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An investigation on surface modified TiO2 incorporated with graphene oxide for dye-sensitized solar cell
Solar Energy 191 (2019) 663–671
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An investigation on surface modified TiO2 incorporated with graphene oxide for dye-sensitized solar cell
T
Soon Weng Chonga, Chin Wei Laia, , Joon Ching Juana, Bey Fen Leoa,b ⁎
a b
Nanotechnology & Catalysis Research Centre (NANOCAT), Level 3, IPS Building, University of Malaya, 50603 Kuala Lumpur, Malaysia Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia
ARTICLE INFO
ABSTRACT
Keywords: Amine functionalized Surface charge Electrophoretic deposition Photoconversion DSSC
In the present study, titanium dioxide (TiO2) nanoparticles were amine functionalized by using (3-aminopropyl) triethoxysilane (APTES) for the incorporation with graphene oxide (GO) to resolve the rapid agglomeration of nanoparticles. The zeta potentiometer revealed a significant increment of surface charge from 8.31 mV (untreated) to 34.5 mV (treated) for the TiO2 nanoparticles which promotes strong electrostatic attraction with GO. Electrophoretic deposition technique was then employed for the fabrication of the photoanodes by generating an electric field for the transportation of the composite material. This work aimed to find the optimum loading content of GO-TiO2 for a higher photoconversion efficiency of the dye-sensitized solar cell (DSSC). Among the samples, the sample loaded with 0.5 g TiO2 showed the lowest emission peak intensity (Raman PL) which indicates efficient trapping of photogenerated electrons. By applying the Kubelka-Munk (K-M) expression, the absorbance spectra were converted into Tauc plots through which the aforementioned sample was found to exhibit a band gap energy of 2.91 eV (suitable for light absorption in the visible light region). The electron transfer efficiency for the sample loaded with 0.5 g TiO2 was relatively higher than the other samples in the DSSCs yielding a photoconversion efficiency of 6.86% because of the effective reduction in electron recombination.
1. Introduction The dye-sensitized solar cell (DSSC) has garnered an enormous amount of interest since it was introduced by Grätzel et al. in 1991 (O'regan and Grätzel, 1991). This interest stems from its satisfactory photoconversion efficiency, ease of fabrication and low production cost. Its structure consists of a sandwich configuration of a photoanode, redox electrolyte and a counter electrode. To date, the DSSC has attained a photoconversion efficiency of 14% which is very promising as a competitor to challenge the silicon based solar cells dominance in the photovoltaic fields (Lee et al., 2017; Selvaraj et al., 2018; Saravanan et al., 2017). The photoanode materials and fabrication techniques are the key factors that determine the effectiveness and performance of DSSCs (Ahmad et al., 2017). The photoanodes are usually prepared by coating semiconductor materials onto conductive glasses. Some of the coating techniques for this purpose are doctor-blade, CVD, spin-coating, RF sputtering, screen printing, ion-implantation, electrochemical/electrophoretic deposition, etc (Chong et al., 2015). Among the aforementioned techniques, the electrophoretic deposition is a fast and
⁎
controllable method to apply target materials onto an electrically conductive surface. This method promotes strong adhesion of targeted material on the electrode which prevents peel off in the DSSC. Electrophoretic deposition features colloidal particles suspended in a liquid medium which are deposited onto an electrode under the influence of an electric field (Chong et al., 2016). In the electrochemical cell system, direct current (DC) is applied to cause the suspended particles to polarize and move towards the oppositely charged electrode creating a relatively compact and homogeneous film. The advantages of this technique are high deposition rate, fast coating speed, relatively high purity and ability to produce thin films with a high degree of control over thickness and morphology (Gurrappa and Binder, 2016; Mozafari et al., 2016; Chang and Zhou, 2018). DSSC is a capacitive device which requires high band gap semiconductor material such as TiO2. During the fabrication process, TiO2 nanoparticles exhibit a strong tendency to agglomerate rapidly because of electrostatic forces (Razmjou et al., 2012). Agglomeration can cause uneven distribution of particles on membranes which lead to change in variables such as membrane surface roughness and hydrophilicity. Typical practice to minimize agglomeration of TiO2 nanoparticles is
Corresponding author. E-mail address: [email protected] (C.W. Lai).
https://doi.org/10.1016/j.solener.2019.08.065 Received 3 July 2019; Received in revised form 18 August 2019; Accepted 26 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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through mechanical modification to disperse the particles physically by shear force (Cao et al., 2006; Rahimpour et al., 2008). Nevertheless, it is hard to break the intraparticle bonds. Thus, the fabrication of nanocomposite requires an efficient dispersion technique. A common method is a chemical treatment by using silane coupling agents such as APTES (Gun'Ko et al., 2000). This silane coupling agent reduces the hydrophilicity and surface energy of the nanoparticles to reduce agglomerations and increases matrix-particle interactions (Gun'Ko et al., 2000; Rong et al., 2006). The presence of amine terminal group on the propyl chain causes APTES to be used commonly in the attachment of organic molecules to hydroxylated metal oxides (Kim et al., 2010; Pasternack et al., 2008). For example, Andrzejewska et al. reported the adsorption of APTES modified organic dyes onto TiO2 surface (Andrzejewska et al., 2004). Some researchers also used APTES to link proteins to TiO2 surface in order to promote live cell adhesion to TiO2 (Balasundaram et al., 2006; Filippini et al., 2001). Although APTES is used for many applications, the dominant conformation of APTES on interfaces is often debated because the factors are dependent on reaction conditions and crystal structure of the nanoparticle. On the other hand, TiO2 exhibits inherent problems such as mismatch of its optical absorption and solar spectrum as well as low quantum yield due to rapid recombination of photogenerated charges (Long et al., 2013). The random transit path of the electron flow in disordered TiO2 nanoparticles increases the probability of charges recombination thus reduces the photoconversion efficiency of the device. To circumvent these obstacles, carbonaceous materials such as graphene with high electrical conductivity and fair ductility have been widely chosen as matrices for metals and metal oxides to improve the photoconversion efficiency. Graphene, a two-dimensional material with sp2 honeycomb lattice-structured C atoms, is one of the most appealing matrices because of its excellent electron conductivity and transparency. The presence of GO would support the migration of photoelectrons and lengthen the lifetime of photoexcited charge carriers in the semiconductor. Therefore, surface grafting of TiO2 with APTES was performed in this work to eliminate agglomeration followed by the incorporation with GO. The resulting nanocomposite was then fabricated into the photoanode of DSSCs by using the electrophoresis method. As a result, improved photoconversion efficiency was achieved.
Fig. 1. FT-IR spectra of (a) P25 and (b) P25 after surface grafting with APTES (Minella et al., 2009).
(DI) was added. Next, 500 mL of water and 15 mL of H2O2 were added into the suspension to stop the oxidation reaction. Subsequently, the 10 mL HCl was added into the suspension and then centrifuged for 15 min at 7000 rpm. The sediment was washed with DI after the supernatant was decanted and then centrifuged again. The purpose of the washing process was to remove metal ions within the suspension (Chen et al., 2013). The suspension was washed for five times and eventually dried in the oven at 90 °C for 24 h. The resulting material was then ground into powder. 2.3. Preparation of amine functionalized TiO2 nanoparticles and GO-TiO2 nanocomposite In this section, the P25 nanoparticles were mixed with 2 mL of APTES in 200 mL toluene and then heated to 80 °C and stirred vigorously for 24 h. This process was repeated for a varied amount of P25 (0.1, 0.5, 1, 1.5, and 2 g). Samples were also extracted to examine the surface charge before and after the surface modification. Meanwhile, GO solution was prepared by adding 2 mg of GO powder into 200 mL of DI and then ultrasonicated for 30 min. After that, the GO solution was added into the P25 mixture; stirred and heated for 24 h. The resulting material was washed with toluene and ether, and then centrifuged and dried.
