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Metal-based semiconductor nanomaterials for photocatalysis        205

           9.7   Conclusion


           Hydrogen generation using solar energy through catalytic water photosplitting and
           catalytic organic photoreforming is one of the most promising technologies for clean
           and sustainable production of energy.
              The literature surveyed points out that, despite considerable efforts devoted to the
           synthesis of metal-based composites for hydrogen production, the activity and stability
           of the water in photocatalytic materials are far from being satisfactory for commercial
           utilization. Current results still record low efficiencies for visible-light-to-hydrogen
           conversion (<3.0% quantum efficiency) for water photosplitting. This difficulty can
           be ascribed mainly to the lack of photocatalytic materials that have simultaneously a
           band-edge potential suitable for overall water splitting and a band gap energy suffi-
           ciently low to be employed in the visible light range.
              To date, no metal-based composite semiconductors have been found that are ca-
           pable of promoting water photosplitting or photoreforming using visible light with a
           photonic efficiency higher than the limit value indicated as being suitable for practical
           applications (>10% at 600 nm).
              The photocatalytic materials for visible light water splitting and photoreforming
           should have proper band gap energy (1.6–2.2 eV) and band alignment, high specific
                      4
           activity (>10  μmol H 2 /h⋅gr), photostability in aqueous electrolyte media, and high
           crystallinity.
              It is also important to observe that, in view of future applications of photocatalytic
           hydrogen production, reduction in costs and toxicity of photocatalysts are key issues.
           The possibility of hydrogen generation through photoreforming processes requires
           large amounts of biomass, high organic load rates, and low hydraulic retention times
           achieving through the use of sludge and sewage.
              However, considering the pioneering results achieved by Fujishima and Honda
           [143] in photocatalysis up to those of the present day, technically and economically
           viable visible light photocatalytic systems for water photosplitting and photoreform-
           ing could become available in the future. Therefore engineering a design for photo-
           catalytic nanomaterials that are active under natural sunlight radiation and that have a
           high photonic efficiency toward hydrogen production will be a key challenge.


           References


              [1]  V. Balzani, N. Armaroli, Energy for a Sustainable World: From the Oil Age to a Sun-
                  Powered Future, Wiley-VCH Verlag, Weinheim, 2010.
              [2]  G.  Centi, R.A.  Van Santen, Catalysis for Renewables: From Feedstock to Energy
                  Production, Wiley-VCH Verlag, Weinheim, 2008.
              [3]  Ullmann’s Encyclopedia of Industrial Chemistry, 2016. Wiley-VCH Verlag, Weinheim,
                  Germany.
              [4]  H.J. Alves, C.J. Bley, R.R. Niklevicz, E.P. Frigo, M.S. Frigo, C.H. Coimbra-Araujo,
                  Overview of hydrogen production technologies from biogas and the applications in
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