This is a pre-print version: Please find the published

This is a pre-print version: Please find the published article at http://pubs.rsc.org/en/Content/ArticleLanding/2012/EE/c2ee03519g#!divAbstract

Cite as S. Roy, et al, Energy Environ. Sci.,5, 7001-7006, 2012. DOI: 10.1039/C2EE03519G

Energy & Environmental Science

Plasma Modified Flexible Bucky Paper as Efficient Counter Electrode in Dye Sensitized Solar Cells

Soumyendu Roy,‡1 Reeti Bajpai,‡1 Ajay Kumar Jena,† Pragyensh Kumar,† Neha kulshrestha,‡ D. S. Misra‡* ‡

Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India and Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay Mumbai 400076, India. †

1

Both these authors have contributed equally. *[email protected].

Platinum (Pt) free counter electrodes (CEs) for dye sensitized solar cell (DSSC) were developed using freestanding flexible single wall carbon nanotube (SWNT) films called bucky papers (BPs). BP was irradiated with microwave plasma, created using a mixture of Ar (1%) and H2 (99%) gases, for 2 hrs. Raman scattering measurements revealed that no significant defects were created in the SWNTs as a result of the treatment. Plasma treated BP (P-BP) developed vertically oriented, micron sized, pillar-like structures on its surface, while its base was still a dense random mesh of SWNTs. This unique flexible film had larger accessible surface area and better catalytic properties. Plasma treatment improved the efficiency of BP based DSSC from 2.44% to 4.02%, which is comparable to Pt thin film (4.08%). P-BP based solar cell operated with an open circuit voltage of 0.73 V and a fill factor of 0.70. It also had much higher efficiencies than films of randomly oriented plasma treated SWNTs. Using electrochemical impedance spectroscopy the charge transfer resistances of P-BP and Pt were found to be 1.46 and 1.73 Ω.cm2, respectively.

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Introduction Dye sensitized solar cells (DSSC) are emerging as one of the key technologies in the field of photovoltaics because of their low cost, simplicity of fabrication and significant performance efficiency. The main electron exchange processes1,2 occurring inside a DSSC are: (1) photo excitation of dye molecules. (2) Transfer of electron into the conduction band of TiO2 (the most popular wide band gap semiconductor material used in DSSC) from the excited dye molecules at the anode. (3) Transportation of electron through the nanocrystalline TiO2 particles to the anode. (4) Reduction of the oxidized dye molecules by I-. This regenerates the dye molecules enabling further absorption of photons. I3- formed as a result of the reaction diffuses to the cathode or counter electrode (CE). (5) Reduction of I3- to I- at the CE by the reaction: I-3 + 2e- = 3I-. This step completes the photon induced charge separation in DSSC as well as redox cycle of the electrolyte. Each of these steps exerts a profound influence on the overall energy conversion efficiency of DSSC. Here we concern ourselves with the catalyst material used to facilitate the reduction of I3- at the CE. Pt has established itself as the most suitable material to catalyze this reaction.1 However, for commercialization and mass usage it is imperative to reduce or eliminate the dependence on Pt because it is not only expensive but is also scarcely available on this planet. Experiments have shown appreciable performance from various carbon based cathode materials such as mesoporous carbon,3 activated carbon,4 carbon black,5 nanocarbon,6 graphene,7 conducting polymer8 and carbon nanotubes (CNT).9-11 In addition to the cathode, CNTs have also been used in other components of DSSC, for example as the transparent and conducting

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coating on electrodes.10 It has been used in composites with TiO2 to improve electron transportation at the anode,10,12 as a replacement for TiO213 and even directly as the photoactive material for generation of charged carriers.14 Efficiency of DSSC using CNT based cathodes has been found to be lower than Pt ones. To overcome this shortcoming the general strategy has been to form composites with Pt,15 or conjugated conducting polymers like PEDOT,16 TiN,17 CoS,18 etc. One aspect that has been largely overlooked is that vertically aligned CNT films may be catalytically more active than randomly oriented ones and some work in this direction have started emerging only very recently.19,20

