Enhanced photoelectrochemical water splitting by a 3D hierarchical sea urchin-like structure: ZnO nanorod arrays on TiO2 hollow hemisphere

A hierarchical sea urchin-like hybrid metal oxide nanostructure of ZnO nanorods deposited on TiO2 porous hollow hemispheres with a thin zinc titanate interface layer is specifically designed and synthesized to form a combined type I straddling and type II staggered junctions. The HHSs, synthesized by electrospinning, facilitate light trapping and scattering. The ZnO nanorods offer a large surface area for improved surface oxidation kinetics. The interface layer of zinc titanate (ZnTiO3) between the TiO2 HHSs and ZnO nanorods regulates the charge separation in a closely coupled hierarchy structure of ZnO/ZnTiO3/TiO2. The synergistic effects of the improved light trapping, charge separation, and fast surface reaction kinetics result in a superior photoconversion efficiency of 1.07% for the photoelectrochemical water splitting with an outstanding photocurrent density of 2.8 mA cm−2 at 1.23 V versus RHE.


Introduction
Hydrogen is an ideal clean fuel with a high energy density of 140 MJ kg −1 [1].In order to preserve fossil fuel as a raw chemical resource for the future and limit carbon dioxide emissions, hydrogen is an ideal candidate as a future energy source.Currently, hydrogen is produced by steam reforming of natural gas or electrolysis of water, which consumes fossil fuel or electricity and generates greenhouse gases [1].Therefore, there has been a motivation to develop green hydrogen produced by renewable energy.Of many techniques, photoelectrochemical (PEC) water splitting is one of the promising simple solutions to harvest and store solar energy 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.into hydrogen fuels.Since TiO 2 was first reported to have the ability in photocatalysis of water splitting in 1972 [2], a significant number of one-dimensional (1D) nano architectures of metal oxide semiconductors, such as nanorods (NRs) [3], nanotubes (NTs) [4], and nanowires (NWs) [5], have been developed for improving the solar to hydrogen (STH) conversion efficiency.
Nanostructured semiconductors (e.g.ZnO [6], BiVO 4 [7], WO 3 [8], SnO 2 [9], and GaN [10]) possess large surface areas and excellent electron transportation.Their good electrical contact with the substrates has attracted extensive investigation into their potential STH PEC water-splitting applications [11][12][13].Among various semiconductors, ZnO and TiO 2 have received significant attention due to their wide direct band gaps [12,14], and have shown the potential of harvesting solar energy applications in photovoltaics [15,16], and PEC water splitting [12,17].In the realm of PEC water splitting, TiO 2 , with a reported maximum efficiency of approximately 0.84% STH [12], grapples with a wide band gap, though it may be an advantage of absorbing a wide spectrum of light, especially in the UV range, hinders its absorption of significant portions of the solar spectrum and rendering it inefficient under visible light [18].Moreover, TiO 2 -based photoanodes exhibit intrinsic shortcomings, including a large band gap, low electron mobility, and short hole diffusion length, leading to limited UV light utilization and rapid carrier recombination [19].Conversely, ZnO, with the highest reported efficiency of around 0.38% [20], encounters similar challenges with its wide band gap, impeding solar spectrum absorption.Furthermore, ZnO is susceptible to photocorrosion in aqueous solutions under ultraviolet illumination, a common concern shared with other materials for water splitting, with degradation exacerbated in higher pH environments, even in the absence of light [21].
A current method to overcome these limitations is the synthesis of heterojunction structures.The integration of ZnO and TiO 2 heterojunctions offers promising solutions to enhance PEC water splitting efficiencies.Notably, such junctions facilitate improved photocarrier separation, which is crucial for mitigating electron-hole pair recombination.The propensity of ZnO to form nanostructures with large surfaceto-volume ratios further enhances the delivery of photogenerated carriers.Moreover, ZnO's intrinsic properties as a wide band gap semiconductor, inherently doped as n-type, underscore its superior electronic conductivity compared to TiO 2 .While the theoretical maximum PEC efficiencies of ZnO/TiO 2 heterojunctions remain unspecified in the literature, recent studies have shed light on practical efficiencies.For instance, investigations into ZnO/TiO 2 core-shell nanorods (NRs) arrays, augmented with Au nanoparticles (NPs), showcased remarkable advancements.A maximum photocurrent density of 3.14 mA cm −12 at 1.2 V versus reversible hydrogen electrode (RHE) was achieved, surpassing the outputs of ZnO NRs and ZnO@TiO 2 arrays by significant margins [22].Similarly, employing ZnO-NS loaded with Au plasmons yielded an efficiency of 0.38% and a photocurrent density of 0.68 mA cm −12 [20].
Several critical factors, including light absorption, charge separation, suitable band edges, and surface reaction kinetics, can determine the efficiency of STH PEC water splitting.Improving sunlight absorption is generally achieved by band gap engineering, although designing morphology and structure with light trapping performance is still limited.Our previous work demonstrated the light-trapping effects of the nanoporous TiO 2 HHS with enhanced utilization of photo energy and increased PEC performance [23].However, its small surface area of 9.8 m2 g −1 with an averaged pore diameter of 75 nm restricts the direct contact with the electrolyte, negatively impacting the oxygen evolution kinetics on the photoanode and the overall PEC performance.To overcome this problem, vertically aligned ZnO NRs were deposited on the nanoporous TiO 2 HHS through the aqueous chemical bath deposition (CBD) method, which forms a sea-urchin-like hierarchy structure.
1D nanostructures, such as nanowires and nanotubes, have been demonstrated to offer direct and fast electron transportation paths with increased effective surface area, resulting in improved PEC performance [24].Consequently, some strategies have been developed to optimize the effective surface area of the electrodes by creating three-dimensional (3D) heterogeneous nanostructured electrodes to promote photocatalytic performances [25,26].Herein, a sea urchinshaped hierarchy composite structure is designed by depositing vertically aligned ZnO NRs (ZNRs) on the surface of TiO 2 HHSs via a combination of electrospraying and CBD methods.With the presence of a zinc titanate (ZnTiO 3 ) thin layer at the interface between the ZNR and HHS, the optimum PEC performance was achieved from the hierarchy sea urchin-like composite photocatalyst with an excellent PEC water splitting efficiency of 1.07%.

