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Energy band engineering of metal 4

oxide for enhanced visible light

absorption

Jiangtian Li, Deryn Chu
US Army Research Laboratory, Sensor and Electronics Device Directorate, Adelphi, MD,
United States

4.1 Introduction

Since the 1970s, when it was discovered that TiO 2 could split water and reduce CO 2
[1,2], the pursuit has continued to produce solar fuels via renewable sunlight, by
mimicking photosynthesis. However, doing so remains one of the major scientific
challenges. This process requires both efficient light absorption and effective charge
carrier transfer for chemical reactions. For commercial applications, long-term stabil-
ity is also a prerequisite. Many catalysts have been reported for this exciting process
[3–6]. In practice, metal oxide semiconductors are the most abundant ones in nature,
and they are more stable in a variety of harsh conditions when used as photocatalysts
[7–12]. Regarding the energetic criteria, only wide band gap semiconductors (e.g.,
TiO 2 and SrTiO 3 ) are thermodynamically able to drive water splitting without applied
external bias. However, the wide band gap of such oxides limits their light absorp-
tion within the ultraviolet region. Some oxides, such as Fe 2 O 3 (E g = 2.0 eV), have ad-
vantages for absorbing visible light, but suffer from high electron affinities and poor
charge carrier mobility and diffusion [13–15].
The major challenge facing metal oxide semiconductors therefore is the balance
between light absorption and charge carrier transfer. Both of them depend on the elec-
tronic energy band structure and are coupled together to determine the ultimate solar
conversion efficiency. Here we summarize the commonly used strategies in this field,
with a focus on engineering the electronic energy band to improve visible light ab-
sorption. In Section 4.2, we look at the basic concepts of the electronic structure of
semiconductors. In Section 4.3, we present the photo-excitation process for photocat-
alytic reaction in semiconductors and describe applications of solar fuel generation by
water splitting and CO 2 reduction. In Section 4.4, we highlight benchmark metal oxide
semiconductors in terms of electronic band structure, such as TiO 2 , Fe 2 O 3 , BiVO 4 ,
and Cu-based p-type oxides. In Section 4.5, we address the recent efforts in electronic
modification/engineering of metal oxides for enhanced light absorption. Several rep-
resentative examples are underlined, including doping, alloying, heterojunction, plas-
monic photosensitization, and multijunction systems. Some concluding remarks and
future research directions are recommended at the end.

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