Page 271 - Multifunctional Photocatalytic Materials for Energy

Multidimensional TiO 2 nanostructured catalysts for sustainable H 2 generation 253

lie between 1.5 and 2.5 eV [146,148–153]. So far, several hundred semiconductors
have proved be capable of producing H 2 via WS under light irradiation. Among vari-
10
0
ous metal-based photocatalysts, metal cations with d or d electronic configurations
have been used to construct CBs, whereas VBs usually consist of nonmetal elements,
such as N, O, S, and Se (Fig. 11.8D) [148]. Moreover, elements containing Ru, Rh,
Pd, Os, Ir, Pt, Ag, and Au are good candidates as co-catalysts for H 2 production.
Because of the surface plasmon resonance (SPR) effects induced by the collective os-
cillations of the conduction electrons, Ag and Au further enhance the H 2 production
rate under visible light irradiation. Additionally, the first-row transition metals and
their oxides, such as Co, Ni, and Fe, are considered to be good Earth-abundant co-
catalysts for hydrogen generation. Among the photocatalysts shown in Fig. 11.8A,
TiO 2 is the most widely studied one for WS because it is stable, noncorrosive, envi-
ronmentally friendly, abundant, and cost-effective. Compared with other approaches,
photocatalysis has proved to be the simplest WS process, which makes it amena-
ble for economically viable large-scale production of H 2 . However, it suffers from
rapid electron-hole pair recombination, and the large band gap (+ 3.2 eV) of TiO 2
limits its utilization of the solar spectrum to only the UV region, which creates low
solar-to-hydrogen efficiencies (< 0.1%). Thus bulk doping and surface engineering
strategies have played a very important role in improving the photogenerated charge
transfer/separation and in shortening the transfer distance of photoinduced charge
carriers to surface reaction sites.
With an additional potential bias provided by a photovoltaic element, photoelec-
trolysis has proved to be a promising route to improving WS with higher efficiency
[154–157]. In this process, electrons at the VB are excited to the CB of the TiO 2 under
+
illumination. Then the amount of H is reduced into hydrogen by the electrons at the
CB, and the generated holes at the VB oxidize water molecules into O 2 (Fig. 11.8B).
As shown in Fig. 11.8C, the electrons can be promoted from the VB to the CB with
irradiation and then driven to the CE via the external circuit, which can significantly
promote the transmission of photocarriers against the electron-hole pair recombina-
tion, despite the electrons and holes reacting with water at two spatially separated
electrodes, all of which leaves the holes on the surface of the TiO 2 electrode [158,159].
Holes on the surface of a TiO 2 electrode are particularly beneficial for environmental
remediation because adsorbed pollutants are easily degraded and harmful by- products
+
diffuse away, while holes at the VB can react with H 2 O 2 to produce O 2 and H ions,
+
and electrons at the CB can react with H ions to generate H 2 [147,160–163]. In prac-
tice, photoelectrocatalytic WS is better able to slow down fast electron-hole pair re-
combinations than is physical separation. However, the inherent imperfections of TiO 2
remain, including fast recombination of electron-hole pairs, poor kinetics, low visible
light absorption, low solar-to-hydrogen ratio, and so on. A detailed and systematic
discussion of bulk doping approaches and surface engineering strategies in photo-
electrocatalytic WS for promoting charge separation and transfer is presented in the
following section, including enlarging the surface area, optimizing surface facets and
band gap positions, increasing visible light absorption, reducing costs, and enhancing
surface reaction kinetics [164–172].