Enhanced photovoltaic performance of silver@titania plasmonic photoanode in dye-sensitized solar cells †

In the present investigation, silver@titania (Ag@TiO 2 ) plasmonic nanocomposite materials with di ﬀ erent Ag content were prepared using a simple one-step chemical reduction method and used as a photoanode in high-performance dye-sensitized solar cells. Transmission electron microscopic images revealed the uniform distribution of ultra-small Ag nanoparticles with a particle size range of 2 – 4 nm on the TiO 2 surface. The incorporation of Ag on the TiO 2 surface signi ﬁ cantly in ﬂ uenced the optical properties in the region of 400 – 500 nm because of the surface plasmon resonance e ﬀ ect. The dye-sensitized solar cells (DSSCs) assembled with the Ag@TiO 2 -modi ﬁ ed photoanode demonstrated an enhanced solar-to-electrical energy conversion e ﬃ ciency (4.86%) compared to that of bare TiO 2 (2.57%), due to the plasmonic e ﬀ ect of Ag. In addition, the Ag nanoparticles acted as an electron sink, which retarded the charge recombination. The in ﬂ uence of the Ag content on the overall e ﬃ ciency was also investigated, and the optimum Ag content with TiO 2 was found to be 2.5 wt%. The enhanced solar energy conversion e ﬃ ciency of the Ag@TiO 2 nanocomposite makes it a promising alternative to conventional photoanode-based DSSCs.


Introduction
Renewable energy sources are the most important approaches for and signify an important method for gaining independence from fossil fuels.Utilizing solar energy is certainly one of the most viable ways to solve the world's energy crisis.Dyesensitized solar cells (DSSCs) have emerged as promising candidates for harnessing solar power because of their low cost, exibility, ease of production, relatively high energy conversion efficiency, and low toxicity to the environment. 1Since rst being introduced by Gratzel and co-workers in 1991, many strategies have been employed to achieve high-performance DSSCs, including novel counter electrodes, electrolytes, dyes, and semiconductor photoanode materials.Among these, the photoanode plays a crucial role in determining the cell performance.So far, titanium dioxide (TiO 2 )-based material is one of the most promising materials for a DSSC due to its low cost, abundance, nontoxicity, safety, large surface area for maximum dye uptake and matched energy and band structure. 2,3However, the major drawback associated with the use of TiO 2 is its random electron transport, which will cause the electronhole recombination process and hence affect the overall performance. 4,5][8][9] In the present decade, surface of TiO 2 has been modied with noble metal nanoparticles such as silver (Ag), with the aim of improving the efficiency of a DSSC.][15] To make use of economically viable Ag to boost the DSSC performance, it is essential to control both the size and distribution of the nanoparticles on the TiO 2 surface.Two method are commonly employed for Ag-TiO 2 nanocomposite preparation: (i) a two-step method involving the chemical and physical adsorption of preformed Ag nanoparticles on the TiO 2 surface [16][17][18] and (ii) the photoreduction of Ag on the TiO 2 surface. 19,20However, these synthetic methods are ineffective because of the aggregation of Ag nanoparticles in the rst method and the difficulty controlling the size of the Ag nanoparticles in the later one.
In the present study, we successfully developed a facile synthesis method to prepare uniformly distributed Ag nanoparticles deposited on TiO 2 using a simple one-step chemical reduction method without adding any stabilizer or surfactant for DSSC application.The as-prepared Ag@TiO 2 plasmonic nanocomposites were characterized using various suitable analytical techniques and used as photoanodes in the DSSCs.The Ag@TiO 2 plasmonic nanocomposite-modied photoanode showed an enhanced solar energy conversion efficiency compared to that of a bare TiO 2 -based DSSC.The effect of the Ag content on the DSSC performance was also investigated.There are many positive aspects of the Ag-modied TiO 2 , including the synergetic interaction of the Ag nanoparticles on the TiO 2 surface, surface plasmon resonance effect, reduction of the band gap, and enhancement of the charge transfer process.These multifunctional properties of the prepared Ag@TiO 2 plasmonic nanocomposite will lead to superior performance in a DSSC.

