H:ZnO Nanorod-Based Photoanode Sensitized by CdS and Carbon

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H:ZnO Nanorod-Based Photoanode Sensitized by CdS and Carbon Quantum Dots for Photoelectrochemical Water Splitting Nguyen Minh Vuong, John Logan Reynolds, Eric Daniel Conte, and Yong-Ill Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08724 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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The Journal of Physical Chemistry

H:ZnO Nanorod-Based Photoanode Sensitized by CdS and Carbon Quantum Dots for Photoelectrochemical Water Splitting

Nguyen Minh Vuong†,§, John Logan Reynolds‡, Eric Conte‡, and Yong-Ill Lee†,*

†

Department of Chemistry, Changwon National University, Changwon 641-773, Republic of Korea ‡

§

Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101 USA

Department of Physics, Quy Nhon University, 170 An Duong Vuong, Quy Nhon, Binh Dinh, Vietnam

ACS Paragon Plus Environment

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ABSTRACT

We report a promising simple strategy to improve the performance of the photoanode for photoelectrochemical (PEC) water splitting. ZnO nanorods on an ITO glass substrate were synthesized by a hydrothermal method following calcinations in air at 500 °C for 2 h and pure ambient hydrogen at atmospheric pressure at 400 °C for 30 min. The hydrogenated ZnO (H:ZnO) sample shows an enhanced photocurrent in comparison to ZnO nanorods. To enhance the absorption in the visible light and near-IR regions, H:ZnO nanorods were sensitized by cadmium sulfide (CdS) nanoparticles and carbon quantum dots (CQDs). The H:ZnO nanorod film sensitized in this way exhibited significantly improved PEC properties after treatment with ambient nitrogen at 400 °C for 30 min. The optimized H:ZnOnanorod sample sensitized by CdS and CQDs yields a photocurrent density of ∼12.82 mA/cm2 at 0 V (vs.SCE) in 0.25 M Na2S and 0.35 M Na2SO3 solution under the illumination of simulated solar light (100 mW/cm2 from a 150 W xenon Arc lamp source). The optimal structure shows a solar-to-hydrogen (STH) conversion efficiency of ∼3.85% (at -0.67 V vs. SCE).The H2 gas generation obtained using this optimal structure consisting of H:ZnO nanorods sensitized by CdS and CQDs was 7.04 mL/cm2 in 1 h. The morphology and properties of the samples were examined by SEM, XRD, TEM, UV-vis absorption, and electrical measurements.

KEYWORDS:

zinc

oxide,

cadmium

sulfide,

carbon

quantum

dots,

up-conversion

photoluminescence.


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1. INTRODUCTION Hydrogen (H2) is considered by many to be the future of fuel because it can be produced by clean and renewable energy sources.1 Hydrogen could provide the answer to the inevitable future energy crisis; as such, it has come to the forefront of alternative energy research. Photoelectrochemical (PEC) water splitting in particular has been widely recognized as one of the most promising methods for H2 gas generation by the scientific community ever since Fujishima and Honda used a titanium dioxide (TiO2) photo-anode to electrolyze hydrogen from water in 1972.2-3 In recent years, beside TiO2, much attention has also been given to zinc oxide (ZnO) as a viable semiconductor for PEC water splitting.4 ZnO has been extensively researched due to its relative abundance, low cost, non-toxicity, and high electron mobility.4-7 Unfortunately, ZnO mainly absorbs in the UV region of the electromagnetic spectrum, which accounts for only 4% of solar energy due to its large energy band gap (>3.1 eV).8-9 This, in turn, yields a very low solarto-hydrogen (STH) conversion efficiency. Therefore, broadening the absorption spectrum of ZnO material to the visible or near-IR region, which accounts for about 45% of the whole solar energy spectrum, has been widely studied; for example, doping with metal10-13 and nonmetal ions,14-15 and hydrogenation6, 16 has received extensive attention to improve light absorption and charge transport. Attempts to enhance the STH conversion efficiency of the electrode in the PEC cell have led to the utilization of narrow band gap materials such as CdS (~2.4 eV),7, 17 CdSe (~1.7 eV), and CdTe (~1.5 eV)18-19 as photoactive sensitizers of photoanodes based on ZnO material due to its attractive light – harvesting characteristic, tunable band gap, easy fabrication, and low cost. The formation of a feasible band alignment with ZnO causes efficient charge separation and transportation. However, the major challenge presented by this material is its photochemical


