Performance Comparison of Pt/g-C₃N₄ Photocatalysts for H₂ Production

Abstract

Graphitic carbon nitride (g-C3N4) is a polymeric semiconductor material which can readily be synthesized by the thermal condensation polymerization of melamine or urea at temperatures around 550 ℃. Owing to its narrow band gap (Eg = 2.6-2.8 eV) and favorable valence band and conduction band levels (+1.6 V and -1.1 V, respectively), g-C3N4-based photocatalysts are been actively pursued for H2 production under visible light. This research project aimed to examine the effect of g-C3N4 morphology on the performance of Pt/g-C3N4 photocatalysts for photocatalytic hydrogen evolution in 10:90 triethanolamine:water mixture under visible light (λ > 420 nm). Firstly, g-C3N4 photocatalysts in the form of nanosheets were synthesized via heating melamine or urea to 550 ℃, and holding at this temperature for 4 h. Experiments were conducted in air (A) or an N2 flow (N), with the obtained g-C3N4 photocatalysts being denoted here as ACN-urea, ACN-melamine, NCN-urea and NCN-melamine, respectively. To prepare the g-C3N4 nanotubes, melamine:urea mixtures in different weight ratios of 1:10, 1:8, 1:6, 1:4; 1:2 and 1:1 were used. On heating, these formed supramolecular complexes which guided nanotube synthesis (the obtained photocatalysts synthesized in air or N2 are denoted here as ACN-x and NCN-x respectively). As the amount of melamine in the precursor mixture increased, the aspect ratio in the g-C3N4 nanotubes decreased. Detailed characterization studies using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy, elemental analysis (C,H,N,O), and UV-visible spectroscopy determined that all the g-C3N4 photocatalysts were crystalline/nanocrystalline and Nrich, with electronic band gaps in the range 2.6-2.8 eV, thus allowing photoexcitation at wavelengths below 460 nm. Next, the various g-C3N4 supports (both ACN-x and NCN-x series) were decorated with Pt nanoparticles at a nominal Pt loading of 5 wt.%. This was achieved in a two-step process involving the adsorption of cationic Pt species on the surface of the photocatalysts using a deposition-precipitation with urea method, followed by a H2 reduction step at 550 ℃ to reduce the cationic Pt species to Pt0 nanoparticles. The Pt/g-C3N4 photocatalysts were then systematically characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and other various standard methods (XRD, FT-IR, N2 physisorption, UV-vis, photoluminescence and elemental analyses). TEM revealed that the average size of Pt nanoparticles in most samples was ~1.6 nm, with the nanoparticles generally being well-dispersed over the g-C3N4 supports. Pt deposition did not alter the structure or electronic properties (e.g. band gap) of the g-C3N4 photocatalysts, though dramatically improved the H2 production performance.H2 production tests were conducted on all the Pt/ACN-x and Pt/NCN-x photocatalysts in a mixture of 10 vol.% triethanolamine and 90 vol.% Milli-Q water under 300 W Xe lamp irradiation (λ > 420 nm). In general, the nanotube-based photocatalysts prepared at the melamine:urea weight ratio of 6:1 offered the best H2 production performance (rates > 5 mmol g-1 h-1), representing some of highest rates yet reported for Pt/g-C3N4-based photocatalysts. The optimized nanotube-based photocatalysts outperformed photocatalyst based on g-C3N4 nanosheets. Results suggests that g-C3N4 nanotubes may be advantageous compared to g-C3N4 nanosheets for photocatalyst H2 production.