Facile synthesis of nanoporous TiC–SiC–C composites as a novel counter-electrode for dye sensitized solar cells

Microporous and Mesoporous Materials 190 (2014) 309–315

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Facile synthesis of nanoporous TiC–SiC–C composites as a novel counter-electrode for dye sensitized solar cells Jie Zhong a,b,c, Yong Peng b, Manyuan Zhou b, Juan Zhao b, Shuquan Liang c,⇑, Huanting Wang d, Yi-Bing Cheng a,b,⇑ a

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology, Wuhan 430074, PR China Department of Materials Engineering, Monash University, VIC 3800, Australia School of Materials Science and Engineering, Central South University, Changsha 410083, PR China d Department of Chemical Engineering, Monash University, VIC 3800, Australia b c

a r t i c l e

i n f o

Article history: Received 11 December 2013 Received in revised form 6 February 2014 Accepted 15 February 2014 Available online 24 February 2014 Keywords: TiC composites Carbon Porous Counter electrode Solar cell

a b s t r a c t Highly mesoporous titanium carbide (TiC) based composites were synthesized through a simple carbothermal reduction of homogenous monoliths which were prepared by a one-pot sol–gel process. Their application properties as a counter electrode of dye-sensitized solar cells (DSSC) were evaluated in this paper. The surface area (116–368 m2/g), average particles size (14–87 nm) and pore size (4–9.2 nm) of the mesoporous composites can be easily tuned by altering chemical compositions of the precursor. Introduction of silicon into the Ti–C–O system significantly reduced the average carbide grain size and also enhanced the mesoporousity of TiC composites. The dye-sensitized solar cell using a PEDOT:PSS doped TiC–SiC–C composite as the counter electrode showed a notable efficiency of 5.7%, which doubled that of pure PEDOT:PSS cell. Potentially, the reported method could be a convenient route to fabricate porous TiC based composites for other functional applications. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Titanium carbide (TiC), a representative transition metal carbide, has high melting point, high mechanical stiffness and hardness, high thermal shock resistance, which has been widely used for cutting tools, polishing pastes, abrasives etc [1]. For functional applications, TiC has received considerable less attention compared to tungsten and molybdenum carbides. Recent work suggests that TiC is a very promising carbide material because of its high thermal and electrical conductivity, high chemical stability and catalysis activity. It has shown potentials in different applications, such as field emission [2], methanol fuel cell [3], Li-ion battery, hydrogen storage etc. [4]. There is still much work remaining to explore the properties of TiC materials and expand their application areas. In order to improve the functional properties, fabricating composite materials with certain nanostructures, such as nano grains, nano porous, membrane, hollow particles or spherical shape is usually required [5–7]. Although the recently reported physical ⇑ Corresponding authors. Address: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology, Wuhan 430074, PR China (Y.-B. Cheng); School of Materials Science and Engineering, Central South University, Changsha 410083, PR China (S. Liang). E-mail addresses: [email protected] (S. Liang), [email protected] (Y.-B. Cheng). http://dx.doi.org/10.1016/j.micromeso.2014.02.029 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

vapor deposition, reactive ballistic deposition and metal-sacrificial co-reduction processes could effectively produce nanostructured TiC materials [8–10], carbothermal reduction is still the most convenient and preferred commercial route due to its low cost [11–13]. The conventional powder mixing technique cannot achieve intimate contacts between titania and carbon particles, resulting in high carbothermal reduction temperature and making it very difficult to produce nano structured carbide materials. In recent years, the sol–gel process has been employed as a suitable approach to synthesis pre-ceramic precursors for producing nano carbide and carbide composites [14]. Moreover, through adjusting composition and microstructure of the precursors through the wet-chemistry route, carbide composites with controlled nanostructures can be obtained after carbonization [13,15]. Dye-sensitized solar cell (DSSC) is the next generation of thin film solar cell device. Since the invention of DSSC, platinum has been employed as a counter electrode (CE) due to its excellent catalytic activity and high electrical conductivity. As a precious metal, Pt is an expensive component of DSSC, which would become a major obstacle for further reduction of the cost of DSSC. Therefore, researchers made various attempts to reduce the amount of Pt employed in the CE by forming Pt composites or alloys [16,17], or just to replace Pt by other low cost materials, such as carbon [18–20], tungsten oxide/carbide ceramics [21,22] and conductive

