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Surface passivation of undoped hematite nanorod arrays via aqueous solution growth for improved photoelectrochemical water splitting
Accepted Manuscript Surface Passivation of Undoped Hematite Nanorod Arrays via Aqueous Solution Growth for Improved Photoelectrochemical Water Splitting Shaohua Shen, Mingtao Li, Liejin Guo, Jiangang Jiang, Samuel S. Mao PII: DOI: Reference:
S0021-9797(13)00978-8 http://dx.doi.org/10.1016/j.jcis.2013.10.063 YJCIS 19185
To appear in:
Journal of Colloid and Interface Science
Please cite this article as: S. Shen, M. Li, L. Guo, J. Jiang, S.S. Mao, Surface Passivation of Undoped Hematite Nanorod Arrays via Aqueous Solution Growth for Improved Photoelectrochemical Water Splitting, Journal of Colloid and Interface Science (2013), doi: http://dx.doi.org/10.1016/j.jcis.2013.10.063
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Surface Passivation of Undoped Hematite Nanorod Arrays via Aqueous Solution Growth for Improved Photoelectrochemical Water Splitting
Shaohua Shen,1,2* Mingtao Li,1 Liejin Guo,1 Jiangang Jiang,1 Samuel S. Mao2
1. International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China. Email: [email protected] 2. Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, United States
Abstract: A facile solution-based strategy was found to be effective for surface passivation of undoped hematite nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe3+ precursor solution. XPS and Raman spectra revealed that noble-metal ions would not be doped into, but lead to surface disorder of hematite. Incident photon to current efficiency (IPCE) of the hematite photoanode grown in HAuCl4 and H2PtCl6 contained precursor solution was increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE (reversible hydrogen electrode). This photoelectrochemical (PEC) enhancement was ascribed to the surface passivation, which resulted in a recombination of
1
photogenerated carriers, as confirmed by ultrafast transient absorption spectroscopy. Keywords: hematite; surface passivation; photoelectrochemical water splitting
1. Introduction An optical band gap of 2.0-2.2 eV makes hematite (α-Fe2O3), the earth abundant, stable and nontoxic material, a very attractive candidate for solar energy conversion via a photoelectrochemical (PEC) water splitting process [1-4]. However, the solar energy conversion efficiency of α-Fe2O3 photoanodes is still far from the theoretical maximum efficiency [5]. It is well recognized that the short hole diffusion length (2−3 nm) and poor electronic conductivity in the bulk [6] is one of the main factors limiting the efficiency of α-Fe2O3 photoanodes. In the past decades, many efforts have been devoted to improving the conductivity of α-Fe2O3 by doping foreign elements such as Ti [7-9], Si [10,11], Zr [12,13], and Sn [14,15], which could in general increase the electron donor densities and certainly benefit charge carrier transport for the enhancement of photocurrent. Wang et al. reported that Ti-doped α-Fe2O3 prepared by a facile deposition-annealing method showed greatly enhanced photocurrent at a relatively low bias (1.83 mA·cm-2 at 1.02 V vs. RHE) due to the improved donor density and reduced electron-hole recombination at the time scale beyond a few picoseconds [16]. Besides foreign element doping, nanostructure design, aiming at improving charge collection efficiency, has been proven to be another effective strategy to shorten the distance the charge carriers have to travel for surface water splitting
reaction
before
recombination.
For
example,
dendritic
α-Fe2O3
nanostructures were developed by Kay et al. and an unprecedented high PEC water
2
splitting efficiency was obtained [17]. This could be attributed to the dendritic nanostructure minimizing the distance photogenerated holes have to diffuse to reach the α-Fe2O3/electrolyte interface. Since α-Fe2O3 nanorod arrays developed by Vayssieres et al. [18], which showed much higher PEC performance than nanoparticulate α-Fe2O3 [19], one-dimensional α-Fe2O3 nanostructures have been attracted growing interest for photoelectrodes, mainly because their design contributes to the channelized electron transport and delayed electron-hole recombination [20-24]. By further doping of metal ions, Ti-doped α-Fe2O3 nanorods [23] and Ni-doped α-Fe2O3 nanotubes [24] achieved very high PEC performances for water oxidation, with IPCE to be ~60% (at 350 nm and 1.53 V vs. RHE) and ~40% (at 400 nm and 0.45 V vs. Ag/AgCl), respectively. This could be attributed to reduced electron-hole recombination as well as improved donor density by metal ions doping. Another inherent disadvantage limiting efficiency is poor oxygen evolution reaction (OER) kinetics and the quick charge carrier recombination at the surface of α-Fe2O3 photoanodes.[3,25,26] To this end, surface treatment has been manipulated to boost PEC performance of α-Fe2O3 photoanodes. Some OER cocatalysts including IrO2 [27], Co-Pi [28-30], CoF3 [31], and CoOx [32,33], etc., were developed to modify the surface of α-Fe2O3 photoanodes by creating surface reactive sites to reduce the activation energy for water oxidation. Given surface states acting as the recombination centers of the photogenerated carriers, a 13-group oxide (Al2O3, Ga2O3, or In2O3) thin overlayer was deposited on the surface of α-Fe2O3 films to passivate the surface trapping states [34,35]. As a result, the PEC performances of α-Fe2O3 films
3
were greatly increased, by reducing the surface electron-hole recombination. Recently, Zhang et al. reported a facile strategy to passivate surface states of α-Fe2O3 photoanodes without introducing any OER catalyst or passivation overlayer [36]. By photoelectrochemically pretreating α-Fe2O3 films in saturated NaCl aqueous electrolyte, the recombination centers of the photogenerated carriers were effectively diminished, which gave rise to great enhancement in PEC performance. In this study, a facile solution-based strategy was found to be effective for surface passivation of α-Fe2O3 nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe3+ precursor solution. The presence of HAuCl4 and H2PtCl6 in precursor solution did not lead to doping of Au or Pt into α-Fe2O3, but induced surface disorder of α-Fe2O3 nanorods. The resulted surface passivation effect was proposed to greatly increase the PEC activity by reducing the charge carrier recombination at the surface of α-Fe2O3 films.
