shell nanoarray decorated with PbS quantum dots for efficient photoelectrochemical water splitting

Journal of Alloys and Compounds 828 (2020) 154449

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Hierarchical ZnO nanorod/ZnFe2O4 nanosheet core/shell nanoarray decorated with PbS quantum dots for efficient photoelectrochemical water splitting Haiyu Jiang a, Yajie Chen a, **, Li Li b, He Liu a, Can Ren a, Xiu Liu a, Guohui Tian a, * a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin, 150080, PR China b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun, 130012, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 January 2020 Received in revised form 19 February 2020 Accepted 20 February 2020 Available online 24 February 2020

The construction of a uniform heterojunction photoelectrode with rational band alignment and component control has been proved to improve the performance of photoelectrochemical water splitting. Here we prepared ZnO nanorod/ZnFe2O4 nanosheet core/shell nanoarray photoanodes, and PbS quantum dots were further decorated to promote charge transport and act as light absorber. The optimized ZnO/ZnFe2O4/PbS photoanode displays a significantly enhanced photocurrent density and excellent photoelectrochemical stability, and its photoconvertion efficiency is 1.34 and 3.54 times higher than that of ZnO/ZnFe2O4 and ZnO nanorod array photoanodes, respectively, and 87.4% of the initial photocurrent density is obtained after 2 h light irradiation. This work provides a facile strategy for the design of highly efficient and stable photoelectrodes in photoelectrochemical water splitting. © 2020 Elsevier B.V. All rights reserved.

Keywords: ZnO/ZnFe2O4/PbS Core/shell heterostructure Nanoarray film Photoanode Photoelectrochemical water splitting

1. Introduction The ever-increasing contemporary energy crisis and environmental pollution have motivated people to develop sustainable green energy. Photoelectrochemical (PEC) water splitting of converting solar energy into hydrogen and oxygen is gaining wide attention due to its advantages of cleanness, efficiency and sustainability [1]. Since TiO2 photoelectrode has been used in the photoelectrochemical water splitting system by Fujishima and Honda in 1972, a great deal of effort has been made on photoelectrochemical water splitting [2]. In order to obtain high photoelectric conversion efficiency in photoelectrochemical system, it is a rational strategy to fabricate efficient and stable semiconductor photoanodes [3e6]. Excellent photoanode materials should possess several good properties including good light absorption, high chemical stability, and appropriate energy band structure [7].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Tian).

(Y. Chen),

https://doi.org/10.1016/j.jallcom.2020.154449 0925-8388/© 2020 Elsevier B.V. All rights reserved.

[email protected]

Therefore, developing effective approaches to fabricate photoanode materials is regarded as a promising pathway. ZnO is a typical photoanode material for photoelectrochemical applications. ZnO nanorod arrays have often been applied for photoelectrochemical water splitting because of its fast electron transport and high surface area [8e10]. However, the negative effects of relatively low chemical stability in the electrolyte solution, poor visible light response, and high recombination rate of photogenerated electron-hole pairs limit the practical applications. Constructing ZnO-based heterostructures have been proved to successfully address these problems [11e15]. For example, 3D ZnO/ CuO branched nanowire arrays were prepared and possessed enhanced surface area and improved gas evolution and light absorption ability, thus resulting in enhanced PEC solar hydrogen production. ZnFe2O4, as a typical n-type semiconductor, is thought as a promising photoelectrode material because of its proper band gap ( 1.9 eV) and good stability [16e18]. Some heterostructures associated with ZnFe2O4 are prepared as photoanodes for photoelectrochemical water splitting [19e22]. In addition, the accomplishment of efficient photoelectrocatalysis also depends on the delicate design and fabrication of catalysts with ideal structures. The core/shell heterostructure catalysts with various

