Abstract

Electrochemical deposition as a liquid phase epitaxial growth method is widely used to fabricate different kinds of hierarchical structures. As a typical heterostructure, TiO2/PbS is widely utilized in the areas of photovoltaics and photocatalysis. Oriented TiO2 nanorod (NR) arrays can provide direct pathways for the electron transport of photoanode. However, the lattice mismatch between TiO2 NR sides and PbS is very large; PbS nanoparticles (NPs) only formed on the top of TiO2 NRs. To solve this problem, TiO2/CdS core/shell nanocables were firstly prepared electrochemically because the lattice ratio between TiO2 and CdS was 0.916; and then, PbS NPs were successfully deposited over CdS shells (the lattice ratio between CdS and PbS was 0.697) to form TiO2/CdS/PbS hierarchical heterostructures. Experimental results demonstrated that the CdS interlayer could effectively promote the growth of PbS NPs on the surface and improve the fill factor and short current density of the photoanodes.

1. Introduction

Various heterostructures have been utilized in the areas of photovoltaics [15] and photocatalysis [6, 7], such as CdS [8, 9], CdSe [10, 11], CdTe [12], PbS [2, 13], PbSe [1418], and AgSe [19] over the surface of TiO2 [2022] and ZnO [2325], with different morphologies such as multilayered films, core/shell nanocables, and spherical dots. Several different chemical methods, including electrochemistry [12, 26, 27], chemical bath deposition (CBD) [28, 29], assembly of quantum dots (QDs) by immobilization via organic linkers [30], and successive ionic layer adsorption and reaction (SILAR) [31, 32], are commonly used to fabricate these heterostructures. Among them, electrochemistry as a liquid-phase epitaxial growth method is easy to fabricate large-area devices with facile and labor-saving superiorities, particularly on nanorod (NR) arrays.

As one of these typical heterostructures, the TiO2/PbS heterostructure has already been widely studied. Chen and coworkers used bath deposition to fabricate a TiO2/PbS counter electrode for QD-sensitized solar cells [33]. Mali et al. fabricated solar cells via SILAR [34]. Sargent’s group utilized colloidal PbS QDs to form heterostructure solar cells [35, 36]. But the separation of the photocarriers in these structures often does not do well due to the low lattice ratio.

Here, we tried to electrically deposit PbS over TiO2 NR arrays which could provide direct pathways for photoelectron transporting from the points of injection at the interfaces of the heterostructures between the two different materials to the transparent conducting oxide (TCO) electrodes. However, we found that PbS nanoparticles (NPs) were only grown on top of the TiO2 NRs due to the low lattice ratio between the sides of TiO2 NR and PbS (about 0.495), so fully covered TiO2/PbS heterostructures were not realized. CdS/PbS heterostructures are also widely used in the area of photovoltaics [37, 38]. In this work, we coated a layer of CdS over TiO2 NRs by electrochemistry because the lattice ratio between TiO2 nanorod and CdS is only 0.916. Then, PbS QDs were deposited on the surfaces of core/shell TiO2/CdS nanocables to form TiO2/CdS/PbS heterostructures (the lattice ratio between CdS and PbS is 0.697). Due to the existence of the CdS interlayer, the fill factor and short current density of the photoanodes were greatly improved.

2. Experimental

2.1. Preparation of TiO2 NR Arrays

TiO2 NR arrays were fabricated through a hydrothermal synthesis [27, 39]. After mixing deionized water and concentrated hydrochloric acid (mass fraction 36.5-38%) of 60 mL each, 2 mL of titanium butoxide was added drop by drop into the solution under vigorous stirring at room temperature. A half hour later, 30 mL of the prepared precursor solution was transferred into a 100 mL stainless steel autoclave with a Teflon liner. And then, three pieces of FTO substrates which had been ultrasonically cleaned by a mixed solution (chloroform, acetone, and 2-propanol with a volume ratio of 1 : 1 : 1) for 60 min were placed with an angle against the wall (the conductive sides faced down) in the Teflon liner. The hydrothermal synthesis took place at 140°C for 14 h. When the autoclave was cooled to room temperature, the substrates were taken out, rinsed with deionized water, and dried in an oven at 150°C.

2.2. Preparation of TiO2/PbS Heterostructures

The preparation of the TiO2/PbS heterostructure was carried out by electrodeposition with a three-electrode system. A Pt sheet, a standard Ag/AgCl electrode, and the TiO2 NR arrays on FTO (4.5 cm2 working area) were used as the counter electrode, the reference electrode, and the working electrode, respectively. An electrolyte containing 0.1 M of Pb(NO3)2 and 0.1 M of thiourea in dimethyl sulphoxide (DMSO)/water (a volume ratio of 1 : 1) was kept at 90°C. After depositions from 2 to 30 min with a constant voltage of 0.5 V, the samples were taken out and washed with deionized water and ethanol, respectively.

