursuing his Ph.D. degree in Inorganic Chemistry at the University of Science and Technology of China, under the supervision of Prof. Yi Xie. His current interests include the synthesis and characterization of nanostructures, especially two-dimensional inorganic graphene analogues and their applications in energy storage and conversion. Yongfu Sun received his BS degree in the Department of Chemistry at Anhui University (2006) and Ph.D. degree in Inorganic Chemistry from the University of Science and Technology of China (2011). After that he has worked as a postdoctoral fellow in the National Synchrotron Radiation Laboratory. In 2013, Dr. Sun joined the Hefei National Laboratory for Physical Sciences at the Microscale as a research associate professor. His current interests include the theoretical computation, controllable synthesis, fine structure characterization, device assembly and energyrelated applications of atomically-thick two-dimensional crystals. Fengcai Lei received her BS degree in Physics from Shandong Normal University, China, in 2011. She is currently pursuing her Ph.D. in Inorganic Chemistry at the University of Science and Technology of China, under the supervision of Prof. Yi Xie. Her current interests include the theoretical computation and the underlying physics during studying the atomically thin inorganic graphene analogues. Jiawei Liu has been admitted to the University of Science and Technology of China in 2011. Now he is learning condensed matter physics for his BS degree. His current interests include the synthesis and characterization of two-dimensional inorganic graphene analogues.
Freestanding atomically-thin cuprous oxide sheets Liang Liang received her BS from Xiamen University, China, in 2012. She is currently pursuing her Ph.D. in Inorganic Chemistry at the University of Science and Technology of China under the supervision of Prof. Y. Xie. Her current interests include the fabrication and application of two-dimensional inorganic graphene analogues.
Tanwei Li received his BS from University of Science and Technology of China in 2004. After that he has worked as a researcher in the Hefei regional center of strategy energy and physical science instrument. His current interests focus on studying the microstructures of the samples.
Bicai Pan received his Ph.D. from the University of Science and Technology of China (USTC, 1993). In 1998, he joined University of Science and Technology of China as an associate professor. After that he worked as a professor of the Department of Physics, USTC (2003). His main interests in research are to reveal and understand some properties of complicated condensed matter systems, using electronic structural calculations at the levels of local density approximation (LDA), tight-
213 binding (TB) and classical molecular dynamics. Especially, he does his efforts to develop new tight-binding potential models. Jingfang Zhou received her BS degree from Xiamen University (1988) and a Ph.D. from the University of South and Australia. She is now a Research fellow of University of South and Australia. Her research interests mainly focus on the study of interactions between particles and interfaces.
Yi Xie received her BS degree from Xiamen University (1988) and a Ph.D. from the University of Science and Technology of China (USTC, 1996). She is now a Principal Investigator of Hefei National Laboratory for Physical Sciences at the Microscale and a full professor of the Department of Chemistry, USTC. She was appointed as the Cheung Kong Scholar Professor of inorganic chemistry in 2000 and elected as a member of the Chinese Academy of Sciences in 2013, also a recipient of many awards, including the Chinese National Nature Science Award (2001 and 2012), China Young Scientist Award (2002), China Young Female Scientist Award (2006) and IUPAC Distinguished Women in Chemstry/Chemical Engineering Award (2013). Her research interests are cutting-edge research at four major frontiers: solid state materials chemistry, nanotechnology, energy science and theoretical physics.
