Solar paint

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2012

Quantum dots coated on titanium dioxide nanoparticles are capable of generating electricity.

KEY CONCEPTS
• Cadmium sulfide and cadmium selenide quantum dots are coated on titanium dioxide nanoparticles and applied to a conductive surface to form a solar paint.
• Upon exposure to sunlight, solar paint generates electricity.
• It’s possible to prepare a multifunctional coating that can not only generate electricity, but also contribute lubricity and corrosion protection.

HARNESSING SOLAR POWER FOR USE AS ELECTRICITY REMAINS a challenging goal. Photovoltaic technology has not been developed to the point of enabling solar energy to be economically viable.

In a previous TLT article, the benefits of a new solar power reflector prepared from silver instead of glass was discussed (1). Silver was found to be lower in weight, more durable and more cost effective. In addition, the reflector has a much larger surface area for the sunlight to impinge on the collector, which leads to greater efficiency.

The development of quantum dots has improved the prospects for increasing the efficiency of solar cells in capturing sunlight. Ian Lightcap, graduate student in the department of chemistry at the University of Notre Dame in South Bend, Ind., says, “Quantum dots are small nanoparticles that act as semiconductors. They can be tuned to absorb light at a certain wavelength. This flexibility enables them to be used across a wide range of wavelengths, including infrared.”

Coatings are discussed a great deal in this column because lubricants function in this respect to provide lubricity and impart corrosion to a metal surface. Measurement of the drying of coatings was discussed in a previous TLT article to learn more about how to improve their durability (2). The formation of a crack at the interface between the coating and the substrate is important in predicting failure.

Use of quantum dots provides the expectation that solar energy can become more cost effective. Currently, research is focused on improving the efficiencies of quantum dot solar cells to make them more economically competitive with silicon-based technology.

One approach to reducing and simplifying the cost of solar cells is to incorporate them into coatings that can be applied onto any conductive surface. The prospect of electricity being generated from such a coating may lead to benefits in a number of consumer and industrial applications. A coating that can generate electricity has now been developed.

COATED TITANIUM DIOXIDE NANOPARTICLES
Lightcap, in collaboration with Prashant Kamat, John A. Zahm Professor of Science in chemistry and biochemistry at the University of Notre Dame and Matthew Genovese, a student at the University of Waterloo in Canada, have incorporated quantum dots into coatings to literally develop solar paint. He says, “We were able to coat titanium dioxide nanoparticles with cadmium sulfide and cadmium selenide quantum dots. When light with the correct wavelength reaches one of the cadmium compounds, it will then cause an electron to escape and be absorbed by the titanium dioxide, which then leads to the generation of electricity.”

Lightcap indicates that there is a major push to improve the fundamental efficiencies of silicon solar cell, possibly through facilitating electron transfer. He adds, “Our approach is a fairly easy and cheap way to potentially realize this goal while moving beyond traditional, silicon-based solar cells.”

Two techniques were used to prepare the solar paint. The first involved physically mixing titanium dioxide and the cadmium salts in a solvent blend of water and t-butanol. The second technique was to use a method known as pseudo-SILAR (sequential ionic layer adsorption and reaction).

Lightcap explains, “A pure SILAR process is a sequential method in which the cation is first introduced into the coating followed by the anion. In the pseudo technique, both the cation and the anion are added at the same time.”

In both cases, mixing of the materials in the solvent blend produced a paste that generates electricity when applied to a conducting surface in the presence of light. Figure 3 shows a yellow solar paint derived from cadmium sulfide and titanium dioxide.


Figure 3. Quantum dots consisting of cadmium sulfide are coated on titanium dioxide and incorporated into a yellow coating, which is applied to a conductive surface. Electricity is generated when the coating is exposed to sunlight. (Courtesy of the University of Notre Dame)

Two sizes of cadmium selenide are prepared by the pseudo-SILAR procedure. Lightcap says, “Particles with average diameters between six and 10 nanometers and between one and six microns were produced.” For the bulk process, the particle size obtained for both cadmium salts is in the micron range.

Scanning electron microscopy shows that a random mix of particles is found in the solar paint. Lightcap says, “We did not see any long range repetition of structure. When the paint is applied, the thickness is made uniform through the use of annealing to remove the solvent. This lack of a repeatable structure works pretty well, even though we are unsure whether it has an effect on the overall performance of the solar paint.”

Electrochemical testing was done to evaluate the solar paint. Lightcap says, “We evaluated the solar paint in a photoelectrochemical cell containing a semiconductor film photoanode, copper sulfide reduced graphene-oxide counter electrode and a sulfide/polysulfide electrolyte.”

The researchers achieved a power conversion efficiency exceeding 1% in these studies. While this is low compared to conventional solar silicon-based solar cells (efficiencies between 10% and 15%), the results show the potential of this technology.

Lightcap indicates that future work will involve making the structure of the solar paint more ordered to see if this will boost its efficiency. A binder will also be included in the paint to see if this will have a positive effect.

Other functionalities, such as lubricity and corrosion protection, can also be formulated into the solar paint. This means that the potential for incorporation of quantum dots may extend the performance attributes of a lubricant coating. It is conceivable that this result will lead to additional uses for coatings in the future.

Additional information can be found in a recent article (3) or by contacting Lightcap at ilightca@ind.edu.

REFERENCES
1. Canter, N. (2009), “Improved Solar Power Reflector,” TLT, 65 (12), pp. 10-11.
2. Canter, N. (2011), “Drying of Coatings,” TLT, 67 (4), pp. 8-9. 
3. Genovese, M., Lightcap, I. and Kamat, P (2012), “Sun-Believable Solar Paint,” ACS Nano, 6 (1), pp. 865-872.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.