Photocatalytic production of hydrogen from water by a p- and n-type polycrystalline iron oxide assembly

Abstract

The photodissociation of water to produce hydrogen has been accomplished using light in the solar range of the electromagnetic spectrum and in the absence of any external potential at 300K. The catalyst for the reaction is a polycrystalline pin diode assembly made out of ~-and Si-doped iron oxide. In 0.1 ¥ Na2S04 solution (pH = 6) the device produces hydrogen catalytically with rates of 1-2 H2 molecules per site per minute and its power conversion effici~ncy is about 0.05%. Iron oxide containing minerals could have played important roles in the photochemical evolution of the planet in the pre-chlorophyl era.-3-We report the successful photochemical conversion of water to hydrogen using light in the solar region of the electromagnetic spectrum and at 300K. The catalysts for this reaction are polycrystalline iron oxide discs that are doped with silicon and magnesium. Iron oxide, Fe203, is a semiconductor with a band gap of 2.2 eV. It becomes n-type when doped with silicon and p-type when doped with magnesium. By connecting n-type and p-type iron oxide polycrystalline pressed discs with a conducting wire, a diode assembly is produced as shown in Figure 1. When this assembly is immersed in a 0.1 M Na2S04 aqueous solution and illuminated with visible light both hydrogen evolution and photocurrent across the samples are detectable ~he hydrogen production rate is in the range of 1 to 2 hydrogen molecules per site per minute and the photocurrent is of the order of 10 IJA/cm 2 • The reaction is thus catalytic and can be carried out for 6-8 hours without any sign of poisoning. When the photoinduced hydrogen production and the corresponding photocurrent eventually decline they can be regenerated'by passing oxygen gas over the catalyst iron oxide surfaces. The poisoning effect is not observed as long as oxygen is continuously passing over.the surfaces.The apparatus that is used to photodissociate water is shown in Figure 2.It consists of a glass cell that houses the iron oxide assembly immersed in the Na2S04 aqueous solution. Photogenerated hydrogen is transported from the cell through a closed circulation loop to a gas chromatograph. Argon is used as a \f carrier gas and is circulated through the loop by means of a mechanical pump. oxygen production. The sample.is illuminated by light from a 500 W lamp which passes through a water filter "and a visible pass filter ( E ~ 2.7 eV). The apparatus also permits photoelectrochemical measurements of p-and n-type iton oxides separately. This is accomplished by a potentiostat and a standard three -4-electrode configu;ration consisting of the iron oxide sample, a reference electrodeand a platinum counter electrode (see Figure 2).Iron oxide discs with n-type behavic:>r were produced by mixing fine powders of a-Fe203 and Si02 so that ° < Si/Si + Fe ( 20 atomic %. Similarly, p-type iron oxide discs were produced by mixing powders of a-Fe203 and ~O so that ° < Yg/ ~ + Fe ( 20 atomic %. By pressing the powders into pellets, heating them in air at 1340-1390 C and allowing them to c...

Hydrogen generation at irradiated oxide semiconductor–solution interfaces

“…As with TiO 2 and WO 3 , Fe 2 O 3 (particularly the a-modification) has been examined in single crystal form, as thin films prepared by CVD [237,239,255,270], pyrolytic conversion of iron [240], and spray pyrolysis [248,[250][251][252][253], or as sintered pellets from powders [241][242][243][244][245][246]. A variety of dopants have been deployed to modify the host [242-245, 247, 250-253] and remarkably, p-type semiconductor behavior has been reported [242,244,245,251] in addition to the more commonly occurring n-type material. The main handicap with Fe 2 O 3 is its rather poor electronic and charge transport characteristics regardless of the method of preparation of the material.…”

Photoelectrochemical Properties of Polycrystalline Mg‐Doped p‐Type Iron (III) Oxide

“…After Morin [40] had first reported unintentionally Mgdoped Fe203 to be p-type after firing in oxygen, Gardner et al [42] have systematically studied the electrical properties of high-purity a-Fe2O3 deliberately doped with 0.01 to 2.0 at.% Mg. Also Somorjai et al [33,35] reported the preparation of p-type Fe2O3 by doping with MgO. Recently, the photogeneration of hydrogen from water was reported to occur in a shortcircuited polycrystalline p/n diode assembly comprising Mg-doped (p-type), and Si-doped (n-type) iron oxide, albeit with a low power-conversion efficiency of 0.05% [32 -371.…”

Nanostructuredα-Fe2O3 in PEC Generation of Hydrogen

“…In the early 1980s, the rate of hydrogen production, using a-Fe 2 O 3 as photoelectrode, was very low and rarely reported [27,29,98]. In 1987, Khader et al reported evolution of 380 mmol hydrogen in 400 h from photocatalytic water dissociation in visible light using a-Fe 2 O 3 .…”