Size Effects on the Photoelectrochemical Activities of Single Wall

7/30/2019 Size Effects on the Photoelectrochemical Activities of Single Wall

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Electrochimica Acta 54 (2008) 821828

Contents lists available at ScienceDirect

Electrochimica Acta

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Size effects on the photoelectrochemical activities of single wallcarbon nanotubes

Chen-Zhong Li a,, Won-Bang Choi b, Cheng-Hsin Chuangc

a Nanobioengineering/Bioelectronics Lab, Department of Biomedical Engineering, Florida International University, EC 2612,

10555 W. Flagler Street, Miami, FL 33174, USAb Department of Mechanical & Materials Engineering, Florida International University, 10555 W. Flagler Street, Miami, FL 33174, USAc Department of Mechanical Engineering & Institute of Nanotechnology, Southern Taiwan University, Taiwan

a r t i c l e i n f o

Article history:

Received 4 April 2008

Received in revised form 19 June 2008

Accepted 19 June 2008

Available online 4 July 2008

Keywords:

Single walled carbon nanotubes

Photoelectrochemistry

Solar cell

Photo energy

Photocurrent

Size effects

Finite

a b s t r a c t

This paper reveals an improved photoelectrochemical activity of single wall carbon nanotube (SWNT)

with respect to enhanced photo-induced currents as they are shortened at a fewnanometer sizes. Raman

spectroscopy, photoelectrochemistry and scanning electronic microscopy (SEM) characteristics indicate

a uniform finite-sized SWNT film with distinct physical and photochemical properties. Unlike p-type

semiconductive pristine SWNTs, both cathodic photocurrent and anodic photocurrent are observed on

shortened SWNT formed thin films. The incident photon conversion efficiency is eight-fold higher com-

pared to the longer SWNTs, suggesting the electronic structure and photoelectrochemical properties of

SWNTs are significantly altered by shortening the length of SWNT. The improved photoelectrochemical

activities open a new perspective to use finite-sized SWNTs in combination with other semiconducting

materials for fabrication of efficient optoelectronic devices, nanotube optical detectors or emitters that

can be operated across a wide range of optical wavelengths.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

The remarkable electronic and mechanical properties of

nanoscale carbon-based materials (fullerene and carbon nan-

otubes) with strong covalent sp2 carbon bonds have a tremendous

potential application for nanoelectronic devices. One of the key

features of nanoscale carbon-based materials (fullerene and car-

bon nanotubes) is their photoactivity and potential to function as

optoelectronic device elements [14]. The extraordinary electron-

acceptor properties of fullerenes have resulted in noteworthy

advances in the application of the carbon material as a basic build-

ing block for the conversion of solar to electrical energy. Single

wall carbon nanotubes (SWNTs) can be viewed as an extension

in one dimension of different fullerene molecular clusters or as a

strip cut form an infinite graphene sheet that rolls up to form a

tube. Fundamental research of band gap fluorescence from indi-

vidual SWNTs provides a basis for applications of SWNT as optical

materials for the solar to electrical energy conversion system [5].

Unlike semiconducting material, silicon, SWNTs have a direct band

gap in the momentum space, which induces interband transitions

Corresponding author. Tel.: +1 305 3480120; fax: +1 305 3486954.

E-mail address: [email protected] (C.-Z. Li).

and direct light absorption and emission without photon interven-

tion. In particular, the seemingly ideal features of optical excitation

together with the recombination across pairs of van Hove peaks

lead to the potential use of carbon nanotubes in the areas of

light-reduced electron-transferchemistry and solarenergy conver-

sion [68]. Indeed, photo-generated currents from CNTs have been

reported by several groups [9,10]. Notably, individual semiconduct-

ingSWNTs in a field-effect transistor (FET) canemit polarizedlight,

while illumination of the FET devices generates significant pho-

tocurrent, in agreement with theoretical estimation by Steward et

al. [11]. Unlike fullerene-based film with n-type (photo-induced

oxidation) photoactivity [2], a p-type photoelectrochemical feature

hasbeen reported by Kamat and co-workers on a SWNT film-based

opto-system [12,13], wherein theydeposited unformed SWNTfilms

on optically transparent electrodes electrophoretically and mea-

sured the resulting photoactivity under excitation by visible light.