2. Experimental section 2.1. Materials Graphite flakes (< 45 μm; ≥99.99%), potassium permanganate (KMnO4; 97%), sulfuric acid (H2SO4; 97%), hydrochloric acid (HCl; 37%), hydrogen peroxide (H2O2; 30%), TiO2 powder (P25; 21 nm; ≥99.5%), (3-aminopropyl)triethoxysilane (APTES; 99%), Fluorine doped tin oxide coated glass slide (FTO) with surface resistivity of ~7 Ω/sq, Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N-719) dye and potassium iodide electrolyte (KI; ≥99%) were purchased from Sigma-Aldrich, Malaysia.
2.4. Preparation of DSSC Prior to assembling the DSSC, the photoanode was first prepared by using the electrophoretic deposition technique. In this method, 0.2 g of GO-TiO2 nanocomposite material was added into a mixture of 5 mL lemon juice and 30 mL DI. The addition of lemon juice in the electrophoretic deposition could strengthen the bond of the thin film onto FTO glasses as well as aid in reduction process (Chong et al., 2016). The experimental setup of the electrophoretic deposition technique was reported in our previous work (Chong et al., 2016). A voltage of 15 V was applied during the deposition process resulting in each FTO glass coated with GO-TiO2 nanocomposite material of 1 cm2 active area. Subsequently, the FTO glasses were immerged into a mixture of anhydrous ethanol and 0.5 mM N-719 dye for 24 h. Then, acetonitrile was used to rinse these dye-sensitized photoanode in order to remove excessive dye molecules. Then, the conductive side of the FTO glass was coated with carbon to serve as the reference electrodes. The photoanode and the reference electrode were sandwiched together by using paper clips followed by droplets of KI electrolyte (0.5 M) between the FTO glasses. Clean wipes were used to remove excess electrolytes.
2.2. Preparation of GO GO was prepared by using simplified Hummer’s method as demonstrated in our previous work (Chong et al., 2015; Chong et al., 2016). Initially, a mixture of 3 g of graphite and 70 mL of H2SO4 was prepared in an ice bath environment. An amount of 9 g KMnO4 was added very slowly while constantly stirring the mixture. An explosion could occur if the temperature of the suspension was not kept under 20 °C because of the exothermic reaction. The suspension was stirred for 30 min at a temperature of 35 °C after all the KMnO4 was added. Then, the temperature was increased to 95 °C after 150 mL of deionized water 664
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Fig. 2. XRD spectra of (i) (a) GO and (b) P25 nanoparticles; and (ii) GO-TiO2 nanocomposites loaded with different contents of TiO2 (a) 0.1 g (b) 0.5 g (c) 1.0 g (d) 1.5 g (e) 2.0 g.
A weak peak at around 1028 cm−1 can be assigned to the stretch vibration of Ti-O-Si moieties formed by the condensation reaction between silanol groups of APTES and hydroxyl groups present on the surface of P25 (Ukaji et al., 2007). Meanwhile, another weak peak at around 1150 cm−1 is attributable to the stretch vibration of Si-O-Si from the condensation reactions between silanol groups (Chen and Yakovlev, 2010). The intense peak 1380 cm−1 is associated with CeH vibrational signals (scissor, CH2 and CH3 deformation vibrations) (Raza et al., 2014) while the band at 1620 cm−1 characterizes NeH bending vibrations of primary amine which can be considered as an evidence of the presence of APTES on P25 (Ukaji et al., 2007; Ng et al., 2013). A weak band present at 2920 cm−1 can be assigned to the alkyl groups [e (CH2)ne] present in APTES (Cheng et al., 2014). Broad absorbance bands appear between 3100 and 3600 cm−1 for the APTES modified sample characterize the stretching vibrations of surface-absorbed water overlapped with the stretching and deformation mode of NHx groups where broad bands near 3200 cm−1 are most likely due to physisorbed water. The appearance of these bands suggest that APTES has been successfully introduced onto P25 surface. The XRD spectra were used to determine the corresponding crystal planes and phases of the composites. Fig. 2 (i) shows the XRD patterns of GO and P25 nanoparticles. As observed in Fig. 2 (i) (b), the peaks are located at 2θ = 25.3, 37.8, 48.1, 53.9, 55.1, 62.7, 68.7 and 75.1° which could be indexed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6) and (2 1 5) crystal planes of P25 anatase (Wang et al., 2012; Tan et al., 2015a; Patrocinio et al., 2015; Low et al., 2017a). The GO used in the reaction had only one peak at 2θ = 11° as seen in Fig. 2 (i) (a). Meanwhile, Fig. 2(ii) shows GO-TiO2 nanocomposites with different contents of TiO2. The disappearance of the GO peak from the GO-TiO2 nanocomposites spectra was because of partial reduction during the electrophoretic process. By applying the Spurr Equation of fA = 1/(1 + 1.26 × IR/IA) where fA is the weight fraction of anatase, fR = 1 − fA is the weight fraction of rutile, IA is the intensity of maximum anatase phase peak, IR is the intensity of maximum rutile phase peak and 1.26 is the scattering coefficient (Samsudin et al., 2015), all GO-TiO2 nanocomposites were calculated to have an anatase content of 82.01% and rutile content of 17.99% which is in good agreement of the relative content of P25′s anatase to rutile phase of 4:1. There is no obvious peak shift for the different contents of TiO2 indicates that the nanoparticles maintain their crystal lattice structure after going through the surface grafting by using APTES as well as the partial reduction during the electrophoretic process. The XPS was used to detect the compositions and the chemical state
2.5. Characterization The functional groups were characterized by using Fourier transform infrared spectroscopy. The model Bruker-IFS 66/S (FT-IR) was operated at a scan from 500 to 4000 cm−1. The morphology and lattice size of GO-TiO2 was observed using a high resolution-transmission electron microscopy (HR-TEM). The machine model JEM 2100-F was operated at 200 kV accelerating voltage. The thickness and surface roughness of the prepared samples were characterized by using a Bruker Multimode 8 Instruments atomic force microscope (AFM), and data were analyzed with the help of Nanoscope Analysis software. To determine the phase of the GO and composites, X-ray diffraction (XRD) was employed. D8 Advance X-Ray Diffractometer by Bruker AXS was used at a scanning rate of 0.033°s-1, 2θ from 2° to 90° with CuKα radiation (λ = 1.5418 Å). The Raman and photoluminescence (PL) spectra were obtained by using the RAMAN microscope with a 514 nm, 5 mW laser focused onto a micro sized spot of 1 μm (Renishaw inVia Microscope, HeCd laser). Meanwhile, the surface charge of TiO2 and composite were analyzed by using the Malvern Zetasizer Nano ZS. The band gap energies of the composite photoanode were measured by UV2600 [UV–Visible-diffuse reflectance spectrophotometer (UV-DRS), Shimadzu Co., Japan] under 400 nm/s scan rate and 200–800 nm wavelength. The PHI Quantera II instrument X-Ray Photoelectron Spectroscopy (XPS) from MIMOS Berhad was used to measure the elemental composition and analyze the surface chemistry of the composite photoanode at wide scan (100–1200 eV) and narrow scan. The voltagecurrent curves of DSSC were determined by Autolab PGSTAT204 under the solar illumination of 100 W lamp (Mercury Xenon Lamp-based Newport 66902 instrument). 3. Results and discussions The P25 nanoparticles were first characterized by zetasizer to determine the surface charge on the P25 nanoparticles before and after surface grafting with APTES. The increment of P25 nanoparticles surface charge from 8.31 to 34.5 mV is a good preliminary indication of successful modification. Subsequently, the surface charge of GO and GO-TiO2 were found to be at −0.347 mV and 23.3 mV respectively. The use of APTES had enhanced the positive charges to ensure strong electrostatic bonds with the GO. The FT-IR spectra were used to examine functional groups on the P25 nanoparticles before and after surface grafting with APTES. Fig. 1 reveal a broad peak between 800 and 500 cm−1 which is assigned to the presence of Ti-O-Ti bonds of P25 (Cheng et al., 2014). 665
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Fig. 3. XPS spectra of (i) wide scan of GO and composite sample loaded with 0.5 g TiO2 (ii) C 1s (iii) O 1s (iv) Si 2p (v) N 1s and (vi) Ti 2p.
