CdS sensitized 3D hierarchical TiO2/ZnO heterostructure for efficient solar energy conversion

For conventional dye or quantum dot sensitized solar cells, which are fabricated using mesoporous films, the inefficient electron transport due to defects such as grain boundaries and surface traps is a major drawback. To simultaneously increase the carrier transport efficiency as well as the surface area, optimal-assembling of hierarchical nanostructures is an attractive approach. Here, a three dimensional (3D) hierarchical heterostructure, consisting of CdS sensitized one dimensional (1D) ZnO nanorods deposited on two dimensional (2D) TiO2 (001) nanosheet, is prepared via a solution-process method. Such heterstructure exhibits significantly enhanced photoelectric and photocatalytic H2 evolution performance compared with CdS sensitized 1D ZnO nanorods/1D TiO2 nanorods photoanode, as a result of the more efficient light harvesting over the entire visible light spectrum and the effective electron transport through a highly connected 3D network.

For conventional dye or quantum dot sensitized solar cells, which are fabricated using mesoporous films, the inefficient electron transport due to defects such as grain boundaries and surface traps is a major drawback. To simultaneously increase the carrier transport efficiency as well as the surface area, optimal-assembling of hierarchical nanostructures is an attractive approach. Here, a three dimensional (3D) hierarchical heterostructure, consisting of CdS sensitized one dimensional (1D) ZnO nanorods deposited on two dimensional (2D) TiO 2 (001) nanosheet, is prepared via a solution-process method. Such heterstructure exhibits significantly enhanced photoelectric and photocatalytic H 2 evolution performance compared with CdS sensitized 1D ZnO nanorods/1D TiO 2 nanorods photoanode, as a result of the more efficient light harvesting over the entire visible light spectrum and the effective electron transport through a highly connected 3D network. C onversion of solar energy to chemical or electric energy by semiconductor nanomaterials has attracted considerable interest [1][2][3] . Conventional dye or quantum dot sensitized solar cells are fabricated using mesoporous films which have large surface area 4 . However, a major drawback of the mesoporous nanoparticle photoanode is the inefficient electron transport due to defects such as grain boundaries and surface traps 5 . For instance, the electron diffusion coefficient of TiO 2 nanoparticles is more than 2 orders of magnitude lower than that of TiO 2 single crystals 6 . On the other hand, high surface area and efficient electron diffusion are both essential to improve the efficiency of solar energy conversion devices.
Over the past decades, zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) nanostructures have attracted much attention due to their unique structure-dependent properties [7][8][9] . Semiconductors with various tailored morphologies, such as nanoparticles (NPs), nanorods (NRs) and nanosheets (NSs) have been extensively studied for photoelectric applications. Furthermore, heterogeneous nanostructures composed of two or three compounds, usually exhibit superior properties compared with that of a single component 10,11 . Therefore, optimal-assembling of multi-component semiconductor nanostructures is an attractive approach for designing next generation solar cells. Up to date, most of the work focused on utilizing one or two kinds of nanostructures to improve the photoelectric conversion performance, such as nanorod/nanoparticle, nanowire/nanosheet et al [12][13][14] . To simultaneously increase the transport efficiency of carriers as well as the surface area, one can assemble hierarchical 0D, 1D and 2D nanostructures on the electrode.
Here we developed a hierarchical heterostructure with multiple components (2D TiO 2 NSs, 1D ZnO NRs and 0D CdS NPs) on FTO for efficient solar energy conversion. TiO 2 and ZnO were chosen in our design due to their low-cost, outstanding chemical stability, and environmentally friendly nature. Both of them are technologically important and have been widely used in the fields of photocatalysis, solar cells, and self-cleaning devices. The integration of ZnO with TiO 2 will facilitate the photo-generated charge separation and give rise to longer carrier lifetime 15 . More importantly, 2D single-crystal TiO 2 NSs with reactive (001) facet exhibit superior photocatalytic activity and could also improve the photoelectric conversion performance [16][17][18][19][20] . Meanwhile, 1D ZnO NRs can significantly improve the electron transport by providing a direct conduction pathway and reducing the number of interparticle hopings [21][22][23][24] . However, the relatively large band gap of 3.2 eV limits further application of TiO 2 and ZnO in the visible light region. CdS, with a narrow direct band-gap (2.4 eV) and flat band potential at 0.66 V (pH 7) 25 , can be used as a photosensitizer for the wide band-gap semiconductors TiO 2 and ZnO. In this work, we synthesize a hierarchical heterostructure, that is, 2D TiO 2 (001) NSs coated with well-aligned 1D ZnO nanorod arrays, on which 0D CdS NPs are anchored. This is referred to as A-TiO 2 /ZnO/CdS, which combines the advantages of TiO 2 , ZnO and CdS. In this optimized heterostructure, 0D CdS NPs gather the visible light, while 1D ZnO NRs arrays which has a high surface area facilitate sufficient CdS deposition and simultaneously harvest UV-light to generate photoelectrons. The 2D TiO 2 NSs work as the efficient charge collection and transport path, meanwhile, provide more sites for ZnO deposition. As a reference, a rutile-TiO 2 NRs/ZnO-NRs/CdS (1D/1D/0D), named as R-TiO 2 /ZnO/CdS, is also fabricated and the influence of different morphologies of the heterogeneous architectures on photoelectric performance is systematically studied.

