Nesting BiVO4 nanoislands in ZnO nanodendrites by two-step electrodeposition for efficient solar water splitting

Journal of Physics D: Applied Physics PAPER • OPEN ACCESS Nesting BiVO4 nanoislands in ZnO nanodendrites by two-step electrodeposition for efficient solar water splitting To cite this article: Ali Can Güler et al 2024 J. Phys. D: Appl. Phys. 57 355110 View the article online for updates and enhancements. You may also like Magnetorheology of carbonyl iron particles coated with polypyrrole ribbons: The steady shear study M Mrlik, M Sedlacik, V Pavlinek et al. - A dimorphic magnetorheological fluid with improved oxidation and chemical stability under oscillatory shear M Sedlacik, V Pavlinek, R Vyroubal et al. - Electrorheological behaviour under oscillatory shear of TiO2 rod-like particles prepared via microwave-assisted moltensalt synthesis M Sedlacik, M Mrlik, V Pavlinek et al. - This content was downloaded from IP address 195.113.97.173 on 03/10/2024 at 12:45 Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 57 (2024) 355110 (15pp) https://doi.org/10.1088/1361-6463/ad5212 Nesting BiVO4nanoislands in ZnO nanodendrites by two-step electrodeposition for efficient solar water splitting Ali Can Güler1, Jan Antoˇ s1, Milan Masaˇ r1, Michal Urbánek1, Michal Machovsk´ y1, Rajesh Dagupati2, Michal ˇ Zitˇ nan2, José J Velázquez2, Duˇ san Galusek2,3 and Ivo Kuˇ ritka1,4,∗ 1Centre of Polymer Systems, Tomas Bata University in Zlin, Tr. T. Bati 5678, 760 01 Zlin, Czech Republic 2Centre for Functional and Surface Functionalized Glass, Alexander Dubcˇek University of Trencˇín, ˇ Studentská 2, SK-911 50 Trencˇín, Slovakia 3Join Glass Centre of the IIC SAS, TnU AD, and FChPT STU, Trencˇín, Slovakia 4Department of Chemistry, Faculty of Technology, Tomas Bata University in Zlín, Vavrecˇkova 5669, 760 01 Zlín, Czech Republic E-mail: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected], [email protected],[email protected] and dusan.g[email protected] Received 15 March 2024, revised 29 April 2024 Accepted for publication 30 May 2024 Published 12 June 2024 Abstract Photoanodes with a large electrochemically active surface area, rapid charge transfer, and broadband light harvesting capacity are required to maximize the photoelectrochemical (PEC) water splitting performance. To address these features, we demonstrate that 3D hierarchal ZnO nanodendrites (NDs) can be sensitized with BiVO4nanoislands by chemical and thermal treatments of electrodeposited Bi metal films. The flat band measurements and optical characterization suggested that the resulting heterojunction had type-II band alignment with a viable charge transfer from BiVO4to ZnO NDs. In parallel, PL analysis revealed inhibition of the charge recombination rate by the electron transfer between BiVO4and ZnO NDs. Upon AM 1.5 G illumination, BiVO4/ZnO NDs heterojunction yielded the highest photocurrent efficiency (0.15 mA·cm−2at 1.2 V vs. NHE), which was attributed to its enhanced surface area (due to the presence of small dendrite branches), extended broadband light absorption extending from UV to visible light regions, and the most efficient interfacial charge transfer as proven by electrochemical impedance spectroscopy (EIS) studies. Besides, the incident photon-to-current conversion efficiency and applied bias photon-to-current efficiency tests confirmed an improved spectral photoresponse of the heterojunction based photoanode, particularly towards the visible light spectrum. The results outline a promising synthesis route for building heterojunctions between visible light active and wide band gap semiconductors for the use as a highly efficient photoanodes in a PEC cell. ∗Author to whom any correspondence should be addressed. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1 © 2024 The Author(s). Published by IOP Publishing Ltd J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al Supplementary material for this article is available online Keywords: ZnO nanodendrites, BiVO4, electrodeposition, heterojunction photoanode, photoelectrochemical 1. Introduction Solar energy conversion to chemical energy (i.e. hydrogen) using semiconducting materials via photoelectrochemical (PEC) water splitting has been regarded as efficient sustainable energy technology since the pioneering demonstration by Honda and Fujishima [1,2]. The overpotential for water oxidation (oxygen evolution reaction (OER)) and water reduction (hydrogen evolution reaction (HER)) in this process is partially compensated by the photoinduced voltage of the light absorber [3,4]. The semiconductor must hence possess an appropriate band alignment with the water redox potentials. The OER and HER could be driven by the PEC water splitting at potentials below 1.23 V and above 0 V with reference to the reversible hydrogen electrode (RHE), respectively [5]. The important argument affecting the solar-to-hydrogen conversion efficiency (STH) are light absorption, charge separation/transport and surface catalytic reaction (charge transfer) [6,7]. Many properties of semiconductor nanorods (NRs) can be significantly altered when their radial dimension (diameter) is equal or below the characteristic length of the interesting solid state phenomena such as the exciton Bohr radius, exciton diffusion length, wavelength of irradiation and phonon mean free path [8,9]. For instance, Song et al fabricated a composite titania thin film composed of quasi-aligned rutile NRs embedded in anatase aggregates and its PEC behavior was significantly enhanced [10]. Recently, more efforts have been devoted on constructing three-dimensional (3D) hierarchical nanostructures from low-dimensional subunits. Compared to 1D morphologies, 3D complex zinc oxide (ZnO) morphologies present a larger active surface area, which results in a more rapid charge transfer process and water oxidation kinetics [11, 12]. This hierarchical structural design can increase the number of light transport paths and thereby improve light harvesting capacity [13,14]. ZnO nanodendrites (NDs) included in this class of nanostructures are very promising candidate for solar energy conversion applications due to their low-cost and a facile synthesis process. In contrast to the conventional methods in which 3D ZnO NDs are grown over sequentially reseeded ZnO NRs surface in aqueous phase [15–17], Wu et al purposed an alternative fabrication method utilizing a supersaturated solution without any sequential seed layer or organic structure-directing agent [18]. Nevertheless, besides all other assets, a wide band gap (∼3.2 eV) of ZnO is considered as the fundamental obstacle to achieving moderate STH efficiency via its utilization in the PEC water splitting. A novel material design is therefore needed to influence the factors mentioned above. Photosensitization with a narrow band gap semiconductor has emerged as an effective means of extension of the optical activity of ZnO NDs into the visible region. Among the different photosensitizers that show photoactivity under visible light, monoclinic bismuth vanadate (BiVO4) was chosen as the light absorber due to its the suitable band gap (∼2.4 eV), appreciable band edge positions for water oxidation and the matched band structure with that of ZnO [19,20]. The monoclinic BiVO4alone demonstrated photocatalytic activity for the O2evolution from an aqueous silver nitrate solution under visible light irradiation [21,22]. It was also reported that the formation of heterojunction photoanode between the wurtzite ZnO and monoclinic BiVO4promotes the charge separation and transport of electrons and holes assisted by the built-in electric field at the interface and light absorption at a wider wavelength range (UV and vis), which then boosts the PEC performance [23,24]. In this sense, Yan et al reported the fabrication of ZnO NRs/BiVO4heterojunction through chemical bath deposition followed by successive ionic layer deposition [25]. They observed that photocurrent