Copper sprouts: Growing new solar cell materials

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  • Published: May 1, 2014
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Copper sprouts: Growing new solar cell materials

Sprouting metal

The schematic illustration of the cross-section and phases observed during copper oxidation. The center photo shows the copper being pushed upward through the grain boundaries to become nano wires. Wustl

A Raman spectroscopy study has looked at how copper produces grass-like nanowires that might one day be exploited in the construction of a new type of solar cell.

By looking at a piece of material in cross section, engineer Parag Banerjee of Washington University in St Louis and his colleagues Fei Wu and Yoon Myung have observed how copper foil can sprout grass-like nanowires. They demonstrated that whereas heating most metals simply leads to the formation of a thin oxide film on the metal's surface, some metals including copper, iron and zinc, produce tiny fibres on their surface nanoscopic in diameter and putatively many micrometres long. Banerjee and colleagues wanted to reveal the growth mechanism of these nanowires.

Single crystal nanowires

"Other researchers look at these wires from the top down," he explains. "We wanted to do something different, so we broke our sample and looked at it from the side view to see if we got different information, and we did." Writing in the journal CrystEngComm with funding from Washington University's International Center for Advanced Renewable Energy & Sustainability (I-CARES) and the McDonnell Academy Global Energy and Environment Partnership (MAGEEP) provided funding for the research, the team used Raman spectroscopy to take a close look at the copper nanowires. The technique revealed that a relatively thick film underpinned the sprouting nanowires. This film comprised two different copper oxides [copper(II) oxide and copper(I) oxide], the spectra showed. Moreover, these oxides had narrow, vertical columns of grain running through them and in between the columns were grain boundaries through which metallic copper could spread from within the bulk metal as the heat was turned up, squeezing out copper nanowires. "We’re now playing with this ionic transport mechanism, turning it on and off and seeing if we can get some different forms of wires,” explains Banerjee, who heads the Laboratory for Emerging and Applied Nanomaterials (LEAN). Critically, the emerging nanowires are single crystals, a characteristic they share with the photovoltaic materials of solar cells, and themselves have no grain boundaries existing as individual and continuous pieces of material.

"If we could take these and study some of the basic optical and electronic properties, we could potentially make solar cells," Banerjee explains. "In terms of optical properties, copper oxides are well-positioned to become a solar energy harvesting material." The explanation for copper nanowire growth might also be exploited by other engineers and materials scientists who would like to use single crystal oxides in scientific research. Manufacturing single crystal copper(I) oxide for research is a rather expensive process with costs of up to approximately US$1500 for just one crystal. Banerjee points out that for many applications a nanowire, single crystal would suffice and be far cheaper to make for investigating a wide range of phenomena.

Multitasking metal

The same copper nanowires might also find use in semiconductor devices where they are sandwiched between two materials, in photocatalysts, or even as electrode components for water splitting systems to produce hydrogen and oxygen.

Related Links

CrystEngComm, 2014, 3264-3267: "Unravelling transient phases during thermal oxidation of copper for dense CuO nanowire growth"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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