**3. The photocatalytic hydrogen evolution applications of metal/ semiconductor hybrid nanocrystals**

## **3.1. Mechanism of photocatalysis on semiconductors and key problems**

Since Fujishima and Honda [79] discovered photoelectrochemical formation of H2 over TiO2 electrode, photocatalytic water splitting and H2 evolution are an attractive solution of global Metal/Semiconductor Hybrid Nanocrystals and Synergistic Photocatalysis Applications http://dx.doi.org/10.5772/61888 315

**Figure 17.** (A)Schematic for preparing Pb–SiO2 yolk–shell nanoparticles and TEM images of Pb cores (B), Pb–SiO2 core–

**Using hard template for metal/semiconductor yolk–shell nanostructure:** By hard template method, shell coating on core is synthesized as middle layer and subsequently removed or etched. The material of middle shell includes SiO2, carbon, polymer, and so on. For example, Au–TiO2, Au–ZrO2, Au–SnO2, and Au–SiO2 yolk–shell nanoparticles are synthesized by coating semiconductor shell on Au–oxides and subsequently etching oxides layer (Figure 18) [74–77]. In most of case, SiO2 is employed as hard template to form the middle layer and etched

For example, Au–ZrO2 yolk–shell nanoparticles can also be synthesized by etching SiO2 layer of Au–SiO2–ZrO2 core–shell nanoparticles (Figure 18A) [74]. It is reported that such Au– ZrO2 yolk–shell nanostructure is stable at high temperature and can be used as catalyst for the oxidation of CO. Taking SiO2 as template, SnO2 hollow nanosphere, and Au–SnO2 yolk–shell nanoparticles can be obtained (Figure 18B). Lou et al. show their work on preparation of Au– SnO2 yolk–shell nanoparticles, etching the SiO2 layer with HF and they found that controlling the size of SiO2 template cage-like and double layer shell of Au–SnO2 can be obtained [75]. The method can also be used for synthesis of yolk–shell with other hybrid material shell [78].

Since Fujishima and Honda [79] discovered photoelectrochemical formation of H2 over TiO2 electrode, photocatalytic water splitting and H2 evolution are an attractive solution of global

**3. The photocatalytic hydrogen evolution applications of metal/**

**3.1. Mechanism of photocatalysis on semiconductors and key problems**

shell nanoparticles (C), and Pb–SiO2 yolk–shell nanoparticles(D). Copyright: ACS, 2011.

314 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

by NaOH, HF, or other reagents.

**semiconductor hybrid nanocrystals**

**Figure 18.** TEM images of (A) Au–ZrO2 yolk–shell nanoparticles [74]. Copyright: Wiley-VCH, 2006. (B) Au–SnO2, yolk– shell nanoparticles [75]. Copyright: Wiley-VCH, 2007. (C) Au–Zr0.5Si0.5O2 yolk–shell nanoparticles [76]. Copyright: Springer, 2014. (D, E) Fe3O4/Au–SiO2 yolk–shell nanoparticles. (F) Au–SiO2 yolk–shell nanoparticles. (G–J) Ag–SiO2 yolk–shell nanoparticles [77]. Copyright: Royal Society of Chemistry, 2010. (K, L) TEM and HAADF-STEM images and EDS maps of Au–GO/TiO2 yolk–shell nanoparticles [78]. Copyright: America Chemistry Society.

energy supply and related environmental issues. Numerous researchers had extensively studied water splitting using semiconductor photocatalysts since the finding [80–87]. The fundamental principle of solar water splitting for semiconductor photocatalysts is shown in Figure 19. Semiconductor materials have a band structure in which the conduction band (CB) is separated from the valence band (VB) by a band gap with a suitable width. When the energy of incident light is larger than that of a band gap, the electrons in the VB of the semiconductor photocatalyst are excited to the CB, while the holes are left in the VB. Therefore, it creates the negative-electron (e-) and the positive-hole (h+) pairs. These photogenerated electron–hole pairs may further be involved in the following possible processes: (1) successfully migrate to the surface of semiconductor, (2) be captured by the defect sites in bulk and/or on the surface region of semiconductor, and (3) recombine to release the energy in the form of heat or photon. If the photoexcited carriers separate and migrate to the surface without recombination, adsorbed species are reduced and oxidized by the photogenerated electrons and holes to produce H2 and O2, respectively (Figure 19).