Results
Morphology and key processes of A-TiO 2 /ZnO/CdS. The fabrication process of CdS sensitized A-TiO 2 /ZnO heterostructure on FTO glass substrate is shown in Figure 1. The 2D single-crystal TiO 2 NSs were grown on FTO by a one-step hydrothermal method 26 . The ZnO NRs arrays were deposited on the surface of TiO 2 via a seed-mediated method 27 , while the seed layer was predeposited by Pulsed Laser Deposition (PLD). After that, CdS was deposited on A-TiO 2 /ZnO by successive ion-layer adsorption and reaction (SILAR) technique 28 , which is based on the successive surface adsorption of ions and thus proceeds via a layer-by-layer buildup of the film 29 .
As shown in Figure 2a, the single-crystalline TiO 2 (001) NSs, with side length of ,2 mm and thickness of ,100 nm, grow vertically or obliquely on the FTO substrate ( Figure S1). The XRD pattern ( Figure 2d) shows that the products are pure anatase phase of TiO 2 with lattice constant a 5 3.7852 Å (JCPDS No. . Compared with the powder diffraction pattern, the (004) diffraction peak is significantly enhanced, indicating the exposing of large percentage of {001} facet. After the deposition of ZnO seed layer by PLD, wellaligned ZnO NRs-arrays with average diameter of 60 nm and length of 800 nm were vertically grown on the TiO 2 {001} and {101} facets, respectively ( Figure S2). Notably, the low-magnification SEM images ( Figure S2a) show that the A-TiO 2 /ZnO heterostructure retain the original morphology of TiO 2 NSs. Such kind of well-aligned ZnO arrays will facilitate efficient photogenerated-electron transfer from ZnO to TiO 2 surface. The XRD pattern ( Figure 2d) shows that all the diffraction peaks of ZnO nanorods can be indexed to a pure wurtzite hexagonal phase of ZnO. Figure 3a shows a typical SEM image of ZnO NRs arrays after the deposition of CdS via SILAR process. It can be found that the surface of ZnO NRs is homogeneously covered by a thin layer of CdS. The coating of CdS NPs on ZnO NRs as shown in Figure 3a is further characterized with energy dispersive X-ray spectroscopy (EDS). The EDS spectrum in Figure 3b reveals that the nanocomposite consists of zinc, cadmium, and sulphur. The titanium can also be detected as the substrate of ZnO NRs arrays.
The ZnO/CdS core-shell structure is further demonstrated with transmission electron microscopy (TEM) images in Figure 3c, which show evidence of morphological changes of the ZnO NRs upon CdS deposition, such as significant surface roughening and diameter increase. The single-crystalline core ( Figure 3d) shows a fringe spacing of 0.26 nm, which matches well with the d-spacing of the (0002) plane of hexagonal ZnO, confirming that the ZnO nanorods are preferentially oriented in the c-axis direction. A lattice fringe spacing of 0.21 nm in the crystallites matches well with the interplanar spacing of the (220) plane of cubic CdS, which demonstrates that the CdS NPs deposited on the surface of ZnO NRs are well-crystallized and the interface between them is of high quality. This will facilitate the photo-generated electron transfer from CdS to ZnO. The XRD pattern of A-TiO 2 /ZnO/CdS ( Figure S3d) does not show the diffraction peak of cubic CdS even though they are well-crystallized. One   possible reason is the overlap between the (111) diffraction peak of CdS and the (110) diffraction peak of SnO 2 (FTO). Another reason is the amount and particle size of CdS NPs is too small to be detected.
