Transparent Ta2O5 Protective Layer for Stable Silicon Photocathode

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Materials and Interfaces

Transparent Ta2O5 Protective Layer for Stable Silicon Photocathode under Full Solar Spectrum Tuo Wang, Shanshan Liu, Huimin Li, Chengcheng Li, Zhibin Luo, and Jinlong Gong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00147 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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Industrial & Engineering Chemistry Research

Transparent Ta2O5 Protective Layer for Stable Silicon Photocathode under Full Solar Spectrum

Tuo Wang,ab Shanshan Liu,ab Huimin Li,ab Chengcheng Li,ab Zhibin Luo,ab and Jinlong Gong*ab

aKey

Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China bCollaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.

Corresponding Authors: [email protected].

ACS Paragon Plus Environment

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Abstract Protective layers are always required for Si-based photoelectrodes to achieve long-term stability in solar water splitting, where TiO2, with excellent chemical stability and the effectiveness of charge carrier transfer, is commonly used. However, previous reports found the performance of TiO2 protected photoelectrode would deteriorate with the formation of Ti3+ traps, which was rarely investigated. This paper demonstrates that AM 1.5G illumination facilitates generation of Ti3+ in TiO2, since TiO2 could be excited by the highly energetic UV photons, and describes the replacement of the classical photoresponsive TiO2 with completely transparent and chemical inert Ta2O5 layers to improve the photostability. Finally, this pn+-Si/Ta2O5/Pt photoelectrode shows a long-term stability over 200 h, a saturated photocurrent density of ∼34.7 mA cm−2 and the maximum applied bias photon-to-current efficiency of 8.1%, largely surpassing the TiO2 protected counterparts. With improved stability, Ta2O5 could be highlighted as a new candidate for the protection of unstable semiconductors.


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Hydrogen production directly from water by sunlight is a promising approach to address the worldwide energy and environmental problems.1-3 P-type Si has received much attention in photoelectrochemical (PEC) systems as the photocathode for hydrogen evolution reaction (HER) because of its earth abundance, low-cost, suitable bandgap and band alignments.4-6 However, one of the key problems of Si cathodes is the formation of electrically insulating SiO2 due to the oxidation of Si by dissolved oxygen species in electrolyte. An efficient solution is to protect Si surface with a layer of stable and conductive material such as TiO2 to suppress or retard oxidation.7-10 It has been reported that a Si photocathode protected by a 100 nm TiO2 layer deposited by atomic layer deposition (ALD) exhibited excellent HER photocurrent stability in 1 M HClO4 for 2 weeks when irradiated by the red light portion of AM 1.5G (λ > 635 nm, 38.6 mW cm-2).11 However, the long term stability of TiO2 protected Si photocathodes under full AM 1.5G spectrum has been rarely studied (Supplementary Table 1). A record high stability of 40 days was achieved using only the red part of solar spectrum (λ > 635 nm).12 In dye sensitized solar cell (DSSC) systems, it has been demonstrated that the critical instability of TiO2 based DSSC under AM 1.5G or UV illumination13 is owing to the highly energetic ultraviolet (UV) photons, as illustrated by the enhanced cell stability when a UV filter (435 nm cut off) was used. This unstable phenomenon has also been observed in other TiO2 protected PEC cathodes such as Cu2O,14-16 InP,8 and CuInS2,17 which significantly limited their lifetimes. Grätzel et al. found the photocurrent of TiO2 protected Cu2O photocathode decreased with time, which was explained by the presence of Ti3+ traps in the TiO2 layer that deformed the protective layer and deteriorated the photocathode performance.14 Indeed, the Ti3+ species has been observed in many TiO2 based systems under UV illumination.18 The unsatisfying performance of TiO2 as a protective layer could be attributed to its relatively narrow bandgap (3.2 eV), which can be excited by the UV