The large photo-current generation of multi wall carbon nanotubes

has also been reported [14]. Despite the significant improvement

of photon-to-current conversion efficiencies achieved using SWNT

conjugated polymer materials [15,16], only very low efficiency was

obtained on pure SWNT films.

It has been reported that the electronic structure of SWNTs

is largely relied on the CNTs diameter, chirality, and length.

Besides the electrical properties, the optoelectronic properties of

0013-4686/$ see front matter 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.electacta.2008.06.059
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822 C.-Z. Li et al. / Electrochimica Acta 54 (2008) 821828

nanotubes may also be tailored by selecting the length accord-

ingly. Venema et al. [17] have prepared carbon nanotubes into

segments of a few tens of nanometers in length and proved that

the band gap increased with a decrease in the tube length. Con-

ceptually, as the length of a SWNT is reduced, the size of SWNT

eventually approaches the limit of a fullerene molecular cluster

with a band gap from 1.5 to 2.3 eV [18,19]. Based on this basis,

finite-sized SWNTs might be considered as promising materials,which are similar to fullerene for manufacturing optoelectronic

devices with better photon-to-current conversion efficiencies. This

paper presents a novel photoelectrochemical property of finite-

sized SWNT in term of an enhanced photoactivity when the length

of SWNT is decreased. An improved photocurrent on finite-sized

SWNT modified gold electrodes; with energy conversion efficiency

eight-fold higher compared to pristine SWNT modified electrodes.

In particular, diode-like characteristics of finite-sized SWNTs are

observed in terms of the generation of both cathodic (electron

current) and anodic (hole current) photocurrent upon the differ-

ent applied potentials. The structure and the physical properties

of the finite-sized SWNT film derived by electrophoretic deposi-

tion are further characterized by Raman and scanning electronic

microscopy (SEM).

2. Instruments and methods

2.1. Instruments

Photoelectrochemical measurements were conducted in a

home-made three electrode cell with a round (5 mm diameter)

quartz window at oxygen-free condition. An Oriel arc lamp source

(Newport Oriel Instruments, Irvine, CA)with an Osram XBO 150-W

xenon lamp was used for UV illumination. A grating monochro-

mator (Oriel) was introduced into the path of the excitation beam

for selecting the wavelengths. The light intensity was measured

with a Melles Griot hotometer (Melles Griot Optics, Irvine, CA).

Currentvoltage characteristics were measured using an EG&G

potentiostat (PerkinElmer, Princeton Applied Research, Oak Ridge,TN).

AC impedance characterization was performed using the same

EGG potentiostat. Impedance obtained at 220 mV versus Ag/AgCl

in the presence of 1 mM ferro/ferri cyanide redox electrolytes was

superimposed on a sinusoidal potential modulation of5 mV. The

data was measured and collected for 31 harmonic frequencies

from 100 kHz to 100MHz. The impedance data obtained from the

bare gold electrode, pristine SWNTs and finite-sized SWNTs mod-

ified gold electrodes was analyzed using the ZSimpWin software

(Princeton Applied Research). In all impedance spectra, symbols

represent the experimental data and the solid lines represent the

fitted curves.

