ZnO-TiO2 CoreâShell Nanowires: A Sustainable Photoanode for

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ZnO-TiO2 Core-Shell Nanowires: A Sustainable Photoanode for Enhanced Photoelectrochemical Water Splitting Kyuwon Jeong, Prashant R. Deshmukh, Jinse Park, Youngku Sohn, and Weon Gyu Shin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00324 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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ACS Sustainable Chemistry & Engineering

ZnO-TiO2 Core-Shell Nanowires: A Sustainable Photoanode for Enhanced Photoelectrochemical Water Splitting Kyuwon Jeong,a# Prashant R. Deshmukh,a#, Jinse Parka, Youngku Sohn,b Weon Gyu Shina,* a

Department of Mechanical Engineering, Chungnam National University, Daejeon-34134, Republic of Korea b

Department of Chemistry, Chungnam National University, Daejeon-34134, Republic of Korea *Corresponding Author: Tel.: +82 42 821 5647; Fax: +82 42 822 5642. E-mail: [email protected] (W. G. Shin)

Abstract We present the synthesis of a unique vertically aligned ZnO-TiO2 core-shell nanowires (NWs) heterostructure on the Si-wafer using a chemical vapour deposition (CVD) method. The structural study shows the well-developed ZnO-TiO2 core-shell NWs heterostructure. This unique ZnO-TiO2 core-shell NWs heterostructure displays the photocurrent density of 1.23 mAcm-2, which is 2.41 times higher than the pristine ZnO NWs. A cathodic shift in the flat band potential and a lower onset potential of ZnO-TiO2 core-shell NWs heterostructure over the ZnO NWs indicates a more favourable properties for photoelectrochemical water splitting with photoconversion efficiencey of 0.53 %. A higher photocurrent density/photoconversion efficiency is due to the effective addition of photogenerated electron-hole separation originating from the ZnO NWs core and the conformal covering of a amorphous TiO2 passivation shell. Therefore, these results suggest that the vertically aligned one-dimentional (1D) ZnO-TiO2 core-shell NWs heterostructure is promising photoanode for solar energy conversion devices.

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Keywords: Chemical vapour deposition, ZnO nanowires, ZnO-TiO2 core-shell nanowires, Photoelectrochemical, Water splitting. Introduction The rising world’s energy consumption, exhausting global fossil fuels and increasing anxiety on the climate change have motivated awareness and development of

an

alternative

clean,

efficient

and

renewable

energy

technologies.

Photoelectrochemical (PEC) water splitting is most favourable, eco-friendly and renewable energy technique for the conversion of chief, clean and an abundant sunlight into hydrogen and oxygen by the use of semiconductors.1-4 PEC water splitting has received great attention since the discovery of photocatalysis in TiO2 by Fujishima and Honda.5 The efficiency and stability performance are crucial features of PEC cells, which are dependent on the semiconductors used. Therefore, many semiconductor materials have explored for the PEC cells, such as, TiO2,6-8 ZnO,9,10 BiVO4,11,12 α-Fe2O3,13,14 and WO3.15-17 Among these metal oxides, ZnO and TiO2 are extensively explored in the photo electrochemical application as crucial materials due to their facile synthesis, natural abundance, low-cost, and low-toxicity to the environment.18-23 Though these materials have revealed the interesting performance, yet the reported efficiency is too low for everyday use. Because of wide band gap, they show a high photoactivity in the UV region and limited in the visible region. Nevertheless, sunlight reaches on the earth’s surface constitutes with small fraction of UV (less than 5%) and maximum of visible light.19,21,23-25 Many efforts have investigated to enhance their photoactivity, such as, composite formation,8,22,26,27 element doping,19,20,28,29 construction of nanostructured 2 ACS Paragon Plus Environment

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materials

(NTs,6,24,30-32

NWs,32-34

NBs,35

NRs,9,35-37)