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portion of sunlight, leading to the generation of Ti3+ sites. Thus, a protective layer that is completely transparent to full solar spectrum might be a better choice for unstable photoelectrodes. Ta2O5 layer can be used as a robust protective layer because of its large bandgap (4.2 eV), superior chemical stability and suitable band alignment with many semiconductors.19 Compared with TiO2, Ta2O5 is inert under solar illumination since it is completely transparent to full solar spectrum. Regardless of the large bandgap, Ta2O5 has a similar conduction band edge position (ECB) with TiO2 (−4.3 and −4.2 eV for Ta2O5 and TiO2, respectively, absolute vacuum scale).20 It has been demonstrated that Ta2O5 layer can effectively transport electrons for Si based photovoltaics as an electron-selective heterocontact.21 Thus, photoelectrons from Si based PEC cathodes should be able to travel to electrolyte via the conduction band of Ta2O5. Meanwhile, Ta2O5 is chemically stable over a wider pH range compared to TiO2 and Al2O3, making it more practical in tandem cells for unassisted water splitting. Therefore, Ta2O5 could be considered as a suitable protective layer for Si photocathodes although the cost of Ta2O5 is higher than classical TiO222. This work describes the importance of photo-inertness of a protective layer to the stability of Si photocathode. We compared the photocurrent and stability of TiO2 and Ta2O5 coated pn+-Si photocathodes under the entire AM 1.5G spectrum (100 mW cm-2) and the red light portion of AM 1.5G (λ > 635 nm, 35 mW cm-2). The similar photocurrents of TiO2 and Ta2O5 protected pn+Si photocathodes demonstrate that electrons can travel through the two protective layers efficiently owing to their small conduction band offset with Si. But pn+-Si/Ta2O5 cathode is stable under full AM 1.5G spectrum (100 mW cm-2), which obviously outperforms TiO2 protected Si cathode that only survives in the red portion of AM 1.5G but fails quickly under full AM 1.5G illumination. By comparing the PEC and optical properties of TiO2 and Ta2O5 protected photocathodes under AM 1.5G and red light, the impact of illumination on the failure mechanism of protective layers has been demonstrated.



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PEC performance comparison between TiO2 and Ta2O5 protected Si photocathodes The pn+-Si/TiO2 and pn+-Si/Ta2O5 photocathodes were fabricated by depositing TiO2 and Ta2O5 thin films on pn+-Si substrates, respectively, followed by photo-electrochemical deposition of Pt nanoparticles as HER cocatalyst. Scanning electron microscopy (SEM) images (Figure S1, ESI†) reveals the smooth surface of the protective layer and the uniform distribution of Pt cocatalyst. The thickness of TiO2 was optimized to be 30 nm (details in Figure S2, ESI†), which is thick enough to explore the influence of light absorption in protective layer and this thickness is comparable with those in previous studies.23-24 The thickness of Ta2O5 layer was optimized to 10 nm for the best electrochemical performance (Figure S3, ESI†). Thicker Ta2O5 layer will render lower photocurrent with higher stability, which results in a more significant superiority of Ta2O5 over TiO2 and upon full AM 1.5G illumination in terms of stability. The thickness and complex optical constants of TiO2 and Ta2O5 layers were obtained from spectroscopic ellipsometry (details in Figure S4, ESI†). The effective transfer of photoelectrons via Ta2O5 lay was evidenced by PEC measurements under simulated AM 1.5G irradiation. The current-potential (J-V) characteristics were compared for pn+-Si/Pt, pn+-Si/TiO2/Pt, and pn+-Si/Ta2O5/Pt electrodes (Figure 1a). The electrochemical properties of our pn+-Si/Pt and pn+-Si/TiO2/Pt electrodes are comparable to benchmark Si photocathodes reported previously,11,

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making them suitable for comparison against Ta2O5

protected counterparts (Table S3). The photocurrent density of pn+-Si/Ta2O5/Pt electrode reaches ∼34.7 mA cm−2 at 0 V vs. RHE, with an onset potential of +0.53 V vs. RHE, which is similar to that of unprotected pn+-Si/Pt electrode and TiO2 protected pn+-Si/TiO2/Pt electrode. The maximum applied bias photon-to-current efficiency (ABPE) reaches 8.1% at 0.25 V vs. RHE for pn+-Si/Ta2O5/Pt electrode, which is slightly lower but still comparable to pn+-Si/Pt and pn+Si/TiO2/Pt electrodes (Figure S5, ESI†). The similar PEC performance between TiO2 and Ta2O5 protected pn+-Si electrodes indicates that Ta2O5 layer also imposes negligible resistance to


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electron transfer from pn+-Si to electrolyte, as the conduction band offset between Ta2O5 and TiO2 is only ~0.1 eV.20 The separation and transport of electron-hole pairs in pn+-Si/TiO2 and pn+-Si/Ta2O5 can be illustrated in their energy band diagrams (Figure 1c and 1d). Different from the similar PEC activity, Ta2O5 protected pn+-Si cathode exhibits a much stronger stability than the TiO2 protected counterpart under AM 1.5G illumination. As expected, the photocurrent of unprotected pn+-Si/Pt electrode decays quickly within 4 h under AM 1.5G at +0.3 V vs. RHE (Figure 1b, orange trace). TiO2 protective layer could improve the stability, but the improvement is very limited under full AM 1.5G (Figure 1b, green trace). When Ta2O5 is used as the protective layer, on the contrary, the photocurrent of pn+-Si/Ta2O5/Pt remains stable for more than 200 h under full AM 1.5G (Figure 1b, blue trace; a more detailed result is shown in Figure S6, ESI†). This drastic difference highlights the superior protective effect of Ta2O5 over TiO2 layer under full AM 1.5G illumination, where our TiO2 is of representative properties as the X-ray photoelectron spectroscopy (XPS) (Figure S7, ESI†) and grazing-incidence X-ray diffraction (GIXRD) patterns (Figure S8, ESI†) are consistent with previously reported results.11, 26

