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Facile hydrothermally synthesized a novel CdS nanoflower/rutile-TiO nanorod heterojunction photoanode used for photoelectrocatalytic hydrogen generation. 2

Selvaraj David, Mahadeo A. Mahadik, Hee-Suk Chung, Jungho Ryu, and Jum Suk Jang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00558 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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ACS Sustainable Chemistry & Engineering

Facile hydrothermally synthesized a novel CdS nanoflower/rutile-TiO2 nanorod heterojunction photoanode used for photoelectrocatalytic hydrogen generation. Selvaraj Davida, Mahadeo A. Mahadika, Hee Suk Chungb, Jung Ho Ryuc and Jum Suk Janga* a

Division of Biotechnology, Advanced Institute of Environmental and Bioscience,

College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea. b

Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do,

54907, Republic of Korea. c

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral

Resources, Daejeon, 34132, Republic of Korea.

*Corresponding author: Jum Suk Jang ([email protected]) Tel.: +82 63 850 0846; fax: +82 63 850 0834.

ACS Paragon Plus Environment


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Abstract With the application of ethylenediamine (EDA) as a solvent, a template, and coordination agents, novel photoanode architecture of the cadmium sulfide (CdS) nanoflower (NF)/rutile(R)-TiO2 nanorod (NR) heterojunction is successfully synthesized using a facile twostep hydrothermal process. The optimized CdS (medium concentration; MC) NF/R-TiO2 NR heterojunction exhibited a greatly enhanced visible-light photoelectrochemical

(PEC)

performance, whereby the highest photocurrent density of 3.23 mA cm -2 at 0.1 V versus Ag/AgCl and a photoconversion efficiency (PCE) of 0.46 % were achieved under solar light irradiation. The optimal photocurrent density of the CdS (MC) NF/R-TiO2 NR heterojunction, photoanode is 2.54 times (60.68 %) higher than that of the pristine R-TiO2 NR, due to an effective light absorption, an appropriate band-edge position, and the charge separation. Furthermore, the open circuit voltage (VOC) was shifted from -0.85 V to -1.34 V due to the grafting of the CdS NF onto the R-TiO2 NR facet; this surface modification reveals the synergistic effect. The optimal CdS (MC) NF/R-TiO2 NR heterojunction showed a PEC hydrogen (H2) generation of 1007.98 µmol after 3 hr. The design of the novel heterojunction photoanode that is proposed in the present strategy can shed light on the fabrication of new, cheap photocatalysts for an effective H2 generation.

Keywords: Hydrothermal process; CdS nanoflower; Synergistic effect; Photoelectrochemical ; Photoconversion efficiency; Hydrogen generation;


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Introduction Since the pioneering work of Fujishima and Honda, the photocatalytic hydrogen (H2) production from water for which the TiO 2 photoanode is used has been pursued over the subsequent decades,1 and photocatalytic technology is now considered a promising option for the resolution of energy demands.2 Among the photocatalytic technologies, photoelectrochemical (PEC) cells are useful devices that are designed for the chemical process of a H2 generation that occurs via the electrolysis of water under solar irradiation.3-6 To date, several catalysts have been designed and tested, and steady progress has been made regarding the field of photocatalytic H2 production.7,8 The development of a robust, inexpensive, and efficient solar water-splitting system, however, is still greatly challenging. Considerable effort has been spent on the development of titanium dioxide (TiO 2) based photocatalysts over the last 40 years; 9 however, the single-semiconductor photocatalysts usually show low photocatalytic activities due to their wide band gap, and their electron/hole can be easily recombined through photoexcitation.10 To solve this problem, a number of efforts including doping with metal or non-metal ions, noble metal modifications, and the fabrication of heterojunction photocatalysts that contain two semiconductors have occurred for the improvement of the photocatalytic activity; 11-16 among these efforts, the heterojunction materials form a rapidly growing catalyst class for the photoelectrocatalytic generation of H 2.17-19 Recently, to overcome the previously mentioned TiO 2-related drawback, cadmium sulfide (CdS) has been widely used with low-band gap materials for the preparation of the heterojunction photocatalysts; because, not only can it efficiently improve the photoexcited electron/hole separation, but it can also broaden the spectral responsive range.20,21 In terms of the CdS/TiO2 heterojunction, several methods such as electrochemical deposition, chemical-bath



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deposition (CBD), and successive ionic-layer adsorption (SILAR) have been employed.22-25 All of these methods have improved the photoactivity of the CdS-sensitized TiO2 heterojunctions. 2631

Besides these methods and achievements, the hydrothermal method is another suitable method that has been employed to continuously amend the fabrication of one-dimensional (1-D), two dimensional (2-D), or three dimensional (3-D) nanostructures for the enhancement of the physicochemical properties of photocatalysts;32 accordingly, many reports on hydrothermallydeposited CdS/TiO2 heterojunctions are available.33-36 However, to the authors’ knowledge, a study has not yet been conducted on the grafted of hydrothermally synthesized CdS nanoflowers (NFs) onto rutile (R)-TiO2 nanorods (NRs) for which ethylenediamine (EDA) was used as both the template agent and the co-ordination agent. Furthermore, this grafting process is use to convert a crystal into an NF in the presence of the hydrothermal process. In this work, a facile hydrothermal route for which EDA was used as a solvent to explore synthesizes the CdS NFs that are grafted onto the R-TiO2 NR facet. Comparative CdS concentration-variation experiments were carried out to show the growth history of the CdS NFs onto the R-TiO2 NR facet, and a possible growth mechanism is preliminarily proposed and discussed. Here, the conduction and the valance-band position of the CdS NFs grafted R-TiO2 NRs were calculated using the ultraviolet-visible diffuse reflection spectroscopy (UV-DRS) and Mott-Schottky (MS) analyses. The photocatalytic activity of the CdS (medium concentration; MC) NF/R-TiO2 NR heterojunction in terms of the generation of H2 from an aqueous solution containing sodium sulfide (Na2S)/sodium sulfite (Na2SO3) reagents was also investigated. Lastly, the charge-transfer mechanism was systematically explored. These results may be helpful for the


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fabrication of chalcogenide/metal-oxide heterojunctions through the application of amended synthesis techniques and a favorable path in the development of a novel photoanode.