and

core-shell

heterostructures.10,18,38,39 Nowadays, core-shell heterostructures have much more attracted in PEC water splitting due to unique physical and chemical characteristics, which fulfil the necessities of an efficient electrode for PEC and solar cell by overwhelming the individual deficiencies.10,18,38-42 Core-shell heterostructure, typically instituting from one-dimensional structure offers a large interfacial area for the rapid charge carrier separation, a short diffusion length, a better absorption of light, improved charge carrier transport and collection efficiency.43,44 There are some examples of core-shell heterostructure used for solar energy related applications. For example; ZnO-Al2O3 and ZnO-TiO2 core-shell NWs in DSCs,45 ZnO and SnO2 surrounded by a TiO2 nanosheet shell in DSSCs,41,46 p-type Fe2O3 based photocathode with the aid of multi-heterojunction and TiO2 coating,47 ZnO-Fe2O3 core-shell NWAs,48 1D ZnO-BiVO4 heterojunction,49 Si-ZnO core-shell NWAs,50 ZnO-TiO2 core-shell NWs,51,52 and ZnO-TiO2 core-brush nanostructures53 for photocatalytic/PEC water splitting applications. Therefore, suitable, well-ordered development of 1D core-shell structure with ideal semiconductors is key to amending the PEC properties. Among the several methods, wet-chemical methods in two-steps have been frequently used in the development of various core-shell or nanocomposite structures. Usually, these methods involve environmentally unfriendly chemicals, surfactants, shape-directing reagents and take longer time (more than 2h) for the development of nanostructured materials.17,35,51 In such methods, any deviations from the pre-set conditions or post annealing process results in the formation of impurity structure.18,54 However, it leftovers a main challenge to develop the nanostructured materials 3 ACS Paragon Plus Environment


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without using them. In addition, while developing the core-shell structure, it is crucial to control the thickness of shell at nm scale, while maintaining their crystalline structure. Since these factors affects the photocatalytic performance of the prepared nanomaterials. Consequently, a simple, unique, high deposition rate and suitable chemical vapour deposition (CVD) method is an alternative to deposit 1D nanostructure (NWs) or core-shell nanostructures.42,55,56 Particularly, CVD is more appropriate for synthesis of high quality vertically aligned ZnO NWs on the Si-wafer with precisely controlled diameter (less than 100 nm) and length (greater than 2 µm) within short time than wet chemical methods.42,55,56 At this point, normal wetchemical methods fails to develop this kind of NWs up to precise level.32,34,51,52 This type of NWs can provide the large surface area for the photocatalytic activity in the PEC study. By considering the literature overview, to the best of our knowledge there is no report available on the synthesis of ZnO-TiO2 core-shell NWs with different TiO2 shell thickens by CVD method. Thus, an optimization and proper development of premium 1D core-shell heterostructure using wide band gap ZnO and TiO2 is yet challenging and vital to enhance the PEC property. Therefore, an attempt has made to prepare the vertically aligned ZnO NWs and further deposition of TiO2 shell to make the ZnO-TiO2 core-shell NWs heterostructure for the enhancement of water splitting efficiency. Accordingly, vertically well-aligned crystalline-amorphous ZnO-TiO2 core-shell NWs heterostructures were synthesized by CVD method on Si-wafer and efficiently studied in PEC water splitting. The outcomes of the study is that the ZnOTiO2 core-shell NWs heterostructure shows the enhanced PEC performance than the 4 ACS Paragon Plus Environment

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pristine ZnO NWs and several ZnO based core-shell structures. Therefore, the present study can be useful for the development of core-shell photoanodes in PEC application. Experimental Method Synthesis of ZnO NWs Vertically well-aligned ZnO NWs were synthesized by means of thermal evaporation of zinc wire (3.18 mm dia., 99.95 % metal basis, ~57.3 g/m, Alfa Aesar) using CVD method under the precisely controlled conditions. Foremost, a thin carbon film was coated on the cleaned p-type Si-wafer. Carbon nanopowder (particle size < 50 nm, trace metal basis ≥ 99 %, Aldrich) and cleaned Si-wafer were placed in the alumina boat and heated up to 1100°C to evaporate the carbon nanopowder for 1 h. The carrier gas (pure N2 gas) for coating of carbon was set to 1 liter per minute (LPM). After that, the alumina boat containing a bulk Zn wire and a carbon coated Siwafer was placed in the centre of tube furnace. Then, ZnO nanostructures were synthesized through thermal evaporation of Zn wire on a carbon coated Si-wafer. Finally, vertically well-aligned ZnO NWs were obtained by varying the experimental parameters such as, temperature, carrier gas (mixture of argon and oxygen) flow rate, and thermal evaporation time. Table S1 (ESI-Electronic Supplementary Information) shows various preparative parameters for the synthesis of vertically well-aligned ZnO NWs.

Synthesis of ZnO-TiO2 Core-Shell NWs In order to make ZnO-TiO2 core-shell NWs heterostructure, TiO2 shell was deposited on the previously synthesized ZnO NWs core with the TTIP (titanium 5 ACS Paragon Plus Environment


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tetraisopropoxide, ≥ 98%, Daejung) precursor using CVD method. A carrier gas of N2 with 0.55 LPM was supplied into the bubbler containing TTIP precursor to generate the TTIP vapour and further dilution (N2) gas with 6.45 LPM was mixed into the carrier gas. The bubbler was maintained at ~ 25°C and the gas line was kept at 60°C to prevent the condensation of TTIP vapour. Temperature inside the tube furnace was maintained at 450°C to acquire a thermal decomposition of TTIP. TTIP vapour was passed into the tube furnace when the temperature was reached at 450°C by opening the valve. A TiO2 nano shell with various thicknesses on the ZnO NWs core was deposited by passing the TTIP vapour into the tube furnace for various flow time ranging from 15-60 min. at 450°C. After that, the tube furnace was allowed to cool down slowly at room temperature. The schematic representation of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure synthesis using CVD method is shown in Figure S1 of supporting information.


