Cu2O Nanorod Arrays

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Energy, Environmental, and Catalysis Applications

Heterogenous p-n Junction CdS/Cu2O Nanorod Arrays: Synthesis and Superior Visible-Light-Driven Photoelectrochemical Performance for Hydrogen Evolution Lijuan Wang, Wenzhong Wang, Yuanlu Chen, Lizhen Yao, Xin Zhao, Honglong Shi, Maosheng Cao, and Yujie Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19530 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Heterogenous p-n Junction CdS/Cu2O Nanorod Arrays: Synthesis and Superior Visible-Light-Driven Photoelectrochemical Performance for Hydrogen Evolution Lijuan Wang,† Wenzhong Wang,*,‡ Yuanlu Chen,† Lizhen Yao,‡ Xin Zhao,‡ Honglong Shi,‡ Maosheng Cao,*,† and Yujie Liang‡ †

School of Material Science and Engineering, Beijing Institute of Technology, Beijing

100081, P. R. China. ‡

School of Science, Minzu University of China, Beijing 100081, P. R. China.

KEYWORDS:

CdS/Cu2O

nanorod

arrays,

heterostructure,

photoelectrochemical, hydrogen evolution

1

ACS Paragon Plus Environment

p-n

junction,

ACS Applied Materials & Interfaces 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

ABSTRACT: Heterogenous p-n junction CdS/Cu2O nanorod arrays have been fabricated by using a facile successive ionic layer adsorption and reaction process to grow Cu2O nanoparticles on the surface of ordered CdS nanorod arrays. The heterogenous p-n junction nanorod arrays exhibit superior photoelectrochemical performance for hydrogen (H2) generation and high stability under visible light irradiation. The highest photocurrent density achieved by heterogenous nanorod array photoelectrode is 4.2 mA cm-2 in sacrificial Na2S and Na2SO3 mixture electrolyte solution at 0 V vs. Ag/AgCl, which is 4 times higher than that of pure CdS nanorod array photoelectrode. In addition, the heterogenous nanorod array photoelectrode achieves incident photon conversion efficiency value of 40.5 % at 470 nm. The photocatalytic hydrogen generation rate of heterogenous nanorod array photoelectrode reaches up to 161.2 μmol h-1, around threefold increase compared with that of bare CdS photoelectrode. Furthermore, the heterogenous p-n junction CdS/Cu2O nanorod arrays show an excellent stability under long light illumination of 7200 s. The improved photoelectrochemical performance, photocatalytic activity and excellent stability of heterogenous nanorod array photoelectrode are resulted from the efficient separation of photoinduced electron-hole pairs, which is achieved by the synergistic effects of CdS, Cu2O, p-n junction and an inner electric field in the photoelectrode. The present work provides a new strategy to fabricate heterogenous photoelectrode. This facile strategy is expected to be utilized to fabricate electrodes of other materials for highly efficient solar-driven water splitting application.

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INTRODUCTION Harnessing solar energy, as an inexhaustible and clean energy source, has been extensively considered as a promising technique for addressing global energy crisis and meeting the growing energy demand in a low-cost and environmentally friendly manner. Nevertheless, there are great technical challenges for utilizing energy from sunlight effectively and economically in practicable energy forms. In the past decades, photoelectrochemical (PEC) water splitting has received much attention for converting solar energy into hydrogen energy with no carbon emission.1-7 Up to now, many semiconductor materials, such as TiO2, BiVO4, WO3, CdS and ZnO, have been used as photocatalysts for PEC water splitting and photocatalysis.8-12 As a visible light response semiconductor, CdS has been extensively studied as an important photoanode material for PEC water splitting, owing to its intrinsic properties such as superior light absorption, proper bandgap and a suitable conduction band level for reduction of H+ to H2.13-15 Therefore, many CdS nanostructures with various morphologies have been applied as photoanodes for PEC water splitting. Especially, it has been proved that one-dimension (1D) CdS nanostructure, such as nanowire and nanorod, exhibits higher PEC water splitting activity than its counterparts with other morphologies, due to the large surface area, high length-to-diameter ratio and short charge path length of nanowire and nanorod.16,17 However, many studies have shown that the photocatalytic performance of pure CdS photoelectrode is significantly limited because of the highly photogenerated carrier recombination.18 Furthermore, CdS nanostructures can occur serious photocorrosion under illumination, because the 3