Chemicals
Acetylacetone, zinc acetate, zinc nitrate hexahydrate, hexamethylenetetramine (HMT), and hydrochloric acid (HCl) were purchased from Fisher Scientific, UK.All chemicals are analytical grade and were used without purification.

Synthesis of sea urchin-like ZnO NRs on TiO 2 HHSs
The synthesis of porous HSS was reported previously [23] on a titanium substrate (2 × 2 cm 2 ) using a specifically designed electrospraying method.ZnO NRs were vertically grown on the surface of the TiO 2 HHS using a CBD method.The annealed TiO 2 HHSs sample (400 °C) was soaked in the 0.10 M zinc acetate seeding solution for an hour.The seeded sample was annealed at 350 °C at a rate of 25 °C min −1 in the air for an hour to convert the zinc acetate into ZnO nanoparticles before the CBD process.The nutrient solution for the CBD growth was composed of a 1:1 molar ratio of zinc nitrate hexahydrate and HMT in DI water with a final Zn 2+ ions concentration of 10.0 mM.The as-prepared samples were vertically positioned in a glass beaker containing 150 ml of growth solution.The solution was heated at 85 °C for 24 h to produce the sea urchin-like structure with ZnO NRs deposited on the surface of TiO 2 HHSs.Finally, the sample was rinsed three times with DI water, dried at room temperature, and annealed to 400 °C-800 °C at a heating rate of 50 °C min −1 .The synthesis procedure is illustrated in figure 1.The samples are named ZTxxx, where xxx is the annealing temperature.
To study the interface structure, the sea urchin-like ZT700 sample was soaked in HCl solution (0.1 M) to dissolve the surface ZnO NRs without damaging the TiO 2 HHS, revealing the interface structure.