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Figure 1. Schematic representation for the synthesis of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructures. Figure 1 shows schematic representation of synthesis of ZnO NWs and ZnOTiO2 core-shell NWs heterostructure. Initially, a thin layer of carbon was coated on Si-wafer. After that, well-aligned ZnO NWs were grown on the carbon coated Siwafer. Finally, a thin layer of TiO2 shell was deposited on the ZnO NWs core. Thus, the ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure was synthesized by CVD method. Results and Discussion Material characterization techniques are given in the supplementary information. FE-SEM (top and tilted view) images of ZnO nanostructures obtained at different temperatures ranging from 550 to 750°C are shown in Figures 2 and S2. As given in the Table S1, during the synthesis of ZnO nanostructures at different temperatures, the flow rate and vaporization time was 0.6 LPM and 30 sec., respectively. Figure S2 (a & e) shows top and tilted view (cross-section) images of ZnO obtained at 550°C. It shows the sparsely ZnO spherical nanoparticles, which have the average diameter of about 100 nm. A complex structure of ZnO NRs and NWs were observed at 600°C (Figure S2 (b & f)). ZnO NRs and NWs have the average length of 500 nm. The average diameters of ZnO NRs and NWs were about 0.3 µm and 30 nm, respectively. When the synthesis temperature was increased to 650°C, the NWs have average diameter of 40 nm and length of 2.5 µm (Figure S2 (c & g)). Further, the high density vertically aligned ZnO NWs were obtained at 700°C temperature as shown in Figure 2 (a & b). The average length and diameter of ZnO 7 ACS Paragon Plus Environment


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NWs are 5 µm and 60 nm, respectively. Figure S2 (d & h) shows the ZnO NWs obtained at 750°C, whose average length and diameter are about 3.5 µm and 100 nm, respectively. We have observed variation in the morphology of ZnO obtained at different temperatures. It is believed that surface morphology of ZnO depends upon the growth temperature, vapour pressure and super-saturation of vapour. As the temperature changes, zinc vapour pressure varies, which determines the supersaturation of Zn vapour giving rise to different ZnO nanostructures.55


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Figure 2. FE-SEM (a) top (inset is magnified view) and (b) tilted view (cross-section) images of ZnO NWs grown at 700°C temperature. XRD patterns (c & d), Raman spectra (e & f), and PL spectra (g & h) of ZnO nanostructures synthesized at various temperature and flow rates of carrier gas (See the Table S1 for detail conditions). FE-SEM (top and tilted view) images of ZnO nanostructures grown at various flow rates of carrier gas are shown in Figure S3, while the temperature and time was 700°C and 30 sec., respectively. Figure S3 (a & d) shows top and tilted view images of ZnO NRs obtained at a carrier gas flow rate of 0.5 LPM. ZnO NRs have the diameter of about 100 nm and length of 1.5 µm. For the 0.6 LPM gas flow rate, the obtained ZnO NWs are discussed in Figure 2 (a & b). ZnO NWs obtained at a carrier 9 ACS Paragon Plus Environment


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gas flow rate of 0.7 LPM is shown in Figure S3 (b & e). In this case, ZnO NWs have the similar diameter and length with that of ZnO NWs obtained for 0.6 LPM gas flow rate. However, growth of ZnO NWs in random direction disturbed their vertical alignment. Further, ZnO NWs continued to grow in random directions at 1 LPM gas flow rate, and have the length of 4.5 µm and the diameter of 90 nm (Figure S3 (c & f). Thus, vertically aligned ZnO NWs are observed at 0.6 LPM gas flow rate and their vertical alignment changed into random growth at higher gas flow rate. Residence time of Zn depends on the flow rate of carrier gas. Residence time is higher at lower flow rate of carrier gas, which causes the short and thick ZnO nanorods. At moderate carrier gas flow rate, residence time is enough to maintain the vertical alignment with long and thin ZnO NWs. Relatively, residence time is very short at higher carrier gas flow rate, which causes the random growth of ZnO NWs. Crystallinity of the obtained ZnO nanostructures at various temperatures and flow rates of carrier gas are characterized by XRD as shown in the Figure 2 (c & d), respectively. All the XRD patterns show very good crystalline nature of ZnO nanostructures obtained at different temperatures and flow rates of carrier gas. The strong characteristics (002) peak at 2θ=34.5° indicates growth of ZnO nanostructures mainly along the c-axis direction. There are other weak peaks at 2θ=31.6°, 36.22°, 47.51°, 56.5°, 62.7°, and 67.8° correspond to (100), (101), (102), (110), (103) and (112) planes, respectively. Thus, the XRD patterns show that ZnO nanostructures have wurtzite crystal structure (JCPDS Card No. 36-1451).9,57 Raman spectroscopy was used to study further crystallinity of ZnO nanostructure. Raman spectra of ZnO nanostructures obtained at various temperatures are shown in Figure 2e. The strong 10 ACS Paragon Plus Environment

