Enhanced light-harvesting by plasmonic hollow gold nanospheres for photovoltaic performance.

A 'sandwich'-structured TiO2NR/HGN/CdS photoanode was successfully fabricated by the electrophoretic deposition of hollow gold nanospheres (HGNs) on the surface of TiO2 nanorods (NRs). The HGNs presented a wide surface plasmon resonance character in the visible region from 540 to 630 nm, and further acted as the scatter elements and light energy 'antennas' to trap the local-field light near the TiO2NR/CdS layer, resulting in the increase of the light harvesting. An outstanding enhancement in the photochemical behaviour of TiO2NR/HGN/CdS photoanodes was attained by the contribution of HGNs in increasing the light absorption and the number of electron-hole pairs of photosensitive semiconductors. The optimized photochemical performance of TiO2NR/HGN/CdS photoanodes by using plasmonic HGNs demonstrated their potential application in energy conversion devices.

Introduction

Recently, quantum dot-sensitized solar cells (QDSSCs) have attracted increasing interest and are considered as a potential alternative to silicon-based photovoltaic devices [1-3]. Titanium dioxide (TiO2) with a wide-band gap (approx. 3.2 eV) is the most commonly used photoanode material in solar cells, which absorbs only ultraviolent light [4-6]. Generally, the light harvesting of QDSSCs is mainly determined by the quantum dot (QD) sensitizers, such as CdS [7-9], CdSe [10-12], CdTe [12] and PbS [13]. Narrow-bandgap QDs have high absorption cross section and absorb light in the visible to infrared (IR) range, which can be combined with wide-bandgap TiO2 semiconductors to form a heterostructure and allow excellent charge transport [14-22].

In the past decades, considerable efforts to augment photovoltaic performance in QDSSCs by perfecting the counter electrode [23], electrolytes [24] and sensitizers [7-13] have been implemented. Noble metal nanoparticles (NPs), especially Au and Ag nanocrystals, have been given significant attention for their ability to couple incident photons in the visible to near IR region through the creation of surface plasmon resonances (SPRs) [25,26]. Plasmonic nanostructures can act as subwavelength scattering elements to fold light into a thin absorber, and also be used antennas to concentrate light energy surrounding them, which finally act as the electron sink for facilitating charge separation [27]. The plasmonic properties can be controlled by the type of the metal, size, shape, surrounding dielectric medium, distance between neighbouring objects and configuration of their ensemble [28-30]. For instance, Zhang et al. decorated CdSe-TiO2 photoanode with Au NPs and found that the photocurrent increased to 85% and the photoconversion efficiency increased to 167% due to the electron sink of Au NPs [31]. Bardhan et al. demonstrated enhanced light harvesting in DSSCs with silica-coated Au nanocubes, resulting in a power conversion efficiency of 7.8% [32]. We also clarified the enhanced photovoltaic performance of CdS- or CdSe-sensitized photoanodes with plasmonic Au NPs as the efficient light scattering elements to couple and trap the sunlight [33,34].

Compared with the solid Au NPs, the hollow gold nanoparticles (HGNs) have stronger plasmon resonance and wider plasmonic tuning range [35,36]. However, as far as we know, there is no investigation of photovoltaic performance of DSSCs by employing HGNs as light scattering centres. In this paper, we introduce the HGNs into the TiO2NR (nanorod) structure by a facile electrophoretic technology (figure 1a). The CdS sensitizer was deposited on the TiO2NR/HGN surface by the successive ionic layer adsorption and reaction (SILAR) to construct the ‘sandwich'-structured photoanodes (figure 1b). The physico-chemical properties of TiO2NR/HGN/CdS photoanodes and their photovoltaic performances had been systematically studied, which would enable us to better understand SPR behaviour of HGNs in a photovoltaic material with adjustable light absorption and photovoltaic response.

Figure 1.

(a) Schematic of surface decoration strategy of TiO2 nanorods electrodes based on electrophoretic deposition in the HGNs colloidal solution. The DC voltage was 35 V. (b) Schematic of separation and transfer of electron-hole pairs in CdS QD-sensitized TiO2 solar cell with plasmonic HGNs.