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Freestanding atomically-thin cuprous oxide sheets for improved visible-light photoelectrochemical water splitting Shan Gaoa,1, Yongfu Suna,1, Fengcai Leia, Jiawei Liua, Liang Lianga, Tanwei Lia, Bicai Pana, Jingfang Zhoub, Yi Xiea,n a
Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026, PR China b Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received 20 March 2014; received in revised form 27 May 2014; accepted 28 May 2014 Available online 11 June 2014
KEYWORDS
Abstract
Cuprous oxide; Non-layered; Atomically-thin; Visible-light; Photoelectrochemical water splitting
Atomically-thin sheet is a new class of two-dimensional materials that bring a wide range of extraordinary properties. However, the lack of intrinsic growth anisotropy makes the synthesis of atomically-thin non-layered materials a great challenge. Here, a versatile rapid-heating strategy is proposed and applied, in which the lamellar hybrid intermediate transforms into the clean and freestanding atomically-thin non-layered Cu2O sheet with 4 atomic thicknesses at 300 1C in less than 8 min. Taking advantage of efficient visible-light harvesting, improved carrier density, and fast interfacial charge transfer as well as facile electrochemical reactions, the ultrathin Cu2O sheets-based photoelectrode yields a photocurrent density up to 3.98 mA cm 2, 36 times higher than that of bulk counterpart. Also, the photoelectrode reaches a visible-lightconversion-efficiency of 26.2% that is superior to most existing reports. The rapid-heating strategy guarantees the full exfoliation of lamellar hybrid intermediate into ultrathin Cu2O sheets with the removal of organic component, opening a new way to produce atomically-thin non-layered materials and holding great promise in the applications of visible-light photoelectrochemical water splitting. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. E-mail address: [email protected] (Y. Xie). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.nanoen.2014.05.017 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Photoelectrochemical (PEC) water splitting using semiconductor materials could provide a promising and environmentally benign pathway to produce clean and renewable hydrogen energy [1–3]. Among the abundant semiconductors, cuprous oxide (Cu2O) has recently attracted extensive attention as a means of solar-to-hydrogen conversion [4–6]. In addition to possessing low cost and environmental friendliness, the fascination also comes from its peculiar conduction band lying 0.7 V negative of the hydrogen evolution potential [4,7], which theoretically points out its feasibility to drive the water splitting into hydrogen. In addition, the high theoretical photocurrent of 14.7 mA cm 2 and light-to-hydrogen conversion efficiency of 18.7% suggest its huge potential applications in solar hydrogen production [8]. Also, the relatively small direct band gap of 2.2 eV endows Cu2O with efficient adsorption of visible light, which accounts for 43% of the incoming solar energy [9]. Moreover, the large absorption coefficient of 3.7 106 cm 1 facilitates efficient light absorption, while the high carrier mobility of 4105 cm2 V 1 s 1 favors for fast separation of photoexcited electron–hole pairs [10,11]. Furthermore, Cu2O can effectively scavenge the photogenerated electrons through adsorbing molecular oxygen and hence constrain the recombination of electron–hole pairs, thus definitely improve the photoconversion efficiency [12]. Despite these excellent advantages of Cu2O, the relatively low photoconversion efficiency, especially in the visible light range, unfortunately hinders the promising applicability. Therefore, further breakthroughs in the design and synthesis of novel Cu2O structures with high visible light water splitting efficiency, hold the key to the development of PEC water splitting. Recently, our studies have shown that the atomicallythick two-dimensional sheets provide the ideal architecture for promoting the solar hydrogen production [2,13]. The peculiar geometric structure can afford the atomic thickness for fast carrier transport from interior to surface, the two-dimensional (2D) conducting channels for quick separation of electron–hole pairs as well as the large surface area for efficient light harvesting and facile surface reactions. Also, the 2D configuration endows them with a much better grain boundary connectivity and allows for intimate contact with the substrate and high interfacial contact area with the electrolyte, thus facilitating fast interfacial charge transfer and electrochemical reactions as well as low corrosion rates. Enlightened by the above discussions, controllable synthesis of the atomically-thick Cu2O sheets is of great importance. As is well-known, controllable exfoliation of anisotropic layered bulk materials have been
S. Gao et al. regarded as the most effective pathway to obtain twodimensional sheets with atomic thickness, owing to the strong in-plane bonds and weak van der Waals interaction between layers [14,15]. However, for the non-layered compounds such as cubic Cu2O, controlling the synthesis of their atomically-thick two-dimensional sheets is extremely challenging on account of the difficulty in bondcleavage and the lack of intrinsic driving force for anisotropic growth. Despite the recent progress in fabricating two-dimensional CdSe and PbS sheets with few atomic thickness [16,17], the hard removal of surface organic surfactant would inevitably obscure their properties. Herein, we describe our efforts to develop a high-yield and scalable fast-heating strategy for fabricating clean and freestanding atomically-thick sheets with non-layered structures, taking the cubic Cu2O as an example (Figure S1) [18]. As shown in Scheme 1, the successful fabrication of the atomically-thin Cu2O sheets takes advantage of an intermediate precursor of lamellar Cu2O–oleate complex microplates (Figures S2 and S3). Notably, the oleate ions play a fundamental role in the formation of this intermediate: the oleate ions initially interact with Cu + to form Cu–oleate complexes via an electrostatic interaction. After hydrothermal treatment at 100 1C for 24 h, the Cu–oleate complexes assemble into lamellar Cu2O–oleate complex intermediate microplates, which are fully characterized in Figures S2 and S3. And then, upon heating at 300 1C for a short time of 8 min in air, the quick release of enormous H2O and CO2 gases contributes to weaken the interaction between the adjacent inorganic layers, and hence fully exfoliate the lamellar Cu2O–oleate complex into clean and freestanding ultrathin Cu2O sheets with 4 atomic thicknesses (Figure 1 and Figures S4 and S5). Of note, this developed fast-heating strategy is simple, high-yield and can be easily scaled up for large-scale synthesis of the atomically-thin non-layered materials.
