Enhanced Photovoltaic Performance by Surface Modification of TiO2 Nanorods with Aminopropyltrimethoxysilane (APTMS)

Modification of TiO2 nanorods through the addition of aminopropyltrimethoxysilane (APTMS) for enhancement of efficiency of solar cells has been studied. Synthesis of TiO2 nanorods was conducted through two major stages of mechanochemical and hydrothermal refluxing at a temperature of 120 °C for 24 hours on various concentration of NaOH. Material characterizations were performed by X-Ray Diffraction (XRD), Fourrier Transform Infrared (FTIR) and Transmission Electron Microscope (TEM). Mechanochemical treatment by ball milling showed that the TiO2 phase changed from anatase into brookite and it decreased of TiO2 crystals size. Morphology transformation of TiO2 to form TiO2 nanorods was showed by rod-shaped from TEM micrographs which are characteristic of the nanorods. FTIR spectra confirmed that amine group of aminopropyltrimethoxysilane (APTMS) were successfully grafted onto the TiO2 nanorods surface. Sensitization of TiO2 used Ruthenium complexes N3 (N3=cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato ruthenium (II)) were able to increase the uptake of TiO2 material to the visible region due to the absorption of visible light by N3 complex-APTMS. Sensitized TiO2 nanorods were prepared for Dye Sensitized Solar Cell (DSSCs) photoanode. The maximum results of the DSSCs (Dye Sensitized Solar Cell) performance was showed that TiO2 material modified by 10 % (v/v) APTMS capable increase efficiency of DSSCs.


Introduction
TiO2 material has been widely used than the other inorganic oxide semiconductor because of its stability owned so it can be used up to several cycles. When TiO2 particle size is reduced to the nanoscale, photovoltaic activity increases as a result of the expansion of light band-gap for quantum size and due to the enhancement of the effective surface area [1]. However, the consequences of the size of nanoscale electrodes is slow electrons movement because of its through a connection between nano particles randomly distributed [2].
In recent years, several studies reported that promising potensial of change TiO2 three-dimensional (3-D) into one-dimensional (1-D) such as nanorods, nanotubes and nanowires have received considirable attention due to their unique properties and novel applications [3,4]. One of the most important and unique properties of TiO2 1-D is that the transfer of electron and holes in semiconducting materials is mainly governed by the quantum confinement phenomenon and transport characteristics related to photons and photons are largely affected by the size and geometrical building of the materials [2,5]. Besides that, modification of nanostructure TiO2 into 1-D is able to provide increase surface area of TiO2 [1]. However, the major problem of application TiO2 nanorods in photovoltaics is dye adsorption onto TiO2 nanorods. Dye adsorption onto TiO2 is highly challenging due to the lack of chemical bonding between semiconductor TiO2 and dye [6]. On the other hand, grafting active functional groups onto particle surface can increase the possibility of chemical bonds formation between TiO2 and dye. Chen and Yakovlev (2010) [7] investigated the adsorption and interaction of organosilanes on TiO2 nanoparticles and confirmed the bonding of functional groups on particle surface was realized through Si-O-Ti bonds.
In this study, the types of silane coupling agent aminopropyltrimethoxysilane (APTMS) was applied as a particle surface modifier for grafting functional groups onto surface of TiO2 nanorods. Modification of semiconductor TiO2 is efforts to increase the effectiveness of photovoltaic semiconductor with the aim of preventing the recombination of electrons (hereinafter abbreviated as e -) and holes (hereinafter abbreviated h + ) to increase the conversion Incident Photon to Current Efficiency (% IPCE). In order to understand the effect of surface modifications on the TiO2 nanorods photovoltaics activity was investigated.

Materials
TiO2 powder was provided from commercial TiO2. Aminopropyltrimethoxysilane (APTMS) was purchased from ABCR, Germany. Sodium hydroxide (NaOH) and ethanol. was provide by Merck. The material was used as received without any pretreatment.