of ZnO NRs/BiVO4 (1.72 mA·cm−2at 1.2 V vs. Ag/AgCl) was higher than that of ZnO NRs photoanode at the same potential. The improvement in photoconversion was associated with the extended spectral response towards the visible spectrum and lower recombination rate of photogenerated charge carrier. Moniz et al successfully synthesized 1D ZnO coupled with nanoparticulate BiVO4and cobalt phosphate (Co–Pi) as a hole acceptor [26]. They found that Co–Pi/BiVO4/ZnO exhibited 12-fold increase in photocurrent (∼3 mA·cm−2) compared to the BiVO4. The enhancement results from higher light absorption, electron flow from BiVO4to ZnO, and hole transfer to Co-Pi for favorable OER. Recently, Yang and Wu constructed novel Co–Pi/BiVO4/ZnO NDs photoanode based on metal organic deposition of BiVO4on the surface of hydrothermally grown ZnO NRs [27]. The resultant electrode yielded an optimized photocurrent density of 3.5 mA·cm−2at 1.23 V vs. RHE after being fully depleted at a low bias potential (0.8 V vs. RHE). Along with the superior light harvesting capability and charge injection efficiency by co-catalyst Co–Pi, the authors claimed that fully depleted junction originating from thin BiVO4shells led to better charge transport, which in turn enhanced the PEC activity. However, synthesis techniques available to produce nanoporous morphology electrodes are quite limited. Kim and Choi presented electrochemically deposited nanoporous BiVO4from bismuth oxyiodide (BiOI) on fluorine-doped tin oxide substrates [28]. They suggested that the voids between 2D crystal structure of BiOI allowed the deposition of ultrathin plates (∼20 nm) by inhibiting the grain formation of 2 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al BiVO4during the conversion process. Kang et al demonstrated that the electrodeposited Bi dendritic electrodes followed by the introduction of a V precursor solution during the oxidation process can lead to BiVO4NPs. The resultant BiVO4thin film had a high surface area and a good electrical continuity among the particles [29]. Using the similar electrodeposition procedure, Bai et al fabricated Cu2O/BiVO4 p–n heterojunction photoanode and obtained the maximum photocurrent density of 1.72 mA·cm−2(1.23 V vs RHE), which is 4.5 times higher than that of pristine BiVO4thin film (∼0.38 mA·cm−2) at the same applied potential [30]. In view of the advantages mentioned above, herein, we introduce a facile strategy for the construction of BiVO4/ZnO NDs heterojunction photoanode with a novel nanostructure for PEC water splitting. The branches with the lengths of 300–400 nm were directly formed on hydrothermally grown primary ZnO NRs with a diameter of 84 nm in the absence of any seed layer or organic structure directing agent to produce ZnO NDs. BiVO4nanoislands nested in ZnO NDs were prepared by electrodeposition of Bi film followed by the introduction of V solution and thermal treatment. A two-step electrodeposition procedure generated an intimate contact at the interface between the constituent semiconductors. BiVO4/ZnO NDs heterojunction was identified to have staggered band arrangement (type II), leading to an efficient photogenerated electron–hole separation and rapid interfacial charge transfer. Under AM 1.5 G illumination, the heterojunction photoanode achieved a higher PEC performance than the BiVO4, ZnO NDs, and ZnO NRs photoelectrodes prepared in this study. This new approach may pave the way for rational design of heterojunctions between narrow band gap and wide band gap semiconductors with optimal structure and compositions. 