Electrochemistry and Photocurrent Response from Vertically-Aligned Chemically-Functionalized Single-Walled Carbon Nanotube Arrays

Abstract

The unique electronic properties of single walled carbon nanotubes ͑SWCNTs͒ can be used to generate a current response from visible light. Vertically aligned single walled carbon nanotube arrays were created on an optically transparent electrode ͑fluorine doped tin oxide coated glass, FTO͒ by a self-assembly process using the hydrophobicity of the nanotube side walls and chemical treatment of both the nanotubes and the FTO substrate. The electrode characteristics were explored using electrochemical approaches. The SWCNTs were further functionalized with light absorbing species such as dye or porphyrin. This led to arrays of SWCNTs chemically attached to the substrate that when exposed to visible light exhibited a prompt current response ͑ϳ5 A/cm 2 , ഛ200 ms͒ and a voltage of ϳ33 mV. This photoresponse behavior was investigated by modifying the attachment conditions and also the SWCNT treatment procedures.Every minute the amount of solar energy incident on the Earth is far greater than the energy provided by fossil fuels in an entire year. 1 However, photovoltaics make up only 0.04% of power generated. 1 This is a result of the high costs involved due to the manufacturing processes that are used and low energy conversion efficiencies. The use of a chemical based photovoltaic that mimics the process of photosynthesis in nature, such as the Grätzel cell 2 -instead of existing thin film semiconductor technology-offers the possibility of an efficient and most important cheaper source of electricity. The novel element in the Grätzel cell was the use of colloidal nanocrystals of titania ͑TiO 2 ͒ as the semiconductor substrate, which provided a much larger surface area and thus increased efficiency. Following the work of Grätzel there have been many attempts to further increase the efficiency of this design by changing almost every parameter. These modifications have included the integration of different dyes, 3 the linkage of one dye molecule to another, 4 the use of titania nanotubes instead of particles, 5 the integration of carbon nanotubes, 6,7 and also the changing of the electrolyte. 8,9 This device design, however, has the possibility to be further improved upon by incorporating carbon nanotubes as the working electrode.Carbon nanotubes ͑CNTs͒ have been widely studied for their electrical conduction properties. Due to their high degree of electron delocalization and high aspect ratio, electrons can exhibit ballistic conduction. 10 Carbon nanotubes that have been modified by photosensitive molecules have been studied with increasing frequency due to their application in optoelectronic devices. [11][12][13][14][15][16][17][18] In most cases it is the same Grätzel cell design but with CNTs mixed in with the TiO 2 to modify their electrical properties, such as decreased resistivity within the TiO 2 layer leading to a markedly increased photocurrent ͑J sc ͒ but a slightly decreased fill factor ͑ff͒ and voltage ͑V oc ͒. 6 The attachment of photosensitive molecules directly to nanotubes can mean highly efficient conduct...

A tool box to ascertain the nature of doping and photoresponse in single-walled carbon nanotubes

“…When the three SWCNT electrodes are irradiated, a negative photocurrent appears at potentials lower than À0.4 V. In previous literature, photoelectroactivity for SWCNTs has been observed in another electrochemical reaction. 56 In our work, the photocathodic process has to be observed at low scan rates (2 mV s À1 ), and the values observed for the photocurrent reached the noticeable value of ca. 20 mA.…”

Dye functionalisation of PAMAM-type dendrons grown from vertically aligned single-walled carbon nanotube arrays for light harvesting antennae

“…[15][16][17] Very little material was needed to produce a photocurrent, making transparent solar cells possible. Additionally, chemical modification of the SWCNT array by other chromophores, for example dye molecules, 16,18 porphyrins, 19 and also dendrons 20 has led to an increased photovoltaic response. Such devices have been investigated primarily through electrochemistry, studying the surface bound concentration and the electron transfer from the redox-active molecule through the nanotube array to the working electrode.…”

“…Transparent vertically aligned SWCNT arrays can be used to generate a prompt and steady photocurrent, 15 and the resultant photoresponse can then be modified by functionalising the array with dye and porphyrin molecules. 18 However, it was found that whilst dye modifying the array increased the current produced, the voltage was decreased. 16,18 One of the most widely studied dendrimer molecules is poly (amidoamine) or PAMAM.…”

“…18 However, it was found that whilst dye modifying the array increased the current produced, the voltage was decreased. 16,18 One of the most widely studied dendrimer molecules is poly (amidoamine) or PAMAM. PAMAM is produced by the successive Michael reactions between methyl acrylate and ethylenediamine.…”

Transition from single to multi-walled carbon nanotubes grown by inductively coupled plasma enhanced chemical vapor deposition

“…Carbon nanotubes (CNTs) are a unique material with many varied applications in the electronics industry such as field emission, 1 electrochemical sensors, 2,3 and photovoltaics. [4][5][6] However, one hindrance to their wide-scale implementation is the ability to produce them both cheaply and with a well-defined structure, both physical and electronic. Current synthesis techniques include arc discharge, 7 laser ablation, 8 and chemical vapor deposition 9 (CVD), with CVD being the most promising approach for high yields.…”