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polymers [23,24] to form Pt-free counter electrodes. Recently, Ptfree TiC based CEs have attracted attentions and many works have been reported by Tingli Ma’s research group [25–29] and other researchers [30,31]. In this study, we report a facile process to synthesize highly mesoporous TiC based composites and their application as CEs for DSSCs. A controllable sol–gel process was carried out to synthesize pre-ceramic precursors. Nanoporous TiC composites with controllable nanostructures were obtained after in-situ carbothermal reduction. Furfuryl alcohol was chosen as the polymerizable carbon source to build an interpenetrated carbon network within the oxides [13]. Through adjusting the amount of carbon and/or doping of silicon species, the surface area, pore structure and nano grain size of the composite can be controlled. Furthermore, the performance of the as-made mesoporous TiC composites as the DSSC counter electrode was evaluated. In order to enhance the coating of the TiC film on the fluorine-doped tin oxide (FTO) glass, a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)) solution, which is a water soluble conductive polymer, is blended with the TiC composites to form a slurry. This work demonstrated that the mesoporous TiC based composite materials are very promising as a solar cell component and the convenient synthesis route presented here would benefit the further studies aiming at commercializing. 2. Experimental 2.1. Sol–gel synthesis of mesoporous TiC composite precursors The sol–gel route employed to synthesize the mesoporous carbide composite gel precursor is described as following. Initially, 9.28 g of poly(ethyleneglycol)-block-poly(propyleneglycol)-block poly(ethyleneglycol) (PEO20PPO70PEO20, Pluronic P-123, Aldrich) was dissolved in 14 g of ethanol absolute to form solution under continuous stirring. Then a required amount of titanium tetraisopropoxide (TTIP, Ti(OCH(CH3)2)4, Alfa Aesar P 97%) was added into the P123 ethanol solution. At the same time, a certain amount of tetraethylorthosilicate (TEOS, Si(OC2H5)4, Aldrich P 98%) was mixed with hydrochloric acid (10 M) and ethanol (4.6 g) at vigorous magnetic stirring. After 15 min of mixing of TTIP and P123, these two solutions were blended under stirring for another 30 min to finish the hydrolysis process. Subsequently, a designed amount of furfuryl alcohol (FA, C5H6O2, Aldrich) was added into the oxide and acid mixture. After continuous stirring for 6 h, the mixture was poured into petri-dishes and put in a vacuum oven at 40 °C to evaporate solvents to obtain oxide–carbon gel monoliths. All chemicals were mixed in polypropylene bottles under magnetic stirring at room temperature. The samples were named as TSC-1 and TSC-1.6 for the silicon contained samples. The values of 1 and 1.6 indicate the molar ratio of FA to total metal precursor. The molar ratio of silicon to titanium species is fixed at 1:3. The silicon free samples were named as TC-1 and TC-1.6. The detailed chemical compositions and names of samples are shown in Table 1. 2.2. Formation of DSSC devices To fabricate the counter electrode, the as-made mesoporous TiC composites were blended with the 1.29 wt% PEDOT:PSS water Table 1 Detailed chemical composition (weight in gram) of samples for sol–gel synthesis. Sample

