Preparation of Nano-Ag-TiO2 Composites by Co-60 Gamma Irradiation to Enhance the Photocurrent of Dye-Sensitized Solar Cells

Abstract

Nano-silver-titanium dioxide (Ag-TiO₂) composites were prepared from commercial TiO₂ (P25, Degussa) and silver nitrate (AgNO₃) by gamma Co-60 irradiation method with various initial concentrations of AgNO₃. The nano-AgTiO₂ composites are utilized as the photoanode for dye-sensitized solar cells (DSCs). Under full sunlight illumination (1000 W/m², AM 1.5), the efficiency of DSCs has improved significantly despite the Ag content of below 1%. The DSC—assembled with 0.75 Ag-TiO₂ (0.75% Ag) photoanode—showed that the photocurrent was significantly enhanced from 8.1 mA.cm⁻² to 9.5 mA.cm⁻² compared to the DSCs using bared TiO₂ photoanode. The unchanged open-circuit voltage resulted in the overall energy conversion efficiency to be increased by 25% from 3.75% to 4.86%. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance is reduced when increasing Ag content, demonstrating that the charge transfer at TiO₂/dye interface was enhanced in the presence of silver nanoparticles.

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1. Introduction

Research and application of new energy resources are essential approaches to reduce dependence on fossil fuels, and solar energy is considered one of the feasible solutions to solve the world’s energy crisis. Dye-sensitized solar cells (DSCs) have promised to replace conventional silicon-based solar cells in the context of using clean solar energy due to their low cost, massive production, and facile process. Thus, DSC technology has been an attractive approach for the large-scale solar panel [1–5]. In DSCs, the cell architecture comprises nanostructured TiO₂ photoanode as an electron conductor, a dye Ru-complex as a light absorber, a redox shuttle for dye regeneration, and a counter electrode to collect electrons and reduce positive charges generated through the cell [1]. Commonly, the DSCs showed efficient solar energy-to-electricity conversion of 10% [1, 6].

Many approaches have been studied to alternately improve the conversion efficiency of DSCs, including researching the novel counter electrode, electrolytes, dyes, and semiconductor photoanode materials. Among these, the photoanode plays a decisive part in determining the performance of cells [1, 7–9]. Many semiconductor materials have been studied to be used as photoanode in DSCs such as TiO₂, ZnO, SnO₂, Nb₂O₃, and SrTiO₃. In particular, TiO₂ has been universally used due to its chemical stability, excellent charge transport capability, low cost, and easy preparation [2, 10, 11]. In DSCs, TiO₂ plays three roles: (i) providing a substrate for dye adsorption, (ii) accepting electrons from the dye’s excited state, and (iii) transporting the electrons from conduction band of TiO₂ to the conducting substrate then to the external circuit [11, 12]. TiO₂ possesses a wide bandgap energy in both common structures: anatase at 3.2 eV and rutile at 3.0 eV. To improve the solar energy-to-electricity conversion efficiency, the surface of TiO₂ are modified with metallic ions such as Fe³⁺ and Zn²⁺, alternatively, metallic nanoparticles such as Au, Ag, and Pt [3, 9, 10, 13, 14]. Study incorporation of Ag nanoparticles onto TiO₂ surface showed that the coupling of semiconductor and metal nanoparticles might yield a photoinduced electron transfer across the interface, which in turn may lead to the increased energy conversion efficiency of DSCs [11, 15–17]. Most of the previous reports showed the enhancement of efficiencies (4.86%) due to the plasmonic effect of Ag nanoparticles at high content (>2.5%) [8]. Many methods have been reported to prepare Ag-TiO₂ composite such as microwave-assisted sol–gel techniques [18], a microwave-hydrothermal technique [19], and UV irradiation [20]. Gamma irradiation has been well known as an effective method due to its simple preparation, massive produce, high efficiency, and eco-friendliness [14, 16, 21].