2. Experimental section 2.1. Preparation of ZnO NDs photoanode The ZnO NR arrays were synthesized by hydrothermal method. Initially, 0.8 M zinc acetate dihydrate (Zn(CH3CO2)2.2H2O, Penta) and 0.8 M diethanolamine (CH2CH2OH)2NH, CDH Fine Chemicals) were dissolved in isopropanol ((CH3)2CHOH, Microchem), and stirred at 50 ◦C for 1 h. After an overnight aging, the sol was spin-coated on cleaned indium tin oxide coated (ITO, 5–15 Ωsq−1, Sigma Aldrich) glass substrates at 3000 rpm for 30 s. The coated substrate was calcined in an ambient atmosphere at 400 ◦C for 1 h to obtain the ZnO seed layer. The typical growth solution consisting of 0.025 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich), 0.025 M hexamethylenetetramine ((CH2)6N4, Lachner), and 0.5 ml polyethyleneimine (PEI, branched, average Mw∼800 by LS, Sigma Aldrich) was preheated for 2 h at 95 ◦C. The aged growth solution turned to yellow straw color. The seeded film with its conductive side facing downwards was immersed in the preheated solution (in yellow straw color) and kept at 95 ◦C for 6 h. The branches of ZnO NDs were formed directly on the ZnO NRs without any assistance from another seeds and organic structure-directing agent, as described elsewhere [31]. The ZnO NRs grown on ITO substrate were first immersed into an aqueous solution of 0.057 M zinc acetate dihydrate and 0.5 M sodium hydroxide (NaOH, Lachner) for 20 min at room temperature. During this process, the etch pits formed that served as growth centers for the formation of nanocactus (NCs), which then turned into branches after 1 h growing at 100 ◦C for in the same solution. The obtained ZnO NDs were carefully rinsed with deionized water and dried at 60 ◦C. It should be mentioned that branch development only started from a supersaturated solution in a metastable state at a high concentration of NaOH. The supersaturation of zinc acetate and NaOH solution can be reached before the solution turns opaque after ∼5 min at room temperature. Therefore, it is very important to immerse ZnO NRs in a clear solution before precipitation. A more detailed information can be found where it was primarily observed [18]. 2.2. Preparation of BiVO4and BiVO4/ZnO NDs photoanodes Scheme 1presents the synthesis procedure to prepare ZnO NDs coated with BiVO4NPs, which could be converted from electrodeposited Bi metal using a modified version of the previously described chemical and thermal treatments [29]. In the first step, the plating solution was prepared by dissolving 20 mM of bismuth(III) nitrate pentahydrate (Bi(NO)3.5H2O, Sigma Aldrich) in 100 ml ethylene glycol (HOCH2CH2OH, Sigma Aldrich) solution. The deposition was achieved by passing ∼0.033 C cm−2at −1 V against the Ag/AgCl electrode. The other details of Bi metal electrodeposition are given in the supplementary information (figure 1S). Following that, 100 µl of dimethyl sulfoxide (DMSO, Sigma Aldrich) containing 150 mM ammonium monovanadate (NH4VO3, Sigma Aldrich) was drop-casted onto the entire Bi film (area =2 cm2) as the second step. The V precursor-incorporated film was calcined at 500 ◦C for 2 h in air. By thermal treatment, Bi and VO2+oxidized to Bi2O3and V2O5, which reacted to form BiVO4. Any residual vanadium pentaoxide (V2O5) on the electrode was removed by soaking it in 0.125 M NaOH solution for 30 s. The resultant pure BiVO4/ZnO NDs photoanode was thoroughly washed by deionized water, and dried at 60 ◦C. For synthesis of BiVO4, all the deposition conditions were the same, except for ITO substrates used as scaffolds. 2.3. PEC measurements The PEC performance tests of BiVO4sensitized ZnO NDs were performed in three-electrode configuration within a plastic cuvette under front-side illumination at 87.5 mW·cm−2 light intensity from a PicoTM solar simulator (G2V Optics) with a standard AM 1.5 G filter. The incident light in UV and visible regions (350–800 nm) is attenuated by 10% (data not shown) while passing throughout the PEC cell cuvette filled with electrolyte solution, corresponding that the light intensity at the sample surface was 78.75 mW·cm−2. 