A sail-shaped ZnO nanowire structure: simultaneously boosting light receiving capacity and carrier transportability

Abstract

Developing a clean and high-efficiency energy source is critical for solving future energy and environmental issues. A promising approach is solar water splitting, using solar energy to dissociate water into H₂ and O₂. However, a complicated mechanism underlies the water splitting process, including light absorbance, photon-to-carrier conversion, and carrier transport/transfer. Designing a suitable photoanode that meets these requirements still is a challenge. Here we demonstrated an alternative photoanode by using a sail-shaped ZnO nanostructure. High light absorbance was achieved due to the increased surface area for the specific nanosail structure. The nanosail structure supports two types of carrier transport mechanisms that provide a direct carrier transport pathway and network, resulting in more photocurrent and conversion efficiency enhancement. This sail-shaped ZnO structure simultaneously increases light receiving capacity and carrier transport ability and shows high potential for water-splitting applications.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s11671-026-04989-6.

Introduction

Solar energy has been gradually regarded as a significant energy source due to its enormous energy. The energy from the sun can be converted into electrical or chemical energy via a photoelectrochemical (PEC) water splitting [1–4]. Recently, some PEC devices made of narrow bandgap metal oxides were proposed, which allowed the absorption of a broad wavelength range and produced the photocurrent in response to different wavelengths [5–10]. However, the main generated photocurrent still resulted from the energy conversion close to the bandgap. The absorbed photon energy cannot be fully converted into photocurrent but generates heat, thus destroying device structures. This phenomenon is partially attributed to the poor carrier transport ability of the metal oxides.

On the other hand, various one-dimensional metal oxide nanowires with a high surface-to-volume ratio were used as PEC host structures to prompt carrier generation and obtain higher carrier collection [11–20]. These nanowires provided a large amount of surface area and strong quantum confinement, which induced band bending and charge separation, further improving carrier exchange and transport. The increase in the surface area enhanced the photocurrent of nanowire devices; however, most randomly arranged nanowires also led to significant surface carrier recombination due to excessive nanowire length and poor crystal quality, and hence limited carrier transport and photocurrent value. Whether from the viewpoint of increasing light absorption by stacking different materials or surface area by designing material geometry [21–25], the PEC host structure’s high carrier transport ability has become an essential prerequisite for developing a high-efficient PEC device.

Studies on improving the carrier transport of PEC host structures have rarely been discussed. Here, we propose a new three-dimensional (3D) sail-shaped host structure composed of single-crystal and specifically-oriented ZnO nanowires. ZnO nanowire was chosen for its encouraging optoelectronic and piezoelectric properties in light-emitting devices, sensors, nanogenerators, and solar devices. ZnO [26–28], with a wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV, not only meets the energy potential requirement for water splitting but allows us to get a clear understanding of carrier transport behaviour because it responds only to UV light. The ZnO nanosail structure is expected to retain the properties of high light absorbance and ample light receiving angle. In addition, the crosslinking node regions between nanosails exhibit single-crystal characteristics, which implies that the photogenerated carriers can move from one nanosail to another without hindrance. The role of the crosslinking nodes in this PEC nanosail device is similar to the carrier transport interconnects in the common electric circuit that enhances carrier transport efficiency. The sail-shaped ZnO structure increases light-receiving capacity and carrier transportability, which is highly promising candidate for water-splitting applications.

Results and discussion

In this study, an r-plane sapphire substrate was used to grow the a-plane ZnO nanowires via a vapour–liquid–solid (VLS) method. First, a thin ZnO film was deposited as the seed layer using a magnetron sputter system. An Au film was deposited on the ZnO film and transformed into Au nanoparticles by a self-assembly process. The Au catalysts decrease the surface diffusion energy to stimulate ZnO nanowire growth in favourable directions. As the growth time increased, the a-ZnO nanowires gradually formed a sail-shaped interlaced structure. Similar growth of the oblique nanowire has been reported in previous studies [29, 30]. Notably, the previous studies produce discrete oblique nanowire arrays via Zn self-catalyzed VLS with isolated nanowires and no crosslinking, while this work forms an interlaced sail-shaped structure through typical Au-catalyzed VLS, in which nanowires gradually contact and crosslink during growth. For comparison, a control sample of c-plane ZnO nanowires on m-plane sapphire substrate was prepared under the same conditions. Figures 1a‐1c show the top-view scanning electron microscopy (SEM) images of the ZnO film, c-ZnO nanowire, and a-ZnO nanosail. The ZnO seed layer is 1 μm thick, as shown in the cross-sectional SEM image of Fig. 1d. Smaller hexagonal c-ZnO nanowires with sizes 100–350 nm were observed on the a-sapphire substrate, which is perpendicular to the substrate surface (Fig. 1e). Interestingly, Fig. 1f displays the nanosails with slope angles 30° or 60° contact each other and create several crosslinking nodes. Notably, the ZnO crosslinking nodes are not pointed contacts but crystallized contacts. The result suggests the occurrence of lateral ZnO growth at the crosslinking nodes. The node width is approximately 20─50 nm, as shown in Fig. 1c.