In this work, we prepared nano-Ag-TiO₂ composites at low Ag content (<1%) via Co-60 gamma irradiation. The nano-Ag-TiO₂ composites were used to prepare the photoanodes for DSCs. The role of Ag on the photoperformance of DSCs were investigated by the current-voltage method and the electrochemical impedance spectroscopy.

2. Experimental

2.1. Materials

TiO₂ (P25, Degussa), AgNO₃ (99.9%, Sigma-Aldrich), two types of ethyl cellulose (EC) powders (5-15 mPa⋅s and 30-50 mPa⋅s, Sigma-Aldrich), ethanol (95%, Sigma-Aldrich), and terpineol (anhydrous 99.9%, Sigma-Aldrich) were commercially available. Fluorine-doped tin oxide (FTO-TEX-8X, $15\Omega /\text{c}{\text{m}}^2$) conductive glass substrates (Dyesol, Australia), electrolyte solution HPE (Dyesol, Australia), ruthenium complex dye N719 (Dyesol, Australia), and platinum paste PT1 (Dyesol, Australia) were used to fabricate the cathode of the DSCs.

2.2. Preparation of Nano-Ag-TiO₂ Composites

4.00 g TiO₂ was dispersed in 20 mL solution of ethanol and distilled water (1/1, $v/v$) by magnetic stirring for 30 minutes; then, the solution was vibrated by ultrasound for 30 minutes. 10 mM AgNO₃ solution was added to the colloidal solution with the various weight ratios of Ag : TiO₂. The mixtures were irradiated via the gamma-radiation from a Co-60 irradiator in the dose range of 10-30 kGy with a dose rate of 1.3 kGy per hour (VINAGAMMA Center, Ho Chi Minh City). Table 1 details the volume of AgNO₃ solution, the weight ratios of Ag : TiO_2, and the dose of gamma-radiation. The irradiated colloidal solutions were centrifuged with speed up 10,000 rpm for 30 minutes to collect the powders; then, the final products were dried in air at 60°C overnight.

Table 1

Preparation of Ag-TiO₂ samples by gamma irradiation.

Sample	$V_{{\text{AgNO}}_3}$ (mL)	${\text{m}}_{{\text{TiO}}_2}$ (g)	Ratio of Ag : TiO2 (% weight)	Dose $\gamma$-irradiation (kGy)
0.25 Ag-TiO2	9.3	4.0	0.25	13.7
0.50 Ag-TiO2	18.5	4.0	0.50	20.4
0.75 Ag-TiO2	27.8	4.0	0.75	27.3

2.3. Fabrication of DSCs

DSCs (an active area of 0.2 cm²) were assembled following our process in previous reports with four steps [22–24].

2.3.1. Ag-TiO₂ Printing Paste Preparation

The Ag-TiO₂ printing paste is composed of Ag-TiO₂ (20% wt.), ethyl celluloses (10% wt.), and terpinol (70% wt.). 4.50 g EC (5-15 mPa⋅s) and 3.50 g EC (30-50 mPa⋅s) was dissolved in absolute ethanol to form 10% EC solution. 0.40 g nano-Ag-TiO₂ composites and 1.40 g terpineol were added to 2.00 g EC solution. The mixture was sonicated in three steps, each for 30 minutes. The final solution was heated in a vacuum oven at 40°C for 10 hours to remove the ethanol and water.

2.3.2. Photoanode Ag-TiO₂ Preparation

The FTO glass, as a current collector, was first cleaned in a detergent solution via ultrasonic for 15 minutes, and then rinsed with distilled water and ethanol. The FTO glass was soaked into a 40 mM TiCl₄ solution at 70°C for 30 minutes and re-washed with distilled water and ethanol. The Ag-TiO₂ paste with a thickness of 12-14 μm was coated on FTO substrate by using screen-printed method. After screen-printing, these coated electrodes were heated at 500°C under airflow for 30 minutes to form the Ag-TiO₂ photoanode.