photogenerated holes induce the self-oxidization of S2- in sulfides before the oxidation of water.19,20 Therefore, it is very important to exploit effective methods to improve the PEC activity of CdS photoelectrode. Many investigations were done to enhance the PEC activity and photo-stability of CdS photoanode, such as decorating with noble metals or carbon-based additives,21,22 loading co-catalysts23 and constructing heterostructures.24 The previous studies have demonstrated that heterostructure fabricated through combining two kinds of semiconductors with suitable band structure can significantly enhance PEC and photocatalytic performances of photoelectrode, because the heterostructure is favorable to facilitating the photo-induced charge separation and increasing interfacial charge transfer efficiency.25 In particular, forming p-n junction between semiconductors is expected to be an efficient protocol that can significantly enhance the PEC and photocatalytic performance of photocatalyst, since the p-n junction can produce an internal electric field to efficiently separate electron-hole pairs.26-27 In addition, an effective electron-hole separation is also favorable to improving the photo-stability of p-n junction heterostructure. For instance, previous studies have shown that ZnO/CuInS2 core-shell nanorods demonstrated an improved PEC water splitting efficiency and photo-stability owning to the efficient separation of electro-hole pairs caused by the p-n junctions established at CuInS2 and ZnO.28,29 Cu2O is a p-type semiconductor and has attracted intense interest for its relatively small band gap (ca. 2.0 eV), superior absorption ability for visible light and suitable band structure for PEC water splitting.30 What’s more, some recent works showed that 4


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Cu2O nanoparticles were usually used to fabricate heterogenous photocatalyst by combining with other semiconductors to enhance PEC performance and photocatalytic activity.31-33 However, there are no any studies for the PEC water splitting activity of p-n heterojunction CdS/Cu2O photoelectrode. Considering intrinsic structure, excellent physicochemical properties and superior photocatalytic activities of CdS and Cu2O, the p-n heterojunction photoelectrode fabricated by coupling CdS nanorod arrays with Cu2O nanoparticles is expected to show superior PEC performance for hydrogen evolution. In this work, the heterogenous p-n junction CdS/Cu2O nanorod arrays have been fabricated via combining a simple hydrothermal reaction with a successive ionic layer adsorption and reaction (SILAR) process for the first time. The p-n junction CdS/Cu2O photoelectrode exhibits superior visible-light PEC performance and photocatalytic hydrogen generation activity compared to pure CdS photoelectrode. Furthermore, the CdS/Cu2O photoelectrodes show excellent photo-stability under long time light illumination. We show that the enhanced PEC performance for hydrogen generation and excellent stability are ascribed to the effective charge transfer and separation in heterogenous p-n junction CdS/Cu2O nanorod arrays, experimentally confirmed by electrochemical impedance and room temperature photoluminescence measurements. The current results offer a new strategy for designing high-efficient photoelectrodes with potential applications in PEC water splitting and photocatalytic hydrogen generation.

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EXPERIMENTAL SECTION Materials. Cd(NO3)2·4H2O, CH4N2S, CuSO4, NaOH, Na2S2O3·5H2O and glutathione were purchased from Alfa Aesar. Fluorine-doped tin oxide (FTO, 12 ohm/sq) glass was bought from Zhuhai Kaivo Optoelectronic Technology Co. Ltd. The chemicals were analytical purity and applied as purchased. Synthesis of heterogeneous CdS/Cu2O nanorod arrays. A hydrothermal reaction and a SILAR process were applied to fabricate the CdS/Cu2O nanorod arrays on FTO substrate. Figure 1 schematically shows the synthesis process of CdS/Cu2O nanorod arrays. The hydrothermal reaction was applied to prepare ordered CdS nanorod arrays. Firstly, the FTO glasses were washed separately in deionized water (18.25 MΩ/cm), ethanol and isopropanol alcohol for 10 min. Then, 1.2 mmol Cd(NO3)2·4H2O, 1.2 CH4N2S, 0.72 mmol glutathione and 40 mL deionized water were transferred to an autoclave (50 mL), followed by vertically placing the cleaned FTO glass in the solution, and heating at 200 ℃ for 8 h. Finally, the as-obtained sample was cooled, washed with deionized water and ethanol in turns, and dried naturally. The Cu2O nanoparticles were grown on CdS nanorod arrays via an easy SILAR process. In a typical SILAR process, the CdS nanorod arrays were firstly immersed into an aqueous solution containing Na2S2O3 (1.0 M) and CuSO4 (1.0 M) for 2 s at 25℃ and subsequently rinsed with deionized water. Afterwards, the nanorod arrays were immersed into NaOH aqueous solution (0.5 M, 100 mL) for 2 s at 70℃ and then rinsed with deionized water. The loading amount of Cu2O nanoparticles was controlled by adjusting the above SILAR cycles, the samples obtained by cycling 5, 6