2.2. Materials

SWNTs obtained from Carbon Nanotechnologies (Houston, TX)

are synthesized by a modified gas phase process and remove

large catalyst particles with a purity of about 99%. The shortened

SWNTs are prepared from pristine SWNTs (Carbon Nanotechnolo-

gies) according to the previous report [20]. Finite-sized SWNTs

were made following an ultrasonic process for over 3 h in dilute

ceric sulfate (CS) at room temperature. Mid-sized SWNTs with

100500nm in length were obtained by controlling the sonica-

tion times. The main by-products in this process were amorphous

carbon and graphitic nanoparticles. Resulting samples were cen-

trifuged (10,000 rpm for 30 min), followed by the removal of the

supernatant. The CNTs were neutralized, dispersed in NaOH and

washed with water followed by recentrifugation. In order to com-

pletely remove CS from the samples, this procedure was repeated

10 times until no UV characteristics (peak at 264 nm) of CS were

observed. The SWNT suspension was further ultrafiltered using

0.1m pore sized membrane filters (Schleicher-Schuell; cellulose

nitrate, diameter 90m) to remove amorphous carbon impurities.

The acid-treated product was purified by gas phase oxidation at a

heating rate of 5 C/min in air from room temperature to 600 C,

leaving the SWNTs with a weight of 40% of the initial raw materi-als.

2.3. Electrode preparation

The SWNT film was electrodeposited on electrode surfaces

according to Kamat and co-workers [12]. Briefly, SWNTs were

negatively charged by mixing with tetraoctylammonium bromide

(TOAB) in tetrahydrofuran (THF). Sonication of the mixture for

30 min yields a stable dark suspension. Then the solution was

washed with THF by several cycles of centrifugation and re-

suspension to remove unbound TOAB in the solution. After the

solvent removal, the charged SWNTs were dried. The dried materi-

als consisting of TOAB modified SWNTs was re-suspended in 10 mL

of THF and sonicated for 20 min. A home-made electrophoretic

deposition cell, including two 8 mm42 mm gold coated siliconplates, a quartz cuvette and a voltage source, was used to deposit

SWNTs on the gold substrate. A 50 V/cm dc voltage was applied on

the twoconducting goldplates and SWNTs started to move towards

the positive electrode. Under the influence of a dc electric field,

SWNTs assembled as stretched bundles were anchored on the pos-

itive electrode within a few minutes. Similarly, the electrophoretic

deposition approach was also employed to cast the carbon black or

graphite film on gold substrates.

3. Results and discussions

The size control of carbonnanotubes wouldallowthe investiga-

tion of the electronic properties at different length scales, towards

the advanced utilization of nanotubes in device applications. Inthis experiment, appropriate conditions for the treatments were

found to depend greatly on the nature of the starting material.

Accordingly, the reaction conditions and production of each step

was carefully examined and characterized by Raman and TEM to

avoid over destruction of SWNTs. To date, numerous methods to

cut SWNTs into shorter segments have been reported including

acidic or fluorinating treatment. We have recently reported that

ceric sulfate (CS)treatmentwith sonication leadsto thetransforma-

tion of SWNTs to nanocrystalline carbon materials. By combining

the wet chemical oxidation process (acidic treatment) and dry

purification process (high temperature oxidation), most impuri-

ties including carbonaceous materials and the metal catalyst have

been removed. By carefully controlling the experimental condi-

tions, finite-sized carbon nanotubes could be obtained, which arerelatively unscathed with a tubular structure. In addition, carbon

blackand graphite powder-based electrodeshavebeen investigated

as control samples to better understand the photoelectrochemical

properties of carbon nanotubes.

3.1. Characterization of SWNT-based film

By varying the sonication time in acid solution, selected length

of SWNTs were obtained. AFM and TEM (Fig. 1) enabled us to

monitor the progressive decrease in SWNT length with different

sonication time. The longer distribution of SWNTs (>1 mm) shown

in Fig. 1A1 and B1 was the pristine SWNTs. After 1 h treatment by

CS, the tubes were cut into shorter segments with length from 200

to 400nm (Fig. 1A2 and B2). Further sonication for 3 h resulted


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C.-Z. Li et al. / Electrochimica Acta 54 (2008) 821828 823