Fig. 1.

ZnO structure and morphology. a–c The top view SEMs of ZnO film, nanowire, and nanosail. d–f Cross-sectional SEM images of the ZnO film, c-ZnO nanowire, and a-ZnO nanosail, respectively

The crystal quality and orientation of the three ZnO structures were investigated by X-ray diffraction (XRD) data, as shown in Fig. 2a. The XRD peaks at 52.57° and 37.87° correspond to () of r-sapphire and () of a-sapphire, respectively. A sharp peak at 34.42° shows the growth of (0002) c-ZnO nanowires on the a-sapphire substrate. Meanwhile, the peak at 56.6° indicates that the () a-ZnO nanosail successfully grew on the r-sapphire substrate. In Fig. 2b, the cross-sectional transmission electron microscopy image of nanosails explains that the two growth orientation directions (labelled A and B) dominated the growth trend of nanosail. The diffraction pattern of the entire crosslinking node region shows more detailed evidence (Fig. 2c). The ordered spot pattern indicates that the crosslinking node region has a single-crystal characteristic. That is, the initial ZnO grains grew vertically from the r-sapphire substrate along the [] direction as well as two lateral growths along the [] and [0001] directions due to the Au catalytic effect. Both vertical and lateral growths resulted in the ZnO nanosails along four oblique directions. The ZnO nanosails labelled A and B correspond to the [] and [] directions, respectively. The oblique nanosails formed with an included angle of 60° from the [0001] direction, and an included angle of 30° was obtained when rotated at 90° around the [] direction (Fig. 2d and e). The nanowires with special structures can effectively improve the light receiving angle range.

Fig. 2.

ZnO nanosail crystalline and orientation. a XRD data of the three ZnO structures. b Cross-sectional TEM image of the ZnO nanosail c Diffraction pattern of the entire crosslinking node region (red circle in Fig. 2b). The oblique nanosails d with an included angle of 60° from the [0001] direction, and e with an included angle of 30° when rotated at 90° around the [110] direction

The UV–vis absorption spectra of the ZnO film, nanowire, and nanosail samples are provided in Fig.S1 (Supporting Information) to compare their optical absorption properties. The photocurrent as a function of applied voltage (0.2–1.8 V versus RHE) was measured in 0.5 M Na₂SO₄ under AM 1.5 illumination, as shown in Fig. 3a. A near-zero dark photocurrent was found for all samples before illumination, while the photocurrent was generated under illumination. Due to defects in the grain boundaries, the ZnO film (i.e., ZnO seed layer) photoanode exhibited a constantly low value of photocurrent density even though the voltage increased. At 1.23 V, the photocurrent densities of the film, nanowire, and nanosail photoanodes are 0.14, 0.6, and 1.72 mA/cm², respectively. The rising rate of the photocurrent density with increasing voltage in the nanosail photoanode was faster than in the nanowire photoanode. The results consider that the nanosail photoanode with crosslinking networks has better carrier transportability. The incident photon-to-current-conversion efficiency (IPCE) was determined as follows:

Fig. 3.

