Hydrothermal growth of highly monodispersed TiO2 nanoparticles: Functional properties and dye-sensitized solar cell performance

Accepted Manuscript Title: Hydrothermal growth of highly monodispersed TiO2 nanoparticles: Functional properties and dye-sensitized solar cell performance Author: M. Navaneethan S. Nithiananth R. Abinaya S. Harish J. Archana L. Sudha S. Ponnusamy C. Muthamizhchelvan H. Ikeda Y. Hayakawa PII: DOI: Reference:

S0169-4332(16)32719-2 http://dx.doi.org/doi:10.1016/j.apsusc.2016.12.019 APSUSC 34566

To appear in:

APSUSC

Received date: Revised date: Accepted date:

14-10-2016 2-12-2016 3-12-2016

Please cite this article as: M.Navaneethan, S.Nithiananth, R.Abinaya, S.Harish, J.Archana, L.Sudha, S.Ponnusamy, C.Muthamizhchelvan, H.Ikeda, Y.Hayakawa, Hydrothermal growth of highly monodispersed TiO2 nanoparticles: Functional properties and dye-sensitized solar cell performance, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.12.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal growth of highly monodispersed TiO2 nanoparticles: Functional properties and dye-sensitized solar cell performance M. Navaneethan1*, S. Nithiananth2, R. Abinaya2, S. Harish1, J. Archana2*, L. Sudha3, S. Ponnusamy2, C. Muthamizhchelvan2, H. Ikeda1, Y. Hayakawa1* 1

Research Institute of Electronics, Shizuoka University

3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan. 2

Department of Physics and Nanotechnology, SRM University, Kattankulathur-603 203, Tamil Nadu, India

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Department of Physics, SRM University, Ramapuram, Chennai, 600 089, Tamil Nadu, India.

*

Corresponding authors Dr. J. Archana – [email protected] JSPS Fellow Assistant Professor, Department of Physics and Nanotechnology, SRM University. Dr. M. Navaneethan JSPS Research Fellow (JSPS Japan), E-mail: [email protected] Tel- +81-534781338, Fax- +81-534781338 Prof. Y. Hayakawa Research Institute of Electronics, Shizuoka University Hamamatsu, Shizuoka 432-8011, Japan. E-mail: [email protected] Tel- +81-534781338, Fax- +81-534781338

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Research highlights 

Monodispersed TiO2 nanoparticles prepared by hydrothermal growth.

 Effect of growth period on the formation of TiO2 nanoparticles is investigated.  Citric acid was effectively controlled the phase and size of TiO2 nanoparticles.  TiO2 nanoparticles coated DSSC exhibits the efficiency of 7.66 %.

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Abstract Monodispersed anatase TiO2 nanoparticles were synthesized by hydrothermal method using citric acid as a capping agent. The effect of citric acid and the growth time on the formation of TiO2, functional properties and dye-sensitized solar cell performances were investigated. X-ray diffraction pattern (XRD) and Raman spectroscopy results revealed that the TiO2 nanoparticles possess the anatase phase. Transmission electron microscopy (TEM) measurement revealed the formation of spherical nanoparticles with monodispersity in size and morphology. An average size of 14 nm was obtained for the growth period of 15 h. The maximum efficiency () of dye-sensitized solar cell was achieved for TiO2 nanoparticles grown for 15 h as 7.66 % which was higher than that of commercial P25 TiO2 (5.23 %) and uncapped nanoparticles (3.68 %).

Keywords Inorganic materials; Semiconductors; Chemical synthesis; Photoconductivity and photovoltaics.

1.

Introduction Dye-sensitized solar cells (DSSCs) have been considered as alternative to semiconductor

solar cells due to their good potential and cost effectiveness [1-5]. It is known that the DSSCs consist of FTO substrate, photoanode material for dye absorption, platinum counter electrode and the electrolyte (iodide/tri-iodide). Among these, the photoanode is considered to be an important factor for the light harvesting and charge transfer properties. In DSSCs, the semiconductor oxide materials with the wide band gap are used as the photoanode material [6]. Titanium-di-oxide (TiO2) has gained good attention due to their unique properties such as well-matched band 3

