Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
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Design Principles for Construction of Charge Transport Channels in Particle-Assembled Water-Splitting Photoelectrodes Ningsi Zhang,† Haoliang Zheng,† Yongsheng Guo,† Jianyong Feng,† Zhaosheng Li,*,†,‡ and Zhigang Zou† †
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Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, People’s Republic of China ‡ Jiangsu Key Laboratory for Nano Technology, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: Charge transport in a photoelectrode largely influences its solar-to-chemical energy conversion efficiency. Especially, particle-assembled photoanode films are always subject to poor interparticle electron transport. The electron transport channel between the particles is usually constructed by using a necking process, in order to overcome the poor interparticle electron transport. In this study, Sb-doped SnO2, as a transparent and degenerate semiconductor, has been found to act as a novel necking material for constructing electron transport channels in particle-assembled photoanode films. As a result, it greatly improves the water-splitting performances of particle-assembled α-Fe2O3 photoanodes. We have formulated design principles for construction of charge transport channels in particle-assembled water-splitting photoelectrodes. KEYWORDS: Photoanode, Particle-assembled electrode, Necking, Charge transport, Solar energy conversion
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splitting.13−17 “Host−guest”, in which photoanode materials (for example α-Fe2O3) are grown on the conductive skeleton, is the other strategy to reduce the electron−hole recombination in the photoanodes.18−23 These methods may reduce the electron−hole recombination in the photoanodes, thus significantly improving their photoelectrochemical performances for solar hydrogen production. In many cases, it is difficult to directly prepare a photoelectrode film, whether on a conductive substrate or conductive skeleton.9,24−28 Photoelectrode materials are first synthesized in the form of dispersed particles and then are deposited onto FTO conductive glass substrates by electrophoretic deposition, gelatinization, or blade coating, thus obtaining a particleassembled photoanode film. Compared with the directly prepared planar films of photoelectrode, the connection between nanoparticles of the particle-assembled film is very loose and there is a great contact resistance between the nanoparticles. In the particle-assembled film, it is requisite to not only increase intraparticle electron transfer of the semiconductor particles but also improve interparticle electron transfer. The bonding between the nanoparticles is generally enhanced by subsequent calcination treatment to enhance
INTRODUCTION The bottleneck that restricts the development of solar hydrogen production by photoelectrochemical water splitting is always the poor water-oxidation performance of photoanodes.1,2 The experimental photocurrent densities of most photoanodes are much lower than their theoretical values for water splitting. One of the important reasons is that the large electric resistance of the semiconductor photoanode seriously hinders the charge transport, resulting in bulk recombination of photoinduced electrons and holes. The photogenerated electrons, as the majority carriers for n-type semiconductor photoanode materials, are required to pass from the space charge layer through the entire photoanode film to the conductive substrate, for example fluorine-doped tin oxide (FTO) coated glass. Therefore, a photoanode material should exhibit good electrical conductivity.3−6 Unfortunately, most photoanode materials have relatively high resistivity, and their photoelectrochemical performances are seriously restricted by poor electron transport.7−12 Two strategies were adopted to overcome the poor electrical conductivity of photoanode films. One is using high-valence ions for n-type doping to increase the electron concentration of photoanodes, thereby enhancing the electrical conductivity of photoanode materials. Doping is an effective means to improve the intraparticle charge transfer of semiconductor materials, thereby effectively increasing the photocurrents for solar water © 2019 American Chemical Society
Received: February 22, 2019 Revised: April 16, 2019 Published: May 14, 2019 10509
DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
Research Article
ACS Sustainable Chemistry & Engineering electrical conductivity.29 In fact, it is difficult to eliminate the grain boundary resistance between the nanoparticles by this method. Therefore, a necking treatment, which is a method of coating nanoparticles with a material having a good conductivity to construct an electron transport channel filled between particles, is widely used in particle-assembled photoanodes.30,31 In our previous work, the effect of grain boundary scattering of polycrystalline grains was weakened by growing large single crystal particles.32 Further, by improving the electron transport between the particles, the photoelectrochemical performances for solar water splitting are significantly prompted.6,33 This study is intended to further discuss the choice of materials for constructing the electron transport channels.