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architectures and adjustable compositions have exhibited significant advantages in photocatalysis and photoelectrocatalysis [23e25]. Recently, one-demensional core/shell heterostructures have been introduced to the PEC field as power photoelectrodes to upgrade photoelectric conversion performance [26]. The inner onedemensional materials (e.g. nanorods, nanowires) endow effective electron access along the axial direction. The outer shell materials can enhance incident light utilization and increase active sites. Moreover, the formed core/shell heterostructures facilitate rapid transfer and transport of electron-hole pairs [27,28]. The above advantages make one-demensional core/shell heterostructure one of the appropriate choices of photoelectrode materials of efficient PEC batteries. Besides, photoabsorption and charge separation could be further enhanced by the introduction of photosensitizers [29,30]. Despite these accomplishments, it remains a great challenge to design and fabricate one-demensional core/shell heterostructures to integrate these compositional and structural advantages mentioned above and realize the improvement of photoelectrocatalytic performance. In this work, we report the design and synthesis of PbS quantum dots decorated ZnO/ZnFe2O4 core/shell nanoarrays photoanodes for high-performance PEC water splitting. The overall synthetic strategy includes several steps, as illustrated in Scheme 1. ZnO nanorod arrays were firstly in situ grown on the FTO glass. Then the wrinkled ZnFe2O4 nanosheets were grown on ZnO nanorods to fabricate ZnO nanorod/ZnFe2O4 nanosheet core/shell nanoarrays via ion exchange reaction and subsequent annealing process [31]. The close contact between ZnO nanorod core and ZnFe2O4 nanosheet shell enables the nanoscale junctions to be built. So photogenerated charge carriers could transport rapidly from the surface to inside and then transfer from the external circuit to the counter electrode, and efficient charge separation could be acquired. Finally, PbS quantum dots were anchored on the ZnO/ZnFe2O4 core/shell nanoarrays using the successive ionic layer adsorption and reaction

method [32]. The obtained PbS quantum dots decorated ZnO/ ZnFe2O4 core/shell nanoarrays combined the structural and functional merits of one-dimensional ZnO/ZnFe2O4 core/shell heterostructure. Impressively, when anchored with visible light photosensitizer PbS quantum dots, the core/shell structural nanoarrays exhibited considerable photocurrent density and high stability in photoelectrochemical water splitting [33]. This study provides an ideal synthetic strategy for the construction of heterostructure photoelectrode systems for efficient photoelectrochemical water splitting. 2. Experimental section 2.1. Materials Zinc acetate dihydrate (Zn(AC)2$2H2O, 99%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Zine nitrate hexahydrate (Zn(NO3)2$6H2O, 99%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Hexamethylenetetramine (HMTA, 99%) was purchased from Xilong Chemical Co., Ltd (Shantou, China). Ferrous sulfate (FeSO4$7H2O, 99.0%) was purchased from Shuangchuan Chemical Reagent Co., Ltd (Tianjin, China). Lead nitrate (Pb(NO3)2, 99.0%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Sodium sulfide (Na2S$9H2O, 98%) was purchased from Tianli Chemical Reagent Co., Ltd (Tianjin, China). Methanol (CH3OH, 99.5%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Ethanol (C2H5OH, 99.7%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). All the chemical reagents were used as received without further purification. 2.2. Synthesis of ZnO nanorod arrays ZnO nanorod arrays (NRs) were prepared by a two-step process

Scheme 1. Synthetic route diagram of ZnO/ZnFe2O4/PbS nanorod arrays.

H. Jiang et al. / Journal of Alloys and Compounds 828 (2020) 154449

3

[34]. Firstly, ZnO seeds layer was deposited on clean FTO class substrate by a dip-coating method. The ethanol solution containing 0.005 M zine acetate dehydrate was vertically dropped on the FTO substrate, dried in an oven, and the above procedure was repeated 10 times. Then the treated FTO substrate was calcined in a muffle furnace at 350
C for 2 h. Secondly, 0.6 mmol zine acetate and 0.6 mmol hexamethylenetetramine were dissolved in deionized water (25 mL) and stirred for 1 h. The above mixture was placed in a 40 mL Teflon-lined stainless steel autoclave, and the FTO glass substrate was placed in the autoclave with the conductive surface facing downward at a certain angle. After reacting under 95
C for 8 h, the autoclave was cooled down to room temperature, the obtained FTO glass covered with ZnO nanorod arrays was washed with distilled water and dried at 60
C.