Experimental section Preparation of lamellar Cu2O–oleate complex intermediate The lamellar Cu2O–oleate complex intermediate was synthesized by a hydrothermal method. In a typical procedure, 99 mg CuCl, 914 mg sodium oleate, 831 mg hexamethylenetetramine (HMT) and 100 mL N2H4 H2O were added into 40 mL distilled water orderly. After vigorous stirring for 10 min, the mixed solution was transferred into 50 mL Teflon cup and heated in a sealed autoclave at 100 1C for 24 h. Upon cooling to room temperature naturally, the final
Scheme 1 Schematic rapid-heating strategy for fabricating clean and freestanding atomically-thin Cu2O sheets, taking advantage of the lamellar Cu2O–oleate complex intermediate.
Freestanding atomically-thin cuprous oxide sheets
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Figure 1 Characterizations for the ultrathin Cu2O sheets with 4 atomic thicknesses: (a) TEM image and the corresponding colloidal ethanol dispersion displaying the Tyndall effect; (b) AFM image, inset yellow line in b corresponds to the height profile in (c); (d) HRTEM and (e) the corresponding fast Fourier transform image; (f) the ideal crystal structure of 4-atomically-thin Cu2O slab along the [01-1] direction.
product was collected by centrifuging the mixture, washed with distilled water and absolute ethanol for several times, and then dried in vacuum at 60 1C overnight for further analysis.
indium tin oxide (ITO)-coated glass at 500 rpm for 30 s with a desktop spin coater (KW-4A). Subsequently, the ITOcoated glasses were put into a 60 1C oven for 30 min in order to evaporate the solvent and then the electrode were obtained for further measurements.
Preparation of atomically-thin Cu2O sheets In a typical procedure, 50 mg lamellar Cu2O–oleate complex intermediate was put in a quartz boat and then heated at 300 1C for 8 min in air, then the sample was allowed to cool down to room temperature naturally. The Cu2O colloidal solution was achieved by dispersed 20 mg calcined product into 10 mL absolute ethanol by ultrasonic for 40 min. The supernatant was removed from the Cu2O colloidal solution which has been put aside for two days. The final product was collected by centrifuging the supernatant at 12,000 rpm for 10 min, and then dried in vacuum overnight for further characterization.
Preparation of bulk Cu2O Bulk Cu2O was synthesized according to a modified method of Zhang et al. [19]. 2 mmol Cu(NO3)2 3H2O was dissolved in the mixed solvent of 5 mL water and 10 mL ethanol under stirring for 30 min, 1 mmol stearic acid were added into the above solution. After sonicating in water for 15 min, the mixture was put into 50 mL Teflon cup and heated in a sealed stainless steel autoclave at 180 1C for 12 h. The product was washed with water and ethanol several times, and then dried in vacuum at 60 1C for further characterization.