Fig. 1. AFM (A) and TEM (B) images showing the pristine SWNTs (A1 and B1); middle-sized SWNTs (A2 and B2); and finite-sized SWNTs (A3 and B3). Magnifications used in

each case are shown on the respective images.

in carbon nanotubes ranging from 10 to 100 nm, which still dis-

played a good tubular structure as indicated in Fig. 1A3 and B3. A

closer inspection of some isolated tubes showed the tendency of

the finite-sized SWNTs to aggregate to form super ropes in order

to minimize the newly exposed surfaces. Although some of SWNTs

might be completely destroyed at thisstage, mostremainedas one-

dimensional structures after being ultra-shortened (Fig. 1B3), and

their tubular structure was evidenced further by Raman measure-

ments as discussed later.

Under the applied dc field (50V), the TOBE-capped SWNTs were

assembled on the positive gold electrode. After only 10 min, the

black SWNTs suspension became transparent due to the depo-

sition of SWNT onto the gold substrate, while a uniform SWNT

layer was formed on the gold surface. The estimated amount of

the deposited SWNTs was 0.2 mg cm2 by weighting the solution

in the deposition cell before and after deposition. SEM was also

used to characterize the SWNT films formed by pristine SWNTs

and shortened SWNTs. As shown in Fig. 2A, many SWNTs bun-

dles and SWNTs loops several micrometers in size were assembled

as a uniformed but rough layer on the electrodes. In contrast, the

deposition of shortened SWNTs resulted in a much smoother film

due to the finite size of the SWNTs ( Fig. 2B). An artificial physi-

cal defect contained SWNT film was selected to measure the film

thickness. The average thickness of the SWNT filmwas determined


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Fig. 2. The SEM images of pristine SWNT-based film (A), and the finite-sized SWNT formed film (B). The inset of (A) shows an enlarged view of pristine SWNT bundles on

the surface.

up to 50m according to the surface profile of SEM image. Fig. 3

shows the Raman spectrum of the pristine SWNT film, the finite-

sized SWNT film and two control samples; graphite and carbonblack powders. Fig. 3 shows a comparison over a broad frequency

rangefrom0to3500cm1, between thespectrums of thesecarbon-

basedfilms on gold substrates withthe sameexcitationlaser source

(531 nm, 2.31eV). As expected, the Raman spectrum of pristine

SWNTs (Fig. 3a) exhibits the radial breathingmode (RBM) frequen-

cies in the range 150190 cm2. However, the lineshape change at

the radial breathing mode of the finite-sized SWNT was noted as a

broadened and suppressed feature, corresponding to the retained

tubular structureand the enrichment of defects [21]. The tangential

stretchG bandin terms of a narrow Lorentzian lineshape1585cm1

with a broad Bret-Wigner-Fano shoulder at 1574 cm1 further con-

firms that the pristine SWNT retains its intact morphology after

the process of electrodeposition, in agreement with the literature

data [22]. Different from the over destructed SWNTs as we previ-

Fig. 3. Raman comparison of (a) pristine SWNT film; (b) finite-sized SWNT film;

(c) graphite-based film; and (d) carbon black taken with 532.1nm (2.31 eV) laser

excitation on gold substrates. Inset shows the enlarged G-bond for comparison.

ously reported [20], in whicha completedisappearance of theRBM

was obtained, the lineshape change at the radial breathing mode

of the finite-sized SWNT was noted as a broadened andsuppressedfeature, corresponding to the retained tubular structure and the

enrichment of defects. After the shorten process, the most obvi-

ous effect is the appearance of a strong band at ca. 1364 cm1 with

an increased 1364 cm1/1585cm1 intensity ratio compared with

longer SWNTs. The increased intensity of the D-band at 1364 cm1

reflects the nanoscale structure of finite size SWNTs that exhibit

higher photon density [23]. Furthermore, the weak Raman inten-

sity at intermediate frequency (5001200 cm1) and the lackof the

other semiconduction associated Lorentzian feature, could be con-

tributed from the vibrations along the direction of the nanotube

axis, indicating the SWNTs are ultra-shortened by the shortening

process.It hasbeen known that an increase of the order in carbona-

ceous materials is reflected by an increase in the frequency of the G

mode as well as a decrease of its bandwidth [24]. Fig. 3b shows theRaman spectrum of the finite-sized SWNT deposited on gold sub-