0.5 M Na2SO4(pH 7) was used as an electrolyte solution that was 3 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al Scheme 1. Schematic representation for the fabrication of BiVO4/ZnO NDs heterojunction photoanode. degassed by nitrogen for 10 min to remove any dissolved oxygen before the PEC measurements. The fabricated electrodes with a fixed surface area of 0.32 cm2, Pt foil and Ag/AgCl (saturated with KCl) were used as the working, counter, and reference electrodes, respectively. The potentials measured vs. Ag/AgCl electrode (EAg/AgCl) were converted to normal hydrogen electrode (NHE) potentials (ENHE) by using the equation: ENHE =EAg/AgCl + 0.2 V.(1) In linear sweep voltammograms, LSV (J-V), the scan rate was 20 mV s−1and the scan range was 0 V–1.5 V (vs. Ag/AgCl) both under dark and illuminated conditions. The incident photon-to-current conversion efficiency (IPCE) tests were also carried out in the three-electrode setup using 12 monochromatic channels of the solar simulator (PicoTM with AM 1.5 G filter) as the light source. The IPCE was calculated using the following equation: IPCE =Jph (λ)×1240 Pmono (λ)×λ(2) where, Jph (in mA·cm−2) is the photocurrent density recorded under monochromatic illumination at wavelength λ(in nm), Pmono (in mW·cm−2) is the light intensity of the monochromatic source at each wavelength, the constant 1240 (hc/ein V·nm) equals to the product of Planck’s constant and the speed of light divided by the charge of an electron. The applied bias photon-to-current efficiency (ABPE), analogue to the STH efficiency with no bias, was calculated by the formula: ABPE =[J(mA.cm−2)×(1.229 − |Vapp|)(V)×ηF Ptotal (mW.cm−2)](3) in which, 1.23 V is the standard state reversible potential for water splitting, Vapp is the applied bias (measured vs. Pt), ηFis the Faradaic efficiency for hydrogen evolution (ηF=1 in this case), and Ptotal is the intensity of the light source. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the same electrolyte and set-up as employed in the photocurrent measurements. The EIS data under illuminated condition was obtained at 10 mV amplitude of AC signal over a frequency range of 100 kHz to 0.1 Hz. The Mott–Schottky (MS) analysis was carried out in dark at 1.5 kHz with 30 potential steps. 2.4. Characterization The crystal structure of the samples was determined by x-ray powder diffractometry (XRD, Panalytical Empyrean DY1098) with Cu Kɑradiation (λ=0.15405 nm) under 40 kV and 45 mA in steps of 0.02◦. Scanning electron microscopy (SEM, Nova NanoSEM 450) was employed to study the morphology of the samples. The elemental composition of the produced heterostructure was determined through energy-dispersive xray spectroscopy (EDX). The microstructure of the films was monitored by transmission electron microscopy (TEM, JEM2100Plus, Jeol). The optical characteristics of the synthesized photoanodes were acquired by a 150 mm InGaAs integrating spheres module of a UV-Vis-NIR absorption spectrometer (Lambda 1050 Perkin Elmer). The photoluminescence spectra at room temperature were obtained by fluorescence spectrophotometer (PL, Fluorolog FL3-21, Horiba) equipped with delta diode laser DD-375 l 376 nm as an excitation source. Raman spectra were collected by a micro-Raman spectrophotometer (Renishaw in Via Reflex) at the excitation wavelength 4 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al of 532 nm. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD spectrometer, Kratos Analytical Ltd) with a monochromatized Al Kαradiation (hν=1,486.7 eV), operated at 150 W (10 mA, 15 kV), was used to explore the elemental composition and oxidation states of the elements on the surface of the BiVO4/ZnO NDs. The XPS spectra were obtained using an analysis area of ∼300 µm×700 µm and analyzed by Casa XPS software. Electrochemical workstation (SP-200 Potentiostat, BioLogic) with EIS was utilized to test the PEC/electrochemical performance as well as interfacial charge transfer properties of the produced electrodes. 