Experimental Section Materials Titanium (IV) butoxide (TB) (C16H36O4Ti) (Aldrich, China), hydrochloric acid (HCL) (Junsei Chemical, Japan), cadmium nitrate tetrahydrate (Cd(NO 3)2.4H2O) (Junsei Chemical, Japan), thiourea (CH4N2S) (Sigma-Aldrich, Germany), and EDA (C2H8N2) (Kanto Chemical, Japan) were acquired for the experiments. All of the reagents were of analytical grade and were used asreceived without further purification.

Synthesis of the R-TiO2 NRs on FTO glass substrate The R-TiO2 NRs were directly grown on transparent fluorine doped tin oxide (FTO) glass substrates using the facile hydrothermal heuristic. The R-TiO2 NRs were prepared according to the report of Liu and Aydil.

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Initially, FTO glass substrates with the dimensions of 1 cm x 2.5

cm were ultrasonically cleaned in acetone, ethanol, and deionized water (DIW) in a series, for which the duration of each cleaning is 10 min, and this was followed by a purging under a nitrogen gas (N2) flow. Typically, 30 mL of DIW was mixed with 30 mL of con-HCL; the mixture was stirred for 5 min at the ambient temperature. Next, 1 mL of TB was rapidly added to the solution followed by 30 min stirring to obtain a clear precursor solution. Furthermore, two pieces of the cleaned FTO substrates were placed in a Teflon-lined stainless steel autoclave with a 120 mL capacity. The prepared precursor solution was then transferred to the autoclave and



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tightly sealed. The hydrothermal reaction was maintained at 150 ° C for 4 h in a fan-forced electric oven. After a natural cooling, the anticipated white color tetragonal R-TiO2 NRs entirely grew onto the FTO substrates, which had been thoroughly washed with copious amounts of DIW and dried in a fume hood for 1 h. Lastly, the R-TiO2 NRs were annealed at 500 ° C for 1 h (5 °C min-1), followed by a natural cooling to room temperature (RT) to promote the crystal structure.

Synthesis of the CdS NFs/R-TiO2 NRs heterojunction photoanode In the typical synthesis process, the CdS NFs were grafted onto the R-TiO2 NRs facet using a hydrothermal process, for which 1 mmol of Cd(NO 3)2.4H2O (MC) is add into a 120 mL Teflon-lined stainless steel autoclave filled with 50 mL of an EDA solution, followed by a 10 min stirring at 300 rpm using a magnetic stirrer. Next, 1.1 mmol of CH 4N2S is add into this solution, and the stirring is continued for another 20 min to obtain a clear solution. One piece of the R-TiO2 NRs is place into the Teflon cylinder; the autoclave is tightly sealed and maintained at 160 ° C for 6 h in a fan-forced electric oven. After a natural cooling, the yellow color CdS NF is graft onto the R-TiO2 NR. Lastly, the deposited films are washed with ethanol three times, dried in oven at 70 ° C for 1 h and then annealed at 350 ° C for 30 min (5 ° C min -1). These assynthesized heterojunctions are represented in this paper as “CdS (MC) NF/R-TiO2 NR heterojunction photoanode”. For a comparison, two other heterojunction photoanodes were synthesized using a similar method for which 0.5 mmol (lower concentration; LC) and 2 mmol (higher concentration; HC) concentrations of Cd(NO3)2.4H2O were used, and the corresponding CH4N2S concentration that were used are 0.55 mmol and 2.2 mmol, respectively; these concentrations were further labeled as “CdS (LC) NF/R-TiO2 NR,” and “CdS (HC) NF/R-TiO2 NR,” respectively.


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Characterization of the n-type semiconductor heterojunction photoanode The surface and cross-sectional morphologies of all of the samples were characterized using field emission scanning electron microscopy (FE-SEM; Carl Zeiss/SUPRA 40VP, Germany). The crystal structures were analyzed according to the X-ray Diffraction (XRD) patterns that were collected on a Bruker D8 diffractometer using copper (Cu) K α radiation. The oxidation state and elemental composition the heterojunction based photoanode were studied using X-ray photoelectron spectroscopy (XPS; PHI Quantera II spectrometer), for which an aluminum (Al) Kα X-ray source was used,) and FE-SEM/X-ray energy dispersive spectrometry (EDX), respectively. The CdS (MC) NF that was grafted onto the R-TiO2 NR heterojunction was characterized using focused ion beam transmission electron microscopy (FIB-TEM; JEOL JEM-3100F) that was operated at 200 kV. The microstructural properties of the resultant nanocomposites were obtained using annular dark field-scanning transmission electron microscopy (ADF STEM) and high resolution transmission electron microscopy (HR-TEM; JEOL JEM-3100F). Solid UV-DRS analysis was conducted using Schimadzu; device (UV-2600 series).

Photoelectrochemical (PEC) measurements The PEC measurements were performed on the COMPACTSTAT electrochemical workstation (Ivium, Netherlands) in a three electrode cell, with the use of the prepared sample as the working electrode (WE), platinum (Pt) foil as the counter electrode (CE), and silver/silver chloride (Ag/AgCl) as the reference electrode (RE). The measurements were performed in an aqueous solution of 0.1 M Na2S and 0.02 M Na2SO3 (pH = 11.5). The illumination source is a solar simulator that provided an irradiation intensity of 100 mWcm -2 on the photoanode. The



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irradiation area of the WE was strictly maintained at 1 cm2. The photocurrent density was measured using linear sweep voltammetry (LSV); intensity modulated photocurrent spectroscopy (IMPS) that was performed in the frequency range of 30 kHz to 0.1 Hz at 0.1 V vs. Ag/AgCl; and MS measurement that were performed under the dark condition. The investigational photo electrochemical impedance spectroscopy (PEIS) data were fitted to the suitable equivalent-circuit model using the Z View (Scribner Associates Inc., USA) software. The photocatalytic H 2 generation was measured in a securely closed reactor for which a 0.1 V vs. Ag/AgCl, the Pt, and the CdS (MC) NF/R-TiO2 NR heterojunction photoanode are the RE, CE, and WE, respectively. Before the light irradiation, N2 gas was purged through the photocatalytic suspension to act as a carrier gas and to complete the removal of the dissolved oxygen in the electrolyte. During the solar-light irradiation, H2 bubbles were generated on the CE electrode surface; they were collected using a syringe and then analyzed via gas chromatography (GC) with a thermal conductivity detector.