strate in comparison with the two reference materials of graphite

powders, sp2 hybridized carbon atoms in a multi-planar hexagonal

structure (Fig. 3c) and the carbon black, amorphous carbon with

a disorder carbon structure (Fig. 3d). The band frequencies of the

graphite film and the carbon black-based film are quite similar to

previously published spectrums of graphite and carbon black pow-

der [25,26]. Clear differences can be extracted from Fig. 3ac when

comparing the bandwidth at the G band of the amorphous carbon

black (Fig. 3d). The narrower G modes indicate a retained polycrys-

talline graphitic structure of SWNTs when they are shortened to

a finite size. Similar to our previously reported data, over 10h of

sonication in CS resulted in the total destruction of SWNTs and no

tubular features were observed.

3.2. Photoelectrochemical measurements

Photocurrent measurements with the electrodes modified with

different carbon materials were performed in an electrolytic

aqueous phosphate buffer solution (0.1 M, pH 8.5) containing

a ferri/ferro cyanide redox couple. The regenerative Fe2+/Fe3+

redox couple facilitates the charge transporter at the electrode

surfaces and thus enables the delivery of photocurrent. As a

ferro/ferricyanide electrolyte is effectively transparent in the

300900 nm region, the deleterious effect in terms of losing inci-

dent photons through competitive electrolyte light absorption will

be prevented. Photocurrents with white light were recorded, con-

firming the photoactivity of the SWNT film on the gold electrodes.


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Fig. 4. (A) Potential dependence of photocurrent, obtained on a finite-sized SWNT modified gold device in the dark (black) and under illumination of white light (red).

(B) Time traces of the cathodic photocurrent at 0.75 V (vs. Ag/AgCl) applied electric field; and (C) Time traces of the anodic photocurrent at +0.75 V (vs. Ag/AgCl) with a

monochromatic incident light chopped, respectively. In (B) and (C), the upper two traces (a) are taken at pristine SWNT films, the middle two (b) are taken at middle-sized

SWNT (200500 nm) formed films, and the lower two (c) are taken at finite-sized SWNT modified electrodes in phosphate buffer solution (0.1M, pH 8.5) that contained

1 mM Fe(CN)63/4 redox species. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

p-Type features of thepristineSWNTs were evident by the cathodic

photocurrentgenerationbecause onlythe photo-generatedholes of

SWNTs were able to migrate from the gold substrate to the counter

electrode. However, with the positive bias on the SWNT modified

surface, no photo-induced anodic current was observed. Fig. 4A

presents photocurrentvoltage curves for the finite-sized SWNT

film measured under dark (dark current) conditions and light illu-

mination (photocurrent). With a positive bias, a manifest increased

anodic current(electron current)was obtainedwith increasing pos-

itive potentials. In both cases, a steady flow of cathodic currents

(photo-induced) was observed, and the photocurrent magnitude

was dependent on the applied potential with respect to a decreas-

ing of cathodic photocurrent (hole current) when the applied

potential becomes negative. The current spikes are visible with

chopping of the incident light. When the light was turned on, in all

three casesthe cathodiccurrents decreased (Fig. 4B), whileonlythe

finite-sized SWNT modified electrode displayed the anodic current

spikes in the opposite direction (Fig. 4B, c).


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Fig. 5. Photoaction spectrum of gold electrodes modified with () finite-sized

SWNTs and () and pristine SWNTs, respectively.

The effectof thelengths of theSWNTs on the generation of pho-

tocurrent was further investigated by using different sized SWNTs.