Freestanding or unsupported films of CNTs are called bucky papers (BPs). These are generally formed by self assembly techniques like vacuum filtration. Although macroscopic in dimension these films retain most of the properties of the constituent 1D nanotubes. In this study we have used single wall carbon nanotube (SWNT) to make the BPs. Such films have been used for several applications like Li+ ion batteries, fuel cells, actuators, heat conductors, field emission, supercapacitor, etc.21-25 They are highly flexible, mechanically robust, have a large accessible surface area, high conductivity and chemical inertness. These properties make them an attractive choice for electrodes in electrochemical devices. There is also a demand for light weight and flexible solar cells for applications like satellites and foldable electronics. 26 BP is especially suited for these purposes.

Low temperature plasma is a versatile tool for fabrication and manipulation of nano-materials and thin films. 27 In this article we propose the use of plasma treated BP (P-BP) as CE in DSSC. On exposure to microwave plasma the densely packed horizontal SWNTs in BP stand up and

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form pillar-like vertical microstructures on the surface. The treatment significantly improves Brunauer–Emmett–Teller (BET) surface area of BP without any additional defect creation in the SWNTs. 24 We found that the BP based CE yielded lower efficiency than conventional sputter deposited Pt thin film (Std Pt). However, P-BP and Std Pt had comparable performances. The efficiencies for both these materials were ≈ 4% and open circuit voltages slightly higher than 0.7 V. Both showed good fill factors of around 0.7. P-BP was redispersed into SWNT bundles and drop-casted on conducting glass electrodes to form films of plasma treated SWNT (P-SWNT). This had much lower efficiency than P-BP. Catalytic properties of the CEs fabricated using all these different materials were further studied using electrochemical impedance spectroscopy (EIS). The observed values of charge transfer resistances (Rct) for both P-BP and Std Pt were low, 1.46 and 1.73 Ω.cm2, respectively.

Experimental Section

Synthesis and plasma treatment of BP: SWNTs were grown by catalytic chemical vapour deposition (CVD) method. CH4 diluted with H2 gas was blown over a heated catalyst kept at a temperature of 960-970 °C and under atmospheric pressure. The catalyst consisted of a solid solution of Co and Mo in MgO (Co:Mo:MgO = 1:0.5:300 by weight) prepared by combustion synthesis.28 The mean diameter of the SWNTs was roughly 1.35 nm.24 The catalyst material was separated from the tubes by dissolution in 11 M HNO3 solution. Purified SWNTs were dispersed and filtered through a

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polyvinylidene fluoride membrane under an applied pressure gradient. BPs used in these experiments were about 1.7-1.8 cm in diameter and 10-12 µm in thickness. Plasma treatment of BP was carried out inside a microwave plasma CVD chamber for a duration of 2 hours. A mixture of H2 and Ar gases with 99% H2 by volume was exposed to microwave radiation (2.45 GHz) to create the plasma. Magnetron power used was ≈ 400 W. The temperature and pressure inside the chamber were kept around 800 °C and 70 Torr, respectively.

Preparation of CEs: For the fabrication of P-SWNT CEs 0.1 mg of P-BP was added to 50 ml of isopropanol and ultra-sonicated for about15 hrs till P-BP is sufficiently redispersed into individual bundles. Roughly 3.8 ml of this solution was drop-casted over an area of approximately 1.5 cm2 of fluorine doped tin oxide (FTO) coated glass (TEC 7, Pilkington U.K., with Sheet resistance 8 ohm/□ and thickness 2.2 mm) giving a CNT loading of roughly 5 µg/cm2. In other experiments BP and P-BP were directly clamped to the anode using FTO glass for support and electrical contact. To assess the efficiency of these solar cells in relation to the industry standards, a CE was formed by sputter depositing a thin Pt film (20-25 nm) onto FTO (Std Pt). This closely resembles conventionally used Pt electrodes. Sputtering was carried out in a Nordiko2000 RF sputtering system. The Pt target used had a purity of 99.999%.