TTIP

TEOS

FA

P123

HCl

TSC-1 TSC-1.6 TC-1 TC-1.6

17.05 17.05 22.736 22.736

4.168 4.168 0 0

7.85 12.65 7.85 12.65

9.28 9.28 9.28 9.28

4 4 4 4

solution using ball-milling at 250 rpm for 4 h. The weight percentage of the carbide composites in the slurry was 30%. The doctor blade method was employed for the coating process and the thickness of the counter electrode was 1 lm. The working electrode consists of 6 lm screen printed P25 TiO2 and 6 lm 400 nm TiO2 scattering layer. N719 Dye was used as sensitizer. The I–V characters of DSSCs made on FTO glass were measured under illumination of 100 mW/cm2. The detailed solar cell device formation was following one of our reported work [32]. 2.3. Characterization and measurements X-ray diffraction (XRD) patterns were recorded on a Philips PW 1140/90 diffractometer with Cu Ka radiation at a scan rate of 2°/ min and a step size of 0.02°. Thermogravimetric analyses (TGA) (Perkin-Elmer, Pyris 1 thermogravimetric analyzer) were conducted at a heating rate of 10 °C/min to 1400 °C in air. Nitrogen adsorption–desorption isotherms at 77 K were determined on a Micromeritics Tristar 3020 sorptometer. The surface area measurements were carried out according to the Brunauer–Emmett–Teller (BET) method. The pore size distribution was calculated with the DFT Plus software (Micromeritics), applying the Barrett–Joyner– Halenda (BJH) model with a cylindrical geometry of the pores. The samples were degassed at 200 °C for 2 h before the measurements. A JEOL 7001F microscope was used to take scanning electron microscopy (SEM) images. Transmission electron microscopy (TEM) images were taken with a Philips Tecnai 20 microscope operated at 200 kV. 3. Results and discussion 3.1. Formation of nanoporous TiC composites and their thermal stability As shown in Scheme 1, the hydrophilic Ti, Si species and polymerized FA molecules were bonded with the amphiphilic P123 during one-pot synthesis of TiC-based nanoporous precursor [33]. Further polymerization producing entangled PFA/P123 chains, along with Ti, Si oxides or and/or hydroxides, which will be further transformed into carbon-supported carbide nano-porous composites [13]. In detailed heat treatment process, calcinations of as-made monolithic gels were first carried out at 550 °C for 5 h under flow of nitrogen to remove the organic components. Fig. 1a shows the typical SEM morphology of calcined oxide/carbon composite and the TEM image in Fig. 1b demonstrates the very fine particles with disordered channel structure [34]. Then the samples were heated at 1450 °C for 1 h under flowing argon. The carbonized samples were broken into micrometer sizes, showing in Fig. 1c. The nano grained carbide composite was formed after carbonization (Fig. 1d). The selected diffraction rings inset of the TEM image suggests the formation of titanium carbide and/or silicon carbide as both of them have the same crystal structure and very similar cell dimension. X-ray diffraction was used to detect the phases of the as-made gel, calcined samples and carbide composites. Four samples have a similar phase-evolution process with increasing of heat treatment temperature. The representative patterns of TSC-1.6 for the as-made gel and oxide/carbon composite are shown in Fig. 2. The gel was amorphous with broad peaks centered at around 22°. Since TTIP has a very high hydrolysis rate, it would form nano titania crystals almost immediately when it meets with water [35]. The forming of amorphous gel suggests that this sol–gel route can effectively control the hydrolysis process of TTIP. This low hydrolysis rate enhances homogeneity of titanium oxide and the carbon source, namely, the poly(furfuryl alcohol) (PFA) network. The

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311

Scheme 1. Illustration of sol–gel synthesized mesoporous TiC based nano composites.

Fig. 1. SEM and TEM morphologies of TSC-1.6 after fired at 550 °C for 5 h (a and b) and 1450 °C for 1 h (c and d).

anatase phase was detected after the calcination at 550 °C. The PFA network decomposed into carbon in which the ultra-fine titania grains was confined [33]. After carbothermal reduction at 1450 °C, no diffraction peaks of titania was observed in the samples, indicating the titania was fully converted into titanium carbide through carbothermal reduction (TiO2 + 3C ? TiC + 2CO "). For TSC-1 and TSC-1.6, the well resolved peaks of the high temperature pyrolyzed samples can be assigned to both the cubic titanium carbide (JCPDS PDF#65-0971) and b silicon carbide (JCPDS PDF#29-1129) as the XRD peaks for both phases are almost fully over lapped. Because the silicon content is much lower than titanium, the splitting of peaks at high index planes such as 311 (72.3°) and 222 (76.1°), is not as clear as reported materials [13]. The sharper peaks observed for TC-1 and TC-1.6 samples indicating the form of larger carbide grains compared to TSC-1 and TSC-1.6, the silicon containing samples. The thermal stability for mesoporous TiC/C and TiC/SiC/C composites in air were tested using thermogravimetric measurements