Fig. 4 also shows time-resolved traces of the photocurrent at 1.0

and +1.0 V applied potential on the gold electrodes modified with

pristine SWNTs (Fig.4B, a),middle-sized SWNTs(200nm < 400nm)

(Fig. 4B, b) andfinite-sizedSWNTs(


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Fig. 6. Nyquist plots with 1mM Fe(CN)63/4 as redox probe for the pristine SWNT

(A) and the finite-sized SWNT (B) modified electrodes. The electrode potential was

0.22 V vs. Ag/AgCl. The solid line is the experimental raw data, and the symbol ()

represents the fitted curves by using the equivalent circuit model shown as inset of

(A). The frequency range was 100kHz to 100 mHz.

remarkably affect the electronic properties of the tube [7,17,23]. Theelectronhole pair could be generated in a high energy azimuthal

sub-band of SWNTs by photo-excitation, followed by a decay into

the lowest sub-band, from which the pair recombines with the

emission of a photon. Photo-excitedelectronhole pairscan be sep-

arated and expected to be driven to the electrode interface by an

applied electric field to generate cathodic or anodic photocurrent.

The so-called flat band potential associated with the kinetics of

electrolyte charge transfer has been predicted and corresponds to

the change of the photocurrent sign [32]. In this case, thedepressed

kinetics of charge transfer of ferri/ferro cyanide on the finite-sized

SWNT layer probably could negatively shift the flat band potential

to produce the measurable anodic photocurrent, whereas pristine

SWNT film under the same range of applied potentials would not

produce a photo-induced anodic current. A quantitative estimateof the flat band potential values for the two cases is a subject of

future studies.

Research predictions have pointed out that mechanical defor-

mation and a high electrostatic field can remarkably affect the

electronic properties of the tube and could be used for engineer-

ing a band gap in nanotube-based molecular scale electronics

[33,34]. Considering the dependence of electronic band structures

of SWNTs on diameter, length and chirality [35], we could suggest

a possible explanation for the improved photocurrent and IPCE of

the finite-sized SWNT. Ideally, the optimal values of the band gap

of semiconducting materials are from 1.2 to 3.3eV, which are com-

parable to theenergies of photons whose frequencies lie within the

visible light, making them have the potential of creating high effi-

ciency photovoltaic devices. Therefore, the lower photocurrent and

IPCE of pristine SWNTs probably could be attributed to the band

gap of SWNT (from 0.3 to 0.7 eV) [34], just outside of the optical

lightrange. It has been shownthat the finite size effects on the elec-

tronic structure of SWNTs results in a reversely increasing bandgap

with reducing SWNT length. Considering the dependence of elec-

tronic band structures of SWNTs on diameter, length and chirality,

the enhanced photocurrent finite-sized SWNTs probably could be

attributed to the band gap shifting closer to the range of incidentlight, which is moreenergetically feasible for sufficientexcitationof

electrons from the valence band into the conduction band. There-

fore, the enhanced photocurrent generation is more likely related

to the photo-excitation of hot holeelectron pair rather than the

charge separation process.

5. Conclusion

Our results have clearly demonstrated an efficient generation of

photocurrents by using finite-sized SWNT filmmodifiedelectrodes.

We observe both anodic photocurrent and cathodic photocurrent

on the photoelectrochemical cell with visible light illumination.

The results present a new concept for the future design of SWNT-

based optoelectronics. The assembly of finite-sized SWNT withother semiconducting solid materials or polymers as solar cell

architecture is of interest for further investigation. Experiments

using SWNTs with different lengths and different substrates are

underway to characterize the photoactivities.

Acknowledgments

This research work is partially supported under grant FA9550-

06-1-0467 and FA9550-07-1-0344 of Department of Defense/Air

Force Office of Scientific Research and 2008 FIU Faculty Research

Award and Wallace H. Coulter Foundation.

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Size Effects on the Photoelectrochemical Activities of Single Wall