Preparation of Cu2O electrodes The two Cu2O electrodes were prepared by uniformly spincoated the ethanol dispersion of ultrathin Cu2O sheets and bulk counterpart (5 mL, 1.0 mg/mL) on the 1.5 cm 2 cm
Characterization The crystallographic information of the as-prepared samples were characterized by using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ=1.54178 Å). Transmission electron microscopy (TEM) images and highresolution TEM image were carried out on a JEOL-2010 TEM at an acceleration voltage of 200 kV. The field emission scanning electron microscopy (FE-SEM) images were performed by means of FEI Sirion-200 SEM. Atomic force microscopy (AFM) study in the present work was measured on a Veeco DI Nano-scope MultiMode V system. X-ray photoelectron spectra (XPS) of the samples were taken on an ESCALAB MKII with Mg Kα (hυ=1253.6 eV) as the excitation source. All binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.8 eV arising from adventitious hydrocarbon. Thermal gravimetric analysis (TGA) of the as-synthesized samples was detected by a Shimadzu TA-50 thermal analyzer at a heating rate of 5 1C min 1 from room temperature to 500 1C with a flowing nitrogen gas stream at 100 mL/min. Room-temperature UV/vis diffuse reflectance spectroscopy (DRS) and UV/vis absorption spectra were recorded on a Perkin Elmer Lambda 950 UV/vis–NIR spectrophotometer. The Fourier transform infrared (FT-IR) spectra were acquired on a NICOLET FT-IR spectrometer in a KBr tablets, scanning from 4000 to 400 cm 1 at room temperature. Photoelectrochemical measurement To investigate the photoelectrochemical performance of the atomically-thin Cu2O sheets as a photocathode for the solar
208 hydrogen generation by water splitting, conventional threeelectrode configuration was adopted. A Platinum wire and a Ag/AgCl reference electrode were used as counter electrode and reference electrode, respectively. 0.5 M Na2SO4 solution (pH = 6.6) was used as the electrolyte. The Na2SO4 electrolyte was purged with nitrogen for 30 min prior to the measurement. The photoelectrochemical performance of the Cu2O photocathodes was measured on an electrochemical station (Zahner IM6ex, Germany) at room temperature with irradiation of a 300 W Xe lamp (PLS-SXE300/ 300UV, Trusttech Co., Ltd. Beijing) with a 420 nm cutoff filter. The intensity of light source used in the measurement is 100 mW cm 2. The linear sweep voltammetry was conducted from 1.0 to 0.6 V vs. Ag/AgCl with a scan rate of 10 mV/s. Mott–Schottky plots were estimated at a frequency of 1 kHz in the dark under the applied potentials from 1.4 to 0.4 V vs. Ag/AgCl. Incident photon conversion to charge carrier conversion efficiency (IPCE) spectra were recorded on a QE/IPCE setup (Newport, USA) by monitoring the photocurrent at different incident wavelengths. And all the kit components were in control of Oriel Tracq Basic V5.0 software automatically. The light from a 300 W Xe lamp was focused through a monochromator (74125 Oriel Cornerstone 260 1/4 m) onto the cell, and the monochromator was incremented through the spectral range (300–450 nm) to generate a photocurrent action spectrum with a sampling interval of 10 nm and a current sampling time of 2 s, where the light intensity and the photocurrent generated were measured with a 2931-C dual channel power/current meter and a 71675 calibrated UV silicon photodetector. The IPCE was calculated from the following equation: IPCE ¼ hcI=λJlight where h is the Planck's constant, c is the speed of light, I is the measured photocurrent density at a specific wavelength, λ is the incident light wavelength and Jlight is the recorded irradiance intensity at a specific wavelength. The electrochemical impedance spectroscopy (EIS) was performed on an electrochemical station (Zahner IM6ex, Germany) in 0.5 M Na2SO4 electrolyte. The Nyquist plots were measured at a bias potential of 0.9 V vs. Ag/AgCl with a frequency of 100 kHz–100 mHz under irradiation of a 300 W