Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen
Summary (2 min read)
Introduction
- InGaN-based materials are recently attracting much attention due to their chemical stability and tunable band gap that can cover the entire visible solar spectrum and improve the solar light harvesting efficiency.
- 10 This can be partially attributed to the lack of compatible and highly conductive substrates, which are indispensable for the efficient separation and transport of the photo-generated charge carriers to the desired water redox interfaces.
- The growth of InGaN NWs usually involved single crystalline substrates, such as semiinsulating sapphire and semi-conducting Si.
- This concept was rarely reported.
Results and Discussion:
- InGaN NWs were grown by PA-MBE on a metallic Ti-coated 2-inch Si (100) substrate and compared with those grown on bare Si (100) substrate.
- While there was no significant difference in the morphology of InGaN NWs grown on Ti or Si surfaces, the normalized photoluminescence (PL) spectra of InGaN NWs grown on Ti exhibited two main characteristics, as shown in Fig. 1(d) .
- Ti also exhibited PL peaks centered at almost the same positions, where the relative intensity of the two peaks increased with increasing the Ti thickness.
- Contact with the electrolyte and under light conditions, there will be an upward band bending due to the transport of the excess charge carriers from the semiconductor NWs to the electrolyte to achieve the electrochemical equilibrium.
- In principle, the difference between the OCP in the dark and under illumination represents the photovoltage caused by the potential difference within the space charge region in the semiconductor resulting from surface Fermi level pinning.
Electrochemical impedance spectroscopy (EIS
- The applied-bias-photon-to-current conversion efficiency (ABPE) was used to evaluate the performance of the photoanodes with respect to the applied potential, as shown in Fig. 4(b) .
- Hence, about 12 times increase in power conversion efficiency was achieved by using a metallic Ti as a charge extraction/collection interlayer.
- As shown in the inset of Fig. 4 (b), the IPCE of the InGaN NWs grown on Si-substrate was significantly improved after using a 300 nm of metallic The inset of Fig. 4(c) shows the difference between the current density measured after almost three hours of continuous irradiation under light and dark conditions indicating a good photoactivity of the photoanode.
- By considering the error of the gas sampling (~5%) and analysis, it is possible to assume that the Faradic efficiency is unity, which emphasis the impact of the semiconductoron-metal approach on the enhanced PEC water splitting performance.
Conclusions
- In conclusion, the growth of InGaN NWs on the conventional Si-substrates showed a poor PEC performance in terms of a low current density and degradation of the photoanode with time, which can be attributed to the formation of the amorphous insulating SiN x interfacial films and the associate effect on charge transport resistance during the water splitting process.
- The authors have then succeeded in putting a Ti layer in between the Si substrate and InGaN NWs.
- The PEC experiments were conducted in a three-electrode configuration cell using InGaN NWs as the working electrode, Ag/AgCl as the reference electrode, and a Pt-mesh as the counter electrode.
- The evolved gases during the water splitting experiment were collected and quantified using two separate gas chromatographs (GCs) SRI 310c.
- The total number of moles was then calculated by multiplying the last value by the reactor head space (60 ml), which was normalized to the surface area of the InGaN NWs photocatalyst.
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Frequently Asked Questions (18)
Q2. What is the effect of light on the surface energy bands of the semiconductor?
Bsemiconductor is brought into contact with an electrolyte under dark conductions, an upward 1 band-bending of the surface energy bands occurs at equilibrium due to excess charge carrier 2 transport from the semiconductor to the electrolyte at the interface.
Q3. What is the simplest way to extract the photo-generated electrons?
Baccelerate the water oxidation step, metal ions co-catalysts are typically deposited on the surface 1 of InGaN NWs to extract the photo-generated holes.2, 4, 25 On the other hand, for the photo-2 generated electrons to be easily transported to the counter electrode and start the hydrogen 3 reduction an Ohmic contact is needed.
Q4. What is the potential of using metallic substrates for the fabrication of photoelectrodes?