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peak of Si-wafer is observed at about 520 cm-1.58 ZnO nanostructures obtained at 550650°C temperatures have a very weak peak at about 438 cm-1 as shown in Figure 2e, which corresponds to the E2 (high) phonon mode of wurtzite ZnO.19,35 Further, increase in synthesis temperature from 650°C to 700 and 750°C, the intensity of E2 (high) phonon mode becomes stronger, which exhibits the increased crystallinity of ZnO NWs produced at 700 and 750°C. Along with this, vibration mode of E2H+E2L at about 375 cm-1 and A1(LO) & E1 (LO) mode at about 581 cm-1 are weakly identified at 750°C (Figure 2e). These peaks show that ZnO NWs obtained at relatively higher temperature have a good crystalline nature with wurtzite phase. The observed A1(LO) & E1(LO) mode was due to oxygen vacancy defect presence in ZnO.19,35,59 Figure 2f shows the Raman spectra of ZnO nanostructures obtained at various flow rates of carrier gas. The characteristic peak observed at 438 cm-1 corresponds to wurtzite ZnO phase.35 When the carrier gas flow rate was increased, the intensity of peak at 438 cm1

was increased slightly. A high crystallinity of ZnO nanostructures is preserved over

the synthesis of various flow rates of carrier gas. Figure 2g shows the photoluminescence (PL) results of ZnO nanostructures obtained at various temperatures. All the PL spectra of ZnO nanostructures obtained at various temperatures show the strong ultraviolet (UV) emission peak at 380 except the ZnO synthesized at 550°C, which shows the peak at 375 nm. In addition, the PL spectra of ZnO nanostructures obtained at 600-650°C temperatures show UV emission peaks at 375 nm and 380 nm.56,60,61 UV emission peaks can be explained by the near band-edge emission of the wide band gap of ZnO, which is attributed to the recombination of free excitons through an exciton-exciton collision process.56,60,61 11 ACS Paragon Plus Environment


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Besides, the broad green emission band from 500-550 nm was observed in the ZnO nanostructures obtained at different temperatures. This green emission band related to zinc interstitial sites and intrinsic defects is due to the zinc and oxygen vacancies present in the band gap of ZnO.62 This green emission band is feeble in all the PL spectra (except at 750°C), which shows the low concentration of oxygen vacancies or structural defects and high crystallinity of ZnO nanostructures, especially ZnO NWs at 700°C.44 Such, a strong UV emission peak and weak green emission band was also observed in needle-like ZnO NWs.61 The increased intensity of green emission band in ZnO NWs synthesized at 750°C indicates the increased structural defects, which causes higher charge carrier recombination as compared with others. This is consistent with oxygen vacancy defect related A1(LO) & E1(LO) mode detected at 581 cm-1 in Raman spectra of ZnO NWs obtained at 750°C. Figure 2h shows the PL spectra of ZnO nanostructures obtained at various flow rates of carrier gas. All the PL spectra show the strong UV emission peak at 380 nm and broad green emission band centered at 520 nm due to the near band-edge emission and defects present in the band gap of ZnO, respectively.44,56 The observed green emission band has weak intensity same as the previous one, which suggest the high crystallinity and low structural defects are conserved during the synthesis of ZnO nanostructures at different flow rates of carrier gas. HR-TEM image and SAED pattern (Figure S5) of ZnO NW shows the high crystallinity and growth along (001) directions. Among the various ZnO nanostructures, ZnO NWs obtained at 700°C, 0.6 LPM carrier gas flow rate and synthesis time of 30 sec. shows the maximum photocurrent density (Explained in PEC section). Therefore, ZnO NWs are considered 12 ACS Paragon Plus Environment

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as suitable for the construction of core-shell heterostructure with variable shell thickness. TEM images of the ZnO-TiO2 core-shell NW heterostructure are shown in Figure 3. The thickness of TiO2 shell was controlled by changing the deposition time of TTIP vapour. In our experiments, the deposition time was set to 15, 20, 25, 30, 45 and 60 minutes. Initially, for 15 min., only small amount of TiO2 particles with thickness of about 1-2 nm were agglomerated on the ZnO NW, as shown in Figure 3a. We observed that, the coating of TiO2 shell is not uniform at the early 15 min., this might be due to the insufficient time for thermal decomposition of TTIP and saturation of nucleation sites of Ti species on the surface of ZnO NW. Further, when the deposition time of TTIP vapour was gradually increased to 20, 25, 30, 45 and 60 min., more active species of Ti were deposited on the ZnO NW surface creating the thin layer of TiO2 shell with the thickness of 5, 10, 15, 28, and 50 nm, respectively (Figure 3 (b-f)).



