Light energy has to be efficiently converted into chemical energy for the sustainable development of humans. Although many photocatalytic devices based on photovoltaic electrolysis have been employed to generate hydrogen (H2) through water reduction, light harvesting and proton reduction in these devices are performed separately, displaying quantum efficiency of approximately 10%–12%.
This article briefly shows how a nano-hybrid photocatalytic assembly allows concomitant photocatalytic production of H2 and pollutant oxidation with up to 20% solar-to-fuel efficiencies.
Challenges Faced in Hydrogen Production
Artificial photosynthesis, that is, artificial conversion of solar light into chemical bonds, has attracted a great deal of interest since it produces storable energy that lacks greenhouse-gas emission. The use of catalysts for photoinduced electron/hole transfer enables the production of molecular hydrogen and oxygen gases from the reduction and oxidation of water, respectively. Conversely, the production of oxygen depends on a multi-step four-electron process, which is not only difficult to achieve but also produces a product that is hard to separate, has low-economic value, and is a direct contender of protons for the light-generated electrons.
Although advances have been made to mitigate the later concerns by separating O2 and H2 evolution in time and space, not many efforts have been made to look for alternative substrates, for example, pollutants, which is capable of enhancing the economic viability and process safety. Furthermore, the usage of H2 goes beyond fuel applications and includes the synthesis of many fine and bulk chemicals, such as drugs, methanol, ammonia, and fragrances. Large-scale production of H2 through water splitting may boost atmospheric oxygen concentration, an unwanted event that can considerably affect the Earth.
Approaches Toward Efficient Conversion of Light
The use of fossil fuels can be mitigated by using low-cost devices for massive solar fuel production. Semiconductor sensitization is another potential strategy, in which the sensitizer harvests sunlight and quickly injects electrons into the TiO2 conduction band (CB). Recently, it was shown that the excitation of Au and Ag LSP resonances enhances charge transfer in dye-sensitized solar cells (DSSCs), photocatalytic oxidations, and photocurrents. Hallett-Tapley and coauthors proposed three reactive processes mediated by LSP - thermal, antenna, or electronic. Among these, the electronic process is applicable for converting light into chemical bonds, inferring the generation of hot electrons.
Figure 1. Nano-hybrid assembly synthesized using a modular design approach. The architecture is comprised of a light absorber and oxidation catalyst (Ag NPs), a molecular wire linker (pABA), a semiconductor (TiO2), a cocatalyst (Ru NPs), and regenerators (Bts).
The generation of electrons upon LSP excitation was established through Au LIII-edge high-resolution X-ray absorption spectroscopy. LSP excitation resulted in an upward shift in the threshold ionization energy (ca. 1.0 eV), along with an increase of Au d-band hole population, which is consistent with the formation of hot electrons. However, hot electrons have a very short lifetime (<100 fs) to promote catalytic reactions, but transferring hot electrons to theTiO2 CB extends their lifetime to microseconds, as demonstrated by transient mid-IR spectroscopy.
Figure 2. The effect of components in hydrogen production and methylene blue (MB+) oxidation. (a) Hydrogen production and (b) methylene blue (MB+) oxidation from the complete silver-based nano-hybrid assembly (Bts-Ag NP–linker–TiO2–Ru NPs), minus the regeneration layer (–Regenerator (Bts)) and without the molecular wire linker (–pABA), upon monochromatic excitation at 405 nm.
Another difficulty to create efficient solar fuels is the conciliation of photo-induced single electron transfer with multi-electron catalysis. Rapid light absorption process produces charged particles, which must be stored or accumulated by systems so that the slow multi-electron catalysis can occur—that is, accumulative charge separation concept.
Figure 3. Effect of linking Ag NPs to TiO2 with a molecular linker to the ultrafast transient infrared absorption signal. Kinetic traces extracted at 2081 cm−1 from transient absorption infrared spectroscopy measurements upon excitation at 405 nm for the Bts-Ag NP–linker–TiO2 (linked system, + linker (pABA)) and Bts-Ag NP–TiO2 (unlinked system, – linker (pABA)).
Metal NPs consist of a vast amount of loosely bonded electrons which, together with LSP capacity to redirect the light flow towards the NP (that is, Poynting vector), make it plausible to generate multi-electrons for each single NP. Tiny metal NPs act as excellent hydrogen evolution catalysts, enabling the accumulation and storage of charge. Gold NPs have been widely employed for conversion and storage of solar energy through LSP. The bottom-up synthesis in solution of alternative silver NPs photocatalytic systems has been sought for both economic and practical reasons.