Characterisation
Scanning electron microscopy (SEM, JSM820M, Jeol) was used to examine the surface morphology and the nanostructured samples at different growth stages.A powder x-ray diffractometer (XRD, D500, Siemens) was used to analyze the crystallinity of the nanostructures.The elemental analysis was carried out by an energy-dispersive x-ray analyzer (EDX).PEC water splitting was undertaken using a standard three-electrode configuration in a 1.0 M aqueous KOH electrolyte solution (pH 13.6).A KCl-saturated Ag/AgCl electrode was used as the reference, and a platinum foil was used as the counter electrode.Sunlight was simulated using a 100 W xenon arc lamp with an AM 1.5 G filter with an illumination area of 5 mm × 5 mm, and the output light power density was adjusted to 100 mW cm −2 .The cyclic voltammetry measurements were measured using a potentiostat with eDAQ software operating at a voltage scan rate of 10 mV per 100 ms.UV-vis diffuse reflectance spectra used to determine optical band gap were acquired using Ocean Optics ISP-REF integrating sphere equipped with an inbuilt tungsten-halogen illumination source 300 nm λ 1000 nm, with MgO reference.EIS was performed in the dark using a Palm Sens 3 (PalmSens BV) electrochemical potentiostat, processed using PS Trace 4.8 (PalmSens BV) and Elchemea Analytical (DTU Energy, Technical University of Denmark).Surface valence band composition was measured using x-ray photoelectron spectroscopy (XPS, Thermo Scientific K-alpha using Al Kαsource).

Growth of sea urchin-like ZnO/TiO 2 sample
Porous TiO 2 HHSs, as shown in figure 2, were successfully synthesized through the electrospraying method developed within the group [23].The top view SEM image in figure 2(A) shows the relative homogeneous size of the TiO 2 HHSs with an average diameter of 3.0 μm.The porous structure can be observed in the magnified SEM image in figure 2(B).The average diameter of the nanopores is 60 nm, determined by the BJH analysis in BET measurement [23].XRD analysis confirmed the formation of a pure anatase phase after annealing at 400 °C.Our previous study demonstrated the light-trapping effects of this TiO 2 HHS structure, which enhanced the PEC efficiency to 0.31% [23].
Although the HHS offers a porous structure with a BET surface area of 9.8 m 2 g −1 and a pore volume of 0.21 cm3 g −1 , its limited contact with the electrolyte restricts the overall PEC performance.To overcome this problem, vertically  XPS data in figure S3 shows the signals of Ti 2p and Zn 2p.There were 4 peaks in each spectrum corresponding to TiO 2 , ZnTiO 3 and ZnO [27].However, due to the 3D architecture of the composite structure, the relative XPS intensity of each component can also be affected by their position in the film.
The annealed urchin-like samples (from 400 to 800 °C for an hour) grown for 24 h showed the evolution of the crystal structures.XRD patterns confirmed the presence of anatase phase of TiO 2 (JCPDS No. 21-1272), rutile phase of TiO 2 (JCPDS No. 21-1276), wurtzite ZnO (JCPDS No. 36-1451), and cubic crystal phase of ZnTiO 3 (JCPDS No. 39-0190) in addition to the diffraction from the titanium substrate (JCPDS # 44-1294).As shown in figure 3, the diffraction peak intensities of the anatase phase increase from 400 °C (ZT400) to 500 °C (ZT500) and start to decrease when the annealing temperature is at 600 °C (ZT600) and above.The initial increase of anatase diffraction intensity indicates the improvement of the crystallinity of the TiO 2 .At 600 °C (ZT600), a few additional diffraction peaks appeared at 2θ of 27.56°, 41.36°, 54.37°, 56.69°, and 69.05°, corresponding to (110), (111), (211), ( 220) and (301) crystal planes for the rutile phase of TiO 2 .The decrease of the anatase peaks with the increase in the rutile peaks confirmed the phase transition from the anatase phase of TiO 2 to the rutile phase of TiO 2 , which is thermodynamically more stable [28].
The XRD patterns in figure 3 also indicate the formation of ZnTiO 3 thin film after annealing at 700 °C (ZT700) with new peaks at 30.0°, 35.4°, and 62.4°, assigned to the (220), (311), and (440) facets of the cubic phase ZnTiO 3 .These peaks become more intense when the annealing temperature is increased to 800 °C.Hence, the formation of ZnTiO 3 is thermally driven.Meanwhile, the diffraction signal from ZnO NRs decreased while more rutile was formed.Cai et al also reported that ZnTiO 3 forms in their sample mixture containing zinc acetate and TTIP annealed at 700 °C [29].It was proposed that the formation of ZnTiO 3 involves the increased mobility of the Ti 4+ and Zn 2+ species.Hence, we expect that the ZnTiO 3 was formed at the interface between the TiO 2 HHS and ZnO NRS at high temperatures.
Meanwhile, the crystal domain size of ZnO increased as the annealing temperature increased.The domain size was calculated from the Sherrer formula with the result listed in table S1.
The formation of ZnTiO 3 and its specific position at the ZnO NRs and TiO 2 HHSs interface was further confirmed by acid treatment of the ZT700 sample to dissolve the ZnO NRs gradually from the from the surface to reveal the underneath structure since both TiO 2 and ZnTiO 3 are stable in acid [30,31].Figure 4(A) shows the morphology of the TiO 2 HHS annealed at 700 °C before ZnO NRs deposition.The annealed TiO 2 HHSs are formed with anatase and rutile phases with a relatively smooth surface and clear visible nanoporous structures.The wall thickness of the HHS is about 300 nm.EDX was carried out to determine the elemental composition changes of the sea urchin-like sample before and after the acid treatment.The quantitative analysis in figure S5 in the Supporting Information reveals the composition of 3.08% at titanium and 96.92% at zinc for the sea-urchin sample, which was changed to 72.33% at titanium and 27.67% at zinc after 80 s of acid treatment.The residual amount of Zn is due to the formation of ZnTiO 3 .
UV-vis absorption was measured using an integrated sphere and a USB spectrometer.The results are shown in figure S6(A), together with the corresponding Tauc plot in figure S6(B).The Tauc plots suggest that the TiO 2 HHS (red curve) has a bandgap energy of 3.08 eV, while ZnO NR deposited on the TiO 2 HSS has a reduced bandgap energy of 2.89 eV.The band gap energies for different materials are summarized in table S2.