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Figure 3. TEM images of ZnO-TiO2 core-shell NW with various TiO2 shell thickness according to deposition time: (a) 15, (b) 20, (c) 25, (d) 30, (e) 45 and (f) 60 minutes. Among the various TiO2 shell coating, the 15 nm TiO2 shell obtained by the 30 min. deposition time shows the maximum photocurrent density (Explained in PEC section). Hence, to get insight of it, the ZnO-TiO2 core-shell NWs heterostructure with 15 nm TiO2 shell thickness is characterized with different characterization techniques and comparative results of ZnO NWs and ZnO-TiO2 core-shell NWs are discussed. Figure 4 shows the (a) XRD, (b) Raman, (c) PL, and (d) UV-vis spectra of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure. XRD and Raman results depicted in Figure 4 (a & b) reflect the well-preserved crystal structure of ZnO NWs even after the coating of TiO2 in ZnO-TiO2 core-shell NWs. The absence of any other structural peaks in the XRD pattern indicates the formation of pure ZnO-TiO2 core-shell NWs 14 ACS Paragon Plus Environment

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even at high temperature. This is better than the solution growth ZnO:Cl/TiO2 coreshell NWs, where adverse effect on the photocurrent density was observed due to formation of ZnTiO3 layer after thermal treatment at 500°C.54 There are no diffraction peaks corresponding to the TiO2 crystal structure, which can be ascribed to the small thickness (15 nm), not enough to detect in the XRD, or formation of amorphous structure of the TiO2 shell on the surface of ZnO NWs core. Raman results (Figure 4b) of ZnO-TiO2 core-shell NWs heterostructures are also cares the XRD results as described before. Thus, the resultant ZnO-TiO2 core-shell NWs heterostructure does not show any significant changes in the XRD and Raman spectra compared to the ZnO NWs. This type of absence in structural changes is also observed in BiVO4/ZnO/TiO2 heterostructure due to the thin layer of shell.63 PL and UV-Vis spectra of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure are shown in Figure 4 (c & d). Though the intensity of broad green emission peak of crystalline ZnO NWs in the PL spectrum is low, still we can observe the small decrease in the intensity after the TiO2 shell coating. This shows the recombination of the electron-hole pairs at surface defects of ZnO NWs was quenched by TiO2 shell.64 Both the ZnO NWs and ZnO-TiO2 core-shell NWs show strong absorbance in the ultraviolet region (Figure 4 d). The ZnO-TiO2 core-shell NWs show much more absorbance in the range of 300-390 nm as compared to ZnO NWs. Moreover, a small red shift in the absorption edge and higher absorbance of ZnO-TiO2 core-shell NWs shows the enhanced photon absorption in the visible region, which is due to the high optical absorption coefficient of TiO2.23



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Figure 4. (a) XRD patterns, (b) Raman spectra, (c) PL spectra, and (d) UV-Vis absorbance spectra of ZnO NWs and ZnO-TiO2 core-shell NWs, (e) high-resolution


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Zn 2p spectra of ZnO NWs and ZnO-TiO2 core-shell NWs, (f) high-resolution Ti 2p spectra of ZnO-TiO2 core-shell NWs. X-ray photoelectron spectroscopy was used to identify the elemental composition of ZnO NWs and ZnO-TiO2 core-shell NWs. Wide scan survey spectra of ZnO NWs and ZnO-TiO2 core-shell NWs are shown in Figure S6a. Figure 4e shows the high-resolution Zn 2p core-level XPS spectra of ZnO NWs and ZnO-TiO2 core-shell NWs. The peak centered at binding energies of 1046 and 1023 eV corresponds to the Zn 2p1/2 and Zn 2p3/2, respectively. A spin-orbit energy separation of 23 eV is in good agreement with literature value of Zn 2p states in the ZnO.40, 65 After the deposition of TiO2 shell, both the Zn 2p1/2 and Zn 2p3/2 peak shifts slightly to higher energy levels with decrease in an intensity, which can be attributed to the significant interface between amorphous TiO2 shell and ZnO NWs core.40 Figure 4f shows the high-resolution core-level XPS spectrum of Ti 2p. For the ZnO-TiO2 coreshell NWs, the locations of binding energies of Ti 2p1/2 and Ti 2p3/2 peaks centered at 466 and 460 eV are in good agreement with the values of Ti4+ assigned in TiO lattice.22 Compared to standard Ti 2p peak locations, which are at 464.3 eV for 2p1/2 and at 458.5 eV for 2p3/2, the Ti 2p peaks in the ZnO-TiO2 core-shell NWs are shifted to higher values. This binding energy difference may be due to a particle size and interface effects between TiO2 particles and ZnO NWs.53 Deconvoluted O1s XPS spectra of ZnO NWs and ZnO-TiO2 core-shell NWs are shown in Figure S6 b & c, respectively, which consists of three peaks. These asymmetric spectra exhibit that there are two kinds of O chemical states according to the binding energy range from 528 to 533 eV, comprising crystal lattice oxygen (OL) and hydroxyl oxygen (OH) with 17 ACS Paragon Plus Environment


