Nickel Catalyst Boosts Solar Hydrogen Generation of CdSe Nanocrystals

Abstract

With the rapid growth of global population and energy demand, exploring renewable and sustainable energy from natural resources such as sunlight is critical. Although solar energy is a promising solution for future energy demand, the solar irradiation on earth is an intermittent source of energy; and, therefore, it requires an efficient and cost effective method to store the excess energy. One of the most attractive approaches is to convert solar energy into the chemical fuels such as hydrogen. For instance, nature is capable of transforming solar energy into carbohydrates through photosynthesis. Likewise, solar energy can be stored in the form of hydrogen through artificial photosynthesis (solar water splitting). Hydrogen gas is an environmentally attractive chemical fuel with high energy density of 140 MJKg . Artificial photosynthesis is a chemical process that involves three critical steps: light harvesting; charge separation; as well as charge transport and transfer. Chromophores, such as dye molecules, or semiconductor materials can be employed as the light-absorbing medium. Photo-bleaching of the dye, however, is a major limitation for their application in solar conversion devices. In comparison with organic or organometallic dye molecules, semiconductor nanocrystals exhibit excellent photostability. Additionally, the small dimension of nanocrystals provides large surface area and short diffusion length for charge carriers. Among different semiconductors, CdSe nanocrystals hold great promise as efficient chromophores for solar water splitting. Bulk CdSe has a favorable bandgap of 1.7 eV for visible light absorption. The conduction band of CdSe is more negative than proton reduction potential, making it suitable for photo-generation of hydrogen. Previous studies have shown that CdSe nanoribbons and nanocrystals exhibit photoactivity for photochemical hydrogen evolution upon irradiation with ultraviolet and visible light in aqueous solution containing sacrificial electron donors such as Na2S/Na2SO3 solution. [3,4] Yet, the hydrogen photo-generation rate was modest, with turnover frequency of 2, which could be attributed to the large overpotential for proton reduction on the CdSe surface as well as the slow interfacial electron transfer between CdSe nanocrystals and electrolyte. Additionally, the relatively poor electrochemical stability of CdSe was another concern. CdSe can be easily oxidized by oxygen unless sacrificial hole scavengers such as S are added in the electrolyte solution. Recently, Han and co-workers at University of Rochester reported a robust and highly active Ni catalyst modified CdSe nanocrystal system for photo-generation of hydrogen in aqueous solution, with a remarkable turnover frequency of >600000 and an extraordinary longevity for hydrogen generation. The catalytic mechanism of Ni-modified CdSe (denoted as Ni-CdSe) nanocrystals is illustrated in Figure 1a. CdSe nanocrystals capped with dihydrolipoic acid (DHLA) were synthesized by a hot injection method and followed ligand exchange method. Aqueous solution containing 1.0m ascorbic acid as sacrificial electron donor was used as electrolyte solution. Ni salt [Ni(NO3)2,] was added as photocatalyst for proton reduction. Upon illumination, photo-excited electrons in CdSe nanocrystals shuttle to Ni catalyst and reduce protons to hydrogen; and photo-excited holes oxidize ascorbic acid to regenerate protons. Figure 1b shows the performance of CdSe nanocrystals for hydrogen production over time in the presence and absence of Ni catalyst. The linear profiles indicate that the hydrogen generation rates are constant for over 360 h, proving that the photoactivities of CdSe nanocrystals are extremely stable. Significantly, the Ni-CdSe nanocrystals boost the rate of hydrogen generation by more than an order of magnitude compared to bare CdSe nanocrystals, indicating Ni is excellent catalyst that facilitates the electron transfer from CdSe to electrolyte for proton reduction. In optimized conditions, Ni-CdSe system achieved a remarkable turnover number over 600000 moles of H2 per mole of catalyst. A quantum yield of 36% was obtained at the irradiation wavelength of 520 nm in aqueous solution. Even higher quantum yield of 60% at 520 nm was achieved in a 1:1 water-ethanol solvent, which is a benchmark value for an aqueous system. Four key innovations were demonstrated in the work of Han et al. , which presents a scientific breakthrough in the development of catalyst for photo-generation of hydrogen. First, the reported Ni-CdSe nanocrystal system showed outstanding photocatalytic performance with turnover number over 600000 moles of H2 per mole of catalyst, which is substantially improved, compared to the previously reported bare CdSe nanocrystals as well as hydrogen evolution catalyst (HER) modified CdSe systems. Second, the Ni-CdSe system demonstrated excellent longevity. Previous studies have showed that the photoactivity of CdSe for hydrogen generation can only last for less than 20 h. Significantly, the work of Han et al. proved that the Ni-CdSe system can be operated in aqueous solution without obvious photoactivity loss for two weeks, [a] G. Wang, Prof. Y. Li Department of Chemistry and Biochemistry University of California, Santa Cruz California 95064 (United States) Fax: (+1)831-459-2935 E-mail : yli@chemistry.ucsc.edu