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10 and 20 times were respectively referenced as CdS-5Cu2O, CdS-10Cu2O and CdS-20Cu2O. The chemical reactions for the formation of Cu2O nanoparticles are given as the following equations34 2Cu2+ + 4S2O32– → 2[Cu(S2O3)] – + [S4O6]2–

(1)

[Cu(S2O3)] – ↔ Cu+ +S2O32–

(2)

2Cu+ + 2OH– → Cu2O + H2O

(3)

Characterization. The crystalline structure of samples was studied by X-ray powder diffraction (XRD) diffractometer using a Rigaku Dmax γA rotation anode. The shape and microstructure feature were evaluated by a Hitachi scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, JEM-2100). X-ray photoelectron spectroscopy (XPS, ESCSLAB 250Xi) was applied to determine the electronic states of elements. The band energy of C 1s (284.6 eV) was used for calibration. A lambda 950 spectrophotometer was applied to study the UV-Vis absorption of the samples. Room temperature photoluminescence properties were studied on a fluorescence spectrophotometer (Edinburgh FLS980) using 325 nm light as excitation source. PEC performance and photocatalytic hydrogen generation. All the PEC performance measurements were conducted in an electrochemical workstation (CHI 660E), in which fabricated CdS/Cu2O nanorod array, a Pt wire and Ag/AgCl were utilized as working, counter and reference electrode, respectively. The measurements were performed under illumination of a Xenon lamp (300 W). The incident light power intensity on the photoelectrode surface was 100 mW cm-2. A mixed 0.35 M 7



Na2S and 0.25M Na2SO3 solution was employed as electrolyte. The incident photon conversion efficiency (IPCE) was measured in a Newport QE/IPCE testing system with a 2936-R dual channel power/current meter. It has been reported that the IPCE = (1240×I)/(λ×Jlight). From this equation, one can find that the IPCE is determined by the measured photocurrent density (I), the incident light wavelength (λ), and the measured power density (Jlight) of monochromatic incident light. Electrochemical impedance spectroscopy (EIS) was measured under visible light irradiation at the open-circuit potential in the frequency from 10 to 100000 Hz. Mott-Schottky curves were collected under illumination at an AC frequency of 1000 Hz. The displacement method was used to collect and measure the volumes of H2 during photocatalytic hydrogen generation experiments.35 RESULTS AND DISCUSSION Characterization. XRD was applied to study the composition of the bare CdS and heterogenous CdS/Cu2O nanorod arrays. As seen in Figure 2a, for bare CdS nanorod arrays, all peaks are readily assigned to those of hexagonal phase CdS crystal (JPCDS card 41-1049), indicating the high purity of the CdS nanorod arrays.36 For CdS/Cu2O nanorod arrays, the XRD patterns also show diffraction peaks of CdS crystal, the peaks of Cu2O crystal are not observed. The absence of the diffraction peaks from Cu2O is probably resulted from the small loading content and the high dispersion of the Cu2O nanoparticles in the heterogenous nanorod arrays. XPS spectra were further used to analyze the composition of CdS/Cu2O heterogenous nanorod arrays and to determine surface chemical states of elements. 8


