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10.1117/2.1200602.0080

Inorganic nanomaterialsthrough chemical designSanjay Mathur, Hao Shen, Sven Barth, and Christian Cavelius

The synthesis of inorganic materials has been revolutionized by the im-

pact of (soft) chemical approaches that allow us to precisely tune the

composition, morphology, and microstructure of the extended solid-

state materials produced.

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 sev-

eral 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 crys-

talline phases at lower temperatures. To overcome thermody-

namic 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 correla-

tion 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 Neverthe-

less, a large number of examples have appeared in recent years

that demonstrate the ability of chemistry and chemists to pro-

duce nanomaterials with controlled properties through the ra-

tional design and tuning of process parameters.1–8

An over-simplified representation of the bottleneck of mate-

rial synthesis is shown in Figure 1. In the first case, the out-

come 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 segrega-

tion, 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-

Figure 1. Outcome control in (a) conventional and (b) molecular level

synthesis.

Figure 2. The molecular route to BaZrO3 ceramics.

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. Es-

sentially, this is a strategy to enhance the probability of the de-

sired 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

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Figure 3. (a) Magnetization curve of GdFeO3 mixed with the

Gd3Fe5O12 phase. (b) The molecular structure of the precur-

sor [GdFe(OR)6(ROH)]2. (c) X-ray diffraction patterns and (d)

tunneling-electron micrographs of GdFeO3 ceramics

Figure 4. In (a) and (b), molecular precursors to Nd-Al materials are

shown. Photoluminescence measurement of an NdAlO3 ceramic, an

NdAlO3/Al2O3 composite, and their corresponding TEM images (c).

Figure 5. (a) Scanning electron micrographs of SnO2 nanowires and

(b) their photo-response behaviors.

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(PriOH)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 ele-

ments in ratios compatible with targeted compositions.1 The pre-

defined metal-ligand interactions facilitate the growth of nano-

materials by lowering the nucleation barriers. Soft-chemistry

methods allow the selective synthesis of metastable compounds.

For example, GdFeO3 (perovskite) is difficult to synthesize be-

cause of the easier formation of the thermodynamically favor-

able Gd3Fe5O12 (garnet) phase. The coexistence of the Gd3Fe5O12

phase with its higher magnetic moment masks the weak ferro-

magnetic signal of GdFeO3: see Figure 3(a). For designed syn-

thesis 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 pro-

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Figure 6. The molecule-to-material tree symbolizing the impact of chemistry in the controlled synthesis of (nano)materials.

duced uniform nanocrystals of GdFeO3—Figure 3(c) 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 be

designed 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(OPri)6(PriOH)]2and [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 photolu-

minescence (PL) intensity of NdAlO3/Al2O3 is much larger than

that observed for pure NdAlO3: see Figure 4(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(OBut)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 demon-

strated 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 poten-

tial 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 precur-

sors to any desired nanomaterial. To demonstrate the strength

of chemical methods in achieving better control over phase pu-

rity 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 address-

ing the design aspects of inorganic material synthesis. However,

this is not trivial due to the uncertainties associated with the

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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 prop-

erty (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 pri-

ority programme on nanomaterials—Sonderforschungsbereich 277—

operating at the Saarland University, Saarbruecken, Germany.

Author Information

Sanjay Mathur, Hao Shen, Sven Barth, and Christian Cavelius

CVD Division

Leibniz-Institut fur Neue Materialien

Saarbrucken, Germany

References

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