Synthesis and characterization of R-TiO 2 /ZnO/CdS. To systema tically study the influence of different morphologies of the hetero geneous architectures on the photoelectric performance, a reference sample was fabricated and characterized. Figure S4 shows the fabrication process of CdS sensitized R-TiO 2 NRs/ZnO NRs heterostructure (R-TiO 2 /ZnO/CdS) on FTO glass substrate, which is similar to that of A-TiO 2 /ZnO/CdS, except that rutile TiO 2 NRs arrays (R-TiO 2 ) are grown on FTO 30 at the beginning instead of A-TiO 2 NSs arrays. Figure S5 shows typical SEM images of the R-TiO 2 NRs arrays grown on FTO surface. The images at different magnifications reveal that the entire surface of the FTO substrate is uniformly covered with TiO 2 NRs. Tetragonal NRs with square top facets are nearly perpendicular to the FTO substrate, and the average diameter is 90 nm. The XRD patterns of the sample show that all the diffraction peaks match well with the tetragonal rutile phase. Absence of some diffraction peaks that are normally present in polycrystalline samples demonstrates that the nanorods are not only aligned but are also single crystalline throughout their length. After the deposition of ZnO seed layer by PLD, ZnO NRs-arrays with average diameter of ,240 nm were vertically grown on TiO 2 NRs film ( Figure S6). The XRD pattern ( Figure S6d) clearly shows the diffraction peaks of wurtzite ZnO and rutile TiO 2 . Figure S7 shows the SEM images of R-TiO 2 /ZnO/CdS heterostructure obtained with different SILAR deposition cycles. The CdS layer deposited on top of the R-TiO 2 /ZnO after 25 SILAR cycles exhibits a flat surface, which is due to the relative smaller surface area of R-TiO 2 /ZnO (top region for CdS deposition) compared with A-TiO 2 /ZnO substrate. Further increase the SILAR deposition to 50 cycles leads to cluster growth and echinus-like CdS microstructures. Thus, for R-TiO 2 /ZnO/CdS, the interface area between ZnO and CdS is much smaller compared with A-TiO 2 /ZnO/CdS heterostructure, while the latter will offer efficient charge transfer between CdS and ZnO. Furthermore, the large exposed surface area of A-TiO 2 /ZnO/ CdS will generate efficient charge transfer between the electrolyte and the photoanodes.

Discussion
UV-vis spectra of the A-TiO 2 /ZnO/CdS, the R-TiO 2 /ZnO/CdS, and the arrays of bare TiO 2 on FTO substrates were characterized (Figure 4a). Compared with the R-TiO 2 NRs arrays (red dotted line), the A-TiO 2 (001) NSs arrays have a stronger absorption in the UV-Vis region (blue dotted line), which is due to its multiple reflection structure of the NSs-arrays. To eliminate the influence of film thickness on the light harvesting efficiency, the cross-sectional morphology was characterized ( Figure S8) and the result shows that the film thicknesses of these two different structures are nearly the same. The hierarchical A-TiO 2 /ZnO/CdS also exhibit a significantly improved absorption in the visible region compared with the R-TiO 2 /ZnO/CdS, therefore, are expected to provide a more efficient light harvesting and an enhanced photoelectric performance. Figure 4b shows the J-V curves of R-TiO 2 , A-TiO 2 , A-TiO 2 /ZnO in dark and under illumination. The A-TiO 2 NSs arrays exhibits a significantly enhanced (3.4 times in photocurrent at 0.8 V) photoelectric response compared with the R-TiO 2 , which is due to its highly reactive (001) facet and good crystallinity. When combined with ZnO NRs arrays, the photocurrent density at 0.8 V is about 4.9 times that of the bare A-TiO 2 , indicating that the TiO 2 /ZnO heterojunction can facilitate the photo-generated charge separation and hence improve the photoelectric performance. With Pt counter-electrodes and polysulfide electrolyte, we obtained a simple solar cell and characterized the photoelectric performance (Figure 4c). When using the A-TiO 2 /ZnO/CdS as the photoanode, a short-circuit photocurrent (J sc ) of 3.24 mA/cm 2 is observed, which is much higher than that of the R-TiO 2 /ZnO/CdS photoanode (1.30 mA/cm 2 ). The cell with A-TiO 2 /ZnO/CdS as the photoanode shows a fill factor (FF) and efficiency of 0.17 and 0.51%, respectively, which exhibit a significant increase when compared with that of the R-TiO 2 /ZnO/CdS photoanode (0.11 and 0.13%). The improved interfacial charge transfer, efficient photo-generated electron collection from the 0D to 2D nanostructures, as well as increased light absorption are the main reasons for the enhanced performance.