2. Experimental α-Fe2O3 nanorod photoanodes were grown on FTO (F-doped SnO2) glass by an aqueous chemical growth method reported previously [18], followed by a high temperature annealing activiation process [14]. The experiment was performed with chemicals of analytic grade in an aqueous solution containing 0.15 M of ferric chloride (FeCl3·6H2O) and 1 M of sodium nitrate (NaNO3), with pH value set at around 1.4 by HCl, in a cap-sealed glass bottle containing two back-to-back FTO glasses (Pilkington, TEC7) leaning against the inner wall of glass bottle. After heated in a regular oven at 100 °C for 24 h, the resultant yellow or light yellow films were
4
washed with distilled water and dried, then annealed at 750 °C for 5 min with ramping rate of 25 °C/min. For surface passivation of α-Fe2O3 nanorod photoanodes, a desired volume of HAuCl4 (0.01 M) aqueous solution was added, with the volume of final aqueous solution kept at 20 mL for heating treatment at 100 °C. A series of surface passivated α-Fe2O3 nanorod photoanodes were obtained by adding different volumes of HAuCl4 aqueous solution (i.e., 0.02, 0.05, 0.2, 0.5, 1, 2, 5 mL), which was denoted as AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5). Scanning electron microscopy (SEM) images were obtained with a Hitachi environmental field emission scanning electron microscope (Model S-4300SE/N) operating in secondary electron detection mode. Spectral transmittance measurements were taken on the samples with a commercial thin film metrology system (Scientific Computing International, FilmTek Par 3000 SE). Raman scattering study was performed on a Jobin Yvon LabRAM HR spectrometer using 514.5 nm irradiation from an argon ion laser at 20 mW. X-ray diffraction (XRD) measurements were performed on a Siemens D5000 diffractometer with Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD) with monochromatic Al Kα radiation (hν = 1486.69 eV) and with a concentric hemispherical analyzer. The ultrafast laser system is based on a Quantronix-designed femtosecond transient absorption laser system which consists of an Er-doped fiber oscillator, a regenerative amplifier, and a diode-pumped, Q-switched, second-harmonic Nd:YLF pump laesr (527 nm, 10 W capacity) [37]. PEC measurements were conducted using a potentiostat (Pine Instruments
5
Bipotentiostat) in 0.5 M NaCl (pH = 6.7) aqueous solution using a three-electrode configuration, with the pristine or surface passivated α-Fe2O3 photoanode as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. N2 gas was continuously bubbled in solution before and during the experiment to remove any dissolved O2 and therefore suppress the reduction of O2 at the counter electrode. A 1 cm2 masked-off, sealed area of the α-Fe2O3 sample was irradiated with a 300 W Xe lamp solar simulator with adjustable power settings through an AM 1.5G filter (Oriel). The light intensity at the sample location in the photoelectrochemical cell was 100 mW·cm-2 as measured by a power detector (Newport). IPCE measurements were performed using a 300 W Xe lamp intergrated with a computer-controlled
monochromator
(Beijing
Optic
Instrument
Factory),
a
photochopper (PARC) and a lock-in amplifier (Signal Recovery) used for photocurrent detection. The absolute intensity of the incident light from the monochromator was measured with a radiometer/photometer (International Light). All IPCE measurements were carried out under potential-controlled conditions, with 0.65 V as applied potential versus Ag/AgCl reference electrode (i.e., 1.23 V vs. RHE, calculated from the Nernst equation: ERHE = EAg/AgCl + E°Ag/AgCl + 0.057·pH).
3. Results and discussion It is well established that α-Fe2O3 nanorod arrays can be facilely fabricated on FTO substrates in an aqueous solution containing Fe3+ as iron precursor [18,19,38]. In this study, HAuCl4 was added into the Fe3+ precursor solution during the solution
6
growth process, then the effect of HAuCl4 on the morphology of α-Fe2O3 nanorod arrays was investigated by SEM images. As shown in Figure 1, the pristine α-Fe2O3 film (Figure 1A) shows the morphology of nanorods as grown in an aqueous solution as previously reported, with average diameter of nanorods to be 50-100 nm after annealed at 750 °C. The film thickness was determined to be 600 nm as shown in inset of Figure 1A. The top view SEM images of AuFe-x (Figure 1B-H) reveals that HAuCl4 in precursor solution did not greatly change the nanorod morphology of obtained AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) films those also have nanorod diameter at 50-100 nm. One can find that all the AuFe-x films (insets in Figure 1C,F,H, not all the cross sectional SEM images for film thickness are shown) have very similar thickness of ~600 nm, when compared to the pristine α-Fe2O3 nanorod film. So, the presence of HAuCl4 in Fe3+ precursor solution will not infuence the morphology of as-grown α-Fe2O3 nanorod arrays. This is quite different from our previous observations that TiCl3 and ZrO(NO3)2 in Fe3+ precursor solution greatly changed the morphology of obtained α-Fe2O3 film, both the nanorod arrays and the film thickness [9, 13]. One possible reason may be related to the successful doping of Zr and Ti into the α-Fe2O3 crystal structure, whereas Au here can not be doped into α-Fe2O3 during the solution growth procedure, as evidenced by XPS analysis results (data not shown) that no obvious Au signals observed for all the AuFe-x films. Thus, it is reasonable that one can not find great morphology change for those AuFe-x films, as compared to the pristine α-Fe2O3 nanorod film. The reason of unseccessful doping of Au into α-Fe2O3 during the solution growth procedure is not clear, but may be
7
related to the good solubility and stability of AuCl4+ in acidic solution. In comparison, Ti, Zr and Sn can be doped into solution-based α-Fe2O3 [9,13,15], possibly due to the slow hydrolysis of Ti, Zr and Sn precursor in the hydrothermal condition. Additional thermodynamic study on oxide/solution interface reaction and the growth of α-Fe2O3 in aqueous solution containing dopant precursor may be helpful for clearly understanding the doping process of foreign metal ions into α-Fe2O3.
