Nature-Mimic ZnO Nanoflowers Architecture: Chalcogenide Quantum

‡Department of Hematology and §Department of Surgery & Central Laboratory, First Affiliated Hospital of Anhui Medical University, Hefei 230022, PR ...
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Nature-Mimic ZnO Nanoflowers Architecture: Chalcogenide Quantum Dots Coupling with ZnO/ZnTiO3 Nanoheterostructures for Efficient Photoelectrochemical Water Splitting Asad Ali,† Xiaodong Li,† Jiangluqi Song,† Siyu Yang,† Wenting Zhang,† Zengming Zhang,† Ruixiang Xia,*,‡ Lixin Zhu,*,§ and Xiaoliang Xu*,†,§ †

Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, The Centre for Physical Experiments, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ Department of Hematology and §Department of Surgery & Central Laboratory, First Affiliated Hospital of Anhui Medical University, Hefei 230022, PR China S Supporting Information *

ABSTRACT: We prepared novel photoanodes structured as FTO/ZnO nanoflowers/ZnTiO3/CdS/CdTeS/ZnS quantum dots (QDs) with highly exposed large specific area and energy coupling. The design of the photoanode expressed significant enhanced photoelectrochemical (PEC) performance such as improved absorption efficiency, reduced recombination rate, and enhanced charge transportation state, resulting in a significant increase in photoelectron current. The multinanoheterostructures photoanode provides a high photocatalytic activity and a maximum photocurrent density up to 9.41 mA/cm2 under AM 1.5 G illumination. The novel cosensitized multinanoheterostructures photoanodes lead to a remarkable and promising application in photoelectrochemical and water splitting reactions.

1. INTRODUCTION Solar-light-driven photoelectrochemical water splitting by semiconductor has gained more attention as an alternative solution for energy and environmental issues, due to the vast supply of water and light resources.1,2 In PEC field, some metaloxide semiconductors such as ZnO, Fe2O3, and TiO2 are considered the most important materials,3,4 among which, ZnO has been extensively used in many applications such as an alternative photoanode material for the PEC cell,5,6 photocatalysis7 and gas sensing.8 However, the wide band gap (Eg = 3.2 eV) of ZnO plus fast recombination of charge carriers give rise to low quantum yields as well as poor photocurrent conversion efficiency. Therefore, it is necessary to enhance the photocatalytic efficiency in visible light irradiation by broadening the absorption range of the photoanode. Recently, various quantum dots (QDs) sensitizers such as CdSe,9−11 CdS,12−14 and CdTe15 have been used to increase the PEC performance of a wide band gap semiconductor such as ZnO. The type-II mode16 in the core−shell nanoheterostructures is very important for the spatial separation, the charge transfer amplification, and minimizing the internal charge recombination, to increase the photocatalytic activity. Some reported chalcogenide QDs such as CdS and CdSe are considered suitable modifiers and can be used to boost up the visible light absorption and solar conversion efficiency in © 2017 American Chemical Society

different quantum-dot-sensitized solar cells (QDSSCs), photoelectrochemical and photovoltaic devices. Simelys et al.17 fabricated one-dimensional nanostructure of ZnO@TiO2 and obtained a photocurrent of 0.40 mA/cm2 at 1.23 V under solar illumination. ZnO nanowires decorated with CdTe QDs exhibited a photocurrent density of 2.0 mA/cm2 at 1.0 V under illumination.18 Moreover, the CdS/CdSe cosensitized photoanodes showed improved performance due to the double sensitizers and of the band edge structure in comparison to sensitized by only CdS or CdSe.4 Yong et al.13 synthesized ZnO/CdS core−shell nanowire heterostructures array for photoelectron chemical cells applications. Dam et al.10 have prepared CdS and CdSe-cosensitized 3D porous ZnO nanosheet photoelectrodes that play a vital role in light-harvesting applications and have photocurrent density of 3.1 mA/cm2 at 1.2 V vs RHE. Jiang et al.15 synthesized CdTe QDs with tunable thickness on ZnO nanorods array to intact the surface of the ZnO nanostructures. Compared to the former published ZnO19,20 or TiO221 substrates, the ZnO/ZnTiO3 used in this work as a core material to provide a broader light absorption range, while Received: May 16, 2017 Revised: September 4, 2017 Published: September 6, 2017 21096