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stability caused by hole – induced anodic corrosion, even in a sacrificial electrolyte,20 and the absorption of photons in the near-infrared (near-IR) region. Recently, improved PEC watersplitting performance was developed through the integration of traditional photoanodes and upconversion photoluminescence (UCPL) materials, such as rare-earth doped materials, graphene quantum dots (GQDs), and carbon quantum dots (CQDs), which are able to absorb both near-IR photons and visible light for conversion to shorter wavelength photons.18,

21-22

Carbon-based

quantum dots exhibit many noticeable advantages compared to rare-earth doped materials such as low cost, easy fabrication, abundance in the earth, and excellent UCPL properties. To date, there have been several reports about the deposition of CQDs on traditional photoanodes for PEC water-splitting applications such as TiO2/CQDs nanotubes,23 TiO2/CdSe/CQDs nanorods in the near-IR region (above 750 nm),18 and TiO2/CQDs.24 In addition, the role of carbon-based quantum dots was investigated thoroughly for the improvement of photo-degradation under a solar light source as illustrated source through UCPL characteristics.8,

25

To the best of our

knowledge, the deposition of CdS and CQDs on hydrogenated ZnO nanorods for the improvement of hydrogen generation efficiency has not yet been reported. Prompted by the success of previous works, we endeavored to fabricate a photoanode for hydrogen generation with high photo-to-electron conversion efficiency by the combination of three materials: H:ZnO, CdS, and CQDs. Herein, the sensitization by CdS nanoparticles, which is a narrow band gap material, could enhance the water-splitting efficiency due to their significant absorption over a wide wavelength range in the visible light region of the solar spectrum.7, 17, 26-27 Meanwhile, CQDs show harvesting ability in the visible and IR region of the solar spectrum with the ability of converting it to light of shorter wavelengths through the UCPL property.21-22,

28

This shorter wavelength emission will be re-absorbed by the CdS and ZnO


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materials, which will excite electron-hole pair generation in these semiconductors and then further improve the hydrogen generation efficiency of the fabricated photoanode. In addition, the reduction of charge recombination due to increasing conductance in H:ZnO is also expected to contribute to the improvement of STH conversion efficiency. 2. EXPERIMENTAL PROCEDURE 2.1. Materials Poly(vinylpyrrolidone) (PVP) (wt 360000, Sigma-Aldrich Co., Ltd), zinc acetatedihydrate (Zn(CH3COO)2 ·2H2O,

99.99%,

Sigma-Aldrich

Co.,

Ltd),

zinc

nitratehexahydrate

(Zn(NO3)2.6H2O, 98%, Sigma-Aldrich Co., Ltd), hexamethylenetetramine (HMTA) (C2H12N4, 99%,

Sigma-Aldrich

Co.,

Ltd),

citric

acid

monohydrate

(C3H4(OH)(COOH)3.H2O),

ethylenediamine (C2H8N2, 99%, Alfa Aesar Co., Ltd), cadmium nitrate tetrahydrate ((Cd(NO3)2.4H2O), 98%, Aldrich Chemical Company, Inc), thioacetamide (C2H5NS, 98%, Alfa Aesar Co., Ltd), sodium sulfate, anhydrous (Na2SO4, ≥98%, SamchungPure Chemical Co., Ltd), sodium sulfide pentahydrate(Na2S.5H2O, 98%, DaeJung Chemical and Metals Co., Ltd), sodium sulfite (Na2SO3, ≥98%, Sigma-Aldrich Co., Ltd), distilled water (18.4 MΩ/cm),N,Ndimethylformamide (DMF) (HCON(CH3)2, ≥99%, Sigma-Aldrich Co., Ltd). 2.2. Preparation of ZnO nanorods on indium tin oxide (ITO) substrate Prior to growing the ZnO nanorods, a ZnO seed layer was prepared on the ITO substrate. A solution of 9.5 mL DMF and 0.8 g of PVP was stirred for 6 h, after which 1.67 g of (Zn(NO3)2.6H2O was added and stirred for an additional 4 h. Of this solution, 50 µL was spincoated onto an indium tin oxide (ITO) substrate for 1 min at a rotation rate of 3000 rpm. The samples were then annealed in air at 450 °C (at a heating rate of 15 °C/min) for 2 h to remove the PVP and form the ZnO nanoparticles. For the hydrothermal synthesis of the ZnO nanorods, a


