Fabrication of Optical Gap Selective Amorphous Carbon Photocatalyst for Hydrogen Production Under Irradiation with Visible Light
Authors: Okafuji Keigo , Kobayashi Ryutaro , Ohtomo Shinpei , Naragino Hiroshi , Honda Kensuke 
Published: February 2020

1. Introduction
TiO2 photocatalyst cannot achieve 10 % quantum efficiency for hydrogen production under the sunlight because it cannot absorb visible light. To utilize visible light efficiently, photocatalysts with an optical gap (Eog) of 1.65 eV are required. Eog of Si-added amorphous carbon (a-C) can be controlled by atom % of introduced Si. When the ratio of Si and C (Si/C) are set in the range from 0.35 to 0.85, a valence band can be located under oxidation potential of water and conduction band over reduction potential of water [1] [2]. Si-added a-C, however, does not exhibit photocatalytic activity because active sites do not exist on the surface. The objective of this study is to add photocatalytic activity to Si-added a-C nanoparticles with high surface area by introducing active sites into them. As an active site for carbon-based photocatalysts, tri-s-triazine structure has been proposed [3]. In this structure, water molecules were adsorbed at lone pairs of nitrogen and electrons were localized at triazine structure at the edge of graphite. Therefore, active sites were introduced into Si-added a-C by incorporating nitrogen atoms into the edge planes of sp2 cluster in a-C.

2. Experimental
Si-added a-C was synthesized at 75 W RF power using RF-PeCVD method. As Si, C and N sources, a mixed solution of tetramethylsilane and 1,1,3,3-tetramethyldisilazane was used. As a source material for active sites, triazine was selected. To synthesize nanoparticles, an aluminum plate with holes (a diameter of 5 mm) was inserted between anode and cathode. Current-Voltage curves were measured in the dark and under UV light (365 nm, 2 mW/cm2). Quantum efficiency for hydrogen production was estimated by measuring absorbance of decomposed indigocarmine after the irradiation of UV light.

3. Results and Discussion
From DFM image, the diameter of a-C nanoparticle was found to be approximately 18 nm in average. Atomic % of C, N, O, and Si at the particles estimated from XPS were 26.2, 2.26, 42.8, and 28.74, respectively. Si/C of the particles was 1.10, which is relatively higher than the target range (from 0.35 to 0.85). Eog was found to be 1.0 eV, narrower than Eog (2.7 eV) predicted from Si/C (1.10). Volume resistivity of the particles (283.9 Ωcm) obtained by Hall-effect measurement was fairly smaller than that of Si-added a-C thin film synthesized without an aluminum plate (4.32×104 Ωcm). It is assumed that the existence of sp2 cluster with a larger diameter induced the reduction of Eog. Electronic structure of Si-added a-C particles was calculated using Eog and reported ionization energy at a-Si1-xCx with the same Si/C [1] [2]. The edge of conduction band (-4.0 eV) is located over the reduction potential of water (-4.44 eV), while the top of valence band (-5.0 eV) is located over oxidation potential of water (-5.67 eV). As a result, the reactivity of Si-added a-C particles toward water oxidation is low. It is confirmed that photocatalytic activity can be evaluated using our system since oxidation potential of indigocarmine is -3.7 eV. The quantum efficiency of Si-added a-C nanoparticles estimated from photocurrents for O2 evolution in the photoelectochemical measurement under exposure to UV light was 0.01 %. The value is relatively low compared to that of the thin film (4.78 %). The difference is supposed to be caused by sp2 clusters which act as recombination centers. In N1s XPS spectra, the peaks attributable to C-N=C of chain and ring were observed. Evidently, the structures with high water adsorptive property are introduced onto particle surfaces. In FTIR spectra, the peaks of deformation vibration of water became noticeable by introducing C-N=C structure. It indicates the improvement of the water adsorptive property. The onset potential of hydrogen evolution in photoelectrochemical measurement was positively shifted with improving the water adsorptive property. This behavior shows the overpotential of hydrogen evolution is lowered. In the test of photocatalytic activity, the resulting nanoparticles can decompose indigocarmine at a rate of 0.118 μmol/min. Consequently, photo-generated holes are found to be utilizable for oxidation of indigocarmine and photo-generated electrons for reduction of water. If all photo-generated electrons were used for hydrogen production, hydrogen production rate is estimated to be 0.059 μmol/min (0.85 % quantum efficiency). The efficiency is quite low. Photocatalytic activity can be improved by decreasing in the size of sp2 cluster.

References
[1]Xu. X et al., J Non-Cryst .Sol 90 pp295-298 (1987).
[2]Andreas. M et al., Thin Solid Films
341 pp68-72 (1999).
[3]X. Wang et al, Nature Mater
8 pp76-80 (2008).

Figure 1