3. Results and discussion BiVO4nanoislands nested in sequential hydrothermally grown ZnO NDs was prepared by electrodeposition of Bi film followed by the introduction of V solution and thermal treatment. The amount of the decorated BiVO4was determined based on measuring the electrode weight before and after coating. Without contribution from ITO, the weight of ZnO NDs was determined as 0.89 mg while the weight of BiVO4deposits on ZnO NDs was determined as 0.12 mg, corresponding to formation of BiVO4/ZnO NDs (∼13.4 wt. %) heterostructure photoanode. The XRD spectra of the produced electrodes are shown in figure 1. The peak positions of ZnO NDs are the same as those of ZnO NRs before growing ZnO branches, which correspond to the characteristic diffraction maxima of hexagonal wurtzite structure [32]. The dominant (002) peaks in the XRD patterns of ZnO NRs as well as ZnO NDs suggest a preferential orientation along the c-axis normal to the substrate rather than an anisotropic orientation. In both samples, the (002) peak at 35◦with the full-width half maximum of less than 0.16◦confirms a high crystallinity. An interesting aspect of this prominent peak is its stronger intensity for ZnO NDs. This change is attributed to the occurrence of branches since the crystalline volume contributes to the diffraction intensity [33]. The XRD patterns of BiVO4were found to be consistent with the characteristic diffraction peaks of monoclinic scheelite [34]. It is discernible that the electrodeposition of BiVO4did not cause any considerable peak shifting or crystal phase change of ZnO NDs. All XRD patterns of BiVO4/ZnO NDs are therefore assigned to the co-existence of BiVO4and ZnO phases without any undesirable impurities. Figure 2S(a) shows the XRD patterns for ITO glass and Bi metal electrodeposited ITO glass before introducing the V precursor (DMSO containing ammonium monovanadate) to form BiVO4. The peak observed at 30.80◦is associated with the (222) plane of ITO [35]. The peaks in the XRD pattern of Bi-deposited ITO film are indexed as the tetragonal phase of β-Bi2O3[36]. The most relevant diffraction planes are labeled, and no other impurity phase was detected. The XRD results indicate that Bi metal reacts with the atmospheric oxygen to form Bi2O3. The Raman spectra of the samples are shown in figure 2. The characteristic weak peak of ZnO at 438 cm−1is observed in the spectra of the ZnO NDs as well as BiVO4/ZnO NDs. Figure 1. XRD patterns of BiVO4, ZnO NRs, ZnO NDs and BiVO4/ZnO NDs photoanodes. Figure 2. Raman spectra of ZnO NDs, BiVO4, BiVO4/ZnO NDs and BiVO4/ZnO NDs without soaking in NaOH solution to remove V2O5. The peaks at 1090 cm−1and 575 cm−1are associated with the defects in ZnO crystals [37]. On the other hand, the peaks around 820, 706, 365, 210 and 145 cm−1in the Raman spectra of BiVO4and BiVO4/ZnO NDs refer to monoclinic BiVO4 [38], implying the formation of the crystalline BiVO4on ND ZnO structures. Raman spectral analysis was also applied to the as-prepared BiVO4/ZnO NDs without soaking in NaOH solution to confirm successful removal of the residual V2O5. Additional vibrational modes appeared in the Raman spectra of BiVO4/ZnO NDs without soaking, when compared with the Raman spectra of BiVO4/ZnO NDs. Among Raman peaks of V2O5, the internal modes between 500–1000 cm−1involve V–O stretching vibrations while the external modes between 200–500 cm−1involve V–O–V bending vibrations [39]. The 5 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al Figure 3. High resolution XPS core level spectra of BiVO4/ZnO NDs (a) Zn 2p, (b) Bi 4f, (c) V 2p and (d) O 1 s. result indicates that the excess of V2O5was firmly eliminated, and pure BiVO4/ZnO NDs was formed. The composition and chemical states in a BiVO4/ZnO NDs heterojunction sample was analyzed by XPS. The peak locations in all XPS spectra were corrected using C 1 s at 284.5 eV. The high resolution XPS spectrum of Zn 2pin figure 3(a) reveals two binding energies of 1021.0 (Zn 2p3/2) and 1044.0 (Zn 2p1/2) eV, signifying that Zn is present in the Zn2+state [40]. The typical binding energies of 158.0 (Bi 4f7/2) and 163.4 (Bi 4f5/2) eV in figure 3(b) denote Bi in the 3+state. As for the V 2pXPS spectrum in figure 3(c), the major peaks appeared at 515.7 (V 2p3/2) and 523.5 eV (V 