Material and methods

Materials and reagents

FTO-coated glass (F : SnO2, resistivity of 14Ω/square and thickness of 300 nm, Nippon Sheet Glass, Japan) was used as transparent conducting oxide substrate. Chloroauric acid (HAuCl4·3H2O, 99.99%), sodium borohydride (NaBH4), cadmium nitrate (Cd(NO3)2), sodium sulfide (Na2S), sodium sulfite (Na2SO3) and titanium butoxide (Ti(OCH2CH2CH2CH3)4, 99%) were purchased from Sigma-Aldrich Co. Trihydrate sodium citrate (TCD, C6H5O3N.2H2O, purity 99%), cobalt chloride hexahydrate (CoCl2·6H2O), hydrochloric acid (HCl, 36%), ethanol (C2H6O), methanol (CH4O) and acetone (C3H6O) were supplied from Eagle Chemical Reagent Co. Ltd. All chemicals were of analytical grade and used as received. All aqueous solutions were prepared using deionized (D.I.) water with a resistivity of 18.2 MΩ cm prepared by Millipore Q purification system.

Preparation of citrate-stabilized hollow gold nanospheres

The citrate-stabilized HGNs are fabricated by using Co NPs as sacrificial templates with starting material of HAuCl4 [37,38], as reported by Zhang et al. [39]. In brief, 75 ml of D.I. water was firstly placed into a three-necked Erlenmeyer bulb with 0.1 mol l−1 TCD in a 35°C water bath, and the solution was deoxygenated by ultrapure argon. After 30 min, 100 μl of CoCl2 (0.4 mol l−1) was added into the flask under vigorous magnetic stirring (2500 r.p.m.), and a certain amount of fresh NaBH4 (1 mol l−1) was added into the mixture solution. The colour of solution was changed from pale pink to brown within tens of seconds, indicating the reduction of Co2+ to cobalt NPs. After the hydrolysis reaction of NaBH4 was finished, 30 ml Co NP colloidal solution was transferred to aqueous solution of HAuCl4 (0.6 mmol l−1, 10 ml) under continuous stirring. Finally, the as-prepared HGNs solution was centrifuged twice and re-dispersed into D.I. water. The solid gold nanoparticles (SGNs) used as a reference were synthesized by the classic Frens' method [35].

Preparation of photoanodes for photovoltaic cells

The FTO glass substrates (10 mm × 20 mm) were cleaned ultrasonically with acetone, ethanol and D.I. water, respectively, and placed into the Teflon-liner at an angle against the wall. Then 0.32 ml of Ti(OCH2CH2CH2CH3)4 was slowly added into the HCl solution within 5 min and transferred into the Teflon-lined stainless steel autoclave (50 ml) for TiO2NR growing at 150°C for 12 h. For the electrophoretic deposition of HGNs on the TiO2NR anode (figure 1b), a two-electrode system was used with the Pt foil as the cathode and the separation distance of two electrodes was 15 mm. A DC voltage of 35 V was applied for 3–13 min to infuse the HGNs on the TiO2NR surface in a 10 ml of HGN dispersion solution (0.04 mM) while ceaselessly stirred. Then the electrodes were dried at 60°C to improve the contact between HGNs and TiO2NR. The TiO2NR/HGN samples were further sensitized in situ with CdS by SILAR deposition by immersing in 0.1 mol l−1 Cd(NO3)2 ethanol solution and 0.1 mol l−1 Na2S methanol : D.I. water solution (1 : 1 by volume) several times. Various quantities of HGNs and CdS were easily controlled by changing the deposition time of HGNs and SILAR cycles for CdS to optimize the photovoltaic performance of photoanodes. In addition, the FTO/TiO2NR/CdS photoanode without HGNs and the FTO/TiO2NR/SGN/CdS photoanode were also prepared as control to explore the effect of HGNs in the photoelectrochemical behaviours of photoanodes.

Characterization

The morphologies of different samples were characterized by transmission electron microscopy (TEM, Philips CM 300 FEG) at 200 kV, and scanning electron microscopy (SEM, Hitachi S4800, Japan) with an energy-dispersive X-ray (EDX) spectrometer. X-ray diffraction (XRD) pattern of sample was determined by a diffractometer (Bruker AXS D8) with Cu Kα radiation (λ = 0.15418 nm) at an accelerating voltage of 40 kV and applied current of 40 mA. The extinction spectra of samples were measured by the UV–vis spectrophotometer (Hitachi U3900).