Figure 4. Evolution of ultrafast transient infrared absorption signal, as Ag NPs connect to TiO2 with a molecular linker. Temporal evolution of the kinetic traces extracted at 2081 cm−1 from transient absorption infrared spectroscopy measurements upon excitation at 405 nm for the (a) Bts-Ag NP–linker–TiO2 (linked system), and (b) Bts-Ag NP–TiO2 (unlinked system). Inserts show the infrared absorption intensity over time extracted at 1.5 ps.
Modular Nano-Hybrid Assembly
Here, a modular nano-hybrid assembly developed using silver NPs created through bottom-up synthesis is presented briefly. The nano-hybrid architectures can achieve long-lived charge separate states, allowing concomitant photocatalytic generation of pollutant oxidation and H2. The nano-hybrid system is made up of a molecular wire linker, natural product-stabilized Ag NPs, Ru NPs as a co-catalyst, and TiO2 as a semiconductor. A flow photoreactor was used to perform experiments. The results demonstrate a significant increase in hydrogen evolution when a molecular wire joins the systems.
Figure 5. Effect of adding the catalyst to the ultrafast transient absorption signal. Kinetic traces extracted at 795 nm from transient absorption spectroscopy measurements upon excitation at 405 nm for the complete system and in the absence of Ru NPs (hydrogen evolution catalyst). The instrument response function is ca. 200 fs.
A considerable rise in the multi-electron photo-oxidation of methylene blue was observed for the complete system, which is in contrast with the low activity of the unconnected system where electron transfer takes place through haphazard collisions between TiO2:Ru NPs (electron acceptor) and Bts-Ag NPs (electron donor).
The solar-to-fuel efficiency or quantum yield of the nano-hybrid system was directly measured, with the assumption that the steady-state amount of H2 produced (150 nmol min−1) at a known photon flux. In addition, a quantum yield of 19.9% ± 0.5%was calculated with the assumption that each photon absorbed has an equal probability of producing a hot electron and a reaction stoichiometry of two electrons for each molecule of H2 formed. At present, the best performing systems based on photovoltaic electrolysis achieve 10%–12% conversion efficiency.
Figure 6. Photoluminescence signal emanated from Ag NPs regenerating layer. (a) Steady-state photoluminescence originated from the complete system during photocatalysis. (b) Steady-state fluorescence profile of betanin in water (10 ppm AcOEt) at pH = 12. (c) Kinetic trace extracted at 515 nm with streak camera time-resolved fluorescence upon excitation at 390 nm of Bts-Ag NPs_pABA_TiO2:Ru NPs in aqueous solution.
While this observation contributes to inferring the increase in hydrogen evolution in the complete system, it leaves behind rationalization regarding the rise in methylene blue degradation relatively to the unconnected system and even more so to the system developed with Ag NPs with PVP termination. The recommended hole stabilization mechanism is susceptible to Ag NPs termination and hence must involve the molecular structures on Ag NPs.
One potential explanation for hole stabilization and the increase of electron-hole pair lifetime is that following the transfer of electrons from Ag NPs to TiO2, the Bts moieties transfer electrons to Ag NPs. Another possibility is the agreement between electron transfer from Ag NPs to TiO2, electron/hole recombination, and electron transfer from Ag NPs to the highest occupied molecular orbital (HOMO) of excited Bts. The last process leads to a stabilized anion-radical of Bts at the Ag NPs’ surface, boosting the persistence of holes.
Figure 7. Charge particles pathways and reaction intermediates in the nano-hybrid assembly leading to a long-lived charge-separated state. Electronic excitation is represented as dashed red arrows; this scheme does not necessarily represent the physical location where events take place. Energy levels were taken from the literature.
To sum up, a silver-based nano-hybrid assembly was developed by integrating natural pigments as well as molecular wire linkers employed in LSP and DSSC-based photoconversion systems. The significant increase in the charge separation state lifetime in comparison to the traditional synthesis of silver systems made it possible to generate hydrogen and oxidative pollutant abatement simultaneously under visible light irradiation. This bottom-up modular design can be adjusted to accommodate preferred applications by altering the identity of a tiny number of basic units, and also the ratio between them. The semiconductor, the linker, the metal co-catalyst, and the plasmonic structure can be permuted and integrated into virtually infinite ways, in parallel to metallic organic frameworks.
This information has been sourced, reviewed and adapted from materials provided by Peafowl Solar Power.
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