2p1/2) indicate that V in BiVO4/ZnO NDs photoanode is present in the oxidation state of V5+[41]. Figure 3(d) demonstrates the O 1 s core level spectrum with two deconvoluted peaks at 529.7 and 531.7 eV. The peak located at 529.7 eV is mainly attributed to the lattice oxygen of ZnO crystal while the peak located at 531.7 eV is related to the hydroxyl (–OH) groups formed on the surface of specimen [42]. Cross-sectional and top-view SEM images and diameter distribution of the primarily grown ZnO NR are respectively shown in supporting information (figures 3S(a)–(c)). Growth of vertically aligned ZnO NRs on a 300 nm thick seed layer after 6 h during the hydrothermal synthesis resulted in homogeneous hexagonal arrays with a length of ∼2µm and an average diameter of 44 nm. Moreover, the spines grew directly on the ZnO NRs to form the NCs at room temperature after 20 min (figure 3S(d)). As shown in figure 4(a), SEM image demonstrates that the spines developed into the branches to form ZnO NDs with high surface area after subsequent hydrothermal growth at 100 ◦C for 1 h. These branches lengths ranged from 300 to 400 nm. The diameter distribution of the primary ZnO NRs within NDs is shown in figure 4(b), demonstrating that the average diameter increased from 44 nm to 84 nm. It should be pointed out that the electrodeposited Bi crystals did not cover the entire ITO surface (figure 2S(b)). This island morphology of Bi is attributed to the poor dissolution of Bi deposits [28,29]. However, as shown in figure 4(c), the ITO surface was almost fully covered with porous, uniform, and nanocrystalline BiVO4. After introducing V source and after heat treatment, the morphology of BiVO4altered significantly compared to that of the original Bi metal deposits. The SEM image of BiVO4/ZnO NDs heterojunction shown in figure 4(d) illustrates that electrodeposition process successfully nested BiVO4nanoislands in and on top of ZnO NDs. The size of the BiVO4nanostructures was about 200–300 nm. It should be noted that there is a difference in appearance between the growth orientation of BiVO4NPs on planar ITO and 3D complex ZnO structures. In the former, the nanoparticles accumulate laterally, in the latter, there is 6 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al Figure 4. SEM images of (a) ZnO NDs, (b) corresponding diameter distribution of ZnO NDs, (c) BiVO4, and (d) BiVO4/ZnO NDs. a vertical growth of the porous small nanoparticles stacked on top of each other appear like a single large cluster. The quality of semiconductor-semiconductor contact is essential for achieving excellent PEC performance of the heterojunction device. BiVO4nanoislands in the upper layer tightly wrap ZnO NDs and intertwine them, leading to an intimidate contact at the interface for the expected good charge separation. Electrodeposition occurs on a conductive substrate by facilitating electrons for the reaction. It was reported that ZnO NRs displayed polarity dependent high electrical conductivity varying between 10.2 and 90.9 S cm−1[43]. This value is much smaller than the electrical conductivity of ITO substrate that is ∼104S cm−1[44]. Equally important is the electrical conductivity of the electrodeposited compounds. Electrons cannot easily reach all the locations at the deposits for the low conductivity phase while electrons are avaliable at all the surface sites for the high conductivity phase [45]. To emphasize again, the cathodic current during the electrodeposition of Bi metal on ZnO reached as high as −1.1 mA, signifying sufficient electrical conductivity of electrodeposited phase. Our results suggest that ZnO in the form of branched NRs provides suitable electrical and transport properties for the uniform formation of BiVO4NPs via electrodeposited Bi metal. Besides, ZnO NDs slightly corroded due to the immersion in the acidic Bi metal plating solution for 5 min. The EDX analysis of BiVO4/ZnO NDs (figure 4S) reveals that the heterostructure is only composed of