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The Journal of Physical Chemistry C ZnTiO3 gives an optical protective layer in improving the optoelectrical stability of ZnO, i.e., to offer a higher and more stable electron mobility and minimize the recombination probability during electron transfer. In contrast, 3D metal oxide materials like ZnO and TiO2 architectures have been reported in the literature.22 The ZnO nanoflowers were used in this work offering a much larger specific surface area for light absorption and electron transportation having shorter distance than other 3D configurations as compared to the traditional 1D to 3D surface configuration such as single plane, nanowires, or nanotrees etc. Therefore, it is observed that the physical features of these 3D oxide films aid to ameliorate the PEC performance when the chalcogenides are sensitized on their surfaces. In this context, 3D ZnO nanoflowers with ZnTiO3 layer were synthesized for the deposition of the quantum dots sensitizers and can provide a path to the electrolyte diffusion.23 More significantly, the 3D ZnO nanostructure on the conducting substrate provides an excellent continues electrontransport pathway for the fast collection of photoexcited carriers compared to nanowires or nanorods,24 minimizing the charge recombination and leads to enhance efficiency and absorption under visible light irradiation.25,26 The most important characteristic of this nanostructure is to overcome the issue of high-energy photons of wide band gap material and integrate low band gap semiconductors in a single nanostructured material.27,28 To the best of our knowledge, this work is the first report on the synthesis of ZnO/ZnTiO3/CdS/ CdTeS/ZnS with multinanoheterostructures photoanodes for the photoelectrochemical water splitting. It is envisioned that such type of work will be a valuable, straightforward paradigm for stability, highly efficient, and the multinanoheterostructures can be reused for energy harvesting and environmentally sustainable applications.

ZnO nanoflowers, assigned as Z. The ZnO nanoflowers obtained at 145 °C for 7 h were used for further deposition. 2.3. Deposition of ZnTiO3 Layer Coated on ZnO Nanoflowers. TBOT (15.84 mL) and HAc (9.6 mL) were mixed and stirred for certain time. Then, the solution was slowly added to 30 mL of 2 mol L−1 HCl solution and was heated at 60 °C. An amount of 13.36 g of Zn(NO3)2 was quickly added to the prepared solution and stirred at 60 °C for 1 h till the homogeneous gel is formed. The prepared gel was then transferred to a crucible and calcined at 600 °C for 3 h to form ZnTiO3 product. This product was changed to white blocks and was ground to transform it into superfine powder. The resultant powder was further calcined at 800 °C for 3 h in order to obtain pure ZnTiO3. The as-prepared ZnTiO3 was ground and preserved for further use. Two solutions were made and designated as sol-A and sol-B. Sol-A was prepared as follows. A 2.5 g amount of ethyl cellulose was dissolved in 19 mL of absolute alcohol at 70 °C. The sol-B was prepared by dissolving 0.6 g of ZnTiO3 in 17 mL of absolute alcohol and sonicated for certain time followed by the addition of terpineol (1.4 g). The sol-A and sol-B were then mixed and stirred at ambient temperature for 3 days. The solvent was evaporated and ZnTiO3 paste was obtained. The paste was cast over ZnO nanoflowers via doctor blade technique. The ZnO nanoflowers covered with ZnTiO3 thin film was heated in a muffle furnace at 450 °C for 1.5 h. The as-synthesized film of ZnO/ZnTiO3 was named ZZ and subsequently used to sensitize with different quantum dots. 2.4. Decoration of ZnO/ZnTiO3 Film with Different Quantum Dots (QDs). First, CdS QDs were successfully synthesized using a successive ionic layer absorption reaction (SILAR) method. In the preparation of CdS QDs, a 0.1 mol L−1 of cadmium acetate in methanol and a 0.1 mol L−1 sodium sulfide in methanol/water (1:1 vol.) were used as the precursor solutions for Cd2+ and S2−, respectively. The as-synthesized ZnO/ZnTiO3 thin film was immersed in the cadmium acetate solution for 1 min and rinsed with methanol, then immersed in the sodium sulfide solution for 1 min and re-rinsed with methanol. This process was repeated for several cycles to grow these QDs effectively. The CdS QDs were successfully grown on the ZnO/ZnTiO3 to get ZnO/ZnTiO3/CdS nanoheterostructures through the aforementioned method and named as ZZS. The CdTeS alloyed QDs were prepared by a modified hydrothermal method.30 The as-prepared material of ZnO/ ZnTiO3/CdS was placed in a solution of CdTeS QDs for several hours to confirm the sensitization of these quantum dots. The FTO substrate was covered with ZnO/ZnTiO3/ CdS/CdTeS multinanoheterostructures and was nominated as ZZST. Moreover, a buffer layer of ZnS was also synthesized on the surface of this photoanode material according to the reported literature.15,23,31 The ZnS equimolar solution, 0.05 M Zn(NO3)2 in ethanol, and 0.05 M Na2S in methanol/water (7:3, v/v) were used for the SILAR process. Meanwhile, the sensitized photoanode was dipped for 1 min per each cycle, repeating the procedure for several times to synthesize ZnO/ ZnTiO3/CdS/CdTeS/ZnS, assigned as ZZSTZ. The FTO substrate (covered with different multinanoheterostructures photoanodes) was used as a working electrode with a platinum sheet as counter electrode. 2.5. Photoelectrochemical Measurements. The photoelectrochemical (PEC) measurements were carried out on the electrochemical workstation (CHI760E) equipped with a