Materials and characterization techniques
Titanium dioxide (P25) was purchased from Acros Organics.Silver nitrate (AgNO 3 ) and sodium borohydride (NaBH 4 ) were purchased from Merck.Indium tin oxide (ITO) conducting glass slides (7 U sq À1 ) were purchased from Xin Yan Technology Limited, China.The N719 (Ruthenizer 535-bisTBA) dye and Iodolyte Z-100 were received from Solaronix.The crystalline phase of the samples was studied via X-ray diffraction (XRD; D5000, Siemens), using copper Ka radiation (l ¼ 1.5418 Å).The morphologies of the prepared samples were examined using a Hitachi-SU 8000 eld emission scanning electron microscope and JEOL JEM-2100 F high-resolution transmission electron microscope.The absorption spectra were assessed using a Thermo Scientic Evolution 300 UV-vis absorption spectrophotometer.Photoluminescence and Raman spectra were collected using a Renishaw inVia 2000 system with an argon ion laser emitting at 325 and 532 nm, respectively.X-ray photoelectron spectroscopy (XPS) measurements were performed using synchrotron radiation from beamline no.3.2 at the Synchrotron Light Research Institute, Thailand.

Synthesis of Ag@TiO 2 nanocomposite materials
The Ag@TiO 2 nanocomposite materials were prepared using a simple one-step chemical reduction method.Briey, 500 mg of TiO 2 were added to aqueous solutions that contained different amounts of AgNO 3 (1, 2.5, 5, 10, and 20 wt%).Each mixture was vigorously stirred for 30 min at room temperature.The reduction of Ag + was carried out by the drop-wise addition of NaBH 4 until the color changed to greenish yellow.The appearance of this greenish yellow color indicated the formation of the Ag@TiO 2 nanocomposite, and the solution was continually stirred for another 30 min.The Ag@TiO 2 nanocomposite was collected and washed with distilled water and ethanol several times by centrifugation.Finally, the product was dried in an oven at 60 C and stored under a dark condition.

Fabrication of Ag@TiO 2 -modied photoanode
Ag@TiO 2 modied photoanodes were fabricated using the following procedure.Initially, 300 mg of the Ag@TiO 2 nanocomposite was mixed in an ethanolic solution and stirred for 30 min.A 0.1 M quantity of TTIP was slowly introduced into the above reaction mixture and stirred until a homogenous solution was obtained.Finally, the Ag@TiO 2 nanocomposites were coated on a conducting side of the ITO using the doctor-blade technique with the aid of scotch-3M tape and the thickness of the lm was $12 mm.In order to obtain a stable photoanode, the lm was dried at room temperature, sintered at 150 C for 30 min in a muffle furnace, and then allowed to cool naturally to room temperature.

Fabrication of DSSCs and evaluation of their performances
The prepared Ag@TiO 2 plasmonic nanocomposite photoanodes were immersed in a ethanolic solution of 0.3 mM N719 (Ruthenizer 535-bisTBA) dye for 24 h at room temperature under a dark condition.The dye-adsorbed plasmonic photoanode was withdrawn from the solution and immediately but gently cleaned with ethanol.A platinum-sputtered ITO was placed on a dye-absorbed photoanode, and they were clamped rmly together.A redox electrolyte (Iodolyte Z-100, Solaronix) solution was introduced into the cell assembly by capillary action.An active area of 0.5 cm 2 was used to measure the cell performance.A 150 W Xenon arc lamp (Newport, Model 69907) containing a simulated AM 1.5G lter with a manual shutter was used as a light source throughout the experiments.Prior to testing the photovoltaic parameter, an Avaspec-2048 ber optic spectrophotometer was used to measure the light illumination intensity.The photocurrent signal measurements (J-V and J-T curves) and electrochemical properties of the fabricated DSSCs were studied by using a computer-controlled VersaSTAT 3 Electrochemical Workstation (Princeton Applied Research, USA).