zinc, bismuth, vanadium, and oxygen. The absence of any other element confirms the purity of the fabricated film. Figure 5. TEM image of BiVO4/ZnO NDs heterojunction. Figure 5represents the TEM image of BiVO4/ZnO NDs heterostructure extracted from a large area of ND arrays. ZnO NDs slightly corroded during the electrodeposition of Bi metal film due to the acidity of the plating solution as 7 J. Phys. D: Appl. Phys. 57 (2024) 355110 A C Güler et al Figure 6. (a) PL spectra (excitation wavelength 335 nm, the peak at 670 nm corresponds to its second harmonics), (b) Absorption plus scattering spectra (A+S=100 −R−T), (c) Nyquist plots under AM 1.5 G irradiation and (d) Mott–Schottky plot of BiVO4and ZnO NDs. semiconductors. was also observed by SEM. The surface morphology examination indicates that a branch grew vertically from the stem of primary ZnO NRs, which have a diameter of about ∼65 nm. Moreover, the size of BiVO4crystals is about ∼200 nm with relatively darker contrast. It can also be deduced that BiVO4 nanoislands are firmly bound to ZnO NDs, leading to a good electrical continuity between two phases. All these results are consistent with SEM observations. The semiconductor heterojunction photoanode is reportedly very effective for improvement of the interfacial charge transportation and separation efficiency [23,46]. To evaluate the recombination rate within the fabricated photoelectrodes PL measurements were performed under the excitation wavelength of 335 nm as its qualitative indication. As seen from figure 6(a), two distinctive emission peaks appeared in the PL spectra of ZnO NRs, ZnO NDs and BiVO4/ZnO NDs. The peak at 376 nm results from the near band edge (NBE) transitions in ZnO [47]. Another wide and intense emission peak for the pristine ZnO samples appeared at 570 nm and was associated with the presence of the oxygen vacancies [48]. As BiVO4deposits on ZnO NDs, a red-shift of the defect-induced emission is observed at 620 nm. Annealing ZnO increases the amount of the oxygen interstitials and decreases the amount of the oxygen vacancies [49]. Therefore, this shift of visible emission could be related to the variation of the local surroundings of the defect sites as a result of annealing the heterojunction sample. In the case of BiVO4, two broad peaks at 505 and 410 nm were attributed to the NBE transition and the deep level defects, respectively [50]. In fact, high PL intensity generally characterizes a high recombination rate of the photogenerated charge carrier [51]. The ZnO NDs NBE emission intensity shows a remarkable reduction after coating it with BiVO4. Consequently, it can be stated that the formation of heterojunction between BiVO4NPs and ZnO NDs can significantly inhibit the recombination rate and decrease the number of crystal defects. The efficiency of charge separation plays a vital role regarding the PEC properties. UV-vis spectrophotometry was also carried out to study the light absorption capacity of the ZnO electrodes associated with the structural transformation and BiVO4photosensitization. Absorption and scattering spectra (A+S=100 −R−T) of the samples were calculated by subtracting reflectance (figure 5S(a)) and transmittance (figure 5S(b)) from 100% incident light and illustrated in figure 6(b). Among the electrodes, the most significant non-zero baseline (λ > 400 nm) was observed for ZnO NDs. The main reason is that the horizontally aligned dense branches surrounding the primary NRs cause scattering. Most photons pass directly through the ITO substrate where they may be internally reflected or escape unmeasured (light loss) [52]. The superior light absorption emerging from structural evolution of the branched ZnO nanostructures was reported previously and justified in a similar way [12,33]. The spectra also indicate that ZnO NRs and ZnO NDs have very similar UV absorption edges at 380 nm while the absorption edge of 8 J. Phys. D: Appl. 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