Figure 1 SEM images of (A) pristine α-Fe2O3 nanorod film, (B) AuFe-0.02, (C) AuFe-0.05, (D) AuFe-0.2, (E) AuFe-0.5, (F) AuFe-1, (G) AuFe-2, (H) AuFe-5, inset is the cross sectional image of each α-Fe2O3 nanorod film.
Spectral transmittance for these α-Fe2O3 nanorod films are shown in Figure 2. When compared to the pristine α-Fe2O3 nanorod film, no obvious change in the optical transmittance could be observed for α-Fe2O3 grown in HAuCl4-contained aqueous solution. All spectra show features typical of α-Fe2O3 films with an absorption onset of ~570 nm [39,40], with band gaps of 2.0-2.1 eV estimated from the Kubelka-Munk equation (inset of Figure 2) [39]. It is widely recoginzed that α-Fe2O3, an indirect semiconductor, has an optical band gap of 2.0-2.2 eV [1-2]. Then, the very 8
similar band gaps should not be the main reason for different PEC activities of these α-Fe2O3 nanorod films as photoanodes for solar water splitting.
Figure 2 Spectral transmittance and (inset) tauc plots of the AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) film.
Figure 3 shows the current–potential plots for the films acting as photoanodes under the irradiation of AM 1.5G light of 100 mW·cm-2 intensity. Dark scan linear sweep voltammagrams from 0 to +1.1 V vs. Ag/AgCl showed a small current in the range of 10-4 mA·cm-2. Upon illumination, the pristine α-Fe2O3 nanorod film showed pronounced photocurrent starting at ~+0.4 V and continues to increase to 0.46 mA·cm-2 at +1.0 V. In comparison to the pristine α-Fe2O3 nanorod film, all the AuFe-x films showed significant enhancement in photocurrent density in the sweep range of +0.4~+1.1 V. Further observation shows that addition of HAuCl4 in the precursor solution gave rise to a strong effect on the photocurrent−potential characteristics. For the AuFe-0.02 film, a very small amount of HAuCl4 could result in remarkable enhancement in photoresponse. As shown in inset of Figure 3, with the increasing 9
amount of HAuCl4 added in the precursor solution, the photocurrent density of the obtained AuFe-x films rapidly increased and then leveled out at ~0.23-0.25 mA·cm-2 at +1.0 V vs. Ag/AgCl. Especially for those AuFe-x films (AuFe-x, x = 0.5, 1, 2, 5) showing highest photoresponse, there is no specific saturation platform of photocurrent observed at more positive potential, which indicates efficient charge separation upon illumination [41]. This may be due to the effective passivation of surface states of α-Fe2O3 nanorods grown in HAuCl4–contained precursor solution. IPCE tests were performed to evaluate the spectral response for water photo-oxidation between 320 and 600 nm (at 1.23 V vs. RHE). As shown in Figure 4, the AuFe-2 film shows IPCE values much greater than the pristine α-Fe2O3 nanorod film, with IPCE increased by a factor of 3.2 (from 3.6% to 11.5%) at 365 nm. Overall these results indicate a positive effect of HAuCl4 in the precursor solution on the water oxidation photoactivity of the obtained α-Fe2O3 nanorod films. Given their very similar morphology and band gap, there should exist other influencing factors contributing to the enhanced photoactivity of α-Fe2O3 nanorod films grown in HAuCl4–contained precursor solution.
10
Figure 3 Photocurrent-potential curves of the AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) film as photoanode, inset shows the change in anodic photocurrent of the AuFe-x films with applied bias of 1.0 V vs. Ag/AgCl as a function of the volume of HAuCl4 aqueous solution added in the precursor solution.
Figure 4 IPCE of pristine α-Fe2O3 nanorod film and the AuFe-2 film as photoanodes at 0.65 V vs. Ag/AgCl (1.23 V vs. RHE).
The crystal structure of pristine α-Fe2O3 nanorod film and the AuFe-2 film was investigated by Raman spectroscopy. As shown in Figure 5, the main peaks of both 11
the films match with the reported values of α-Fe2O3 [42-44]. But unlike the pristine α-Fe2O3 nanorod film, a very obvious Raman band located at around 657.0 cm-1 was observed for the AuFe-2 film. While no Fe 2p XPS peak assigned to Fe2+ species (in the phase of FeO or Fe3O4) was observed (Figure S1), this Raman peak can be assigned to the surface disorder of α-Fe2O3 [45,46]. Such disorder was also detected by Raman spectra in doped hematite films [42-44]. In this study, it has been evidenced by XPS that no Au was doped into α-Fe2O3, and then the surface disorder should be resulted from the surface passivation of α-Fe2O3 which was grown in the HAuCl4-contained precursor solution. Monitoring on this additional Raman peak was also conducted by Cao et al. to investigate the surface passivation of α-Fe2O3 films [47]. They indicated that no observable Raman peak at 657.0 cm-1 means no surface passivation effect. Such surface disorder should be related to the lattice defects of α-Fe2O3 crystals, as detected by Raman spectroscopy, which has been considered as a useful tool to evidence the surface disorder [42-47]. However, further investigation deep into the detailed structure of such surface disorder is still of great necessity to reveal its inherent effects on the enhanced PEC activities of α-Fe2O3 films. In others’ previous studies, surface passivation was proposed as an effective method to enhance hematite PEC performance. As reported by Barroso et al., surface treatment of α-Fe2O3 films with a Ga2O3 thin layer that could play the role of surface passivation, resulting in a reduction of electron-hole recombination [48]. Zhang et al. developed a photoelectrochemical pretreatment method to passivate the surface states of undoped α-Fe2O3 photoanodes, which was proven to effectively reduce the
12
recombination centers of the photogenerated carriers [36]. In order to investigate the effect of surface passivation on the electron-hole recombination behavior, we used ultrafast transient absorption spectroscopy was used to investigate the charge carrier dynamics in α-Fe2O3 films following photoexcitation with a 500 nm pump. Figure 5B shows the normalized ultrafast transient absorption profiles of pristine α-Fe2O3 nanorod film and the AuFe-2 film for 0 – 10 and 0 – 300 ps time windows. By fitting the kinetic traces to a triple exponential of 1.8, 60, and 4600 ps, we find that the overall charge carrier decay for both α-Fe2O3 films is very fast. For this material, whose early time dynamics are dominated by intrinsic properties of α-Fe2O3 but not strongly influenced by morphology or thickness [25], this observation is typically attributed to the high density of electronic states in the bandgap caused by both internal and surface defects. At longer lifetimes, the absorption intensity of the decay profile for the AuFe-2 film is higher than that of the pristine α-Fe2O3 film. This result may indicate a reduction in electron–hole recombination in the AuFe-2 film at ultrafast time scales. By excluding the metal ion doping effect, this reduced electron–hole recombination should be resulted from surface passivation of α-Fe2O3, which contributed significantly to the enhanced PEC performance of the AuFe-x films.