(Fig. 3). Under flowing air, the adsorbed water was first removed which is evident by the first weight loses step (finished around 100 °C) for TSC-1 and TSC-1.6. This is not found for TC-1 and TC1.6, which indicates the TiC/SiC/C samples have higher absorbability to moisture, compared to TiC/C composites. The significant weight-increasing phenomenon, which is the typical oxidation of TiC nanocrystals in air, can be observed for all carbide/carbon samples between 200 and 500 °C. This oxidation started at a relatively lower temperature for SiC contained samples, TSC-1 and TSC-1.6 (290 °C), compared to TC-1 and TC-1.6 (380 °C). This may be due to the incorporation of silicon components decreased the size of TiC grains in the composites, which promoted the contact area of oxygen and TiC crystals. The high weight-gain of TSC-1 and TC-1 suggests the high TiC content in the composites. The abrupt weight loss step finishing at around 650 °C is caused by carbon oxidation. The further weight-grain steps observed for TSC-1 and TSC-1.6 between 1000 and 1200 °C, are typical SiC oxidation [36], confirming the generation of SiC in the carbothermal

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TiC/β−SiC

Table 2 Detailed N2-sorption results and carbon content of mesoporous TiC composites.

Anatase

Samples

SBET/ m2 g1

SMIC/ m2 g1

DA/ nm

VP/ cm3 g1

Average particle size (nm)

Carbon content (wt%)

TSC-1 TSC-1.6 TC-1 TC-1.6

177 368 116 324

21 59 9 47

9.2 4 8.4 4.6

0.40 0.35 0.25 0.39

26 14 87 36

25 41 8 34

Intensity(Counts)

TC-1.6

TC-1

SBET BET surface area, SMIC Micropore area, DA Average pore size, VP Volume of pores.

TSC-1.6 TSC-1 TSC-1.6 oxide-carbon TSC-1.6 gel 20

30

40

50

2-Theta(

60

70

80

O

Fig. 2. X-ray diffraction patterns of carbide composites and their precursor.

reduction procedure. The oxidation of SiC is slighty delayed with decrease of carbon contents from 1000 °C (TSC-1.6) to 1050 °C (TSC-1). This result suggests the composites have proper stability below 300 °C in atmospheric environment. The calculated residue carbon in the samples is shown in Table 2. TC-1 has the lowest carbon content, 8 wt%. 3.2. Microstructures and porosities of carbide composites The mesoporous structures of the composites were verified by nitrogen-sorption isotherm measurement. Fig. 4 shows the N2-sorpthion isotherms for TSC-1, TSC-1.6, TC-1 and TC-1.6 and their pore-size distribution (PSD) curves were inserted,