This unanticipated instability of TiO2 under full solar spectrum is significantly different from

those under the red part of solar spectrum (λ > 635 nm).11 Although TiO2 protected Si-based photoanodes have been reported for stable performance under full AM 1.5G, the case of photocathodes is very different due to the highly reductive photo-generated electrons accumulated on the cathode surface, which may lead to the partial reduction of the oxide protective layer. Considering the different bandgap of Ta2O5 (~4.2 eV) and TiO2 (~3.2 eV), we propose that the degraded stability of pn+-Si/TiO2/Pt electrode may be related to the spectral range of illumination.



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Figure 1. (a) J-V curves for the pn+-Si/Pt (orange), pn+-Si/Ta2O5/Pt (blue), and pn+-Si/TiO2/Pt (green) electrodes. The samples were irradiated by AM 1.5G light (100 mW cm-2) and measured in 1 M HClO4 (pH 0). (b) PEC stability test for the pn+-Si/Pt (orange), pn+-Si/Ta2O5/Pt (blue), and pn+-Si/TiO2/Pt (green) electrodes at an applied bias of 0.3 V vs. RHE. The energy band diagrams of (c) pn+-Si/TiO2 and (d) pn+-Si/Ta2O5.

Impact of irradiation on stability To prove our hypothesis, we compared the PEC performance of pn+-Si/TiO2/Pt electrode under full AM 1.5G irradiation (100 mW cm-2) and the red light portion of AM 1.5G (λ>650 nm, 35 mW cm-2) (Figure 2a). The range of red light is selected to be comparable with previous reports,11,

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which avoids the UV excitation of TiO2. Under red light illumination, the pn+-

Si/TiO2/Pt electrode exhibits no obvious photocurrent decay over 2.5 h (Figure 2a, red trace). This is in good agreement with previously reported studies.11, 26 However, when AM 1.5G light was


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adopted, pn+-Si/TiO2/Pt electrode undergoes a dramatic decay in photocurrent (Figure 2a and 1b, green trace). The loss of Pt cocatalyst from electrodes might be a possible reason for the instability.27-28 Indeed, replatinization of the same amount of initial Pt (~2.5 μg) could restore the onset potential and saturation current (Figure 2b, dark green trace). Furthermore, inductively coupled plasma mass spectrometry (ICP-MS) indicated the existence of dissolved Ti species in the electrolyte after PEC measurements under AM 1.5G light (Table S2, ESI†). Thus, it could be assumed that the falling off of Pt is resulted from the dissolution of TiO2 film, which is initiated by the illumination of AM 1.5G light.

Figure 2. (a) PEC stability comparison of pn+-Si/TiO2/Pt electrode under the red portion (λ>650 nm, 35 mW cm-2) and full AM 1.5G light (100 mW cm-2) at 0.3 V vs. RHE. (b) J-V characteristics of the as-synthesized pn+Si/TiO2/Pt electrode (green), and the same electrode after stability test (purple) and replatinization (dark green).

To confirm this assumption, the stability of pn+-Si/TiO2 at −0.8 V vs. RHE without Pt cocatalyst (to enable the ellipsometric measurement) was compared under dark, AM 1.5G light (100 mW cm-2) and red light portion of AM 1.5G (λ > 650 nm, 35 mW cm-2) (Figure 3a). The current density of TiO2 coated sample was nearly unchanged in 4.5 h under red light illumination, while a significant change of current density could be evidenced under AM 1.5G illumination. On the contrary, pn+-Si/Ta2O5 sample exhibits constant photocurrents under both AM 1.5G light and red light illumination (Figure 3b). Therefore, one may conclude that the dissolution of TiO2 film



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is related to the exposure of AM 1.5G light, where the highly energetic UV portion might be the direct reason.