Preparation of photo anodes: Titania (TiO2) slurry was made in ethanol with solid loading of 6 % v/v. Poly-ethylene glycol (PEG) (MW approx. 600, Thomas baker 99.9%) was used as dispersant as well as binder. Composition of the slurry was 7.2 g TiO2 (P25 Degussa, mean particle size 25 nm, anatase:

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rutile:: 75:25) , 7.2 ml PEG 600 and 21 ml ethanol. The mixture was roller milled for 24 hrs. The TiO2 paste was coated on a FTO glass using doctor blade technique. The TiO2 films had an area of about 0.16 cm2 and a thickness of ~ 8-9 µm. These films were dried in air and sintered at 4500C for 1 hour. TiO2 coated FTO was dipped in a dye (Dyesol) made of 0.3 mM N3 (cisbis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)) in EtOH for 24 hours at room temperature. Almost a monolayer of dye molecules get adsorbed onto the surface of TiO2 nanoparticles. These serve as the photo anodes.

Assembling the DSSC: DSSC was formed in the following three steps: (1) a surlyn sheet of 60 µm thickness was placed across the periphery of theTiO2 film. This serves as a sealer as well spacer between photoanode and CE. (2) A few drops of electrolyte were smeared over the TiO2 film. Composition of the electrolyte was 0.6 M PMII (1-methyl-3-propylimidazolium iodide, 98 % Aldrich) + 0.5 M LiI (anahydrous, 98 %, Merck) + 0.05 I2 (99.8 %, Thomas Baker) + 0.5 M 4-tert butyl pyridine (TBP) (96%, Aldrich) in acetonitrile (Merck). (3) The CE was placed over the TiO2 photo anode and the whole arrangement was clamped tightly.

Characterization: Surface microstructure and morphology of BP was examined by scanning electron microscope (SEM) (JEOL, JSM-6400). High resolution transmission electron microscope (HRTEM) and Raman spectrometer used for imaging and probing the SWNTs are JEOL, JEM 2100F and Horiba Jobin Yvon, HR 800, respectively. The photovoltaic response of the solar cells (current voltage characteristics) were measured on a Keithley 2420 source meter (least count: 50 µV and

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100 pA) under illumination from Newport class A solar simulator. Solar radiation of intensity 100 mW.cm-2 for air mass 1.5 G was provided by the device. EIS measurements were performed on the DSSCs using Autolab PGSTAT 302N (Metrohm). The cells were kept under the same illumination as above and a DC bias was applied so as to maintain an open circuit condition. Frequency of applied AC signal was varied from, roughly 1Hz to 105 Hz and amplitude kept at 0.05V. Z View version 3.2c was used to fit the experimental EIS data with circuit model.

Results and discussion Figure 1(a) presents HRTEM images of as synthesized SWNTs and figure 1(b) shows an individual SWNT bundle under high magnification. SEM images of BP surface before and after the plasma treatment are shown in figures 2 (a) - (c). The BP can be folded and unfolded without causing any visible damage (see inset of figure 2(a)). Surface morphology of BP undergoes dramatic changes after irradiation with H2 + Ar plasma. Layers of CNT material on the BP surface stand up and are then etched by the plasma to produce vertical pillar-like microstructures.24 The flexibility of BP remains intact after the treatment. Vertical arrays of CNTs can also be produced by alternative methods like chemical vapor deposition (CVD) on lithographically patterned catalyst. But these suffer from disadvantages like the presence of metal catalysts and weak adherence of CNTs to the substrate. Presence of the substrate itself can also lead to undesirable effects in several applications. P-BP on the other hand is substrate-free and composed only of CNTs. D band in the Raman spectrum of BP does not change much as a result of the treatment as can be seen in figure 2(d). This indicates that although SWNT material is being sputtered away by the plasma, those that remain have their sp2 crystallinity intact. This is somewhat counter intuitive but has always been observed in our experiments. Accordingly no