PEC performance and energy conversion efficiency. a Photocurrent density as a function of applied voltage. b IPCE and c APCE of the three ZnO photoanodes at 1.23 V versus RHE condition. d ABPE for the ZnO film, nanowire, and nanosail photoanodes

where is the photocurrent density, is the wavelength of the incident light, and is the optical power density of incident light. IPCE indicates PEC device performance, considering the efficiencies for light absorbance, carrier generation, and carrier transport across the solid to solid-liquid interfaces. Generally, it represents the external quantum efficiency of PEC devices. As shown in Fig. 3b, the maximum IPCEs of the ZnO film, nanowire, and nanosail photoanodes at 370 nm are 16.31%, 39.27%, and 73.15%, respectively. Nanosail photoanode exhibited a high integrated IPCE enhancement, 1.86 times and 3.72 times the IPCE of the nanowire and film cases, respectively. To further understand the intrinsic properties of ZnO nanosails in light absorbance and conversion efficiency, absorbed photon-to-current efficiency (APCE) was calculated, which describes the collected photocurrent per absorbed incident photon. The APCE is 28%, 60%, and 92% for the film, nanowire, and nanosail photoanodes, respectively, at 370 nm wavelength (Fig. 3c). To our knowledge, the 92% efficiency in the ultraviolet region is the highest APCE for the ZnO-based PEC devices. The APCE in the nanosail cases is higher by 1.53 times and 3.28 times than for the nanowire and the film photoanode, respectively. APCE is synonymous with the internal quantum efficiency of PEC devices [31]. At the same absorbance, the nanosail structure produced higher conversion efficiency and obtained higher photocurrent. This result predicts that the nanosail structure has a unique carrier transport mechanism compared to the nanowire structure, thus providing better carrier transport. Meanwhile, the absorbance () of the nanosail is 1.2 times that of the nanowire structure, which further supports the superior photoelectric conversion properties originating from the unique sail-shaped architecture.

Applied bias photon-to-current efficiency (ABPE) is one of the most critical indicators for PEC device performance. ABPE measures the net output power of a device in a unit of incident light power, and it is calculated as follows:

where is the photocurrent density, is the applied bias between the working and counter electrodes, and is the power density of incident light whose value is 100 mW/cm². As shown in Fig. 3d, the nanosail photoanode exhibited considerable enhancement in the whole voltage range, and its maximum ABPE was 0.54% at 0.78 V versus RHE. Because the ABPE calculation uses the AM 1.5 standard (wavelength range from UV to mid-IR at total power intensity of 100 mW/cm²), there may be an underestimation of the photoresponse of the ZnO nanosail structure. We modified the ABPE formula, and the ABPE of the nanosail was increased to 9.7% when only the photoresponse of UV light was considered.

As mentioned above, a complicated mechanism underlies the PEC process, including light absorption, photon-to-current conversion, and carrier transfer. Carrier transfer characteristics are further analysed by electrochemical impedance spectrum at a bias of 1.23 V for the three photoanodes, as shown by the Nyquist plots in Fig. 4a. The charge transfer resistance values for the film, nanowire, and nanosail photoanodes are 14.15, 6.28, and 3.06 kΩ, respectively. This result shows that the charge transfer resistance was gradually reduced by improving the crystal quality and providing a direct transport path, corresponding to using the nanowire and nanosail photoanode. Moreover, the non-vertical low-frequency tail observed in the Nyquist plot of the nanosail photoanode may be attributed to Warburg-type impedance behavior, which is generally related to ion diffusion/mass transport in the electrolyte solution [32, 33]. Based on the unique interconnected nanosail architecture and relevant literature support, we infer that this feature could also indicate the presence of abundant charge carriers accumulated and transferred between the nanosail surfaces, providing efficient charge transfer channels for the redox reactions.

Fig. 4.

Electrical properties of the three ZnO nanostructures a Nyquist plots. b The Mott─Schottky plot in dark conditions for ZnO film, nanowire, and nanosail photoanode, corresponding to a carrier concentration of 6.4 × 10¹⁷ cm^− 3, 4.4 × 10¹⁸ cm^− 3, and 5.9 × 10¹⁸ cm^− 3, respectively

Moreover, Mott─Schottky (M─S) plot in dark conditions was drawn to judge the flat-band potential of ZnO photoanodes. The M–S relationship describes the capacitance of the space charge layer of the photoanode as a function of the applied potential.

where is the relative permittivity of ZnO, is the permittivity of vacuum,  A is the surface area,  e is the electron charge, is the carrier concentration,  k is the Boltzmann constant,  T is the temperature,  V is the applied potential, and is the flat-band potential. A lower flat-band potential of 0.20 V of the nanosail photoanode compared to that of the film (0.36 V) and nanowire (0.37 V) photoanodes, as shown in Fig. 4b, indicates a more favourable band position. It leads to greater upward band bending and faster carrier separation. In addition, the positive slopes of the M─S curve enclose the n-type feature of photoanodes. A smaller slope implies that the nanosail structure has a high carrier concentration.