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oxide photoanodes. The conductivity of the doped tin dioxide can reach up to 104 S/cm.34−37 From the viewpoint of conductivity, the doped SnO2 is more suitable as a necking material than TiO2. For semiconductor photoanode materials, the choice of necking materials must also consider whether their conduction band positions are lower (more positive) than those of photoanodes or not. In the particle-assembled photoanode, the electron transport route is from the nanoparticle photoanode to necking materials and then to FTO substrates. As shown in Scheme 2a, photogenerated electrons need to pass over the barrier of 0.57 eV in the process, transporting from the αFe2O3 conduction band to the FTO via titanium oxide. If pure SnO2 is used instead of TiO2 as the necking material, the barrier that the electrons need to cross in this process is 0.28 eV as shown in Scheme 2b. Considering the effect of doping, the conduction band position of Sbdoped SnO2 (Sb:SnO2) should be lower, as indicated in Scheme 2c.38−41 From the perspective of conduction band matching, Sb:SnO2 is more advantageous than TiO2 as a necking substance. On the basis of the above reasons of electrical conductivity and conduction band matching, we used Sb-doped SnO2, instead of TiO2, as the necking substance of the α-Fe2O3 photoelectrodes. The particle-assembled α-Fe2O3 film has been prepared onto FTO substrates, XRD patterns of which are shown in Supporting Information Figure S1. The photoelectrochemical performances of the particleassembled α-Fe2O3 films have been measured in 1 mol/L NaOH solution under irradiation of AM 1.5G standard simulated sunlight (100 mW/cm2). As shown in Figure 1, the photocurrent density of the
EXPERIMENTAL SECTION
In a typical particle-assembled photoanode, the electron transport paths with or without necking treatment are shown in Scheme 1.
Scheme 1. Electron Transport Pathways in ParticleAssembled Photoanode Film: (a) without and (b) with the Necking Treatment
Photoinduced electrons should transfer from the photoanode to the conductive substrate. p-type semiconductors are not suitable as necking materials for particle-assembled photoanodes (n-type semiconductor). Due to the electric field direction of the p/n junction, the electron transport direction is from the necking material to the photoanode, which will obstruct the water oxidation process. As a result, n-type semiconductors, instead of p-type semiconductors, are commonly used as necking materials for photoanodes. As an n-type necking material for constructing an electron transport channel for particle-assembled photoanodes, it is necessary to have a good visible light transmittance and a sufficiently small resistivity. Based on these two requirements, the degenerate semiconductors with wideband gaps, such as SnO2, In2O3, and ZnO, are reasonable choices. Although In2O3 and ZnO have good electrical conductivity, SnO2 is a more reasonable choice for its chemical stability in a strongly alkaline environment, which is in favor of accelerating water oxidation over iron
Figure 1. Photocurrents of iron oxide photoanodes with different necking treatment conditions: AM 1.5G simulated sunlight irradiation (100 mW/cm2); 1 mol/L NaOH solution. particle-assembled α-Fe2O3 films without any necking treatment is less than 0.1 mA/cm2 under the bias of 1.6 VRHE, and the photocurrent density of the samples treated with TiCl4 (in fact TiO2 as necking
Scheme 2. Energy Barrier of Electron Transfer in Particle-Assembled α-Fe2O3 Photoanodes: (a) TiO2 as Necking Material, (b) SnO2 as Necking Material, and (c) Sb:SnO2 as Necking Material
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DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
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material) is 0.25 mA/cm2. By adding 6% NbCl5 in TiCl4, the photocurrent density of the particle-assembled α-Fe2O3 films is further increased to 0.80 mA/cm2, because the pentavalent Nb element is added to TiCl4 for increasing the electron concentration in the TiO2 necking material. For the 9% SbCl3-doped SnCl4-treated sample (in fact 9% Sb-doped SnO2 as necking material), the photocurrent density of the particle-assembled α-Fe2O3 films reached 1.15 mA/cm2 at 1.6 VRHE. The use of 9% Sb-doped SnCl4 as a reagent for necking treatment can exhibit the best photoelectrochemical performances for solar water splitting. Figure S2 also showed that the samples with necking treatment 9% Sb-doped SnO2 had the highest monochromatic incident photon-to-electron conversion efficiency (IPCE). It is proved that 9% Sb:SnO2 can act as a good necking substance of the α-Fe2O3 photoanodes. In the preparation process of FTO glass, the doping concentration of F element determines the conductivity of the tin dioxide film.21−23 Similarly, the doping concentration of Sb also affects the electrical conductivity of the electrical connection layer in our experiments, thus affecting the photoelectrochemical performances of the particleassembled α-Fe2O3 films. Sb3+ is easily formed in SnO2 when the Sb doping concentration is low, while Sb5+ is more easily formed when the Sb doping concentration is higher.42−44 The low valence state Sb3+doped SnO2 may behave as a p-type semiconductor, while the high valence state Sb5+-doped SnO2 behaves as an n-type semiconductor. The saturated photocurrents density of the particle-assembled α-Fe2O3 films treated with different Sb doping concentrations of SnCl4 are shown in the Figure 2.