Xe lamp. The measured spectra were fitted by using the ZSimpWin program (from Echem software). Moreover, dye absorption experiments were carried out to obtain the actual surface area of the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart, according to the previous report [20]. By studying the adsorption of the soluble dye—methylene blue on the fabricated Cu2O-based film photoelectrode, the actual surface area of the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart were determined to be 68 cm2 and 39 cm2, respectively. Although the actual surface area difference between the ultrathin Cu2O sheets-based photoelectrode and bulk counterpart was tremendous, it is not as large as expected. This could be ascribed to the following two reasons: (1) more space in the loosely packed bulk Cu2O-based film photoelectrode helps to adsorb more dye molecules (Figure S6b), thereby leading to their relatively larger actual surface area and (2) the atomic thickness endowed the atomically-thin Cu2O sheets with remarkably more grain boundaries, which inevitably leaded to their interface overlap in the
S. Gao et al. fabricated film photoelectrode. Thus, the ultrathin Cu2O sheets-based photoelectrode could not expose 100% surface area to the electrolyte. Calculation method The first-principles density functional theory (DFT) calculations were performed using a plane wave basis set with the projector augmented plane-wave (PAW) method [21–23]. The exchange–correlation interaction is described within the generalized gradient approximation (GGA) in the form of PW91 [24,25]. The energy cutoff is set to 400 eV, and the atomic positions are allowed to relax until the energy and force are less than 10 4 eV and 10 2 eV/Å, respectively. The [01-1]-oriented cubic Cu2O quadruple layer are simulated by periodically repeating the four atomic layers along the [01-1] direction of the unit cell. Each Cu2O quadruple layer model is separated by a vacuum region of 15 Å. A 2 2 two-dimensional unit cell was used to mimic the Cu2O quadruple layer with four atomic thicknesses. The compared calculations of bulk counterpart were performed within a supercell constructed from a standard unit cell of Cu2O lattice.
Results and discussion Owing to the fact that it is currently infeasible to perform Xray diffraction (XRD) characterization on an individual ultrathin sheet, the XRD measurement is performed on the collected powder sample accumulated by the as-obtained freestanding ultrathin sheets. The corresponding XRD peaks in Figure S4a could be indexed to cubic Cu2O, corresponding to JCPDS no. 78-2076. In addition, their X-ray photoelectron spectroscopy in Figure S5a–c demonstrated the formation of pure Cu2O, further verified by the corresponding IR spectrum in Figure S3a [26,27]. This suggested the absence of oleate on the surface of the as-obtained Cu2O sample. As shown in Figure 1a and Figure S4b, the nearly transparent feature and the clear Tyndall effect indicated the ultrathin thickness of the as-obtained Cu2O sheets, further verified by their atomic force microscopy (AFM) image in Figure 1b and c. Also, compared to the bulk counterpart, the Raman bands of the ultrathin Cu2O sheets shifted towards lower wavenumbers, which could be ascribed to the phonon confinement effect and reasonably inferred their ultrathin thickness [2,28]. Moreover, the high-resolution TEM image in Figure 1d illustrated that the ultrathin Cu2O sheets possessed a [01-1] orientation, confirmed by their corresponding FFT image in Figure 1e. Their AFM image and the corresponding height profile in Figure 1b and c showed a uniform thickness of 0.62 nm, corresponding to the 0.60 nm thickness of 4 atomically-thick Cu2O slab along the [01-1] direction. Notably, the surface energy of cubic Cu2O typically follows the sequence (111)o(100)o(110) [19], indicating that the high-energy (110) facet could exhibit the highest reactivity. Considering the (110) and (01-1) facets are the equivalent facets, the (01-1) facet could also possess the large surface energy with a high activity. As such, upon the formation of Cu2O–oleate complex, the oleate ions tended to preferentially adsorb on the surface of (01-1) facet and hence impede the crystal growth along the [01-1] direction [19], thus finally resulting in the [01-1] orientation of the