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The survey spectrum in Figure 2b displays that the heterogenous nanorod array contains Cd, Cu, S, O, and C elements. The XPS spectra of the heterogenous CdS/Cu2O nanorod arrays are shown in Figure 2c-f. In Figure 2c, the binding energies at 404.7 and 411.4 eV can be respectively indexed to Cd 3d 5/2 and Cd 3d 3/2 of Cd2+ in CdS7. In Figure 2d, the band energies at 162.5 and 161.2 eV can be indexed to S 2p 1/2 and S 2p 3/2, which are ascribed to S2- in CdS.37 In Figure 2e, the band energies at 932.3 and 952.1 eV originate from Cu 2p 3/2 and Cu 2p 1/2, showing the existence of Cu1+ in the heterogenous nanorod array.38 In addition, there are no satellite peaks at the high energy side of Cu 2p 1/2 and Cu 2p 3/2, proving that only Cu2O nanoparticles are formed on the CdS nanorod surface.39 The of O 1s spectrum is presented in Figure 2f. The main peak at 530.6 eV corresponds to the oxygen species of Cu2O, while the weak peak at 531.8 eV may come from the surface hydroxyl group or the adsorbed H2O on the surface of the heterogenous nanorod array.40 Figure 3a shows the SEM images of pure CdS nanorod arrays and the corresponding enlarged SEM picture (inset) of a CdS nanorod, showing that the ordered CdS nanorods are composed of regular hexagonal prisms with smooth lateral surface and flat top surface. The cross-section size of nanorod is around 300-350 nm in diameter. Figure 3b presents the SEM image of heterogenous CdS/Cu2O nanorod array obtained by cycling 10 times in SILAR process. One can see that the shape of CdS nanorods has no significant change after growing Cu2O nanoparticles. However, after the deposition of Cu2O nanoparticles, both the top and lateral surfaces of nanorods become rough and are coated with uniform Cu2O nanoparticles. TEM 9



images were also employed to characterize the heterogenous CdS/Cu2O nanorod arrays. The TEM images of pure CdS and heterogenous CdS/Cu2O nanorods are shown in Figure 3c, d, respectively. TEM images show that the surface of bare CdS nanorod is smooth. Nevertheless, the surface of CdS nanorods becomes rough after the deposition of Cu2O nanoparticles. Figure 3e exhibits the high resolution TEM (HRTEM) image of heterogenous CdS/Cu2O nanorod. A high-magnification HRTEM image of the selected interface region of the heterogenous nanorod (marked by a dashed rectangle in Figure 3e) is shown in Figure 3f, demonstrating that the Cu2O nanoparticles are tightly grown on the edge of the CdS nanorods. The lattice plane with spacing of 0.25 nm can be observed, which corresponds to the (111) crystalline planes of the cubic phase Cu2O crystal.30 Moreover, the CdS and Cu2O interface can be clearly seen and the two components contact intimately, which gives the direct evidence for the formation of heterojunction between Cu2O nanoparticle and CdS nanorod. The loading contents of Cu2O nanoparticles were controlled by adjusting the SILAR deposition cycles. Figure 4 displays the SEM images of the heterogenous CdS/Cu2O nanorod arrays prepared with different SILAR cycles. In Figure 4a, the SEM image demonstrates that the CdS nanorods show smooth surface before loading Cu2O nanoparticles. As seen in Figure 4b, a few Cu2O nanoparticles with small size are grown on the surface of CdS nanorods after 5 SILAR cycles. However, the loading amount and size of Cu2O nanoparticles increase with prolonging the cycle number to 10 times (CdS-10Cu2O) as shown in Figure 4c. Moreover, after 20 SILAR 10


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cycles, many Cu2O nanoparticles grow on the surface of CdS nanorods. In addition, some Cu2O nanoparticles grow together to form aggregate with larger size as seen in Figure 4d (CdS-20Cu2O). The light absorption spectra of the samples are presented in Figure 5. For pure CdS nanorod arrays, a significant absorption edge is at 540 nm, which matches well with the intrinsic bandgap absorption of CdS crystal.41,42 Whereas for the heterogenous CdS/Cu2O nanorod arrays, we note that the light absorption from 540 to 700 nm is enhanced after the loading of Cu2O nanoparticles. The enhanced light absorption of heterogenous CdS/Cu2O nanorod array is beneficial for improving its PEC activity. In addition, it can be found that the absorption intensity of the heterogenous nanorod arrays shows gradually increase with increasing the loading contents of Cu2O nanoparticles. PEC performance and photocatalytic hydrogen generation. Figure 6a shows the transient visible light responses and photocurrent densities of pure CdS and CdS/Cu2O photoelectrodes at 0 vs. Ag/AgCl. The photocurrent of each photoelectrode rises rapidly and maintains constant after the light turns on. The photocurrent density of pure CdS nanorod array photoelectrode is 0.96 mA cm-2. After the growth of Cu2O nanoparticles,

