TiO₂ NRA was grown on the fluorine-doped SnO₂-coated conducting glass (FTO) substrate (resistance 25 Ω/square, transmittance 85%) by a hydrothermal method as described in the literature 26 . Briefly, 30 mL deionized water was mixed with 30 mL concentrated hydrochloric acid (36.5% to 38.0% by weight). The mixture was stirred for 5 min followed by an addition of 1 mL titanium butoxide (98%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). After stirring for another 5 min, the mixture was transferred into a Teflon-lined stainless steel autoclave of 100-mL volume. The FTO substrate was placed at an angle against the wall of the Teflonliner with the conducting side facing down. After a hydrothermal treatment at 150°C for 20 h, the substrate was taken out and immersed in 40 mM TiCl₄ aqueous solution for 30 min at 70°C. The TiCl₄-treated sample was annealed at 450°C for 30 min.

As illustrated in Figure 1, the photodeposition procedure was conducted in two steps. Firstly, the as-prepared TiO₂ NRA was immersed into the ethanol solution containing Ag⁺. The solution was prepared by dissolving 0.2 g polyvinylpyrrolidone (K90, MW = 1,300,000, Aladdin Chemical Co., Ltd., Shanghai, China) in 20 mL pure ethanol, followed by adding 0.2 mL of AgNO₃ aqueous solution (0.1 M) dropwise. Irradiation was carried out from the direction of TiO₂ film with a high-intensity mercury lamp for a given period. After irradiation, the substrate was taken out, washed with ethanol, and transferred into methanol solution consisting 1 M Na₂S and 2 M S. The sulfurization reaction was conducted at 50°C for 8 h. Finally, the photoanodes were passivated with ZnS by dipping into 0.1 M Zn(CH₃COO)₂ and 0.1 M Na₂S aqueous solution for 1 min alternately.

The counter electrode was prepared by dripping a drop of 10 mM H₂PtCl₆ (99.99%, Aldrich Company, Inc., Wyoming, USA) ethanol solution onto FTO substrate, followed by heating at 450°C for 15 min. Ag₂S-sensitized TiO₂ nanorod (NR) photoanode and Pt counter electrode were assembled into sandwichstructure using a sheet of a thermoplastic frame (25-μm thick; Surlyn, DuPont, Wilmington, USA) as spacer between the two electrodes. The polysulfide electrolyte consisted of 0.5 M Na₂S, 2 M S, 0.2 M KCl, and 0.5 M NaOH in methanol/water (7:3 v/v). An opaque mask with an aperture was coated on the cell to ensure the illuminated area of 0.16 cm².

X-ray diffraction (XRD) measurements were carried out using a RAD-3X (Rigaku Corporation, Tokyo, Japan) diffractometer with Cu-Kα radiation. The morphology of the films was observed by field emission scanning electron microscopy (FESEM, S4800, Hitachi Ltd., Tokyo, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Beijing, China). To prepare the TEM sample, TiO₂ NRs together with Ag₂S QDs were scratched from the FTO substrate and dispersed in ethanol by sonication. The UV–vis absorption spectra of TiO₂ NRA and Ag₂S-deposited TiO₂ NRA were recorded in the range from 350 to 800 nm using a Hitachi U-3010 spectroscopy. The photocurrent density-voltage (J-V) characteristics of solar cells were examined by a Keithley 2400 sourcemeter (Keithley Instruments, Inc., Cleveland, USA) under illumination by a solar simulator (AM 1.5 G). Incident light intensity was calibrated by standard silicon solar cell and light intensity meter (FZ-Aradiometer) simultaneously. The stability of the solar cell was measured by electrochemical workstation (pp211; Zahner, Elektrik GmbH & Co.KG, Kronach, Germany) with continuous illumination on the solar cell.

Figure 2 shows the FESEM images of TiO₂ NRA grown on the FTO substrate (FTO/TiO₂) viewed from top (a) and cross-section (b). The TiO₂ film is composed of separate NRs with consistent orientation, forming a uniform array that covered the entire surface of the substrate. The top view of FTO/TiO₂ shows that the top surface of NRs contains many step edges facilitating further growth. The NRs are tetragonal in shape with square top facets, consistent with the growth habit of tetragonal crystal structure. The average side length of the top squares is 200 nm, and the space between them is about the same size. The cross-section view of FTO/TiO₂ shows that the NRs are 2 to 3 μm in length with smooth sides. At the bottom of the TiO₂ NRA, a thin layer composed of short disordered NRs adhering to the FTO substrate is found. The compact layer may reduce the recombination of electron from the FTO to the electrolyte in the working course of QDSSCs by segregating them.