2.3.3. Platinum Cathode Preparation

The FTO glass was treated in 0.1 M HCl in ethanol in an ultrasonic bath for 15 minutes and washed with acetone. The platinum cathode on the FTO substrate (Pt/FTO) was prepared by the screen-printing method using platinum paste PT1. The cathode Pt/FTO was annealed at 450°C for 30 minutes.

2.3.4. DSC Assembly

Both electrodes were arranged into sandwich-type cells by using a ply of melted surlyn at 190°C for 30 seconds. The dye solution (10 mM N719 in DMF) was injected successively into the cells through a hole in the back of platinum cathode, soaking in 4 hours and removing the DMF solvent. The cell was cleaned with acetonitrile for three times before being injected with electrolyte. The hole was then scaled using a quick-drying adhesive. The DSC assembly was performed in a nitrogen-filled glove-box to avoid oxygen and water.

2.4. Structural Characterization

The crystalline structures of nano-Ag-TiO₂ composites were characterized by X-ray diffractometer D8 Advanced (Bruker, Germany) with a copper anode ($\lambda \text{K}\alpha =1.54\text{Å}$). The XRD patterns were acquired in the 2$\theta$ range of 20°-80° (0.02° per second). The particle size of nano-Ag-TiO₂ was analyzed by transmission electron microscopy (TEM) images on a TEM 1400 (JEOL, Japan). The UV-vis spectra of nano-Ag-TiO₂ composites were recorded on a UV-vis spectrophotometer (Jasco-V630, Japan). The chemical composition of the materials was analyzed by EDS method using HITACHI S-4800 FE-SEM/EDS instrument.

2.5. $I$-$V$ Characterizations

The photovoltaic characteristics ($I$-$V$) of the cells were recorded by Keithly 2400 source meter. The light source was a solar simulator from a 450 W halogen lamp with an infrared filter (AM 1.5). The incident light intensity was 1000 W/m² calibrated with a standard Si solar cell. Electrochemical impedance spectroscopy (EIS) of DSCs was carried out by an AUTOLAB 302N apparatus (Ecochemie, Netherlands) in the frequency range of 0.1 Hz-100 kHz and under illuminations of 1000 W/m².

3. Results and Discussion

3.1. Structural Characterization of Nano-Ag-TiO₂ Composites

Figure 1 illustrates the XRD patterns of commercial TiO₂ (Degussa P25) and nano-Ag-TiO₂ composites. All diffraction peaks can be indexed in the anatase phase (Tetragonal, space group I41/amd) and rutile phase (Tetragonal, space group P42/mnm). Structural conservation of TiO₂ indicates that the $\gamma$-irradiation with Co-60 irradiator does not affect the crystalline structure of TiO₂ as well as the ratio of anatase phase and rutile phase. No diffraction peak of Ag was observed in XRD patterns of the Ag-TiO₂ samples due to the low content of Ag nanoparticles (below 1%). To determine the existence of Ag naoparticles in composites, other techniques (TEM, EDS, and UV-VIS) were applied.

[figure omitted; refer to PDF]

Figure 2 exhibits the TEM images of the nano 0.75 Ag-TiO₂ composite. We observed the well-defined TiO₂ nanoparticles (bright color) in the range 10-25 nm and the nano-Ag (dark color) on the background of TiO₂ particles. The EDS pattern of 0.75 Ag-TiO₂ (Figure 3) composite powder confirms the existence of Ag on the composite.

[figures omitted; refer to PDF]

[figure omitted; refer to PDF]

Figure 4(a) shows the UV-vis spectra of the samples in powder. We observed that the band-edge absorption of nano-Ag-TiO₂ composites shifted towards the red wavelength (redshift) and the plasmon resonance effect of the silver nanoparticles appeared in the range of wavelength 500-550 nm. The results verified the formation of nano-Ag on TiO₂ by the gamma Co-60 irradiation. Based on the Kubelka-Munk plot, the bandgap energy ($E_{\text{g}}$) of nano-Ag-TiO₂ composite in powder (Table 2) dropped slightly as compared to TiO₂ ($E_{\text{g}}=3.1\text{eV}$) following the increase of Ag content.