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mixture of 0.04 M of aqueous solution that included Zn(NO3)2.6H2O and C6H12N4 (1:1 ratio) was prepared based on a previous study.26 The seeded substrates were placed in this solution at 90 °C for 4 h to allow the ZnO nanorods to grow. The hydrothermal reaction was repeated one more time to increase the length of the ZnO nanorods. The duration of the second reaction was controlled at 0, 2, 4, and 6 h. This means that the length of the ZnO nanorods was controlled by a total growth time of 4, 6, 8, and 10 h. After the growth of the ZnO nanorods , the samples were annealed in air at 500 °C for 2 h and then in hydrogen at 400 °C for 30 min. 2.3. Preparation of CQDs The CQDs were synthesized by mixing 0.5 M citric acid with 0.5 M ethylenediamine in 10 mL of DI water.29 Then the mixing solution was transferred to a Teflon pot in an autoclave and heated for 12 h at 180°C. After the reaction, the reactors were allowed to cool to room temperature naturally. The product (a brown-black solution) was subjected to dialysis in order to obtain the CQDs. 2.4. Deposition of CdS nanoparticles The CdS nanoparticles were directly grown on the H:ZnO and H:ZnO/CQDs nanorod surfaces by soaking these electrodes in an aqueous solution of 10 mM of Cd(NO3)2 as a source of Cd2+ and 10 mM of C2H5NS as a source of S2- at 80 °C for 30 min, followed by rinsing with deionized water and natural drying. The optimal dipping time of 30 min for CdS growth was employed to achieve the highest efficiency as shown in our previous study.26-27 2.5. Fabrication of the photoanode Figure S1 (Supporting Information) shows the fabrication process of nanostructures film photoanodes. First, ZnO nanorods were grown on the seed layer containing the ITO substrate by a hydrothermal method followed by annealing in air (500 °C/2 h) and hydrogen gas (400 °C/30


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min). Then the film of hydrogenated ZnO (H:ZnO) nanorods was in turn coated with CdS nanoparticles (or CQDs) and CQDs (or CdS nanoparticles) to obtain the H:ZnO/CdS/CQDs (or H:ZnO/CQDs/CdS) structure. Finally, the fabricated films were treated in ambient nitrogen at 400 °C for 30 min to improve the structural stability and crystallinity. Herein, the CQD coating was applied by an electro-deposition method using a two-electrode system at an applied potential of 10 V for 5 min with Pt wire as counter electrode. The electrolyte was an aqueous solution of 0.1 mg/mL of CQDs and the current density was 1 mA/cm2. 2.6. Characterization of the films The surface morphological, structural, and optical properties of the fabricated structures were investigated by field emission scanning electron microscopy (FE-SEM, MIRA II LMH, Tescan, USA) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan), X-ray diffraction (XRD Panalytical, Netherlands), using Cu Kα radiation with a Ni filter, UV-vis-NIR spectroscopy (Jasco V670, Japan), and fluorescence measurements using an FP-6500 spectrofluorometer (JASCO, Tokyo, Japan). Conductance measurements were recorded using a 6487 Keithley instrument in a chamber with controls for temperature and gas flow as previously reported.30-31 2.7. PEC measurements and generated H2 gas collection PEC

properties

were

measured

in

a

three-electrode

electrochemical

system

(Potentiostat/Galvanostat Model 263A), using Hg2Cl2/Hg in saturated KCl as the reference electrode, a platinum (Pt) grid as counter electrode and the fabricated nanostructural films formed on ITO as the working electrode. The electrolyte utilized for the ZnO structures consisted of 0.5 M Na2SO4, whereas that for the CdS-contained structures consisted of 0.25 M Na2S and 0.35 M Na2SO3, as sacrificial agents. A simulated sunlight source, a 150 W Xe lamp (UXL –


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150MO) with an intensity of 100 mW/cm2 coupled with an AM 1.5 G filter, was also employed to evaluate the solar–to–hydrogen (STH) conversion efficiency of the photoanodes. All the measurements were performed with front-side illumination of the photoanodes. The area of the working electrode exposed to the electrolyte was fixed at 1 cm2 by insulating epoxy resin. The STH conversion efficiency was calculated with the following equation:26,32

η=

I ( E rev − E app )

(1)

I0

where I is the photocurrent density (mA/cm2) at the measured bias, I0 is the irradiance intensity of the incident light (100 mW/cm2) (AM 1.5G), and Erev is the standard state-reversible potential (1.23 V vs. NHE). Eapp = Emeas - Eaoc is the applied potential, where Emeas is the electrodepotential (vs. SCE) of the working electrode at which the photocurrent was measured under illuminationand Eaoc is the electrode potential (vs. SCE) of the same working electrode under open circuit conditions under the same illumination and with the same electrolyte. The H2 gas generated in the PEC water-splitting process was collected in a home-made device (Figure S2, Supporting Information). The device consisted of a hollow glass matrix with a pore diameter of 2 cm. The H2 gas was collected by monitoring the electrolyte liquid column in a test tube. 3. RESULTS AND DISCUSSION 3.1. Morphology and characteristics Figure 1 (a-e) shows the SEM images of ZnO, H:ZnO, H:ZnO/CQDs, H:ZnO/CQDs/CdS, and H:ZnO/CdS/CQDs nanostructures. We can see that the ZnO nanorods grew vertically on the transparent conductive ITO glass. The diameter and the length of the ZnO nanorods are in the range of 50 – 200 nm and ~ 1.5 µm, respectively. A similar morphology is observed after hydrogenation for 30 min at 400 °C (Figure 1b). The distance between the nanorods is of the