the

photocurrent

density

of

the

CdS/Cu2O

heterostructure

photoelectrode achieves a significant improvement. Moreover, the loading contents of Cu2O nanoparticles show an obvious effect on the photocurrent density of CdS/Cu2O heterostructure

photoelectrode.

The

photocurrent

density

of

CdS-10Cu2O

photoelectrode is 4.2 mA cm-2, over 4 times that of the bare CdS photoelectrode. 11



However, when the cycles further increase to 20 times, the photocurrent of photoelectrode (CdS-20Cu2O) decreases to 1.7 mA cm-2, indicating that the excess Cu2O nanoparticles possibly is unfavorable for the charge transport and separation at the interface of CdS and Cu2O.43 The experiment measurements show that both low and excess loading contents of Cu2O nanoparticles are not beneficial for the efficient charge transfer and separation of heterogenous CdS/Cu2O nanorod arrays. Figure 6b shows the plots of the current density versus bias for the photoelectrodes at a bias from -0.9 to 0.9 V vs. Ag/AgCl. The photocurrent densities of both pure CdS and CdS/Cu2O photoelectrodes show improvement with increasing applied bias. The pure CdS photoelectrode exhibits inapparent improvement for photocurrent density, while the current densities of CdS/Cu2O heterostructure photoelectrodes improve significantly. The results demonstrate that appropriate loading of Cu2O nanoparticles not only significantly improves visible light absorption but also efficiently separates the photoinduced electrons and holes, leading to the remarkable enhancement for PEC water splitting performance. To quantify the photoelectric conversion efficiency with incident light of disparate wavelengths, the IPCE measurements were conducted on pure CdS and CdS/Cu2O photoelectrodes at 0 V vs. Ag/AgCl. Figure 7 displays the IPCE curves for the as-obtained CdS and CdS/Cu2O photoelectrodes, showing that all photoelectrodes exhibit a broader light response ranging from 400 to 590 nm. The pure CdS nanorod array photoelectrode shows a very low IPCE value (∼8%) and exhibits photoresponse up to 560 nm. After the loading of Cu2O nanoparticles, the CdS/Cu2O photoelectrodes 12


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achieve much higher IPCE values. When the SILAR cycle is 10 times, the photoelectrode (CdS-10Cu2O) shows a very broad IPCE plateau ranging from 400 to 480 nm with an IPCE value of 40.5% at 470 nm, which is over 5 times to the pure CdS photoelectrode. The photocatalytic H2 generation experiments for bare CdS and CdS/Cu2O photoelectrodes were carried out at 0 V vs. Ag/AgCl in Na2S and Na2SO3 mixture electrolyte solution under visible light irradiation. A linear increase of H2 generation with the illumination time can be observed over the two photoelectrodes (Figure 8a), indicating that the photoelectrode is very stability during continuous measurements. Figure 8b displays the H2 production rates of pure CdS and CdS-10Cu2O photoelectrodes. The measurements show that the bare CdS photoelectrode exhibits a low rate of H2 generation (57.14 μmol h-1). Nevertheless, the H2 production rate of CdS-10Cu2O photoelectrode reaches to 161.2 μmol h-1, about 3 times over that of CdS photoelectrode. The results demonstrate that the increase of hydrogen production is not as great as the increase of photocurrent density of CdS/Cu2O nanorod arrays, which may be resulted from the measurement error of the displacement method used to collect and measure the volumes of H2 during photocatalytic hydrogen generation experiments. Moreover, it is noteworthy that the CdS-10Cu2O photoelectrode shows excellent stability during four cycling experiments without renewing the electrolyte and the photoelectrode (Figure 8c). In addition, under visible light irradiation, a linear increase of H2 generation with the illumination time can be found in each cycle. The above results reveal the crucial role of Cu2O nanoparticles in improving the 13