[figures omitted; refer to PDF]

Table 2

Bandgap energy ($E_{\text{g}}$) of the as-prepared nano-Ag-TiO₂ composites in powder and Ag-TiO₂ photoanode films, calculated by the Kubelka-Munk plot.

	Bandgap (eV)
	As-prepared nano-Ag-TiO2 composites	Ag-TiO2 photoanode films
TiO2	3.1	3.1
0.25 Ag-TiO2	3.1	3.1
0.50 Ag-TiO2	3.0	3.1
0.75 Ag-TiO2	2.8	3.1

Following the fabricating process of photoanodes, nano-Ag-TiO₂ composites were calcinated at 500°C for 30 minutes. We keep track of the photoproperties of photoanodes, with the UV-vis spectra visible in Figure 4(b). The UV-vis spectra of photoanodes in Ag-TiO₂ changed significantly. The disappearing of the plasmonic effect in nanosize, as well as the blueshift, was observed due to the agglomeration of Ag nanoparticles after the annealing process. The calculated bandgap of the four photoanodes was approximated in 3.1 eV. Many researches indicated the role of plasmon resonance effect of Ag nanoparticles to increase the performance of DSCs [11, 16, 19, 25]. The lack of plasmonic effect of low Ag content was detailed in DSCs Performance section.

3.2. DSCs Performances

We fabricated the DSCs using nano-Ag-TiO₂ composites as well as TiO₂-P25 as the photoanode and studied the photoperformance under the 1000 W/m² intensity light. The DSCs’ performances’ results were gathered in Figure 5 and Table 3. The DSCs assembled from TiO₂-P25 photoanode receive a short-circuit current ($I_{\text{sc}}$) of 8.12 mA/cm², open-circuit voltage ($V_{\text{oc}}$) of 0.77 V, and fill factor (FF) of 67%; the overall photocurrent conversion efficiency ($ɳ$) is calculated to be 3.75%. In the case of Ag-TiO₂ photoanodes, the photoperformance of DSCs essentially increased. It is noted that the open-circuit voltages ($V_{\text{oc}}$) were nearly unchanged and stabilized around 0.77 V, indicating that the energy structure of photoanodes (Fermi level) is unvaried. The short-circuit current ($I_{\text{sc}}$) enhanced gradually with the Ag content; particularly, the 0.75 Ag-TiO₂ photoanode exhibited the highest $I_{\text{sc}}$ to 9.56 mA/cm² as compared to 8.12 mA/cm² with TiO2-P25 photoanode. The photoefficiency ($\eta$) also improved significantly to 4.86%. We believed that the Ag nanoparticles at low content played as the electron-bridge between TiO₂ and the current collector FTO which limited the grain boundaries’ effect across the TiO₂ matrix. We described the mechanism of photoelectron transfer of Ag-TiO₂ photoanodes in DSCs in Figure 6. When the photoanodes Ag-TiO₂/N719 were illuminated under sunlight AM 1.5, N719 dye was excited and transformed to N719${}^{\ast }$ following by ultra-fast electron injection from N719${}^{\ast }$ to the conduction band (CB) of TiO₂ semiconductor. Due to the lower energy (Fermi level) of Ag-CB than TiO₂-CB, the photoelectrons can collect on Ag particles and transfer facilely in TiO₂ matrix to the current collector FTO. Moreover, the recombination of ${e_{\text{CB}}}^{–}$ and ${h^{+}}_{\text{VB}}$ on TiO₂ particles can be restricted which also enhances the photocurrent in DSCs.

[figure omitted; refer to PDF]

Table 3

Performance parameters of DSCs based on photoanodes nano-Ag-TiO₂ and TiO₂-P25.

