Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen

TLDR: In this article, the InGaN nanowires (NWs) were grown on a metallic Ti/Si template for improving the water splitting performance compared to a bare Si substrate, and the open circuit potential of the epitaxially grown NWs on metallic Ti was almost two times higher than when directly grown on the Si substrate.

Abstract: Water splitting using InGaN-based photocatalysts may make a great contribution to future renewable energy production systems. Among the most important parameters that need to be optimized are those related to substrate lattice-matching compatibility. Here, we directly grow InGaN nanowires (NWs) on a metallic Ti/Si template, for improving the water splitting performance compared to a bare Si substrate. The open circuit potential of the epitaxially grown InGaN NWs on metallic Ti was almost two times higher than when directly grown on the Si substrate. The interfacial transfer resistance was also reduced significantly after introducing the metallic Ti interlayer. An applied-bias-photon-to-current conversion efficiency of 2.2% and almost unity faradaic efficiency for hydrogen generation were achieved using this approach. The InGaN NWs grown on Ti showed improved stability for hydrogen generation under continuous operation conditions, when compared to those grown on Si, emphasizing the role of the semiconductor-on-metal approach in enhancing the overall efficiency of water splitting devices.

Full text

Water Splitting over Epitaxially Grown InGaN Nanowires
on-Metallic Titanium/Silicon Template: Reduced
Interfacial Transfer Resistance and Improved Stability
Item Type Article
Authors Ebaid, Mohamed; Min, Jungwook; Zhao, Chao; Ng, Tien Khee;
Idriss, Hicham; Ooi, Boon S.
Citation Ebaid M, Min J, Zhao C, Ng TK, Idriss H, et al. (2018) Water
Splitting over Epitaxially Grown InGaN Nanowires on-Metallic
Titanium/Silicon Template: Reduced Interfacial Transfer
Resistance and Improved Stability. Journal of Materials
Chemistry A. Available: http://dx.doi.org/10.1039/c7ta11338b.
Eprint version Post-print
DOI 10.1039/c7ta11338b
Publisher Royal Society of Chemistry (RSC)
Journal Journal of Materials Chemistry A
Rights Archived with thanks to Journal of Materials Chemistry A
Download date 09/08/2022 22:11:29
Link to Item http://hdl.handle.net/10754/627346

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Journal of
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Materials for energy and sustainability
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ISSN 2050-7488
Volume 4 Number 1 7 January 2016 Pages 1–330
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1
Classification: General Business Use
Water Splitting to Hydrogen over Epitaxially Grown InGaN Nanowires on Metallic 1
Titanium/Silicon Template: Reduced Interfacial Transfer Resistance and Improved 2
Stability 3
4
Mohamed Ebaid
1
, Jung-Wook Min
1
, Chao Zhao
1
, Tien Khee Ng
1
, Hicham Idriss
2,*
, and Boon S. 5
Ooi
1,*
6
7
1
Photonics Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 8
23955-6900, Saudi Arabia 9
2
SABIC-Corporate Research and Development Center (CRD) at KAUST, Thuwal 23955, Saudi 10
Arabia 11
12
*Email: boon.ooi@kaust.edu.sa and IdrissH@sabic.com 13
14
15
16
Water splitting using InGaN-based photocatalysts may have a great contribution in future 17
renewable energy production systems. Among the most important parameters to solve are those 18
related to substrate lattice-matching compatibility. Here, we directly grow InGaN nanowires 19
(NWs) on a metallic Ti/Si template, for improving water splitting performance compared to a 20
bare Si substrate. The open circuit potential of the epitaxially grown InGaN NWs on metallic Ti 21
was almost two times more than when directly grown on Si substrate. The interfacial transfer 22
resistance was also reduced significantly after introducing the metallic Ti interlayer. An applied-23
bias-photon-to-current conversion efficiency of 2.2% and almost unity Faradic efficiency for 24
hydrogen generation were achieved using this approach. The InGaN NWs grown on Ti showed 25
improved stability of hydrogen generation under continuous operation conditions, when 26
compared to those grown on Si, emphasizing the role of the semiconductor-on-metal approach in 27
enhancing the overall efficiency of water splitting devices. 28
29
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2
Classification: General Business Use
Introduction 1
InGaN-based materials are recently attracting much attention due to their chemical 2
stability and tunable band gap that can cover the entire visible solar spectrum and improve the 3
solar light harvesting efficiency.
1
Several InGaN-based photoelectrolysis systems have already 4
been demonstrated including single photoelectrodes
2-5
, dual-photoelectrodes
6
, and monolithic 5
tandem cells.
7-9
However, the commercial applications of these structures are hindered by the 6
low solar-to-hydrogen (STH) energy conversion efficiency, which is limited to values far from 7
that achieved by III-V compound semiconductors such as GaInP/GaAs tandem cells.
10
This can 8
be partially attributed to the lack of compatible and highly conductive substrates, which are 9
indispensable for the efficient separation and transport of the photo-generated charge carriers to 10
the desired water redox interfaces.
2
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, we investigated the combination of 13
both nitride NWs with silicon substrate in the presence of a metallic interlayer. 14
The growth of InGaN NWs usually involved single crystalline substrates, such as semi-15
insulating sapphire and semi-conducting Si. For instance, high aspect ratio InGaN NWs are 16
typically grown by metalorganic chemical vapor deposition (MOCVD) on sapphire using metal 17
catalyzed vapor-liquid-solid (VLS)
3
or selective area growth (SAG) techniques.
11
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.
12
The 20
prerequisite of growing InGaN NWs by PA-MBE under nitrogen rich conditions led, however, to 21
the formation of inherent amorphous SiN
x
insulating layers between the GaN/Si 22
heterointerface.
13
The formation of the insulating SiN
x
thin film was reported to increase the 23
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3
Classification: General Business Use
interfacial resistance and introduce a large conduction band offset with n-GaN that can resist the 1
charge carrier transport during the water splitting process.
14
Therefore, the growth of InGaN 2
NWs on substrates made of conventional metals can provide new opportunities for enhancing the 3
photo-generated charge carrier transport that can significantly improve the STH efficiency. 4
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.
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. 9
Furthermore, metal substrates can act to mitigate the current crowding effects at the InGaN 10
NWs/substrate interface leading to enhanced carrier collection and increased effective surface 11
area for water splitting reactions.
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.
17-18
Although this approach can greatly contribute to improving the overall 14
STH of water splitting, this concept was rarely reported.
2
This can be attributed to the difficulties 15
of tuning the growth conditions to obtain high density of vertically aligned InGaN NWs.
15, 17
16
In this study, we investigate the photoelectrochemical (PEC) performance of high quality 17
InGaN NWs grown on a metallic Ti thin film deposited on Si substrates. Ti thin film was 18
considered for integration with silicon substrate for both scalability and cost-effectiveness, ideal 19
for future industrial uptake in both template-substrate production and PEC cell deployment. 20
Here, the Si substrate was only used as a low-cost mechanical support for Ti thin film. The use of 21
Ti carries the following advantages. It is well known that Ti-metal is a good Ohmic contact for n-22
type GaN,
19
and the electrical resistivity of Ti is approximately 4.2x10
-5
Ohm.cm, which is much 23
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DOI: 10.1039/C7TA11338B

Frequently asked questions

Q1. How many electrons will be generated by the photo-generated holes?

The photo-generated holes will then diffuse to the 18 InGaN NWs/electrolyte interface to start the water oxidation reaction and the photo-generated 19 electrons will migrate towards the bottom n-GaN to the counter electrode to reduce hydrogen 20 ions into hydrogen molecules. 

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.