Flexible InGaN nanowire membranes for enhanced solar water splitting

III-Nitride nanowires (NWs) have recently emerged as potential photoelectrodes for efficient solar hydrogen generation. While InGaN NWs epitaxy over silicon is required for high crystalline quality and economic production, it leads to the formation of the notorious silicon nitride insulating interface as well as low electrical conductivity which both impede excess charge carrier dynamics and overall device performance. We tackle this issue by developing, for the first time, a substrate-free InGaN NWs membrane photoanodes, through liftoff and transfer techniques, where excess charge carriers are efficiently extracted from the InGaN NWs through a proper ohmic contact formed with a high electrical conductivity metal stack membrane. As a result, compared to conventional InGaN NWs on silicon, the fabricated free-standing flexible membranes showed a 10-fold increase in the generated photocurrent as well as a 0.8 V cathodic shift in the onset potential. Through electrochemical impedance spectroscopy, accompanied with TEM-based analysis, we further demonstrated the detailed enhancement within excess charge carrier dynamics of the photoanode membranes. This novel configuration in photoelectrodes demonstrates a novel pathway for enhancing the performance of III-nitrides photoelectrodes to accelerate their commercialization for solar water splitting. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (160.4236) Nanomaterials; (260.5130) Photochemistry; (350.6050) Solar energy. References and links 1. N. Kannan and D. Vakeesan, “Solar energy for future world: A review,” Renew. Sustain. Energy Rev. 62, 1092–1105 (2016). 2. N. S. Lewis and D. G. Nocera, “Powering the planet: Chemical challenges in solar energy utilization,” Proceedings of the National Academy of Sciences 103(2006). 3. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, and D. G. 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Introduction
Urged by increasing energy demands and rising average global temperature, there are enormous global efforts to shift the world energy dependence from fossils fuels to renewable energy sources [1].Conventional solar electricity in particular, while representing an abundant energy source, suffers from intermittency and dependence on weather conditions and thus require an efficient energy storage and transport system [2,3].In this regards, solar water splitting represents a clean, renewable and storable energy source where solar energy is converted into hydrogen chemical energy through splitting water molecule into hydrogen and oxygen [4].
The semiconductor photoelectrodes required to constitute an efficient kinetic path for such a reaction, need to fulfil several optical, chemical and morphological criteria [5].Having proper energetics as well as a tunable energy bandgap, indium gallium nitride (InGaN) semiconductor alloy efficiently absorbs solar radiation while straddling the electrochemical potentials of the oxygen and hydrogen evolution reactions, allowing for an efficient solar hydrogen generation [6][7][8].Furthermore, InGaN nanowires (NWs) have large surface-tovolume ratio and are capable of withstanding high indium incorporation without defects nucleation [9], and therefore, are excellent candidate for solar hydrogen generation photoelectrodes [10][11][12][13][14].To achieve high crystalline quality as well as economic production, InGaN NWs, previously employed for solar hydrogen generation, were epitaxially grown on silicon (Si) substrates [11,15,16].Unfortunately, besides having a comparatively low electrical conductivity, growth of InGaN NWs on Si leads to the formation of an amorphous insulating silicon nitride (SiNx) interfacial layer, during the nitrogen rich nucleation phase [17,18].These issues highly impede transport of excess charge carriers from the NWs to the counter electrode and thus reduce the device performance.
We present an unconventional technique to resolve the detrimental effect of growing InGaN NWs on Si substrate without risking their high crystalline quality.Specifically, we fabricated the first substrate-free InGaN NWs membrane photoanodes, based on lift-off and transfer techniques for solar hydrogen generation (Fig. 1).The membrane photoelectrodes consisted of lifted-off InGaN/GaN NWs forming an ohmic contact with a high electrical conductivity metallic layer and thus, allowing for an efficient excess charge carrier extraction and transport towards the counter electrode (inset of Fig. 1).As further presented below, due to their large aspect ratio, the prepared membranes could be efficiently bended and thus be attached to a plethora of media including rigid and flexible platforms.The presented strategy represents a novel pathway to enhance the solar water splitting efficiency of InGaN NWs to further drive III-nitride photoanodes towards commercialization.