of GO before and after the incorporation with TiO2. The nanocomposite sample loaded with 0.5 g TiO2 was selected for the XPS because it yields the highest photoconversion efficiency among the samples. Fig. 3 (i) (a) reveals that GO possess two major peaks which are the C 1s and O 1s peaks. After the amine functionalized TiO2 was incorporated with the GO, the C 1s peak was reduced while the O 1s was increased because of the joint oxygenated group and surface area distribution with the Ti functional group. A few important peaks such as the Ti 2s, Ti 2p, N 1s, Si 2s, Si 2p, Ti 3s and Ti 3p (Fig. 3 (i) (b)) also emerged after the incorporation of GO with TiO2. The peak at 284 eV in Fig. 3 (ii) corresponds to the aromatic C-C bonds while the peak at 286.8 eV corresponds to the C-O group (Tan et al., 2015a). Meanwhile the peak located at 285.5 eV is attributed to the sp2 carbon defects (Cheng et al., 2013). The absence of Ti-C peak at 281 eV suggests that our GO was not doped into the O2-TiO2 lattice
(Tan et al., 2015a). On the other hand, the presence of oxygenated functional groups on GO allows it to be attracted towards the amine functionalized TiO2 through electrostatic interaction. Fig. 3 (iii) shows the O 1s XP spectrum of the composite which can be deconvoluted into three peaks located at 529.3, 530.2 and 531.5 eV. The two peaks at 529.3 and 530.2 eV can be assigned to TiO2 and TiOH respectively (Song et al., 2010). This 530.2 eV peak is an important feature because the amount of surface hydroxyl groups act as the main coupling location for the silane groups from APTES. The other peak at 531.5 eV was attributed to Si-O-Ti bonds (Abad et al., 2010). Meanwhile, there is an intense sharp peak at 102.2 eV in the Si 2p XP spectrum (Fig. 3 (iv)) which indicates a clear evidence of successful attachment of APTES onto the surface of TiO2 (Cheng et al., 2014). There are three peaks (399.3, 400.5 and 401.7 eV) that constitute the N 1s XP spectra in Fig. 3 (v). The peak at 399.3 eV was ascribed to the 666
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Fig. 4. Reaction pathway of amine functionalized TiO2.
Fig. 5. AFM topology of (a) 2D height, (b) 3D height, and (c) line profile of composite sample loaded with 0.5 g TiO2 thin film.
1991). Whereas, the Ti core level XP spectra depicted in Fig. 3 (vi) indexed the Ti 2p1/2 and Ti 2p3/2 at binding energies of 463.9 and 458.6 eV which gives a difference of 5.3 eV that indicates the presence of normal states of Ti (Umrao et al., 2014). The Atomic Force Microscopy (AFM) was used to characterize the height profiles and surface topology of GO-TiO2 nanocomposite. The height profile, two-dimensional (2D) and three-dimensional (3D) of the
presence of NH2 groups in APTES coated on the TiO2 while the peaks at 400.5 and 401.7 eV could be attributed to the hydrogen-bonded amino or protonated amino-groups (Cheng et al., 2014; Song et al., 2010). The higher intensity of peak 399.3 eV compared to 400.5 eV also implies that APTES was mainly attached to TiO2 by silanized bonding leaving free amino groups to react with GO as illustrated in Fig. 4 (Pasternack et al., 2008; Cheng et al., 2014; Arranz et al., 2008; Vandenberg et al., 667
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Table 1 Surface analysis of GO-TiO2 thin-films of various ratio (0.1, 0.5, 1.0, 1.5, 2.0 g) after electrophoresis at 15 V for 5 min. Sample
GO-TiO2 GO-TiO2 GO-TiO2 GO-TiO2 GO-TiO2
Roughness Parameters
(0.1 g) (0.5 g) (1.0 g) (1.5 g) (2.0 g)
Thickness (Mean), nm
Roughness (Rmax), nm
Surface Roughness (Ra), nm
134.6 147.7 151.5 162.7 164.8
143.154 147.067 148.231 151.334 151.896
126.1 127.4 127.8 128.3 128.6
Fig. 8. PL spectra of GO-TiO2 nanocomposites where the TiO2 content is (a) 0.5 g (b) 1.5 g (c) 2.0 g (d) 1.0 g (e) 0.1 g.
electrophoresis process. Fig. 6 shows the HR-TEM images of the GO-TiO2 and its lattices. The two observed fringe spacings were measured to be 0.35 nm and 0.38 nm. The 0.35 nm corresponds to the (1 0 1) crystallographic plane of anatase TiO2 while the 0.38 nm is the inter-planar distance for GO (Shalaby et al., 2015; Yang et al., 2015). The image also shows the TiO2 nanoparticles were not agglomerated because of the modification by using APTES. The GO could not have been observed if clumps of agglomerated TiO2 were attached on its surface. The Raman spectroscopy was used to characterize the disorder or structural defect in nanocrystallites. Fig. 7 shows the Raman spectra of GO-TiO2 nanocomposites with different contents of TiO2. Five characteristic peaks of anatase TiO2 can be observed. The symmetric stretching vibrations, Eg are located at 143 cm−1, 196 cm−1 and 637 cm−1. A symmetric bending vibration, B1g is found at 395 cm−1 while an anti-symmetric vibration, A1g is seen at 515 cm−1. The rutile bands are not visible in the Raman spectra implies that the Raman can only show the presence of TiO2 but not the exact crystal structure (Samsudin et al., 2015). The percentage of exposed active anatase facets can be obtained by dividing the intensity of A1g with the dominant Eg peak. There are also two typical peaks in the spectra of GO which are the D band at 1350 cm−1 and G band at 1606 cm−1 (Low et al., 2017b). In comparison to our previously reported GO, the ratio ID/IG of the nanocomposite sample (0.5 g) has increased from 0.54 to 0.83 (Chong et al., 2016). This increase describes a change in the electronic conjugation state of GO from the incorporation with TiO2. The size of the sp2 domains become smaller for the composite compared with GO. The G band and 2D band are blue shifted for the composite samples compared to GO which might be attributed to the tensile strain from the introduction of APTES binder as well as the improved free charge carriers. The PL was used to determine the electron trapping efficiency, investigate the emission mechanism and charge transfer within a specific material. The PL intensity provides information on the recombination rate of charge carriers. Fig. 8 shows the spectra of PL for GO-TiO2 composites with varying content of TiO2. It can be observed from Fig. 8(a) that the sample loaded with 0.5 g of TiO2 in the nanocomposite displayed the lowest emission peak intensity which indicates efficient trapping of photogenerated electrons. Low peak intensities imply that a larger amount of photogenerated electrons are trapped and efficiently transferred across the Schottky barrier while high intensity peaks correspond to rapid charge recombination (Tan et al., 2015b). However, insufficient amount of TiO2 used to synthesize the
Fig. 6. HR-TEM images of composite sample loaded with 0.5 g TiO2 and the lattice spacings of GO and TiO2.
Fig. 7. Raman spectra of GO-TiO2 nanocomposites.
composite sample loaded with 0.5 g TiO2 thin-film were shown in Fig. 5. In the 3D image, the highest point of the sample surface was represented by light pink areas while the valley or sample pores were represented by dark red region. The height profile shows a thin film thickness of 147.7 nm was achieved by using the electrophoresis method for 5 min as shown in Table 1. As previously reported in our work, the thin film obtained by using lemon juice during the electrophoresis method promotes well-adhered chemically stable thin layer of the deposited film (Chong et al., 2016). The combination of citric acid and ascorbic acid in lemon juice introduced more surface adsorptive sites which promotes adhesion on the FTO glass during the 668
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Fig. 9. Tauc plots of GO-TiO2 nanocomposites where the TiO2 content is (a) 0.5 g (b) 1.5 g (c) 2.0 g (d) 1.0 g (e) 0.1 g. Table 2 Estimated band gap energies of GO-TiO2 nanocomposites. Sample
Band gap energy (eV)
0.1 g 0.5 g 1.0 g 1.5 g 2.0 g P25
2.58 2.91 3.08 3.10 3.11 3.20
composite will retard the photocatalytic performance of the material. When the amount of TiO2 in the composite is too low [Fig. 8(e)], the material is unable to trap photogenerated electrons effectively which resulted in the highest intensity peak compared to other samples. The amount of GO become significant which increased the probability of collision between electrons and holes, thus promoting recombination of charges. The reason of the low electrons trapping efficiency could be attributed to the light harvesting competition between GO and TiO2 which lead to reduced photocatalytic performance.
