Electrochemical Epitaxial Growth of TiO 2 / CdS / PbS Nanocables

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China Department of Chemistry and Physics, Louisiana State University, Shreveport, LA 71115, USA

As one of these typical heterostructures, the TiO 2 /PbS heterostructure has already been widely studied.Chen and coworkers used bath deposition to fabricate a TiO 2 /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 TiO 2 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 TiO 2 NRs due to the low lattice ratio between the sides of TiO 2 NR and PbS (about 0.495), so fully covered TiO 2 /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 TiO 2 NRs by electrochemistry because the lattice ratio between TiO 2 nanorod and CdS is only 0.916.Then, PbS QDs were deposited on the surfaces of core/shell TiO 2 /CdS nanocables to form TiO 2 /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.

Experimental
2.1.Preparation of TiO 2 NR Arrays.TiO 2 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 2propanol 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 TiO 2 /PbS Heterostructures.The preparation of the TiO 2 /PbS heterostructure was carried out by electrodeposition with a three-electrode system.A Pt sheet, a standard Ag/AgCl electrode, and the TiO 2 NR arrays on FTO (4.5 cm 2 working area) were used as the counter electrode, the reference electrode, and the working electrode, respectively.An electrolyte containing 0.1 M of Pb(NO 3 ) 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 TiO 2 /CdS Heterostructures.The preparation of the TiO 2 /CdS heterostructure was carried out by electrodeposition with the same three-electrode system stated above.An electrolyte containing 0.2 M of Cd(NO 3 ) 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.) electrolyte (0.25 M Na 2 S and 0.35 M Na 2 SO 3 in deionized water), while a Zolix SS150 Solar Simulator was used as the illumination source with a power of 100 mW/cm 2 .

Results and Discussion
3.1.TiO 2 /PbS Heterostructures.Vertical TiO 2 NR arrays fabricated on FTO by hydrothermal synthesis are shown in Figure 1(a).The TiO 2 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 μm 2 .
With these as-prepared TiO 2 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 TiO 2 NRs.
3.2.TiO 2 /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 TiO 2 and PbS to fabricate TiO 2 /CdS/PbS heterostructures for our photoanodes.
With the as-prepared TiO 2 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 TiO 2 NRs, there were more defect centers which could be good for the epitaxial growth.
Furthermore, we deposited PbS over the prepared TiO 2 / 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 TiO 2 /CdS nanocables.Through these observations, the growth process is summarized in Figure 2     Journal of Nanomaterials 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 TiO 2 core and CdS shell and the d spacings of TiO 2 (001) and CdS (100) were 0.29 nm and 0.36 nm, respectively.In Figure 3(b), it shows the TEM image of TiO 2 /CdS/PbS QDs deposited with CdS for 10 min and PbS for 10 min.Except the d spacings of TiO 2 (001) and CdS (100), the d 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 TiO 2 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 TiO 2 (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 TiO 2 NRs took place along their c-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 TiO 2 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 TiO 2 /CdS/PbS heterostructure, and the typical peaks of all components could be found.Proved from the XRD pattern, the lattice constants of the CdS (JCPDS 77-2306) shown in Table 1 are a = b = 0 4136 nm, c = 0 6713 nm.The lattice ratio between TiO 2 (a = 0 4517 nm) and CdS (a = b = 0 4136 nm) is 0.916.Because of the high lattice ratio, CdS could be deposited all over the TiO 2 nanorods and the core/shell TiO 2 /CdS nanocables were therefore formed.The lattice ratio between CdS (a = b = 0 4136 nm) and PbS (a = b = c = 0 5936 nm) 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 (TiO 2 ) 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   5 Journal of Nanomaterials with CdS over TiO 2 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 TiO 2 /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 TiO 2 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 TiO 2 NRs and they did not grow on the sides.Shown in Figure 6 and Table 1, the photocurrent density of TiO 2 /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 TiO 2 /CdS was 2.17 mA cm -2 when CdS was deposited for 20 min.After TiO 2 /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).

Conclusions
With a suitable lattice distance between PbS and TiO 2 , CdS was selected to coat on the surface of TiO 2 nanorods and then PbS QDs were epitaxially grown all over the surface of TiO 2 /CdS nanocables to form a TiO 2 /CdS/PbS heterostructure.This strategy solves the difficulty to directly grow PbS QDs on TiO 2 NR arrays and makes use of the optoelectronic property of PbS QDs for superior photovoltage characteristics of the TiO 2 /CdS/PbS photoanode.

2. 4 .
Preparation of TiO 2 /CdS/PbS Heterostructures.The preparation of the TiO 2 /PbS/CdS heterostructure was formed in two steps.(1) Following Section 2.3, the TiO 2 /CdS heterostructure was firstly fabricated.(2) Following Section 2.2, PbS QDs were deposited over TiO 2 /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 TiO 2 /CdS heterostructures were taken by a JEM-2100F high-resolution transmission microscope.The absorption characterizations of all samples were measured

Figure 1 :
Figure 1: (a, b) SEM images of TiO 2 NR arrays.(c) SEM images of TiO 2 /PbS deposited by electrochemistry for 20 min.(d) SEM images of the top of TiO 2 /PbS.

Figure 2 :
Figure 2: (a, b) SEM images of TiO 2 /CdS nanocable arrays.(c) SEM image of TiO 2 /CdS/Pb.(d) Schematic diagram of the growth process of the TiO 2 /CdS/PbS heterostructures.(e) Relative band edges of TiO 2 , CdS, and PbS (left) and the proposed band edges of TiO 2 /CdS/PbS termed by Fermi level alignment (right).
(d).CdS grew over the surface of TiO 2 NRs, and the core/shell TiO 2 /CdS structures were formed.The relative band edges of TiO 2 , CdS, and PbS are shown in Figure2(e) with their band gaps, respectively, to be 3.20, 2.25, and 1.28 eV.When the TiO 2 /CdS/PbS heterostructure is formed, their band edges at the interfaces would be termed by Fermi level alignment shown in the right part of Figure2(e).

Figure 3 :
Figure 3: (a) TEM image of a TiO 2 NR coated with a CdS shell.(b) TEM image of TiO 2 /CdS/PbS.(c) HRTEM image shows the interface and crystalline structure of TiO 2 /CdS.(d) HRTEM image shows the interface and crystalline structure of TiO 2 /CdS/PbS.

Figure 5 :
Figure 5: UV-Vis absorption spectra of TiO 2 NRs (black line); TiO 2 /CdS deposited with CdS for 5 min (blue line), 10 min (green line), and 20 min (red line); and TiO 2 /CdS/PbS deposited with CdS for 10 min and PbS for 10 min (wine line).The electrolyte for CdS contained 0.2 M of Cd(NO 3 ) 2 and 0.2 M of thiourea; the electrolyte or PbS contained 0.1 M of Cd(NO 3 ) 2 and 0.1 M of thiourea.

Figure 6 :
Figure 6: Photocurrent density-output potential difference (J-V) curves of TiO 2 /CdS deposited with CdS for 10 min (green line), 20 min (red line), and 30 min (blue line); TiO 2 /PbS deposited with PbS for 20 min (orange line); and TiO 2 /CdS/PbS deposited with CdS for 20 min and PbS for 20 min (wine line).