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What factors prompted you and your coauthor to undertake the work in this paper?

I developed the concept of low-temperature aqueous chemical growth of highly oriented metal oxide nanorod-arrays without template, undercoating, surfactant, or applied field more than 10 years ago. It worked very well for a-Fe₂O₃ and ZnO, for instance. The request, to do exactly the same for SnO₂ in order to fabricate a new generation of more efficient transparent conducting substrate for photovoltaic devices, came from Dr. M. Graetzel when he invited me to work in his lab.

The major challenge (compared to the other systems) was that the crystal structure of rutile SnO₂ (cassiterite) is centrosymmetric; that is, the crystal symmetry does not allow anisotropic growth along the c-axis (due to the presence of a symmetry element called mirror planes). All previous attempts in the literature failed and only non c-axis structures were produced. The c-axis orientation is crucial as it represents the highest electronic density path (for electrons) to be conducted along and thus, to be collected efficiently.

How did you conduct the work, and did the resultant material conform to your aims or expectations?

Following the same concepts developed during my Ph.D. work back in the '90s (i.e., growth under low water-oxide interfacial tension at thermodynamically stable conditions), the specific approach here was to overcome the centrosymmetry of the structure by promoting the growth of the most stable faces of cassiterite, that is the (110) faces which, in tetragonal symmetry, would result in c-axis elongated rod with a square cross section.

"...chemistry is truly a powerful (and very cheap) tool to build advanced nanostructures at low cost and large scale..."

The other challenge to cope with was the presence of metal cations of higher oxidation state (+4), which are very reactive in water (and sometimes with only moisture in the air, e.g., Ti) and thus require acidic solutions to exist as solvated ions. However (and that was the dilemma), a basic medium is necessary to form most transition metal oxides in aqueous solutions (the so-called alkalinization of metal cations).

Our strategy was to use a simple solution of hydrous SnCl₄ salt as the single source of Sn(IV) ions (stabilized at very low pH) in the presence of a stable common (and highly water-soluble) amine (i.e. urea). The latter would slowly decompose when heated in aqueous solutions around 90-95°C, producing the necessary raise in pH to form the dioxide by olation and oxolation reactions. The synthesis is rather slow and the proportion very strict (compared to zinc or iron oxide) but the outcome is so much more rewarding.

For the first time (and truly as exactly as predicted), the fabrication of very large arrays (hundreds of cm²) of vertically oriented (c-axis) rod with square cross section and (110) side faces of crystalline tin dioxide (SnO_2,cassiterite) was finally demonstrated (by "simply" heating up a bottle containing common salts and a glass (or any other) substrate in a regular laboratory oven at low temperature)…and as I vividly recall (from the very dark electron microscopy room), it did put a smile on my face; you know when theory and experiment agreed.

What, in particular, about your methods was different/better than prior attempts to synthesize SnO₂ nanorod arrays?

Most of the prior attempts were carried out by physical (gas phase) techniques where the crystal symmetry dictated the final morphology and crystal orientation, and, given the inherent centrosymmetry of the structure, no successful results were ever recorded.

This work was indeed the first demonstration of c-axis growth of SnO₂ nanorod arrays. It also clearly showed the beauty (and superiority?) of materials chemistry in general and the growth against crystal symmetry in particular (in this case, by promoting the most stable side faces to grow along the forbidden axis).

How was this paper received by the community?

As for the other papers related to vertically oriented nanorod-based structures, many groups around the world have been inspired and investigated various physical and chemical properties (with or without referring to the actual paper). It also definitely proved, if necessary, that chemistry is truly a powerful (and very cheap) tool to build advanced nanostructures at low cost and large scale—and not only as ultrafine powders but also as vertically oriented nanorod-arrays.

Where have you taken this work since the publication of this paper?

"This work was indeed the first demonstration of c-axis growth of [tin oxide] nanorod arrays."

We focused mostly on the academic aspect; that is, for instance, the in-depth studies (angular dependent soft x-ray absorption and resonant inelastic x-ray scattering at synchrotron radiation facilities) of the electronic structure. The SnO₂ rods are indeed growing along a forbidden axis and are also quantum-confined in the lateral dimension (~2 nm), thus showing unusual and very interesting behavior.

We are also conducting experiments probing their morphology-property relationships as well as developing novel quantum rod-based hetero-nanostructures such as visible light active quantum dots grown on quantum rods. In addition, the actual aqueous chemical growth mechanism (involving zero-charged intermediates) is currently being completed and will be communicated shortly.

Are there currently any practical applications for SnO₂ nanorod arrays?

The major applications are for transparent conducting oxides (TCOs), as those arrays can be grown directly onto bare glass, thus creating better conducting properties, thus leading to more efficient devices. In addition, SnO₂ is a very well-known material for gas sensing. For many years, scientists have tried to develop nanoparticles exhibiting (110) faces, which are the most active for oxygen-based compound adsorption/decomposition (due to the shortest distances between Sn atoms).

It is now available at low cost and large scale, as vertically oriented quantum rods (no grain boundaries and with minimum charge recombination) and with the highest electronic density (c-axis) onto a variety of substrates (glass, TCOs, silicon wafers, polypropylene, Teflon, etc...). Provided that the rather crucial (technological) issue of nanocontacts (schottky and/or ohmic) of vertically oriented structures can be solved, we should most certainly see nanodevices (room-temperature gas sensors, anodes for photovoltaics and solar hydrogen generation) based on such structures in the near future.

Dr. Lionel Vayssieres
Nanomaterials Scientist and R&D Consultant
National Institute for Materials Science (NIMS)
International Center for Materials NanoArchitectonics (MANA)
Tsukuba, Ibaraki, Japan
and
Lawrence Berkeley National Laboratory
Chemical Sciences Division
Berkeley, CA, USA

LIONEL VAYSSIERES' MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:

Vayssieres L, "Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions," Advan. Mater. 15(5): 464-6, 4 March 2003 with 795 cites. Source: Essential Science Indicators from Clarivate.

ADDITIONAL INFORMATION:

• Read a previous interview with Lionel Vayssieres.

KEYWORDS: SOLAR-CELLS, ELECTRON INJECTION, OXIDE NANOWIRES, CHEMICAL GROWTH, THIN FILMS, SURFACE, ZNO, MORPHOLOGIES, TEMPERATURE, FABRICATION, SN02, CRYSTAL SYMMETRY, C-AXIS ORIENTATION, ELECTRON DENSITY PATH, METAL IONS, OXIDATION STATE, SNCL4 SALT SOLUTION, LOW COST, LARGE SCALE, ELECTRONIC STRUCTURE, MORPHOLOGY-PROPERTY RELATIONSHIPS, QUANTUM ROD-BASED HETERO-NANOSTRUCTURES.

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• 2010
• November 2010 - Author Commentaries
• Lionel Vayssieres on Controlled Aqueous Growth of Tin Oxide Nanorod Arrays