Page 216 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for photocatalysis 201

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H 2 /h⋅gr, sacrificial agent: methanol [115]) and Sr/tantalates (1.1⋅10 μmol H 2 /h⋅gr, sac-
rificial agent: methanol [116]) under UV-A light irradiation, and over Ni/CdS nanorods
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(6.3⋅10 μmol H 2 /h⋅gr, sacrificial agent: ethanol [117]), CdS/RGO (5.6⋅10 μmol
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H 2 /h⋅gr, sacrificial agent: 10% lactic acid [118]), Pt/CdSe-CdS (4.0⋅10 μmol
H 2 /h⋅gr, sacrificial agent: isopropanol [119]), and N/Zn,Ga-mixed oxide-Rh/Cr 2 O 3
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(>3.7⋅10 μmol H 2 /h⋅gr, sacrificial agent: methanol [120]) under visible light irradi-
ation. Currently, the best photocatalysts for hydrogen generation by photoreforming
under visible light are CdS-based materials [120a].

9.6 Operating variables affecting photocatalyst activity

Because oxygen gas competes with protons as electron scavengers and reacts with
gaseous hydrogen to form water, the presence of oxygen gas in the reaction system
negatively affects the activity during both water photosplitting and photoreforming
processes. The main variables affecting the activity of the photocatalyst toward hydro-
gen generation by water photosplitting and photoreforming are the size of the particles
and their structure, morphology and crystallinity, synthetic procedures, band gap en-
ergy, loading of the co-catalyst, nature and concentration of the sacrificial agent, pH
of the solution, and operating temperature [10,15].
The structure and the morphology of photocatalytic material depend on the syn-
thetic procedure adopted. Different temperatures and reagents used during synthesis
generate particles characterized by different sizes, shapes, and crystallinities.
The amorphous/crystalline ratio must be considered. The nature of the crystallo-
graphic phase can affect the band gap [121]. The presence of structural defects in
the bulk of semiconductors affects electrical properties of the material by generating
localized electronic states that trap charge carriers [17,122]. Therefore it is desirable
to prepare particles without structural defects and impurities.
The particle size is crucial for the activity of the materials. First, it affects the band
gap of the semiconductor [121,123].
Second, the surface area of the photocatalyst is strongly dependent on the
particle size [15]. Particles with a smaller diameter (i.e., nanoparticles) are more
active because they have a higher surface area (high density of surface catalytic
sites).
Moreover, the amount of inner surface defects is proportional to the particle size.
Large particles can be represented as consisting of multi-aggregates of ideal crystals
with different finite sizes [124]. Smaller particles exhibit larger portions of highly
crystalline areas. Nanoparticles consist of only one ideal crystalline particle with finite
dimensions (Fig. 9.10).
Finally, the likelihood of recombination between charge carriers, before occurrence
of surface reactions, is favored in larger-size particles (Fig. 9.10).
The calcination process adopted during photocatalyst synthesis may influence its
surface area and crystallinity. An effective calcination aims to improve the stability
and the crystallinity of the material. Nevertheless, beyond an optimum calcination
temperature, the photocatalytic activity generally decreases because of agglomeration