photocatalytic

hydrogen

generation

activity

for

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CdS/Cu2O

heterogenous

photoelectrode. Long-time photocurrent stability of photoelectrode is also important for practical applications of PEC cells. The photocurrent stability of pure CdS and CdS-10Cu2O photoelectrodes was examined via photocatalytic hydrogen generation experiment. From Figure 8d, it can be found that the photocurrent density of CdS-10Cu2O photoelectrode maintains at 3.7 mA cm-2 after light illumination of 7200 s. whereas pure CdS photoelectrode decays obviously with prolonging illumination time. The results experimentally show that the CdS-10Cu2O photoelectrode exhibits an excellent stability during a long light illumination. The superior photo-stability of heterogenous nanorod array photoelectrode possibly results from the effective separation of photoinduced electron-hole pairs. Mechanism. EIS were applied to study the charge transfer and separation of the obtained photoelectrodes. Figure 9a displays the Nyquist plots of pure CdS and CdS-10Cu2O photoelectrodes obtained under visible light illumination at open circuit potential (OCP). The measured values of OCP for pure CdS and CdS-10Cu2O photoelectrodes are -0.555 and -0.728 V, respectively. It has been demonstrated that the smaller semicircle arc diameter in Nyquist plot means the lower charge transfer resistance and the efficient charge transportation and separation.44-46 As seen in Figure 9a, the CdS-10Cu2O photoelectrode shows a smaller semicircle arc diameter than that of bare CdS electrode, showing that the loading of Cu2O nanoparticles can reduce the charge-transfer resistance of the heterogenous photoelectrode. It can be seen that there 14


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are two semicircles in the Nyquist plots. The large semicircle in low-frequency range is associated to electron transfer resistances at the photoelectrode/electrolyte interface, and the small semicircle in high-frequency range corresponds to the counter electrode/electrolyte interface. For further quantitative analysis, the equivalent circuit model has been constructed based on the experimental EIS data (inset of Figure 9a), and the relative fitting parameters are listed in Table 1.47 In the model, the small semicircle is fitted to charge-transfer resistance (Rct1) and constant phase element (CPE1), while the large semicircle is fitted to Rct2 and CPE2. Rs is the serious resistance. As shown in Table 1, the calculated Rct2 values for CdS-10Cu2O photoelectrode is 103 Ω, which much lower than that of pure CdS photoelectrode (718 Ω), suggesting that the formation of CdS/Cu2O heterostructure can efficient drive the charge transfer and separation. The transfer electron lifetime (τe) for recombination can be determined by Bode phase plots, as shown in Figure 9b. τe can be evaluated by following equation: τe = 1/(2πfmax), where fmax is the characteristic maximum frequency of the low frequency region. From the Bode phase plots, one can see that the characteristic maximum frequency of CdS-10Cu2O photoelectrode locates in the lower frequency position, which suggests that the τe in the CdS-10Cu2O photoelectrode is larger than that in pure CdS photoelectrode. The larger τe shows more effective suppression of photoinduced electron-hole pair recombination, leading to significant enhancement for PEC performance. Mott-Schottky (M-S) curves were employed to study the carrier density for the as-obtained photoelectrodes. The M-S plots of pure CdS and CdS-10Cu2O 15



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photoelectrodes were recorded at an AC frequency of 1000 Hz under visible light irradiation (Figure 9c). The pure CdS photoelectrode shows a positive slope, demonstrating an n-type semiconductor behavior. However, for the CdS-10Cu2O photoelectrode, a p-n junction characteristic can be observed as demonstrated with an obvious inverted “V-shape”, indicating that p-n junctions are formed at the p-type Cu2O and n-type CdS interface.48,49 The donor density of the as-prepared photoelectrodes can be obtained using the following equation: ଵ