PEC water splitting performance
The PEC performance of the sea urchin-like ZnO/TiO 2 sample was investigated as a function of annealing temperature ranging from 400 to 800 °C in 1.0 M KOH electrolyte (pH 13.6) under the illumination of a 100 mW cm −2 solar simulator with an AM1.5 G filter.PEC water splitting energy conversion efficiency (η) of the sea urchin like structure was calculated by equation (1).It has been reported that the appropriate ratio between anatase and rutile phases plays an important role in determining the photocatalytic performance of the sample.The (101) anatase XRD peak intensity is shown in figure 5(C).Initially, the increase in calcination temperature improves the crystal quality of the TiO 2 HHS.However, above 600 °C, the anatase is subjected to a phase transition to form rutile TiO 2 (figure 5(D)).The similar curvature between the anatase diffraction intensity and the maximum output power reveals some correlation between the crystal quality and the photoconversion output power of TiO 2 HHSs.A decrease in the photoconversion output power is observed when the annealing temperature was further increased to 750 °C due to the increase in the rutile phase of TiO 2 .
It is important to note that the maximum PEC efficiency was achieved at the temperature of 700 °C when a thin interface layer of ZnTiO 3 was formed between the ZnO NRs and TiO 2 HHSs.To confirm the positive effects of this interface layer, the PEC performances from the photoanodes of ZnO NRs, TiO 2 HSS, and the sea urchin-like ZnO/TiO 2 annealed at 600 °C (ZT600) and 700 °C (ZT700) were directly compared in figure 6.The ZnO NRs and TiO 2 HHSs photoanodes were calcined at 400 °C and 650 °C, respectively, for achieving their optimal PEC performances.The length, diameter, and density of the ZnO NRs were similar to those grown on TiO 2 HHSs, synthesized under identical experimental conditions.
Figure 6(A) represents the corresponding transient I-V curves.The onset potentials of ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 photoanodes were estimated to be −0.39,−0.42, −0.63, and −0.72 V Ag/AgCl , respectively.The negative shifts in the photocurrent onset potentials for the ZT600 and ZT700 photoanodes are attributed to the improved charge separation with reduced electron-hole recombination.ZT700 has a more negative onset potential, suggesting the presence of the ZnTiO 3 interface layer has improved the charge separation further.The short circuit current, I sc , measured at the potential of 0.82 V Ag/AgCl from ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 electrodes were measured to be 0.28, 1.07, 1.79, and 2.8 mA cm −2 , respectively.The PEC output powers and the PEC efficiencies under AM1.5G solar illumination are presented in figure 6(B).The photoconversion efficiencies of ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 photoanodes were calculated to be 0.12, 0.25, 0.61 and 1.07%, respectively.The highest PEC performance was achieved from the sea-urchin sample annealed at 700 °C.In addition, photocurrents under a constant bias of 0.23 V Ag/AgCl with the light on-off cycles for 160 s were recorded in figure 7. The long-term (2 h) stability test result is shown in figure S7.There is no noticeable decrease in the photocurrent density for all the photoanodes, confirming the stable PEC performance.
The electrochemical property of ZT700 with respect to the TiO 2 HHS was investigated with electrochemical impedance spectroscopy (EIS) with the corresponding Nyquist plots shown in figure S8.The smaller radius from the ZT700 suggests a reduced interface resistance between the composite electrode and the electrolyte, which is related to the increased surface area from the ZnO nanorods.This reduced interface resistance is responsible for the increased PEC efficiency for ZT700.
PEC stability is related to structural stability.After a 2-h PEC experiment, the sample was analyzed with XRD and SEM. Figure S9(A) shows no significant changes in the crystal structure of the 3D hierarchical components, while figure S9(B) confirms that the sea urchin-like morphology was maintained.
It is worth noting that all the photoanodes, particularly the TiO 2 HHS and ZnO NRs, show a repeated initial anodic photocurrent spike when the light was switched on, as shown in figure 8(A), which is due to the accumulation of charges at the interface of semiconductor and electrolyte, resulting from the slow oxygen evolution reaction kinetics and carrier trapping [33].These anodic photocurrent spikes have been used to assess the lifetime of the charge carrier of the photoanodes, which can be related to the general rate of charge recombination [33].