was measured at a frequency ranging from 100 kHz to 0.01 Hz with the amplitude of 5 mV. The Mott-Schottky curves under dark condition were measured at a frequency of 1 KHz within potential range from 1 V to 0.35 V vs RHE. The wavelength dependent incident photon current efficiency (IPCE) equation can be calculated as follows:

2.3. Preparation of hierarchical ZnO/ZnFe2O4 core/shell nanoarray

ERHE ¼ EAg=AgCl þ 0:059 pH þ 0:1976 V

The FTO glass covered with ZnO nanorod arrays was immersed in a ferrous sulfate solution with different concentrations of 0.05 M, 103 M, and 104 M and stirred for 7 min. Then the substrate was taken out and rinsed with distilled water, and dried in an oven. Finally, the substrate was annealed at 530
C for 2 h with a heating rate of 2
C min1. The ZnO/ZnFe2O4 nanosheets arrays prepared in 0.05 M, 103 M, and 104 M ferrous sulfate solution were labeled as ZnO/ZnFe2O4-1, ZnO/ZnFe2O4-2, and ZnO/ZnFe2O4-3, respectively. PbS quantum dots decorated ZnO/ZnFe2O4 was prepared via the successive ionic layer adsorption and reaction (SILAR) methed. 0.01 M Pb(NO3)2 and 0.01 M Na2S$9H2O methanol solutions were first prepared by resolving Pb(NO3)2 and Na2S$9H2O into methanol solution, respectively. Then the FTO substrate was soaked in the Pb(NO3)2 (0.01 M) and Na2S$9H2O (0.01 M) solutions in sequence and stirred for 1 min. Then the substrate was rinsed with methanol and deionized water, and dried in an oven. The above process was repeated 3 times. 2.4. Characterization The X-ray diffraction (XRD) data of the as-synthesized products were obtained from X-ray diffractometer using Cu Ka radiation (SmartLab 9 KW, l ¼ 0.15405 nm, 40 kV, 100 mA). The morphology and structure of the obtained samples were characterized by scanning electron microscopy (SEM, SIGMA 500), and the microstructure of samples was characterized by transmission electron miscroscopy (TEM, JEM-F200) operated at 200 kV. The surface chemical composition and element valence of samples was detected by using X-ray photoelectron spectroscopy (XPS, KratosAXISULTRA DLD, Al Ka X-ray source). The optical properties of the samples were analyzed by UVevis diffuse spectra (DSR) on an UVevis spectrophotometer (Lambda 950, PerkinElmer). 2.5. PEC measurements The PEC performances of the as-prepared sample were measured on multifunctional BAS100B electrochemical measurement system in 0.1 M Na2SO4 electrolyte using a standard threeelectrode configuration. The prepared films were used as working electrode, Ag/AgCl electrode and Pt foil were used as reference electrode and counter electrode, respectively. The photocurrent voltage (i-v) curves were measured with an AM 1.5 filter in the potential range of 0.35 Ve1.35 V [vs reversible hydrogen electrode (RHE)] at a scanning speed of 10 mV s1 using a 300 W Xe lamp (100 mW cm2) as the light source. The transient photocurrent density (i-t) curves were evaluated under chopped light irradiation (light on/off period 300 s) at a fixed electrode potential of 0.6 V vs RHE. The electrochemical impedance spectroscopy (EIS)

IPCE ð%Þ ¼

. h i ðl  pin Þ  100% 1024  iph

(1)

Where iph, l and pin represent the photocurrent density (mA cm2), the wavelength (nm) of incident radiation and the intensity of the incident light, respectively. The measured potential vs the Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE) according to the Nernst equation.