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O), N-methyldiethanolamine (MDEA) (C5H13NO2), titanium dioxide (CP), sodium borohydride (NaBH4), tetrabutyl titanate (TBOT, CP), acetic acid (HAc, AR), selenium powder (Se, CP), hydrochloric acid (HCl, 36− 38 wt %), tellurium powder (Te), absolute alcohol (AR), methanol (AR), sodium hydroxide (NaOH), ethyl cellulose M70 (CP), terpineol (CP), cadmium acetate (Cd(CH3COO)2.2H2O, CP), sodium sulfide (Na2S.9H2O, AR), 3-mercaptopropionic acid (MPA), cadmium chloride (CaCl2), sodium sulfite (Na2SO3, AR), and sublimed sulfur (S, CP) all were obtained from Sinopharm Chemical Reagent Co., Ltd. and have been used directly without further treatment. Other materials, such as FTO substrate, scotch tape, and sealing film were all received from the commercial sources. DI water was used as per requirement. 2.2. Synthesis of ZnO Nanoflowers. ZnO nanoflowers on a FTO substrate were synthesized by a modified reported method.29 Equimolar amounts of Zn(NO3)2·6H2O and MDEA were added to 40 mL of DI water and stirred for 30 min. NaOH was used to set the pH at 12. The resultant solution with FTO was then put in a Teflon beaker and transferred to a stainless-steel autoclave. The autoclave was sealed and heated up to temperature 145 °C for 4−7 h. After completing the reaction, the autoclave was naturally cooled, and the FTO substrate covered with ZnO nanostructures was washed several times with DI water and then dried at room temperature to get 21097

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showing that the ZnO nanoflowers were successfully grown on the FTO substrate. In Figure 1a and a-I, only small flowers were observed when the reaction was carried out for 4 h. The mature and well-defined flowers were obtained after 7 h of reaction as given in Figure 1b. This behavior can be corelated to the MDEA, which acts as a ligand to control the morphology of nanoflowers. When the time was increased from 4 to 7 h, MDEA (as base) produced more −OH ions, which trigger the flower’s structure. The optimized reaction time is very necessary to get fully grown flowers. The insets of a-I and b-I show the magnified SEM images of flowers that are grown symmetrically and enhance the fill rate between the intervals of the nanoflowers. In Figure 1b, ZnO nanoflowers have diameter ranges from 366 to 401 nm. The ZnO nanoflowers on the FTO substrate are grown homogeneously with large petals. The wellarranged nanoflowers with large petals would provide large specific surface area as compared to the other traditional nanostructures such as nanorods, nanoneedles, and so on. Literature survey reveals that the large specific surface area is of great importance for the electron transportation channels that could provide a potential application for the recombination and photogenerated electrons in QDSSCs. According to our knowledge, ZnTiO3 for the first time was deposited on the surface of the ZnO nanoflowers through doctor blading as shown in Figure 1c. The cross-sectional study of the ZnO nanoflowers and zinc titanate were performed through SEM having thickness of about 212 and 664 nm as shown in Figure 1d, respectively. 3.2. TEM and HRTEM Analysis. The TEM analysis was done to confirm the microstructure of the ZZST multinanoheterostructures. The low magnification of TEM images show that ZnO nanoflowers cores were coated with uniform and compact shells of CdS and CdTeS QDs. A typical highresolution TEM image is shown in Figure 2a, the d-spacing of

