Vol. 31 No. 3
May-June 2009

What Is "Materials Chemistry"?

by Peter Day, Leonard Interrante, and Anthony West

The words “materials” and “chemistry” have only been linked relatively recently, yet “materials chemistry” now accounts for a significant fraction of chemical science. The phrase has often been used quite indiscriminately, so IUPAC launched a project to try to define it. This article presents some of the background and conclusions to the study.

The Rise of Materials Chemistry
What do the following topics in contemporary chemistry have in common? (1) Using NH₃ incorporated in simple inorganic solids as a medium for storing hydrogen; (2) designing and assembling chemically patterned or anisotropically shaped colloidal particles into arrays with predefined geometries; (3) predicting the structures of molecular and hybrid crystals through computer simulations; (4) creating porous crystals of metal oxides templated by mesoporous silica cages. The answer is that all were the subjects of recent articles published in one of the main international journals devoted to materials chemistry.¹ In view of this ubiquity, it is surprising that as recently as 20 years ago, the words “materials” and “chemistry” were rarely linked together. Now, in 2009, “materials chemistry” represents one of the major growth sectors in pure and applied chemistry and accounts for a significant fraction of all publications in the chemical sciences.

Several straightforward measures verify these assertions. As an indication of its broad global outreach, entering the phrase “materials chemistry” into the Google search engine results in a number of hits comparable to those of traditional chemistry disciplines, such as “physical chemistry,” “organic chemistry,” “inorganic chemistry,” or “macromolecular chemistry.”

Figures for citations covering these major subdisciplines in the journals of one leading chemical society are as follows (2006 figures):

The figure for materials chemistry is especially impressive, because it has risen from nearly zero in just a short period of time.

The number of articles submitted to Chemistry of Materials, one of the major journals in the field, increased 18-fold over the first 19 years of its existence. Similar increases in submissions over the last decade have been noted in other journals that focus partly or entirely on materials-related chemistry, such as the Journal of Materials Chemistry (RSC), Advanced Materials and Advanced Functional Materials (Wiley), Nature Materials (NPG), Journal of Solid State Chemistry (Elsevier), and many others dealing with more specialized areas of materials chemistry and materials science. The number of citations of papers from these materials chemistry journals, and their corresponding “Impact Factors,” has also increased enormously.

Increasingly, however, the phrase “materials chemistry” is being bandied about indiscriminately, often by those merely in search of a buzzword. What does it really mean?

Defining Materials Chemistry
Given that the phrase “materials chemistry,” although coined only recently, has become so popular within the chemical community, it is pertinent to ask how materials chemistry should be defined—what it is and (perhaps more importantly) what it is not. Because one of IUPAC’s roles is to provide internationally agreed-upon definitions and standards, it was reasonable to take up this question; a project was launched in 2005 with this goal:

To assemble, collate, and disseminate information about the scope of the newly emerging discipline of materials chemistry, leading to an authoritative definition of the subject within the family of chemical sciences.

The objective was not so much to produce lists of specific topics or categories of compounds and phenomena, which would quickly become out of date, but to establish some principles that could be deployed by IUPAC and the chemical community at large to help structure this new discipline within the broad family of chemical sciences.

The Origins of Materials Chemistry
Chemistry began, and largely continues today, to be inextricably associated with preparing, processing, and utilizing “materials,” both natural and synthetic. Early examples include tanning and dying skins and fibers; extracting metals from their ores and the developing cement and concrete for construction.

Following World War II, “solid-state chemistry” developed as a distinct branch of knowledge focused on inorganic compounds. In parallel, and quite separately, “coordination chemistry” developed within inorganic chemistry, concentrating on molecular species in which a metal ion was enveloped by (mostly organic) ligands, in contrast to the solid state in which compounds had continuous non-molecular lattices. “Organometallic chemistry,” concerned exclusively with molecular species, evolved some 10 years later. “Organic chemistry,” which originated in the 19th century, also concentrated on discrete molecules, albeit sometimes very large ones, leading to the emergence of “macromolecular chemistry” (effectively, polymer chemistry). In the early to mid–20th century, synthetic polymers began to revolutionize the science and technology of materials, a development that continues today as one of the main areas of research that can be included under the materials chemistry umbrella.