Photoelectrochemical performances of samples were performed on a CHI 630D electrochemical system (Chenhua Instruments, Shanghai, China). The system consisted of three electrodes and a transparent quartz cell, which was filled with Na2S (0.2 mol l−1) and Na2SO3 (0.2 mol l−1) electrolyte (25 ml). A Pt foil was used as a counter electrode with Ag/AgCl/KCl as a reference electrode. All the samples (10 mm × 20 mm) with impregnated area of 1.5 cm2 were employed as working electrodes. The curves of photovoltage and photocurrent versus times were recorded by the electrochemical system using a 150 W Xe lamp (filtered, λ ≥ 300 nm) as the light source. The illumination intensity near the sample surface was about 100 mW cm−2. Incident photon conversion efficiency (IPCE) was measured on an IPCE measurement system (Solar Cell Scan 100, Zolix) with a 150 W Xe lamp aligned into a monchromator scanning. All measurements were carried out in ambient air and at room temperature without encapsulation.

Results and discussion

Microstructures of TiO2NR/HGN/CdS photoanode

The TEM image of concentrated HGNs is shown in figure 2a. The strong contrast difference with a bright centre and a much darker edge confirms their hollow architecture, and their average outer diameter and inner one are about 33 ± 2.7 nm and 19 ± 2.3 nm, respectively, as observed from the size distribution histograms (figure 2b). Figure 2c displays the SPR absorption band centred at 575 nm, which corresponds to HGNs [35,39,40]. The XRD pattern of the as-prepared HGNs on the glass substrate (figure 2d) shows four diffraction peaks at (2θ) 38.0°, 44.2°, 64.6° and 77.8°, respectively, which correspond to (111), (200), (220) and (311) crystallographic planes [41], confirming the face-centred cubic gold (JCPDS, 04-0784).

Figure 2.

(a) TEM image, (b) size distribution histograms, (c) UV–vis absorption spectrum and (d) XRD spectrum of HGNs.

The SEM micrograph in figure 3a presents well-aligned TiO2NRs uniformly grown on the FTO substrates. The cross-sectional SEM image of TiO2NR with HGNs decorated in figure 3b indicates that the length of TiO2NR is about 1.75 µm. The HGNs distributed on the TiO2NRs surface are brighter than TiO2 (figure 3b,c). From the cross-sectional and surface SEM images of TiO2NR/HGN/CdS sample, we can see that many CdS QDs are well adsorbed onto the side walls of TiO2NR/HGN via SILAR. The EDS result shows the existence of TiO2, CdS and gold (figure 3f). The XRD peaks at (2θ) 36.0° and 62.7° (figure 4a) correspond to (101) and (002) planes of rutile structure of TiO2 [42] (JCPDS, 21-1276), respectively. The XRD peaks at (2θ) 26.6° and 43.7° are assigned to the (111) and (220) planes of cubic CdS structure (JCPDS, 65-2887) in figure 4b,c. The crystallographic peaks of HGNs are indistinguishable in TiO2NR/HGN/CdS sample due to the low content of HGNs (figure 3d). Furthermore, the TEM and high-resolution TEM images for TiO2NR/HGN and TiO2NR/HGN/CdS were also recorded (figure 5). The (111) facet of gold (lattice fringe spacing of 0.236 nm) and (101) facet of TiO2NR (lattice fringe spacing of 0.314 nm) are easily distinguished from the high-resolution TEM image (figure 5b), and the (111) crystal face of CdS with the lattice fringe spacing of 0.336 nm is also shown in figure 5d.

Figure 3.

SEM images of (a) TiO2NR, (c) TiO2NR/HGN and (e) TiO2NR/HGN/CdS hybrids. Cross-sectional SEM micrographs of (b) TiO2NR/HGN and (d) TiO2NR/HGN/CdS hybrids. The electrophoretic deposition time of HGNs was 7 min, and the number of SILAR cycles applied to deposit CdS was 7. (f) EDS result of TiO2NR/HGN/CdS hybrid. The electrophoretic deposition time of HGNs was 9 min, and the number of SILAR cycles applied to deposit CdS was 7.

Figure 4.

(a) XRD patterns of TiO2NR, TiO2NR/CdS and TiO2NR/HGN/CdS. Detailed XRD patterns of these three samples in the range (2θ) (b) from 26° to 29° and (c) from 36° to 45°. The electrophoretic deposition time of HGNs was 13 min, and the number of SILAR cycles applied to deposit CdS was 7.

Figure 5.

TEM images of (a) TiO2NR/HGN and (c) TiO2NR/HGN/CdS hybrids. High-resolution TEM images of (b) TiO2NR/HGN and (d) TiO2NR/HGN/CdS hybrids.

