Heterojunction Photoelectrode of Polyaniline/ZnS Film/ZnO Nanorod on FTO Glass

To enhance the absorption of visible light for wide-band-gap semiconductors, methods such as sensitizing with nanoparticles or quantum dots, and band gap engineering using dopants have been reported. However, these can cause lattice mismatch, inherent disorders, or imperfect charge balance, which serve as recombination sites and significantly reduce the photocatalytic efficiency. Herein, photoelectrodes of PANI/ZnS/ZnO on F:SnO2 (FTO) were fabricated to analyze these issues and examine their interface microstructural and photocatalytic properties. ZnO nanorods (NRs) were grown on FTO glass by potentiostatic electrodeposition, and ZnS and PANI films were coated by liquid processes. The PANI films were applied as a visible light sensitizer and photocorrosion prevention layer for ZnS/ZnO photoelectrodes. Subsequently, the prevention effect of photocurrent loss due to the photocorrosion of semiconductors was systematically investigated. The photocurrent of the PANI/ZnS/ZnO photoelectrode measured at 0.5V under white light illumination was five times higher than that of the ZnO NR photoelectrode. This was attributed to photocorrosion prevention and visible light absorption of PANI layers, due to proper energy band alignment of the hybrid heterojunction semiconductors.


Introduction
The solar energy received by the earth consists of ultraviolet (~5%) and visible light (~43%). Nanostructures are important not only for visible-light absorption in the solar spectrum, but also to obtain a high photocatalytic efficiency for semiconductor photoelectrodes. There has been extensive research on various nanostructured photoelectrodes such as nanowires, nanorods, and nanotubes H. Kim [1]- [6]. Many multilayered heterojunction nanostructures with great penetration depth have been also reported for the absorption of long wavelengths in the visible light spectrum, as well as the efficient charge separation and transfer of excited electrons and holes [7] [8] [9] [10].
One of the methods to enhance visible-light absorption is to adhere inorganic semiconductors with narrow-band-gap materials such as nanoparticles or quantum dots (QDs) on the surface of wide-band-gap semiconductors, as a sensitizer [11] [12] [13]. Another way is to manipulate the band gap sufficiently to obtain a visible light spectrum response of 1.7 -2.9 eV by doping impurities such as carbon, nitrogen, or hydrogen [14] [15]. Regarding visible-light sensitizers, studies on chalcogenide compound QDs such as CdS (Eg = 2.4 eV), CdSe (1.7 eV), and CdSSe (1.9 eV) have been reported [16], as well as those on plasmonic Ag and Au nanoparticles [17], transition metal dichalcogenide sheets such as MoS2 and WS2 [18], and metal halide perovskites such as CH3NH3PbI3 and CsPbBr3 [19]. However, it is important to develop alternative sensitizer materials, owing to the significant disadvantages of the abovementioned materials, such as the harmful nature of Cd, high cost of noble metals, and unstable moisture process.
In band gap engineering through doping of single or several impurities, the recombination probability of excited electrons and holes is very high near doping sites, which include deteriorative crystal defects such as lattice mismatch at the interface between the sensitizers and wide-band-gap semiconductors, unstable inherent disorders [20], and charge imbalance among dopants. Hence, even though the absorption of visible light is improved by doping, these recombination sites can cause significant loss of photocatalytic current during the separation and transfer of visible-light-excited electrons and holes.
Meanwhile, there are few reports on the photocorrosion of sensitizers such as QDs and nanoparticles attached on wide-band-gap semiconductors or those of semiconductors themselves. As alternatives, polymeric materials have recently been proposed as a prevention layer for photocorrosion. However, systematic studies on the photocatalytic performance using these films have not been reported thus far.
In this study, conductive organic polyaniline (PANI, ΔELUMO-HOMO = 2.4 eV) was applied for the core/shell composites of ZnS film/ZnO nanorods (NRs) on F:SnO2 (FTO) substrates. Subsequently, the performance of PANI as a sensitizer as well as a prevention layer to improve the photocatalytic properties, was evaluated.

Methods
In this study, to fabricate the hybrid core/shell heterojunction photoelectrodes, af-

Preparation of ZnO NRs/FTO Glass Photoelectrode
ZnO NRs were grown on FTO (resistance = 8 Ωm) glass by potentiostatic electrodeposition [21] [22]. The FTO glass was placed in an aqueous solution of 0.5 mM ZnCl2 and 0.1 M KCl under oxygen bubbling in a three-electrode electrochemical cell, comprising a counter electrode of Pt mesh and reference electrode of Ag/AgCl/sat. KCl. Electrodeposition was carried out at −1.0 V for ~3 h at room temperature.

Particulate ZnS Film Coating on ZnO NRs
A particulate ZnS film was coated on the surface of the ZnO NRs by immersing the ZnO NR electrode in an aqueous 0.32 M Na2S9H2O solution for ~12 h. Sulfurization by anion-exchange reaction was carried out in a deionized water bath at 60˚C. Na2S9H2O was used as an S 2− anion source to form ZnS on the surface of the ZnO NRs, according to the following reaction: The electrode was then washed with deionized water and absolute ethanol several times and dried at 80˚C for 2 h in an oven [23] [24].