Starting in the late 1960s and continuing into the 1970s, the simple subdivision of chemical compounds as extended network solids or largely isolated molecules began to break down with the increasing interest in solid-state properties of molecular and polymeric metal-organic and organic compounds. From one side, solid-state chemists discovered more complex lattices, often with a mixture of inorganic and organic components; from the other, synthesis specialists began to attack selected targets, not so much for their behavior as individual molecules (e.g., reactivity, catalysis) as for the resulting properties when the units were packed into lattices. An additional strand was the increasingly important part played by molecular chemistry—organic, inorganic, and macromolecular—in fabricating integrated circuitry for microelectronics (photo-resists, molecular beam epitaxy precursors, etc.).

The discovery of conducting polymers in the 1970s and the widespread use of liquid crystals in displays gave these developments increased impetus; from the purely inorganic side, the spectacular developments in high-temperature superconductivity in the late 1980s drew attention to the opportunities for unlooked-for properties provided by complex metal-oxide structures. The latter in particular focused attention on careful, chemically based control of composition, phase purity, and microstructure. Later, and equally significantly, came the phenomenon of colossal magneto-resistance in perovskite structures, with similar implications.

Two other developments in the last decade illustrate how the field that we now recognize as materials chemistry has grown increasingly complex in its ramifications. First, chemical routes are increasingly being used to synthesize extended structures, either in the form of discrete (“zero-dimensional”) clusters, such as dendrimers and tailored metal-organic clusters, or in selectively modified semiconductor or metal (“two-dimensional”) surfaces by attaching electro-active molecules. Second, the entire field of nanoscience developed over the last decade depends on chemical design and control (nanotubes, “functionalized” metal, semiconductor particles, etc.).

Finally, and perhaps with the greatest long-term potential for expansion, chemistry has moved into the area of biomaterials. Starting with bulk (although highly textured) substances, such as bone or spider silk, scientists are developing heterogeneous structures involving surface modification and the medically important issue of biocompatibility. Among biomaterials, one can distinguish between materials produced by life-forms and those made by humans, through chemical synthesis, for use in living organisms as prostheses or other purposes. However, this distinction is becoming increasingly blurred as chemists adapt natural materials to meet specific needs in non-biological applications or develop synthetic materials designed to bio-degrade in nature.

Given the multiplicity of substances and techniques just summarized that cluster under the portmanteau phrase “materials chemistry,” one may legitimately ask whether enough common ground exists between them to constitute a valid new subdivision of chemical science.

What Is a Material?
A key to defining “materials chemistry” lies in defining what constitutes a “material” versus a chemical. Some dictionary definitions follow:

• the matter from which an article, fabric, or structure is made²
• the matter from which a thing is made³
• a physical substance from which things can be made⁴
• a substance having properties that make it useful in machinery, structures, devices, and products⁵

To devise a more generic definition, the ideas of functionality and application (at least, the potential for application) should be considered. A material is something with properties that give it the potential for a particular application, either structural, as with a building material; functional, as with materials used to make devices (electronic, optical, or magnetic); or biological, with biomedical applications. A material is generally thought of as a solid or an organized liquid (e.g., liquid crystal) in which interactions between the entities forming the assemblage play large roles in determining the resulting properties.

Another key concept is that of “emergent properties,” as understood in the new science of complexity. Materials are assemblages of subunits. The properties of a material emerge from the way these subunits are put together. Although a single molecule has properties related to its chemical structure, which remain constant irrespective of its state of aggregation, the properties of a material depend on how its subunits are assembled. Some such emergent properties are called “collective,” because they are only found in assembled samples. For example, ferromagnetism is a property not of a single atomic or molecular unit but only of an ensemble of units. In addition, properties can arise from structural defects, and materials made of the same chemical subunits but with different defects can have different properties. This relationship between structure and property could be used to define a material and differentiate it from a chemical.

Characterizing Materials Chemistry
Building on the definition of a material, it is now possible to create an approach to the cohesive body of practice called “materials chemistry” by developing several key aspects.