Freestanding atomically-thin cuprous oxide sheets atomically-thick Cu2O sheets. Therefore, all the abovementioned results verified the formation of clean and freestanding ultrathin Cu2O sheets with four atomic thicknesses. It is noticeable that the atomically-thin nanosheets are usually accompanied by obvious structural distortion [2,13], which hence leads to the increase of density of states (DOS) at the edge of valence band or conduction band. The increased DOS near the Fermi level could lead to increased carrier mobility as well as decreased band gap. As such, for the ultrathin Cu2O sheets with four atomic thicknesses, it is reasonable that they also possess increased surface structural distortion compared with the bulk counterpart, which hence leads to increased DOS at the edge of valence band or conduction band. To verify this viewpoint, densityfunctional (DFT) calculations were performed to study the electronic structure of the atomically-thin Cu2O sheets and bulk counterpart. As shown in Figure 2a, the calculated DOS clearly revealed that the atomically-thin Cu2O sheets possessed greatly increased DOS at the edge of valence band and also exhibited extended conduction band edge with respect to bulk counterpart, which was further confirmed by their calculated charge density contour in Figure 2b and c. This implied that the ultrathin Cu2O sheets with 4 atomic thicknesses favored for higher carrier mobility
209 [2] and narrower band gap, thus ensuring promoted PEC efficiency. Taking advantage of atomic thickness, large surface area and obviously increased DOS, the ultrathin Cu2O sheets with 4 atomic thicknesses would bring on remarkably improved PEC properties. Thus, the photoelectrode of the ultrathin Cu2O sheets was prepared by spin coating the corresponding ethanol dispersions for testing their water splitting performances (Figure S6a). As depicted in Figure 3c and Figure S6a, the large specific surface area and relatively smooth surface of the ultrathin Cu2O sheets-based photoelectrode facilitated the greatly increased visible light harvesting and intimate contact with the ITO substrate and electrolyte, while obvious light transmittance and reflection usually occurred in the loosely packed bulk particles (Figure S6b) [20]. As shown in Figure 3a, both the photocathodes comprising of the ultrathin Cu2O sheets and bulk counterpart possessed negligible dark currents at the applied potentials between 1.0 and 0.4 V vs. Ag/AgCl. Upon the irradiation of 300 W Xe lamp with a 420 nm cutoff filter (λ4420 nm), the photocathode of the ultrathin Cu2O sheets displayed pronounced photocurrent density up to 3.98 mA cm 2 at 1.0 V vs. Ag/AgCl, remarkably higher than that of the bulk counterpart (0.11 mA cm 2), indicating their
Figure 2 (a) Calculated density of states (DOS) for the ultrathin Cu2O sheets with 4 atomic thicknesses and bulk counterpart, in which the shadow part gives the increased DOS at the valence band edge of the atomically-thin Cu2O sheets. (b and c) The charge densities for the valence band maximum of the ultrathin Cu2O sheets and their bulk counterpart.
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Figure 3 Photoelectrochemical properties for the ultrathin Cu2O sheets and bulk counterpart: (a) photocurrent vs. Ag/AgCl curves at 300 W Xe lamp irradiation (λ4420 nm). (b) Incident photon-to-current conversion efficiency. (c) UV/vis diffuse reflectance spectra. (d) Electrochemical impedance spectra. (e) Mott–Schottky plots. (f) I–t curves at 0.9 V vs. Ag/AgCl at irradiation of a 300 W Xe lamp (λ4420 nm; I=photocurrent density and t =time).