13
Figure 5 (A) Raman spectra and (B) Normalized ultrafast transient absorption decay profiles of pristine α-Fe2O3 nanorod film and the AuFe-2 film in 0-10 ps (inset) and the 0-300 ps windows. They are fit simultaneously using a nonlinear least-squares fitting algorithm to a triple-exponential decay convolved with a Gaussian, representing the cross correlation of the 500 nm pump and 585 nm probe pulses. The solid lines are fitted curves.
As discussed above, HAuCl4 has played a surface passivation role for α-Fe2O3 nanorods grown an aqueous solution. In this study, H2PtCl6 was also added into the Fe3+ precursor solution (see experimental details in Supplementary Material) to further confirm the positive effects of surface passivation on the PEC activity of α-Fe2O3 nanorod films as photoandoes for water splitting. Very similar to HAuCl4, H2PtCl6 in Fe3+ precursor solution did not give rise to Pt doping in α-Fe2O3 as no Pt XPS signal was observed (XPS spectra are not shown), or greatly differ the nanorod morphology, film thickness (Figure S2) as well as optical band gap (Figure S3) of the obtained mPtFe (m = 0.5, 1, 2, 5, 10) films when compared to the pristine α-Fe2O3 film. Figure S4 shows the Raman spectra of the mPtFe (m = 0.5, 1, 2, 5, 10) films. All the films display typical Raman features of α-Fe2O3. By compared to the pristine 14
α-Fe2O3 nanorod film, the mPtFe films shows an additional Raman peak at ca. 657.0 cm-1, especially for the 2PtFe film the most obvious peak could be described. As discussed in the case of the AuFe-x films, this additional Raman peak means surface disorder of α-Fe2O3, and hence surface passivation which would lead to the enhancement in PEC water splitting by reducing electron-hole recombination. As shown in Figure S5, all the mPtFe films display greatly enhanced PEC performances. The highest PEC activity was observed for the 2PtFe film, with IPCE value 3.6 times higher than that of the pristine α-Fe2O3 nanorod film (12.9% vs. 3.6% at 365 nm, Figure S6). This could be explained by the surface disorder of the 2PtFe film as revealed by the Raman peak at ca. 657.0 cm-1 in Figure S4, which leads to surface passivation and reduced electron-hole recombination. One can also observe that the addition amounts of H2PtCl6 in the precursor solution gives rise to a strong effect on the PEC activities. With increasing amounts of H2PtCl4, the photocurrents increase first and reach to the maximal value for the 2PtFe film, and then begin to decrease as the amount of H2PtCl6 further increases (inset of Figure S5). This is quite different from the change tendency of the AuFe-x films depending on the amounts of HAuCl4 in Fe3+ precursor solution (Figure 3). It could be related to the different mechanisms of HAuCl4 and H2PtCl6 (e.g., Au3+ and Pt4+) influencing the surface structure of the α-Fe2O3 nanorods grown in the aqueous solution, which still needs further exploration for clear understanding the surface passivation role of HAuCl4 and H2PtCl6. Anyhow, in the present study, the surface passivation effects of HAuCl4 and H2PtCl6 in Fe3+ precursor solution was evidenced to effectively enhance the PEC activities of
15
solution-based undoped α-Fe2O3 nanorod films.
4. Conclusions In summary, a solution growth method was found to enhance the PEC performance of hematite nanorod films by surface passivation. It was supposed that noble-metal chlorides (e.g., HAuCl4 or H2PtCl6) in the Fe3+ precursor solution would not be doped into, but induced surface disorder of hematite. The surface disorder led to surface states passivation of hematite, and hence a reduction of electron-hole recombination as evidenced by ultrafast transient absorption spectroscopy. As a result, the hematite nanorod films grown in the Fe3+ aqueous solution containing noble-metal chlorides, HAuCl4 or H2PtCl6, showed much higher PEC activities that the pristine hematite nanorod film, with IPCE increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE. This study puts forward a new way to develop undoped hematite photoanodes for efficient water splitting. Further investigation is still of great necessity to understand the surface states of hematite.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 51102194, and no. 51121092), the Doctoral Program of the Ministry of Education (no. 20110201120040), and the Nano Research Program of Suzhou City (ZXG2013003). One of the authors (S. Shen) was supported by the Fundamental Research Funds for the Central Universities. The authors thank
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the Prof. Jin Z. Zhang’s lab at University of California, Santa Cruz for their contribution of the ultrafast transient absorption spectroscopy measurement.