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order of hundreds of nanometer, which is favorable for the deposition of CQDs and CdS. Although bare ZnO and H:ZnO nanorods reveal quite a smooth surface, the deposition of CQDs (Figure 1c), CQDs/CdS (Figure 1d), and CdS/CQDs (Figure 1e) causes the surface of the H:ZnO nanorods to become more rough. The deposited enveloping layers are uniform, covering the H:ZnO nanorods entirely. The traces of CQDs on the H:ZnO nanorods are clearly visible as shown in Figure 1c, although they cannot be distinguished in the presence of an underlying CdS layer (Figure 1e) due to the resolution limit of the SEM. The fabricated CQDs show particle diameters of 3–5 nm as shown in Figure 1f. A uniform dispersion without aggregation is propitious for electro-deposition on H:ZnO nanorods.

Figure 1. (a-e) FESEM images of ZnO, H:ZnO, H:ZnO/CQDs, H:ZnO/CQDs/CdS, and H:ZnO/CdS/CQDs nanorods, respectively, with ZnO growth time of 4 h. (f) TEM of CQDs. The inset in (a) is the cross-section SEM image of ZnOnanorods at a growth time of 4 h.


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Figure 2. (a) XRD patterns of ZnO (2), H:ZnO (3), H:ZnO/CQDs (4) and N2-treated H:ZnO/CdS/CQDs (5) nanorods. (b) UV-vis absorption spectrum of ZnO, H:ZnO, H:ZnO/CQDs, H:ZnO/CdS/CQDs, and N2-treated H:ZnO/CdS/CQDs nanorods. Figure 2a shows the X-ray diffraction patterns of ZnO, H:ZnO, H:ZnO/CQDs, and H:ZnO/CdS/CQDs (N2 treatment). The XRD pattern of the hydrogen-treated ITO substrate is also shown for comparison. All of them exhibited a hexagonal wurtzite ZnO structure with lattice parameters of a = 3.25 Å and c = 5.21 Å [JCPDS 36-1451]. The strong diffraction peak centered at the scattering angle of 34.5° for the (002) diffraction plane of the wurtzite type of ZnO, which dominates the other peaks, provides evidence that the growth process of ZnO


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nanorods is highly oriented in the 〈001〉 direction on the ITO substrate. As shown in Figure 2a, there is a decrease of diffraction peak intensity although no phase change was observed during hydrogenation at 400 °C for 30 min. This could be due to the increase in defect density in the structure of the ZnO nanorods, which has also been observed in recent studies of the hydrogenated TiO2 structure.33 There is almost no change in the diffraction peaks after deposition of CQDs on the H:ZnO nanorods. This could be due to the very small amount of CQD decoration required to obtain a diffraction peak at a scattering angle of around 25° as shown in Figure S3 (Supporting Information) for CQDs powder. The diffraction peaks at 26.6°, 44.0°, and 52.2° corresponding to the (111), (220), and (311) planes are indicative of a cubic CdS structure [JCPDS 75-1546]. The peak broadening observed for CdS suggests that the CdS grown on the surface of the ZnO nanorods takes particulate forms. The crystallite size of CdS was about 3.2 nm which was determined by the XRD pattern using the Scherrer formula.13 The UV-vis optical absorption spectra of the fabricated nanostructural films are shown in Figure 2b. The absorption edge of the films is determined from the intersection of the sharply decreasing region of the spectrum and its baseline.34 The spectra of ZnO show a strongly absorbed edge at around 402 nm. The absorptivity in the UV region is due to both absorption and scattering, whereas that in the visible light region is consistent with scattering by nanorods with a small diameter only. Importantly, in comparison with ZnO, H:ZnO displays higher absorptivity of visible light and especially in the IR region (>850 nm). Because they have the same crystal structure and morphology, the absorptivity of H:ZnO in the visible light and IR regions is not due to a light scattering effect, indicating that H:ZnO absorbs wavelengths in these regions. The ability of H:ZnO to absorb light in the visible and IR regions can be attributed to impurity states which were introduced in the bandgap of ZnO during hydrogenation. The absorption intensity in