The potential of employing metallic substrates for the fabrication of photoelectrodes 5 stems from the utilization of the high thermal and electrical conductivity and the potentially high 6 optical reflectivity of metals as well as the freedom of enabling scale-up for field deployment.
Q5. What is the effect of the small conduction band offset between n-InGaN?
The small conduction band offset between n-InGaN and n-GaN 21 will allow electrons to readily tunnel through, while the transport of holes in the same direction 22 will be hindered by the large valence band offset, which will help in charge separation.
Q6. What is the role of metal in the improvement of water redox?
15 7 The semiconductor-on-metal approach can play a vital role in the improvement of carrier 8 extraction/collection efficiency that is crucial to accelerate the water redox reaction rates.
Q7. What is the effect of the combination of 13 nitride NWs with silicon substrate?
Motivated by the inert nature of wide bandgap and wide 11 compositional tunability of group-III nitride, as well as the feasible single crystal growth of 12 nitride-based nanowires (NWs) on scalable silicon substrate, the authors investigated the combination of 13 both nitride NWs with silicon substrate in the presence of a metallic interlayer.
Q8. How many times increase in power conversion efficiency was achieved by using a metallic Ti?
about 12 times increase in power conversion efficiency was achieved by 2 using a metallic Ti as a charge extraction/collection interlayer.
Q9. How did the InGaN NWs retain their initial current density after continuous illumination for 5?
The InGaN NWs on Ti could also retain the same initial current density after continuous 6 illumination for 5 hours and continuously produce hydrogen and oxygen gases with a Faradic 7 efficiency close to unity.
Q10. How many times did the inGaN NWs undergo a stability test?
The amount of the evolved hydrogen and oxygen gases during the stability test were then 5 measured using gas chromatography for the InGaN NWs grown on 300 nm Ti sample.
Q11. What is the key advantage of the InGaN NWs?
Another key advantage is that, 1 without a metal interlayer, an amorphous SiNx dielectric thin film was known to have formed 2 when InGaN NWs were directly grown on Si substrates.
Q12. What is the corresponding resistance of the InGaN NWs?
Bsemicircle at high frequency in a Nyquist plot represents the charge transfer at the InGaN 1 NWs/electrolyte interface while its diameter is equivalent to the charge transfer resistance.
Q13. What is the effect of the growth of InGaN NWs on Si?
Based on the results 13 shown by OCP and Nyquist plots, the authors can conclude that the growth of semiconductor NWs on a 14 metallic substrate reduces the interfacial charge transport resistance, which is poised to enhance 15 the water splitting reaction.
Q14. What is the effect of the photo-generated charge carriers on the Ti surface?
The effective transport of the photo-generated charge carriers in the 13 InGaN NWs grown on Ti as evident in the significant change in OCP (~500 mV vs RHE), is 14 about two times that of the Si-substrate (~270 mV vs RHE).
Q15. What is the ABPE of the photoanodes?
18 The applied-bias-photon-to-current conversion efficiency (ABPE) was used to evaluate 19 the performance of the photoanodes with respect to the applied potential, as shown in Fig. 4(b).
Q16. What is the effect of the Faradic efficiency on the PEC water splitting performance?
By considering the error of the gas sampling (~5%) and analysis, it is possible 13 to assume that the Faradic efficiency is unity, which emphasis the impact of the semiconductor-14 on-metal approach on the enhanced PEC water splitting performance.
Q17. How many InGaN NWs have been grown?
On the other 18 hand, plasma-assisted molecular beam epitaxy (PA-MBE) has been dedicated for the growth of 19 vertically-aligned InGaN NWs with high In-content on Si under nitrogen-rich conditions.
Q18. What is the significance of the growth of III-V NWs on metallic substrates?
16 The growth of III-V NWs on metallic substrates has already 12 been demonstrated for the optoelectronic devices, which showed significant improvement of the 13 device efficiency.