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Graphic Abstract
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Research Highlights
HAuCl4 in precursor solution leads to surface passivation of hematite Surface passivation effectively improves photoelectrochemical performance of hematite nanorods A reduction in electron-hole recombination occurs in surface passivated hematite nanorods
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S0021-9797(13)00978-8 http://dx.doi.org/10.1016/j.jcis.2013.10.063 YJCIS 19185
To appear in:
Journal of Colloid and Interface Science
Please cite this article as: S. Shen, M. Li, L. Guo, J. Jiang, S.S. Mao, Surface Passivation of Undoped Hematite Nanorod Arrays via Aqueous Solution Growth for Improved Photoelectrochemical Water Splitting, Journal of Colloid and Interface Science (2013), doi: http://dx.doi.org/10.1016/j.jcis.2013.10.063
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Surface Passivation of Undoped Hematite Nanorod Arrays via Aqueous Solution Growth for Improved Photoelectrochemical Water Splitting
Shaohua Shen,1,2* Mingtao Li,1 Liejin Guo,1 Jiangang Jiang,1 Samuel S. Mao2
1. International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China. Email: [email protected] 2. Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, United States
Abstract: A facile solution-based strategy was found to be effective for surface passivation of undoped hematite nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe3+ precursor solution. XPS and Raman spectra revealed that noble-metal ions would not be doped into, but lead to surface disorder of hematite. Incident photon to current efficiency (IPCE) of the hematite photoanode grown in HAuCl4 and H2PtCl6 contained precursor solution was increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE (reversible hydrogen electrode). This photoelectrochemical (PEC) enhancement was ascribed to the surface passivation, which resulted in a recombination of
1
photogenerated carriers, as confirmed by ultrafast transient absorption spectroscopy. Keywords: hematite; surface passivation; photoelectrochemical water splitting
1. Introduction An optical band gap of 2.0-2.2 eV makes hematite (α-Fe2O3), the earth abundant, stable and nontoxic material, a very attractive candidate for solar energy conversion via a photoelectrochemical (PEC) water splitting process [1-4]. However, the solar energy conversion efficiency of α-Fe2O3 photoanodes is still far from the theoretical maximum efficiency [5]. It is well recognized that the short hole diffusion length (2−3 nm) and poor electronic conductivity in the bulk [6] is one of the main factors limiting the efficiency of α-Fe2O3 photoanodes. In the past decades, many efforts have been devoted to improving the conductivity of α-Fe2O3 by doping foreign elements such as Ti [7-9], Si [10,11], Zr [12,13], and Sn [14,15], which could in general increase the electron donor densities and certainly benefit charge carrier transport for the enhancement of photocurrent. Wang et al. reported that Ti-doped α-Fe2O3 prepared by a facile deposition-annealing method showed greatly enhanced photocurrent at a relatively low bias (1.83 mA·cm-2 at 1.02 V vs. RHE) due to the improved donor density and reduced electron-hole recombination at the time scale beyond a few picoseconds [16]. Besides foreign element doping, nanostructure design, aiming at improving charge collection efficiency, has been proven to be another effective strategy to shorten the distance the charge carriers have to travel for surface water splitting
reaction
before
recombination.
For
example,
dendritic
α-Fe2O3
nanostructures were developed by Kay et al. and an unprecedented high PEC water
2
splitting efficiency was obtained [17]. This could be attributed to the dendritic nanostructure minimizing the distance photogenerated holes have to diffuse to reach the α-Fe2O3/electrolyte interface. Since α-Fe2O3 nanorod arrays developed by Vayssieres et al. [18], which showed much higher PEC performance than nanoparticulate α-Fe2O3 [19], one-dimensional α-Fe2O3 nanostructures have been attracted growing interest for photoelectrodes, mainly because their design contributes to the channelized electron transport and delayed electron-hole recombination [20-24]. By further doping of metal ions, Ti-doped α-Fe2O3 nanorods [23] and Ni-doped α-Fe2O3 nanotubes [24] achieved very high PEC performances for water oxidation, with IPCE to be ~60% (at 350 nm and 1.53 V vs. RHE) and ~40% (at 400 nm and 0.45 V vs. Ag/AgCl), respectively. This could be attributed to reduced electron-hole recombination as well as improved donor density by metal ions doping. Another inherent disadvantage limiting efficiency is poor oxygen evolution reaction (OER) kinetics and the quick charge carrier recombination at the surface of α-Fe2O3 photoanodes.[3,25,26] To this end, surface treatment has been manipulated to boost PEC performance of α-Fe2O3 photoanodes. Some OER cocatalysts including IrO2 [27], Co-Pi [28-30], CoF3 [31], and CoOx [32,33], etc., were developed to modify the surface of α-Fe2O3 photoanodes by creating surface reactive sites to reduce the activation energy for water oxidation. Given surface states acting as the recombination centers of the photogenerated carriers, a 13-group oxide (Al2O3, Ga2O3, or In2O3) thin overlayer was deposited on the surface of α-Fe2O3 films to passivate the surface trapping states [34,35]. As a result, the PEC performances of α-Fe2O3 films
3
were greatly increased, by reducing the surface electron-hole recombination. Recently, Zhang et al. reported a facile strategy to passivate surface states of α-Fe2O3 photoanodes without introducing any OER catalyst or passivation overlayer [36]. By photoelectrochemically pretreating α-Fe2O3 films in saturated NaCl aqueous electrolyte, the recombination centers of the photogenerated carriers were effectively diminished, which gave rise to great enhancement in PEC performance. In this study, a facile solution-based strategy was found to be effective for surface passivation of α-Fe2O3 nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe3+ precursor solution. The presence of HAuCl4 and H2PtCl6 in precursor solution did not lead to doping of Au or Pt into α-Fe2O3, but induced surface disorder of α-Fe2O3 nanorods. The resulted surface passivation effect was proposed to greatly increase the PEC activity by reducing the charge carrier recombination at the surface of α-Fe2O3 films.