Photovoltaic performance of TiO2NR/HGN/CdS photoanode

Figure 6 displays the UV–vis absorption spectra of different photoanodes. The absorption of bare TiO2NR is featureless above 380 nm, but TiO2NR/HGN shows an obvious absorption in the visible region between 550 and 700 nm with respect to TiO2NR, which is attributed to the plasmon resonance absorption peak arising from HGNs [35,43]. As compared to the HGNs colloid in D.I. water (figure 2c), the plasmon band of HGNs for TiO2NR/HGN is red-shifted and broadened owing to the high refractive index of rutile TiO2 (2.55–2.76) surrounding HGNs. Since the plasmon resonance frequency of HGNs is sensitively dependent on the surrounding matter, the local electromagnetic field enhancement around the HGNs might cause the red shift of absorption and increase light absorption of the surrounding photosensitizer [34]. This means that the optical feature of TiO2NR/HGN is a synergistic effect between TiO2NR and HGNs. The broad peak from 380 to 480 nm corresponds to the absorption of CdS QDs. Compared with TiO2NR/CdS, there is a significant absorption enhancement of TiO2NR/HGN/CdS photoanode in the region from 550 to 750 nm, which is attributed to the behaviour of HGNs. When considering the TiO2NR/HGN and TiO2NR/HGN/CdS photoanodes, the obvious red shift of the plasmon band of HGNs could be attributed to the HGNs-mediated near-field enhancement effect and near-field coupling with photosensitive semiconductor [34]. Mahmoud's study indicated that HGNs possessed stronger plasmonic electromagnetic field and wider plasmon resonant wavelengths than the special SGNs with same size via the finite difference time domain (FDTD) calculations, which hinted that the HGNs might be more suitable as the scatter centre to concentrate photons and enhance the light absorption when compared to the SGNs [44]. To explore the light-harvesting capabilities of HGNs, the optimum amount of the HGNs was found by varying the density of HGNs on the TiO2NR surface with the electrophoretic deposition time from 3 to 13 min (the number of SILAR cycles applied to deposit CdS was 4). It can be seen that as the amount of HGNs in the photoanodes increases (from 0.6 to 3.3 wt% obtained from EDS results), the absorption of TiO2NR/HGN/CdS in the visible region between 500 and 700 nm is intensified by the impact of SPR absorption peak arising from HGNs when the electrophoresis times of HGNs increases from 3 to 9 min (figure 7a), and tends to stabilize with the electrophoresis times rising up to 13 min. Besides, we observe a slight red shift in the optical absorption of TiO2NR/HGN/CdS, which may be attributed to an increase in HGNs clusters at a certain high amounts [45].

Figure 6.

UV–vis absorption spectra and photographs of TiO2NR, TiO2NR/HGN, TiO2NR/HGN/CdS and TiO2NR/CdS hybrids. The electrophoretic deposition time of HGNs was 13 min, and the number of SILAR cycles applied to deposit CdS was 7.

Figure 7.

(a) UV–vis absorption spectra, (b) photocurrent density, (c) open-circuit voltage and (d) IPCE of TiO2NR/HGN/CdS hybrids with varied amounts of HGNs conducted by varied electrophoretic deposition times. The number of SILAR cycles applied to deposit CdS was 4.