Figure 8(B) shows the normalized lnD plots as a function of illumination time.The transient time constant (τ) is defined as the time when lnD = 1, which reflects the lifetime of the charge carriers of the sample [33].The values of ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 photoanodes were determined to be 0.38, 0.97, 2.42, and 3.18 s, respectively.These measurements demonstrate that the charge recombination of ZT600 and ZT700 photoanodes is more than three times slower than ZnO NRs and TiO 2 HHSs photoanodes, respectively, which is conducive to photocatalytic performance.
The possible mechanism for the enhanced photocurrent densities of ZT600 and ZT700 may be attributed to the unique urchin-like morphology, which facilitates a large effective surface area of ZnO NRs in addition to the light trapping by the TiO 2 HHS [23].The coupling between ZnO NRs and TiO 2 HHSs can improve electrolyte infiltration, helping to maintain electrolyte concentration in the vicinity of reaction sites, which may also contribute to improving PEC efficiency.
More importantly, the reduced charge recombination for the ZT600 photoanode with respect to the separated ZnO NRs and TiO 2 HHS can be attributed to the appropriate band edge alignment between the coupled ZnO and TiO 2 , as shown in figure 9. From the Tauc plot in figure 6S(B), TiO 2 HSS has a band gap energy of 3.08 eV, while ZnO has a smaller band gap energy of 2.89 eV.The band edges of ZnO and ZnTiO 3 were determined by XPS and UV-vis absorption measurements [34].The conduction band of ZnO is 0.13 eV higher than TiO 2 due to the difference in Fermi levels, which produces an internal electric field to drive the photogenerated  electrons from ZnO to TiO 2 and the holes in the opposite direction.The constructed 3D hierarchical heterojunctions form a typical type II staggered band structure configuration responsible for the improved charge separation.This is the dominant mechanism for the improved photocurrent density of the ZT600 photoanode over the ZnO NRs and TiO 2 HHSs photoanodes.
By annealing the sea-urchin sample to 700 °C, the photoconversion efficiency was further increased from 0.61% (ZT600) to 1.07%, accompanied by the increase in the charge separation due to the formation of a thin layer of ZnTiO 3 at the interface of ZnO NRs and TiO 2 HHSs.The energy diagram of this 3D hierarchical sea urchin-like structure is shown in figure 8(B).The hierarchical structure is formed with two sequential interfaces of ZnO/ZnTiO 3 and ZnTiO 3 /TiO 2 .The relative positions of the band edges suggest that a type II staggered junction is formed between ZnTiO 3 and TiO 2 , while a type I straddling junction is formed between ZnO and ZnTiO 3 .Combining these two junctions creates a gradually increased conduction band position from ZnO through ZnTiO 3 towards TiO 2 , which can more effectively transfer excited electrons compared to the interface between ZnO and TiO 2 without the ZnTiO 3 layer.For the valence bands, the ZnTiO 3 has the least positive potential, and it forms a hole reservoir collecting the excited holes from both TiO 2 HHSs and ZnO NRs.However, since only a small energy difference presents between valence bands of ZnO and ZnTiO 3 (0.06 eV), the accumulated holes in the ZnTiO 3 can be easily transferred to the ZnO NRs to facilitate the oxidation of water without too much energy cost.As such, the thin layer of ZnTiO 3 acts as a barrier to prohibit the hole transfer while conducting electrons freely to the TiO 2 , resulting in the decreased electron-hole recombination in ZT700 with an enhancement of the PEC water splitting efficiency.However, a further increase in the thickness of the ZnTiO 3 barrier layer could decrease the hole transfer from TiO 2 to ZnO, reducing PEC performance, such as observed in ZT750 and ZT800.The comparison of the PEC performance with the literature values is shown in table 1, where the comparison is made with nanostructured TiO 2 and ZnO photoanodes.Our 3D hierarchy sea urchin-like ZnO/ZnTiO 3 /TiO 2 offers a superior performance after annealing at 700 °C due to its unique hierarchical structure.
In summary, the novel 3D hierarchical sea urchin-like ZnO/ZnTiO 3 /TiO 2 sample significantly improved the PEC water splitting compared with the ZnO NRs and TiO 2 HHSs.The enhanced performance is attributed to the synergetic effects of the improved contact with the electrolyte with a large effective surface area of the ZnO NRs, enhanced light trapping by the HHS, and improved charge separation due to the ZnTiO 3 barrier layer.Therefore, the proposed technique to synthesize 3D sea urchin-like ZnO NR aligned arrays on TiO 2 HHSs represents a significant advance for PEC production.