஼మ

=

ଶ

ఌబ ఌೝ ௘ேವ ஺మೞ

ቀܸ௔௣௣௟ − ܸி஻ −

௞் ௘

ቁ

（4）

Where ε0 and εr are separately the vacuum permittivity and the dielectric constant of semiconductor, e is the elementary charge, ND is the donor density, As is the surface area of electrode, Vappl and VFB are separately the applied potential and the flat-band potential, k is the Boltzmann constant, and T is the temperature.50 According to the slope of M-S plots, the donor densities of pure CdS and CdS-10Cu2O photoelectrodes are 1.05×1020 and 3.29×1020 cm-3, respectively. The increase of donor density for CdS-10Cu2O photoelectrode could be related to enhanced charge separation, which is caused by the p-n junction in the heterogenous photoelectrodes. The separation and transfer behavior of photoinduced carriers for the as-prepared photoelectrodes was further investigated by room temperature PL measurements. Figure 9d exhibits the PL spectra of the CdS and CdS/Cu2O nanorod array photoelectrodes. It is obvious seen in Figure 9d that the CdS nanorod arrays show a emission peak at around 540 nm, which corresponds to the recombination of photoexcited electrons and holes of the CdS crystal.51 Nevertheless, the intensity of 16


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the CdS/Cu2O nanorod arrays is significantly deceased, proving that the loading of Cu2O nanoparticles can effectively facilitate the separation of photoinduced charges in heterostructure.7,52 Therefore, these results experimentally give the evidence for the efficient separation of photoinduced carriers and enhanced PEC water splitting performance for heterogenous CdS/Cu2O nanorod array photoelectrode. According to experimental results, a proposed mechanism has been used to elucidate the charge transfer and separation for CdS/Cu2O nanorod arrays as presented in Figure 10. According to the proposed mechanism, the electrons photoexcited to the conduction band (CB) of Cu2O will move to the CB of CdS. The transferred electrons and the photogenerated electrons of CdS can transfer to the Pt counter electrode through an external circuit to split water into hydrogen (Figure 10a). Meanwhile, the holes that are separated and remained on the valence band (VB) of CdS will flow to the VB of Cu2O and then react with the sacrificial agents of Na2S and Na2SO3 in the electrolyte (Figure 10a). Obviously, p-n junctions formed in the interface between n-type CdS nanorods and p-type Cu2O nanoparticles are favorable for the transport and separation of photoproduced charges in heterogenous nanorod array photoelectrode. Figure 10b displays the energy band structure of CdS/Cu2O nanorod arrays. Prior to contact, both CB and VB levels of p-type Cu2O are over those of n-type CdS, whereas the Fermi level (EF) of Cu2O is below that CdS. After contacting to construct the p-n junction heterostructure, the energy level of Cu2O moves up, while that of CdS moves down, resulting in the formation of an equilibrium state for Fermi levels between CdS and Cu2O. Consequently, the CB and VB edges of Cu2O 17



are much higher than those of CdS. Consequently, an inner electric field at the CdS and Cu2O interface is built under the thermodynamic equilibrium condition.31 Both CdS and Cu2O can be excited to generate electrons and holes under visible light illumination. The electrons excited to the CB of Cu2O can migrate to that of CdS, the holes can transfer from the VB of CdS to that of Cu2O. Furthermore, the inner electric field can greatly facilitate the migration and separation of photoinduced electron-hole pairs, resulting in enhanced PEC performance. CONCLUSIONS The heterogenous p-n junction CdS/Cu2O nanorod arrays have been successfully fabricated by combining a hydrothermal method with a SILAR process. Used as photoelectrode, the heterogenous p-n junction CdS/Cu2O nanorod arrays show significantly enhanced PEC performance compared with pure CdS nanorod arrays. The IPCE value achieved over heterogenous p-n junction CdS/Cu2O nanorod array photoelectrode is as high as 40.5 % at 470 nm without applying bias potential. The photocatalytic hydrogen generation rate of heterogenous nanorod array photoelectrode is 161.2 μmol h-1, about 3 times higher than that of bare CdS photoelectrode. Moreover, the heterogenous p-n junction CdS/Cu2O nanorod array photoelectrode show an excellent stability under long light illumination. The enhanced PEC water splitting performance, photocatalytic H2 generation activity and high photo-stability of heterogenous p-n junction CdS/Cu2O nanorod array photoelectrode are attributed to the effective separation of photoexcited electro-hole pairs, which is achieved by the synergistic actions of CdS, Cu2O, p-n junction and inner electron field in the 18


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photoelectrode. This work provides an effectively synthetic strategy that is expected to show promising potential applications in fabricating other electrodes with enhanced PEC performance. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (M.C.). * E-mail: [email protected] (W.W.). ORCID Wenzhong Wang: 0000-0003-1366-3498 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China under Grant nos. 61575225, 11074312, 11374377, 11474174, 11404414, 51132002 and 51372282.