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increasing binding energy. The highest peak centered at 530 eV can be assigned to the lattice oxygen bonded to Zn+ or Ti+ ions in ZnO or ZnO-TiO2. The other two peaks with binding energies of 531.4 eV and 532.4 eV are attributed to oxygen vacancies or defects and chemisorbed or dissociated oxygen species in the ZnO NWs and ZnOTiO2 core-shell NWs surface.53,66 In this way, the structural study reflects welldeveloped ZnO-TiO2 core-shell NWs heterostructure. ZnO nanostructures obtained at various temperatures and flow rates of carrier gas were successively investigated for maximum photocurrent density as shown in Figure S7. The details of PEC characterization is given in the supporting information. ZnO nanostructures obtained at various temperatures and flow rates of carrier gas show the very low dark current of ~ 0.1 µAcm-2 over the entire potential scan. But, these nanostructures show the enhanced photocurrent density under the light illumination. Among these, vertically well-aligned ZnO NWs obtained at 700°C and 0.6 LPM carrier gas flow rate shows the maximum photocurrent density of 0.51 mAcm-2 at + 0.8 V vs. RHE under the light illumination. The maximum photocurrent density is due to the high density, high surface-to-volume ratio and superior charge transport path due to one-dimensional structure of ZnO NWs. It has shown that wellaligned 1D nanostructure gives maximum photocurrent as compared with randomly orientated nanostructures and compact nanoparticles due to its charge collection efficiency and direct path to photogenerated electrons.7 Therefore, TiO2 shell was deposited on ZnO NWs to pursue the ZnO-TiO2 core-shell NWs heterostructure. Synthesis and characterizations of ZnO-TiO2 coreshell NWs heterostructure are explained in the preceding sections. Figure 5a shows 18 ACS Paragon Plus Environment

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the photocurrent density of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure with variable TiO2 shell thickness. Photocurrent density of ZnO NWs is explained in the previous section. ZnO-TiO2 core-shell NWs heterostructure shows the very low dark current similar to the ZnO nanostructures, which is of ~ 0.1 µAcm-2 over the entire potential scan. Nevertheless, ZnO-TiO2 core-shell NWs heterostructure shows an enhanced photocurrent density, much higher than the pristine ZnO NWs under the light illumination. Pointedly, a linear increase in the photocurrent density with positive potential observed for all the ZnO-TiO2 core-shell NWs heterostructure, which indicates efficient charge separation in core-shell NWs upon illumination. Among these, ZnO-TiO2 core-shell NWs heterostructure with 15 nm TiO2 shell shows significantly lower onset potential (~ +0.2 V vs. RHE), which would be beneficial in the PEC water splitting at a lower potential. Photocurrent density of ZnO-TiO2 coreshell NWs with increasing TiO2 shell thickness shows the increasing trend up to optimum shell thickness and then decreases. When the thickness of TiO2 shell is 15 nm, the photocurrent density increased to 1.23 mAcm-2 at + 0.8 V vs. RHE, which is 2.41 fold higher than photocurrent density of ZnO NWs at the same conditions. When the thickness of TiO2 shell is further increased to 30 and 50 nm, the photocurrent density decreased to 0.6 and 0.25 mAcm-2 at + 0.8 V vs. RHE, respectively. Figure 5b shows the PEC water splitting efficiency of ZnO NWs and ZnO-TiO2 core-shell NWs with various thickness of TiO2 shell. As the maximum photocurrent density was observed at 15 nm TiO2 shell thickness, accordingly maximum PEC water splitting efficiency was observed at 15 nm TiO2 shell thickness in the ZnO-TiO2 core-shell NWs. PEC water splitting efficiency decreases at lower and higher thickness of TiO2 19 ACS Paragon Plus Environment