Figure 3. PEC stability test for the (a) pn+-Si/TiO2 electrode and (b) pn+-Si/Ta2O5 electrode at applied biases of −0.8 V vs. RHE under red light (λ>650 nm, 35 mW cm-2) and AM 1.5G light (100 mW cm-2). Dashed line indicates that the measurements were suspended and resumed at 1.5 h for ellipsometric measurements.

UV-induced defects in protective layers and the impact on instability We suspect that the dissolution of TiO2 film under AM 1.5G irradiation is caused by the formation of Ti3+ defects. However, XPS failed to show significant differences due to oxidation of Ti3+ species in ambient before the samples could be loaded into the XPS chamber. Thus, the formation of Ti3+ species in TiO2 protected photocathodes under AM 1.5G was evidenced by spectroscopic ellipsometry, immediately after the PEC measurements (within 60 seconds) to avoid the oxidation of Ti3+ defect sites. The optical constants, refractive index (n) and extinction coefficient (k), could be extracted from spectroscopic ellipsometry, where k could be used to illustrate the change in light absorption of the film and further confirm structural changes. It has been shown that Ti3+ can extend the photoresponse of TiO2 from the UV to the visible light region.29-30 Thus, the change of k for TiO2 film at 0 h, 1.5 h and 4.5 h under AM 1.5G illumination at −0.8 V vs. RHE in 1 M HClO4 could be used to demonstrate the change of lightmater interaction of TiO2 layer (Figure 4a).


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Before illumination, the TiO2 film shows an UV absorption edge at 3.34 eV, with no absorption in visible light range (Figure 4a, black trace). Upon AM 1.5G illumination for 1.5 h, a weak absorption tail emerges at ~ 3 eV (Figure 4a, blue area),31-33 which could be attributed to the generation of Ti3+ species. After 4.5 h measurement, an apparent absorption peak could be observed at ~ 3 eV as a result of the optical absorption of Ti3+ species (Figure 4a, red area). Further, the TiO2 film thickness of pn+-Si/TiO2 electrodes through the 4.5 h stability test was compared under dark, red light (λ>650 nm, 35 mW cm-2) and AM 1.5G light (100 mW cm-2) (−0.8 V vs. RHE, 1 M HClO4, Figure 4b). Under dark condition, the TiO2 exhibits negligible thickness change when pn+-Si/TiO2 electrode was biased at −0.8 V vs. RHE for 4.5 h (Figure 4b, black trace), indicating the electrochemical inertness of TiO2 film. Under AM 1.5G illumination, however, the thickness of the TiO2 protective layer decreased drastically from 309 Å to 242 Å after 1.5 h, and finally decreased to 196 Å after 4.5 h (Figure 4b, green trance), indicating the dissolution of TiO2 film. Under red light illumination, on the contrary, the TiO2 thickness remained unchanged (Figure 4b, red trace). The dissolution of TiO2 layer under AM 1.5G explains the instability of pn+-Si/TiO2 electrode in PEC stability test (Figure 3a), confirming that the optical absorption of TiO2 under AM 1.5G is main reason of unstable PEC performance. The difference in current density and protective layer thickness under red light and AM 1.5G illumination highlights the impact of full solar spectrum on the stability of pn+-Si/TiO2 electrode, while the change of extinction coefficient suggests the formation of Ti3+. In this study, pn+Si/TiO2 electrode works as the cathode, leading to the accumulation of electrons that accelerates the formation of Ti3+ species. The Ti3+ formation motivates the hydroxylation of TiO2 and results in the dissolution of TiO2 layer, which is responsible for the degradation of PEC performance under AM 1.5G illumination.



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Figure 4. (a) The extinction coefficient (k) and (b) thickness change of TiO2 film of pn+-Si/TiO2 electrode for PEC stability test the at applied biases of −0.8 V vs. RHE under AM 1.5G light (100 mW cm-2) and red light (AM 1.5G cutoff650 nm, 35 mW cm-2, Figure 3b). The Ta2O5 thickness of pn+-Si/Ta2O5 was also unchanged under −0.8 V vs. RHE in 1M HClO4, regardless of the light resources (Figure 4d), which is in line with the PEC stability results. On the other hand, the extinction coefficient (k) of Ta2O5 on the pn+-Si/Ta2O5 electrode is not changed after 4.5 h stability test at applied biases of −0.8 V vs. RHE under AM 1.5G light, red light or darkness (Figure 4c). The refractive index (n)


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of Ta2O5 is also not changed after the stability test (Figure S9, ESI†). These indicate that the optical property and structure of Ta2O5 are not affected by the photon energy of incident light. Compared with TiO2 (bandgap 3.2 eV), the complete transparency of Ta2O5 (bandgap 4.2 eV) to the full solar spectrum could explain its superior stability. Thus, it could be concluded that the excellent protective effect of the Ta2O5 film lies in its chemical inertness and optical transparency to sunlight.