alignment with dyes [7]. It is regarded as a promising material preferred as a heterogeneous photo catalyst in solar cells [8,9]. TiO2 exists in three crystalline polymorphs such as rutile, anatase and brookite. Among those, rutile is the most stable phase, whereas anatase and brookite are in metastable phases [10]. However, the anatase phase has been highly employed in wide applications such as DSSCs, photo catalysts, sensors etc [11, 12]. In order to synthesis TiO2 nanoparticles several methods such as solvothermal, sol gel laser ablation, hydrothermal were adopted [13-15]. Jong Ho Park et al., synthesized the TiO2 nanoparticles by solvothermal method and investigated the fractal dimension of the material [16]. N.Okubo et al., fabricated the anatase TiO2 by pulsed laser ablation method. They suggested that the particle size increased with the increase of gas pressure irrespective with the increase of flow rate [17]. Huaming yang et al., successfully prepared the TiO2 nanoparticles with the crystal size of about 16 nm by sol gel method and performed the photo catalytic studies [18]. In comparison with the other methods, hydrothermal method is a simple and inexpensive method to prepare well crystalline materials. However, the fast hydrolysis process leads to the formation of irregular phase and morphology. In order to synthesize the nanoparticles without agglomeration, it is necessary to use the capping agent. It is reported that the carboxylic acids have strong affinity with TiO2 material [19]. The carboxylic acid with a long hydrocarbon chain is considered as an important surfactant for the synthesis of titania nanoparticles. Wang et al., synthesized the TiO2 nanoparticles using decylamine as the capping agent [20]. Weller et al., reported the oleic acid capped TiO2 nanoparticles [21]. It is very important to identify the capping ligand which offers unique size reduction and better morphology. Graham et al., [22] investigated the nanoparticle-nanotube interactions in the solution and studied the effect of pH and the ionic strength using citric acid as the capping agent.

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The ionogenic carboxylic acid groups on the surface of citrate capped gold nanoparticles and multi walled carbon nanotubes determined the surface charge of the nanostructures in the solution. Dmitri et al., synthesized the silver nanoparticles and studied the initio preferential surface coordination with the citric acid. They investigated the chemical reduction and demonstrated that the blocking of different surfaces of crystals can be used to prevent chemical activity at some surfaces. In particular, citric acid is considered as an effective capping agent capable of blocking surfaces from chemical reactivity [23]. In our previous report, rutile phase TiO2 nanorods with the length of 5–7 m was obtained using citric acid as a capping ligand under the influence of mixture of hydrochloric acid and water [24]. However, anatase phase and highly size confined TiO2 nanostructures are required to achieve the high efficiency in the DSSC’s. In the present work, we report synthesize of monodispersed anatase TiO2 nanoparticles by facile hydrothermal method using citric acid as a capping agent. The systematic investigations are carried out on the functional properties TiO2 nanoparticles. The photoanodes are fabricated using spray technique and DSSC performances are studied.

2.

Experimental procedure All the chemicals were used as received without further purification from WAKO

chemicals, Japan. A 25 mL of Titanium tri chloride was added to the 250 mL of water under vigorous magnetic stirring. 15 g of citric acid was added to the above solution as the capping agent. The stirring was continued to obtain the transparent color. Then the solution was transferred to a Teflon-lined stainless steel autoclave and hydrothermal growth was carried out at 200 °C. The growth period was varied as 5, 15, 25 and 45 h, respectively. Finally, the resultant

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powder was annealed at 350 °C for 1 h. The photoanodes were prepared by spray deposition method and it was adopted from our previous report [25]. TiO2 powders were dispersed in ethanol and ground using mortar for 15 min. The solution was ultrasonicated for 30 min and 5 drops of triton-X were added to the solution as a binder. The solution was sprayed on a transparent conducting glass (F-doped SnO2 (FTO), Nippon Sheet Glass, 8.7 Ω/square, transparency of 80 % in the visible range) at 150 °C by spray deposition method. The prepared TiO2 films were annealed at 530 °C for 2 h. The resulting photoanodes were soaked in an ethanol solution containing 0.03 M of ditetrabutylammonium cis-bis (isothiocyanato) bis (2,2”-bipyridyl-4,4’ dicarboxylato) ruthenium (II) (N719) for 15 h. The DCCS photoanode was clamped firmly with a Pt coated counter electrode (FTO) to form a sandwich type cell. A redox electrolyte solution was filled in between the electrodes to form the cell by capillary action. The electrolyte was composed of 0.6 M dimethylpropylimidazolium iodide, 0.1 M lithium iodide, 0.01 M iodide and 0.5 M tetrabutylpyridine in acetonitrile (FUNCHEM, Tomiyama electrolyte company, Japan). The crystalline phase of the samples was measured using a Rigaku (Japan) X-ray diffractometer (XRD, RINT-2200, CuKα radiation) with 0.02°/sec as the step interval. Raman characterization was performed using a helium-neon laser at room temperature. Ultravioletvisible (UV-vis) spectra were measured using a Shimadzu (Japan) 3100 PC spectrophotometer. Fourier transform infrared (FTIR) spectra were recorded by JASCO MFT 2000 using KBr pellet technique. X-ray photoelectron spectrum (XPS) was measured using a Shimadzu ESCA 3100. The surface morphologies of the samples were observed with a JEOL JSM 6320F microscope (field-emission scanning electron microscope, FESEM). The transmission electron microscope (TEM) images were recorded using a JEOL JEM 2100F microscope at an accelerating voltage of