superior photoactivities. Figure 3b displays the incidentphoton-to-current-conversion efficiency (IPCE) data of the Cu2O photocathodes, the photocathode of the ultrathin Cu2O sheets showed an onset IPCE at around 680 nm, which corresponded to a band gap of about 1.82 eV and also fairly consisted with their absorption edge of 1.92 eV (Figure 3c). It is interesting that the band gap of the ultrathin Cu2O sheets is much smaller than 2.17 eV for the bulk counterpart (Figure 3c), indicating the ultrathin Cu2O sheets could
efficiently harvest visible light. As stated above, the narrowed band gap of the ultrathin Cu2O sheets could be ascribed to their increased surface structural distortion, which leads to the increased DOS at the valence band edge as well as the extended conduction band edge (Figure 2a). In fact, the structural distortion in semiconductors has also been demonstrated to narrow its band gap and hence enhance the visible and infrared absorption [29,30]. Importantly, an IPCE up to 26.2% was achieved for the
Freestanding atomically-thin cuprous oxide sheets photocathode of the ultrathin Cu2O sheets at 500 nm, obviously higher than the 3.3% value for the bulk counterpart, implied that the photogenerated carriers could realize efficient separation and transport in the ultrathin Cu2O sheets-based photocathode. To further understand the promoted PEC performances of the ultrathin Cu2O sheets-based photocathode, their electrochemical impedance spectroscopy was performed in dark conditions at a bias potential of 0.9 V vs. Ag/AgCl. As displayed in Figure 3d, the interfacial charge transfer resistance for the ultrathin Cu2O sheets-based photoelectrode was eminently lower than that of the bulk counterpart, suggesting that their large surface and [01-1] orientation allowed for better grain boundary connectivity and intimate contact with the ITO substrate [2,31], which could also be verified by the SEM image of the ultrathin Cu2O sheets-based photoelectrode in Figure S6a. In this case, the 4 atomic thicknesses endowed the ultrathin Cu2O sheets with faster carrier transport from inside to surface and hence decreased the recombination chance of electron– hole pairs [2,13,32], thus increasing the efficiency of visible-light water splitting and improving the stability of the photocathode. To further investigate the carrier transport in the ultrathin Cu2O sheets-based photoelectrode, the capacitance measurements were employed to estimate the flat band potential (Vfb) and carrier density (ND), which can be obtained according to the following Mott–Schottky Equation [20]: 2 kT Csc 2 ¼ V V fb e0 e0 ε0 εr ND A2 where CSC is the capacitance of semiconductor space charge layer, ND is the carrier density, Vfb is the flat band potential, e0 is the electron's charge, ε0 is the vacuum permittivity, εr is the dielectric constant of the semiconductor (εr of Cu2O is 7.6), A is the actual surface area, V the applied potential, T is the absolute temperature and k is the Boltzmann constant. ND could be calculated by the following equation: " #1 2 dð1=C2 Þ ND ¼ dV e0 ε0 εr A2 The slopes in Figure 3e and Figure S7 were negative, indicating that both the ultrathin Cu2O sheets and bulk counterpart were p-type semiconductors [33]. The flat-band potential for the ultrathin Cu2O sheets-based photoelectrode was determined to be 0.93 V vs. Ag/AgCl, which was more positive than the 1.13 V value for the bulk counterpart, reflecting the greatly decreased carrier recombination [34]. The positive Vfb shift could be attributed to the adequately increased hole density that led their energy band downward [35]. The ND for the ultrathin Cu2O sheets was calculated to be 4.4 1014 cm 3, roughly 4 times higher than the 1.1 1014 cm 3 for the bulk material. The much increased ND would help to enhance the charge transport in the ultrathin Cu2O sheets and at the interface between the ITO and the ultrathin Cu2O sheets [2]. Notably, the photocorrosion of the photoelectrode could be reduced by the increase of carrier mobility [8]. Therefore, it is reasonable that the photoelectrode of the ultrathin Cu2O sheets with four atomic thicknesses would also possess enhanced carriers mobility and improved photostability.
211 To investigate the stability of the photoelectrode, the photocurrent stability measurements were carried out at 0.9 V vs. Ag/AgCl under 300 W Xe lamp irradiation with irradiation intensity of 100 mW cm 2 (λ4420 nm). As shown in Figure 3f, despite the ultrathin Cu2O sheetsbased photocathode behaved relatively large photocurrent density variations in the first 20 min, its photocurrent possessed a stable value of 1.26 mA cm 2 and exhibited negligible variations in the following 10 h. By contrast, as depicted in Figure S8, the bulk counterpart showed serious I–t fluctuations, demonstrating the high photostability of the ultrathin Cu2O sheets-based photoelectrode. Meanwhile, gas chromatography has also been used to measure the H2 and O2 content during the photoelectrochemical water splitting. As shown in Figure S9, the H2 evolved rate was roughly two times larger than the O2 evolved rate, consisting with the principle of the water splitting into H2 and O2. Also, both the experimental H2 and O2 evolved rates were fairly consisting with the corresponding theoretical H 2 and O2 evolved rates, indicating that there was no obvious photo-corrosion occurred on the ultrathin Cu2O sheets-based photoelectrode. To further clarify the stability of the ultrathin Cu2O nanosheets-based photoelectrode, XPS spectra and XRD pattern were conducted to examine the composition of the samples before and after stability test (10 h). As shown in Figure S10, there were no noticeable difference in the XPS spectra and XRD patterns of the ultrathin Cu2O nanosheets before and after stability test, indicating that the Cu2O nanosheets were neither reduced nor oxidized after photoelectrochemical measurements. Therefore, all the above-mentioned results unambiguously demonstrated that the photoelectrode consisting of the ultrathin Cu2O sheets is stable in 0.5 M Na2SO4 solution (pH = 6.6) under illumination.