Results and Discussion The synthetic route and growth mechanism for the CdS NF and nanaowire (NW) that were grafted onto the photoanode of the R-TiO2 NR facet is illustrated in Figure 1. Initially, the R-TiO2 NRs were vertically grown onto the FTO substrates using the hydrothermal route at 150 ° C for 4 h, followed by an annealing at 500 ° C for 1 h, as shown in Figure 1 (a). Subsequently, due to the various Cd2+ and S2- ion precursor concentrations, which are represented as LC, MC, and HC, the different CdS morphologies were grafted onto the R-TiO2 NRs using a second-step hydrothermal step at 160 ° C for 6 h. Further, the CdS NF and NW that were grafted onto the R-


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TiO2 NR heterojunctions were annealed at 350 ° C for 30 min, as shown in Figure 1 (b). With an increase of the Cd2+ ion concentration, the rod diameter and growth rate of the CdS NF and NW were increased. Herein, the formation of the CdS NF onto the R-TiO2 NR is mainly in the vertical direction first. Moreover, a further increasing of the CdS concentration proved favorable for the coalescence of the neighboring grains through the emergence of self-formed bridges and the minimization of the gaps between the R-TiO2 NRs. The proposed reaction mechanism for the preparation of the R-TiO2 NR and the CdS NF as shown in (Figure S1). Thus, the hydrothermal technique can effectively improve the grafting of the CdS NF onto the R-TiO2 NR facet. This technique resulted in the formation of a CdS NF (MC)/R-TiO2 NR heterojunction nanostructure network for an efficient visible-light absorption and an effective photogenerated charge transfer in the photoanode. The effect of manifold CdS precursor concentrations onto the R-TiO2 NR facet in terms of the microstructural evolution was studied using FE-SEM. Figure 2 depicts the top and cross-sectional images of the prepared samples. Figure 2 (a) indicates the formation of the vertically aligned R-TiO2 NRs with small square grids that are perpendicular to the FTO substrate. Regarding the R-TiO2 NRs, the diameter is from 100 nm to 175 nm and the average length is ~ 1.7 μm (inset of Figure 2 (a)). However, the color of the R-TiO2 NRs changes from white to yellow after the CdS grafting, and the morphologies of all of the CdS grafted RTiO2 NR samples were changed (Figure. 2 (b-d) (LC, MC, and HC, respectively)). Figure 2 (b) depicts the initial CdS (LC) NF

growth onto the R-TiO2 NR facet, wherein the inset clearly

shows the initial growth stage of the CdS NF; due to the lower CdS precursor concentration, only a few CdS NFs were grafted and it was not possible to sheathed the R-TiO2 NR facet. Furthermore, when the CdS precursor concentration was increased from the LC to MC, a large number of CdS NFs was obtained; they were entirely sheathed onto the R-TiO2 NR facet, and



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this result shows the greater surface roughness, as shown in Figure 2 (c). It was presumed that this heterojunction would be favorable for the improvement of the visible light absorption and a higher PEC performance compared with the growth of the R-TiO2 NR and the CdS (LC) NF onto the R-TiO2 NR facet heterojunctions. When the CdS precursor concentration was further increased from the MC to the HC; the NF was converted into the (NW form). Figure 2 (d) reveals that the R-TiO2 NR facet was non-uniformly covered with the CdS (HC) NWs; however, the inset of Figure 2 (d) shows that the CdS NWs were inserted into the spaces between the RTiO2 NRs. Therefore, an increase of the CdS precursor concentration affects the morphology as well as the growth mechanism. In addition, (Figure S2 (a-d)) shows the enlarged forms of the cross-sectional views for the pristine R-TiO2 NR and the CdS grafted R-TiO2 NR. Moreover, a detailed study of the CdS/TiO2 heterostructure can be achieved with the XRD and FIB-TEM analyses. The phase purity and crystallinity of these prepared samples were investigated using the XRD analyses. The XRD patterns of the manifold CdS grafted R-TiO2 NR are shown in Figure 3, where the symbols F, T, and C show the XRD peaks that correspond with the FTO substrates of the R-TiO2 NR, CdS NF, and CdS NW, respectively. The XRD pattern of the R-TiO2 NR shows peaks at 2θ = 36.03°, 41.2°, 54.3°, 62.5°, and 69.6° that correspond well with the (011), (111), (121), (002), and (031) planes of the tetragonal R-TiO2 NR (reference code: 01-077-0440), respectively (Figure S3). Depending on the CdS precursor concentration, two different crystal phases were obtained for the CdS in the CdS grafted R-TiO2 NR. As shown in Figure 3 (a), a CdS peak is not evident due to the very low CdS precursor concentrations (cubic; LC), which might be the result of the high CdS dispersion and the low grafted CdS NF content. Furthermore, the CdS-peak intensity was gradually increased with respect to the increasing of the CdS