potentiostat and a custom-built three-electrode quartzwindowed photoelectrochemical cell. The incident light from a 300W Xe lamp was filtered to match AM 1.5 G spectrum with an intensity of 50 mW/cm2 as measured by a radiometer (OPHIR, Littleton, CO). In the PEC measurements, an electrolyte containing a mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution was used. The as-prepared photoanode was employed as a working electrode while the saturated calomel electrode (SCE) and platinum sheet served as reference and counter electrodes, respectively (see the Schematic diagram). The electrochemical impedance spectroscopy (EIS) was measured on the same workstation at an applied potential of 0.5 V with frequency ranges from 0.1 Hz to 50 kHz and modulation amplitude of 5 mV. The incident photon-to-electron conversion efficiency (IPCE) spectra were collected using a Newport 150 W Xe lamp coupled to a monochromator, by varying the wavelength of the incident light from 300 to700 nm with two electrodes under 0 V bias. The incident light intensity was calibrated by standard silicon photodiode. 2.6. Material Characterization. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to study the detailed morphology of decorated ZnO nanoflowers materials by JEOL JEM 1400 and the elemental analysis was conducted on HRTEM equipped with an energy dispersive spectroscopy (EDS) attachment. The X-ray diffraction measurement was conducted on Rigaku D/max diffractometer Cu Kα radiation (λ = 0.15418 nm). The surface morphologies of the samples were examined by using field-emission electron microscopy (FESEM JEOL Hitachi SU8000). The elemental contents of the samples were obtained with the help of X-ray photoelectron spectroscopy (XPS) by ESCLAB 250 spectrometer. The photoluminescence (PL) emission spectra at room temperature were analyzed by using a steady-state/lifetime spectrofluorimeter (FLUOROLOG-3-TAU, Jobin Yvon) equipped with xenon (Xe) lamp having an excitation wavelength of 325 nm. The UV−vis diffuse reflectance spectrum (DRS) was measured by a Shimadzu Solid 3700 UV−vis NIR spectrometer facilitated with an integrating sphere using BaSO4 as the baseline.

3. RESULTS AND DISCUSSION 3.1. Scanning Electron Microscopy. The morphology of ZnO nanostructures is schematically illustrated in the Figure 1,

Figure 1. (a and b) SEM images of ZnO nanoflowers, (c) ZnO/ ZnTiO3, (d) Cross-sectional SEM image of ZnO nanoflowers and ZnTiO3.

Figure 2. (a) TEM/HRTEM analysis of ZnO/ZnTiO3/CdS/CdTeS (ZZST), (b) SAED, (c) elemental mapping of the multinanoheterostructures. 21098

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Figure 3. XPS spectrum of ZnO/ZnTiO3/CdS/CdTeS (ZZST): (a) Zn 2p, (b) Cd 3d, (c) Ti 2p, (d) O 1s, (e) S 2p, and (f) Te 3d.

3.3. X-ray Photoelectron Spectroscopy. Electronic states and chemical composition of the ZZST multinanoheterostructures were measured by XPS and the results are given in Figure 3, showing the presence of Zn, O, Ti, Te, Cd, and S. The XPS survey spectra is provided in Figure S3. The C spectrum observed at 285.3 eV was used as a reference element. The two symmetric peaks appeared at 1021.6 and 1044.7 eV are assigned to Zn 2p3/2 and Zn 2p1/2 and exhibited that Zn chemical state is bivalent in Figure 3a, respectively. The hydrothermal reaction changes the metastable zinc species into a stable state with the highest valence. The high-resolution XPS spectrum of Cd 3d in Figure 4b exhibits two peaks, 3d5/2 at 404.7 eV and 3d3/2 at 411.5 eV, which demonstrated Cd2+ oxidation state in the as-prepared ZZST multinanoheterostructures.32 The spin−orbit coupling with Ti 2p given in the