(2)

3. Results and discussion The morphology of the prepared samples was characterized by scanning electron microscopy (SEM). From SEM images shown in Fig. 1a, d, we can observe ZnO nanorods array structure. The surface of nanorods is smooth, and their diameters are about 60e110 nm. After ZnFe2O4 nanosheets were grown on the surface of ZnO nanorods, the formed core/shell structure ZnO/ZnFe2O4 maintained the vertical nanoarray morphology. ZnFe2O4 nanosheets can be wrapped around the entire nanorods (Fig. 1b, e), and the diameter of the ZnO/ZnFe2O4 NRs is increased to 150e230 nm. Due to the annealing treatment, the ZnFe2O4 nanosheets exhibit a porous structure compared the smooth solid surface of ZnFe2O4 precursor nanosheets (Fig. S1). Meanwhile, with the increase of concentration of FeSO4$7H2O solution, the ZnFe2O4 nanosheets become larger, and the amount of ZnFe2O4 nanosheets also gradually increased (Fig. S1). SEM images of the as-obtained ZnO/ZnFe2O4/PbS also keep the whole nanoarray morphology, and PbS quantum dots are uniformly anchored on the ZnO/ZnFe2O4 NRs (Fig. 1c, f). Fig. 2a displayed the TEM image of the ZnO/ZnFe2O4/PbS NRs. It can be observed that the surface of ZnO nanorod is covered with porous nanosheets. Fig. S2 shows the TEM and HRTEM images of ZnO nanorods, and a prominent (100) crystal plane of ZnO can be found in HRTEM image. The diameter of the ZnO nanorods is about 80 nm, and the diameter of ZnO/ZnFe2O4 core/shell NRs is about 200e500 nm. The HRTEM image in Fig. 2b reveals that the lattice fingers with a space of 0.280, 0.485, and 0.209 nm correspond to the (100) lattice plane of ZnO, (111) lattice plane of ZnFe2O4, and the (220) lattice plane of PbS, respectively. The STEM element mapping images (Fig. 2deh) indicate that the Zn and O element are distributed throughout the core/shell structure. But the Fe, S, and Pb elements are mainly distributed on the outer side of the core/ shell structure, further indicating the successful formation of core/ shell structure ZnO/ZnFe2O4/PbS. The crystal structure of the ZnO/ZnFe2O4/PbS heterojunction was characterized by X-ray diffraction (XRD) as shown in Fig. 3. The diffraction peaks at 31.85
, 34.55
, 36.357
, 56.74
, and 63.091
can be assigned to the crystal planes of (100), (002), (101), (110), and (103) of the ZnO phase (JCPDS 79e0205). Apart from the diffraction peaks of ZnO, another two diffraction peaks at 26.51
and 37.84
coincide with the (110) and (200) crystal planes of FTO phase (JCPDS 77e0451). In the XRD pattern of ZnO/ZnFe2O4, the diffraction peaks at 30.02
, 35.36
, 42.97
, and 62.39
coincide with the (220), (311), (400), and (440) crystal planes of the ZnFe2O4 (JCPDS 89e1009). The diffraction peaks at 26.07
, 31.19
, 43.23
, 51.18
, and 53.63
crystal planes can be assigned to the crystal planes of (111),

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H. Jiang et al. / Journal of Alloys and Compounds 828 (2020) 154449

Fig. 1. SEM images of the samples at different magnifications: (a, d) pristine ZnO, (b, e) ZnO/ZnFe2O4, and (c, f) ZnO/ZnFe2O4/PbS NRs.

Fig. 2. TEM (a) and HRTEM (b) images of the ZnO/ZnFe2O4/PbS. Scanning transmission electron microscopy (STEM) image (c) and corresponding elemental mapping images (deh) of the ZnO/ZnFe2O4/PbS.

