Page 205 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for photocatalysis 191

Finally, the catalyst should be (i) photo-hydrostable in an aqueous environment
without undergoing photocorrosion processes [14] and (ii) easily recoverable from the
mixture at the end of the process.
To date, several heterogeneous semiconductors satisfying some of the above-
mentioned requirements have been developed for photocatalytic production of hydro-
gen. In this regard, an energy conversion efficiency for the photocatalytic hydrogen
production can be defined as [15,16]

Output energy asH R × DG o
QE = 2 % = H 2 H O %
2
Input energy (incident light ) IS ×

æ mol ö o æ J ö æ W ö
,
where R H 2 ç ÷ , DG HO ç ÷ , I ç 2 ÷ and S (cm ) are the rate of hydrogen
2
è s ø 2 è mol ø è cm ø
generation, the standard Gibbs free energy for generating one mole of hydrogen from
−1
water (237 kJ mol ), the specific power of the radiation source, and the irradiated sur-
face, respectively. This expression is reported if hydrogen is produced through water
photosplitting only.
A different form of light-to-hydrogen conversion efficiency can be used if photore-
forming is also considered,

R ××D H o
2
QE = H 2 comb %
×
IS
o
where ∆H comb is the standard enthalpy change for the combustion reaction of hydro-
−1
gen with oxygen (−282 kJ mol ).
However, the QE value reported for a certain number of photocatalysts does not
take into account the adopted catalyst load. On the other hand, the activities of differ-
−1
ent photocatalysts (μmol H 2 ⋅(s⋅gr catalyst ) ) cannot be directly compared to each other
because the operating conditions adopted in the experiments (e.g., the effective spe-
cific light source) are quite different.
In the periodic table, shown in Fig. 9.4, the main metallic elements used, mainly in
the form of oxides or sulfides, as photocatalysts for photochemical hydrogen produc-
tion are highlighted [15,17]. Observe that the oxidation states of the elements in these
6+
5+
4+
5+
4+
4+
3+
3+
4+
photocatalysts are the highest (Ti , Zr , Nb , Ta , W , Ce , Ga , In , Ge ,
5+
4+
Sn , Sb ) [14]. In the table, the transition-metal cations with empty d or f orbitals
0
0
have d and f electronic configuration, respectively, whereas the metal cations with
10
filled d orbitals have a d electronic configuration.
Doping photocatalytic materials with selected transition-metal elements, used as
3+
2+
co-catalysts in zero-valent (Au, Ag, Cu, Pt, etc.) or higher oxidation state (Cr , Ni ,
3+
+4
Rh , Ru ), can contribute to the design of novel visible-light-driven materials with
an increased quantum efficiency (QE).
n
It is noteworthy to stress that some metal compounds with d (0 < n < 10) elec-
tronic configurations, such as ZnO or CdS, are expected to be effective photoactive