Optical properties of Ag@TiO 2 nanocomposite materials
In the present synthetic method, the formation of the Ag@TiO 2 nanocomposite takes place through the adsorption of Ag + ions on the TiO 2 surface, followed by the chemical reduction of Ag + by NaBH 4 at room temperature in the absence of any stabilizer and surfactant (Fig. 1a).The physical appearances of the Ag@TiO 2 with different quantities of Ag (wt%) are shown in Fig. 1b, shows that the nanocomposite becomes darker in color with increasing Ag content.
The UV-vis absorption spectra of the TiO 2 and Ag@TiO 2 were recorded and are shown in Fig. 2. The TiO 2 did not show any absorbance in the visible region (Fig. 2A) because of the wide band gap ($3.2 eV).The deposition of Ag on the TiO 2 surface signicantly inuenced the absorption in the visible regions of 450 and 500 nm, which was due to the surface plasmon resonance (SPR) band of Ag nanoparticles. 19,21A considerable shi in the adsorption edge toward the visible region was also observed for the Ag@TiO 2 sample.The presence of Ag nanoparticles signicantly inuenced the visible light absorption properties of TiO 2 .The band-gap energy (E bg ) of the prepared TiO 2 and Ag@TiO 2 were calculated using a well-known Tauc's plot method. 22,23The relations of (ahn) 2 versus hn for the TiO 2 and Ag@TiO 2 are shown in Fig. 2B and C. It can be observed that the band-gap energy values of TiO 2 decreased from 3.36 eV to 3.22 eV with the addition of Ag nanoparticles.This may have been due to the presence of Ag, which decreased the absorbance band edge of TiO 2 close to the visible region.
Understanding the charge recombination process of a photoanode material is crucial because it can signicantly inuence the photovoltaic performance of a DSSC.The TiO 2 will absorb incident photons with sufficient energy equal to or higher than the band-gap energy.This will produce photoinduced charge carriers (h + /e À ), and the recombination of photoinduced electrons and holes will release energy in the form of photoluminescence.Hence, a lower PL intensity indicates less charge recombination.The TiO 2 showed a broad and high PL intensity at around 580 nm due to the high photoinduced charge carrier recombination, whereas the PL intensity was minimized upon the addition of Ag on the TiO 2 surface (Fig. 3A).This was mainly attributed to the formation of the Schottky barrier at the Ag-TiO 2 interface, which could act as an electron sink to efficiently prevent the electron-hole recombination process. 24The Ag@TiO 2 with 2.5 wt% Ag showed the lowest PL emission intensity, which indicated the least electron-hole recombination compared to Ag contents of 1, 10, and 20 wt% on the TiO 2 (Fig. 3B).

Crystalline properties of Ag@TiO 2 nanocomposite materials
The X-ray diffraction patterns indicated that the TiO 2 and Ag@TiO 2 were composed of mixed anatase and rutile phases Hence, the peaks were indistinguishable in Ag@TiO 2 .
To further evaluate the phase identication of the TiO 2 and Ag@TiO 2 , Raman spectroscopy was performed in the range of 100-1000 cm À1 , and the results are shown in Fig. 5.][27] This clearly indicated that the TiO 2 and Ag@TiO 2 nanoparticles contained a mixture of anatase and rutile phases.No signals related to Ag particles were identied for the samples because of the relatively low concentration of Ag loaded onto the TiO 2 and its weak Raman scattering.An interesting observation is that the peak intensities were reduced by the presence of Ag, but the position of the Raman signal remained the same and was broadened.This indicated that the interaction between the Ag and TiO 2 affected the Raman resonance of the TiO 2 . 28This observation showed the successful deposition of Ag on the TiO 2 surface without any phase transition. 22,27

XPS analysis of Ag@TiO 2 nanocomposite materials
The XPS spectra of the TiO 2 and Ag@TiO 2 were recorded to understand their chemical nature and are shown in Fig. 6.Fig. 6A shows the Ti 2p core level spectra for both samples, in which two peaks are observed at 454.1 and 459.9 eV corresponding to the binding energies of the Ti 2p 3/2 and Ti 2p 1/2 core levels due to the presence of the Ti(IV) state. 29Aer the deposition of the Ag nanoparticles, it is obviously observed that the Ti 2p peak was shied to lower binding energies due to its surrounding chemical environment. 30Fig. 6B shows the O 1s spectra of the TiO 2 and Ag@TiO 2 , and the binding energy of the O 1s state of the samples is located at 530.9 eV, which is assigned to the bulk oxides (O 2À ) in the P25 lattice.The O 1s is slightly shied to higher binding energies which indicate the increase in electron density around O atoms due to the interaction of residue precursor with TiO 2 . 21The binding energy found for the Ag 3d 5/2 and Ag 3d 3/2 levels are 367.5 and 373.5 eV, respectively (Fig. 6C), with a peak separation of 6 eV due to the  metallic silver. 21The XPS analysis provided support for the existence of elements such as Ti, O, and Ag in the nanocomposite materials.

Morphological studies of Ag@TiO 2 nanocomposite materials
The microscopic morphologies of the as-prepared samples were studied using FESEM, TEM, and HRTEM.Fig. S1a † shows the FESEM results for TiO 2 , which appear to be spherical with a uniform size.Upon the addition of Ag, no signicant change in morphology was observed for the lm (Fig. S1b †).
The EDX analysis results are shown in Fig. S1c, † which shows Ti, O, C, and Ag.The C peak found in the EDAX spectrum is a result of carbon tape.Further, TEM images of TiO 2 and Ag@TiO 2 (2.5 wt% Ag) were also recorded and are shown in Fig. 7a and b, respectively.The TEM image shows that the Ag@TiO 2 nanoparticles are spherical in shape, with the TiO 2 particles having a size range of 20-25 nm.Fig. 7b clearly shows the deposition of distributed spherical and smaller Ag nanoparticles (2-4 nm) on the surface of TiO 2 .Fig. 7c depicts the selected area electron diffraction (SAED) pattern of the nanocrytalline TiO 2 particles.This pattern clearly reveals bright concentric rings, which are due to the diffraction from the (211), ( 200), (004), and (101) planes of anatase TiO 2 .The lattice resolved HRTEM image of the Ag@TiO 2 (Fig. 7d) shows dspacing values for the lattice fringes of 2.28 A and 2.50 A, which correspond to the (200) and (101) rutile planes of TiO 2 , respectively; whereas the interplanar spacing of 1.89 A was assigned to the (200) plane of anatase TiO 2 .In Fig. 7e, the areas of bright contrast on the element maps correlate with the Ti, O, and Ag signal maps.