(200), (220), (311), and (222) of PbS (JCPDS 78e1054) plane in the diffraction peak of ZnO/ZnFe2O4/PbS. Meanwhile, it can be found that some diffraction peaks are overlapped. Except for the diffraction peaks of ZnFe2O4, ZnO, PbS and FTO glass, no other new diffraction peak was found, which proves the successful synthesis

of ZnO/ZnFe2O4/PbS. The UVevis diffuse reflectance spectra (DRS) of different samples are shown in Fig. 4a. The light absorption edges of ZnO and ZnFe2O4 are approximately located at 420 and 680 nm, respectively. Obviously, compared with ZnO, the light absorption of the ZnO/

H. Jiang et al. / Journal of Alloys and Compounds 828 (2020) 154449

Fig. 3. XRD patterns of ZnO, ZnO/ZnFe2O4, and ZnO/ZnFe2O4/PbS NRs array on FTO substrates.

Fig. 4. (a) UVevis absorption spectra of ZnO, ZnFe2O4, ZnO/ZnFe2O4, and ZnO/ZnFe2O4/ PbS NRs, (b) The corresponding plots of (ahv)2 vs photo energy (hv).

ZnFe2O4 extends to the visible light region due to the introduction of ZnFe2O4. The further decoration of PbS quantum dots enable the as-fabricated ZnO/ZnFe2O4/PbS to exhibit more absorption over 680 nm. Therefore, it is expected that ZnO/ZnFe2O4/PbS has good PEC performance. The band gaps were also calculated according to the plots of (ahv)2 against hv [35]. The results of the calculated band gaps show that ZnO/ZnFe2O4/PbS can be irradiated under visible light illumination, and the difference of band gaps of the various components enables more photogenerated charges to be generated and separated quickly, thus improving the PEC performance. X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the surface chemical composition and elemental valence of ZnO/ZnFe2O4/PbS. The XPS survey scan spectrum in Fig. 5a confirms the presence of Zn, Fe, O, S, and Pb elements in the ZnO/ZnFe2O4/PbS without apparent impurity materials. Fig. 5b shows that the Zn 2p XPS spectrum has two strong asymmetric sub-peaks at 1044.29 and 1021.18 eV, corresponding to the Zn 2p1/2 and Zn 2p3/2, respectively, and the corresponding spin-orbit splitting energy is 23.11 eV. The results indicates the divalent oxidation state of Zn [36]. In O 1s XPS spectrum shown in Fig. 5c, the peak at 529.65 eV can be assigned to the lattice oxygen binding of Fe/ZneO. The peak at 531.4 eV can be assigned to surface-absorbed groups  such as O 2 or OH . In Fig. 5d, the Fe 2p peak contains two sets of sub-peaks due to the spin-orbit coupling of Fe 2p1/2 and Fe 2p3/2 states [37]. The peak at 725.23 eV is well consistent with the Fe 2p1/ 2, and the peaks at 711.5 and 713.83 eV correspond to the Fe 2p3/2. In addition, the shoulder peaks at 732.78 and 719.7 eV originate from shakeup process 2p1/2 and 2p3/2 and are marked as satellite peaks. The above analyses confirmed Fe exists mainly as Fe3þ. In Fig. 5e, the S 2p peaks at 162.99 and 161.18 eV correspond to S 2p1/2 and S 2p3/2, respecgtively, which are derived from the S2 in PbS. Another small peak at 168.16 eV is evidenced by the presence of oxygen on the surface of the sample. It is caused by oxide contamination [38].