Conclusions
A novel, hierarchical sea urchin-like composite nanostructure is developed with the aligned arrays of ZnO NRs deposited on the porous TiO 2 HHSs.A ZnTiO 3 layer is formed at the interface between ZnO NRs and TiO 2 HHSs after annealing at 700 °C.The hierarchy structure forms a unique electronic band structure with a type II, staggered junction between ZnTiO 3 and TiO 2 , which is directly coupled to a type I, straddling junction between ZnO NRs and ZnTiO 3 thin film.This electronic structure offers a smooth directional electron movement from ZnO to TiO 2 while accumulating holes at the ZnTiO 3 shared with ZnO.Each of the components in the hierarchy structure offers different effects in improving the PEC water-splitting performance.The ZnO NRs offer a large contact area with the electrolyte, and TiO 2 HHSs are responsible for trapping the light, while ZnTiO 3 improves the charge separation.An optimum photoconversion efficiency of 1.07% was achieved from the sea-urchin sample annealed at 700 °C with the corresponding photocurrent density of 2.8 mA cm −2 at 1.23 V versus RHE.The specially designed 3D urchin-like ZnO/TiO 2 represents a significant advancement in applying nanostructured photocatalysts for PEC hydrogen generation.

Figure 1 .
Figure 1.The synthesis process for producing a 3D hierarchical sea urchin-like structure by depositing ZnO nanorods on hollow hemisphere TiO 2 .