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precursor concentration, as depicted in Figures 3 (b-c). The diffraction peaks of the CdS grafted R-TiO2 NR that correspond to the hexagonal (MC, HC) phases at 2θ = 26.5°, 43.7°, and 54.4° correspond well with the (0 0 2), (1 1 0), and (0 0 4) planes (reference code: 03-065-3414). However, as the overall XRD peaks intensity of the CdS in the CdS grafted R-TiO2 NR heterojunctions is very small, further proof of the existence of the CdS is given in the XPS results. The compositional and chemical states of the elements in the heterostructured CdS (MC) NF/R-TiO2 NR were characterized by XPS. Figure 4 depicts the Cd, S, Ti, and O elements that are present in the heterojunction photoanode at various spin orbital positions and oxidation states. The high-resolution XPS spectrum of the Figure 4 (a) shows the spikes of the Cd 3d3/2 and the Cd 3d5/2 at the binding energies (BE’s) of 411.55 eV and 404.77 eV, respectively. The 6.78 eV spin orbit separation of the Cd2+ between the Cd3d3/2 and the Cd3d5/2 revealed that cadmium (Cd) is in the state of +2. Furthermore, Figure 4 (b) shows the spikes of S 2p1/2 and S 2p3/2 at the BE’s of 162.19 eV and 160.99 eV, respectively, indicating the presence of the S 2- species. These results confirmed that the CdS is present in the heterojunction photoanode as indicated in previously reports.38,39 Figure 4 (c) shows that the titanium (Ti) spikes that are located around 464.02 eV and 458.20 eV could be assigned to Ti 2p 1/2 and Ti 2p3/2 with BE’s difference of 5.82 eV, and this confirms that the titanium (Ti) is in the state of +4. Figure 4 (d) shows the BE that corresponds to O1s and reveals one sharp spike at 529.49 eV;40,41 this result also supports the grafting of the CdS NF onto the R-TiO2 NR facet in the heterojunction photoanode. The typical XPS survey spectrum of the CdS (MC) NF/R-TiO2 NR heterojunction (Figure S4) to confirm the Cd, S, Ti, and O elements in the single spectrum. The XPS results are therefore consistent with the XRD and EDX results.



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To obtain the details of the CdS presence in the CdS grafted R-TiO2 NR heterojunction, the FE-SEM/ EDX was performed (Figure S5); wherein the FE-SEM/ EDX spectra of the CdS (MC) NF/R-TiO2 NR heterojunction is shown at different Cd(NO 3)2.4H2O and CH4N2S concentrations. The EDX spectrum displays the characteristic peaks that correspond with the gradual increasing of the BE states of the Cd and the S (Figure S5 (a-c)) from the lower CdS concentration (LC) to the higher CdS concentration (HC). The elemental analysis was carried out for the Cd, S, Ti, and O, and their average atomic percentage ratios are listed in the table (Figure S5 (d)). Moreover, the CdS precursor concentration increased the Cd 2+/S2- ion ratio, thereby gradually increasing the atomic percentage for the heterojunction photoanodes. However, as the CdS (MC) NFs/R-TiO2 NRs were deposited onto the FTO substrate, a predominant peak that corresponds to tin (Sn) is evident in the spectra. To study the detailed growth of the CdS on the R-TiO2 NR and the interface between the grafted CdS NF and the R-TiO2 NR, the CdS grafted TiO2 NRs were characterized using FIBTEM and EDX mapping. Figure 5 shows a typical cross-sectional FIB-TEM image and the EDX mapping for the CdS (MC) NF/R-TiO2 NR heterojunction photoanode. Figure 5 (a) presents the STEM image that shows that the CdS NFs were successfully attached onto the R-TiO2 NR facet. Furthermore, Figures 5 (b-e) shows the EDX elemental mapping of the S, Cd, O, and Ti in the CdS (MC) NF/R-TiO2 NR heterojunction. This mapping demonstrates that the CdS NF was homogeneously distributed into the facet of the R-TiO2 NR, thereby indicating that the CdS NFs were successfully grafted onto the R-TiO2 NR facet, as well as some of the CdS that had been incorporated into the space on the metal oxide facet. The corresponding EDX spectrum results are shown in Figure 5 (f). In addition to this, the EDX analysis was carried out at three different positions of the heterojunction photoanode (Figure S6); here, it is obvious that the percentages of


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the Cd and the S are higher at the top facet of the R-TiO2 NR, and they decreased from the middle to the bottom of the CdS (MC) NF/R-TiO2 NR heterojunction (Figures S6 (b-d)). A consideration of the EDX spectrum together with the mapping data suggests the successful grafting of the CdS NF onto the R-TiO2 NR facet. This result proposes that metal sulphide can be used to change the metal oxide facet morphology for the attainment of an effective light absorption and the charge transport mechanism. Figure 6 shows the (a) ADF-STEM, (b) HR-TEM, and (c) EDX analysis of the CdS (MC) NF/R-TiO2 NR heterojunction photoanode. With respect to the microstructure of the product, the incorporated TiO2 and CdS nanostructures in Figures 6 (c-f), which are also well matched with the FE-SEM image, were found. To confirm the chemical composition, the EDX mapping result (yellow box in Figure 6 (a)) clearly displays the elemental distribution of the Ti, O, Cd, and S. For microstructural evidence, the product was subjected to a high resolution (HR) ADF-STEM analysis (red box area). In the HR ADF-STEM results, the single crystalline structures of the RTiO2 NR and the CdS NF are apparent in Figure 6 (b). The measured d-spacing on the CdS(MC) NF/R-TiO2 NR lattice fringe corresponds well with the XRD result. The optical properties of the pristine R-TiO2 NR and the manifold CdS grafted R-TiO2 NR samples were investigated using ultraviolet visible diffuse reflectance spectra in the region of 330 nm to 600 nm. Figure 7 (A) shows the UV-vis absorbance spectra, and Figure 7 (B) shows the tauc plots. The UV-vis absorbance curves of the pristine R-TiO2 NR and the CdS grafted R-TiO2 NR shows the different absorption behaviors. The pristine R-TiO2 NR exhibited a strong absorption under the UV light with an absorption edge at 397.4 nm, as depicted in Figure 7 ((A) (a)). After the grafting of the CdS NF onto the pristine R-TiO2 NR, the light source was shifted from the UV region to the visible region. Figure 7 ((A) (b-d)) show an obvious red shift of the exhibited absorption edge of