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In Fig. 5f, the Pb 4f XPS peaks at 141.85 and 137.13 eV correspond to Pb 4f5/2 and Pb 4f7/2, respectively [39]. The results indicate that Pb and S elements exist in the form of Pb2þ and S2, respectively, which confirming the formation of PbS quantum dots on the surface of ZnO/ZnFe2O4 NRs. The photoelectrochemical (PEC) behaviors of the prepared photoanodes were investigated by measuring the photocurrent response. As shown in Fig. 6a, dark current densities of these photoelectrodes are less than 0.06 mA cm2. Under illumination, the pristine ZnO NRs reached 0.158 mA cm2 at 0.6 V (vs RHE). Comparably, the ZnO/ZnFe2O4 NRs shows a higher photocurrent density of 0.309 mA cm2 at 0.6 V (vs. RHE) than ZnO NRs. This improvement is mainly attributed to the effective charge separation and enhanced visible light absorption [40]. Meanwhile, the increased active sites generated from the nanorod/nanosheet core/ shell nanoarray structure can also promote the enhancement of PEC performance. Especially, the obtained ZnO/ZnFe2O4/PbS NRs exhibits the highest photocurrent density of 0.577 mA cm2 at 0.6 V (vs RHE), which is almost four times higher as compared to the pristine ZnO NRs photoelectrode. The enhanced photocurrent density comes from the significantly increased visible light absorption and accelerated transfer and separation of charge carriers caused by the decoration of PbS quantum dots. Meanwhile, the water contact angle (7
) of ZnO/ZnFe2O4/PbS film is smaller than that of ZnO/ZnFe2O4 (11.2
) and ZnO (15.2
) films (Fig. S3), confirming its excellent hydrophilicity. Since the photoelectrocatalytic reaction occurs at the electrode/electrolyte interface, the hydrophilicity of the film has a considerable influence on the photoelectrocatalytic performance. The excellent hydrophilicity of the ZnO/ZnFe2O4/PbS film can contribute to the improvement of the PEC performance. The transient photocurrent curves of the ZnO-based photoelectrodes measured under the repeated on/off photoperiod conditions at 0.6 V (vs RHE) were shown in Fig. 6b. When the illumination is terminated, the photocurrent rapidly drops to near zero, and once it is re-illuminated, the photocurrent value rapidly recovered, which indicates that the charge transport in the photoelectrode is very rapid due to the direct electron transfer pathway provided by the 1D core/shell structure array. The result indicates that the synergistic effects of 1D nanoarray structure, the formation of ZnFe2O4eZnO heterojunction, and PbS quantum dots decoration accelerate the separation of photogenerated electrons and holes. The plots (Fig. 6c) of photoconversion efficiencies as a function of the applied bias were calculated using Equation (3) [41].

.

h ¼ I ð1:23－ VRHE Þ Jlight

(3)

Where I represent the measured photocurrent density (mA cm2), V represents the applied bias potential versus reversible hydrogen electrode (RHE), and Jlight is the irradiance intensity (100 mW cm2, AM 1.5G). In Fig. 6c, ZnO/ZnFe2O4/PbS photoanode shows the highest efficiency of 0.39% at 0.42 V vs. RHE, which is about 1.34 and 3.54 times higher than that of ZnO/ZnFe2O4 NRs (0.29% at 0.11 V vs RHE) and ZnO NRs (0.11% at 0.69 V vs RHE) photoelectrode, respectively. To study the photoelectrochemical response of the photoelectrodes from different incident light wavelengths, the IPCE values were measured at 0.6 V vs. RHE. As shown in Fig. 6d, the ZnO/ZnFe2O4/PbS NRs exhibits higher IPCE values than both ZnO and ZnO/ZnFe2O4 NRs from 350 to 800 nm. The IPCE of ZnO/ ZnFe2O4/PbS reaches 54.99% at 350 nm, which is higher than that of ZnO/ZnFe2O4 NRs (45.18%) and ZnO NRs (28.65%). The IPCE values are well matched with the UVevis absorption spectra, indicating that the ZnO/ZnFe2O4 NRs can more effectively utilize the absorbed

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Fig. 5. XPS survey scan spectrum (a) and high resolution XPS spectra of (b) Zn 2p, (c) O1s, (d) Fe 2p, (e) S 2p, and (f) Pb 4f of the as-prepared ZnO/ZnFe2O4/PbS.