2. Experimental α-Fe2O3 nanorod photoanodes were grown on FTO (F-doped SnO2) glass by an aqueous chemical growth method reported previously [18], followed by a high temperature annealing activiation process [14]. The experiment was performed with chemicals of analytic grade in an aqueous solution containing 0.15 M of ferric chloride (FeCl3·6H2O) and 1 M of sodium nitrate (NaNO3), with pH value set at around 1.4 by HCl, in a cap-sealed glass bottle containing two back-to-back FTO glasses (Pilkington, TEC7) leaning against the inner wall of glass bottle. After heated in a regular oven at 100 °C for 24 h, the resultant yellow or light yellow films were
4
washed with distilled water and dried, then annealed at 750 °C for 5 min with ramping rate of 25 °C/min. For surface passivation of α-Fe2O3 nanorod photoanodes, a desired volume of HAuCl4 (0.01 M) aqueous solution was added, with the volume of final aqueous solution kept at 20 mL for heating treatment at 100 °C. A series of surface passivated α-Fe2O3 nanorod photoanodes were obtained by adding different volumes of HAuCl4 aqueous solution (i.e., 0.02, 0.05, 0.2, 0.5, 1, 2, 5 mL), which was denoted as AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5). Scanning electron microscopy (SEM) images were obtained with a Hitachi environmental field emission scanning electron microscope (Model S-4300SE/N) operating in secondary electron detection mode. Spectral transmittance measurements were taken on the samples with a commercial thin film metrology system (Scientific Computing International, FilmTek Par 3000 SE). Raman scattering study was performed on a Jobin Yvon LabRAM HR spectrometer using 514.5 nm irradiation from an argon ion laser at 20 mW. X-ray diffraction (XRD) measurements were performed on a Siemens D5000 diffractometer with Cu Kα radiation. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD) with monochromatic Al Kα radiation (hν = 1486.69 eV) and with a concentric hemispherical analyzer. The ultrafast laser system is based on a Quantronix-designed femtosecond transient absorption laser system which consists of an Er-doped fiber oscillator, a regenerative amplifier, and a diode-pumped, Q-switched, second-harmonic Nd:YLF pump laesr (527 nm, 10 W capacity) [37]. PEC measurements were conducted using a potentiostat (Pine Instruments
5
Bipotentiostat) in 0.5 M NaCl (pH = 6.7) aqueous solution using a three-electrode configuration, with the pristine or surface passivated α-Fe2O3 photoanode as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. N2 gas was continuously bubbled in solution before and during the experiment to remove any dissolved O2 and therefore suppress the reduction of O2 at the counter electrode. A 1 cm2 masked-off, sealed area of the α-Fe2O3 sample was irradiated with a 300 W Xe lamp solar simulator with adjustable power settings through an AM 1.5G filter (Oriel). The light intensity at the sample location in the photoelectrochemical cell was 100 mW·cm-2 as measured by a power detector (Newport). IPCE measurements were performed using a 300 W Xe lamp intergrated with a computer-controlled
monochromator
(Beijing
Optic
Instrument
Factory),
a
photochopper (PARC) and a lock-in amplifier (Signal Recovery) used for photocurrent detection. The absolute intensity of the incident light from the monochromator was measured with a radiometer/photometer (International Light). All IPCE measurements were carried out under potential-controlled conditions, with 0.65 V as applied potential versus Ag/AgCl reference electrode (i.e., 1.23 V vs. RHE, calculated from the Nernst equation: ERHE = EAg/AgCl + E°Ag/AgCl + 0.057·pH).
3. Results and discussion It is well established that α-Fe2O3 nanorod arrays can be facilely fabricated on FTO substrates in an aqueous solution containing Fe3+ as iron precursor [18,19,38]. In this study, HAuCl4 was added into the Fe3+ precursor solution during the solution
6
growth process, then the effect of HAuCl4 on the morphology of α-Fe2O3 nanorod arrays was investigated by SEM images. As shown in Figure 1, the pristine α-Fe2O3 film (Figure 1A) shows the morphology of nanorods as grown in an aqueous solution as previously reported, with average diameter of nanorods to be 50-100 nm after annealed at 750 °C. The film thickness was determined to be 600 nm as shown in inset of Figure 1A. The top view SEM images of AuFe-x (Figure 1B-H) reveals that HAuCl4 in precursor solution did not greatly change the nanorod morphology of obtained AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) films those also have nanorod diameter at 50-100 nm. One can find that all the AuFe-x films (insets in Figure 1C,F,H, not all the cross sectional SEM images for film thickness are shown) have very similar thickness of ~600 nm, when compared to the pristine α-Fe2O3 nanorod film. So, the presence of HAuCl4 in Fe3+ precursor solution will not infuence the morphology of as-grown α-Fe2O3 nanorod arrays. This is quite different from our previous observations that TiCl3 and ZrO(NO3)2 in Fe3+ precursor solution greatly changed the morphology of obtained α-Fe2O3 film, both the nanorod arrays and the film thickness [9, 13]. One possible reason may be related to the successful doping of Zr and Ti into the α-Fe2O3 crystal structure, whereas Au here can not be doped into α-Fe2O3 during the solution growth procedure, as evidenced by XPS analysis results (data not shown) that no obvious Au signals observed for all the AuFe-x films. Thus, it is reasonable that one can not find great morphology change for those AuFe-x films, as compared to the pristine α-Fe2O3 nanorod film. The reason of unseccessful doping of Au into α-Fe2O3 during the solution growth procedure is not clear, but may be
7
related to the good solubility and stability of AuCl4+ in acidic solution. In comparison, Ti, Zr and Sn can be doped into solution-based α-Fe2O3 [9,13,15], possibly due to the slow hydrolysis of Ti, Zr and Sn precursor in the hydrothermal condition. Additional thermodynamic study on oxide/solution interface reaction and the growth of α-Fe2O3 in aqueous solution containing dopant precursor may be helpful for clearly understanding the doping process of foreign metal ions into α-Fe2O3.