Fig. 11. DSSC performance of GO-TiO2 nanocomposites photoanode with different contents of TiO2 (0.1, 0.5, 1.0, 1.5 and 2.0 g).
Fig. 10. Energy diagram for the GO-TiO2 nanocomposites.
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Table 3 VOC, JSC, FF, and η for DSSC calculated from Fig. 10. GO-TiO2 nanocomposites with different contents of TiO2, (g)
Open-Circuit Voltage, VOC (V)
Current Density, JSC (mA/cm2)
Maximum Voltage, Vmax (V)
Maximum Current, Imax (A)
Fill Factor, FF
Efficiency, η (%)
0.1 0.5 1.0 1.5 2.0
0.59 0.73 0.60 0.39 0.64
16.04 16.21 18.62 19.18 18.35
0.19 0.49 0.34 0.32 0.46
15.87 14.00 16.68 11.00 10.00
0.32 0.58 0.52 0.47 0.39
3.03 6.86 5.71 3.67 4.58
The optical properties of GO-TiO2 nanocomposites sample are analyzed by UV-DRS. From Fig. 9 (i), Tauc plots of rGO-TiO2 nanocomposites are transformed from the reflectance spectra by applying the Kubelka-Munk (K-M) expression (Eqs. (1) and (2)) (Morales et al., 2007).
F (R ) =
F (R )
(1
R)2 2R
h = A (h
with 0.5 g TiO2 was found to provide the highest photoconversion efficiency (6.86%) as compared to the other samples. A suitable ratio of GO-TiO2 is important to trap photogenerated electrons efficiently which in turn increased the lifetime of the photo-excited electron-hole pairs and retard the electron-hole recombination to enhance the photocatalytic performance. After the optimum content of TiO2 is exceeded, the characteristic of TiO2 will overpower the effect of the GO content which is to decrease the band gap energy of TiO2. Meanwhile, the surface grafting of TiO2 by using APTES has overcome the rapid agglomeration tendency of nanoparticles, thus increasing the photoconversion efficiency of DSSCs. A future prospect of this work would be replacing the P25 with self-synthesized TiO2 to achieve higher photoconversion efficiency.
(1)
Eg )1/2
(2)
The band gap energies of the composites are estimated through a Tauc plot of the K-M function with linear extrapolations (Fig. 9) and compiled in Table 2. P25 by itself exhibits a large band gap energy of 3.2 eV hindering it from absorbing light in the visible range. The change in band gap energies indicate that the incorporation of GO with APTES treated P25 was effective. The hypothesis was verified by studying its performance in DSSC under visible light irradiation. It is well known that a typical Schottky junction barrier will be formed at the contact interface between graphene and P25 (Liu et al., 2014). The heterojunctions will suppress the recombination of electronhole pairs in P25 nanoparticles where the GO nanosheets serve as efficient electron trap aiding electron-hole separation. The Schottky junction barrier facilitates the electron capture thus increasing the lifetime of the photo-excited electron-hole pairs and retard the electronhole recombination to enhance the photocatalytic performance. This process can be envisioned as a two-step transition of an electron from the conduction band to the valence band which meet each other in the trap. Once the trap is filled, it cannot accept another electron. The electron occupying the trap then moves into an empty valence band state to complete the recombination process. This process can also be referred as the Shockley-Read-Hall (SRH) recombination. As shown in Fig. 10, P25 which consists of the coupling of two phases allows vectorial displacement of electrons from the anatase phase to the rutile phase and retards the recombination of electron-hole pairs (Miyauchi et al., 2000). The incorporation of GO with P25 further prolong the lifetime of electrons which favours enhanced photoactivity of the composite material. The GO-TiO2 thin-films were assembled into photoanodes of DSSCs in order to investigate the electrical performance by using AutoLab PGSTAT204. The constructed DSSCs were examined with voltage swept from 0 to 0.8 V. The power of the light source used was 100 W whereas the dimension of the DSSCs active area was fixed at 1 cm2 (Low et al., 2019). As a result, the current-voltage characteristics were plotted in Fig. 11. As observed from Table 3, the 0.5 g GO-TiO2 thin-film DSSC was found to have the highest efficiency compared to other samples. This result is in good agreement with the Raman PL because the 0.5 g sample has the best efficiency in the trapping of photogenerated electrons. This result implies that a larger amount of photogenerated electrons are trapped and efficiently transferred across the Schottky barrier.
Acknowledgments This research work was financially supported by the University of Malaya Grant (No. RP045B-17AET and RP045D-17AET), ImpactOriented Interdisciplinary Research Grant (No. IIRG018A-2019) and Global Collaborative Programme - SATU Joint Research Scheme (No. ST009-2018). References Abad, J., et al., 2010. Characterization of thin silicon overlayers on rutile TiO 2 (110)−(1× 1). Phys. Rev. B 82 (16), 165420. Ahmad, M.S., Pandey, A., Rahim, N.A., 2017. Advancements in the development of TiO2 photoanodes and its fabrication methods for dye sensitized solar cell (DSSC) applications. A review. Renew. Sustain. Energy Rev. 77, 89–108. Andrzejewska, A., Krysztafkiewicz, A., Jesionowski, T., 2004. Adsorption of organic dyes on the aminosilane modified TiO2 surface. Dyes Pigm. 62 (2), 121–130. Arranz, A., et al., 2008. Influence of surface hydroxylation on 3-aminopropyltriethoxysilane growth mode during chemical functionalization of GaN surfaces: An angleresolved X-ray photoelectron spectroscopy study. Langmuir 24 (16), 8667–8671. Balasundaram, G., Sato, M., Webster, T.J., 2006. Using hydroxyapatite nanoparticles and decreased crystallinity to promote osteoblast adhesion similar to functionalizing with RGD. Biomaterials 27 (14), 2798–2805. Cao, X., et al., 2006. Effect of TiO2 nanoparticle size on the performance of PVDF membrane. Appl. Surf. Sci. 253 (4), 2003–2010. Chang, J., Zhou, Y., 2018. Surface modification of bioactive glasses. In: Bioactive Glasses. Elsevier, pp. 119–143. Chen, J., et al., 2013. An improved Hummers method for eco-friendly synthesis of graphene oxide. Carbon 64, 225–229. Chen, Q., Yakovlev, N.L., 2010. Adsorption and interaction of organosilanes on TiO2 nanoparticles. Appl. Surf. Sci. 257 (5), 1395–1400. Cheng, G., et al., 2013. Novel preparation of anatase TiO2@ reduced graphene oxide hybrids for high-performance dye-sensitized solar cells. ACS Appl. Mater. Interfaces 5 (14), 6635–6642. Cheng, F., et al., 2014. UV-stable paper coated with APTES-modified P25 TiO2 nanoparticles. Carbohydr. Polym. 114, 246–252. Chong, S.W., Lai, C.W., Hamid, S.B.A., 2015. Green preparation of reduced graphene oxide using a natural reducing agent. Ceram. Int. 41 (8), 9505–9513. Chong, S.W., Lai, C.W., Abd Hamid, S.B., 2016. Controllable electrochemical synthesis of reduced graphene oxide thin-film constructed as efficient photoanode in dye-sensitized solar cells. Materials 9 (2), 69. Filippini, P., et al., 2001. Modulation of osteosarcoma cell growth and differentiation by silane-modified surfaces. J. Biomed. Mater. Res.: Off. J. Soc. Biomater., Japanese Soc. Biomater., Austral. Soc. Biomater. Korean Soc. Biomater. 55 (3), 338–349. Gun'Ko, V., et al., 2000. Features of fumed silica coverage with silanes having three or two groups reacting with the surface. Colloids Surf., A 166 (1–3), 187–201. Gurrappa, I., Binder, L., 2016. Electrodeposition of nanostructured coatings and their characterization—a review. Sci. Technol. Adv. Mater. Kim, J., et al., 2010. Investigations of chemical modifications of amino-terminated organic films on silicon substrates and controlled protein immobilization. Langmuir 26