Research Article
RESULTS AND DISCUSSION
Since the low-concentration Sb3+-doped SnO2 may behave as a p-type semiconductor, the p/n junction formed by Sb:SnO2 and α-Fe2O3 hinders electron transport, and thus the saturation photocurrent density of the particle-assembled α-Fe2O3 films with necking of (1−3%) Sb:SnO2 is comparable with the samples with bare SnO2 necking. The saturation photocurrent density reaches approximately 0.3−0.4 mA/cm2 at the Sb doping concentration of 0−3%. Thereafter, as the Sb doping concentration in SnO2 increases, Sb:SnO2 exhibits n-type characteristics and acts as an electron transport channel, so the saturated photocurrents of the particle-assembled α-Fe2O3 films are significantly improved. The saturated photocurrent density of the particle-assembled α-Fe2O3 films reaches a maximum at the Sb doping concentration of 9%. When the doping concentration of Sb exceeds 9%, Sb5+ can no longer enter the lattice to replace Sn4+, and excess Sb5+ exists in the gap of lattice or precipitates in the form of Sb2O5. The excess Sb becomes an electron scattering center, which causes the conductivity to decrease. Therefore, the saturated photocurrent density shows a downward trend. As shown in the XPS spectra of Figure 3a, the binding energy of Sn 3d5/2 in undoped SnO2 is 486.4 eV, while that in Sb-doped SnO2 is 487−488 eV. The increase of binding energy indicates that Sb element is effectively incorporated into the SnO2 lattice. As illustrated in Figure 3b and Figure S3, the binding energy of Sb 3d for 9% Sb- and 3% Sb-doped SnO2 corresponds to Sb5+ and Sb3+, respectively. This could be used as evidence for valence changes of Sb doping in SnO2 as discussed above. In order to study the role of necking treatment for improving photoelectrochemical performances, the electrochemical impedance spectroscopy was adopted to analyze the pristine, SnCl4-treated and Sb-doped SnCl4-treated α-Fe2O3 samples (Figure 4). The fitting data of the electrochemical impedance spectrum in Figure 4 is shown in Table S1, which is similar to the literature.45 Compared with the bare sample, the bulk resistance (R1) and interfacial resistance (R2) of the sample subjected to necking treatment were significantly reduced, and this performance was more pronounced as the Sb doping content increased. Although the system resistance (Rs) is slightly increased, it can be seen that when the Sb doping concentration is 9%, the bulk resistance and the interface resistance of the photoelectrode are indeed minimal (see Table S1); meanwhile, the photocurrent density of the film is the largest. This result indicates that the necking treatment effectively improves the electron transport
Figure 2. Effects of Sb doping concentration in SnO2 on the photocurrent density of the particle-assembled α-Fe2O3 film at 1.6 VRHE.
Figure 3. X-ray photoelectron spectroscopy of (a) Sn 3d in Sb:SnO2 and (b) Sb 3d in Sb:SnO2. 10511
DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
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Figure 4. (a−c) AC electrochemical impedance spectrum of iron oxide photoanodes with different necking treatment conditions at 1.6 VRHE; (d) circuit used in the fitting.
Figure 5. X-ray photoelectron spectroscopy of α-Fe2O3 photoanodes with different necking treatment conditions: (a) Fe 2p for none; (b) Fe 2p for SnCl4 necking; (c) Fe 2p for 9% Sb-doped SnCl4 necking; (d) Fe 2p for TiCl4 necking and calcined at 750 °C for 1 h.