photons in the visible light region and promote charge carriers separation after the decoration of PbS quantum dots. To gain a deeper understanding of the charge transport behavior at the electrode/electrolyte interface, electrochemical impedance spectroscopy (EIS) measurements were performed under AM 1.5 illumination (Fig. 6e) and in dark condition (Fig. S4). In the Nyquist diagram, the radius of the circle of Nyquist plot is related to the charge transfer process of the corresponding electrode/electrolyte interface. The smaller radius indicates the lower charge transfer resistance, thus implying slower charge recombination rate [42]. Obviously, the charge transfer resistance of the ZnO/ZnFe2O4 NRs is smaller than that of ZnO NRs, which indicates that the heterojunction formed between ZnO and ZnFe2O4 results in a decrease in charge recombination rate. Compared with ZnO/ZnFe2O4 NRs, the charge transfer resistance of ZnO/ZnFe2O4/PbS NRs is further reduced, indicating that the decoration of PbS quantum dots is more favorable for the separation of charge carriers, providing the optimal transport path of charge carriers. As shown in Fig. 6f, The Mott-Schottky (MS) measurements were carried out to determine the carrier density when Schottky barrier is formed between the photoanode and the electrolyte under dark conditions. The MS curves of all thin-film

photoelectrodes are positive slopes, indicating that the thin-film materials are all n-type semiconductors [43]. The linear portion of the slope of the MS curve can also be used to determine carrier density. The carrier density of the photoanode materials can be calculated using the following equation [44]:

h  . . i1 dV N ¼ ð2=εε0 e0 Þ d 1 C 2

(4)

Here, N is the carrier density, C is the depletion capacitance, e0 is the electron charge of 1.602  1019 C, ε0 is the dielectric constant of vacuum (8.854  1012 F/m), ε is the dielectric constant of 18, and V is the potential applied. Thus, the carrier densities of ZnO, ZnO/ ZnFe2O4, and ZnO/ZnFe2O4/PbS are calculated to be 2.27  1019, 5.33  1019, 4.08  1020, respectively. The increased carrier density of ZnO/ZnFe2O4/PbS contributed to the improvement of photocurrent. Stability is an important property of photoelectrode. As shown in Fig. S5, the photocurrent decay ratio of the ZnO NRs photoanode is about 48.7% after 2 h irradiation. The severe photocorrosion comes from its susceptibility to oxidation [45]. However, the photocurrent values of ZnO/ZnFe2O4 NRs and ZnO/ZnFe2O4/PbS

H. Jiang et al. / Journal of Alloys and Compounds 828 (2020) 154449

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Fig. 6. Photoelectrochemical measurements of the photoelectrodes in 0.1 M Na2SO4: (a) Photocurrent density-potential curves under AM 1.5 irradiation. (b) Photocurrent densitytime curves measured at 0.6 V vs RHE under chopped simulated illumination. (c) The calculated photoconversion efficiencies. (d) Incident photon to current conversion spectra. (e) EIS Nyquist plots of the different photoelectrodes under AM 1.5 irradiation. (f) Mott-Schottky plots of the different photoelectrodes.

NRs are well maintained. Especially 87.4% of the initial photocurrent of ZnO/ZnFe2O4/PbS is obtained after 2 h irradiation, demonstrating the high stability and remarkably decreased electron-hole pair recombination in ZnO/ZnFe2O4/PbS. In order to understand the origin of the photoelectrocatalytic properties of ZnO/ZnFe2O4/PbS, the band gap energy diagram of composite was illustrated with Fig. 7. The energy band position of ZnO and ZnFe2O4 were determined according to the results of UVevisible absorption spectra (Fig. 4b) and XPS valence band spectra (Fig. S6). The valence band and conduction band of ZnFe2O4 are more negative than the corresponding bands of ZnO, indicating the type-II energy band alignment in the composite. After illumination, ZnO, ZnFe2O4, and PbS will produce electrons from valence band to conductive band. The photogenerated electrons on the conduction band of PbS will migrate into the conduction band of ZnFe2O4 and then transfer to the conduction band of ZnO through the gradient energy band route [46]. 4. Conclusions In conclusion, we have designed and prepared ZnO/ZnFe2O4 nanorod/nanosheet core/shell nanoarray photoanode decorated with PbS quantum dots for photoelectrochemical water splitting system. Compared with pure ZnO nanorod arrays, the ZnFe2O4

Fig. 7. Schematic of the potential energy dragram for the ZnO/ZnFe2O4/PbS electrode.

porous nanosheets grown on the outside of ZnO nanords can significantly enhance the visible light absorption, accelerate charge carriers transfer and separation, and inhibit photocorrosion.