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200 kV. The current density and voltage characteristics were measured at an air mass of AM1.5 (100 mWcm-2 of simulated sunlight) by JASCO solar stimulator equipped with Keithley picoammeter.

3.

Results and discussion Figure 1(a-1),(b-2) shows the TEM and HRTEM images of the uncapped TiO2

nanoparticles respectively. The overview TEM image (Figure 1 (a-1)) of the uncapped nanoparticles indicated the irregular morphology due to the presence of both the spherical shaped nanoparticles with the size of 20 nm and the nanorods with the size of 100 - 200 nm. Figures 1(b1), (b-2) represent the TEM and HRTEM images of the citric acid-capped TiO2 nanoparticles synthesized for 5 h, respectively. The TEM image clearly showed the formation of spherical particles without any agglomerations. It demonstrated that the citric acid effectively passivated the surface during the nucleation which avoided the agglomerations and restricted the polydispersity in morphology. The lattice fringes were clearly seen in the HRTEM image which was the evidence of the crystalline nature of the citric acid-capped TiO2 nanoparticles. Figure 1(b-3) shows the size distribution. From the histogram it was observed the particle size distributed in the range of 6 – 14 nm with the maximum at 9 nm. Figure 1(c-1), (c-2) represents the TEM and HRTEM images of the TiO2 nanoparticles synthesized for 15 h respectively. It represented the same morphology of spherical shape but the size of the particles increased when compared to the particles grown for 5 h. Figure 1(c-3) shows that the size distribution was in the range of 10 - 20 nm with the maximum distribution at 14 nm. Figure 1(d-1), (d-2) represents the TEM and HRTEM images of the TiO2 nanoparticles grown for 25 h respectively. The morphology of the particles were irregular in shape. From the HRTEM it was found that some

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particles showed elongated shape. Figure 1(d-3) shows that the size distribution was in the range of 18 - 35 nm with the maximum distribution at 27 nm. Figure 1(e-1), (e-2) represent the TEM and HRTEM images of the TiO2 nanoparticles grown for 45 h respectively. The particles had the rod-like morphology. From the HRTEM it was evidence that the particle size increased both in length and diameter. Figure 1(e-3) shows that size distribution was in the range of 23 - 52 nm with the maximum distribution at 34 nm. It clearly showed that the size of the nanoparticles gradually increased from 9 to 34 nm by increasing the growth period from 5 to 45 h. From the TEM analysis, it was clear that the prolonged growth period up to 45 h resulted the polydispersity in size and irregular morphology. The following factors are considered (1) the smaller nanoparticles tend to attach with the bigger particles when the growth period is over 25 h under hydrothermal growth conditions, (2) At higher growth period, the citric acid may not effectively passivate the surface due to the low melting point and boiling point as compared to that of the growth temperature as 200 °C. Figure 2 (a) and (b) shows the TEM and HRTEM images of commercial P25 Degussa TiO2 nanoparticles. The images show that P25 TiO2 had irregular morphologies with spherical nano particles, elongated nano cubes. Sizes of the nanoparticles were in the range of 20 – 80 nm. Moreover, the HRTEM image represents the amorphous and crystalline nature of the P25 TiO2 nanoparticles. Figure 3(a) depicts the XRD pattern of the sample of uncapped and citric acid capped TiO2 at different growth periods of 5, 15, 25 and 45 h. The phase compositions of all the samples were identified from the XRD pattern. All the diffraction peaks were indexed to (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of the crystal structure of anatase TiO2 phase and it matched with card (JCPDS: 21-1272). Whereas the uncapped-material shows rutile phase such as (110), (101), (111), (211) and (220). It was demonstrated that citric acid was acted as a

