Highly Efficient Visible-Light Driven Carbon Particles/g-C3N4 Photocatalysts with Enhanced Photocatalytic H2 Production
Authors: Nakai Yuto , Katsumata Hideyuki , Suzuki Tohru , Kaneco Satoshi 
Published: February 2020

As a typical metal free inorganic semiconductor, graphitic C3N4 (g-C3N4) has attracted intensive attention for H2 generation, pollutant degradation and CO2 reduction. It is well-known that the band gap of g-C3N4 is about 2.7 eV, which can absorb visible light up to 460 nm. Furthermore, the conduction band minimum of g-C3N4 is extremely negative, so photogenerated electrons should have high reduction ability. However, the photocatalytic efficiency of the pure g-C3N4 is limited by the high recombination rate of its photo-generated electron–hole pairs. One of the techniques for increasing the separation efficiency of photo-generated electron–hole pairs is to load a nonmetal element into a photocatalyst. For the increase of the photocatalytic activity, an extensively studied carbonaceous candidate is carbon particles with tunable size and band gap, abundant hydrophilic surface groups, and long term stability. Particularly, carbon particles can act as spectral converters to overcome the contradiction between optical absorption and chemical reaction dynamics of various semiconductors with UV and visible light activity because of its large cross-section of upconversion absorption and a wide range of emission spectra that boost the energy band-gap matching. In this study, novel carbon particles and g-C3N4 composite photocatalysts were prepared through a facile two-step calcination using carbon particles and melamine as a starting material.　The synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), nitrogen-sorption, UV-Vis diffuse reflectance spectra (DRS), photoluminescence spectra (PL), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Photocatalytic activity of carbon particles modified g-C3N4 (C/g-CN) was evaluated by photocatalytic hydrogen production under visible irradiation (λ ≧ 420 nm).

Firstly, 3.6 g of glucose was dissolved in distilled water (40 mL) to form a clear solution, then 40 mL of concentrated HCl solution was dropped into the solution of glucose. Afterward, the mixture solution was transferred into a 100 mL Teflon liner followed by hydrothermal treatment at 180 ℃ for 6 h. After hydrothermal reaction, the black precipitates were centrifuged and get brown carbon particles solution. Carbon particles and g-CN composite photocatalysts were prepared by two step calcination of the mixture of carbon particles and melamine. Before calcination, 5 mL of carbon particles suspension in ethanol was added into 3g of melamine, then ethanol was evaporated. This obtained ingredient was applied two times calcination, first heating at 550 ℃ for 3h, second heating at 500 ℃ for 10h. Physicochemical measurement of the samples were characterized by XRD, FT-IR, XPS, nitrogen-sorption, DRS, PL, SEM, and TEM. The pyrex column vessel reactor (inner volume: 123 mL) was used for the photocatalytic H2 production from aqueous solution (40 mL) containing 10 vol% triethanolamine (TEA) as a sacrificial donor. 2 wt% Pt loaded on the surface of the photocatalyst by the in situ photodeposition method using H2PtCl6. Before irradiation, N2 gas was bubbled into the reaction solution for 30 min to remove a dissolved O2. Typically, 40 mg of the photocatalyst were added into the reaction solution. A 300 W Xe lamp with a UV cut filter (λ<420 nm) was applied as a light source. The concentration of H2 production from the aqueous TEA solution was analyzed by GC with TCD.

The rate of hydrogen production for pure g-CN was approximately 7.9 μmol/g・h. After modified carbon particles, photocatalytic performance was higher. The highest photocatalytic hydrogen evolution rate for 0.5 wt% C/g-CN was approximately 340 μmol/g・h, which was about 43 times faster than that of pure g-CN.

DRS spectra showed that the absorbing wavelength region expanded after loading carbon particles. Especially, 5.0 wt% C/g-CN could be absorbed over NIR. However, the photocatalysts which is the highest photocatalytic performance is 0.5 wt% C/g-CN. This result was attributed to the light shading effect. PL spectra represented the peak intensity of C/g-CN were weaker than that of pure g-CN. In this case, this result was implied to preventing recombination of electron-hole pairs which derived from carbon particles loaded.

In conclusion, the highest photocatalytic hydrogen evolution rate for 0.5 wt% C/g-CN was approximately 340 μmol /g・h, which was about 43 times higher than that of pure g-CN. The improved photocatalytic activity was attributed to the fact that carbon particles acted electron reservoirs to trap the electrons for promoting charge separation and light harvesters to enhance the visible light absorption.