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the heterojunction (LC = 482 nm, MC = 498 nm, and HC = 504 nm) that is due to the synergic effects that are exerted by the CdS onto the TiO 2. The vitol role of the CdS can improve the strong visible light absorption behavior, the high specific facet area, and a higher crystallinity, which are helpful in the attainment of a higher photocurrent during the photocatalytic process.42,43 The prepared heterojunction photoanodes showed the strongest absorbance, and the order of the absorbance intensities is as follows: CdS(HC) NW/R-TiO2 NR > CdS (MC) NF/RTiO2 NR > CdS (LC) NF/R-TiO2 NR. The superior absorbance of the CdS (HC) NW/R-TiO2 NR heterojunction photoanode compared with the other photoanodes can be attributed to the exposure of the highly active CdS NW facets that partially encapsulated the R-TiO2 NR, as shown in the FE-SEM results, and this is in sound agreement with the previously mentioned XRD results. Moreover, the spectra of the CdS/TiO 2 heterojunction photoanode show UV-Vis absorption spectra that correspond with the combination of the CdS and TiO 2 spectra and represent the extended solar light response range of the PEC performance. Therefore, thus heterojunction photoanode was revealed as a promising material for the renewable H 2 fuel generation under visible light irradiation. The photographs of the manifold CdS grafted R-TiO2 NR heterojunction photoanodes and the pristine R-TiO2 NR photoanode are shown in the inset of Figure 7 (A). The inset shows, the gradual increase of the CdSprecursor concentration onto the R-TiO2 NR, whereby the increased thickness of the CdS gradually increased the yellow color. These findings are in agreement with the FE-SEM and XRD results. The energy band gap of the pristine R-TiO2 NR and heterojunction of the CdS/TiO2 can be accurately calculated with the use of the Kubelka–Munk function. Figure 7 (B) shows the plot of the (αhʋ)2 versus the hʋ, and by extrapolating the straightest line to the vertical segment of the spectra, it is extended to intersect with the hʋ axis


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that is used to obtain the direct band gap (Eg).44 The measured band gap values of the pristine RTiO2 NR, CdS (LC) NF/R-TiO2 NR, CdS (MC) NF/R-TiO2 NR, and CdS (HC) NF/R-TiO2 NR, heterojunctions were approximately related to the light responsibility with the energy band gaps of 3.12 eV, 2.57 eV, 2.49 eV, and 2.46 eV, respectively. Photoelectrochemical (PEC) measurements were carried out in a three-electrode cell in 0.1 M Na2S·9H2O and 0.02 M Na2SO3 aqueous electrolytes under solar light irradiation at 100 mW cm−2. Figure 8 (A) shows the linear sweep voltammetry (LSV) results of the pristine RTiO2 NR, CdS (LC) NF/R-TiO2 NR, CdS (MC) NF/R-TiO2 NR, and CdS (HC) NF/R-TiO2 NR, in the potential range of -1.5 V to 0.5 V vs. Ag/AgCl. The observed dark current densities for all of these four photoanodes are negligible; the pristine R-TiO2 NR exhibited a relatively low photoresponse (1.24 mA cm-2 at 0.1 V vs. Ag/AgCl) over the whole potential frame. In contrast, the photocurrent densities were significantly increased with the increasing of the CdS precursor concentration up to a certain level. That is, 2.14 mA cm -2 for the CdS (LC) NF/R-TiO2 NR, 3.23 mA cm-2 for the CdS (MC) NF/R-TiO2 NR, and 2.62 mA cm-2 for the CdS (HC) NW/RTiO2 NR at 0.1 V vs. Ag/AgCl that are due to the synergistic effect that is for the efficient charge transfer and surface modification to improve the light harvesting. 45,46 It is interesting to note that the photoanode of the CdS (HC) NW/R-TiO2 NR heterojunction decreased the photocurrent density compared with that of the CdS (MC) NF/RTiO2 NR heterojunction, which can be mainly explained by three possible reasons, as follows: (i) A thicker CdS layer formed on the R-TiO2 NR facet, and this will increases the internal recombination of the charge carriers and slows down the charge separation.47 (ii) Due to the “HC” CdS precursor concentration; the CdS NF was converted into the CdS NW; additionally, the CdS NWs non-uniformly sheathed the R-TiO2 NR facet, as clearly shown in Figure 2 (d), and;



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these NWs will lead to a decrease of the photocatalytic activity due to the numerous trapped states on the valence band, and they will also affect the poor charge mobility. (iii) The increased CdS amount can create a boundary resistance between the CdS and the R-TiO2 NR. The electron-hole recombination occurred progressively along with the grafting of the CdS onto the R-TiO2 NR.48 Furthermore, the open circuit voltage (V OC), i.e., the voltage corresponding to J = 0, is an approximated measure of the flat band potential, which is an important parameter for the semiconductor photoanode.49 The obtained values for the VOC and the short circuit current for the pristine TiO 2 NR and the manifold CdS grafted R-TiO2 NR (Table S 1). Among the CdS grafted R-TiO2 NR the CdS (MC) NF/R-TiO2 NR shows a VOC value that is more negative than those of the other studied samples, demonstrating the shift of the fermi energy to a more negative potential. Moreover, to confirm the importance of the grafting of the CdS (MC) NF onto the R-TiO2 NR, instead of EDA, an aqueous solvent was used in the previous preparation method for the “MC” CdS concentration. It evident that the CdS (MC) nanograin/R-TiO2 NR heterojunction that was prepared in the aqueous solvent shows a lower photocurrent response (2.7 mA cm -2 at 0.1 V vs. Ag/AgCl (Figure S7)) than the CdS (MC) NF/R-TiO2 NR heterojunction. In addition, the systematic study of the CdS precursor concentration choice of Cd(NO3)2.4H2O (1.5 mmol) interval was checked and obtained the maximum photocurrent density values of 3.82 mA cm -2 (Figure S8). This enhanced PEC performance was explained with the aid of the FE-SEM (combination of the NF and the NW) and PEIS results. Figure 8 (B) shows the transient photocurrent response (TPR) measurement of the previously mentioned photoanodes. All of the photoanodes show relatively better PEC performances under the on and off irradiation cycles. The straight line represents the stability of the four photoanodes,