Sample	$V_{\text{oc}}$ (V)	$I_{\text{sc}}$ (mA/cm2)	FF	$ɳ$ (%)
TiO2	0.77	8.12	0.67	3.75
0.25 Ag-TiO2	0.79	8.90	0.67	4.83
0.50 Ag-TiO2	0.79	9.25	0.67	4.88
0.75 Ag-TiO2	0.77	9.56	0.64	4.86

[figure omitted; refer to PDF]

To clarify the role of Ag-TiO₂ photoanodes on the photoefficiency of DSCs, the electrochemical impedance spectroscopy (EIS) was performed at the $V_{\text{oc}}$ under illuminations of 1000 W/m² in the frequency range of 0.1 Hz-100 kHz. The Nyquist plots and Bodes plots are presented in Figure 7; the analysis of EIS spectra is detailed in Table 4. The equivalent circuit is given in the inset of Figure 7. The EIS spectra of DSCs in Nyquist plot (Figure 7(a)) show two semicircles, corresponding to two processes: (i) electron transfer in cathode platinum and (ii) electron transfer in TiO₂ network and from TiO₂-CB to triiodide in electrolyte via reaction (1), called recombination-process. $$\begin{array}{}\text{(1)}& {\text{TiO}}_2/\text{e}+{{\text{I}}_3}^{-}\to {\text{TiO}}_2+3{\text{I}}^{–}\end{array}$$

[figures omitted; refer to PDF]

Table 4

Cathode charge transfer resistances ($R_{\text{ct}}$) and recombination resistances ($R_{\text{r}}$) of DSCs measuring at $V_{\text{oc}}$ under 1 sun illuminate.

Sample	$R_{\text{Pt}}$ (ohms)	$R_{\text{r}}$ (ohms)	$C_{\mu }$ (μF)	${\tau }_{\text{n}}$ (mS)	$f_{\max }$ (Hz)
TiO2	2.9	43.1	210	7.4	37.3
0.25 Ag-TiO2	4.1	36.5	233	7.6	37.3
0.50 Ag-TiO2	4.5	36.6	230	7.2	37.3
0.75 Ag-TiO2	3.7	36.5	235	6.7	37.3

At high frequencies, we observed a negligible variation of electron transfer resistance ($R_{\text{Pt}}$) in cathode platinum. At intermediate frequencies, the recombination resistances ($R_{\text{r}}$) were decreased drastically (from $43.1\Omega$ for photoanode TiO₂ and $36.6\Omega$ for photoanodes Ag-TiO₂) that reveal the role of Ag electron-bridge to facilitate the electron transfer in TiO₂ network. According to the research of Wang et al. [4], the reducing recombination resistance $R_{\text{r}}$ reflects the fast electron transfer in the photoanode whereby the photoperformances were beneficial. Moreover, the chemical capacitance of conduction band electron ($C_{\mu }$) was also increased that indicated the electron lifetime (${\tau }_{\text{n}}$)—composed of resistance and capacitance (${\tau }_{\text{n}}=R_{\text{r}}{C}_{\mu }$)—was slightly decreased. Furthermore, the characteristic frequency of $R_{\text{r}}$ was stabilized at 37.3 Hz in Bode plots (Figure 7(b)), indicating stable free electron lifetime.

4. Conclusions

In conclusion, we demonstrate the direct preparation of nano-Ag-TiO2 composites by the $\gamma$-irradiation method from a Co-60 irradiator and the Ag-TiO₂ showed a capability as photoanode in dye-sensitized solar cells. The DSCs—based on nano-0.75 Ag-TiO₂ composite photoanode—presented an encouraging performance with $V_{\text{oc}}$ of 077 V, $I_{\text{sc}}$ of 9.56 mA/cm², fill factor of 0.64, and photoefficiency of 4.86%. Studying the role of Ag by EIS, we observe only a reduction of recombination resistance in photoanode due to the formation of Ag electron-bridge that improves the electron transfer process but do not reduce electron lifetime.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

This research work was supported by Vietnam National University Ho Chi Minh City through grant number HS2015-18-01.

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