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significant difference between SWNT and P-SWNT were seen inside HRTEM. I-V characteristics of the DSSCs under 1 sun illumination (AM 1.5, 100 mW.cm-2) are shown in figure 3. Cell performance and catalytic response of all the CEs are summarized in Table I. Important cell parameters include short circuit current (Jsc), open circuit voltage (Voc), power conversion efficiency (η) and fill factor (FF). These are connected by the following equations: 1,2

FF =

Pmax P .100 and η = max , where Pmax is the maximum power output from the cell and Pin J scVoc Pin

is the input optical power (100 mW/cm2). BP based DSSC had an efficiency of 2.44% which improved significantly after plasma treatment. We found that the performance of DSSC based on P-BP and Std Pt films were similar. Both showed efficiency of about 4%. Jsc of Std Pt was higher than P-BP, 8.38 and 7.83 mA.cm-2 respectively. However the FF of P-BP (= 0.7) was slightly better. P-BP was redispersed into isolated SWNT bundles and drop-casted on FTO to form a film of randomly oriented plasma treated SWNT (P-SWNT). However, this rearrangement resulted in the loss of efficiency (see Table I). η of P-SWNT film was close to that of ordinary BP. Also the shape of its IV characteristic differed from the expected rectangle-like. This may be due to unstable attachment of CNTs to FTO. Low efficiency of P-SWNT indicates that there is no significant change in the catalytic properties of individual SWNTs on plasma irradiation. The improvement in the efficiency of P-BP is solely due to the modification of geometry and structure of its surface. Plasma treatment increases the BET surface area of BP by almost three times.24 Additionally the vertical alignment of SWNTs exposes more edges of the nanotubes. Edges are known to be more reactive than the sidewalls. These factors lead to more efficient I-3 reduction at the cathode. It is worthwhile to note that back reflection of light from the cathode has been known to improve absorption at anode and hence the efficiency of DSSC.2 Pt film is reflective but BP is not. Thus P-BP is catalytic enough to overcome this drawback and work as

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efficiently as Pt thin film. Although perfectly crystalline basal plane of graphite is known to have very little catalytic effect29 CNTs with their high conductivity and large specific surface area have been used in several electrocatalytic applications, in addition to DSSC.21,22,30-32 Previously there have been several attempts at developing Pt free DSSC. B. Fan, et al developed CE using composites of conducting polymer poly(3,4-ethylenedioxythiophene) or PEDOT and multiwall CNT.16 These were about 76% as efficient as Pt electrodes. Recently Guo-ran Li, et al used TiN nanoparticles dispersed on multiwall CNT surface as CE.17 The performance of this electrode was similar to Pt thin film electroplated on FTO. Jeng-Yu Lin, et al made a CoS/CNT composite film by electrophoresis of CNTs onto a FTO substrate followed by CoS electrodeposition.18 The composite CE outperformed Pt. S. I. Cha, et al produced unique micron sized balls from densely packed CNTs and used this to produce CEs.33 The best efficiency that they could obtain was 85% of that of a Pt electrode. In 2011 Tingli Ma, et al published an extensive study in which they investigated nine different forms of carbon material as CE.11 Among these, carbon dye, ordered mesoporous carbon and to some extent multiwall CNTs showed performance comparable to Pt. Very recently films of vertically aligned CNTs have also been used as CE. In reference 20 the authors showed that vertical CNTs grown directly on FTO electrodes by CVD have higher efficiency than both Pt and randomly oriented CNTs. However, the authors had taken no steps to remove the catalyst metal nanoparticles used to grow the CNTs. These particles have been shown to contribute significantly towards the electrocatalytic activity of CNTs.34 In our case the acid purification step after synthesis of SWNTs dissolve all free metal and metal oxide impurities. K. S. Lee, et al prepared nitrogen doped vertically aligned CNT films that could be transferred to any substrate. These CEs performed much better than undoped films and comparably to Pt.19 In both references 19 and 20 the authors have used multiwall CNT. In

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comparison SWNT are thought to be better catalysts because of their larger aspect and surface to volume ratios.