Application-Oriented or Curiosity-Driven
Much of the focus of materials chemistry in discovering and developing materials that may be exploited for desired applications. Although this is an essential motivating factor, structure-property relations crucial for further advances also need to be studied and developed. Chemists may generate new materials before their potential applications have been conceived. The discipline must include the ability to synthesize, to study, and to assess new materials.

Structural, Functional, or Biological
Today, many materials chemists are synthesising functional device materials, and the discipline is often seen as directed towards producing materials with function—electrical, optical, or magnetic. The production of structural materials, such as alloys, composites, and plastics, is seen traditionally as the province of materials scientists. Polymer science has not always been strongly connected historically to other materials chemistry, largely due to the number and strength of journals devoted to macromolecular chemistry alone. However, with the development of conducting polymers, the materials chemistry and polymer science communities are moving closer together. The development of new nanostructured, smart materials is also uniting communities and bringing together the science involved in functional and structural materials. Materials chemistry encompasses both structural and functional materials. Structural properties, such as strength or flexibility, should be considered types of functionality. The fact that chemists are currently more interested by other types of functionality may change in the future, and, certainly, a huge expansion in the area of biological materials is anticipated.

Designing and Processing Materials
The concept of “design” is important in defining the work of materials chemists. Rather than purely investigate properties, the materials chemist tries to manipulate the synthetic process to produce a desired function. The relationship between the method of synthesis and the design of the final product is crucial to a materials chemist.

Characterization and Analysis
Characterization techniques are important to the work of all chemists. However, although many mainstream chemists are primarily concerned with characterizing chemical or molecular structures, materials chemists are often interested in looking at structures at all levels, ranging from defect and unit-cell scales to nano- and microstructures. Microscopy in all its forms—from optical to electron to scanning probe—is important in the work of materials chemists. “Analysis” can take the form of a theoretical analysis or a model of a material’s electronic or molecular/crystal structure and even the interactions that occur when a molecule is present in a solid state or in another medium. In this manner, important insight can be gained regarding the structure-macroscopic property relationships of a material.

What Is Not Materials Chemistry?
It may be agreed that simply synthesizing a new chemical substance in nano- or macroscopic form is not materials chemistry but just chemical synthesis. For it to be considered materials chemistry, an element of application, function, or design needs to be present. For example, in the case of “nanomaterials,” there should be some indication of special or potentially useful properties that result directly or indirectly from the small size or exceptionally high surface-to-volume ratio of the substance. Work on novel materials linked to a particular property must be included as materials chemistry, because chemists may generate new types of materials with previously unknown properties, thus leading to unimagined applications.

Research in nonmaterials chemistry is directed toward building understanding of the science of chemistry itself, of how matter is composed and interacts, and of how fundamental properties arise. It is also quite properly concerned with synthesizing and identifying completely new assemblies of atoms in the form of molecules and solids that may demonstrate unsought behaviors. In addition, nonmaterials chemistry tends to focus on reactivity, which makes it a vital resource for the chemical industry, whether pharmaceutical or petro-chemical.

Toward a Working Definition
A definition was previously suggested in 1992 in an article by one of the Working Group members in the Materials Research Society Bulletin⁶ and again later in a chapter of a book on materials chemistry,⁷ which is: “Chemistry related to (or directed at) the preparation, processing, and analysis of materials.” A number of definitions of materials chemistry can be found on the websites of university chemistry departments. Here are a few examples:

The branch of chemistry aimed at the preparation, characterization, and understanding of substances/systems that have some specific useful function (or potentially useful function). (University of Wisconsin)

Materials chemistry involves the synthesis and study of materials that have interesting and potentially useful electronic, magnetic, optical, and mechanical properties. (Washington University)

Materials chemistry is a relatively new discipline centered on the rational synthesis of novel functional materials using a large array of existing and new synthetic methods. (University of Oregon)

Materials chemistry differs from classical chemical research in that it is generally concerned with interactions that arise from organizing molecules, polymers, and clusters over length scales beyond typical small molecule dimensions (nanometres to centimetres). (Massachusetts Institute of Technology)

In addition, a number of definitions were suggested during the workshop (see acknowledgements below):

Materials chemistry is the chemistry of the design, synthesis, and characterization of assemblies of molecules whose properties arise from interactions between them.