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and the next five chopped curves indicate the faster charge transfer rate from the CdS to the TiO2.50 A remarkable enhancement of the PEC performance of the CdS (MC) NF/R-TiO2 NR heterojunction photoanode was observed, which could be attributed to the improved visible light absorption of the R-TiO2 NR facet via the CdS NF. The current density of these photoanodes under illumination follows a descending order, as follows: CdS (MC) NF/R-TiO2 NR > CdS (HC) NW/R-TiO2 NR > CdS (LC) NF/R-TiO2 NR > R-TiO2 NR. Furthermore, the photo conversion efficiency (PCE) of all of the synthesized photoanodes were calculated using the following equation: 51 ɳ (%) = jph[E0rev - |Eapp|] X 100/(I0)

(1)

Where jph is the photocurrent density (mA cm -2), jph E0rev is the total power output, jph|Eapp| is the electrical power input, I0 is the power density of the incident light (100 mW cm -2), and E0rev is the standard reversible potential that is 1.23 V vs. NHE. The applied potential is derived by Eapp= Emeas - Eaoc, where Emeas is the electrode potential (vs. Ag/AgCl) of the working photoanode, at which the photocurrent is measured under illumination, and E aoc is the electrode potential (vs. Ag/AgCl) of the same working photoanode under the open-circuit condition. As shown in Figure 8 (C), the CdS (MC) NF/R-TiO2 NR heterojunction photoanode shows a PCE of 0.46 % at 0.1 V, which is significantly larger than those of the CdS (HC) NW/R-TiO2 NR (0.35 %), CdS (LC) NF/R-TiO2 NR (0.31 %), and R-TiO2 NR (0.18 %) at the same applied potential. An important factor for the improvement of the efficiency of the CdS (MC) NF/R-TiO2 NR among the studied samples is the enhanced photogenerated charge separation that, harvests visible light. In addition, the comparative literature survey of the CdS/TiO 2 photoanode is



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summarized (Table S2). When compared with the whole literature survey, the present report contains the higher photocurrent density. The PEIS studies were carried out to investigate the charge transport properties of the prepared samples. The PEIS results were fitted using the equivalent circuit that is shown in the inset of Figure 8 (D), and the values of the fitted parameters are listed in Table 1. The photoanode of the CdS (MC) NF/R-TiO2 NR heterojunction displayed the smallest charge transfer resistance among the four samples, indicating the fastest charge transport. The size of the semicircle radius on the Nyquist plot is related to the electron transport resistance. The smaller arc radius on the PEIS Nyquist plot suggests a more effective separation of the photogenerated electron/hole pairs and a faster interfacial charge transfer.52,53 In the plot Z and Z’ signify the real and imaginary components of the impedance, respectively; R S is the substrate resistance as well as the external electric circuit in the PEC cell; and R ct1 is the photoanode/electrolyte interface together with the constant phase element 1 (CCPE1). In addition to this, the IMPS and intensity modulated voltage spectroscopy (IMVS) results of the pristine R-TiO2 NR and heterojunction of the CdS (MC) NF/R-TiO2 NR (Figure S9). The charge transfer and life time of the photoanodes were determined and correlated with the PEC performance. Figure S9 (A) shows the IMPS plots for (a) the pristine R-TiO2 NR and (b) the CdS (MC) NF/R-TiO2 NR heterojunction photoanodes, which are displayed as semicircles in the complex plane. The electron transport time (τd) can be calculated using the equations τd = 1/(2πfIMPS,min), and the shortest electron transport time was measured for the CdS (MC) NF/RTiO2 NR heterojunction/electrolyte (τd = 343.03 μs), suggesting a lesser recombination and a more effective transfer of electrons compared with the pristine R-TiO2 NR/electrolyte (τd = 431.8 μs).54 This result indicates that the effective charge transport in the CdS (MC) NF/R-TiO2


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NR is faster than that the R-TiO2 NR. Furthermore, the charge life time of the photoanode was determined using the IMVS and PEIS analyses. Figure S9 (B) shows the IMVS plots of the (a) Pristine R-TiO2 NR and (b) CdS (MC) NF/R-TiO2 NR heterojunction, which are displayed as semicircles in the complex plane. The electron life time (τn) can be calculated using the equations τ n = 1/(2πfIMVS,min). The calculated τn values for the pristine R-TiO2 NR and the CdS (MC) NF/R-TiO2 NR are 0.29 s and 0.63 s, respectively; indicating that the CdS (MC) NF/R-TiO2 NR heterojunction photoanode possesses a longer electron life time than the pristine R-TiO2 NR. Moreover, the electron lifetime of the photoanodes were again conformed by the PEIS data shown in Figure 8 (D). The electron life time was calculated using the Rct1 and CCPE1, values (τn = Rct1 × CCPE1). The lower Rct1 values revealed larger electron lifetimes.55 Furthermore, the electron life times of the previously mentioned photoanodes; A longer life time was obtained for the CdS (MC) NF/RTiO2 NR heterojunction photoanode compared with the other studied samples (Table S3), thereby supporting’s the lower recombination rate and higher PEC performance. The VFB that was studied using the MS plots (Figure S9 (C)). Depending upon the MS plots, the observed positive slopes of the capacitance versus the applied potential (1/C 2 vs. V) verify that the pristine R-TiO2 NR and the CdS (MC) NF/R-TiO2 NR are n-type semiconductors. Moreover, it can be calculated as -0.73 V and -0.96 V vs. Ag/ AgCl, respectively. This greater negative shift of the VFB leads to a greater band bending in the space charge layer, and it can promote the separation of the photoelectrons and holes and decrease the recombination rate.56 These VFB values are further used to explain the charge transfer mechanism of the CdS (MC) NF/R-TiO2 NR heterojunction photoanode.