Photovoltaic performances of Ag@TiO 2 plasmonic nanocomposite-modied photoanode-based DSSCs
The photovoltaic performances of the Ag@TiO 2 plasmonic nanocomposite-modied photoanode-based DSSCs with different Ag contents were evaluated under simulated solar AM 1.5G irradiation.Their obtained photocurrent density and photovoltage (J-V) curves are shown in Fig. 8, and their evaluated photovoltaic parameters are listed in Table 1.The Ag@TiO 2 plasmonic photoanode (2.5 wt% of Ag) showed a higher efficiency (4.86%) than the unmodied TiO 2 (2.57%).The enhanced photovoltaic performance may have been due to the plasmonic effect and rapid interfacial charge transfer that arose from the Ag nanoparticles on the TiO 2 .The optimization of Ag on TiO 2 is essential from the economic and high-performance perspectives for a DSSC.Although increasing the Ag content on the TiO 2 surface beyond 2.5 wt% showed a decrease in the conversion efficiency to 3.59%, the observed results clearly revealed that the conversion efficiency of a DSSC was increased with an increase in the Ag content of the photoanode until it reached a maximum of 2.5%.Then, a further increase in the Ag content eventually led to the decrease in conversion efficiency (Fig. 8a and Table 1).The decrease in the efficiency at high Ag loading is due to the free standing/excess Ag in the composite may oxidized to Ag(I) 13,31 and eroded by the electrolyte. 13The oxidation of the Ag will act as new recombination centre, thus reducing the number of the charge carrier led to decrease in the J sc and V oc .Consequently, the overall conversion efficiency of the DSSC would have deteriorated.Moreover, the addition of Ag more than 2.5 wt% might result in decrease of the active surface area of TiO 2 interacted with the dye molecules.Hence, the recombination between the electrons and holes will increase leads to decrease of J sc value.Consequently, the overall conversion efficiency of the DSSC would have deteriorated.The relationships between the photovoltaic parameters and Ag content on the TiO 2 surface are represented in Fig. S2(a-c) † for better understanding.

Electrochemical behaviours of Ag@TiO 2 plasmonic photoanode-based DSSCs
In order to gain deeper insight into the interfacial charge transfer process within the fabricated DSSC, the electrochemical impedance spectra (EIS) were recorded in a frequency range between 0.01 Hz and 100 kHz, and are shown in Fig. 9.A well-dened semicircle in the middle-frequency region can be observed for the TiO 2 and Ag@TiO 2 based DSSCs.The intersection of a high-frequency semicircle at the real axis represents the equivalent series resistance of the device (R s ); the arc in the middle-frequency range between 1 and 1000 Hz represents the charge transfer resistance (R ct ) between the dye-adsorbed    photoanode and the electrolyte interface. 16,18From the Nyquist plot (Fig. 9A), the R s values for the TiO 2 and Ag@TiO 2 nanocomposite-based DSSCs are 21.03U and 23.96 U, respectively.The R ct value increased from 8.77 U to 9.01 U aer the addition of the Ag.This increase in the R ct value could affect the open-circuit voltage (V oc ) and ll factor (FF) of the DSSC device.Therefore, the origin of the higher J sc in the Ag@TiO 2 is expected to arise from the device resistance (R s ) and charge transport dynamics determined by the electron lifetime (s n ) and R ct .Based on the Bode phase plots in Fig. 9B, the frequency was apparently shied to a lower frequency region with the addition of Ag.The maximum frequencies (u max ) in the middlefrequency region of the Bode plots for TiO 2 and Ag@TiO 2 were 2511.89Hz and 1995.26Hz, respectively.Because u max is inversely associated with the electron lifetime s n ¼ 1/(2pf), 32,33 a decrease in u max indicates a reduced rate for the chargerecombination process in the DSSC.Electrons with longer s n values will survive from the recombination.Therefore, it will be characterized by a larger R ct .Furthermore, Table 2 and Fig. S3 † summarize the results of the Nyquist plot.The Ag@TiO 2 exhibited a faster electron transport time (s s ¼ R s C m ) 32,34,35 than the TiO 2 .Hence its electron lifetime (s n ¼ R ct C m ) 32,34,35 was signicantly increased and survived from the recombination.The photovoltaic performance of the DSSC is clearly reected by the charge collection efficiency (h c ) 32,34,35 derived from h c ¼ (1 + R s /R ct ) À1 .Eventually, the charge collection efficiency was signicantly increased with the addition of Ag.We can conclude that because of the longer s n and larger R ct , the devices fabricated using Ag@TiO 2 showed improved J sc values compared to TiO 2 .