Moreover, band position calculations (see detail in supporting information) suggest that TiO 2 , ZnO and CdS have the staggered Z-scheme energy potentials that can reduce the recombination of the photogenerated carriers within the three semiconductors. Under solar light irradiation, the 0D CdS NPs are excited and the as-photoinduced electrons migrate to the 1D ZnO, which could be subsequently collected by the 2D TiO 2 NSs and finally migrate to the FTO substrate. Meanwhile, the redox electrolyte (sulfide/polysulfide) scavenges the holes and thus ensures regeneration of the CdS.
To further investigate the role of each component, the photoelectric performance was characterized under different light irradiation. As shown in Figure S9, under visible light irradiation, the sample still shows photoelectric response, which is arises from the CdS NPs. Under the full spectrum light irradiation, the photocurrent density is higher than that under UV light, indicating that such a hierarchical composite could utilize both UV and visible band of solar light.
Photocatalytic H 2 production from water using semiconductor photocatalysts is an attractive process for solving energy and environmental problems. As shown in Figure 5a, the hierarchical heterostructures exhibit efficient H 2 evolution. Under visible light irradiation, the H 2 evolution rate of A-TiO 2 /ZnO/CdS is 13.3 mmol h 21 cm 22 , which is 2.6 times that of R-TiO 2 /ZnO/CdS. The higher activity under UV-visible light irradiation is due to the excitation of all the three components as well as enhanced charge separation. Figure 5c displays the stability of A-TiO 2 /ZnO/CdS for H 2 evolution under UV-visible light irradiation. There is no obvious activity decrease after each run, indicating the good stability of our sample. Furthermore, the action spectra were obtained using monochromatic light at intensity of 4.3 mW cm 22 . The quantum efficiency (QE) at each centered wavelength of the monochromatic light was calculated from the ratio of twice the number of H 2 molecules to the number of incident photons using the following equation: QE 5 (2 3 the number of H 2 molecules/the number of incident photons) 3 100%. Figure 5b and 5d show the action spectra of H 2 evolution and quantum efficiencies. For both samples, the H 2 evolution rate and quantum efficiencies decreased obviously at around 520 nm with band-pass filters, revealing that the visible-light response for H 2 evolution is due to the band-gap transition of CdS 31 . The QE of H 2 evolution for A-TiO 2 /ZnO/CdS obtained using a 460-nm band-pass filter was still as high as 5.5%, which is much higher than that of R-TiO 2 /ZnO/CdS (2.1%).
In summary, we have developed a well-aligned, structurally-optimized heterostructure with multiple components (2D TiO 2 NSs, 1D ZnO NRs and 0D CdS NPs) for efficient solar energy conversion. The structure2property relationship is systematically studied by comparing with a reference photoanode, R-TiO 2 NRs/ZnO NRs/CdS NPs. The designed A-TiO 2 /ZnO/CdS heterostructure shows a significantly enhanced photoelectric performance and H 2 evolution activity as a result of improved light absorption and efficient electrons transport through a highly connected 3D network. We believe that this strategic design of 3D architectures from 2D to 0D nanocomponents can be further extended to the synthesis of various heterostructures with promising applications in clean energy, optoelectronics, sensing and so on. Substrate Preparation. Fluorine-doped tin oxide (FTO) glasses (1 cm 3 2 cm) were cleaned ultrasonically with acetone and ethanol for 10 min, respectively, and then dried with nitrogen before TiO 2 growth.

Materials
Growth of TiO 2 Arrays on FTO Substrate. (1) Anatase TiO 2 (001) tetragonal nanosheet arrays (A-TiO 2 ) were synthesized with an improved method similar to that described by Yang 26 . In a typical procedure, 1 mL of tetrabutyl titanate was mixed with 60 mL hydrochloric acid (5 M) under vigorous stirring for 10 min, then 0.5 g (NH 4 ) 2 TiF 6 was added into the solution and stirred for another 10 min. The resulting solution was transferred in a dried Teflon autoclave. After that, two pieces FTO glasses were placed at the bottom of the Teflon autoclave with the conductive side facing up. The Teflon autoclave was kept at 180 uC for 16 h. After cooling down to room temperature, the FTO substrates were taken out and rinsed with deionized water thoroughly and then dried at 60 uC. (2) Rutile TiO 2 nanorod arrays (R-TiO 2 ) were synthesized following similar procedure except (NH 4 ) 2 TiF 6 was removed, and the FTO substrates were placed at an angle against the wall of the Teflon autoclave with the conducting side facing down. The hydrothermal synthesis was conducted at 150uC for 6 h.