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phase directing ligand to achieve only the anatase phase. Figure 3(b) illustrates the Raman spectra of the prepared samples. Generally, the anatase phase has six fundamental vibrational modes such as [A1g + 2 B1g + 3 Eg] and the rutile phase has four fundamental vibrational modes such as [A1g + B1g + B2g + Eg]. The citric-acid capped nanoparticles grown at different growth period had the four Raman peaks at 145, 395, 519 and 642 cm-1 which can be assigned to Eg, B1g, A1g and Eg modes of the anatase phase [24]. The uncapped TiO2 had the Raman peaks at 237, 395, 450, 518 and 640 cm-1. Where the peaks 237 and 450 cm-1 can be assigned to Eg modes of the rutile phase and the remaining lines belong to the anatase phase as mentioned above [26]. Thus it confirms that the uncapped-TiO2 material had the mixture phase of anatase and rutile. It has good agreement with the XRD data. The hydrolytic stability is considered to be an important issue. Hobbel et al., [27] studied the effect of the multi ligands on the hydrolysis process of various metal complexes such as Al, Zr and Ti. In addition they had explained that the hydrolytic stability was strongly dependent on the structure of the ligand. Livage et al., [28] reported that the suppression of the hydrolysis was possible by complexing the metal ions with the ligands such as EDTA. They explain that the condensation reactions will be forced by this complexation due to the charge generated from these complexes. The hydrolysis rate will be directly affected by the molecular fragment which departs with the pair of electrons in the bond cleavage (leaving group). The leaving group will donate the electron and weaken the bond of the other ligand. Thus it separates the nucleofugal group from the metal center. In the present case, the citric acid played a determinative role as a ligand to obtain the agglomerated free TiO2 nanoparticles. The carboxylic functional group favors the conjugate system which reduces the Lewis basicity of the bonding oxygen and it limits the charge donation to the metal center [29]. It is worthy to note that the existence of mixed 9

phases of anatase and rutile were observed when there was no ligand. When the citric acid was added, only anatase phase was formed. The main reason is that the citrate ions substitute the chlorine ions of Titanium tri chloride during the hydrolysis process. Thus the citrate ion forms a strong coordination with the Ti4+ ions and highly stabilizes the molecule. By face shared linking it favors the formation of anatase TiO2 molecule. The possible formation mechanism is illustrated in the Figure.4. It is evidenced that the citric acid-capped TiO2 nanoparticles show the anatase phase with the average size of 6 - 14 nm for 5 h growth period. Then the average size of the particle increased as 10 - 20 nm and 18 – 35 nm with the spherical morphology for the higher growth period of 15 and 20 h respectively. The rod-like morphology occurs with the size of 23 – 52 nm for 45 h growth. The driving force of the crystal growth is the reduction of the surface energy [30]. The two primary nanocrystals attach together and result the rod - like morphology. In hydrothermal reaction the two possible growth mechanisms are reported as oriented attachment and repeated nucleation. In our work the favorable growth mechanism is considered as the oriented attachment of the primary crystals. The optical absorption spectra of the TiO2 nanoparticles are shown in Figure 5 (a). From the spectra it is clear that the uncapped-particles did not show any significant onset in the region of 300 – 400 nm. There was no significant absorption onset in the uncapped particles. This may be due to the presence of both the rutile and anatase phase. The capped-nanoparticles showed the clear absorption onset in the region of 350 – 380 nm. This may be attributed to the intrinsic band gap absorption of TiO2. It was found that the incident light was greatly absorbed by the citric acid capped nanoparticles and enriched the light harvesting. Figure 5 (b) shows the typical FTIR absorption spectra of the uncapped and citric acid-capped TiO2 nanoparticles at various growth periods. The uncapped-TiO2 does not show any significant vibration peaks in the region

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of 1000 – 3500 cm-1. It indicates that the uncapped-TiO2 did not have any organic molecules. The IR band at 3400 cm-1 indicated the presence of the Ti-OH stretching vibrations. The peak at 2400 cm-1 corresponded to the atmospheric CO2. The citric acid-capped TiO2 shows significant vibrational peaks in the region of 1000 - 3500 cm-1. In particular, it had several vibrational peaks in the region of 1100 - 1800 cm-1 which was considered to be the finger print region of citric acid. The peak at 1195 cm-1 was corresponded to the C-O stretching of citric acid. The peak at