2.3. Preparation of TiO2/CdS Heterostructures

The preparation of the TiO2/CdS heterostructure was carried out by electrodeposition with the same three-electrode system stated above. An electrolyte containing 0.2 M of Cd(NO3)2 and 0.2 M of thiourea in dimethyl sulphoxide (DMSO)/water (volume ratio of 1 : 1) was kept at 90°C. After depositions from 2 to 30 min with a constant voltage of 0.66 V, the samples were taken out and washed with deionized water and ethanol, respectively.

2.4. Preparation of TiO2/CdS/PbS Heterostructures

The preparation of the TiO2/PbS/CdS heterostructure was formed in two steps. (1) Following Section 2.3, the TiO2/CdS heterostructure was firstly fabricated. (2) Following Section 2.2, PbS QDs were deposited over TiO2/CdS nanocables.

2.5. Characterizations

Field emission scanning electron microscopy (FESEM, JEOL JSM-6700) was used to examine the microstructures of the samples. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images of TiO2/CdS heterostructures were taken by a JEM-2100F high-resolution transmission microscope. The absorption characterizations of all samples were measured with a UV-3150 spectrophotometer. A Rigaku D/max-2500 X-ray diffractometer (XRD) with Cu Kα radiation () was used at room temperature to analyze the crystal structures of TiO2 and CdS. With an electrochemical workstation (Corrtest CS150), photoelectrochemical properties were tested under the three-electrode system where the samples, Pt sheet, and Ag/AgCl electrode worked as the working electrode, counter electrode, and reference electrode, respectively, in a polysulfide redox couple (S2-/SO32-) electrolyte (0.25 M Na2S and 0.35 M Na2SO3 in deionized water), while a Zolix SS150 Solar Simulator was used as the illumination source with a power of 100 mW/cm2.

3. Results and Discussion

3.1. TiO2/PbS Heterostructures

Vertical TiO2 NR arrays fabricated on FTO by hydrothermal synthesis are shown in Figure 1(a). The TiO2 NRs grew orderly with a quadrangular prism morphology indicating well-crystallized structures. Their sides were very neat and smooth, which were different from their rugged tops (Figure 1(b)). Therefore, there were a lot of lattice defects at the tops. These NRs were about 2 μm long, and we could count that the planar density was 8-12 NRs per μm2.

With these as-prepared TiO2 NRs, PbS was deposited electrochemically, and the SEM images show the morphologies of samples deposited for 20 min (Figures 1(c) and 1(d)). In Figure 1(c), it is clear to see that the quadrangular prism NRs were covered by PbS NPs (diameter around 300 nm) which were obviously distinguished in the SEM image over the top of the TiO2 NRs.

3.2. TiO2/CdS/PbS Heterostructures

The CdS/PbS heterostructure has already been widely used in the areas of photovoltaic and photocatalysis. Here, we would insert a CdS layer between TiO2 and PbS to fabricate TiO2/CdS/PbS heterostructures for our photoanodes.

With the as-prepared TiO2 NRs, CdS was firstly deposited by the electrochemical method. Figures 2(a) and 2(b) show the morphologies of samples deposited with CdS for 30 min. The morphologies of the NRs changed from quadrangular prisms to cylinders whose diameters became ~50 nm larger, and the surface was no longer smooth. In Figure 2(b), it is clear to see that the quadrangular prism NRs were covered by CdS shells which were easily distinguished in the SEM image. Comparing with the smooth sides of TiO2 NRs, there were more defect centers which could be good for the epitaxial growth.

Furthermore, we deposited PbS over the prepared TiO2/CdS nanocables at the same conditions. In Figure 2(c), the SEM image shows the morphology of the sample deposited with CdS for 10 min and PbS for 10 min. It is clear to see that there are dense PbS QDs adhered all over the TiO2/CdS nanocables. Through these observations, the growth process is summarized in Figure 2(d). CdS grew over the surface of TiO2 NRs, and the core/shell TiO2/CdS structures were formed. The relative band edges of TiO2, CdS, and PbS are shown in Figure 2(e) with their band gaps, respectively, to be 3.20, 2.25, and 1.28 eV. When the TiO2/CdS/PbS heterostructure is formed, their band edges at the interfaces would be termed by Fermi level alignment shown in the right part of Figure 2(e).

The core/shell rod structure (nanocable) could also be seen from the TEM image with CdS deposited for 15 min (Figure 3(a)), while in the HRTEM image (Figure 3(c)) it is easy to distinguish the interface between TiO2 core and CdS shell and the spacings of TiO2 (001) and CdS (100) were 0.29 nm and 0.36 nm, respectively. In Figure 3(b), it shows the TEM image of TiO2/CdS/PbS QDs deposited with CdS for 10 min and PbS for 10 min. Except the spacings of TiO2 (001) and CdS (100), the spacing of PbS (200) is shown to be 0.30 nm (Figure 3(d)).