Figure 1 SEM images of (A) pristine α-Fe2O3 nanorod film, (B) AuFe-0.02, (C) AuFe-0.05, (D) AuFe-0.2, (E) AuFe-0.5, (F) AuFe-1, (G) AuFe-2, (H) AuFe-5, inset is the cross sectional image of each α-Fe2O3 nanorod film.
Spectral transmittance for these α-Fe2O3 nanorod films are shown in Figure 2. When compared to the pristine α-Fe2O3 nanorod film, no obvious change in the optical transmittance could be observed for α-Fe2O3 grown in HAuCl4-contained aqueous solution. All spectra show features typical of α-Fe2O3 films with an absorption onset of ~570 nm [39,40], with band gaps of 2.0-2.1 eV estimated from the Kubelka-Munk equation (inset of Figure 2) [39]. It is widely recoginzed that α-Fe2O3, an indirect semiconductor, has an optical band gap of 2.0-2.2 eV [1-2]. Then, the very 8
similar band gaps should not be the main reason for different PEC activities of these α-Fe2O3 nanorod films as photoanodes for solar water splitting.
Figure 2 Spectral transmittance and (inset) tauc plots of the AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) film.
Figure 3 shows the current–potential plots for the films acting as photoanodes under the irradiation of AM 1.5G light of 100 mW·cm-2 intensity. Dark scan linear sweep voltammagrams from 0 to +1.1 V vs. Ag/AgCl showed a small current in the range of 10-4 mA·cm-2. Upon illumination, the pristine α-Fe2O3 nanorod film showed pronounced photocurrent starting at ~+0.4 V and continues to increase to 0.46 mA·cm-2 at +1.0 V. In comparison to the pristine α-Fe2O3 nanorod film, all the AuFe-x films showed significant enhancement in photocurrent density in the sweep range of +0.4~+1.1 V. Further observation shows that addition of HAuCl4 in the precursor solution gave rise to a strong effect on the photocurrent−potential characteristics. For the AuFe-0.02 film, a very small amount of HAuCl4 could result in remarkable enhancement in photoresponse. As shown in inset of Figure 3, with the increasing 9
amount of HAuCl4 added in the precursor solution, the photocurrent density of the obtained AuFe-x films rapidly increased and then leveled out at ~0.23-0.25 mA·cm-2 at +1.0 V vs. Ag/AgCl. Especially for those AuFe-x films (AuFe-x, x = 0.5, 1, 2, 5) showing highest photoresponse, there is no specific saturation platform of photocurrent observed at more positive potential, which indicates efficient charge separation upon illumination [41]. This may be due to the effective passivation of surface states of α-Fe2O3 nanorods grown in HAuCl4–contained precursor solution. IPCE tests were performed to evaluate the spectral response for water photo-oxidation between 320 and 600 nm (at 1.23 V vs. RHE). As shown in Figure 4, the AuFe-2 film shows IPCE values much greater than the pristine α-Fe2O3 nanorod film, with IPCE increased by a factor of 3.2 (from 3.6% to 11.5%) at 365 nm. Overall these results indicate a positive effect of HAuCl4 in the precursor solution on the water oxidation photoactivity of the obtained α-Fe2O3 nanorod films. Given their very similar morphology and band gap, there should exist other influencing factors contributing to the enhanced photoactivity of α-Fe2O3 nanorod films grown in HAuCl4–contained precursor solution.
10
Figure 3 Photocurrent-potential curves of the AuFe-x (x = 0.02, 0.05, 0.2, 0.5, 1, 2, 5) film as photoanode, inset shows the change in anodic photocurrent of the AuFe-x films with applied bias of 1.0 V vs. Ag/AgCl as a function of the volume of HAuCl4 aqueous solution added in the precursor solution.
Figure 4 IPCE of pristine α-Fe2O3 nanorod film and the AuFe-2 film as photoanodes at 0.65 V vs. Ag/AgCl (1.23 V vs. RHE).
The crystal structure of pristine α-Fe2O3 nanorod film and the AuFe-2 film was investigated by Raman spectroscopy. As shown in Figure 5, the main peaks of both 11
the films match with the reported values of α-Fe2O3 [42-44]. But unlike the pristine α-Fe2O3 nanorod film, a very obvious Raman band located at around 657.0 cm-1 was observed for the AuFe-2 film. While no Fe 2p XPS peak assigned to Fe2+ species (in the phase of FeO or Fe3O4) was observed (Figure S1), this Raman peak can be assigned to the surface disorder of α-Fe2O3 [45,46]. Such disorder was also detected by Raman spectra in doped hematite films [42-44]. In this study, it has been evidenced by XPS that no Au was doped into α-Fe2O3, and then the surface disorder should be resulted from the surface passivation of α-Fe2O3 which was grown in the HAuCl4-contained precursor solution. Monitoring on this additional Raman peak was also conducted by Cao et al. to investigate the surface passivation of α-Fe2O3 films [47]. They indicated that no observable Raman peak at 657.0 cm-1 means no surface passivation effect. Such surface disorder should be related to the lattice defects of α-Fe2O3 crystals, as detected by Raman spectroscopy, which has been considered as a useful tool to evidence the surface disorder [42-47]. However, further investigation deep into the detailed structure of such surface disorder is still of great necessity to reveal its inherent effects on the enhanced PEC activities of α-Fe2O3 films. In others’ previous studies, surface passivation was proposed as an effective method to enhance hematite PEC performance. As reported by Barroso et al., surface treatment of α-Fe2O3 films with a Ga2O3 thin layer that could play the role of surface passivation, resulting in a reduction of electron-hole recombination [48]. Zhang et al. developed a photoelectrochemical pretreatment method to passivate the surface states of undoped α-Fe2O3 photoanodes, which was proven to effectively reduce the
12
recombination centers of the photogenerated carriers [36]. In order to investigate the effect of surface passivation on the electron-hole recombination behavior, we used ultrafast transient absorption spectroscopy was used to investigate the charge carrier dynamics in α-Fe2O3 films following photoexcitation with a 500 nm pump. Figure 5B shows the normalized ultrafast transient absorption profiles of pristine α-Fe2O3 nanorod film and the AuFe-2 film for 0 – 10 and 0 – 300 ps time windows. By fitting the kinetic traces to a triple exponential of 1.8, 60, and 4600 ps, we find that the overall charge carrier decay for both α-Fe2O3 films is very fast. For this material, whose early time dynamics are dominated by intrinsic properties of α-Fe2O3 but not strongly influenced by morphology or thickness [25], this observation is typically attributed to the high density of electronic states in the bandgap caused by both internal and surface defects. At longer lifetimes, the absorption intensity of the decay profile for the AuFe-2 film is higher than that of the pristine α-Fe2O3 film. This result may indicate a reduction in electron–hole recombination in the AuFe-2 film at ultrafast time scales. By excluding the metal ion doping effect, this reduced electron–hole recombination should be resulted from surface passivation of α-Fe2O3, which contributed significantly to the enhanced PEC performance of the AuFe-x films.
