Inorganic nanomaterials through chemical design

Traditional methods of synthesizing and processing inorganic materials were designed to overcome intrinsic energy barriers such as slow reaction states and large diffusion path lengths. Empirically, the supply of energy was optimized through several intermittent grinding (mechanical) and heating (thermal) steps. Although successfully applied to bulk materials, these top-down methods have limited application in the synthesis of nano-sized materials, which demand recipes to synthesize crystalline phases at lower temperatures. To overcome thermodynamic impediments, several bottom-up procedures based on the application of molecular precursors have been employed, which have successfully reduced diffusion lengths and produced well-defined materials under milder conditions.

However, synthetic methods for transferring the short-range chemical order present in the precursor state to infinite correlation lengths in three dimensions are not well understood, and they drastically restrict the predictability of inorganic syntheses when compared to the domain of organic materials.¹ Nevertheless, a large number of examples have appeared in recent years that demonstrate the ability of chemistry and chemists to produce nanomaterials with controlled properties through the rational design and tuning of process parameters. ^1–8

An over-simplified representation of the bottleneck of material synthesis is shown in Figure 1. In the first case, the outcome of the reaction (OUT 1–3) is ungoverned, implying that the reaction parameters will lead to products accessible within the thermodynamic space with almost equal probability. As a result, abnormal grain growth, de-mixing of elements, phase segregation, and the formation of side-products, are unavoidable: see Figure 1(a)

We are using discrete chemical precursors as molecular seeds to grow nanomaterials by inducing positional control on phase-building elements. This approach offers channeled output and the possibility of tuning the chemical parameters to achieve a chemically-controlled synthesis of the material of interest. Essentially, this is a strategy to enhance the probability of the desired reaction while simultaneously reducing the likelihood of unwanted reactions, see Figure 1(b)

The success of this chemical route to nanomaterials is due to the molecular precursors: these transform into solid phases at much lower temperatures than those required for conventional procedures. Since the elements are chemically linked, diffusion is either not necessary or the path lengths are too short, which augments the advantages of chemical processing. For instance, perovskite BaZrO₃ could be prepared in nanocrystalline and monophasic form at 600°C using [BaZr(OH)(OPrⁱ)₅(Prⁱ OH)₃]₂ as the molecular precursor (see Figure 2). On other hand, higher temperatures (>1000°C) would be required to process the solid-solution of Ba and Zr salts, the final product of which would contain undesired phases (BaO, ZrO₂ and Ba₂ZrO₄).⁹

We have shown that solid-state structures can be templated using well-defined molecular clusters containing metallic elements in ratios compatible with targeted compositions.¹ The pre-defined metal-ligand interactions facilitate the growth of nanomaterials by lowering the nucleation barriers. Soft-chemistry methods allow the selective synthesis of metastable compounds.

For example, GdFeO₃ (perovskite) is difficult to synthesize because of the easier formation of the thermodynamically favorable Gd₃ Fe₅O₁₂ (garnet) phase. The coexistence of the Gd₃Fe₅O₁₂ phase with its higher magnetic moment masks the weak ferromagnetic signal of GdFeO₃: see Figure 3(a). For designed synthesis of GdFeO₃, a single-molecular framework containing Gd and Fe ions in the desired ratio (Gd:Fe = 1:1) was used: see Figure 3(b).¹⁰ Controlled hydrolysis of the Gd-Fe precursor produced uniform nanocrystals of GdFeO₃—Figure 3(c)(d) and (d)—that were found to be stable up to 1200°C (the transformation into the garnet phase usually occurs above 900°C).¹⁰

It's not only stoichiometric complex oxides that can bedesigned by choosing the appropriate cation ratio in the precursor framework: this is true of nanocomposites too. Due to the pre-defined Nd:Al ratio,[NdAl(OPrⁱ₆(PrⁱOH)]₂ and [NdAl₃(OPrⁱ)₁₂(PrⁱOH)] produce nanophasic NdAlO₃ and NdAlO₃/Al₂O₃ composite, respectively: see Figure 4(a) and (b).¹¹ Comparative evaluation of the optical properties of Nd³⁺ ions in NdAlO₃ and NdAlO₃/Al₂O₃ has revealed that photoluminescence (PL) intensity of NdAlO₃/Al₂O₃ is much larger than that observed for pure NdAlO₃: see Figure 4(c) (c). Enhancement of the optical properties of the oxide-oxide composite is attributed to the influence of the Al₂O₃ matrix on the electronic structure of the Nd³⁺ ions in the NdAlO₃ particles.

Using molecular precursors also has advantages for tuning the morphology. Tin oxide (SnO₂) nanowires of different diameters were conveniently grown by combining the chemical influence of a single molecular precursor [Sn(OBu^t)₄] with vapor-liquid-solid growth (catalyst-assisted chemical-vapor deposition, see Figure 5(a)).¹² Upon illumination with UV photons (370nm), the nanowires exhibit interesting photo-conductance that can be modulated by tuning the wire diameter. This has been demonstrated for samples with radial dimensions in the 50–1000nm range. The stable photo-response of SnO₂ samples over several on-off cycles—shown in Figure 5(b)—demonstrates their potential for application in UV detectors or optical switches. Here the nanowires can act as resistive elements whose conductance changes via charge-transfer processes.

In the context of the chemical design of inorganic materials, the challenge is to develop a customized assembly of molecular building blocks that would facilitate synthesis of suitable precursors to any desired nanomaterial. To demonstrate the strength of chemical methods in achieving better control over phase purity and the composition of the final materials, we would like to have insight into the transformation of chemical properties (bond type and order, coordination state, auxiliary ligands, etc.) from the moleculular to the material level. The application of quantum-mechanical calculations is a viable way of addressing the design aspects of inorganic material synthesis. However, this is not trivial due to the uncertainties associated with the validity of synthetic procedures for delivering solids with the desired compositions and properties.

In summary, chemistry offers a great deal of potential and promise, more than may be apparent today, for the materials of tomorrow. The chemistry/materials-science interface is highly fertile ground on which to grow new materials by design, and to impose precise control over composition, structure, and property (see Figure 6).

The authors are grateful to the Saarland state and central government for providing financial assistance. Thanks are also due to the German Science Foundation (DFG) for supporting this work as part of the priority programme on nanomaterials—Sonderforschungsbereich 277—operating at the Saarland University, Saarbruecken, Germany.