Catalytic hydrogen production from renewables is a promising method for providing energy carriers in the near future. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. The most promising semiconductor at present is TiO2. The successful design of these catalysts relies on a thorough understanding of the role of the noble metal particle size and the TiO2 polymorph. Here we demonstrate that Au particles in the size range 3–30 nm on TiO2 are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3–12 nm range. The high hydrogen yield observed makes these catalysts promising materials for solar conversion.
Figures at a glance
Figure 1: Structural analysis of a typical Au/TiO2 photocatalyst of this study for hydrogen production showing strong contact between Au and the TiO2 support. 
a, A high-resolution transmission electron microscopy image of the 2 wt% Au/TiO2 anatase photocatalyst. The dark area (i) is Au and the lighter surrounding area (ii) is TiO2. This can be confirmed by Fourier transform analysis of these selected areas. b, Lattice fringes of the anatase support. c, Characteristic Au (111) and (200) lattice fringes at 0.24 and 0.20 nm.
Figure 2: Particle size distribution of Au on TiO2 photocatalysts for hydrogen production, as determined by high-angle annular dark-field scanning TEM. 
The mean Au particle size on anatase (A) is smaller than that on rutile (R). 1A, 2A, and so on, refer to 1wt%, 2wt%, and so on, of Au.
Figure 3: H2 photo-production from ethanol over Au/TiO2 anatase (A) and rutile (R) as a function of Au loading. 
The left-hand y-axis presents the normalized rate per unit area for Au/TiO2 anatase nanoparticles and the right-hand y-axis is that for Au/TiO2 rutile nanoparticles. The x-axis presents the weight % of Au on the TiO2 support. Particle size distribution of Au is shown in Fig. 2. Note the two orders of magnitude difference in the rate of hydrogen production between the anatase catalysts and the rutile catalysts.
Figure 4: Dark-field TEM (top) and XPS Au 4f (bottom) of the Au/TiO2 anatase and rutile photocatalysts. 
The main observation is the similarity of the electronic properties of Au despite a one order of magnitude difference in the particle size. The XPS Au 4f7/2 binding energy is consistent with zero-valent Au (dashed line). The nominal wt% of Au is indicated in each part.