respectively. All four samples demonstrated type IV curves, as defined by IUPAC, characteristic of mesoporous materials. TSC-1 and TC-1 show type H2 like hysteresis loops, suggesting pores with narrow and wide sections and possible interconnecting channels. The PSD curves of TSC-1 and TC-1 display multi-peaked pore distribution at 2.2, 3.3 and 7.4 nm for TSC-1 and 2, 3.2 and 9.2 nm for TC-1. The higher carbon content samples, TSC-1.6 and TC-1.6, show type H3 hysteresis loops with sloping adsorption and desorption branches covering a large range of relative pressure (P/P0 = 0.42– 0.95). TSC-1.6 and TC-1.6 have only one sharp pore peak at around 3.7 nm. Since the carbothermal reduction at high temperature will inevitably destroy the disordered channel structure in the oxides/ carbon composites, the spaces between the carbide grains and pores in residue carbon become the origins of high surface area of TiC composites. The narrow connection channels between carbide grain-gaps form ‘‘ink-bottle’’ type pore structure. The smaller pores shown in the PSD curves (TC-1, TSC-1) are possibly corresponded to small necks while the large pores are the cavities [37]. No distinct large pore observed for TSC-1.6 and TC-1.6 is probably due to the high residue carbon still occupying the spaces between carbide grains. The BET surface areas, micropore areas, desorption average pore diameters for TSC-1, TSC-1.6, TC-1 and TC-1.6 samples are summarized in Table 2. TSC-1 and TC-1 have relatively low BET surface areas and the value almost tripled in TSC-1.6 and TC-1.6 with increasing of carbon. The average pore size of TSC-1 and TC-1 (9 nm) are much larger than TSC-1.6 and TC-1.6 (4 nm). This may be due to the high microporousity within the PFA-derived residue carbon for TSC-1.6 and TC-1.6 [38]. Introduction of silicon

130 125 120

TC-1

115

Weight %

110 105

TSC-1

100 95 90 85

TC-1.6

80 75

TSC-1.6

70 200

400

600

800

1000

1200

O

Temperature/ C Fig. 3. Thermo gravimetric curves for mesoporous TiC/C and TiC/SiC/C samples.

1400

313

20

250

3

200 150

5 0

100

100 1

10

Pore Diameter (nm)

50

3

TSC-1 0

0

150

100

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

200 150 100 50 0 1

0.6

10

Pore Diameter (nm)

0.8

1.0

25

250

20 15

200

10

TC-1.6

150

5 0 1

50

100

10

3

3

Pore Volume (cm /g)

0.0

Pore Volume (cm /g)

Quantity Adosorbed (cm /g)

TSC-1.6

10

3

200

15

Pore Volume (cm /g)

300

Pore Volume (cm /g)

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Pore Diameter (nm)

50

TC-1 0

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

150 100 50 0 1

0.6

10

Pore Diameter (nm)

0.8

1.0

Relative Pressure (P/Po) Fig. 4. Nitrogen sorption isotherms and pore sized distributions for TiC based mesorporous nano composites.

Fig. 5. SEM images of the mesoporous TiC nano composites, (a) TSC-1, (b) TSC-1.6, (c) TC-1 and (d) TC-1.6.

species into the titanium carbide compositions also enhances the mesoporosity of the samples, although its effect on the pore structure is not as significant as carbon. For example, with the addition of silicon, the BET surface area was increased from 324 m2/g for TC1.6 to 368 m2/g for TSC-1.6, while the surface areas for TC-1 and TSC-1 were only 116 and 177 m2/g, respectively. Although the surface area is low, TSC-1 has the highest cumulative pore volume (0.40 m3/g), which may be attributed to its large average pore size (9.2 nm).

It should be noted that silicon containing samples have very fine grains and uniform mesoporous structures (Fig. 5a and b), while the grain size in TC-1.6 is non-uniform, ranging from tens to a hundred nanomerters (Fig. 5d). The uniform interconnected pore structure cannot be observed in TC-1 and TC-1.6 (Fig. 5c and d). The distinct morphology change with the silicon addition can be highlighted between TSC-1 (Fig. 5a) and TC-1 (Fig. 5c). The grain size of the carbides decreased from over 80 to <30 nm when silicon was included. This suggests that the binary carbide

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2

Current density (mA/cm )

12

10

8

6

TSC-1 TSC-1.6 TC-1 TC-1.6 PEDOT:PSS Pt

4

2

0 0

100

200

300

400

500

600

700

800

900

Voltage (mV) Fig. 6. Photocurrent-voltage plots for the DSSC devices using mesoporous TiC composites and PEDOT:PSS mixture as the counter electrode.