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the visible light and IR regions increases after the deposition of CQDs on H:ZnO. This could be due to the increasing diameter of the H:ZnO nanorods which would enhance light scattering. The absorption edge of H:ZnO/CdS/CQDs and nitrogen-treated H:ZnO/CdS/CQDs nanostructured films shifts from 538 to 560 nm, respectively, indicating that these structures can strongly increase light absorption in the visible region. It was obvious that, after nitrogen annealing, the absorptivity was enhanced strongly and the absorption edge was red-shifted, which might be attributed to the enhanced crystallinity and larger size distribution of the decorated CdS nanoparticles. 3.2. PEC Performance All PEC measurements were performed in a three-electrode electrochemical system, using Hg2Cl2/Hg in saturated KCl as the reference electrode, the platinum grid as counter electrode, and the fabricated nanostructural films formed on ITO as the working electrode. The electrochemical properties of ZnO, H:ZnO, and H:ZnO/CQDs were investigated by linear sweeps in 0.5M Na2SO4 electrolyte solution with a scan rate of 50 mV/s under simulated sunlight illumination at 100 mW/cm2 from a 150 W xenon lamp coupled with an AM 1.5G filter. The role of CQDs in the PEC measurement was observed by the light enhancement of the photocurrent density as shown in Figure 3a. The H:ZnO and H:ZnO/CQDs samples showed an increasing photocurrent density at biases higher than 0.45 V(vs SCE), compared to the ZnO sample that was annealed in air (Figure 3a). This enhancement of the photocurrent density is consistent with previous work, which reported results obtained with ZnO nanorods6 and TiO2.33 The improved photocurrent density of the H:ZnO compared to ZnO annealed in air was attributed to decreased hole trapping and increasing conductivity of the film.6


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Figure

3.

(a)

Comparison

of

the

photocurrent

density

of

ZnO,

H:ZnO,

and

H:ZnO/CQDsnanorods in 0.5 M Na2SO4 electrolyte at a scan rate of 50 mV/s as a function of applied potential (vs. SCE) under simulated solar light illumination of 100 mW/cm2. (b) Relationship between ln(R) and 1/T for the ZnO and H:ZnO nanorod structures in Ar (the straight lines show the fitting). The structure of the device used in the conductance test is shown in the inset image of (b).


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In order to confirm the change in conductance after hydrogenation of the ZnO nanorods, these nanostructures were prepared on an Al2O3 substrate, which was patterned with two Au bar-type electrodes as shown in Figure 3b (inset). The conductance of the fabricated nanorod devices, as fundamental properties of electronic materials, was measured with varying temperatures in ambient argon. Prior to the conductance measurements, the devices were heated to 300 °C in the given ambient conditions to remove the effect of adsorbed water molecules on the surface of the nanorods. The I–V curves of both ZnO and H:ZnO are linear (not shown here), indicating the formation of Ohmic contacts between the nanorods and Au electrodes. The conductance of the nanorods was calculated from the I–V curves for the ZnO and H:ZnO structures and is shown in Figure S4 (Supporting Information). The measurements performed under ambient Ar conditions are not affected by the oxygen ionosorption effect; the results revealed a monotonic increase in the conductance with increasing temperature for all devices. This relationship indicates a dominant semiconducting behavior. Therefore, treatment of the ZnO nanorods in hydrogen did not alter the semiconducting conductance–temperature behavior. Nevertheless, the effect of hydrogenation was observed for the ZnO nanorods, in which the conductance increased significantly in H:ZnO compared to ZnO. The increase in the conduction of the hydrogenated ZnO can be explained by the formation of defects in the ZnO lattice. If the conductivity of a transition metal oxide is established by electrons hopping via dopant impurity levels, then the resistance of the oxide can be expressed by the following equation:35-36

 ∆E a R(T) = Ro exp   k BT

  

(2)

where R(T) is the resistance of a semiconductor at the absolute temperature T, ∆Ea is the thermal activation energy for hopping, and kB is the Boltzmann constant. The temperature-