Figure 5. A proposed mechanism for the instability of pn+-Si/TiO2/Pt electrode under AM 1.5G light (100 mW cm-2).

Based on our experimental results and previous investigations, a schematic diagram of pn+Si/TiO2/Pt electrode is illustrated to explain the instability of TiO2 protective layer under full AM 1.5G (Figure 5). Two steps are involved during the dissolution of TiO2. Ti3+ species and O vacancies would be generated when TiO2 is irradiated under UV in water, which has been well documented previously (Figure 5b).34-37 The electron rich environment of the cathodes also



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favours the generation of Ti3+ species. The coordinatively unsaturated Ti3+ species are highly unstable, which tends to further react with water to generate Ti4+, leaving two adjacent Ti-OH groups per vacancy (Figure 5c). As a result, the photoexcitation induces the hydroxylation of TiO2 layer, leading to the generation of unstable OH-coordinated surfaces, which in turn destabilizes the protective layer and results in the loss of Pt catalysts (Figure 5c). So the photocurrent of pn+-Si/TiO2/Pt electrode is gradually reduced during the stability test under AM 1.5G irradiation. For comparison, the bandgap of Ta2O5 (4.2 eV) is large enough to ensure its optical inertness to sunlight, preventing the dissolution of the protective layer. Therefore, pn+Si/Ta2O5/Pt electrode shows much higher durability than pn+-Si/TiO2/Pt electrode, especially when UV is included in the incident light. In conclusion, this paper identified the origin of instability for TiO2 protected Si photocathodes under full AM 1.5G illumination, which could be attributed to the generation of Ti3+ species in TiO2 layer under the UV portion of illumination. On the contrary, the Ta2O5 coated Si photocathodes show dramatically enhanced stability compared with TiO2 owing to its higher transparency to sunlight. The pn+-Si/Ta2O5/Pt photoelectrode realized a stable H2 production for 200 h with a current density of ∼34.7 mA cm−2 at 0 V vs. RHE and ABPE of 8.1% at 0.25 V vs. RHE under AM 1.5G irradiation in 1 M HClO4. This work not only proves a method to improve the stability of Si photocathode, but also highlights the importance of using a transparent protective layer to prevent defect formation in the protective layer under AM 1.5G. ASSOCIATED CONTENT Supporting Information Available: Experimental section, supporting table of performance literatures comparison, additional SEM images, J-V curves, J-t curves, GIXRD characterization and XPS patterns AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] ORCID Jinlong Gong: 0000-0001-7263-318X Tuo Wang: 0000-0002-9862-5038 URL of the group website http://gonglab.org Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the National Key R&D Program of China (2016YFB0600901), the National Natural Science Foundation of China (21525626, U1463205, 21722608, 51861125104), and the Program of Introducing Talents of Discipline to Universities (B06006) for financial support. REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (2) Hisatomi, T.; Kubota, J.; Domen, K., Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43, 7520-7535. (3) Weber, M. F.; Dignam, M. J., Splitting water with semiconducting photoelectrodes—Efficiency considerations. Int. J. Hydrogen Energy 1986, 11, 225-232. (4) Oh, J.; Deutsch, T. G.; Yuan, H.-C.; Branz, H. M., Nanoporous black silicon photocathode for H2 production by photoelectrochemical water splitting. Energy Environ. Sci. 2011, 4, 1690-1694. (5) Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.; Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S., Photoelectrochemical hydrogen evolution using Si microwire arrays. J. Am. Chem. Soc. 2011, 133, 1216-1219.



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Jinlong Gong studied chemical engineering and received his BS and MS degrees from Tianjin University and his PhD degree from the University of Texas at Austin under the guidance of C. B. Mullins. After a stint with Professor George M. Whitesides as a postdoctoral fellow at Harvard University, he joined the faculty of chemical engineering at Tianjin University, where he currently holds a Cheung Kong Chair Professorship. He has served on the editorial boards for several journals including Chemical Society Reviews, Chemical Science, and AIChE Journal. He also serves as an Associate Editor for ACS Sustainable Chemistry & Engineering. He is an elected Fellow of the Royal Society of Chemistry. He has published more than 200 papers in peer-refereed journals and has been listed as a coinventor on 73 patents and applications. His research interests include catalytic conversions of green energy, utilization of carbon oxides, and synthesis and applications of nanostructured materials.


Transparent Ta2O5 Protective Layer for Stable Silicon Photocathode

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