Growth of ZnO Nanorod-Arrays on TiO 2 Arrays. ZnO nanorod-arrays were synthesized by a seed-mediated method 27 . First, ZnO seed layer was deposited on the surface of TiO 2 arrays by Pulsed Laser Deposition (PLD). The distance between the ZnO target and the substrate was ,50 mm. The vacuum chamber was evacuated to a base pressure of 1 3 10 25 Pa and then retained at an oxygen partial pressure of 3 Pa. The energy at the target surface and repetition rate of the laser was 52 mJ and 5 Hz, respectively. Hydrothermal ZnO growth was carried out by suspending the abovementioned substrates upside-down in an aqueous solution containing 25 mM zinc nitrate hexahydrate and 25 mM hexamethylenetetramine at 90 uC. The typical growth time is 4 hours. The samples were then removed from solution, rinsed with deionized water, and dried at 50 uC. The ZnO nanorod-arrays modified TiO 2 films were mentioned as A-TiO 2 /ZnO and R-TiO 2 /ZnO, respectively.
CdS Deposition. CdS was deposited on the A-TiO 2 /ZnO and R-TiO 2 /ZnO samples by successive ion-layer adsorption and reaction (SILAR) technique 28 . Typically, the TiO 2 /ZnO films were successively immersed in two different aqueous solutions for 20 s each, first in 50 mM Cd(NO 3 ) 2 in methanol and then in 50 mM Na 2 S in methanol/water (151/v5v). Between each immersion step, the samples were rinsed with methanol for 20 s to remove excess ions that were weakly bound to the surfaces. This immersion cycle was repeated for up to 50 cycles and, after finishing the coating, the samples were dried at 40 uC. The as-prepared samples were named as A-TiO 2 / ZnO/CdS and R-TiO 2 /ZnO/CdS, respectively.
Fabrication of solar cells. Platinum (Pt) coated silicon wafers counter-electrodes were prepared by PLD. Solar cells were prepared by assembling and bonding the TiO 2 /ZnO/CdS electrodes with the Pt counter-electrodes. The two electrodes were separated by a 60 mm thick polypropylene spacer, and the internal space of the cell was filled with a polysulfide electrolyte (1.0 M S, 1.0 M Na 2 S and 0.1 M NaOH in deionized water).
Characterization. X-ray diffraction patterns were obtained using a Shimadzu XRD-6000 X-ray diffractometer with Cu Ka radiation (l 5 1.5418 Å ). Field emission scanning electron microscopy images were obtained using a JSM-7600F microscope. The EDS was taken on an OXFORD INCA X-Max Energy Dispersive X-Ray Spectroscopy. High-resolution transmission electron microscopy (HRTEM) measurements were carried out on JEM-2010 operated at 200 KV. J-V measurements were performed using a Keithley 2400 SourceMeter with a custom-made LabTracer program. A 300 W Xenon Arc Lamp (Newport) with appropriate filter was used as the light source.
Photocatalytic Hydrogen Production Activity Tests. Typically, the TiO 2 /ZnO/CdS electrode (0.3 cm 2 ) was immersed in 4 mL of aqueous solution containing 0.1 M Na 2 S and 0.1 M Na 2 SO 3 bubbled with Ar, and sealed with a rubber septum. A 300 W Xenon Arc Lamp (Asahi Spectra, LAX-C100) with appropriate filter was used as the light source to provide simulated sunlight (AM 1.5G). For visible-light hydrogen production, a 420 nm cutoff filter was used to remove UV light. The amount of H 2 in the gas phase was measured using a Shimadzu GC-8A gas chromatograph equipped with an MS-5A column and a thermal conductivity detector (TCD). To obtain an action spectrum in this reaction system, photocatalytic H 2 formation was carried out under irradiation of monochromated light from a Xe lamp (Asahi Spectra; 4.3 mW cm 22 ) with band-pass filters (light width: 65 nm).