system derived from the sol–gel synthesis route can effectively refine the crystal grains and promote the homogeneity of nanocomposites. The hydrolysis of silicon alkoxide usually forms a continuous 3D framework other than nano particles occurred in the titanium alkoxide systems. The increase of carbon concentration would also decrease the grain size because the residue carbon restricts the growth of carbide grains. 3.3. Applications of TiC composites as DSSC counter electrodes Since PEDOT:PPS showed great catalysis ability for reduction of  the I 3 to I in redox electrolytes of DSSC, many researches worked on PEDOT:PPS or its composite counter electrodes to improve the efficiency of the solar cells [23,39–43]. We incorporated a commercial PEDOT:PSS water solution with the as-made TiC composites to form a mixture slurry and investigated their applications as counter electrodes for DSSCs. Fig. 6 shows the photocurrent density-voltage curves of the DSSC cells that were made using different carbide materials. The detailed photovoltaic performances of those cells are shown in Table 3. The DSSC device using PEDOT:PSS along as a counter electrode shows a relatively low efficiency of 2.35%. When the as-made mesoporous TiC composites were applied as the CE, the efficiency was improved except for sample TSC-1.6. The highest efficiency of 5.7% was obtained for TSC-1, and its fill factor (FF) reaches 65%, which marks a significant improvement comparing to the device using solely PEDOT:PSS as a counter electrode. This efficiency reached 84% of that when using Pt as a counter electrode. TC-1 has the highest short-circuit photocurrent density (Jsc), 11.46 mA/cm2. The highest open-circuit voltage (Voc) 794 mV is obtained for TSC-1. However, the FF is still Table 3 Performance of DSSC with different counter electrode. Counter electrodes

Jsc mA/cm2)

Voc mV

Fill Factor %

g%

TSC-1/P TSC-1.6/P TC-1/P TC-1.6/P Pt PEDOT:PSS

11.13 9.94 11.46 7.81 10.48 6.38

782 765 769 794 828 636

65.4 30.1 44.8 44.7 78.3 57.9

5.7 2.29 3.95 2.8 6.8 2.35

low for both samples, around 45%. The efficiencies for TSC-1.6, TC1 and TC-1.6 were only 2.29%, 3.95% and 2.8%. Higher efficiency for these devices were observed, namely 5.07%, 5.57% and 3.87%, respectively, when the light intensity was decreased to 100 W/m2. The results indicate that the high carbon containing TiC composites are not suitable for the application as a counter electrode in DSSCs. Decreasing of carbon content of the carbide composites (from TSC-1.6 to TSC-1 and from TC-1.6 to TC-1) improves the efficiency of the solar cell, suggesting that nano carbide crystals act more efficient as catalysts compared to the porous carbon. The silicon components significantly decreased the average grain size (87 nm of TC-1 and 26 nm of TSC-1) and enhanced meosporosity (116 m2/g of TC-1 and 177 m2/g of TSC-1), which increased the contact area of carbide nanocrystals with PEDOT:PSS and electrolyte. This may be the reason that TSC-1 shows the best catalysis results in DSSC. The experiments here have provided a new counter electrode candidate and the possible guideline to enhance the efficiency of DSSCs using TiC composites by further modification of its compositon and nanostructure. 4. Conclusion In summary, we demonstrated a convenient route to synthesize highly mesoporous TiC based nanocomposites and their applications as counter electrodes in DSSCs. The BET surface area (116–368 m2/g), average particles size (16–87 nm) and pore size (4–9.2 nm) can be tuned by altering the chemical composition. Including silicon into the titanium carbide composite can significantly reduce the average particles size and pore size and increase porosity of the composites. Increasing carbon content shows a similar effect as silicon on the nanostructure, but it demonstrates a negative effect on the efficiency of the DSSC as a counter electrode. The highest efficiency, 5.7% is achieved in the TiC/SiC composition with a low carbon content. The material has a surface area of 177 m2/g and average particle size of 26 nm. This shows a significant improvement compared to the pure PEDOT:PSS counter electrode and reaches 84% performance of the cell containing Pt as a counter electrode. The proper thermal stability of the TiC composite also provides potential applications as catalyst or electrodes in the environment up to 300 °C.

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