Synthesis of TiO2 nanorods
The ratio of ball :TiO2 = 20:1 was inserted into a planetary ball milling at the rotation speed of 1000 rpm for 4 hours. 3 g of TiO2 from the milling process was then added to aqueous NaOH solutions at various consentration of 8, 10 and 12 M. The mixture was then refluxed for 24 hours at a temperature of 120 °C. The mixture was neutralized with 0.1 M HCl until reach pH 7. The mixture was filtered and dried at a temperature of 60 °C for 12 hours and calcined at 400 °C for 2 hours.
2.3. Surface modification of TiO2 nanorods 0.5 g of TiO2 nanopowder, was dispersed in 2 ml ethanol to obtain a pasta. Then, the pasta was coated on FTO (Flourine doped Tin Oxide) glass with slip casting method. Then, the silane coupling agents of APTMS with different concentrations (5, 10 and 15%) were added in the thin layer of TiO2 and allowed for 1 minute. After that, the the thin layer of TiO2 was washed with ethanol and water for at least 2 cycles to remove excessive silanes. The modified particles as thin film FTO/TiO2-APTMS were dried in an oven at 40 oC for 30 minutes

Photovoltaics characteristic performance
Thin layer of FTO/TiO2-APTMS was soaked in a dye solution of N3 Ruthenium complexes (3 x 10-4 M) for 24 hours. Whereas, a counter electrode was prepared by addition of carbon powder onto FTO glass. Both electrodes were sealed and given a distance to place electrolyte solution. After the electrolyte was inserted, the photoanoda cell was tested by a IV Keithley 2602A measurment by cell illumination cell by a halogen lamp (1000 W/m2).

Synthesis of TiO2 nanorods
A material TiO2 nanorod has been synthesized from TiO2 nanoparticle that has been treated ball milling with the rotation speed of 1000 rpm for 4 hours. Figure 1 shows the results of XRD pattern of TiO2 before and after ball milled treatment.Transformation of the X-ray diffraction pattern (Fig. 1) shows the revolution in anatase phase TiO2 into brookite phase TiO2. This mechanochemical technique of TiO2 preparation has also been carried out by Dutta et al (2002) [8] and Rezaee et al (2011) [9]. Dutta et al (2002) have reported the phase transformation of TiO2 crystals that can occur through mechanochemical ball milling treatment [8]. Those processes were influenced by the rotational speed and agitation time. We used ball milled TiO2 to prepare TiO2 NRs. Synthesis of TiO2 NRs has been done by hydrothermal refluxing method at a temperature of 120 o C using a strong base of NaOH on various concentration of 8, 10, and 12 M for 24 hours. The hydrothermal process can be divided into four stages, e.g 1) the synthesis of nano-TiO2 in an alkaline solution, 2) alkaline hydrolysis through bond disconnection Ti-O-Ti, 3) re-polymerisation by condensation process 4) crystallization growth through annealing process. If TiO2 is reacted with a strong base it will hydrolyse, this is splitting the polymer chains of TiO2 (depolymerization) by the presence of OHgroups. Depolymerization occurs because the strength of OHions that can break the Ti-O-Ti in the polymer. A result of subsequent alkaline hydrolysis was neutralized by the addition of HCl. At pH 7, it will obtain the most optimal condensation process. Condensation processes are H2O releasing at the two ends of the polymer chain due to polymerization process. The condensation material result was heated at a temperature of 400 o C to obtain TiO2 nanorods. TiO2 after calcination at 400 o C were characterized using XRD shown in Figure 2. NaOH that used to synthesize TiO2 nanorods influence the TiO2 structure. XRD pattern of TiO2 nanorods ( Figure 2) shows that anatase TiO2 phase is as most abundance. Rutile phase TiO2 was increasing when the NaOH was added about 12 M. These results are related to the synthesis results reported by Tsai and Teng (2006) that the nano TiO2 powder can be synthesized from a mixture of rutile and anatase phase at various concentration of NaOH [10].
Physicochemical treatment of TiO2 influence both phase and size of TiO2 crystal. Calculation using Scherrer equation resulted that the ball milled TiO2 composed by anatase TiO2 54.07 nm in crystal size, and brokite TiO2 41.19 nm in crystal size. While after hydrothermal refluxing process, the crystal size of TiO2 decrease into 7.65 nm. Gribb and Banfield (1997) have described that tendencious anatase phase transform to more stable phase of rutile if the crystal size extent to about 30 nm [11]. But the brokite phase was rare occur because of lower stability properties. Brokite phase in most case will develop at annealed in a range temperature of 200 -500 o C [12]. TiO2 annealed at a higher than 500 o C shows the lower anatase as well as brokite content, finally no found both of them at annealed result at more higher than 800 o C [13].  preparation results using NaOH 10 M , showed flaking 3D structure, then has formed TiO2 2D structure ( Figure 3b ). We further discoverred 1D structure of TiO2 in preparation results using NaOH 12 M. Structural transformation of the ball milled TiO2 to 1D TiO2 nanorods was characterized from pore size distribution data. 1D TiO2 nanorods have an average surface area 79 m 2 /g, but ball milled TiO2 only have it about 7.56 m 2 /g. It was concluded that structural transformation to 1D nanorods capable increase surface area. Increasing of surface area will be increase dye adsorption onto TiO2 surface. It will gain good performa for dye sensitized TiO2 photoanode.