4. Conclusion Thin-film GO-TiO2 of different weight content of TiO2 have been prepared into DSSCs. Based on the results, the composite sample loaded 670
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Raza, M., et al., 2014. Interaction and UV-stability of various organic capping agents on the surface of anatase nanoparticles. Materials 7 (4), 2890–2912. Razmjou, A., et al., 2012. The effect of modified TiO2 nanoparticles on the polyethersulfone ultrafiltration hollow fiber membranes. Desalination 287, 271–280. Rong, M., Zhang, M., Ruan, W., 2006. Surface modification of nanoscale fillers for improving properties of polymer nanocomposites: a review. Mater. Sci. Technol. 22 (7), 787–796. Samsudin, E.M., et al., 2015. Influence of triblock copolymer (pluronic F127) on enhancing the physico-chemical properties and photocatalytic response of mesoporous TiO2. Appl. Surf. Sci. 355, 959–968. Saravanan, S., et al., 2017. Efficiency improvement in dye sensitized solar cells by the plasmonic effect of green synthesized silver nanoparticles. J. Sci.: Adv. Mater. Devices 2 (4), 418–424. Selvaraj, P., et al., 2018. Enhancing the efficiency of transparent dye-sensitized solar cells using concentrated light. Sol. Energy Mater. Sol. Cells 175, 29–34. Shalaby, A., et al., 2015. Structural analysis of reduced graphene oxide by transmission electron microscopy. Bul. Chem. Commun. 47 (1), 291–295. Song, Y.-Y., Hildebrand, H., Schmuki, P., 2010. Optimized monolayer grafting of 3aminopropyltriethoxysilane onto amorphous, anatase and rutile TiO2. Surf. Sci. 604 (3–4), 346–353. Tan, L.-L., et al., 2015a. Visible-light-active oxygen-rich TiO2 decorated 2D graphene oxide with enhanced photocatalytic activity toward carbon dioxide reduction. Appl. Catal. B 179, 160–170. Tan, L.-L., et al., 2015b. Visible-light-active oxygen-rich TiO2 decorated 2D graphene oxide with enhanced photocatalytic activity toward carbon dioxide reduction. Appl. Catal. B 179, 160–170. Ukaji, E., et al., 2007. The effect of surface modification with silane coupling agent on suppressing the photo-catalytic activity of fine TiO2 particles as inorganic UV filter. Appl. Surf. Sci. 254 (2), 563–569. Umrao, S., et al., 2014. A possible mechanism for the emergence of an additional band gap due to a Ti–O–C bond in the TiO2–graphene hybrid system for enhanced photodegradation of methylene blue under visible light. RSC Adv. 4 (104), 59890–59901. Vandenberg, E.T., et al., 1991. Structure of 3-aminopropyl triethoxy silane on silicon oxide. J. Colloid Interface Sci. 147 (1), 103–118. Wang, P., et al., 2012. Enhanced photoelectrocatalytic activity for dye degradation by graphene–titania composite film electrodes. J. Hazard. Mater. 223, 79–83. Yang, Y., et al., 2015. One-step hydrothermal synthesis of surface fluorinated TiO2/reduced graphene oxide nanocomposites for photocatalytic degradation of estrogens. Mater. Sci. Semicond. Process. 40, 183–193.
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An investigation on surface modified TiO2 incorporated with graphene oxide for dye-sensitized solar cell
T
Soon Weng Chonga, Chin Wei Laia, , Joon Ching Juana, Bey Fen Leoa,b ⁎
a b
Nanotechnology & Catalysis Research Centre (NANOCAT), Level 3, IPS Building, University of Malaya, 50603 Kuala Lumpur, Malaysia Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia
ARTICLE INFO
ABSTRACT
Keywords: Amine functionalized Surface charge Electrophoretic deposition Photoconversion DSSC
In the present study, titanium dioxide (TiO2) nanoparticles were amine functionalized by using (3-aminopropyl) triethoxysilane (APTES) for the incorporation with graphene oxide (GO) to resolve the rapid agglomeration of nanoparticles. The zeta potentiometer revealed a significant increment of surface charge from 8.31 mV (untreated) to 34.5 mV (treated) for the TiO2 nanoparticles which promotes strong electrostatic attraction with GO. Electrophoretic deposition technique was then employed for the fabrication of the photoanodes by generating an electric field for the transportation of the composite material. This work aimed to find the optimum loading content of GO-TiO2 for a higher photoconversion efficiency of the dye-sensitized solar cell (DSSC). Among the samples, the sample loaded with 0.5 g TiO2 showed the lowest emission peak intensity (Raman PL) which indicates efficient trapping of photogenerated electrons. By applying the Kubelka-Munk (K-M) expression, the absorbance spectra were converted into Tauc plots through which the aforementioned sample was found to exhibit a band gap energy of 2.91 eV (suitable for light absorption in the visible light region). The electron transfer efficiency for the sample loaded with 0.5 g TiO2 was relatively higher than the other samples in the DSSCs yielding a photoconversion efficiency of 6.86% because of the effective reduction in electron recombination.
1. Introduction The dye-sensitized solar cell (DSSC) has garnered an enormous amount of interest since it was introduced by Grätzel et al. in 1991 (O'regan and Grätzel, 1991). This interest stems from its satisfactory photoconversion efficiency, ease of fabrication and low production cost. Its structure consists of a sandwich configuration of a photoanode, redox electrolyte and a counter electrode. To date, the DSSC has attained a photoconversion efficiency of 14% which is very promising as a competitor to challenge the silicon based solar cells dominance in the photovoltaic fields (Lee et al., 2017; Selvaraj et al., 2018; Saravanan et al., 2017). The photoanode materials and fabrication techniques are the key factors that determine the effectiveness and performance of DSSCs (Ahmad et al., 2017). The photoanodes are usually prepared by coating semiconductor materials onto conductive glasses. Some of the coating techniques for this purpose are doctor-blade, CVD, spin-coating, RF sputtering, screen printing, ion-implantation, electrochemical/electrophoretic deposition, etc (Chong et al., 2015). Among the aforementioned techniques, the electrophoretic deposition is a fast and
⁎
controllable method to apply target materials onto an electrically conductive surface. This method promotes strong adhesion of targeted material on the electrode which prevents peel off in the DSSC. Electrophoretic deposition features colloidal particles suspended in a liquid medium which are deposited onto an electrode under the influence of an electric field (Chong et al., 2016). In the electrochemical cell system, direct current (DC) is applied to cause the suspended particles to polarize and move towards the oppositely charged electrode creating a relatively compact and homogeneous film. The advantages of this technique are high deposition rate, fast coating speed, relatively high purity and ability to produce thin films with a high degree of control over thickness and morphology (Gurrappa and Binder, 2016; Mozafari et al., 2016; Chang and Zhou, 2018). DSSC is a capacitive device which requires high band gap semiconductor material such as TiO2. During the fabrication process, TiO2 nanoparticles exhibit a strong tendency to agglomerate rapidly because of electrostatic forces (Razmjou et al., 2012). Agglomeration can cause uneven distribution of particles on membranes which lead to change in variables such as membrane surface roughness and hydrophilicity. Typical practice to minimize agglomeration of TiO2 nanoparticles is
Corresponding author. E-mail address: [email protected] (C.W. Lai).