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The enhanced PEC activity of the optimized CdS (MC) NF/R-TiO2 NR heterojunction photoanode was further examined for the H 2 generation using a sacrificial reagent as the electrolyte under solar-light irradiation at the intensity of 100 mW cm -1. Figure 9 shows the photocurrent response of the stability line and the H 2 generation at various time intervals for 0.1 V vs. Ag/AgCl. The inset photography image clearly shows the three electrode cell setup. During the solar-light irradiation, the optimized heterojunction photoanode reached the Pt electrode via the FTO substrate upon its generation of a considerable number of electrons/holes, the detailed charge transfer mechanism is given as a schematic illustration (Figure 10). These electrons can react with the 2H+ ion to generate H2 gas at the Pt electrode; the large amount of small-size H2 bubble was collected using the syringe, and the bubbles were measured using GC at various time intervals. The obtained H2 in the PEC system shows a linear relationship with the irradiation time, whereby a continuous and steady H2 generation is denoted. The highest H2 evolution for the optimized heterojunction photoanode of 1007.98 µmol was achieved after 3 hr, as shown in Figure 9. Meanwhile, during the H2 generation, the stability of the optimized heterojunction photoanode was checked to confirm the stability of the photoanode for its commercialization potential. After the application of the irradiation over a long-term period, the photoresponse decayed due to a rapid recombination of the photogenerated charges, whereby the surface oxidation of the photoanode occurred via its own photogenerated holes.57,58 Even though the PEC H2 generation rates of the heterojunction materials were significantly enhanced in terms of the CdS (MC) NF/R-TiO2 NR heterojunction compared with the pristine R-TiO2 NR heterojunction, the former suffered from the stability issue. To improve the photostability, charge separation, and transportation of the CdS (MC) NF/R-TiO2 NR heterojunction, various surface treatments like metal oxide (NiO 2, RuO2, BiVO4, and Rh2O3)


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passivation and layered double hydroxide (LDH) loading, or the traditional noble-metal cocatalysts, and effective heterojunctions, need to be used in the future studies. Figure 10 shows a schematic diagram of the charge-transfer mechanism in the CdS NF grafted R-TiO2 NR heterojunction during the photoelectrochemical H2 generation. The rate of PEC H2 generation depends upon (i) the absorption of semiconducting materials, (ii) charge separation followed by migration of these photogenerated carriers in semiconductor materials; (iii) surface chemical reactions between these carriers with various compounds.59 The appropriate band alignment of the constituent materials in the heterojunction allows for an efficient electron/hole pairs transfer.60 This resulted in a synergistically higher photocurrent density of CdS (MC) NF/R-TiO2 NR heterojunction compared with the pristine R-TiO2 NR photoanode. Under the dark condition, when heterojunction photoanode inserted into the electrolyte, the equilibrium of the Fermi levels was established at the interface of the CdS NF and the R-TiO2 NR and electrolyte.61 However, under the solar-light irradiation due to the multiple reflections in CdS NF, the light-propagation path was extended in the CdS (MC) NF /RTiO2 NR heterojunction and then this heterojunction photoanode utilize maximum incident solar light. Furthermore, in addition to the light absorption of the CdS (MC) NF /R-TiO2 NR heterojunction, the potential of the conduction band (CB) of CdS NF in the CdS (MC) NF/RTiO2 NR heterojunction photoanode is slightly negative (- 0.03 V) compared with the corresponding CB potential of the R-TiO2 NR (+0.18 V). So, the photogenerated electrons of CdS NF in the heterojunction photoanode will easily migrate to the CB of the R-TiO2 NR and then move toward Pt counter electrode (CE) through the external circuit. 62 This mechanism suggests that Pt electrode with higher work function due to proper band positions and alignments in heterojunction photoanode can easily induce electron transfer to the hydrogen cations in



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electrolyte to generate the H2 gas at the CE.63 Meanwhile, the excited holes can be separated and transferred quickly from the higher valance band (VB) of the R-TiO2 NR (2.94 V) to the lower VB of the CdS NF (2.46 V).64 Moreover, the photogenerated holes formed in the VB of the CdS NF can also react with the sacrificial reagents (Na2S/Na2SO3). The possible reaction mechanism occurring at both heterojunction photoanode (working electrode; WE) and Pt (CE) electrode during the PEC H2 generation was discussed as follows: 65-68 CdS-TiO2 + hv  CdS (h+ + e-)-TiO2(h+ + e-)

(2)

CdS (h+ + e-)-TiO2 (h+ + e-)  CdS VB (2h+) + TiO2 CB (2e-)

(3)

TiO2 CB (2e-)  FTO (2e−)  Pt (2e−)

(4)

Pt (2e−) + 2H+  Pt + H2

(5)

CdS VB (2h+) + 2S2-  S22-

(6)

S22− + SO32− → S2O32− + S2−

(7)

SO32- + 2OH-+ CdS (2h+)  SO42- + H2O

(8)

Under the simulated solar light irradiation, the electrons are excited from VB to CB of both CdS NF and R-TiO2 NR semiconductor as shown in equation 2. As shown in Figure 10, due to appropriate band positions of CdS NF and R-TiO2 NR the photoelectrons generated in CdS NF transfer to the CB of the R-TiO2 NR and then reach the Pt (CE) electrode to generates H2 gas (equation 4 and 5).Meanwhile, the photogenerated holes are accumulated at the VB of the CdS NF and then react with sulfide (S2−) ions and generate S22−−, which is in turn recycled back into the S2− by the SO32−.69 As a result, the present CdS (MC) NF/R-TiO2 NR heterojunction photoanode enhances the photoconversion efficiency through its effective charge separation and light absorption.