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Figure Captions

Figure 1. Schematic illustration for the fabrication procedure of heterogenous CdS/Cu2O nanorod arrays. Figure 2. (a) XRD patterns of pure CdS and heterogenous CdS/Cu2O nanorod arrays. (b) The survey XPS spectra of heterogenous CdS/Cu2O nanorod arrays. High-resolution XPS spectra of heterogenous CdS/Cu2O nanorod arrays: (c) Cd 3d, (d) S 2p, (e) Cu 2p and (f) O 1s. Figure 3. (a) SEM image of the pure CdS nanorod arrays. The inset is enlarged SEM image of a single CdS nanorod. (b) SEM image of the as-fabricated heterogenous CdS/Cu2O nanorod arrays. TEM images of pure CdS nanorod arrays (c) and heterogenous CdS/Cu2O nanorod arrays (d). (e) HRTEM image of heterogenous CdS/Cu2O nanorod arrays. (f) Enlarged HRTEM image of the area marked with the red rectangle in Figure 3e. Figure 4. SEM images of the heterogenous CdS/Cu2O nanorod arrays obtained at different SILAR cycles: (a) 0, (b) 5, (c) 10 and (d) 20 cycles. Figure 5. UV-Vis absorption spectra of the CdS, CdS-5Cu2O, CdS-10Cu2O and CdS-20Cu2O nanorod arrays. Figure 6. (a) Visible light responses and photocurrent densities at 0 V vs. Ag/AgCl and (b) the photocurrent density versus applied bias curves of the CdS, CdS-5Cu2O, CdS-10Cu2O and CdS-20Cu2O photoelectrodes. 28


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Figure 7. IPCE curves of the CdS, CdS-5Cu2O, CdS-10Cu2O and CdS-20Cu2O photoelectrodes at 0 V vs. Ag/AgCl. Figure 8. (a)Time-dependent amounts and (b) the average rate of photocatalytic H2 production over pure CdS and CdS-10Cu2O photoelectrodes at 0 V vs. Ag/AgCl. (c)The photocatalytic H2 generation stability of CdS-10Cu2O photoelectrodes. (d) The long-time photocurrent stability of pure CdS and CdS-10Cu2O photoelectrodes. The surface area of photoelectrodes used for photocatalytic hydrogen generation is 1.5×4.0 cm2. Figure 9. (a) Nyquist plots, (b) Bode plots, (c)Mott-Schottky plots, and (d) Room temperature PL spectra of the pure CdS and heterogenous CdS/Cu2O photoelectrodes. The insets in Fig. 9a depict the equivalent circuit model used to fit the EIS data (left) and EIS spectra in high-frequency region (right). Figure 10. (a) The proposed photogenerated carriers transfer process in the heterogenous CdS/Cu2O photoelectrode and (b) the energy-band schematic diagram for heterogenous p-n junction CdS/Cu2O nanorod arrays. Table 1. The fitting parameters related to the equivalent circuit in Fig. 9a for the as-prepared photoelectrodes.

29



Figure 1

30


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Figure 2

31



Figure 3

32


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Figure 4

33



Figure 5

34


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Figure 6

35



Figure 7

Figure 7

36


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Figure 8

37



Figure 9

38


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Figure 10

39



Rs

Rct1

CPE1

(Ω cm2)

(Ω cm2)

(sn/Ω cm2)

CdS

10.2

7.07

1.194E-4

CdS-10Cu2O

9.92

6.91

1.962E-5

ndl1

Page 40 of 41

Rct2

CPE2

ndl2

(Ω cm2)

(sn/Ω cm2)

0.642

718

5.262E-5

0.73

0.735

103

4.618E-3

0.65

Samples

Table 1

40


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TOC

41


Cu2O Nanorod Arrays

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