Furthermore, the photocurrent and photovoltage of TiO2NR/HGN/CdS photoanodes were examined by constructing a photoelectrochemical cell system with the Ag/AgCl/KCl as a reference electrode and platinum foil as a counter electrode in the Na2S/Na2SO3 electrolyte (0.2 mol l−1). The current densities in all samples are almost zero without photoillumination (figure 7b). Compared to TiO2NR/CdS, the TiO2NR/HGN/CdS samples show superior photoelectrochemical properties by introducing HGNs. The photocurrent responses of the TiO2NR/HGN/CdS photoanodes are strongly dependent on the deposition time of HGNs, in which the photocurrent of TiO2NR/HGN/CdS photoanode presents a first increased and then decreased tendency with the maximum obtained at the electrophoresis times of 7 min (TiO2NR/HGN7/CdS with 2.7 wt% HGNs obtained from EDS result). The photovoltage also shows a similar variation trend with the maximum obtained for the TiO2NR/HGN5/CdS or TiO2NR/HGN7/CdS system (figure 7c). The schematic representation of separation and transfer of electron-hole pairs in CdS QD-sensitized TiO2 solar cell with plasmonic HGNs is demonstrated in figure 1b. Upon photoillumination, CdS QDs will preferentially absorb incident light to produce photogenerated electron-hole pairs. The electrons are then injected into the conduction band of TiO2NR and further rapidly transported to the FTO substrate, while the holes are got by the electrolyte, resulting in the photocurrent in the circuit and open-circuit voltage. When the plasmonic HGNs are introduced into the TiO2NR/CdS system, the HGNs would absorb the incident light, scatter and trap it into the CdS film by multiple scattering, causing an increase of effective optical path length in the cell (figure 1b) [33,46]. This could favour the strong local-field enhancement around the HGNs and near-field coupling with the CdS semiconductor, increasing the absorption of CdS and the number of electron-hole pairs [27]. However, the intensity of localized electric field is weakened with the reduced ‘hot spots' when the HGNs greatly overlap [33] (as the recombination centres of the electrons and holes), resulting in the dramatic drop in the photovoltaic response for the TiO2NR/HGN9/CdS and TiO2NR/HGN13/CdS systems. The IPCE result measured in the 300–600 nm wavelength range (figure 7d) follows a similar trend to that observed in the photocurrent responses spectra (figure 7b). The IPCE spectral characteristics correspond well to the absorbance spectra of the CdS-sensitized photoanodes. It can be observed that HGN-decorated TiO2NR/CdS photoanodes show improved photoactivity from 380 to 530 nm compared with the TiO2NR/CdS, especially in the visible region from 450 to 530 nm due to the main contribution from HGNs. These results demonstrate that excitation of the HGNs SPR is responsible for the improved photoelectrochemical properties of TiO2NR/HGN/CdS structures. When using the SGNs with similar size to the HGNs, the photoelectrochemical properties of TiO2NR/SGN/CdS structures are obviously inferior to the TiO2NR/HGN/CdS structures, demonstrating the preponderance of HGNs in light harvesting.

For further optimize the photovoltaic performance of TiO2NR/HGN/CdS photoanode, the amount of CdS QDs was adjusted by changing the SILAR cycles. The UV–vis absorption spectra of TiO2NR/HGN/CdS photoanodes are enhanced by increasing the number of SILAR cycles (n = 2, 4, 6, 7) (figure 8a). Furthermore, the absorbance spectra related to plasmon mode of HGNs in the range from 550 to 700 nm show a red shift with the increasing number of SILAR cycles, indicating the increased coupling and synergistic effect between HGNs and CdS. Additionally, the photocurrent density and open-circuit voltage increase with the number of SILAR cycles up to 4 (figure 8b,c). However, those parameters further decrease when the number of SILAR cycles is up to 6 and 7. This phenomenon may be attributed to the following reasons: firstly, the excessive SILAR cycles would lead to the conglomeration of CdS QDs, resulting in the poor charge injection efficiency due to the large QDs; secondly, the aggregation of CdS QDs may form excessive grain boundaries between CdS NPs, which would act as potential barriers and enhance the recombination possibilities of electron-hole pairs, leading to a decrease in photovoltaic behaviours [34,47,48]; thirdly, the conglomeration of excess CdS crystal nucleus may hinder the diffusion of the electrolyte into the TiO2NR/HGN/CdS photoanode, which also limits the efficiency of charge separation and charge extraction, as shown in the IPCE responses (figure 8d).

Figure 8.

(a) UV–vis absorption spectra, (b) photocurrent density, (c) open-circuit voltage and (d) IPCE of TiO2NR/HGN/CdS hybrids with different numbers of SILAR cycles applied to deposit CdS. The electrophoretic deposition time of HGNs was 7 min.

Conclusion

In summary, the ‘sandwich'-structured TiO2NR/HGN/CdS photoanode was fabricated by using plasmonic HGNs as light concentrators to effectively enhance photovoltaic performance. The light scattering and trapping of plasmonic HGNs and the near-field coupling of plasmonic HGNs with the photosensitive layers increase the absorption and the number of photoproduced electron-hole pairs of semiconductor. The TiO2NR/HGN/CdS photoanode with optimized design presents an obvious predominance in the improvement of photovoltaic performances such as photocurrent, open-circuit voltage and incident photon-to-current conversion efficiency when compared to the TiO2NR/CdS one. The plasmonic light-trapping concept would have potential applications in QDs-sensitized film solar cells and related energy conversion devices.