Sunlight‐Driven Production of Methylcyclohexane from Water and Toluene Using ZnSe : Cu(In,Ga)Se₂‐Based Photocathode

Abstract

A three‐dimensional reaction field composed of a thick, porous TiO2 layer decorated with Pt and an anionic ionomer was fabricated on the surface of a membrane photocathode assembly (MPA). This MPA comprised (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 (ZnSe : CIGS) particles that promoted the photoelectrochemical (PEC) production of methylcyclohexane (MCH) from toluene and water. The Pt/TiO2 layer provided selective active sites for MCH production as well as reactant supply pathways. A tandem PEC cell consisting of the Pt/TiO2‐modified ZnSe : CIGS MPA and a photoanode exhibited spontaneous MCH production with a solar‐to‐MCH (STMCH) conversion efficiency as high as 0.32 %. This is the first‐ever report of sunlight‐driven overall MCH production without any assistance of external bias voltages and demonstrates the feasibility of driving this reaction as an artificial photosynthesis.

Electrochemical Evaluation for Multiple Functions of Pt‐loaded TiO2 Nanoparticles Deposited on a Photocathode

“…The transparency of the TiO 2 in the visible light region should ensure the light transmission to the ZnSe : CIGS particles even when using such a thick layer. The EDS line analyses of the surface of the Pt/TiO 2 ‐modifed (1500 rpm) photocathode revealed that the distribution of Pt well agreed with that of Ti and O as shown in Figure e, supporting the fact that Pt was highly uniformly dispersed over the whole TiO 2 layer . In the previous report, the high‐resolution transmission electron microscopy (HRTEM) and the scanning transmission electron microscopy (STEM) observations have also revealed that 2–6 nm of crystalline Pt particles were highly dispersed throughout the TiO 2 layer .…”

“…Figure shows the scanning electron microscopy (SEM) images for the ZnSe : CIGS particles and the Pt/TiO 2 ‐modified photocathodes. The ZnSe : CIGS photocathodes were fabricated by the particle transfer (PT) method and modified with various thicknesses of the Pt/TiO 2 layer by spin coating at different rotating speeds. ZnSe : CIGS particles synthesized via the flux method were comprised of triangular‐shaped crystals with a relatively wide particle size range from submicron to several micron‐order (see Figure a), similar to that in a previous report .…”

“…In addition, the Pt/TiO 2 modification evidently improved the IPCE especially in the long wavelength region at not only a negative potential (0 V RHE ; see Figure d), but also at a relatively positive potential (0.3 V RHE ; see Figure e), while the IPCE of the Pt/TiO 2 ‐modified photocathode drastically decreased in the UV light range. The reduced IPCE in the UV light range should be caused by the light shielding by the TiO 2 , indicating that the bandgap photoexcitation in TiO 2 did not contribute to the PEC reaction and that the TiO 2 layer did not hinder the light absorption of ZnSe : CIGS in the visible light range . Such light shielding by the TiO 2 layer in the UV light range should be one possible reason for the decreased photocurrent generated by the ZnSe : CIGS photocathode modified with an overly thick Pt/TiO 2 layer (almost 2 μm; 1000 rpm) as shown in Figure a.…”

“…In the present study, TiO 2 nanoparticles were employed as transparent and conductive supports for the Pt co-catalysts, and the Pt-loaded TiO 2 nanoparticle (Pt/TiO 2 ) layer was deposited on the particulate ZnSe : CIGS photocathodes. [12] Since the TiO 2 nanoparticles possess a high specific surface area and are transparent in the visible-light region, it should be possible to increase the number of the active sites on the photocathode surface along with minimizing the harmful influence of light shielding. A TiO 2 laminar structuring on a photoelectrode is known to improve both the photocurrent and stability due to formation of a heterojunction with the light absorbing material.…”

“…The particulate photocathode prepared by the PT method was composed of almost a monolayer of photocatalytic particles firmly anchored on the backside metal electrode (see Figure 2a), which was consistent with previous reports. [4,14] After spin coating of the colloidal TiO 2 and subsequent Pt photodeposition, the surface of the ZnSe : CIGS particles was covered by a relatively thick micronorder TiO 2 layer [12] (see Figures 2b-d). Spin coating at 2500, 1500, and 1000 rpm rotation speeds resulted in approximately 0.6, 1.3, and 1.8 μm thicknesses, respectively.…”