1400 cm-1 was corresponded to COO- and the peak at 1720 cm-1 attributed to the C=O

stretching of the carboxyl group of citric acid as can be seen from the spectra of 5 and 15 h grown samples [31, 32]. It clearly demonstrates that the citric acid was effectively passivated the surface of the TiO2 nanoparticles. On the other hand, it is observed that the 25 and 45 h grown samples did not have the strong peaks related to carboxylic group. It confirmed that the citric acid can be liberated from the surface of the TiO2 nanoparticles at higher growth ◦

temperature of 200 C for longer growth period at hydrothermal condition. These results can be directly correlated with the increase of the size of the TiO2 nanoparticles at longer growth period as evidenced by TEM analysis. Further confirmation for the electronic levels of the samples was analysed by X-ray photoelectron spectroscopy (XPS). The binding energies obtained in the XPS analysis were corrected by reference to C1s at 284.60 eV. Figure 6 represents the XPS spectra obtained from Ti and O regions of TiO2 nanoparticles. Figure 6 (a) shows two strong peaks at 459.5 and 464.9 eV which correspond to the binding energies of Ti 2p3/2 and Ti 2p1/2. In Figure 6 (b), there was a strong peak at 530.8 eV, which was attributed to signature of the lattice oxygen O1s in the Ti-OTi bonds [33, 34]. All the samples exhibited the similar peak values in the Ti and O core level spectra. No obvious peaks for other elements of impurities were observed. The similar XPS

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spectra of Ti and O regions for commercial P25 was reported [35].

3.1

DSSC performance The DSSCs were fabricated using the nanoparticles synthesized at various growth periods.

Figure 7 shows the current density versus voltage (I-V) characteristics measured for 5, 15, 25 and 45 h. Figure 8 shows the dependency of device parameters at various growth periods. The values of Voc, Isc, FF and conversion efficiencies () of the DSSCs are listed in the Table.1. From the table it is clear that the Voc and FF show somewhat constant whereas the Isc shows an increasing behavior from 12.02 to 16.59 mA cm-2 when the average particle size increased from 9 nm (5h) to 14 nm (15 h). When the particle size increased as 27 nm (20 h) and 34 nm (45 h), the Isc started to drop from 16.59 to 13.44 and 12.16 mA cm-2. The overall conversion efficiency () shows the similar behavior of Isc thus the efficiency increased from 5.44 to 7.66 % ((5 h) to (15 h)) and it decreased as 6.45 and 5.61 % ((25 h) to (45 h)). It is clear that the DSSC performance was highly dependent on the Isc factor. When compared with the above data, the growth period 15 h was optimized to yield the maximum efficiency 7.66 % with the average particle size of 14 nm. The decrease in the efficiency as the particle size increased may be due to the minimal of surface area for the greater absorbance of dye molecules. For the comparison, I-V characteristics were measured for uncapped and P25 TiO2 nanoparticles coated devices as shown in Figure 9. The uncapped-TiO2 nanoparticles coated device exhibited the efficiency of 3.86 %, Isc of 10.38 mA cm-2, Voc of 0.59 V and FF of 0.62. The low efficiency can be attributed to the mixed crystal structure, irregular morphology and polydispersity in size. Whereas, P25 TiO2 nanoparticles coated device shows the efficiency of 5.23 % with the following device parameters such as Isc of 11.11 mA cm-2, Voc of 0.69 V and FF of 0.68. However, the obtained efficiency

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from P25 TiO2 nanoparticles coated device and uncapped-TiO2 nanoparticles coated device were less as compared to that of citric acid capped-TiO2 nanoparticles coated device. Therefore, size confinement and monodispersity in morphology significantly improves the efficiency of DSSC. A comparison was made between the DSSC performance of the materials developed in this work and those of other reported TiO2 nanostructures, as shown in Table 2. It can be seen that the TiO2 nanoparticles produced in this work exhibits the enhanced efficiency of 7.66 %, which is higher than any other reported value. 4.

Conclusions TiO2 nanoparticles have been successfully synthesized using facile hydrothermal method.

The effect of citric acid on the formation of TiO2 nanoparticles has been studied. The functional properties of the TiO2 nanoparticles were investigated by TEM, XRD, Raman spectroscopy, UV-vis spectrophotometery, FTIR spectroscopy and XPS analysis. It was found that citric acid promoted the nucleation for anatase phase formation through the coordination of carboxylic groups with the titanium complexes. The effects of various growth periods (5, 15, 25 and 45 h) have been investigated. TiO2 nanoparticles coated photoanodes were employed for DSSC fabrication. It was found that the maximum efficiency () of 7.66% was obtained for 15h growth period. Acknowledgements This work was financially supported by Grant-in-Aid for Scientific Research (B) (25289087), Grant-in-Aid for JSPS Fellows (24-12363, 25-13360) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the cooperative research projects of the Research Institute of Electronics, Shizuoka University. The authors would like to thank Center for Instrumental Analysis, Shizuoka University, Hamamatsu, Japan for the

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characterization techniques. The authors would like to thank Prof. K. Murakami of the NanoDevice Process Laboratory, for extending his support to the I-V measurements. J. Archana would like to thank JSPS, Japan, for their award of JSPS research fellowship.