To further investigate the catalytic activity of the electrode materials we performed electrochemical impedance spectroscopy (EIS) measurements. Nyquist plots for the different materials are shown in figure 4(a). Circuit model used to fit the experimental EIS data is shown in inset of figure 4(a). Charge transfer resistance (Rct) is a measure of the ease of electron exchange between the electrode and the electrolyte. It is directly proportional to the diameter of the semicircles that appear in Nyquist plots. Constant phase element (CPE) is used to model the capacitance which develops at the electrode/electrolyte interface due to the accumulation of ions at the electrode surface. For an inhomogeneous and rough electrode like the ones used in these experiments, CPE is used to simulate the AC response instead of an ideal capacitor. The element Rs accounts for the resistance within the bulk of the electrolyte and the electrodes, the contact resistances at the various joints, etc. As can be seen from figure 4(a) the Nyquist plots consist of two dominant semicircles. The smaller one in the high frequency region (close to the origin) is due to the CE and the larger one in the low frequency domain is due to the TiO2 film at the anode.5,16,35 At even lower frequencies a third semicircle has been reported which is due to the diffusion of ions from bulk to the electrode.35 Since the prime concern of this study is to compare the catalytic activities of the CEs we have avoided the diffusion region. Hence Warburg impedance element has also been omitted while modeling. In the circuit used to mimic the EIS set up, the first block consisting of Rct1 and CPE1 represents the CE while the second block represents the TiO2 anode. Similar circuits have been used to model DSSC systems earlier.5,35 The quantity of interest here is Rct (see table I). The values obtained for P-BP and Std Pt are 1.46

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and 1.73 Ω.cm2, respectively. The best reported values in literature are generally of the order of 1 Ω.cm2.1,3,5 Figure 4(b) shows the bode plots. Variation in phase of the impedance of DSSC is plotted against frequency of the input AC voltage. The high frequency peak corresponds to the reactions at the CE. It has been shown through simulations and experiments that the intensity of this peak drops with decrease in Rct.36 In our case both P-BP and Std Pt have peaks with much lower intensity than either SWNT or BP, with that of P-BP being the lowest. Low Rct for I-/I3redox couple in EIS translates into higher efficiency of the DSSC. Thus the results of EIS corroborate well with the performance of DSSCs. Finally there is scope for further optimization of the effectiveness of P-BP. For example, one can increase the defects in SWNTs by changing the plasma parameters or the composition of the gas mixture or dope the SWNTs by introducing gases like nitrogen and diborane during the plasma treatment.

Conclusion Substrate-less, freestanding, flexible SWNT films or BPs were used as CE in DSSC. It had an efficiency of 2.44%. The surface morphology of BP changed dramatically on exposure to microwave plasma formed using a mixture of Ar and H2 gases. The more or less smooth surface of BP is replaced by arrays of vertically aligned pillar shaped microstructures. The catalytic property improved, resulting in a higher efficiency of 4.02%, which is comparable to that of a conventional Pt thin film CE. Voc (0.73 V) and FF (0.70) of P-BP based DSSC were slightly better than that of Pt. However, the Jsc was lower at 7.83 mA.cm-2. EIS studies further confirmed the superior electrocatalytic properties of P-BP. Rct for I-/I3- redox couple obtained with P-BP, Pt and BP electrodes were 1.46, 1.73, and 2.11 Ω.cm2, respectively. When P-BP is redispersed and assembled into a film of randomly oriented P-SWNTs the efficiency falls back to the level of BP,

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confirming the importance of surface microstructure of P-BP and vertical alignment of nanotubes. The properties of P-BP appear promising and there is further scope for modification and improvement. This material can be instrumental in developing Pt free DSSC especially in cases where flexibility is important.

Acknowledgement Authors acknowledge Prof. Parag Bhargava form Dept. of Metallurgical Engineering and Materials Science, IIT-B for allowing unconstrained use of his lab for DSSC measurements. Authors also acknowledge CRNTS, IIT-B for facilitating HRTEM imaging and Raman spectroscopy done in this study.