For the particle-assembled α-Fe2O3 photoanodes with necking treatment of Sb-doped SnO2, their photoelectrochemical performances may be also improved, if Sn element is
performance, thereby improving the photoelectrochemical performance of the photoanodes. 10512
DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
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ACS Sustainable Chemistry & Engineering incorporated into the crystalline structure of α-Fe2O3 during the annealing process. In order to exclude the influence of this factor, the samples were analyzed by X-ray photoelectron spectroscopy (XPS). The corresponding test results are shown in Figure 5. Comparing Figure 5a−c, the difference in peak position corresponding to Fe 2p3/2 is only 0.14 eV at the maximum. Considering the fitting error and the accuracy of instrument measurement, it can be considered that the binding energy has no chemical shift; that is, the binding state of Fe atom and O atom has not changed. The peaks at 714.5 eV in Figure 5b,c, which may be masked by the satellite peak of Fe 2p3/2, correspond to Sn 3p3/2. It can be concluded that no Sn4+ was doped into α-Fe2O3 after the necking process. For comparison, the sample necking with TiCl4 was calcined at 750 °C for 1 h. The Ti elements diffuse into the crystal lattice of Fe2O3 to form an effective doping. Without the interference of Sn 3p3/2, the binding energy of Fe 2p in Figure 5d is significantly different from those in Figure 5a−c (∼0.9 eV). In addition, the Mott− Schottky test in Figure S4 also showed no significant change in the carrier concentrations of the samples, indicating that Sn4+ or Ti4+ doping did not occur during the necking process. Therefore, the reduction in bulk resistance may be caused by the charge transport channels, which is formed by the Sb-doped SnO2. Figures S5 and S6 show the surface topography of the α-Fe2O3 photoanode samples with and without necking treatment. The necking treatment did not cause any damage to the electrode topography. The surface of the particles in the Figure S5 is smooth, and the morphology does not change. Therefore, the effects of surface topography can be ruled out when comparing the performance of various necking treatment samples. The change in performance may be independent of the surface topography of the sample. From the high-resolution TEM images of Figure 6, there is a coating layer on the surface of the
necking treatment affects the light absorption of the photoelectrode must be investigated.46−48 The UV−visible absorption spectra and transmission spectra of the α-Fe2O3 photoanodes are given in Figure 7 and Figure S9, respectively. The samples
Figure 7. UV−visible absorption spectra of α-Fe2O3 photoanodes with different necking treatment conditions.
after treatment with TiCl4 or SnCl4 have an obvious increase in absorption, while their absorption edges are similar to that of the as-prepared α-Fe2O3 photoanode without any necking treatment. Under the irradiation of Xe lamp, the Sb-doped SnCl4 necking sample was stable in 1 mol/L NaOH at 1.6 VRHE and the photochemical stability is similar to that of the sample treated with TiCl4 necking (shown in Figure 8). Performance degradation might come from not only corrosion of the necking material in the alkaline solution but also enrichment of the gas product in the pores.
Figure 6. HR-TEM pictures of α-Fe2O3 photoanode (a, b) with and (c, d) without necking treatment.
Figure 8. Photochemical stability of α-Fe2O3 photoanodes treated with 9% Sb-doped SnCl4 and 6% Nb-doped TiCl4 as necking reagent and without necking reagent.
necked sample, while the sample without necking treatment has a clean surface. Element mapping of the α-Fe2O3 photoanode with SnO2 necking is shown in Figure S7. Figures 6, S7, and S8 (energy dispersive X-ray spectra) confirm that the necking treatment does form an oxide coating on the surface of the nanoparticles. As a conventional transparent conductive oxides material, tin dioxide has a good visible light transmittance. Whether the
Tin dioxide can completely replace titanium dioxide as the necking treatment material of α-Fe2O3 photoanode, and it performs well in photoelectrochemical performance and stability under alkaline conditions. The α-Fe2O3 photoanode with Sb-doped SnO2 as the necking treatment has better photoelectrochemical performance than the α-Fe2O3 photoanode treated with Nb-doped TiO2. Therefore, Sb-doped SnO2 is more advantageous than TiO2 as an electron transport channel 10513
DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
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ACS Sustainable Chemistry & Engineering
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for constructing an iron oxide photoelectrode. The photoelectrochemical performance of the α-Fe2O3 photoanode increases with the increase of the conductivity of the electron transport channel, and the optimal Sb doping concentration is 9%. The improvement in the performance of the α-Fe2O3 photoelectrode is attributed to the increased conductivity of the film, rather than the doping of the Sn element.
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CONCLUSION In this study, Sb-doped SnO2 was found to be a good necking material for particle-assembled α-Fe2O3 photoanodes. For the first time, we summarized some basic principles of a good necking material for particle-assembled photoanodes. The first principle is that the good necking material may be an n-type degenerate semiconductor with good conductivity. The second one is that the conduction band position should be more positive than that of the particle-assembled photoanodes, which favors the transfer of the photogenerated electrons from the photoanode particles to the necking material. Another is that good necking material should exhibit good chemical stability in an alkaline electrolyte and thermal stability.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01067. Physical and chemical characterization of α-Fe2O 3 photoanodes (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jianyong Feng: 0000-0003-4275-8306 Zhaosheng Li: 0000-0001-8114-0432 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Key Research and Development Program of China (Grant No. 2018YFA0209303), the National Natural Science Foundation of China (Grant Nos. U1663228 and 21473090), and the Project Funded by the Priority Academic Program Development of Jangsu Higher Education Institutions.
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DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
Research Article
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DOI: 10.1021/acssuschemeng.9b01067 ACS Sustainable Chem. Eng. 2019, 7, 10509−10515