Modified of TiO2 nanorods
The modification process of APTMS on the TiO2 nanorods surface is illustrated in Figure 4(2). Different concentrations of silane coupling agents to TiO2 have a significant influence on FTIR spectra. As can be seen in Figure 4, the transmission FTIR spectra of TiO2, TiO2-APTMS 5% , TiO2-APTMS 10%, and TiO2-APTMS 15% were shown. From spectra of TiO2-APTMS the peaks below 700 cm −1 , which were assigned to Ti-O and Ti-O-Ti bonding of titania were ignored in this case because of their over saturated absorption [7]. The stretching vibration of absorbed water as well as surface hydroxyl groups (OH) which presented in the TiO2 were confirmed by the broad absorption band between 3400 and 3200 cm −1 and the low intensity peak at 1640 cm −1 [7]. After surface  [14]. Furthermore, the peak corresponding to Si-O-Si bond was observed at around 1040 cm −1 indicating the condensation reaction between silanol groups. The N-H bending vibration of primary amines (-NH2) was observed as a broad band in the region 1605-1560 cm −1 , and another low intensity peak on the shoulder of titania peak at 1140 cm −1 was assigned to the C-N bond. The appearance of these bands demonstrated that amine functional groups in organosilane were grafted onto the modified surface.

Photoelectrochemical solar cells perform of modified TiO2 NRs
Modification of surface TiO2 nanorods with APTMS was able to increase adsorption N3 complexes. APTMS has a amine group with a lone pair electrons which is caused the chemical bond between TiO2 and N3 complexes more easy. Amount of the dyes was higher adsorbing onto modified TiO2 compared with it onto unmodified TiO2 NRs. Figure 5 shows the current density-voltage curves of the dye sensitized solar cells based on the TiO2 nanorods without modification and TiO2 with modification on various concentration of APTMS (5,10 and 15 % (v/v)) composites. The values of short-circuit current density (Jsc), open-circuit voltage (Voc) and overall power conversion efficiency (η) are summarized in Table 1.  Overall, photovoltaic performance test was showed that modification of TiO2 nanorods can increase efficiency of solar cell. Howover, modification of APTMS on concentration 15 % doesn't provide optimum efficiency. Modification of high concentration was likely to occur polymerization from APTMS, which the polimerization will increase the resistivity of material [15]. Polymerization was triggered by bond formation at the end of the chain, it is causing hole recombination and the efficiency will be lower [16].

Conclusion
In this study, the effect of surface modification TiO2 nanorods with silane coupling agent (APTMS) was investigated. The characterization using TEM and FTIR confirmed that surface modification has been successfully grafted onto TiO2 surface through Ti-O-Si chemical bonds. By the modification of APTMS 10 % (v/v) on the surface TiO2 nanorods have been capable improved cell performance which was efficiency of 1.13 %.