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shell. The maximum water splitting efficiency of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure is 0.22 and 0.53% at +0.8 V vs. RHE, respectively. The observed efficiency of ZnO-TiO2 core-shell NWs is higher than the other ZnO based core-shell type structures reported previously (Table S2, ESI). In addition, the observed PEC efficiency for ZnO NWs is higher than the reported value of Al-doped ZnO photoanodes.67 In the present study, the enhanced PEC performance is mainly due to the welldeveloped ZnO-TiO2 core-shell NWs heterostructure by uniform coating of amorphous TiO2 shell on ZnO NWs core. ZnO-TiO2 core-shell NWs heterostructure shows the high absorbance in the UV-vis region compared to the ZnO NWs. In addition, vertically aligned dense core-shell structure helps to reduce the reflection losses by absorbing incoming light after multiple reflections. Besides, lower diameter and higher length of ZnO NWs as compared to the reported ZnO electrodes32,34,51,52 for water splitting provides a higher surface to volume ratio for more electrode/electrolyte interface. Consequently, more electron-hole pairs are generated which are effectively separated due to the strong interfacial interaction, band bending alignment and changes in the quasi-Fermi energy level at the core-shell (n/n) heterojunction.35 Furthermore, adsorption and surface reactions on the amorphous TiO2 shell are easier than those on pure crystalline, which further enhances the charge separation by passivating the ZnO NWs surface.68-70 Shalom et al.70 have shown that the improved performance of CdS QD sensitized mesoporous solar cell after the thin coating of an amorphous TiO2. In addition, the TiO2 coating significantly reduces the recombination of electron and holes arising on the surface of ZnO NWs, as evidenced by PL results. 20 ACS Paragon Plus Environment

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Therefore, effective separation and transport of charge-carriers due to 1D core-shell heterostructure are the most influential factors as compared to the absorption of light, typically, which are considered as predominant factors to determine the PECs performance. At lower deposition time of TiO2, electrolyte may encounter ZnO core directly due to the less and non-uniform deposition amount of TiO2, which cannot contribute much more to the separation of photogenerated charges showing the low photocurrent density. The highest photocurrent density at 15 nm TiO2 shell is due to the optimum thickness for effective separation of photogenerated charge carriers.67 It is reported that the thickness-controlled BiVO4/ZnO/TiO2 heterostructured thin film exhibits better water splitting efficiency mainly due to the improved charge separation efficiency, albeit the negligible light absorbance.63 Also, the amorphous structure of TiO2 shell gives higher charge carrier density and longer recombination life time as previously proved with help of scanning Kelvin probe microscopy (SKPM) in the literature.71 On the other side, charge transfer path increases at higher thickness of TiO2 shell triggering higher recombination and lower separation of charge carriers, which decreases the photocurrent density.67

Seo et al.

64

observed reduced

photovoltaic efficiency owing to the decreased electron mobility in too thick TiO2 coating on ZnO in the organic solar cell. Optimum thickness and amorphous structure of shell in the core-shell heterostructure plays an important role in the present study for the enhancement of the photocurrent density. Hence, current CVD method can be used in the development of core-shell heterostructure to control these kind of factors.



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Chronoamperometric (photocurrent vs. time) and electrochemical impedance spectroscopy (EIS) were performed for the best ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure electrodes to obtain the photoresponse, stability and charge transfer process at electrode/electrolyte interface, respectively. Figure 5c shows the It curves of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure under the chopped light with 60 s on/off cycles at +0.8 V vs. RHE over the 900 seconds. Both the photoanodes show a very low dark current of ~ 0.1 µΑcm-2, when the light was turned off. Upon illumination with light, we observed a spike in the photoresponse and the photocurrent quickly returned to a steady state, which shows that photogenerated electrons are quickly transported from TiO2 shell to ZnO core NWs. Only a small decay in photocurrent density (~13 µA over 900 s) in ZnO-TiO2 coreshell NWs heterostructure was observed. This result shows the fast photoresponse and reappearance of the same photoresponse over 900 seconds. Furthermore, there is no degradation of the electrode material observed from the clear transparent electrolyte solution, which suggest that there may not be any structural or morphological changes in the electrodes. Thus, the observed results indicates the stable nature of photoanodes during the PEC process. The stability reported in the present work over the time is quite comparable to the reported stability of tungsten-copper co-sensitized TiO2 NTs (1200 s),72 ZnO NRs49 and ZnO NRs/BiVO4 heterojunction (1000 s).49 The achieved fast photoresponse and chemical stability can be ascribed to the deposition of thin TiO2 shell, which allows fast and efficient transfer of the photogenerated electrons from TiO2 to the ZnO NWs.73


