Mesoporous CuFe₂O₄ Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties**

Abstract

Metal oxide‐based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid‐liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2O4 (CFO) thin film photoanodes were prepared by dip‐coating and soft‐templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)‐block‐poly(propylene oxide)‐block‐poly(ethylene oxide) (Pluronic® F‐127), polyisobutylene‐block‐poly(ethylene oxide) (PIB‐PEO) and poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide) (Kraton liquid™‐PEO, KLE). The non‐ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron‐hole recombination at the defective surface. This interpretation is confirmed by intensity‐modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP‐SPS). The lowest surface recombination rate was observed for the ordered KLE‐based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Soft-templated, mesoporous Co3O4 thin films for electrocatalysis of the oxygen evolution reaction

“…52 Therefore, Pluronic® F-127 based mesoporous frameworks can be generally considered as a suitable nanoarchitecture for electrochemical applications, since this polymer is capable of creating mesopores below 15 nm (depending on the calcination temperature). 51 Conclusively, the presented citric acid route in combination with Pluronic® F-127 as SDA and cobalt nitrate as precursor, results in the formation of structurally intact Co 3 O 4 mesostructures with good electronic conductivity, high surface areas, and thus a remarkable OER performance.…”

“…Interestingly, most mesoporous TMO thin films are reported to be prepared without chelating agents such as citric acid, as was shown, for example, for the synthesis of ordered mesoporous CoFe 2 O 4 , 50 CuFe 2 O 4 , 51 indium tin oxide, 52 and high-entropy oxide. 53 In conclusion, the feasibility of forming those mesoporous TMOs thin films strongly depends on the complexation behaviour of the corresponding TM.…”

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications

“…The ne tuning of the nanoporous framework is of importance with respect to the mass transport, 54 signicantly inuencing the PEC properties, especially in metal oxides. 55 The Laberty-Robert group synthesized CuO thin lms by a sol-gel-based dip-coating approach and presented that the photoresponse depends on the crystallinity and microstructure. 43 The best-performing dense CuO thin lm (−0.94 mA cm −2 at 0 V vs. RHE) was found for a calcination temperature of 400 °C and 500 nm thickness.…”

“…The fine tuning of the nanoporous framework is of importance with respect to the mass transport, 54 significantly influencing the PEC properties, especially in metal oxides. 55 …”

“… 59 Related to these findings, a systematic study on the surface area and pore order of mesoporous CuFe 2 O 4 photoanodes postulated a general trend that the photoinduced water and sulfite oxidation reactions are predominantly determined by the surface area of the photoabsorber. 55 In particular, the sulfite oxidation, which is a thermodynamically down–hill reaction (Δ G < 0), 60 is accelerated at high-surface-area materials since sulfite is a fast hole acceptor. 61 Therefore, the sulfite oxidation is dominated by the number and availability of reactive surface sites.…”