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dependent resistances of the ZnO and H:ZnO structures are shown in Figure 3b, and were obtained by using the logarithm of equation (2). For the H:ZnO structure, we estimated an activation energy in the measured temperature range (25–250 °C) of 37 meV. This value is smaller than the thermal activation energy of 125 meV observed for the ZnO nanorod structure. The decrease in activation energy of the hydrogenated ZnO structure might be due to increasing donor density arising from hydrogen insertion (Hi) shallow donors or alternatively from oxygen vacancies (Vo). The hydrogen treatment of ZnO nanorods can create impurities in the bandgap, and this observation is in agreement with previous cases of In-doped CdS,37 Al- and Sb-doped CdTe,38 and Au-WO339 which showed a decrease in activation energy with increasing dopant concentrations. In summary, the hydrogenation process of the ZnO nanorods showed significantly increasing trends in conductance. This will be helpful for the rapid transfer of electrons within the ZnO nanorods, resulting in more efficient electron collection. Thus, the ZnO nanorods were subjected to the hydrogenation process before the application of the CdS and CQDs coating in the next steps of the preparation of the photoanodes. The electrochemical properties of the ZnO/CdS, H:ZnO/CdS, H:ZnO/CQDs/CdS, and H:ZnO/CdS/CQDs with a CQD deposition time of 5 min and ZnO growth time of 4 h was investigated by linear sweeps with scan rate of 50 mV/s under simulated sunlight illumination (Figure 4a). An aqueous solution of 0.25 M Na2S and 0.35 M Na2SO3 was utilized as an electrolyte and sacrificial reagent. The current-voltage characteristics were measured and analyzed as shown in Figure 4a. The photocurrent density at 0 V (vs. SCE) of ZnO/CdS (4.04 mA/cm2) and H:ZnO/CdS (4.81 mA/cm2) is considerably higher than that of bare ZnO (0.26 mA/cm2) and H:ZnO (0.22 mA/cm2). This observed enhancement is attributable to the enlargement of the spectral range in which visible light absorption was found to occur, as


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summarized in Figure 2b, which results from the decoration with CdS nanoparticles. Moreover, it was found Figure 4a that the photocurrent density of the H:ZnO/CdS nanorods was higher than that of ZnO/CdS nanorods. This revealed that the effect of hydrogen treatment of the ZnO nanorods was useful for the enhancement of the photocurrent density. This phenomenon could be attributed to the enhancement in the conductivity of the H:ZnO nanorods as observed in the above result. As also shown in Figure 4a, the photocurrent density at 0 V (vs. SCE) of H:ZnO/CQDs/CdS (5.75 mA/cm2) and H:ZnO/CdS/CQDs (6.32 mA/cm2) shows an enhancement of 19.5 % and 31.4 % compared to that of H:ZnO/CdS, respectively. This demonstrates that the introduction of CQDs can effectively improve the photoelectrochemical water-splitting property. It is noteworthy that the sequence of H:ZnO coated with CdS followed by the introduction of CQDs outperformed the sequence of ZnO:H followed by CQDs deposition and then CdS (Figure 4a). This can be ascribed to the partial re-dissolution of CQDs in aqueous solution during CdS deposition at 80 °C in the fabrication process of the H:ZnO/CQDs/CdS nanostructures as well as the obstruction of charge transfer processes between CdS and H:ZnO when CQDs are located in the middle of these layers. 28 In short, the photoresponse results reveal that the H:ZnO/CdS/CQDs structure achieves the highest photocurrent density among the fabricated photoanode structures. Thus, this structure is considered as the optimal structure for the PEC water-splitting photoanode in this work.


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Figure 4. (a) Photocurrent density characteristics of ZnO/CdS, H:ZnO/CdS, H:ZnO/CQDs/CdS, and H:ZnO/CdS/CQDs photoanodes with ZnOnanorods growth time of 4 h and CQDs deposition time of 5 min in 0.25 M Na2S and 0.35 M Na2SO3 electrolyte solution at a scan rate of 50 mV/s under simulated solar light illumination of 100 mW/cm2. (b) Schematic diagram to illustrate the operation of PEC hydrogen generation using the H:ZnO/CdS/CQDs nanorodsphotoanode.


