A Stable and Efficient Hematite Photoanode in a Neutral Electrolyte for Solar Water Splitting: Towards Stability Engineering

Abstract

water splitting. So far, most studies employing hematite photoelectrodes were carried out in strongly basic conditions. [ 10,[14][15][16][17] It usually shows the poor performance (smaller water oxidation photocurrents and/or higher onset potential) and low stability in neutral electrolytes, although there is no systematic study on the effects of the electrolytes. In contrast, our modifi ed hematite photoanode showed almost the same performance in a neutral electrolyte at pH 7 as in a strongly basic electrolyte at pH 13.6. The modifi cation of the hematite photoanode was carried out by simply dipping the bare hematite thin fi lm in a solution containing phosphate ions and annealing it under mild conditions. The treatment led to the hematite thin fi lm being uniformly modifi ed with phosphate ions on its surface. The phosphate ions effectively prevent deterioration of performance in activity and stability in neutral electrolyte. There have been some reports of hematite photoanodes active for solar water splitting performed in neutral electrolytes, especially when the semiconductors are decorated by cobalt phosphate (Co-Pi) as an oxygen evolution reaction (OER) co-catalyst. [14][15][16][17][18][19] The Co-Pi works best in a phosphate buffer, yet the purpose and operating principle are distinguished from the present study as discussed later. Hence, this work represents the fi rst attempt at stability engineering by correlating surface electrostatic state of hematite and its performance and stability in electrolytes with different pH values.As described in the Experimental Section, the surface of the hematite photoanode was modifi ed by dipping the bare hematite thin fi lm in an aqueous sodium phosphate solution and annealing it under mild conditions. In the transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping images shown in Figure 1 , phosphorus is uniformly distributed over the hematite surface as the hematite surface was successfully coated with phosphate ions. It was also confi rmed by scanning electron microscopy (SEM) and SEM-energy dispersive X-ray spectroscopy (EDS; Figure S1 of the Supporting Information). The surface morphology does not change after the surface modifi cation with phosphate ions, as shown in the SEM images of Figure S1 (Supporting Information). According to the EDS results, phosphorus certainly exists on the hematite surface treated with a solution containing phosphate ions. On the other hand, the bare hematite surface showed no phosphorus on the surface.To identify the chemical state of the surface species, X-ray photoelectron spectroscopy (XPS) analyses were carried out for the bare and the phosphate ion-modifi ed hematite thin fi lm (Pi-Fe 2 O 3 ). Figure 2 A shows a Fe 2p 3/2 peak at 711.2 eV, a 2p 1/2 peak at 724.4 eV, and a 2p 3/2 satellite peak at 719 eV, corresponding to binding energies of Fe 3+ . These features show

A multijunction of ZnIn2S4 nanosheet/TiO2 film/Si nanowire for significant performance enhancement of water splitting

“…To realize highly efficient water splitting and achieve practical PEC application, photoelectrodes should meet several requirements, including high sunlight absorption efficiency, matching energy band levels, and good corrosion resistance. Since the first demonstration of water splitting with TiO 2 by Fujishima and Honda in 1972 [4], aggressive research efforts have been focused on the use of various semiconducting materials and structures to build blocks of photoelectrodes [5][6][7][8][9][10]. Among these electrode materials, Si (E g = 1.12 eV) exhibits high light absorption over a broad range of solar spectrum, Nano Res.…”

Nanocube-based hematite photoanode produced in the presence of Na 2 HPO 4 for efficient solar water splitting

“…The beneficial effects of phosphate groups have also been reported for Fe 2 O 3 film. The negative electrostatic field formed by the surface phosphate groups is considered to promote charge separation and extraction of photoexcited holes to the electrode surface [37]. In our case, higher photocurrent was observed for the PM-Fe 2 O 3 electrode compared with the PS-Fe 2 O 3 , which might be due to the more phosphate groups coating on the PM-Fe 2 O 3 surface resulting in a stronger negative electrostatic field.…”

“…The surface will be negatively charged by deprotonation at pH higher than its IEP, but becomes positively charged at pH lower than its IEP. As reported in literature [37], an electrostatic field could be built up through the surface negative electrical charge, which facilitates photoexcited hole injection to the electrolyte solution, leading to efficient separation of photogenerated electron-hole pairs. As depicted in Fig.…”

“…Recently, hematite photoanode was reported to be modified with phosphate ions on its surface for stable and efficient solar water splitting. [37]. The phosphate ion modified Fe 2 O 3 electrode was prepared by simply immersing the hematite thin film in an aqueous sodium phosphate solution and then annealing it at 350 °C.…”

Elaboration and characterization of nanoplate structured α-Fe2O3 films by Ag3PO4

“…The doped Ag effectively increased the charge density in the near-surface of hematite, and also improved the surface oxidation ability, thus led to enhanced PEC performances. Apart from metal ions, phosphate ions were reported to modify a-Fe 2 O 3 by simply immersing hematite film in an aqueous sodium phosphate solution (Kim et al, 2014). A negative electrostatic field is formed by the phosphate ions on hematite surface, which promotes extraction of holes to the electrode surface and suppresses surface charge recombination.…”