Operation principle of Ag@TiO 2 plasmonic photoanode-based DSSC
The operation principle of the DSSC based on the Ag@TiO 2 -modied photoelectrode under illumination is shown in Fig. 10.During light irradiation, the dye absorbs incident light and promotes electrons to the excited state.An excited electron is injected into the conduction band of the TiO 2 nanoparticles.The dye is then oxidized by receiving the electron from the electrolyte through the redox system, and is ready to be used again.The electrolyte itself will regenerate via the platinum counter-electrode by an electron passing through the external circuit.In our study, the Ag deposited onto the TiO 2 surface not only acted as an electron sink for the photoinduced charge carriers but could also be used as a scattering element for plasmonic scattering to trap the light and near eld coupled with the dye molecules. 36This will eventually improve the optical absorption of the dye, resulting in a signicant enhancement of the photocurrent (Fig. 10).Whenever the TiO 2 comes to contact with Ag, both will undergoes Fermi level equilibration to each other, resulted in the formation of Schottky barrier and thus led to large numbers of electron are accumulated at the surface of the metal (Ag) nanoparticles.This accumulation of electrons on the Ag nanoparticles shied the position of the Fermi level closer to the conduction band of TiO 2 . 37The Schottky barrier formed between the Ag and TiO 2 helps to ow the electrons from the Ag to TiO 2 conduction band via rapid interfacial charge transfer process and were collected by the current collector (ITO), thus improve the photocurrent generation under irradiation. 38Once the electrons being transfer to the conduction band of TiO 2 , the photo-excited electrons from the dye, N719, will start to accumulate on Ag surface again.Furthermore, the formation of Schottky barrier at the Ag-TiO 2 interface, which could act as an electron sink, also will help to prevent the electron-hole recombination process. 24he similar report also available for the metal-semiconductor (Ag-TiO 2 ) photoanode sensitized with N719 based DSSC. 21

Conclusion
In conclusion, plasmonic silver nanoparticles modied titania (Ag@TiO 2 ) nanocomposite materials with various Ag contents were synthesized by simple chemical reduction method without using any stabilizer and surfactants.The as-prepared Ag@TiO 2 plasmoinc nanocomposite materials were used as photoanode in the dye-sensitized solar cells to investigate the solar to electrical energy conversion ability.The incorporation of Ag on the TiO 2 surface signicantly inuenced the optical properties in the region of 400-500 nm because of the surface plasmon resonance effect and the formation of 2-4 nm sized Ag nanoparticles on the TiO 2 was conrmed through the HRTEM.The DSSC assembled with the Ag@TiO 2 -plasmonic photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency (4.86%)  than that of bare TiO 2 (2.57%) under an AM 1.5G simulated solar irradiation of 100 mW cm À2 , due the surface plasmon resonance effect of Ag nanoparticles present in the nanocomposites.The inuence of the Ag content on the overall efficiency was also investigated, and the optimized Ag content with TiO 2 was found to be 2.5 wt%.The enhanced solar energy conversion efficiency of the Ag@TiO 2 plasmonic nanocomposite makes it a promising alternative to conventional photoanode-based DSSCs.

Fig. 1
Fig. 1 (a) Schematic representation of formation of Ag on TiO 2 surface and (b) physical appearance of as-prepared Ag@TiO 2 nanocomposites with different Ag content.

Table 2
Electrochemical parameters of TiO 2 and Ag@TiO 2 nanocomposite-based DSSCs a Photoanode R s (U) R ct (U) C m (mF) s s (ms) s n (ms) h c (%) a The electrochemical impedance spectra (EIS) were recorded at an applied bias of À0.7 V in the frequency range of 0.01 Hz to 100 kHz.R s : device resistance; R ct : charge transfer resistance; C m : chemical capacitance; s s : electron transport time; s n : electron lifetime; h c : charge collection efficiency.