the lattice fringes of the ZnO, ZnTiO3, CdS, and CdTeS are estimated to be ∼0.161, ∼0.177, ∼0.194, and ∼0.359 nm which correspond well with the planes (110), (105), (220), and (111), respectively. The distinct selected area electron diffraction (SAED) pattern is shown in the Figure 2b. The agglomerate shown in the TEM image is composed of nanocrystallites, as suggested from the corresponding SAED pattern. The first four diffraction rings with planes (012), (101), (220) and (222) of SAED can be indexed with ZnTiO3 (Powder Diffraction File (PDF) 26−1500), ZnO (PDF 361451), CdTeS (PDF 15−0770) and CdS (PDF 21−0829), respectively, corresponding to a pseudo multinanoheterostructures. The SAED pattern confirms that the product is polycrystalline of the ZZST nanoheterostructures. The ZnO/ ZnTiO3 has sharp interface so that CdTeS crystal structure may resemble with CdS in the low magnification range, whereas its interface also depends on the thickness of the nanoshell. Furthermore, the multinanoheterostructures ZnO/ZnTiO3, CdS and CdTeS QDs can be observed through elemental mapping as shown in Figure 2c. All the elements, namely, Zn, O, Cd, Te, Ti, and S, were randomly found in the Figure 2c. In the elemental distribution, it was observed that ZnO and ZnTiO3 are indicated as the inner layers, while CdS and CdTeS are the outer layers coated on the surface of the inner layers. The XRD data are well-matched with HRTEM and TEM/ SAED results (Figures S1 and S2). It was found that both CdS and CdTeS QDs were grown in random directions to each other, due to which their interface becomes continuous and very compact and plays a key role in the photogenerated carrier transfer along the radial direction of the nanoshells. In HRTEM images, it is noted that CdS and CdTeS layers have a number of grain boundaries, which are not higher than that along radial orientation of the inner layer materials. While such phenomena are very vital for the enhancement of the PEC properties and suppression of carrier recombination.

Figure 4. UV−vis diffuse reflectance spectra of ZnO nanoflowers (Z), ZnO/ZnTiO3 (ZZ), ZnO/ZnTiO3/CdS (ZZS), ZnO/ZnTiO3/ CdTeS (ZZT), ZnO/ZnTiO3/CdS/CdTeS (ZZST), and ZnO/ ZnTiO3/CdS/CdTeS/ZnS (ZZSTZ) multinanoheterostructures. 21099