Figure 2 (
C) shows the morphology of the sea urchin thin film formed after 24 h of CBD deposition of ZnO NRs.The surfaces of the TiO 2 HHSs are completely covered by highdensity, vertically aligned ZnO NRs perpendicular to the surface of the HHSs.The nanorods appear uniform in size with an average diameter of 230 nm ± 43 nm, measured from the magnified SEM image in figure 2(D).The diameter of the sea urchin structure is about 9.3 μm, increased from the 3.0 μm of the HHS.Hence, the estimated length of the ZnO NRs is about 3.15 μm, determined by the CBD growth duration.The ZnO NR growth process was also monitored by SEM and XRD, as shown in figures S1 and 2 in the supporting information (SI).As the duration of ZnO NR growth increases, the sea urchin-like structure becomes well-constructed with the increase in the density, diameter, and length of the vertically aligned ZnO NRs.The XRD spectra in figure S2 also show an increase in the ZnO diffraction intensities while no changes in the TiO 2 diffraction intensities.

Figure 2 .
Figure 2. SEM images of TiO 2 HHSs annealed at 400 °C for 18 h with an initial heating rate of 1 °C min −1 .(A) Top view and (B) magnified images of TiO 2 HHSs.(C) The hierarchy sea urchin-like structures of ZNRs deposited on TiO 2 HHS and (D) the magnified sea-urchin structure.

Figure 4 (
B) shows the morphology of the sea urchin-like ZT700 sample after dipping in an HCl solution (pH 1.0) for 80 s.The surface ZnO NRs were removed, and the sample surface shows a layer of crystallized nanostructures with multi facets with a wall thickness of 450 nm.Hence, the film thickness of the ZnTiO 3 on the surface of TiO 2 HHSs is about 150 nm.The XRD spectra in figure 4(C) reveal the gradual reduction of the ZnO diffraction peaks while the diffraction signals from TiO 2 and ZnTiO 3 were gradually more intense.After 80 s of acid treatment, no ZnO signals were observed, indicating the complete removal of ZnO NRs.The evolution of diffraction intensities for ZnO is presented in figure S4(A).Meanwhile, the peak intensity ratio between the anatase (101) and rutile (110) was roughly maintained constant during the acid-washing processes.Hence, no significant change in the crystallinity of the TiO 2 HHS is expected, and the rough surface of the acid treat sample can only be attributed to the formation of ZnTiO 3 .The gradual increase in the XRD intensities from the (220) and (311) planes in ZnTiO 3 following the acid washing is shown in figure S4(B).

Figure 3 .
Figure 3. XRD patterns of sea urchin-like ZnO/TiO 2 sample grew at 24 h at different annealing temperatures.

Figure 5 (
A) represents the PEC output power, and figure 5(B) demonstrates the maximum output power from the sea urchin-like ZnO/TiO 2 samples.The hydrogen generation efficiency increased initially as the annealing temperature increased, with a maximum output power of 1.07 mW cm −2 from the ZT700 sample.

Figure 4 .
Figure 4. SEM images of TiO 2 HHS (A) before depositing and (B) after removing ZnO NRs with 80 s of acid treatment.(C) the change of XRD signals during the acid treatment.

Figure 6 .
Figure 6.(A) IV curves of ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 together with dark current measured from ZT700 in 1.0 M KOH electrolyte (pH 13.6) under simulated sunlight of 100 mW cm −2 with an AM1.5 G filter and (B) corresponding PEC output power and conversion efficiency under AM1.5 G solar irradiation.

Figure 7 .
Figure 7. Amperometric I t curves of ZnO NRs, TiO 2 HHSs, ZT600, and ZT700 photoanodes in 1.0 M KOH electrolyte (pH 13.6) at an applied voltage of 0.23 V Ag/AgCl at 100 mW cm −2 for 160 s with repeated light on-off cycles.

Figure 8 .
Figure 8. (A) Anodic photocurrent dynamics of the photoanodes recorded in 1.0 M KOH electrolyte (pH 13.6) at an applied potential of 0.23 V Ag/AgCl at 100 mW cm −2 and (B) the normalized plots of photocurrent density (lnD) as a function of illumination time.

Figure 9 .
Figure 9. Band gap schematic diagrams representing the charge-transfer process in (A) ZT 600 and (B) ZT 700.