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Figure 4b shows a schematic diagram for the entire PEC water-splitting process from the stepwise structure under solar light illumination. First, the conduction band edge of CdS shifts to a more negative position due to the alignment of Fermi levels and forms the stepwise structure. This permits the rapid transfer of the photo-generated electrons from CdS to ZnO and improves the separation of electron-hole pairs, while preventing the injection of excited electrons from the LUMO level of CQD to the conduction band of CdS. Second, the large band gap ZnO semiconductor material can absorb UV light (which occupies about 4% of the solar spectrum), whereas the CdS material absorbs visible light photons due to its narrow band gap. This UV and visible light absorption excites the electron-hole pair generation in materials. Third, when the CQDs are introduced into the system, they can absorb near-IR photons and visible light, and convert to shorter wavelength photons through the UCPL effect to excite CdS and ZnO, which may increase the number of photo-generated electron-hole pairs. Finally, the electrons would be transferred to the Pt counter electrode under the assistance of a small external potential to enable hydrogen generation from the reduction of protons (2H+ + 2e-→ H2). The holes remaining in the valence bands move to the photoanode/electrolyte interface and are consumed by the hole scavengers (S2-) of the sacrificial agent (Na2S and Na2SO3). It was determined above that the optimal sequence of substances was ZnO:H/CdS/CQDs; thus, H:ZnO/CdS/CQDs(5min) nanorod photoanodes with different growth times for the ZnO nanorods (4, 6, 8, and 10 h) were selected for further study. Figures 5 (a-d) show the crosssection SEM images of these structures. The lengths of the H:ZnO/CdS/CQDs(5min) nanorods are ~1.5, ~2.4, ~3.2, and ~4.3 µm corresponding to ZnO growth times of 4, 6, 8, and 10 h, respectively. The photoelectrochemical property of the H:ZnO/CdS/CQDs(5min) nanorod photoanodes with different growth times of ZnO nanorods (4, 6, 8, and 10 h) was studied using


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current – voltage curves in linear sweeps (as shown in Figure 5e). The photocurrent density (Figure 5e) and corresponding STH efficiency, which is calculated from equation (1) (Figure 5f) increase with increasing ZnO growth time and reaches a maximum value at a ZnO growth time of 8 h (12.15 mA/cm2 at 0 V and 3.85 % at -0.67 V), which is attributed to a larger specific area, thus providing more sites for photoelectrochemical reactions. The photocurrent density value for the optimized structure was about 92% higher than that with a ZnO growth time of 4 h. However, the photocurrent density declined after prolonged ZnO growth (10 h). This lower photocurrent density is attributed to the increasing diffusion length of the photogenerated electrons due to the longer and larger nanorods (as shown in Figure S5). On the other hand, potential growth of nanorods leads to an encounter with other nanorods (Figure S5b), which reduces the contact area between the photoanode and electrolyte. In conclusion, the highest photoresponse from the H:ZnO/CdS/CQDs (5 min) structure is realized by an optimal ZnO growth time of 8 h. The extension of the distance across which electrons are transported and the reduction in the nanorods/electrolyte contact area resulted in a reduction of the photoresponse in the H:ZnO/CdS/CQDs (5 min) structure with prolonged ZnO growth. The above results showed that the photocurrent density of the photoanodes improved after introduction of CQDs into the system. Thus, the effect of the amount of CQDs was also investigated and analyzed in this study. As seen in Figure 6, the photocurrent density and STH efficiency increased when the deposition time of the CQDs was increased from 2.5 to 7.5 min, which indicates that introducing an adequate amount of CQDs can effectively improve the photoanode performance. A further increase in the amount of CQDs (deposition time of 10 min) led to a decline in the STH efficiency as shown in Figure 6. As mentioned above, the photocurrent density improvement with the introduction of CQDs is attributed to IR and visible


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light harvesting, followed by its conversion to shorter wavelength photons through the UCPL effect, which excites the CdS and ZnO to form additional electron-hole pairs. Besides, it is believed that CQDs are able to accept photo-generated electrons from the CdS nanoparticles,28 which would cause a reduction in the transfer of electrons to the Pt counter electrode for H2 gas generation. Competition between these factors enables a suitable amount of deposited CQDs on the photoanode to achieve optimal H2 generation performance. In addition, the effect of CQDs through UCPL property on higher PEC activity was confirmed as illustrated in Figure S6. The H/ZnO(8h)/CdS/CQDs (7.5 min) structure showed an enhanced photocurrent clearly under the illumination of light with 900 nm wavelength (15.7 µW/cm2), while it was not observed in the H:ZnO(8h)/CdS photoanode. This enhanced photocurrent can be attributed to the efficient light harvesting of CQDs with near-IR and the subsequent excitation of CdS by CQDs’ UCPL property.