**Figure 19.** Process in photocatalytic water splitting [80]. Copyright: Royal Society of Chemistry, 2009.

For effective water splitting, important points in the semiconductor photocatalyst materials are their band structure, including the band gap and positions of VB and CB. The bottom level of the CB has to be more negative than the redox potential of H+ /H2 (0 V vs. NHE), while the top level of the VB be more positive than the redox potential of O2/H2O (1.23 V) [80]. Therefore, a minimum band gap of 1.23 eV is required, while a much larger band gap (usually >2.0 eV) is often needed for appreciable water splitting reaction due to additional overpotential associated with each electron transfer and gas evolution step [84]. The band edge positions of some semiconductor photocatalysts are shown in Figure 20. The band structure of a semicon‐ ductor is not only determined by its own crystal phase and vacancies but can also be modified by the introduction of foreign elements into the bulk or surface of the semiconductor. Take the most studied photocatalyst TiO2 as an example, the larger band gap (~3.2 eV) of TiO2 limits its utilization of the solar spectrum to only the ultraviolet (UV) region (wavelength λ < 400 nm) [85]. The solar spectrum has a very small fraction of UV light (ca. 5%) in comparison with those of visible light (400 < λ < 800 nm, ca. 43%) and near-infrared (NIR) light (800 < λ < 2500 nm, ca. 52%) [83]; therefore, studies on visible-light-driven photocatalysts are more important for practical applications. There are two efficient strategies to make TiO2 as visible-light-driven photocatalysts as shown in Figure 2B [84]: (1) narrow the band gap of TiO2 to make it absorb visible light by introducing other elements into TiO2 and (2) modify the TiO2 surface with other visible light active materials (dye or quantum dot) as a light harvester to sensitize TiO2.

Metal/Semiconductor Hybrid Nanocrystals and Synergistic Photocatalysis Applications http://dx.doi.org/10.5772/61888 317

If the photoexcited carriers separate and migrate to the surface without recombination, adsorbed species are reduced and oxidized by the photogenerated electrons and holes to

**Figure 19.** Process in photocatalytic water splitting [80]. Copyright: Royal Society of Chemistry, 2009.

of the CB has to be more negative than the redox potential of H+

For effective water splitting, important points in the semiconductor photocatalyst materials are their band structure, including the band gap and positions of VB and CB. The bottom level

top level of the VB be more positive than the redox potential of O2/H2O (1.23 V) [80]. Therefore, a minimum band gap of 1.23 eV is required, while a much larger band gap (usually >2.0 eV) is often needed for appreciable water splitting reaction due to additional overpotential associated with each electron transfer and gas evolution step [84]. The band edge positions of some semiconductor photocatalysts are shown in Figure 20. The band structure of a semicon‐ ductor is not only determined by its own crystal phase and vacancies but can also be modified by the introduction of foreign elements into the bulk or surface of the semiconductor. Take the most studied photocatalyst TiO2 as an example, the larger band gap (~3.2 eV) of TiO2 limits its utilization of the solar spectrum to only the ultraviolet (UV) region (wavelength λ < 400 nm) [85]. The solar spectrum has a very small fraction of UV light (ca. 5%) in comparison with those of visible light (400 < λ < 800 nm, ca. 43%) and near-infrared (NIR) light (800 < λ < 2500 nm, ca. 52%) [83]; therefore, studies on visible-light-driven photocatalysts are more important for practical applications. There are two efficient strategies to make TiO2 as visible-light-driven photocatalysts as shown in Figure 2B [84]: (1) narrow the band gap of TiO2 to make it absorb visible light by introducing other elements into TiO2 and (2) modify the TiO2 surface with other visible light active materials (dye or quantum dot) as a light harvester to sensitize TiO2.

/H2 (0 V vs. NHE), while the

produce H2 and O2, respectively (Figure 19).

316 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 20.** (A) Relationship between band structure of semiconductor and redox potential of water splitting [80]. Copy‐ right: Royal Society of Chemistry, 2009. (B) Donor level (a), acceptor level (b), and mid-gap states (c) formed by metal ion doping. (C) The mechanism of H2 production for (a) dye-sensitized and (b) QDs-sensitized semiconductor [82]. Copyright: Royal Society of Chemistry, 2015.

After excited charges are created, efficient charge separation is next crucial factor determining the light to fuels conversion efficiency. Crystal structure, crystallinity, and particle size strongly affect the step [86]. The defects operate as trapping and recombination centers between photogenerated electrons and holes, resulting poor photocatalytic activity. Therefore, improv‐ ing the crystalline quality will decrease the amount of defects and improve photocatalytic activity. If the particle size becomes small, the distance that photogenerated electrons and holes have to migrate to reaction sites on the surface becomes short, and this results in a decrease in the recombination probability. Also, the fabrication of junction structure has been recognized as an effective strategy to avoid charge recombination in semiconductors.

The surface catalytic reaction is a successive step of charge separation. The important points for this step are surface character (active sites) and quantity (surface area). Even if the photo‐ generated electrons and holes possess thermodynamically sufficient potentials for water splitting, they will have to recombine with each other if the active sites for redox reactions do not exist on the surface. Cocatalysts such as Pt, NiO, and RuO2 are usually loaded to introduce active sites for H2 evolution [87], which will be discussed later.

The processes of the photocatalytic reaction on a semiconductor photocatalyst involve light absorption, charge separation, carrier migration, and surface catalytic reactions. Therefore, developing band gap engineering to narrow down the band gap of semiconductor materials for absorbing broader spectrum of solar energy and materials engineering to tune the physical properties (crystal structure, crystallinity, and particle size) for gaining efficient charge separation and migration and creating enough active sites are the key problems of improving photocatalytic hydrogen evolution efficiencies.