13
Figure 5 (A) Raman spectra and (B) Normalized ultrafast transient absorption decay profiles of pristine α-Fe2O3 nanorod film and the AuFe-2 film in 0-10 ps (inset) and the 0-300 ps windows. They are fit simultaneously using a nonlinear least-squares fitting algorithm to a triple-exponential decay convolved with a Gaussian, representing the cross correlation of the 500 nm pump and 585 nm probe pulses. The solid lines are fitted curves.
As discussed above, HAuCl4 has played a surface passivation role for α-Fe2O3 nanorods grown an aqueous solution. In this study, H2PtCl6 was also added into the Fe3+ precursor solution (see experimental details in Supplementary Material) to further confirm the positive effects of surface passivation on the PEC activity of α-Fe2O3 nanorod films as photoandoes for water splitting. Very similar to HAuCl4, H2PtCl6 in Fe3+ precursor solution did not give rise to Pt doping in α-Fe2O3 as no Pt XPS signal was observed (XPS spectra are not shown), or greatly differ the nanorod morphology, film thickness (Figure S2) as well as optical band gap (Figure S3) of the obtained mPtFe (m = 0.5, 1, 2, 5, 10) films when compared to the pristine α-Fe2O3 film. Figure S4 shows the Raman spectra of the mPtFe (m = 0.5, 1, 2, 5, 10) films. All the films display typical Raman features of α-Fe2O3. By compared to the pristine 14
α-Fe2O3 nanorod film, the mPtFe films shows an additional Raman peak at ca. 657.0 cm-1, especially for the 2PtFe film the most obvious peak could be described. As discussed in the case of the AuFe-x films, this additional Raman peak means surface disorder of α-Fe2O3, and hence surface passivation which would lead to the enhancement in PEC water splitting by reducing electron-hole recombination. As shown in Figure S5, all the mPtFe films display greatly enhanced PEC performances. The highest PEC activity was observed for the 2PtFe film, with IPCE value 3.6 times higher than that of the pristine α-Fe2O3 nanorod film (12.9% vs. 3.6% at 365 nm, Figure S6). This could be explained by the surface disorder of the 2PtFe film as revealed by the Raman peak at ca. 657.0 cm-1 in Figure S4, which leads to surface passivation and reduced electron-hole recombination. One can also observe that the addition amounts of H2PtCl6 in the precursor solution gives rise to a strong effect on the PEC activities. With increasing amounts of H2PtCl4, the photocurrents increase first and reach to the maximal value for the 2PtFe film, and then begin to decrease as the amount of H2PtCl6 further increases (inset of Figure S5). This is quite different from the change tendency of the AuFe-x films depending on the amounts of HAuCl4 in Fe3+ precursor solution (Figure 3). It could be related to the different mechanisms of HAuCl4 and H2PtCl6 (e.g., Au3+ and Pt4+) influencing the surface structure of the α-Fe2O3 nanorods grown in the aqueous solution, which still needs further exploration for clear understanding the surface passivation role of HAuCl4 and H2PtCl6. Anyhow, in the present study, the surface passivation effects of HAuCl4 and H2PtCl6 in Fe3+ precursor solution was evidenced to effectively enhance the PEC activities of
15
solution-based undoped α-Fe2O3 nanorod films.
4. Conclusions In summary, a solution growth method was found to enhance the PEC performance of hematite nanorod films by surface passivation. It was supposed that noble-metal chlorides (e.g., HAuCl4 or H2PtCl6) in the Fe3+ precursor solution would not be doped into, but induced surface disorder of hematite. The surface disorder led to surface states passivation of hematite, and hence a reduction of electron-hole recombination as evidenced by ultrafast transient absorption spectroscopy. As a result, the hematite nanorod films grown in the Fe3+ aqueous solution containing noble-metal chlorides, HAuCl4 or H2PtCl6, showed much higher PEC activities that the pristine hematite nanorod film, with IPCE increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE. This study puts forward a new way to develop undoped hematite photoanodes for efficient water splitting. Further investigation is still of great necessity to understand the surface states of hematite.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (no. 51102194, and no. 51121092), the Doctoral Program of the Ministry of Education (no. 20110201120040), and the Nano Research Program of Suzhou City (ZXG2013003). One of the authors (S. Shen) was supported by the Fundamental Research Funds for the Central Universities. The authors thank
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the Prof. Jin Z. Zhang’s lab at University of California, Santa Cruz for their contribution of the ultrafast transient absorption spectroscopy measurement.
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Graphic Abstract
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Research Highlights
HAuCl4 in precursor solution leads to surface passivation of hematite Surface passivation effectively improves photoelectrochemical performance of hematite nanorods A reduction in electron-hole recombination occurs in surface passivated hematite nanorods
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