3.3. Phase Composition and Structure

In Figure 4, there are two new peaks of the XRD pattern of TiO2 NRs (blue line) compared with the pattern of FTO (black line), and the two peaks, respectively, correspond to the different planes of tetragonal phase rutile TiO2 (JCPDS 88-1175) exhibited as quadrangular prisms in the SEM images. The two predominant peaks at 36.4° and 63.2°, respectively, indexed to the (101) and (002) planes suggesting that the growth of the TiO2 NRs took place along their -axis on the FTO substrate proved in Figure 1(a). When CdS was electrochemically deposited (shell), two new diffraction peaks (orange line) appeared at 24.84° and 43.74° which, respectively, indexed to the (100) and (110) planes of the hexagonal CdS (JCPDS 77-2306). Meanwhile, the intensity of the peak around 36° was higher, because this peak was not only from the (101) plane of the tetragonal TiO2 but also from the (102) plane of the hexagonal CdS. When PbS was deposited (nanoparticle), there were several new diffraction peaks (green line) that appeared at 25.96°, 30.07°, 43.06°, 50.76°, and 53.41° which, respectively, indexed to the (111), (200), (220), (311), and (222) planes of the galena PbS (JCPDS 05-0592). The red line represents the XRD pattern of the TiO2/CdS/PbS heterostructure, and the typical peaks of all components could be found.

3.4. Morphologies Controlled by the Lattice Ratio

From the SEM and TEM images, TiO2 NRs grew along the [001] direction. The lattice constants of the TiO2 NRs (JCPDS 88-1175) and PbS NPs (JCPDS 05-0592) shown in Table 1 are, respectively, , , and . At the sides of the TiO2 NRs, the lattice ratio between TiO2 () and PbS () is 0.495. At the top of TiO2 NRs, the lattice ratio between TiO2 () and PbS () is 0.761. Therefore, PbS NPs were deposited on the top of TiO2 NRs.

Proved from the XRD pattern, the lattice constants of the CdS (JCPDS 77-2306) shown in Table 1 are , . The lattice ratio between TiO2 () and CdS () is 0.916. Because of the high lattice ratio, CdS could be deposited all over the TiO2 nanorods and the core/shell TiO2/CdS nanocables were therefore formed. The lattice ratio between CdS () and PbS () is 0.697, so PbS could easily deposit on CdS.

3.5. UV-Vis Absorption

The UV-Vis absorption spectra of all samples with different treatments are shown in Figure 5. Comparing the black line (TiO2) and the blue line (deposited with CdS for 5 min), it is clear to see that the absorption range was broadened from the UV region to the visible light region with CdS over TiO2 NRs. Comparing the blue line (deposited with CdS for 5 min) to the green (deposited with CdS for 10 min) and red lines (deposited with CdS for 20 min), the absorption ranges of TiO2/CdS were further broadened with the increase in deposition time (more CdS). When PbS was deposited, the absorption range was extended to the near infrared region shown as the wine line. With a larger area of absorption range, the photoanode could be excited by more photons and a higher short current density would be achieved.

3.6. Photovoltaic Performance of the Electrodes

In Figure 6, it shows the photocurrent density-output potential difference (J-V) curves of the photoanodes fabricated by different conditions. When PbS was deposited over TiO2 NRs (orange line), the photovoltage characteristics are given in Table 2 with the photocurrent density, output potential difference, and fill factor, respectively, to be 3.36 mA cm-2, 0.90 V, and 0.26. The rather low fill factor was consistent with the SEM images that PbS NPs just grew on the top of TiO2 NRs and they did not grow on the sides. Shown in Figure 6 and Table 1, the photocurrent density of TiO2/CdS increased at first and then decreased and the output potential differences had no obvious change with more CdS deposition time. The highest photocurrent density of TiO2/CdS was 2.17 mA cm-2 when CdS was deposited for 20 min. After TiO2/CdS/PbS formed, the absorption range was broadened and PbS NPs densely covered all over the nanocables. Thus, it is clear to see that the photocurrent density and fill factor were largely increased from 3.36 mA cm-2 to 7.83 mA cm-2 and from 0.26 to 0.63 (Figure 6 and Table 2).

4. Conclusions

With a suitable lattice distance between PbS and TiO2, CdS was selected to coat on the surface of TiO2 nanorods and then PbS QDs were epitaxially grown all over the surface of TiO2/CdS nanocables to form a TiO2/CdS/PbS heterostructure. This strategy solves the difficulty to directly grow PbS QDs on TiO2 NR arrays and makes use of the optoelectronic property of PbS QDs for superior photovoltage characteristics of the TiO2/CdS/PbS photoanode.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

There is no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51772123 and 11674127), Jilin Province Science Fund for Excellent Young Scholars (20170520129JH), and Board of Regents Support Funds Professorship.