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Figure 5. (a-d) Cross–section morphology of H:ZnO/CdS/CQDs photoanodes with ZnO nanorods growth time of 4, 6, 8, and 10 h, respectively. (e) Photocurrent density and (f) calculated STH conversion efficiencies for photoanodes in (e) as a function of applied potential (vs. SCE) under simulated solar light illumination of 100 mW/cm2 in 0.25 M Na2S and 0.35 M Na2SO3 electrolyte solution at a scan rate of 50 mV/s. The H:ZnO(8h)/CdS/CQDs(7.5 min) structure exhibited the highest photocurrent density and STH efficiency of 12.82 mA/cm2 (at 0 V vs. SCE) and 3.85 % (at -0.67 V vs. SCE), respectively (Figure 6). These observed data are comparable with or higher than recently reported values for ZnO/CdS and ZnO/CdS/CdSe. For example, ZnO/CdS core-shell nanowire arrays have an STH efficiency value of 3.53%,40 and Al-doped H:ZnO/CdS nanorods using an ITO free electrode,3 vertical ZnO/CdS/CdSe nanowire arrays,41 ZnO/CdS nanotube arrays synthesized by an electrochemical method,42 and an urchin-like ZnO/CdS nanowire array26 show a slightly lower photocurrent density of 5.03, 9.75, 10.64, and 12 mA/cm2, respectively. Therefore, the hydrogen generation and the stability of the photoanode was investigated using the optimal


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H:ZnO(8h)/CdS/CQDS (7.5 min) structure. The design of the homemade device for hydrogen gas collection is shown in Figure S2. The photoanode containing the nanorods was illuminated from the frontside. Under light illumination (100 mW/cm2), H2 gas production was observed at the Pt cathode. Figure 7a shows the collected H2 gas volume as a function of time at -0.67 V (vs. SCE). We renewed the gas collection device every 50 min under illumination, and this was considered as one gas collection cycle. After the first cycle, we carried out H2 gas collection with two more cycles. An average H2 gas evolution rate of ~7.04 mL/h/cm2 was obtained from the optimal H:ZnO(8h)/CdS/CQDS (7.5 min) structure. A linear relationship between the volume of H2 gas and the illumination time was observed for the duration of the measurement (150 min), suggesting that the volume of gas is produced enduringly during continuous operation. In addition, the photocurrent density of the photoanode was recorded during each H2 gas collection cycle in Figure 7a as shown in Figure 7b. The results show that the measured photocurrent densities are nearly the same, supporting that the fabricated photoanode is very stable in the process. However, for practical application, the long-term chemical stability would have to be tested.


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Figure 6. (a) Photocurrent densities and (b) calculated STH conversion efficiencies for the H:ZnO/CdS/CQDs photoanodes with ZnO growth time of 8 h and CQDs deposition times of 2.5 (black curve), 5.0 (red curve), 7.5 (blue curve), and 10 (dark-cyan curve) min as a function of applied potential (vs. SCE) in 0.25 M Na2S and 0.35 M Na2SO3 electrolyte solution at a scan rate of 50 mV/s under simulated solar light illumination of 100 mW/cm2.


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Figure 7. (a) Measured H2 gas collection of optimal H:ZnO(8h)/CdS/CQDs(7.5min) photoanode structure at -0.67 V (vs. SCE) as a function of time in 0.25 M Na2S and 0.35 M Na2SO3 electrolyte solution under simulated solar light illumination of 100 mW/cm2. (b) Dependence of the

corresponding

photocurrent

density

on

the

time

obtained

for

the

optimal

H:ZnO(8h)/CdS/CQDs(7.5min) photoanode structure during the H2 gas collection cycles in (a).


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4. CONCLUSIONS In conclusion, we present a promising way to develop an efficient for PEC hydrogen generation. Using hydrogenated ZnO nanorods coated with CdS and CQDs, we have successfully created a photoanode capable of increasing the STH conversion efficiency in comparison to ZnO coated separately with the individual components. In detail, the optimal structure of H:ZnO(8h)/CdS/CQDS (7.5 min) nanorods showed the highest photocurrent density of 12.82 mA/cm2 at 0 V (vs.SCE) and STH efficiency of 3.85 % at -0.67 V (vs.SCE). The CdS is credited with increasing the absorption of the photoanode in the visible light region of the spectrum, while the up-conversion properties of the CQDs are credited with increasing the absorption of light of near-IR wavelengths. This, along with the reduction in electron-hole recombination attributed to the hydrogenation process, has successfully increased the STH conversion efficiency and provides a framework for future PEC testing.


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ASSOCIATED CONTENT Supporting Information accompanies this paper. AUTHOR INFORMATION Corresponding Author: * Yong-Ill Lee: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Regional University Research Program (NRF 2014008793) through the National Research Foundation of Korea (NRF) and IRES program of National Science Foundation (No. 1358222) in USA.

ABBREVIATIONS PEC, photo electrochemical; ITO, indium tin oxide; CQDs, carbon quantum dots; GQDs, graphene

carbon

quantum

dots;

STH,

solar-to-hydrogen;

UCPL,

up-conversion

photoluminescence.


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H:ZnO Nanorod-Based Photoanode Sensitized by CdS and Carbon

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