https://doi.org/10.1016/j.solener.2019.08.065 Received 3 July 2019; Received in revised form 18 August 2019; Accepted 26 August 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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through mechanical modification to disperse the particles physically by shear force (Cao et al., 2006; Rahimpour et al., 2008). Nevertheless, it is hard to break the intraparticle bonds. Thus, the fabrication of nanocomposite requires an efficient dispersion technique. A common method is a chemical treatment by using silane coupling agents such as APTES (Gun'Ko et al., 2000). This silane coupling agent reduces the hydrophilicity and surface energy of the nanoparticles to reduce agglomerations and increases matrix-particle interactions (Gun'Ko et al., 2000; Rong et al., 2006). The presence of amine terminal group on the propyl chain causes APTES to be used commonly in the attachment of organic molecules to hydroxylated metal oxides (Kim et al., 2010; Pasternack et al., 2008). For example, Andrzejewska et al. reported the adsorption of APTES modified organic dyes onto TiO2 surface (Andrzejewska et al., 2004). Some researchers also used APTES to link proteins to TiO2 surface in order to promote live cell adhesion to TiO2 (Balasundaram et al., 2006; Filippini et al., 2001). Although APTES is used for many applications, the dominant conformation of APTES on interfaces is often debated because the factors are dependent on reaction conditions and crystal structure of the nanoparticle. On the other hand, TiO2 exhibits inherent problems such as mismatch of its optical absorption and solar spectrum as well as low quantum yield due to rapid recombination of photogenerated charges (Long et al., 2013). The random transit path of the electron flow in disordered TiO2 nanoparticles increases the probability of charges recombination thus reduces the photoconversion efficiency of the device. To circumvent these obstacles, carbonaceous materials such as graphene with high electrical conductivity and fair ductility have been widely chosen as matrices for metals and metal oxides to improve the photoconversion efficiency. Graphene, a two-dimensional material with sp2 honeycomb lattice-structured C atoms, is one of the most appealing matrices because of its excellent electron conductivity and transparency. The presence of GO would support the migration of photoelectrons and lengthen the lifetime of photoexcited charge carriers in the semiconductor. Therefore, surface grafting of TiO2 with APTES was performed in this work to eliminate agglomeration followed by the incorporation with GO. The resulting nanocomposite was then fabricated into the photoanode of DSSCs by using the electrophoresis method. As a result, improved photoconversion efficiency was achieved.
Fig. 1. FT-IR spectra of (a) P25 and (b) P25 after surface grafting with APTES (Minella et al., 2009).
(DI) was added. Next, 500 mL of water and 15 mL of H2O2 were added into the suspension to stop the oxidation reaction. Subsequently, the 10 mL HCl was added into the suspension and then centrifuged for 15 min at 7000 rpm. The sediment was washed with DI after the supernatant was decanted and then centrifuged again. The purpose of the washing process was to remove metal ions within the suspension (Chen et al., 2013). The suspension was washed for five times and eventually dried in the oven at 90 °C for 24 h. The resulting material was then ground into powder. 2.3. Preparation of amine functionalized TiO2 nanoparticles and GO-TiO2 nanocomposite In this section, the P25 nanoparticles were mixed with 2 mL of APTES in 200 mL toluene and then heated to 80 °C and stirred vigorously for 24 h. This process was repeated for a varied amount of P25 (0.1, 0.5, 1, 1.5, and 2 g). Samples were also extracted to examine the surface charge before and after the surface modification. Meanwhile, GO solution was prepared by adding 2 mg of GO powder into 200 mL of DI and then ultrasonicated for 30 min. After that, the GO solution was added into the P25 mixture; stirred and heated for 24 h. The resulting material was washed with toluene and ether, and then centrifuged and dried.
2. Experimental section 2.1. Materials Graphite flakes (< 45 μm; ≥99.99%), potassium permanganate (KMnO4; 97%), sulfuric acid (H2SO4; 97%), hydrochloric acid (HCl; 37%), hydrogen peroxide (H2O2; 30%), TiO2 powder (P25; 21 nm; ≥99.5%), (3-aminopropyl)triethoxysilane (APTES; 99%), Fluorine doped tin oxide coated glass slide (FTO) with surface resistivity of ~7 Ω/sq, Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N-719) dye and potassium iodide electrolyte (KI; ≥99%) were purchased from Sigma-Aldrich, Malaysia.
2.4. Preparation of DSSC Prior to assembling the DSSC, the photoanode was first prepared by using the electrophoretic deposition technique. In this method, 0.2 g of GO-TiO2 nanocomposite material was added into a mixture of 5 mL lemon juice and 30 mL DI. The addition of lemon juice in the electrophoretic deposition could strengthen the bond of the thin film onto FTO glasses as well as aid in reduction process (Chong et al., 2016). The experimental setup of the electrophoretic deposition technique was reported in our previous work (Chong et al., 2016). A voltage of 15 V was applied during the deposition process resulting in each FTO glass coated with GO-TiO2 nanocomposite material of 1 cm2 active area. Subsequently, the FTO glasses were immerged into a mixture of anhydrous ethanol and 0.5 mM N-719 dye for 24 h. Then, acetonitrile was used to rinse these dye-sensitized photoanode in order to remove excessive dye molecules. Then, the conductive side of the FTO glass was coated with carbon to serve as the reference electrodes. The photoanode and the reference electrode were sandwiched together by using paper clips followed by droplets of KI electrolyte (0.5 M) between the FTO glasses. Clean wipes were used to remove excess electrolytes.
2.2. Preparation of GO GO was prepared by using simplified Hummer’s method as demonstrated in our previous work (Chong et al., 2015; Chong et al., 2016). Initially, a mixture of 3 g of graphite and 70 mL of H2SO4 was prepared in an ice bath environment. An amount of 9 g KMnO4 was added very slowly while constantly stirring the mixture. An explosion could occur if the temperature of the suspension was not kept under 20 °C because of the exothermic reaction. The suspension was stirred for 30 min at a temperature of 35 °C after all the KMnO4 was added. Then, the temperature was increased to 95 °C after 150 mL of deionized water 664
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Fig. 2. XRD spectra of (i) (a) GO and (b) P25 nanoparticles; and (ii) GO-TiO2 nanocomposites loaded with different contents of TiO2 (a) 0.1 g (b) 0.5 g (c) 1.0 g (d) 1.5 g (e) 2.0 g.