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The Journal of Physical Chemistry C Figure 3c is deconvoluted into two peaks at about 458.1 eV (Ti 2p3/2) and 464.2 eV (Ti 2p1/2), respectively, and corresponded well to Ti4+ ions. The peaks of the Ti 2p after the formation of ZnTiO3, shifts toward the larger binding energy direction. The phenomena are due to emergence of Zn atoms between the layers of nanosheets leading to the change in electron dispersion around Ti atoms. The binding energy of both Zn 2p and Ti 2p are shifted toward higher in comparison to the reported values of pure ZnO and TiO2. The O 1s peak as in Figure 4d can be deconvoluted into peaks at around 531.1 and 532.1 eV, which correspond to Zn−O bonds of ZnO and surface hydroxyl species, respectively. The effect of OH− radicals adsorbed on the surface of TiO2 has a peak assigned to 532.1 eV. The Zn species located at the interlaminations in ZnTiO3 has no surfactant unsaturated bonds. The XPS of S 2p shows two peaks at 161.3 and 162.3 eV which are associated with S 2p3/2 and S 2p1/2 and are attributed to the hybrid chemical bond species to S2− of CdS.33 The contribution of these peaks arouses from S 2p bound to ZZ in the form of S2−, which play a key role in the attachment of CdS and CdTeS QDs to the ZnTiO3 nanoparticles through electron donor− acceptor mechanism. Moreover, at a nanoscale level the ZnO nanoflowers and QDs have strong van der Waals forces associated with short-range forces. Semiconductor QDs possess the dipole moment as S has electron acceptor while Ti is an acceptor on the surface of ZnTiO3. To direct assemble the system, strong driving forces are required which arise from dipole−dipole interactions. It was observed that Te 3d peak was split due to Te−Cd bonding in the CdTeS.34 Te 3d5/2 and Te 3d3/2 have binding energies located at 576.3 and 585.2 eV, respectively. 3.4. UV−Vis Absorption Spectra. The optical absorption properties of the as-prepared photoanodes were examined by UV−vis diffuse reflectance spectrometer. The absorption spectra as a function of wavelength for the bare Z and ZZ with different sensitizers such as CdS and CdTeS quantum dots are shown in Figure 4. The bare Z exhibits a light absorption onset at 375 nm, which is low because of its wide band gap. The addition of ZnTiO3 to ZnO nanoflowers further reduced the light absorption. This further reduction in absorption can be attributed to the incorporation of Ti as ZnTiO3. It is observed that ZZ in the visible light region shows minimum absorption due to its wide band gap. The core materials were decorated with CdS and CdTeS QDs, the absorption edges redshift of about 500 and 565 nm are observed which are consistent with energy band gap of the bulk counterparts (Eg, CdS ≈ 2.4 eV and CdTeS ≈ 2.3 eV). As the CdTeS QDs has lower band gap as compared to that of the CdS QDs, the redshift observed in ZZST is larger than that in ZZS. It is worth considering that the doubly sensitized (from CdS to CdTeS) ZZSTZ multinanoheterostructures extended the absorption range as well as increased their absorbance as compared to the singly sensitized (from CdS) photoanodes. In brief, the enhanced visible light absorption of the multinanoheterostructures photoanodes can be ascribed to the presence of visible-light-responsive CdS and CdTeS, which lead to the cosensitization effect.35 Therefore, the visible-light-driven photocatalytic activity can be expected from these multinanoheterostructures photoanodes. 3.5. Photoluminescence Spectroscopy. The photoluminescence (PL) spectra of different samples were measured at 25 °C, by using a xenon lamp with 325 nm excitation wavelengths and the results are shown in Figure 5. The ZnO

Figure 5. Photoluminescence spectra of ZnO nanoflowers at 25 °C (Z), ZnO/ZnTiO3/CdS (ZZS), ZnO/ZnTiO3/CdS/CdTeS (ZZST), and ZnO/ZnTiO3/CdS/CdTeS/ZnS (ZZSTZ) multinanoheterostructures.

PL spectrum consists of two parts, i.e., excitonic near band edge emission and the defects related deep level emission on the visible region.36 The ZnO has UV excitonic emission peak exhibiting visible luminescence due to extrinsic and intrinsic defects. The origin of these different emissions, especially the green emission considered controversial. This may arise due to the synthesis procedure of ZnO nanoflowers and to the complex nanostructure. The ZnO UV emission depends on some different factors, i.e., the direct transition from conduction band to valence band, transition of zinc interstitial (Zni) defect level to valence band, transition from the conduction band to zinc vacancy (VZn) which is a result of blue−green emission of the material, and transition from Zni to VZn.36 The ZnO nanoflowers have a stronger emission peak at 390 nm and can be ascribed to band-to-band transition. However, it was observed that the strong and sharp UV band edge emission peak of the ZnO corresponds to the free excitons recombination process. In the literature,37 it was studied that the emission is due to different defects in ZnO nanocrystals. The emission intensity becomes weaker for ZnO nanoflowers and can be corelated to annealing at 500 °C for 30 min. In this system, as CdS and CdTeS QDs were introduced into ZZ to broaden the absorption range from UV to UV−vis, which enhances the photocurrent conversion efficiency. It was observed that the CdS and CdTeS QDs can accept the photogenerated electron hole pairs, which decreased its PL intensity and significantly enhanced the transient photocurrent.38 It is noteworthy that the incorporation of these QDs as a double sensitizer to the core materials lead to the separation of photogenerated electron hole pairs.39 The UV emission is also dependent on the crystal orientation and grain size. The spatial charge separation and resulted PL quenching should be the major contribution to the reduced PL intensity of the CdS and CdTeS coating on ZZ. As shown in Figure 5, ZnS buffer layer grown on the multinanoheterostructures further decreases the PL intensity. The green emission is endorsed to the near-band-edge emission of CdS and CdTeS, associated with the free-exciton recombination in CdS and CdTeS at room temperature.36 The PL emission spectra of all these samples sensitized with different QDs on the surface of ZZ 21100