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Figure 5. (a) Photocurrent density, (b) Water splitting efficiency of ZnO NWs and ZnO-TiO2 core-shell NWs at various TiO2 shell thickness, (c) Amperometric I-t 23 ACS Paragon Plus Environment


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curves and (d) Nyquist plots of ZnO NWs and ZnO-TiO2 core-shell NWs, (Inset is enlarged view of high frequency region). (e) Schematic representation of the possible mechanism of photogenerated charge separation at the interface of ZnO-TiO2 coreshell NWs heterostructure with their corresponding energy band diagram during the PEC water splitting. Figure 5d shows Nyquist plots of ZnO NWs and ZnO-TiO2 core-shell NWs in the dark and illumination. Both electrodes show the much smaller charge transfer resistance under the illumination than the dark conditions. Moreover, in both situations, ZnO-TiO2 core-shell NWs indicates a higher electron-transfer process at the electrode/electrolyte interface than the ZnO NWs, which implies a higher electron mobility after the thin coating of TiO2 shell. A positive slope determined from the Mott−Schottky (Figure S8) plots (1/C2 vs. V) suggests the n-type nature of the ZnO NWs and ZnO-TiO2 core-shell NWs.48 As shown in Figure S8, when extra plotted 1/C2 to zero, the intercept at the X-axis corresponds to the flat band potential (VFB), which is estimated to be -0.53 V and -0.74 V vs. Ag/AgCl for ZnO NWs and ZnOTiO2 core-shell NWs, respectively. An inferior flat band potential shows the more efficient charge-separation and transportation in ZnO-TiO2 core-shell NWs heterostructure than the ZnO NWs.72 Thus, the EIS results also supports to the observed higher photocurrent density and efficiency for the ZnO-TiO2 core-shell NWs heterostructure. Considering all the above results, a representative scheme for the charge separation process at the interface of ZnO-TiO2 core-shell heterostructure with their corresponding energy band diagram is illustrated in Figure 5e. When the 24 ACS Paragon Plus Environment

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photoelectrode is illuminated, separation of the photogenerated electron-hole pairs occurs due to the electric field. Photogenerated electrons in the conduction band of TiO2 shell are collected at the Si-wafer through the ZnO NWs before recombination, and then travel towards the Pt electrode through the external circuit. These photogenerated electrons reduce the water to form hydrogen by reacting with hydrogen ions in the electrolyte. Meanwhile, photogenerated high-energy holes in the valence band of ZnO NWs will be efficiently transported toward the electrode surface through the valence band of TiO2 due to the built-in electric field, and involved in the oxidation of water. Therefore, the improved photocurrent is observed in the ZnO-TiO2 core-shell NWs. The excellent electron collection efficiency of ZnO-TiO2 core-shell NWs is due to the high electron mobility of 1D ZnO NWs and the presence of TiO2 shell, which increases the charge separation efficiency. Conclusions CVD deposited vertically aligned crystalline-amorphous ZnO-TiO2 core-shell NWs heterostructure shows the enhanced photocurrent density of 1.23 mAcm-2, which is 2.41 fold higher than the pristine ZnO NWs. This enhancement in the photocurrent density is due to the synergistic effect of the crystalline-amorphous core-shell heterostructure ensuing the efficient separation and quick transportation of charges through the 1D core-shell heterostructure. ZnO-TiO2 core-shell NWs heterostructure exhibits fast photoresponse as well as steady photocurrent density over the 900s. Mott-Schottky analysis of ZnO NWs and ZnO-TiO2 core-shell NWs heterostructure reveals the n-type semiconductor. A cathodic shift in the flat band potential and lower onset potential of ZnO-TiO2 core-shell NWs heterostructure as compared with ZnO 25 ACS Paragon Plus Environment


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NWs demonstrates the improved electron transfer and more favourable energy band positions for the PEC water splitting. Therefore, this type of core-shell NWs heterostructure can be efficiently used in solar water-splitting applications. Conflicts of Interest There are no conflicts to declare. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2014-008356). Supporting Information Synthesis parameter table, FE-SEM images of ZnO nanostructures, HR-TEM image and SAED pattern of ZnO NW, XPS spectra of ZnO NWs and ZnO-TiO2 core-shell NWs, PEC results of ZnO nanostructures, Mott-Schottky plots, and PEC performance table of ZnO based core-shell nanostructures. Authors Contributions # Kyuwon Jeong, and Prashant R. Deshmukh equally contributed.

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Graphical Abstract


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Synopsis Well-developed ZnO-TiO2 core-shell NWs heterostructure demonstrate the excellent photoconversion efficiency towards the hydrogen generation as a clean/green and sustainable energy source.


ZnO-TiO2 CoreâShell Nanowires: A Sustainable Photoanode for

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