Semiconductors binary oxides

For nonstoichiometric compounds, the general rule is that when there is an excess of cations or a deficiency of anions, the compound is an n-type semiconductor. Conversely, an excess of anions or deficiency of cations creates a / -type semiconductor. There are some compounds that may exhibit either p- or n-type behavior, depending on what kind of ions are in excess. Lead sulfide, PbS, is an example. An excess of Pb + ions creates an n-type semiconductor, whereas an excess of ion creates a /7-type semiconductor. Similarly, many binary oxide ceramics owe their electronic conductivity to deviations from stoichiometric compositions. For example, CU2O is a well-known / -type semiconductor, whereas ZnO with an excess of cations as interstitial atoms is an n-type semiconductor. A partial list of some impurity-controlled compound semiconductors is given in Table 6.9. 

Table 8 contains a compilation of studies on other binary oxides that have been examined for their applicability to drive the photoelectrolysis of water. As cited earlier, general reviews are available on many of the oxides listed in Table 8.32,62,65 Other than Ti02, Fe203 and WO3 are two of the most widely studied among the binary oxide semiconductors, and studies on these oxides have continued to appear right up to the time of the writing of this Chapter.

In closing this Section, comparative studies on binary oxide semiconductors are available62,65,353,383 including one study383 where the electron affinities of several metal oxides (used as anodes in photoelectrolysis cells) were calculated from the atomic electronegativity values of the constituent elements. These electron affinity estimates were correlated with the Vih values measured for the same oxides in aqueous media.383... 

NajO, CaO), through semiconductors (VO2), to metallic conductors (Cr02, RUO2). Binary oxides also include two of the rare examples of ferromagnetic compounds, EuO and Cr02. 

An X-ray analysis carried out on the insulator-insulator solids and on the insulator-semiconductor solids revealed that they were amorphous or badly crystallized. Since the preparation temperature was rather low, the presence of bulk compounds such as silicates, spinels and titanates is very unlikely although the incipient formation of disordered surface compounds cannot be excluded. Thus the solids should be considered intimate mixtures in which the chemical features of each component are not changed in a significant way. Indeed the I.R. measurements also support the idea that interaction between the components of the binary oxides is not strong. 

Since only very few binary oxides show promise as photoelectrodes for water splitting, most notably WO3 and Fe203, PEC research activities have now partly shifted toward ternary and more complex metal oxides (see, e.g.. Chaps. 5 and 6). One of the best known examples is BiV04, which is an n-type semiconductor with a... 

The authors [42] considered only the carriers localized on the defects in the simple binary oxides like MgO, Hf02, Sn02, which are typically wide-gap semiconductors and where the electron-electron correlations are negligibly small [46]. 

Today, we do not have any technical problems in fabricating binary oxides, solid electrolytes, and standard semiconductors with specified electrophysical, chemical, and structural properties. In the literature, one can find a great number of works devoted to the elaboration of various techniques for deposition of sensing materials (Nenov and Yordanov 1996 Will et al. 2000 Van Tassel and Randall 2006 Viswanathan et al. 2006 Vayssieres 2007 Milchev 2008 Tiemann 2008). These methods were analyzed in detail by Korotcenkov (2010). However, for more complicated oxides (for example, binary oxides modified by various additives), there are stiff many problems to be resolved when attempting deposition of these materials. 

The electronic structures of both compounds have been well studied experimentally. The experimental data show that the MgO crystal is a wide-bandgap insulator Eg = 7.8 eV) titanium dioxide Ti02 in the rutile structure is a semiconductor with an experimental bandgap of approximately 3 eV. These differences are reproduced in the band strucrure of these two binary oxides, calculated in [623] by HF and LDA LCAO methods and shown in Figures 9.5 and 9.6, respectively. The details of the AO basis-set choice and BZ summation can be found in [623]. 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Abstract This review highlights how molecular Zintl compounds can be used to create new materials with a variety of novel opto-electronic and gas absorption properties. The generality of the synthetic approach described in this chapter on coupling various group-IV Zintl clusters provides an important tool for the design of new kinds of periodically ordered mesoporous semiconductors with tunable chemical and physical properties. We illustrate the potential of Zintl compounds to produce highly porous non-oxidic semiconductors, and we also cover the recent advances in the development of mesoporous elemental-based, metal-chalcogenide, and binary intermetallic alloy materials. The principles behind this approach and some perspectives for application of the derived materials are discussed. 

One of the potential strategies for the synthesis of compound semiconductors is the pyrolysis of a single-molecule precursor that incorporates the elements of a compound into a single molecule. For a number of binary materials, predominantly metal oxides but also compound semiconductors, such as GaAs [101] or metal alloys [102-104], it has been demonstrated that the use of single-molecule precursors (SMPs), which contain both components for the respective material in one molecule rather than applying two independent... 

Binary Selenides. Most binary selenides are formed by heating selenium in the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts in aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH Se [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the tellurides. Selenides of the alkali, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are insoluble in water. Polyselenides form when selenium reacts with alkali metals dissolved in liquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

The next five chapters deal with deposition of specific groups of semiconductors. In Chapter 4, II-VI Semiconductors, all the sulphides, selenides, and (what little there is on) tellurides of cadmium (most of the chapter), zinc (a substantial part), and mercury (a small part). (Oxides are left to a later chapter.) This chapter is, understandably, a large one, due mainly to the large amount of work carried out on CdS and to a lesser extent on CdSe. Chapter 5, PbS and PbSe, provides a separate forum for PbS and PbSe, which provided much of the focus for CD in earlier years. The remaining sulphides and selenides are covered in Chapter 6, Other Sulphides and Selenides. There are many of these compounds, thus, this is a correspondingly large chapter. Chapter 7, Oxides and Other Semiconductors, is devoted mainly to oxides and some hydroxides, as well as to miscellaneous semiconductors that have only been scantily studied (elemental selenium and silver halides). These previous chapters have been limited to binary semiconductors, made up of two elements (with the exception of elemental Se). Chapter 8, Ternary Semiconductors, extends this list to semiconductors composed of three elements, whether two different metals (most of the studies) or two different chalcogens. 

---