A weak peak at around 1028 cm−1 can be assigned to the stretch vibration of Ti-O-Si moieties formed by the condensation reaction between silanol groups of APTES and hydroxyl groups present on the surface of P25 (Ukaji et al., 2007). Meanwhile, another weak peak at around 1150 cm−1 is attributable to the stretch vibration of Si-O-Si from the condensation reactions between silanol groups (Chen and Yakovlev, 2010). The intense peak 1380 cm−1 is associated with CeH vibrational signals (scissor, CH2 and CH3 deformation vibrations) (Raza et al., 2014) while the band at 1620 cm−1 characterizes NeH bending vibrations of primary amine which can be considered as an evidence of the presence of APTES on P25 (Ukaji et al., 2007; Ng et al., 2013). A weak band present at 2920 cm−1 can be assigned to the alkyl groups [e (CH2)ne] present in APTES (Cheng et al., 2014). Broad absorbance bands appear between 3100 and 3600 cm−1 for the APTES modified sample characterize the stretching vibrations of surface-absorbed water overlapped with the stretching and deformation mode of NHx groups where broad bands near 3200 cm−1 are most likely due to physisorbed water. The appearance of these bands suggest that APTES has been successfully introduced onto P25 surface. The XRD spectra were used to determine the corresponding crystal planes and phases of the composites. Fig. 2 (i) shows the XRD patterns of GO and P25 nanoparticles. As observed in Fig. 2 (i) (b), the peaks are located at 2θ = 25.3, 37.8, 48.1, 53.9, 55.1, 62.7, 68.7 and 75.1° which could be indexed to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6) and (2 1 5) crystal planes of P25 anatase (Wang et al., 2012; Tan et al., 2015a; Patrocinio et al., 2015; Low et al., 2017a). The GO used in the reaction had only one peak at 2θ = 11° as seen in Fig. 2 (i) (a). Meanwhile, Fig. 2(ii) shows GO-TiO2 nanocomposites with different contents of TiO2. The disappearance of the GO peak from the GO-TiO2 nanocomposites spectra was because of partial reduction during the electrophoretic process. By applying the Spurr Equation of fA = 1/(1 + 1.26 × IR/IA) where fA is the weight fraction of anatase, fR = 1 − fA is the weight fraction of rutile, IA is the intensity of maximum anatase phase peak, IR is the intensity of maximum rutile phase peak and 1.26 is the scattering coefficient (Samsudin et al., 2015), all GO-TiO2 nanocomposites were calculated to have an anatase content of 82.01% and rutile content of 17.99% which is in good agreement of the relative content of P25′s anatase to rutile phase of 4:1. There is no obvious peak shift for the different contents of TiO2 indicates that the nanoparticles maintain their crystal lattice structure after going through the surface grafting by using APTES as well as the partial reduction during the electrophoretic process. The XPS was used to detect the compositions and the chemical state
2.5. Characterization The functional groups were characterized by using Fourier transform infrared spectroscopy. The model Bruker-IFS 66/S (FT-IR) was operated at a scan from 500 to 4000 cm−1. The morphology and lattice size of GO-TiO2 was observed using a high resolution-transmission electron microscopy (HR-TEM). The machine model JEM 2100-F was operated at 200 kV accelerating voltage. The thickness and surface roughness of the prepared samples were characterized by using a Bruker Multimode 8 Instruments atomic force microscope (AFM), and data were analyzed with the help of Nanoscope Analysis software. To determine the phase of the GO and composites, X-ray diffraction (XRD) was employed. D8 Advance X-Ray Diffractometer by Bruker AXS was used at a scanning rate of 0.033°s-1, 2θ from 2° to 90° with CuKα radiation (λ = 1.5418 Å). The Raman and photoluminescence (PL) spectra were obtained by using the RAMAN microscope with a 514 nm, 5 mW laser focused onto a micro sized spot of 1 μm (Renishaw inVia Microscope, HeCd laser). Meanwhile, the surface charge of TiO2 and composite were analyzed by using the Malvern Zetasizer Nano ZS. The band gap energies of the composite photoanode were measured by UV2600 [UV–Visible-diffuse reflectance spectrophotometer (UV-DRS), Shimadzu Co., Japan] under 400 nm/s scan rate and 200–800 nm wavelength. The PHI Quantera II instrument X-Ray Photoelectron Spectroscopy (XPS) from MIMOS Berhad was used to measure the elemental composition and analyze the surface chemistry of the composite photoanode at wide scan (100–1200 eV) and narrow scan. The voltagecurrent curves of DSSC were determined by Autolab PGSTAT204 under the solar illumination of 100 W lamp (Mercury Xenon Lamp-based Newport 66902 instrument). 3. Results and discussions The P25 nanoparticles were first characterized by zetasizer to determine the surface charge on the P25 nanoparticles before and after surface grafting with APTES. The increment of P25 nanoparticles surface charge from 8.31 to 34.5 mV is a good preliminary indication of successful modification. Subsequently, the surface charge of GO and GO-TiO2 were found to be at −0.347 mV and 23.3 mV respectively. The use of APTES had enhanced the positive charges to ensure strong electrostatic bonds with the GO. The FT-IR spectra were used to examine functional groups on the P25 nanoparticles before and after surface grafting with APTES. Fig. 1 reveal a broad peak between 800 and 500 cm−1 which is assigned to the presence of Ti-O-Ti bonds of P25 (Cheng et al., 2014). 665
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Fig. 3. XPS spectra of (i) wide scan of GO and composite sample loaded with 0.5 g TiO2 (ii) C 1s (iii) O 1s (iv) Si 2p (v) N 1s and (vi) Ti 2p.
of GO before and after the incorporation with TiO2. The nanocomposite sample loaded with 0.5 g TiO2 was selected for the XPS because it yields the highest photoconversion efficiency among the samples. Fig. 3 (i) (a) reveals that GO possess two major peaks which are the C 1s and O 1s peaks. After the amine functionalized TiO2 was incorporated with the GO, the C 1s peak was reduced while the O 1s was increased because of the joint oxygenated group and surface area distribution with the Ti functional group. A few important peaks such as the Ti 2s, Ti 2p, N 1s, Si 2s, Si 2p, Ti 3s and Ti 3p (Fig. 3 (i) (b)) also emerged after the incorporation of GO with TiO2. The peak at 284 eV in Fig. 3 (ii) corresponds to the aromatic C-C bonds while the peak at 286.8 eV corresponds to the C-O group (Tan et al., 2015a). Meanwhile the peak located at 285.5 eV is attributed to the sp2 carbon defects (Cheng et al., 2013). The absence of Ti-C peak at 281 eV suggests that our GO was not doped into the O2-TiO2 lattice
(Tan et al., 2015a). On the other hand, the presence of oxygenated functional groups on GO allows it to be attracted towards the amine functionalized TiO2 through electrostatic interaction. Fig. 3 (iii) shows the O 1s XP spectrum of the composite which can be deconvoluted into three peaks located at 529.3, 530.2 and 531.5 eV. The two peaks at 529.3 and 530.2 eV can be assigned to TiO2 and TiOH respectively (Song et al., 2010). This 530.2 eV peak is an important feature because the amount of surface hydroxyl groups act as the main coupling location for the silane groups from APTES. The other peak at 531.5 eV was attributed to Si-O-Ti bonds (Abad et al., 2010). Meanwhile, there is an intense sharp peak at 102.2 eV in the Si 2p XP spectrum (Fig. 3 (iv)) which indicates a clear evidence of successful attachment of APTES onto the surface of TiO2 (Cheng et al., 2014). There are three peaks (399.3, 400.5 and 401.7 eV) that constitute the N 1s XP spectra in Fig. 3 (v). The peak at 399.3 eV was ascribed to the 666
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Fig. 4. Reaction pathway of amine functionalized TiO2.
Fig. 5. AFM topology of (a) 2D height, (b) 3D height, and (c) line profile of composite sample loaded with 0.5 g TiO2 thin film.
1991). Whereas, the Ti core level XP spectra depicted in Fig. 3 (vi) indexed the Ti 2p1/2 and Ti 2p3/2 at binding energies of 463.9 and 458.6 eV which gives a difference of 5.3 eV that indicates the presence of normal states of Ti (Umrao et al., 2014). The Atomic Force Microscopy (AFM) was used to characterize the height profiles and surface topology of GO-TiO2 nanocomposite. The height profile, two-dimensional (2D) and three-dimensional (3D) of the
presence of NH2 groups in APTES coated on the TiO2 while the peaks at 400.5 and 401.7 eV could be attributed to the hydrogen-bonded amino or protonated amino-groups (Cheng et al., 2014; Song et al., 2010). The higher intensity of peak 399.3 eV compared to 400.5 eV also implies that APTES was mainly attached to TiO2 by silanized bonding leaving free amino groups to react with GO as illustrated in Fig. 4 (Pasternack et al., 2008; Cheng et al., 2014; Arranz et al., 2008; Vandenberg et al., 667
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Table 1 Surface analysis of GO-TiO2 thin-films of various ratio (0.1, 0.5, 1.0, 1.5, 2.0 g) after electrophoresis at 15 V for 5 min. Sample
GO-TiO2 GO-TiO2 GO-TiO2 GO-TiO2 GO-TiO2
Roughness Parameters
(0.1 g) (0.5 g) (1.0 g) (1.5 g) (2.0 g)
Thickness (Mean), nm
Roughness (Rmax), nm
Surface Roughness (Ra), nm
134.6 147.7 151.5 162.7 164.8
143.154 147.067 148.231 151.334 151.896
126.1 127.4 127.8 128.3 128.6
Fig. 8. PL spectra of GO-TiO2 nanocomposites where the TiO2 content is (a) 0.5 g (b) 1.5 g (c) 2.0 g (d) 1.0 g (e) 0.1 g.