NWs me
After growing Ti (60 nm) an [24].Since an fabricated dev to the contac through nitrog

TEM im
Following th specimen was (Fig. 4(a)).T metal contact was deposited from the stem peaks assigne present withi expanded reg spectra origin extract the e elemental ma signals were c in Fig. 4(a).V the EDX spe extracted from line profiles r attributed to formation of Therefore, the based on ther expected, the highly affecte Therefore, as high Au comp    no plateau in the photocurrent density was observed, which denoted that not all of the photogenerated holes take part of the charge carrier transfer across the InGaN/electrolyte interface [31].The presence of surface states, which were investigated by previous OCP measurements, is the main cause for the trapping/recombination of the photogenerated charge carriers.Indeed, several published reports showed similar photocurrent behaviour for InGaN NWs (and other nanostructures) in solar-based water splitting [11,12,[32][33][34].
To shed light on the kinetics of charge carriers, responsible for the enhanced performance of the NWs membrane, the dynamic behaviour of the devices was studied through EIS.Under illumination and zero applied bias, a single frequency perturbation potential (10 mV), alternating from 1 Hz to 10 kHz, was applied over the samples and the counter electrode and the changes in impedance were recorded.Figures (a) and (b) showed the collected Nyquist plot of the NWs on silicon substrate (blue dots) and NWs membrane (red dots), respectively.The insets, present the equivalent electrical circuits used to model the charge carrier dynamics which provided a good fitting (black lines) for the EIS data.The circuits were derived based on the photo-stationary state of the electronic charge carriers within the photoanodes during water splitting, depicted in Fig. (c) and (d).The first RC element in the equivalent circuit for the NWs on silicon substrate (R CT Q CT ) represented the hole transfer process from the InGaN NWs to the electrolyte, where R CT describes the resistance to charge transfer and Q CT is a constant phase element (CPE) which describes the double layer capacitance [35].While the double layer capacitance, generated from the bulk space-charge region and the Helmholtz layer is ideally modelled by capacitor, the surface roughness and non-uniform chemical composition associated with NWs, caused the interface to be best described by a CPE [36,37].The second RC element (R 2 Q 2 ) described the GaN/silicon interface which hinders electrons transfer due to the presence of the large bandgap insulating SiN x interfacial layer, Fig. 5(b) [17,18].Finally, there is a series resistance (R sol,sub ), which describes the solution and substrate cumulative resistance.The values of the fitting parameters are presented in Table 1.
On the other hand, the equivalent electrical circuit, which presented a good fitting of the NWs membrane EIS data (black curve), only contained R CT , Q CT , and R sol,sub elements (Fig. 6(b)).The absence of the series R 2 Q 2 elements in the equivalent circuit revealed that indeed, an ohmic contact formed between the NWs and the back-metal contact which could facilitate excess charge carriers transfer.Furthermore, the values of the CPE elements (Q CT ) from both structures were within the same range, which demonstrated that the main difference in performance was not due to the charge transfer layer, but rather due to the enhancement of the collection of the excess charge carriers.Finally, the difference in magnitude between R sol,sub for the membrane (46 Ω) and for the NWs on silicon (1.3 kΩ), demonstrated the higher resistance from the comparatively lower substrate electrical conductivity and its effect on the charge carrier dynamics, as previously anticipated.Having es on silicon, the samples were mA/cm 2 for th Si-substrate sh higher equilib described mec

Conclusions
In conclusion, we have demonstrated the fabrication of a flexible substrate-free novel InGaN NWs membrane photoanode structure.The LSV characterization of the NWs membrane photoanode revealed an enhancement of the photocurrent densities as well as a lower turn on potential, as compared to regular NWs on bulk silicon substrate.As confirmed by the EIS measurements, such an increase in performance was ascribed to more efficient excess charge carrier extraction from the NWs due to the Ohmic contact formed by the deposited metals, as compared to the silicon wafer which forms the interfacial SiN x insulating layer.The demonstrated membrane technique represents a novel pathway for enhancing InGaN NWs photoanode performance that could be implemented alongside other techniques, such as cocatalysts [39], sidewall passivation [34], surface protection [40,41] or bandgap engineering [11] to further drive III-nitride photoanodes towards commercialization.

Funding
BSO, TKN and CZ acknowledge funding support from King Abdulaziz City for Science and Technology (KACST) Technology Innovation Center (TIC) for Solid State Lighting, grant no.KACST TIC R2-FP-008.King Abdullah University of Science and Technology (KAUST) baseline funding, grant no.BAS/1/1614-01-01.

Disclosures
The authors declare that there are no conflicts of interest related to this article.
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Fig. 7 .
Fig. 7. Stability measurements of the NWs membrane (red) and NWs on silicon (blue) under continuous water splitting operation.The photo-anodes were biased at 0.4 V.