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bias of 1.1 V of the bare Z, ZZ, ZZS, ZZT, ZZST, and ZZSTZ multinanoheterostructures photoanodes are 0.84, 1.21, 1.54, 3.91, 7.05, and 9.41 mA/cm2, respectively. At the same bias, the cosensitization effect of these QDs as a double sensitizer photoanode exhibits much higher photocurrent density in comparison to that of a single sensitizer photoanode. The incorporation of these QDs into ZZ increases the light absorption that produces the light-induced electron hole pairs and generates a strong electric field around these QDs. This electric field reinforces the separation of electron hole pairs on the surface of ZnO nanoflowers and ZnTiO3 nanoparticles. These generated electron hole pairs can be separated from the surface potential in which the electrons move to the counter electrode, and holes shift to the working electrode surface.42,43 Herein, several aspects are included for the enhancement: (i) The surface morphology of the Z plays a key role in the efficient charge separation and transport properties and also possesses superior light harvesting efficiency, while the exposed nanoflowers in the electrolyte solution favor more nanoparticles. (ii) The ZZ photoanodes decorated with QDs in the visible region have strong absorption as compared with that of the bare Z and ZZ photoanodes that raised the use of solar energy. (iii) The band-edge structure of these photoanodes provides an efficient path for the transportation of charges and establishes a higher resistance to carry excited electrons back to the electrolyte, which gives rise to high performance of the decorated electrodes. The ZZ and CdS nanoheterostructures are beneficial to collect the excited electrons of CdS from CdTeS to ZZ in the stepwise band-edge structure. (iv) Last, the broader spectral response of CdS/CdTeS than ZZS or CdTeS interface will boost the effective surface area, resulting in the improved photocurrent density. The band edges of these semiconductors QDs favor charge separation that arises from the Fermi level’s realignment. A solution containing a redox couple in which semiconductors are brought closer to each other gain identical Fermi levels of the semiconductors and the solution after electrostatic equilibrium, which causes the upward and downward shifts of the band-edges for the semiconductors. However, more work needs to be done for the improved photocurrent conversion efficiency of a multi sensitizing system. To better understand the PEC performance for water oxidation behavior of these photoanodes, the electrochemical impedance spectroscopy (EIS) for assessing the charge transfer resistance was measured, and the results are given in Figure 7a. In the Nyquist plot, the smaller arc corresponds to low charge transfer impedance at the electrode−electrolyte interface for the associated electrode.44,45 The multinanoheterostructures photoanodes represent a smaller arc than that of the ZnO nanoflowers, which ensure effective charge carrier separation. The smallest arc of ZZSTZ represents the most effective charge separation among the other photoanodes. The ZZST has a smaller impedance because of the larger visible light absorption range, giving rise to higher photoinduced charge carrier density and faster hole transfer kinetics than ZZS. The photoanodes with larger impedance have the poor PEC performance. The fitted apparent charge transfer resistance are 1459 Ω (ZZSTZ), 1924 Ω (ZZST), 2184 Ω (ZZS), 3041 Ω (ZZ), and 4589 Ω (Z) in the Nyquist plot was observed, which suggests the overall high conductivity of ZZSTZ and is conducive to photogenerated carrier migration. Figure 7b indicates the Bode plots that reflect the efficient electron lifetime of these photoanodes. The characteristic peak frequency shifts to the

show that the optical properties are extremely sensitive to their defects densities and morphology. 3.6. Photoelectrochemical Performance. The PEC analyses of these multinanoheterostructures photoanodes were conducted through photoelectrochemical workstation using a mixture of 0.25 M Na2S and 0.35 M Na2SO3 solutions as reported40,41 and results are given in Figure 6. Scheme 1

Figure 6. Current density vs applied potential characteristics (10 mV/ s) of ZnO nanoflowers (Z), ZnO/ZnTiO3 (ZZ), ZnO/ZnTiO3/CdS (ZZS), ZnO/ZnTiO3/CdTeS (ZZT), ZnO/ZnTiO3/CdS/CdTeS (ZZST), and Zno/ZnTiO3/CdS/CdTeS/ZnS (ZZSTZ) multinanoheterostructures. The inset shows the dark scan.