electrophoresis process. Fig. 6 shows the HR-TEM images of the GO-TiO2 and its lattices. The two observed fringe spacings were measured to be 0.35 nm and 0.38 nm. The 0.35 nm corresponds to the (1 0 1) crystallographic plane of anatase TiO2 while the 0.38 nm is the inter-planar distance for GO (Shalaby et al., 2015; Yang et al., 2015). The image also shows the TiO2 nanoparticles were not agglomerated because of the modification by using APTES. The GO could not have been observed if clumps of agglomerated TiO2 were attached on its surface. The Raman spectroscopy was used to characterize the disorder or structural defect in nanocrystallites. Fig. 7 shows the Raman spectra of GO-TiO2 nanocomposites with different contents of TiO2. Five characteristic peaks of anatase TiO2 can be observed. The symmetric stretching vibrations, Eg are located at 143 cm−1, 196 cm−1 and 637 cm−1. A symmetric bending vibration, B1g is found at 395 cm−1 while an anti-symmetric vibration, A1g is seen at 515 cm−1. The rutile bands are not visible in the Raman spectra implies that the Raman can only show the presence of TiO2 but not the exact crystal structure (Samsudin et al., 2015). The percentage of exposed active anatase facets can be obtained by dividing the intensity of A1g with the dominant Eg peak. There are also two typical peaks in the spectra of GO which are the D band at 1350 cm−1 and G band at 1606 cm−1 (Low et al., 2017b). In comparison to our previously reported GO, the ratio ID/IG of the nanocomposite sample (0.5 g) has increased from 0.54 to 0.83 (Chong et al., 2016). This increase describes a change in the electronic conjugation state of GO from the incorporation with TiO2. The size of the sp2 domains become smaller for the composite compared with GO. The G band and 2D band are blue shifted for the composite samples compared to GO which might be attributed to the tensile strain from the introduction of APTES binder as well as the improved free charge carriers. The PL was used to determine the electron trapping efficiency, investigate the emission mechanism and charge transfer within a specific material. The PL intensity provides information on the recombination rate of charge carriers. Fig. 8 shows the spectra of PL for GO-TiO2 composites with varying content of TiO2. It can be observed from Fig. 8(a) that the sample loaded with 0.5 g of TiO2 in the nanocomposite displayed the lowest emission peak intensity which indicates efficient trapping of photogenerated electrons. Low peak intensities imply that a larger amount of photogenerated electrons are trapped and efficiently transferred across the Schottky barrier while high intensity peaks correspond to rapid charge recombination (Tan et al., 2015b). However, insufficient amount of TiO2 used to synthesize the
Fig. 6. HR-TEM images of composite sample loaded with 0.5 g TiO2 and the lattice spacings of GO and TiO2.
Fig. 7. Raman spectra of GO-TiO2 nanocomposites.
composite sample loaded with 0.5 g TiO2 thin-film were shown in Fig. 5. In the 3D image, the highest point of the sample surface was represented by light pink areas while the valley or sample pores were represented by dark red region. The height profile shows a thin film thickness of 147.7 nm was achieved by using the electrophoresis method for 5 min as shown in Table 1. As previously reported in our work, the thin film obtained by using lemon juice during the electrophoresis method promotes well-adhered chemically stable thin layer of the deposited film (Chong et al., 2016). The combination of citric acid and ascorbic acid in lemon juice introduced more surface adsorptive sites which promotes adhesion on the FTO glass during the 668
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Fig. 9. Tauc plots of GO-TiO2 nanocomposites where the TiO2 content is (a) 0.5 g (b) 1.5 g (c) 2.0 g (d) 1.0 g (e) 0.1 g. Table 2 Estimated band gap energies of GO-TiO2 nanocomposites. Sample
Band gap energy (eV)
0.1 g 0.5 g 1.0 g 1.5 g 2.0 g P25
2.58 2.91 3.08 3.10 3.11 3.20
composite will retard the photocatalytic performance of the material. When the amount of TiO2 in the composite is too low [Fig. 8(e)], the material is unable to trap photogenerated electrons effectively which resulted in the highest intensity peak compared to other samples. The amount of GO become significant which increased the probability of collision between electrons and holes, thus promoting recombination of charges. The reason of the low electrons trapping efficiency could be attributed to the light harvesting competition between GO and TiO2 which lead to reduced photocatalytic performance.
Fig. 11. DSSC performance of GO-TiO2 nanocomposites photoanode with different contents of TiO2 (0.1, 0.5, 1.0, 1.5 and 2.0 g).
Fig. 10. Energy diagram for the GO-TiO2 nanocomposites.
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Table 3 VOC, JSC, FF, and η for DSSC calculated from Fig. 10. GO-TiO2 nanocomposites with different contents of TiO2, (g)
Open-Circuit Voltage, VOC (V)
Current Density, JSC (mA/cm2)
Maximum Voltage, Vmax (V)
Maximum Current, Imax (A)
Fill Factor, FF
Efficiency, η (%)
0.1 0.5 1.0 1.5 2.0
0.59 0.73 0.60 0.39 0.64
16.04 16.21 18.62 19.18 18.35
0.19 0.49 0.34 0.32 0.46
15.87 14.00 16.68 11.00 10.00
0.32 0.58 0.52 0.47 0.39
3.03 6.86 5.71 3.67 4.58
The optical properties of GO-TiO2 nanocomposites sample are analyzed by UV-DRS. From Fig. 9 (i), Tauc plots of rGO-TiO2 nanocomposites are transformed from the reflectance spectra by applying the Kubelka-Munk (K-M) expression (Eqs. (1) and (2)) (Morales et al., 2007).
F (R ) =
F (R )
(1
R)2 2R
h = A (h
with 0.5 g TiO2 was found to provide the highest photoconversion efficiency (6.86%) as compared to the other samples. A suitable ratio of GO-TiO2 is important to trap photogenerated electrons efficiently which in turn increased the lifetime of the photo-excited electron-hole pairs and retard the electron-hole recombination to enhance the photocatalytic performance. After the optimum content of TiO2 is exceeded, the characteristic of TiO2 will overpower the effect of the GO content which is to decrease the band gap energy of TiO2. Meanwhile, the surface grafting of TiO2 by using APTES has overcome the rapid agglomeration tendency of nanoparticles, thus increasing the photoconversion efficiency of DSSCs. A future prospect of this work would be replacing the P25 with self-synthesized TiO2 to achieve higher photoconversion efficiency.
(1)
Eg )1/2
(2)
The band gap energies of the composites are estimated through a Tauc plot of the K-M function with linear extrapolations (Fig. 9) and compiled in Table 2. P25 by itself exhibits a large band gap energy of 3.2 eV hindering it from absorbing light in the visible range. The change in band gap energies indicate that the incorporation of GO with APTES treated P25 was effective. The hypothesis was verified by studying its performance in DSSC under visible light irradiation. It is well known that a typical Schottky junction barrier will be formed at the contact interface between graphene and P25 (Liu et al., 2014). The heterojunctions will suppress the recombination of electronhole pairs in P25 nanoparticles where the GO nanosheets serve as efficient electron trap aiding electron-hole separation. The Schottky junction barrier facilitates the electron capture thus increasing the lifetime of the photo-excited electron-hole pairs and retard the electronhole recombination to enhance the photocatalytic performance. This process can be envisioned as a two-step transition of an electron from the conduction band to the valence band which meet each other in the trap. Once the trap is filled, it cannot accept another electron. The electron occupying the trap then moves into an empty valence band state to complete the recombination process. This process can also be referred as the Shockley-Read-Hall (SRH) recombination. As shown in Fig. 10, P25 which consists of the coupling of two phases allows vectorial displacement of electrons from the anatase phase to the rutile phase and retards the recombination of electron-hole pairs (Miyauchi et al., 2000). The incorporation of GO with P25 further prolong the lifetime of electrons which favours enhanced photoactivity of the composite material. The GO-TiO2 thin-films were assembled into photoanodes of DSSCs in order to investigate the electrical performance by using AutoLab PGSTAT204. The constructed DSSCs were examined with voltage swept from 0 to 0.8 V. The power of the light source used was 100 W whereas the dimension of the DSSCs active area was fixed at 1 cm2 (Low et al., 2019). As a result, the current-voltage characteristics were plotted in Fig. 11. As observed from Table 3, the 0.5 g GO-TiO2 thin-film DSSC was found to have the highest efficiency compared to other samples. This result is in good agreement with the Raman PL because the 0.5 g sample has the best efficiency in the trapping of photogenerated electrons. This result implies that a larger amount of photogenerated electrons are trapped and efficiently transferred across the Schottky barrier.
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