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Conclusion In summary, the CdS (MC) NF/R-TiO2 NR heterojunction photoanode was successfully synthesized through the use of a two-step hydrothermal process for an efficient light absorption and charge separation. A systematic study was performed to investigate the effects of the CdS concentration on the PEC performance of the CdS NF/R-TiO2 NR heterojunctions under solar light irradiation. Among the as-prepared samples, the CdS (MC) NF/R-TiO2 NR heterojunction photoanode exhibited the improved photocurrent density of 3.23 mAcm-2 at 0.1 V vs. Ag/AgCl, and higher PCE of 0.46% due to the lesser recombination, a longer lifetime, and a speedier charge separation. In addition, the PEC H2 generation of the optimized heterojunction photoanode increased to a value as high as 336 µmol/hr. The charge transportation mechanism for the CdS grafted R-TiO2 NR is proposing based on the author’s findings. Further, the synergistic effect of the efficient visible-light absorption by the CdS NF; can be used to improve the charge transfer in the optimized heterojunction photoanode; it provides a better PEC performance and useful insights for the design of novel chalcogenide/metal-oxide heterojunction materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Field emission scanning electron microscopy (FE-SEM) for surface and cross sectional view, Xray Diffraction (XRD) pattern for pristine R-TiO2 nanorod, X-ray photoelectron spectroscopy (XPS) survey spectrum, Energy dispersive spectrometry (EDX) spectrums, Focused ion beam transmission electron microscopy (FIB-TEM), Photoelectrochemical measurements: linear sweep voltammetry (LSV), Transient-photocurrent response, intensity modulated photocurrent



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spectroscopy (IMPS), intensity modulated photo voltage spectroscopy (IMVS), and MottSchottky (MS) respectively.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Jum Suk Jang: 0000-0001-6874-8216 Notes The authors declare no competing financial interest.

Acknowledgments We acknowledge the support from the Basic Science Research Programs through the National Research Foundation of Korea (NRF) that is financed by the Ministry of Education, Science and Technology (2012R1A6A3A04038530), as well as the Public Technology Program of the Korea Ministry of Environment (MOE) that is based on Environmental Policy (2014000160001).


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1D

Zr:α-Fe2O3/FTO

performance.

photoanode

Dalton

Trans.,

for

efficient

2017,

10.1039/c6dt04472g


46,

solar-light-driven 2377-2386.

DOI:

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Table 1. Estimated Values of the PEIS Parameters, as Calculated by the Equivalent Circuit.

Photoanodes/PEIS-fitted parameters

Rs

Rct1

CCPE1

(Ω)

(Ω)

(μF)

Pristine R-TiO2 NR

30

4433

3.43

CdS (LC) NF/R-TiO2 NR

36

1521

0.11

CdS (MC) NF/R-TiO2 NR

36

1285

0.25

CdS (HC) NW/R-TiO2 NR

34

1403

0.14



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Figure captions Figure 1. Schematic illustration for the preparation of; (a) pristine R-TiO2 NR and (b) manifold CdS-precursor concentrations for the CdS/R-TiO2 NR heterojunctions. Figure 2. Top FE-SEM images of: (a) pristine R-TiO2 NR, (b) CdS (LC) NF/R-TiO2 NR, (c) CdS (MC) NF/R-TiO2 NR, and (d) CdS (HC) NW/R-TiO2 NR heterojunctions. Inset shows the corresponding cross-sectional views. Figure 3. XRD patterns for the CdS/TiO2 photoanode synthesized at different CdS concentrations: (a) CdS (LC) NF/R-TiO2 NR, (b) CdS (MC) NF/R-TiO2 NR, and (c) CdS (HC) NW/R-TiO2 NR heterojunctions. Figure 4. High-resolution XPS spectra of the CdS (MC) NF/R-TiO2 NR heterojunction for: (a) Cd 3d, (b) Sp 2p, (c) Ti 2p, and (d) O1s. Figure 5. (a) FIB-TEM and EDS-mapping images of the elements of the CdS (MC) NF/R-TiO2 NR heterojunction: (b) Cd, (c) S, (d) Ti, and (e) O of . (f) EDX spectrum of the CdS (MC)-NF /R-TiO2-NR heterojunction. Figure 6. Results for the CdS (MC) NF/R-TiO2 NR heterojunction photoanode: (a) ADF-STEM, (b) HR-TEM, and (c) to (f) EDX . Figure 7. (A) UV-vis absorbance spectrum and (B) tauc plots of the: (a) pristine R-TiO2 NR, (b) CdS (LC) NF/R-TiO2 NR, (c) CdS (MC) NF/R-TiO2 NR, and (d) CdS (HC) NW/R-TiO2 NR heterojunction photoanodes. Figure 8. (A) Linear sweep voltammetry, (B) ) Transient-photocurrent response, (C) PCE, and (D) PEIS (inset equivalent circuit) under the solar light irradiation of the; (a) pristine R-TiO2 NR,


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(b) CdS (LC) NF/R-TiO2 NR, (c) CdS (MC) NF/R-TiO2 NR, and (d) CdS (HC) NW/R-TiO2 NR heterojunction photoanodes. Figure 9.

The photocurrent stability and H2 generation for the CdS (MC) NF/R-TiO2 NR

heterojunction (inset represents the cell setup). Figure 10. Schematic illustration of the PEC H2 generation; the band alignment, and the chargetransfer mechanism for the CdS (MC) NF/R-TiO2 NR heterojunction.



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Figures

Figure 1


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Figure 2


T (002)

F

T (031)

C - CdS

F

F

T (121)

F

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F - FTO T - TiO2 T (111) C (110)

C (101)

c

C (100)

F

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

T (011)


F

b

a 20

30

40

50

60

2  (Degree) Figure 3


70

80

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Figure 4



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Figure 5


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Figure 6



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Figure 7


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Figure 8



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Figure 9


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Figure 10



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Table of Contents (TOC) Graphical abstract

Synopsis: CdS nanoflowers prepared by hydrothermal method using ethylenediamine as a solvent, template and co-ordination agent to enhance the charge separation and H 2 generation.


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Facile hydrothermally synthesized a novel CdS nanoflower/rutile-TiO2 nanorod heterojunction photoanode used for photoelectrocatalytic hydrogen generation. Selvaraj Davida, Mahadeo A. Mahadika, Hee Suk Chungb, Jung Ho Ryuc and Jum Suk Janga* a

Division of Biotechnology, Advanced Institute of Environmental and Bioscience,

College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea. b

Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do,

54907, Republic of Korea. c

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral

Resources, Dajeon, 34132, Republic of Korea. Corresponding author: Jum Suk Jang ([email protected]) Tel.: +82 63 850 0846; fax: +82 63 850 0834.



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Table of Contents

CdS nanoflowers prepared by hydrothermal method using ethylenediamine as a solvent, template and co-ordination agent to enhance the charge separation and H2 generation.


Rutile

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