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[38] D. Zhang, H. Yin, Z. Li, Y. Zhou, T. Yu, J. Liu, Z. Zou, Controllable electrophoresis deposition of TiO2 mesoporous spheres onto Ti threads as photoanodes for fiber-shaped dye-sensitized solar Cells, RSC Adv. 5 (2015) 65005-65009. [39] J.T. Park, D.K. Roh, R. Patel, E. Kim, D.Y. Ryu, J.H. Kim, Preparation of TiO2 spheres with hierarchical pores via grafting polymerization and sol–gel process for dye-sensitized solar cells, J. Mater. Chem. 20 (2010) 8521–8530. [40] Y.H. Jung, K.H. Park, J. Seok, D.H. Kim, C.K. Hong, Effect of TiO2 rutile nanorods on the photoelectrodes of dye-sensitized solar cells, Nanoscale Res. Lett. 8 (2013) 37-42.

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Figure Captions Figure 1. (a-1) TEM and (a-2) HRTEM images of uncapped -TiO2 nanostructures. Figure 1. (b-1) TEM, (b-2) HRTEM images, and (b-3) histogram of size distribution of TiO2 nanoparticles for growth of 5 h period. Figure 1. (c-1) TEM, (c-2) HRTEM images, and (c-3) histogram of size distribution of TiO2 nanoparticles for growth of 15 h period. Figure 1. (d-1) TEM, (d-2) HRTEM images, and (d-3) histogram of size distribution of TiO2 nanoparticles for growth of 25 h period. Figure 1. (e-1) TEM, (e-2) HRTEM images, and (e-3) histogram of size distribution of TiO2 nanoparticles for growth of 45 h period. Figure 2. (a) TEM and (b) HRTEM images of commercial P25 Degussa TiO2 nanoparticles. Figure 3. (a) XRD patterns and (b) Raman spectra of TiO2 nanoparticles with growth periods of 5, 15, 25 and 45 h. .Figure 4. Formation mechanism for the citric acid capped TiO2 nanoparticles Figure 5. (a) Optical absorption spectra and (b) FTIR spectra of f TiO2 nanoparticles with growth periods of 5, 15, 25 and 45 h. Figure 6. XPS spectra of TiO2 nanoparticles with growth periods of 5, 15, 25 and 45 h. (a) Ti 2p3/2, (b) O1s. Figure 7. (a) I-V characteristics curves of citric acid capped TiO2 nanoparticles at growth periods of 5, 15, 25 and 45 h. Figure 8. Relationship between DSSC device parameters at various growth periods. Figure 9. I-V characteristics curves of uncapped and P25 Degussa TiO2 nanoparticles coated DSSC devices.

19

Figure 1. (a - 1) and (a - 2)

Figure 1. (b - 1), (b - 2) and (b - 3)

20

Figure 1. (c - 1), (c - 2) and (c - 3)

Figure 1. (d - 1), (d - 2) and (d - 3) 21

Figure 1. (e - 1) and (e - 2)

Figure 2 (a) and (b). 22

Figure 3 (a) and (b)

23

Figure. 4

24

Figure 5 (a) and (b)

25

Figure 6 (a) and (b)

26

Figure 7.

Figure 8. 27

Figure 9.

Table.1 Device parameters of DSSC Growth period

5h

15 h

25 h

45 h

FF

0.64

0.67

0.66

0.65

Jsc (mA/cm2)

12.02

16.59

13.44

12.16

Voc (V)

0.64

0.69

0.72

0.69

Eff (%)

5.54

7.66

6.45

5.61

28

Table 2 Comparison table of DSSC performances. Materials

Morphology

Capping agent

Ƞ (%)

Reference

Mesoporous

Mesoporous

Dodecylamine

6.01

[36]

TiO2

spheres

Mesoporous

Nanoparticles

EDTA-Na2

5.22

[37]

spheres

Hexadecylamine

3.80

[38]

Nanoparticles

Polyethylene

2.5

[39]

TiO2 Mesoporous TiO2 Mesoporous TiO2

glycol

TiO2

Nanorods

PVP

6.16

[40]

TiO2

Nanoparticles

Citric acid

7.66

This work

29