Hot Hole-Deriven Water Splitting via LSPR Metal Nanostructures

The localized surface plasmon resonance (LSPR) in metal nanostructures is one of the most efficient materials for the futuristic energy, environmental science and industry. The new era is the ability to significantly drive and promote photocatalytic reactions and photodetection which acts as an interfacial energy transfer to adsorbate molecules and semiconductors. 

The combination of plasmonic noble metallic nanostructures with semiconductors for plasmon-enhanced visible light-driven water splitting (WS) has attracted considerable attention. WS is one of the most capable way to save solar energy into other useful energy applications. In WS, solar energy is converted to chemical energy mainly in the form of hydrogen and oxygen.

Some of the review reports indicate that the highest reported quantum efficiency for overall WS achieved is 57% with NiO/NaTaO₃:La photocatalyst under the excitation wavelength of 270 nm [1]. Its large scale commercial applications are still lacking due to low photocatalytic WS efficiency.

P- and n-type semiconductors are involved photogenerated holes which involve interfacial electron transfer process. These reaction take part in both O₂ evolution reaction and photogenerated electrons which move towards the counter electrode to be involved during the H₂ evolution reaction. This is known as photoelectrochemical WS cell design.

Plasmonic hot carriers are important parameter in LSPR species because of their low harvesting and conversion efficiencies hamper and wider use due to their ultrashort lifetime and mean-free path. It can be also be generated in a non-equilibrium state which holds a great promise [2].

Recently, many reports are on LSPR architecture which particularly focused on the various parameters such as the size, shape, and distribution of the metallic nanostructure. These parameters can be used to manipulate initial hot carrier population and the efficiency of transfer to acceptors, thus providing additional opportunities to extract and harness of hot carriers more efficiently.

More effort has been put in this area with many researches, most of the studies are with hot carrier transfer remain in the domain of hot electrons limited with minimum exploration. Although, hot holes can provide more energetic kinetics, the relative to hot electrons will behave as a more efficient energy converter.

It has required a complete understanding of plasmonic hot hole generation, injection and examinations of hot hole flux based on plasmonic architectures have to be experimentally conducted.

In this type of noble metals (Au or Ag) coupled semiconductor composite photocatalysts, the visible light response can be tuned and enhanced via engineering the shape and size of the metal nanostructure since the surface plasmon resonance (SPR) property are highly depends on such structural parameters. 

 

Recently, Song et al., have reported on “Plasmonic Hot Hole-Driven Water Splitting on Au Nanoprisms/P-Type GaN” in ACS Energy Letters [3]. They have reported direct photoelectrochemical (PEC) experimental proof on which the injection of plasmonic hot holes depends on the size of the metallic nanostructures. Their results have clearly indicate that a plasmonic template with smaller Au nanoprisms exhibits higher external and internal quantum efficiencies, leading to a significant enhancement of both oxygen evolution and hydrogen evolution reactions. They have also verified their results with these outcomes stemmed from the enhanced hot hole generation with higher energy and transfer efficiency driven by enhanced field confinement. Their reports have also provide a simplistic strategy by which futuristic solar energy conversion and photocatalysis applications based on plasmonic hot holes can be expedited.

The following are the main observations from their research reports: 

• The Au nanoprisms were directly decorated on p-GaN materials as a plasmonic nanoantenna and exciter of the hot holes. 
• This interfacial structure have facilitated hot hole injection and transport effectively from the metal to the semiconductor. 
• p-GaN behaves as a hole-transporting layer, thus driving the charge separation between hot holes and electrons effectively and inhibiting undesirable charge recombination. 
• This platform is appropriate for extracting and utilizing only the energetic hot hole-driven charge fluxes in PEC reactions, clarifying the impacts of the plasmonic hot holes during the oxygen evolution reactions (OER) with different Au sizes.

In this study, they have performed PEC studies of Au nanoprism/p-GaN templates to solve the V-I characteristics of the plasmonic photocathode, linear sweep voltammetry (LSV) measurements. It has measured in the range of -1.2 to 0.4 V_RHE under light irradiation with a wavelength longer than 425 nm and a power intensity of 100 mW/cm².

From these studies, they have demonstrated that the photocurrent with potential variation shows cathodic behaviour. Their results are mainly consistent with various previous research studies. The pristine GaN didn’t showed PEC activity even with both the conditions like with and without light irradiation.

In this study, they have designed with differently sized Au nanoprism distributed on GaN and all these photocathodes have exhibited negligible catalytic performance without light irradiation and in contrast, the photocurrents in Au nanoprism devices were significantly improved compared with the pristine GaN, indicating that the plasmonic hot carrier fluxes across the Au and p-type GaN crystal junction dominated the PEC characteristics. It has been clearly observed that the highest photoinduced current density of −2.75 μA cm⁻² was achieved in the 90 nm Au/p-GaN photocathode which is approximately twice as high as the outcome of −1.44 μA cm⁻² for 220 nm Au.

Our SNB team have mainly emphasize this new research article to enrich our viewer’s knowledge about the area of intense research on hot hole-deriven water splitting via LSPR Metal Nanostructures. Particularly, their results have proved that the size of nanoprisms has governed both the activity of WS and the photocurrent enhancement which also indicates the improvement in the reaction activity with smaller nanoprisms [4]. Their experimental and theoretical analyses are jointly verified with the size of the plasmonic nanostructure which governed the hot-hole behavior, stemming from the enhanced generation of hot holes with higher energy and enhanced transfer efficiency driven by the stronger field confinement.

Hence, this work will provide a new idea for the development of hot hole applications and guides for the design of future hot carrier technologies.

References

1. J. Zhang, and X. Wang, Angew Chem Int Ed., 54 (2015) 7230−7232.
2. A. M. Brown, et al., ACS Nano, 10 (2016) 57−966.
3. K. Song, et al., ACS Energy Lett., 6 (2021) 1333−1339.
4. R. Jiang, et al., Adv. Mater., 26 (2014) 5274−309.