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Specially, the decoration of PbS quantum dots further improves visible light absorption and charge transfer and separation. The optimized ZnO/ZnFe2O4/PbS exhibited much higher photocurrent than the ZnO/ZnFe2O4 and ZnO films. The wavelength dependent incident photon current efficiency of the ZnO/ZnFe2O4/PbS film was significantly enhanced in the entire light region by the introduction of ZnFe2O4 nanosheets and PbS quantum dots compared to the original ZnO nanorod arrays. The enhanced photoelectrochemical performance of ZnO/ZnFe2O4/PbS was mainly attributed to a greater generation of charge carriers upon illumination as well as their efficient separation and fast transport. The ZnO/ZnFe2O4/PbS core/shell heterostructure nanoarrays have a promising application in the photoelectrochemical systems for stable water splitting. Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. CRediT authorship contribution statement Haiyu Jiang: Methodology, Investigation, Writing - original draft. Yajie Chen: Validation, Writing - review & editing. Li Li: Data curation. He Liu: Data curation. Can Ren: Data curation. Xiu Liu: Methodology. Guohui Tian: Writing - review & editing, Supervision. Acknowledgments This work was supported by the National Natural Science Foundation of China (51772079), the Natural Science Foundation of Heilongjiang Province of China (B2017009). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154449. References [1] T. Hisatomi, J. Kubota, K. Domen, Recent Advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520e7535. [2] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37e38. [3] P. Lianos, Review of recent trends in photoelectrocatalytic conversion of solar energy to electricity and hydrogen, Appl. Catal. B Environ. 210 (2017) 235e254. [4] H. Magnan, D. Stanescu, M. Rioult, E. Fonda, A. Barbier, Epitaxial TiO2 thin film photoanodes: influence of crystallographic structure and substrate nature, J. Phys. Chem. C 123 (2019) 5240e5248. [5] R.Z. Chen, C. Zhen, Y.Q. Yang, X.D. Sun, J.T.S. Irvine, L.Z. Wang, G. Liu, H.M. Cheng, Boosting photoelectrochemical water splitting performance of Ta3N5 nanorod array photoanodes by forming a dual co-catalyst shell, Nanomater. Energy 59 (2019) 683e688. [6] Y.D. Wang, W. Tian, C. Chen, W.W. Xu, L. Li, Tungsten ttioxide nanostructures for photoelectrochemical water splitting: material engineering and charge carrier dynamic manipulation, Adv. Funct. Mater. 29 (2019) 1809036. [7] H. Lin, X. Long, Y.M. An, D. Zhou, S.H. Yang, Three-dimensional decoupling cocatalyst from a photoabsorbing semiconductor as a new strategy to boost photoelectrochemical water splitting, Nano Lett. 19 (2019) 455e460. [8] X.F. Long, L.L. Gao, F. Li, Y.P. Hu, S.Q. Wei, C.L. Wang, T. Wang, J. Jin, J.T. Ma, Bamboo shoots shaped FeVO4 passivated ZnO nanorods photoanode for improved charge separation/transfer process towards efficient solar water splitting, Appl. Catal. B Environ. 257 (2019) 117813. [9] D. Commandeur, G. Brown, P. McNulty, C. Dadswell, J. Spencer, Q. Chen, Yttrium-doped ZnO nanorod arrays for increased charge mobility and carrier density for enhanced solar water splitting, J. Phys. Chem. C 123 (2019) 18187e18197. [10] D. Commandear, G. Brown, E. Hills, J. Spencer, Q. Chen, Defect-rich ZnO nanorod arrays for efficient solar water splitting, ACS Appl. Nano. Mater. 2

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