Materials chemistry is the understanding, synthesis, processing, and exploitation of compounds or substances in their assembled form.

Materials chemistry is the synthesis, processing, characterization, understanding, and exploitation of compounds that have useful or potentially useful properties and applications.

We propose the following working definition, based on a synthesis of the above suggestions and the currently accepted meaning of materials in most dictionaries and books:

Materials chemistry comprises the application of chemistry to the design, synthesis, characterization, processing, understanding, and utilization of materials, particularly those with useful, or potentially useful, physical properties.

This definition draws upon the existing definitions for the terms “chemistry” and “materials,” while acknowledging that the materials that have been (and are likely to continue to be) of particular interest to the practitioners are generally those that have certain properties—e.g., mechanical, electrical, magnetic, optical, etc.—that make them useful, or potentially useful, in a functional sense. Thus, the keywords “useful” and “properties” were added to further define the materials that are most likely to be the subject of investigation in this field as well as to acknowledge the fact that functionality, or the prospect of functionality, is a major driver for research and development in the field.

Acknowledgements
The Task Group is grateful for the input it received from practitioners and from the journals principally devoted to the subject, as well as from representatives of other IUPAC divisions, members of the chemistry community who contributed to the debate, and the RSC staff. In particular, we appreciate the efforts of Graham McCann and Rachel Brazil, who organized a workshop sponsored by the Materials Chemistry Forum of RSC in London on 12 April 2006. MCF brought together representatives not only of the relevant subject groupings within RSC but also from contiguous disciplines, such as physics and materials science, all of whom contributed, in addition to international speakers.

References

1. J. Mater. Chem., 2007, 17, (41); ibid., 2007, 17, (47); ibid., 2008, 18, (19); ibid., 2008, 18, (20); ibid., 2008, 18, (24).
   Oxford English Dictionary, 1971, Oxford University Press, Oxford.
2. Concise Oxford Dictionary, 8th ed., 1990, Clarendon Press, Oxford.
3. Webster’s Collegiate Dictionary, 11th ed., 2003, Merriam-Webster.
4. M. Cohen, ed., 1974, “Materials Science and Engineering: Its Evolution, Practice and Perspectives,” Materials Science and Engineering, 37, 1; M.B. Bever, 1986, Encyclopaedia of Materials Science and Engineering, Vol. 1.
5. L.V. Interrante, “Materials Chemistry—A New Subdiscipline?” MRS Bulletin, January 1992, p. 4.
6. L.V. Interrante and M. Hampden-Smith, eds., 1998, “Introduction to Materials Chemistry,” in Chemistry of Advanced Materials—An Overview, chap. 1, Wiley-VCH, New York.

Image Credits
Cover images of the Journal of Materials Chemistry above are reproduced with permission. Copyright 2008, 2009, Royal Society of Chemistry; <www.rsc.org/Publishing/Journals/JM/>. Cover images of Chemistry of Materials Chemistry above are reproduced with permission. Copyright 2007, 2008, 2009, American Chemical Society; <http://pubs.acs.org/toc/cmatex/current>.

Peter Day <[email protected]> chaired this IUPAC project and is a professor from Davy Faraday Research Laboratory, University College, in London, UK; Leonard Interrante <[email protected]>, task group member, is an editor for Chemistry of Materials, and secretary of the IUPAC Inorganic Chemistry Division, and professor at Rensselaer Polytechnic Institute, in Troy, New York, USA; Anthony West <[email protected]>, task group member and past president of the IUPAC Inorganic Chemistry Division, is a professor at the University of Sheffield, UK. Other task group members were M. Prato, chair of the Editorial Board of Journal of Materials Chemistry and a professor at the Universita Trieste, Italy; and Y. Shirota, former member of the Editorial Board of Journal of Materials Chemistry and a professor at Osaka University, Japan.

www.iupac.org/web/ins/2005-001-1-200