Scheme 1. Schematic Diagram of the Experimenta

a

The ZnS layer is neglected for that it is only to control the surface defects and have no effect for the energy coupling.

illustrates the incident photon generates electron−hole pairs, transport and separation of charges, and oxidation and reduction reactions of electrolyte. The current density versus potential characteristics of the multinanoheterostructures photoanodes under dark and illumination were measured, while saturated calomel electrode (SCE) was used as a reference at a scan rate of 10 mV/s. In dark, the photocurrent density and applied potential of Z, ZZ, and ZZSTZ photoanodes exhibited typical rectifying behavior with current responses less than 1.07 mA/cm2 as shown in the inset of Figure 6. In this work, it was observed that under illumination the bare Z photoanode has the lowest photoresponse current density in comparison to that of ZnTiO3 and cosensitized CdS, CdTeS QDs photoanodes. The photocurrent density under 21101

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Figure 8. IPCE spectra of Z, ZZS, and ZZSTZ multinanoheterostructures photoanodes.

charge separation, transport of photoinduced electron hole pairs, and light harvesting, as well as having excellent behavior in PEC water splitting. The ZZ photoanodes decorated with CdS/CdTeS QDs layers act as blocking layers to protect the ZnO core from the electrolyte and QDs. The best optical and improved photoelectrochemical performance of these materials such as bare Z, ZZ, ZZS, ZZT, and ZZSTZ multinanoheterostructures photoanodes provide a feasible route to fabricated photocatalysts. The resultant 3D heterojunctions demonstrated better light absorption and charge transport properties as compared to those of 1D and 2D heterojunctions. Moreover, ZZSTZ multinanoheterostructures photoanodes showed the enhanced PEC performance. These materials provide a maximum photocurrent density up to 9.41 mA/cm2 under AM 1.5 G illumination. The observed findings conclude that the promising achievements could be due to the improved absorption efficiency, reduced recombination rate including charge transportation, and enhanced photoelectron collection process.

Figure 7. (a) Nyquist plots of the impedance spectra. (b) Bode plots of the multinanoheterostructures.

lower value of these multinanoheterostructures photoanodes imply a longer electron lifetime than the ZZ or ZZS photoanodes. This analysis shows that the introduction of CdS and CdTeS QDs help in promoting the charge separation and prolonging the lifetime of the photogenerated electron. IPCE analysis was carried out for Z, ZZS, and ZZSTZ to assess the wavelength-dependent light-harvesting efficiency of the multinanoheterostructure photoanodes. The wavelength was set from 300 to 700 nm under 0 V bias, and the following equation was used to calculate IPCE performance. IPCE (%) =

(1240)I Jlight λ

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04701. XRD study analysis, XRD patterns, TEM image, and XPS spectrum (PDF)

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(1)

In eq 1, I and λ represent the photocurrent density and wavelength of incident light respectively, where Jlight is the observed irradiance at a certain wavelength. Photoactivity for the prepared sensitized multinanoheterostructures photoanodes were observed in the light region from 300 to 590 nm. A maximum of 52% IPCE value was recorded for designated sample ZZSTZ. This is two times higher when compared with that of Z and quite encouraging for wavelength-dependent light harvesting efficiency as shown in the Figure 8. The obtained results exhibit that the prepared 3D ZnO nanoflowers can boost the photochemical reactions by providing large expose surface area to the sensitized QDs.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wenting Zhang: 0000-0001-8924-2003 Zengming Zhang: 0000-0001-8245-9955 Xiaoliang Xu: 0000-0002-5549-9051 Notes

The authors declare no competing financial interest.

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7. CONCLUSION The surface morphology of ZnO/ZnTiO3 nanoflowers on the top of FTO decorated with different QDs greatly increased the

ACKNOWLEDGMENTS Thanks to Dr. Zhonglin Wang for kind help in measurements and discussions. A.A. thanks the China Scholarship Council for 21102

DOI: 10.1021/acs.jpcc.7b04701 J. Phys. Chem. C 2017, 121, 21096−21104

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The Journal of Physical Chemistry C

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