The photodeposition of Ag₂S QDs was conducted by two separate processes: photoreduction of Ag⁺ to Ag and sulfurization of Ag to Ag₂S. Photocatalytic properties of TiO₂ play an essential role in the reduction of Ag⁺. The mechanism of TiO₂ photocatalytic-reduction metal ions was described in the literature 27 . The main reaction processes of photoreduction Ag⁺ are as follows (reactions 1 to 4): (1) Typically, TiO₂ surface exhibits strong adsorptivity for Ag⁺, and the adsorption equilibrium is reached soon after immersing FTO/TiO₂ in Ag⁺ ethanol solution in the dark. (2) UV irradiation (λ < 400 nm) excites TiO₂ to generate electron–hole pairs. (3) The electrons reduce the Ag⁺ adsorbed preferentially on the surface to Ag, (4) while the holes are irreversibly scavenged by ethanol. Continuous reduction of Ag⁺ can produce Ag nucleates on the surface of TiO₂ forming a Schottky junction between them. The charge-separation generated electrons are partially transferred to the Ag clusters from TiO₂ 28 . Oxidation and reduction processes are carried on at the surface of TiO₂ and Ag, respectively, as illustrated in Figure 3. Consequently, the reduction on the surface of Ag enables the crystal nucleus to grow up. After the photoreduction, the sulfurization reaction of Ag clusters occurs spontaneously, owing to the low reaction Gibbs energy of −47.1 kJ/mol 29 .

TiO 2 + Ag + → TiO 2 Ag +

TiO 2 + hv → TiO 2 e − + h +

e − + Ag + → Ag

h + + C 2 H 5 OH → C 2 H 4 OH

Photoreduction rate of Ag⁺ by TiO₂ in ethanol solution is so rapid that the electrode turned to silvery-white within 3 min after immersing FTO/TiO₂ in the solution. To verify the effect of photocatalytic properties of TiO₂ on the reduction process, the ethanol solution containing Ag⁺ was irradiated in the same condition but in the absence of TiO₂, and no silver was observed in 10 h. Similar results were also observed when immersing FTO/TiO₂ in the Ag⁺ solution in the dark, consistent with the proposed photoreduction mechanism.

Figure 4 shows XRD patterns of FTO/TiO₂ (a), FTO/TiO₂/Ag (b), and FTO/TiO₂/Ag₂S (c) electrodes. XRD patterns of FTO/TiO₂ electrode reveal that the synthesized TiO₂ NRs are tetragonal rutile structure (JCPDS card no. 21–1276). The enhanced (101) peak indicates the NRs are well-crystallized and grow in consistent orientation. In the XRD pattern of FTO/TiO₂/Ag electrode (b), all peaks indexed as TiO₂ crystal have been weakened while the outstanding diffraction peaks of silver (silver-3C, syn JCPDS card no. 04–0783) emerged. This proves the large coverage of crystallized Ag on the surface of TiO₂ nanostructure as a result of the photoreduction process. As compared with curve b, the XRD pattern of FTO/TiO₂/Ag₂S electrode shows five diffraction peaks which agreed well with acanthite Ag₂S (JCPDS card no. 14–0072), suggesting a conversion of Ag to Ag₂S. Additionally, the outstanding peaks of Ag in curve b are not observed in curve c which indicates that the reaction between Ag and S has been completed thoroughly.

Figure 5 displays a SEM image of a top view of FTO/TiO₂/Ag₂S electrode with 10-min photoreduction (a) and a TEM image of single NR stripped from the FTO/TiO₂/Ag₂S electrode (b). The two images clearly show that TiO₂ NRs are coated by a layer of Ag₂S crystallites not only on the top surface but also on the four side faces. The top view of FTO/TiO₂/Ag₂S electrode shows that the small steps within the top face of TiO₂ NR observed in SEM image of FTO/TiO₂ electrode (Figure 2a) are invisible due to the coverage of Ag₂S crystallites. The interval between NRs is reserved as well as the ordered TiO₂ NRA structure. The TEM image (b) shows that the entire NR is coated with QDs from the bottom to the top. Most of the QDs that covered the surface of NR disperse well with an average diameter of 10 nm. A closer observation of the Ag₂S QDs attached with TiO₂ NR can be obtained by the high resolution transmission electron microscope (HRTEM) images (Figure 5c,d). The NR grows along the [001] direction, and lattice fringes with interplanar spacing d ₁₁₀ = 0.321 nm are clearly imaged. The Ag₂S QDs anchoring on the side surface of TiO₂ NR are composed of small crystallites as observed by the fringes which correspond to the (121) planes of Ag₂S.

