Hydrogen-evolving semiconductor photocathodes: nature of the junction and function of the platinum group metal catalyst
01 Jan 1982-Journal of the American Chemical Society (American Chemical Society)-Vol. 104, Iss: 25, pp 6942-6948
TL;DR: In this paper, the Fermi level difference of p-InP and H/sup +//H/sub 2/ (0.9 +/- 0.2 eV) was shown to be a function of metal work functions.
Abstract: Noble metal incorporation in the surface of p-type semiconductor photocathodes to catalyze hydrogen evolution leads to efficient solar to chemical conversion if a set of energetic and kinetic criteria are satisfied: (1) the semiconductor-catalyst junction barrier height must be equal to or greater than that of the semiconductor H/sup +//H/sub 2/ junction; (2) the recombination velocity of photogenerated electrons at the semiconductor-catalyst interface must be low; (3) the overpotential for hydrogen evolution at solar cell current densities (approx.30 mA/cm/sup 2/) must be minor. Because of substantial differences in the vacuum work functions of Pt, Rh, Ru, and the (redox potential of the) H/sup +//H/sub 2/ couple, the barrier heights for junctions of each of the four systems with p-InP ought to vary widely. Yet experiments show that all p-InP(M)/H/sup +//H/sub 2/ junctions, where M = Pt, Rh, Ru, or no metal, have essentially the same approx.0.7-V gain in onset potential for hydrogen evolution relative to Pt/H/sup +//H/sub 2/. We attribute the similarity to the known lowering of metal work functions upon hydrogen alloying. Such alloying increases the barrier height and thereby the gain in onset potential over that anticipated from the vacuum work functions. The barrier height, measured as themore » limiting value of onset potential gain at high irradiance, approaches in all cases the Fermi level difference of p-InP and H/sup +//H/sub 2/ (0.9 +/- 0.2 eV). That Fermi level pinning by interfacial states is not the cause of the similar barriers is evident from the reversible decrease in onset potential with hydrogen depletion and by a unity diode perfection factor of the p-InP(Rh)/H/sup +//H/sub 2/ photocathode, which indicates no measurable interfacial recombination of photogenerated carriers. In agreement, the quantum efficiency of carrier collection (hydrogen evolution) nears unity.« less
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TL;DR: The biggest challenge is whether or not the goals need to be met to fully utilize solar energy for the global energy demand can be met in a costeffective way on the terawatt scale.
Abstract: Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2
8,037 citations
TL;DR: In this article, three major ways to utilize nanostructures for the design of solar energy conversion devices are discussed: (i) mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells.
Abstract: The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices: (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utiliza...
2,119 citations
TL;DR: In this paper, the preparation of highly reactive titanium oxide photocatalysts and the clarification of the active sites as well as the detection of the reaction intermediates at the molecular level are discussed.
1,554 citations
TL;DR: In this article, the structural and electronic compatibility between graphitic carbon nitride (g-CN) and co-catalysts has been studied, and it has been shown that g-CN alone shows very poor electrocatalytic activities for water splitting and relies on surface co catalysts to activate its photocatalytic functions.
Abstract: The production of chemical fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Photocatalytic water splitting is a promising route to capture, convert, and store solar energy in the simplest chemical compound (H2). [1] Since the initial report of a photoelectrochemical cell using Pt-TiO2 electrodes for hydrogen evolution by Fujishima and Honda in 1972, considerable studies have been focused on the development of highly efficient and stable photocatalyst powder systems, and especially on using earth-abundant semiconductors and co-factors for water splitting. In practice, the achievement of the conversion of solar energy into hydrogen necessitates the spatial integration of semiconductors and co-catalysts to form surface junctions, so as to optimize the capture of light and to promote charge separation and surface catalytic kinetics. The construction of effective surface junctions is therefore of vital importance, and not only strongly depends on the properties, such as crystal structure, band structure, and electron affinity, of both semiconductors and catalysts but also on the interface between the two materials. In photocatalysis, an ohmic contact between photocatalysts and cocatalysts can allow the prompt migration of light-induced charge, thus resulting in an efficient photocatalytic reaction. Recently, we found that graphitic carbon nitride (g-CN), a polymeric melon semiconductor with a layered structure analogous to graphite, meets the essential requirements as a sustainable solar energy transducer for water redox catalysis; these requirements include being abundant, highly-stable, and responsive to visible light. g-CN is indeed a new type of visible-light photocatalyst that contains no metals, and has a suitable electronic structure (Eg = 2.7 eV, conduction band at 0.8 V and valence band at 1.9 V vs. RHE) covering the water-splitting potentials. An improvement in the efficiency of H2 production has been demonstrated by the introduction of nanohierarchical structures into g-CN. It is noted that, like many other photocatalysts, g-CN alone shows very poor electrocatalytic activities for water splitting and relies on surface co-catalysts to activate its photocatalytic functions. The co-catalyst cooperates with the light harvester to facilitate the charge separation and increases the lifetime of the photogenerated electron/hole pair, while lowering activation barriers for H2 or O2 evolution. Thus, the use of a co-catalyst leads to an increase in overall photocatalytic performance, including activity, selectivity, and stability. Generally, the efficiency of a given photocatalytic system is dependent on the ability of the co-catalysts to support reductive and/or oxidative catalysis. In particular, the structural characteristics and intrinsic catalytic properties of a co-catalyst are important. However, the study of the structural and electronic compatibility between g-CN and co-catalysts has been limited so far. The co-catalysts used are mainly platinum group metals or their oxides, which are scarce and expensive. Photocatalytic/catalytic systems based on abundantly available materials are certainly desirable for large-scale hydrogen production for future energy production based on water and sunlight. Among various hydrogen-evolution reaction (HER) catalysts, molybdenum sulfur complexes have received a lot of attention. MoS2 was found to be a good electrocatalyst for H2 evolution, and the HER activity stemmed from the sulfur edges of the MoS2 crystal layers. [10] When grown on graphene sheets, nanostructured MoS2 exhibited excellent HER activity owing to the high exposure of the edges and the strong electronic coupling to the underlying planar support. Incomplete cubane [Mo3S4] + clusters and amorphous MoS2 are also proven HER catalysts. Some of these HER catalysts have been used in photocatalytic H2 production and they exhibited a remarkable promoting effect. MoS2 has a similar structure to graphite; it has a layered crystal structure consisting of S Mo S “sandwiches” held together by van der Waals force. The fact that g-CN and MoS2 have analogous layered structures should minimize the lattice mismatch and facilitate the planar growth of MoS2 slabs over the g-CN surface, thus constructing an organic–inorganic hybrid with graphene-like thin layered heterojunctions (Scheme 1a). Such a distinct nanoscale structure has some advantages. It can increase the accessible area around the planar interface of the MoS2 and g-CN layers and diminish the barriers for electron transport through the co-catalyst, thus facilitating fast electron transfer across the interface by the electron tunneling effect. Also, thin layers can lessen the light blocking effect of the co-catalyst, thus improving the light utilization by g-CN. Importantly, the intrinsic band structures [*] Y. Hou, J. Zhang, G. Zhang, Y. Zhu, Prof. X. Wang Research Institute of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn
791 citations
TL;DR: In this paper, the authors discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on their own work on cobalt and iron complexes and nickel-molybdenum alloys.
Abstract: Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.
614 citations