Broader context Efforts on a worldwide scale are on to develop technologies that can meet the ever increasing demands for energy in a way that is environmentally sustainable. Photovoltaic devices are expected to play a major role. Dye sensitized solar cells (DSSCs) are new generation solar cells that stand out because of their ease of fabrication. One disadvantage of DSSC is the use of a precious and scarce metal, Pt. Carbon based materials, especially carbon nanotube (CNT) have been touted as a possible replacement for Pt. Electrocatalytic capabilities of CNTs have been well established thorough numerous experiments in the recent past. Herein, flexible substratefree single wall CNT (SWNT) films called bucky papers (BPs) were used as counter electrodes in DSSC. By a microwave plasma treatment procedure the BPs were converted into a unique material (P-BP) that had vertical SWNT microstructures on its surface. P-BP based DSSC had

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performance characteristics comparable to that of Pt and superior to BP and films of randomly oriented plasma-treated SWNTs. Charge transfer resistance of P-BP for I-/I3- redox couple measured by electrochemical impedance spectroscopy was also found to be slightly lower than Pt. We expect further optimization and modifications can lead to more improvements in the properties of P-BP. It can become a suitable substitute for Pt, especially in flexible solar cells.

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CE

η (%)

Jsc (mA.cm-2)

Voc (V)

FF

Rct (Ω.cm2)

Std Pt

4.08

8.38

0.72

0.68

1.73

P-SWNT

2.21

6.18

0.67

0.53

2.60

BP

2.44

5.85

0.71

0.59

2.11

P-BP

4.02

7.83

0.73

0.70

1.46

Table I: Performance characteristics of the DSSCs made using the different cathode materials. The corresponding charge transfer resistances are also shown here.

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(a)

(b)

Fig. 1 (a) HRETM images of as synthesized SWNT bundles. (b) High magnification image showing individual tubes within an isolated bundle.

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(a)

(b)

(c) intensity (arbitrary units)

(d)

BP P-BP

G band

2D band

D band RBM P-BP

BP 200

400

1500

2000 -1

frequency shift (cm )

2500

3000

Fig. 2 (a) SEM image of BP surface. The inset is a photograph of BP folded like a scroll, demonstrating the flexibility of the material. (b) Surface morphology of BP after plasma treatment. Vertical pillar shaped microstructures can be seen. Inset shows magnified image of an individual pillar. The scale bar is 1 µm. (c) Low magnification image showing the uniform distribution of pillars all over the BP surface. Images (a-c) were taken by keeping the samples in a tilted position inside SEM, making an angle of about 600 with the horizontal. (d) Typical Raman spectrum of BP before and after the treatment. Low intensity of D band indicates that SWNTs have good crystallinity which remains intact even after the plasma irradiation.

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-2

Current Density (mA.cm )

9

6

3

Std Pt P-BP BP P-SWNT

0

-3 0.0

0.2

0.4

0.6

Cell Voltage (V)

0.8

Fig. 3 Current-voltage (IV) characteristics of DSSCs formed with different CEs under 1 sun illumination (AM 1.5, 100 mW.cm-2).

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4.2

25

(b)

P-SWNT Std Pt BP P-BP

2

-Z" (Ω.cm )

3.6

20

-Phase (°)

3.0

(a) 2.4 1.8 1.2

15

Circuit Fit

10

P-SWNT Std Pt P-BP BP

0.6

5

0.0 4

2

Z' (Ω.cm )

8

12

10

0

1

10

2

10

10

3

4

10

10

5

Frequency (Hz)

Fig. 4 (a) Nyquist plots obtained from EIS of different DSSCs. The first semicircle is due to redox reactions at the CE while the second one corresponds to the TiO2 anode. The curves have been normalized w.r.t. area for better comparison. The smaller the diameter of the first semicircle the lower is the Rct of the CE. Circuit model used to fit the EIS spectra of DSSCs has been shown in the inset. (b) Bode plots obtained from EIS spectra. The subdued peak in the high frequency region for P-BP indicates its superior catalytic activity towards I-/I3- redox couple.

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