Figure 6 shows the absorption spectra of FTO/TiO₂ electrode and FTO/TiO₂/Ag₂S electrodes with different photoreduction times (t _p). The absorption edge around 400 nm is consistent with bandgap of rutile TiO₂ (3.0 eV). While Ag₂S QDs are deposited on TiO₂ NRs, absorption spectra are successfully extended to visible wavelength. With t _p increasing from 3 to 15 min, the absorption range changes from 400 to 520 nm until covering the entire visible spectrum; moreover, the absorbance obviously increases. The bandgap of bulk Ag₂S is 1.0 eV. The redshift of absorption edge for FTO/TiO₂/Ag₂S electrodes with prolonged t _p indicates the fact that the size of Ag₂S QDs gradually increases, and the quantization effect of ultrasmall QDs gradually vanishes. The enhanced absorbance is due to the increased amount of deposited Ag₂S QDs.

Figure 7 shows J-V characteristics of solar cells fabricated with different photoanodes under AM 1.5 illumination at 100 mW/cm². The photovoltaic properties of these cells are listed in Table 1. TiO₂/Ag₂S cell with t _p = 3 min possesses a much higher J _sc and a decreased V _oc compared with bare TiO₂ solar cell. The increased J _sc value is attributed to the sensitization of TiO₂ by Ag₂S QDs, while the slightly decreased V _oc value is mainly due to the band bending between Ag₂S QDs and TiO₂. With t _p increasing from 3 to 10 min, the J _sc is promoted from 4.15 to 10.25 mA/cm². The improved J _sc value is caused by an increasing loading amount of Ag₂S QDs and a broaden absorption spectrum (as shown in Figure 6). Meanwhile, the V _oc values are slightly improved, which is probably due to electron accumulation within TiO₂ shifting the Fermi level to more negative potentials. The optimal solar cell performance is obtained with a η of 0.98% and a superior J _sc of 10.25 mA/cm² when t _p = 10 min. The J _sc value is much higher than those of other reported Ag₂S QD sensitized solar cells even though they were prepared by a TiO₂ nanoparticle matrix with a larger surface area. The enhancement in J _sc is a result of the synergy of larger QD loading amount and fine connection between QDs and TiO₂. Compared with typical QDSSCs based on other narrow bandgap semiconductors (e.g., CdS and CdSe), the V _oc values of Ag₂S-QDSSCs are quite low which are almost equivalent to half of the others (CdS-QDSSCs, 0.6 to 0.7 V). Despite of the high J _sc values owing to a broad absorption spectrum, η is limited by the low V _oc values. When t _p was elongated to 15 min, η decreases sharply with a halving J _sc and a lower Fill factor (FF). This phenomenon is speculated to be caused by too long deposition time which results in excess Ag₂S nanoparticles generated on TiO₂ NRs, consequently decreases effective electron injection and increases recombination rate. The slightly reduced FF as t _p increases also indicates that recombination rate rises with growing amount of loading Ag₂S nanoparticles.

The J-V curves of a Ag₂S QD-sensitized solar cell measured at three different light intensities are shown in Figure 8. The photovoltaic performance parameters are listed in Table 2. The η reaches a value of 1.25% at 47 mW/cm² solar intensity. The J _sc value accumulates to 11.7 mA/cm² as incident light intensity increases to 150 mW/cm² (150% sun). However, J _sc produced by per unit light power is decreased by a factor of 40.9 compared with lower light level condition of 47% sun. This suggests that the incident light is not effectively converted into electricity at a higher photon density, which may be attributed to a lower rate of photon capture due to the insufficient QDs loading on TiO₂ nanorods. By employing longer TiO₂ NRs, the response of the photocurrent should be promoted to be linear with the incident light intensity, and a higher conversion efficiency should be reached at full sunlight.

The photostability of Ag₂S-QDSSC was measured by illuminating it at 100 mW/cm² sunlight for 2 h and characterized by recording the J _sc and V _oc of the device (Figure 9). During illumination, the J _sc remained relatively steady with a drop less than 5%, and the V _oc fluctuated within 2%. This shows that the Ag₂S QDs are robust in resisting photo corrosion in the presence of polysulfide electrolyte, and the small decline is probably caused by the loss of the solvent.