PANI Sensitizer Coating on Particulate ZnS Film/ZnO NRs
The PANI sensitizer was coated by successive ionic layer adsorption and reaction using ammonium persulfate as an oxidizing agent. First, 0.4 M aniline was dissolved in 1 M sulfuric acid, which served as a cationic precursor in the first beaker. The second beaker contained an oxidant solution of 0.2 M ammonium persulfate in 1 M sulfuric acid, which served as an anionic precursor. The electrodes were then immersed into the aniline solution for 60 s for the surface polymerization of PANI on the particulate ZnS film/ZnO NR. Subsequently, the electrodes were immersed in the ammonium persulfate solution for 30 s [25] [26].

Microstructural Characterization
The microstructures of the photoelectrodes were characterized by field-emission scanning electron microscopy (FE-SEM; JSM-6500 F, JEOL), field-emission transmission electron microscopy (FE-TEM; 200 kV/JEM-2100F HR, JEOL), and X-ray diffraction (XRD, SWXD, Rigaku). Ultraviolet-visible (UV-vis) absorbance spectra of the electrodes were obtained using an ultraviolet-visible-near infrared (UV-VIS-NIR) spectrophotometer (UV 2600, Shimadzu). XPS was performed using a Thermo VG ESCALAB 250 instrument equipped with a microfocused, monochromatic A1 Kα X-ray source (1486.6 eV) and magnetic lens. The X-ray spot size was 500 µm (15 kV, 150 W). The spectra were acquired in the constant analyzer energy mode with a pass energy of 150 eV and 40 eV for overview scans and high-resolution scans, respectively. The photocurrent densities of the photoelectrodes were measured using a potentiostat (AMT VERSASTAT 3, Princeton Applied Research) with a three-electrode cell comprising Pt mesh as the counter electrode and Ag/AgCl/sat. KCl as the reference electrode separated by a proton exchange membrane in aqueous 0.5 M Na2SO4 (pH = 7.15) electrolyte. Using a 1 kW xenon lamp (Newport) with the infrared wavelengths filtered out by water, 1 cm 2 of the working electrode was exposed. The light irradiance, measured using a thermopile detector, was 100 mW/cm 2 .

Microstructural Characterization
In this study, photocatalytic properties of ZnS film/ZnO NR heterojunctions with PANI layers were investigated. For heterojunction composite fabrication, after the ZnO NRs were grown on FTO substrates by electroplating, ZnS layers were formed via sulfurization followed by PANI coating via cationic surface polymerization. The cross-sectional and surface FE-SEM images of ZnO NRs (Figure 1) show that the hexagonal ZnO NRs, with diameters of 300 -500 nm and lengths of 0.5 -1.5 μm, were grown vertically.  planes, because Zn 2+ concentration is much higher in the top surfaces due to higher electroplating potential. However, many voids are observed on the surface of ZnO NRs (Figure 1) Figure 1(e) and Figure 1(f), respectively. As shown in Figure 1(f), the smooth surface of a ZnO NR is surrounded by both ZnS film composed of stacked agglomerated nanoparticles and PANI film.
TEM images of PANI/ZnS film/ZnO NRs (Figure 2(a) and Figure 2(b)) show that a core electrodeposited ZnO NR is surrounded by both ~5-nm-thick PANI film and ~30-nm-thick particulate ZnS film. Thus, this core/shell structure should prevent direct contact of ZnO NRs with the electrolyte in the photocatalytic cells, thus preventing deterioration of photocatalytic current due to photocorrosion.
ZnS films on ZnO NRs can be typically formed by several methods like chemical absorption of ZnS nanoparticles and vapor-liquid-solid (VLS) methods. However, for direct adhesion of ZnS nanoparticles on the ZnO surface, micro-voids can occur in the sites between the ZnS nanoparticles. The lattice mismatch between the ZnS nanoparticles and ZnO NRs can also cause the interface defects. Thus, annealing processes are necessary for reduction of these crystalline defects.
In the VLS methods using Zn and S powder sources, non-uniform adsorption of   (162 eV) peaks were observed for ZnS films. On the other hand, N 1s (399 eV) and C 1s (285 eV) peaks were observed for PANI. The N 1s (399 eV) peak, consisting of imine R 3 -N = C-R 1 R 2 (398.5 eV), amine R 1 R 2 -NH (399.7 eV), and protonated nitrogen -N + = (401.9 eV) signals, originated from the N element of the aniline monomer, as shown in Figure 4(b) [28].

Photoelectrochemical Characterization
The UV-Vis absorption spectra of PANI/ZnS film/ZnO NRs, ZnS film/ZnO NRs, and ZnO NR electrodes were measured as shown in Figure 5(a). All the spectra present a strong absorption in the UV region between 300 and 400 nm. The band gaps of each layers were estimated from the Tauc plots [29] as shown in Figure   5(b

Conclusion
In this study, PANI/ZnS/ZnO/FTO electrodes were studied on their interface and photocatalysis. Under white light illumination, the PANI/ZnS film/ZnO NR electrode showed five times higher photocurrent density than ZnO NRs. This was due to the anti-photocorrosion and visible light absorption of PANI layer, and also provided proper energy band alignment of the hybrid heterojunction (PANI/ZnS film/ZnO NRs) to efficiently transfer and separate excited electrons and holes. It is expected that this hybrid heterojunction photoelectrode with proper band alignment using organic materials could be utilized for other applications such as photovoltaics, solar CO2 fuel conversion, and solar pollutant decomposition.