Conclusion In conclusion, clean and freestanding ultrathin Cu2O sheets with 4 atomic thicknesses were first artificially synthesized via a high-yield and scalable rapid-heating strategy at 300 1C in less than 8 min, taking advantage of the lamellar Cu2O–oleate complex intermediate. Benefiting from efficient visible-light harvesting, improved carrier density, and fast interfacial charge transfer as well as facile electrochemical reactions, the atomically-thin Cu2O sheets-based photoelectrode exhibited an IPCE up to 26.2% at 500 nm, strikingly higher than 3.3% for the bulk counterpart. Such a visible-light conversion efficiency is also superior to most existing reports, strongly highlighting their charming photoactivity. Moreover, the photocathode comprising of the atomically-thin Cu2O sheets displayed a photocurrent of 3.98 mA cm 2, which is over 36 times higher than that of the bulk counterpart. This work develops a versatile fast-heating strategy to fully exfoliate the lamellar hybrid intermediate into atomically-thin sheets with the removal of organic component, opening new avenues for fabricating atomicallythin non-layered materials and hence bringing a series of unprecedented performances.
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Acknowledgments This work was financially supported by National Natural Science Foundation of China (21331005, 11079004, 90922016, 21201157, 11321503), Chinese Academy of Science (XDB01020300), Program for New Century Excellent Talents in University (NCET-13-0546), Fundamental Research Funds for the Central Universities (WK2310000022) and Australian Research Council (ARC) Discovery Project (Project ID: DE120101788).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.05.017.
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Freestanding atomically-thin cuprous oxide sheets Liang Liang received her BS from Xiamen University, China, in 2012. She is currently pursuing her Ph.D. in Inorganic Chemistry at the University of Science and Technology of China under the supervision of Prof. Y. Xie. Her current interests include the fabrication and application of two-dimensional inorganic graphene analogues.
Tanwei Li received his BS from University of Science and Technology of China in 2004. After that he has worked as a researcher in the Hefei regional center of strategy energy and physical science instrument. His current interests focus on studying the microstructures of the samples.
Bicai Pan received his Ph.D. from the University of Science and Technology of China (USTC, 1993). In 1998, he joined University of Science and Technology of China as an associate professor. After that he worked as a professor of the Department of Physics, USTC (2003). His main interests in research are to reveal and understand some properties of complicated condensed matter systems, using electronic structural calculations at the levels of local density approximation (LDA), tight-
213 binding (TB) and classical molecular dynamics. Especially, he does his efforts to develop new tight-binding potential models. Jingfang Zhou received her BS degree from Xiamen University (1988) and a Ph.D. from the University of South and Australia. She is now a Research fellow of University of South and Australia. Her research interests mainly focus on the study of interactions between particles and interfaces.
Yi Xie received her BS degree from Xiamen University (1988) and a Ph.D. from the University of Science and Technology of China (USTC, 1996). She is now a Principal Investigator of Hefei National Laboratory for Physical Sciences at the Microscale and a full professor of the Department of Chemistry, USTC. She was appointed as the Cheung Kong Scholar Professor of inorganic chemistry in 2000 and elected as a member of the Chinese Academy of Sciences in 2013, also a recipient of many awards, including the Chinese National Nature Science Award (2001 and 2012), China Young Scientist Award (2002), China Young Female Scientist Award (2006) and IUPAC Distinguished Women in Chemstry/Chemical Engineering Award (2013). Her research interests are cutting-edge research at four major frontiers: solid state materials chemistry, nanotechnology, energy science and theoretical physics.