The carrier transfer behaviour of the nanosail structure during PEC process was schematically shown in Fig. 5. Due to its unique architecture, the large surface area of the nanosail structure enables efficient light absorption and carrier generation at the interfaces, leading to a ridged potential barrier and carrier depleted region. This ridged potential barrier facilitated enhanced carrier separation [34, 35]. The nanosail photoanode hence created a strong photoresponse and more photocurrent. Such a high potential barrier may impede carrier transfer between nanowires when they touch each other. In our study, the crystallized crosslinking regions of the a-ZnO nanosails provide a smaller inner electric field [36], reducing the barrier height and enabling carriers to move easily between nanosails. As a whole, the PEC performance improvement is mainly attributed to the unique carrier transfer mechanisms enabled by the crosslinking nodes in the nanosail structure. Meanwhile, it should be noted that the performance is also affected by the enhanced light-harvesting ability and increased surface area. On the nanosail surface, the photogenerated carriers are highly excited and result in a strong band bending (i.e., ridged potential barrier) that intensifies the photoresponse and photocurrent. At the same time, the photogenerated carriers pass the crosslinking nodes of nanosails through the depletion zones owing to the lower potential barrier. These crosslinking nodes serve as the interconnects between the nanosail networks. ZnO nanosails with crosslinking nodes exhibited high PEC performance in light absorbance, conversion efficiency, and carrier transport and are suitable host building block candidates for developing highly efficient water-splitting devices.

Fig. 5.

Carrier transport mechanism. a Schematic illustration for the carrier transport behavior at the nanosail/electrolyte interface and crosslinking nodes. b Photogenerated carriers are excited on the surface to form obvious band bending and boost photoresponse; c High potential barrier hinders carrier transmission between adjacent nanowires; d Carriers efficiently migrate through crosslinking nodes with lowered barrier height

Conclusions

In summary, we demonstrated the performance of the crossing-node-containing ZnO nanosails during the PEC process. Besides the large surface area of the nanosails achieving high light absorbance, the nanosail structure also supports two carrier transport mechanisms that promote photoresponse and photocurrent, resulting in conversion efficiency enhancement. The nanosails hold promise as multifunctional platforms for PEC devices with high light harvesting and carrier transport in future water-splitting applications.

Method

ZnO nanostructure fabrication

A magnetron sputtering system (FJL560) deposited a 1 μm ZnO film as a seed layer on the r-plane sapphire substrate. Subsequently, an Au catalyst of 20 nm thickness was deposited on the ZnO film using the FJL560 sputter. A mixture of ZnO powder (Alfa Aesar, 99.99%) and graphite (~ 200 mesh, 99.9995%) with a 2:1 ratio was used as a material source and placed into a double-zone tube furnace. The ZnO nanosail structure was grown at 860℃ for 8 min. For comparison, the ZnO nanowire structure was fabricated on an m-plane sapphire substrate under the same conditions.

PEC measurements

The electrochemical measurements were carried out in a standard three-electrode electrolytic cell with 0.5 M Na₂SO₄ (PH = 7) aqueous solution serving as an electrolyte at room temperature. A platinum wire and Ag/AgCl (in a 3.0 M KCl solution) were used as counter and reference electrodes, respectively. The light source for PEC measurement was a 300 W Xenon lamp with an AM 1.5 filter (PLX-SXE300, Perfectlight). The PEC cell chamber was made of quartz that transmitted UV and visible light. The IPCE spectra of the three ZnO photoanodes were acquired by using a Xenon lamp and a monochromator (Omini-λ150, Zolix) equipped with a Si detector (Model 71640, Oriel). The EIS was measured in a frequency range of 0.1–10⁵ Hz with 1.23 V (vs. RHE) as the initial potentials and an AC potential amplitude of 5 mV under AM 1.5 illumination.

Supplementary information

Below is the link to the electronic supplementary material.

Author contributions

C.Y.L. grew nanosail ZnO nanowires. C.Y.L. and P.W.S. supported the optical measurements. C.Y.L., P.W.S., and K.C.S. analyzed the experimental data, and wrote the manuscript. P.W.S. and K.C.S. supervised the research. All authors discussed the results and commented on the manuscript.

Funding

This work was supported by the Fujian Provincial Department of Science and Technology of China (Grant No. 50021125).

Data availability

Author declares that data used in this paper is available to this manuscript. Corresponding author will share the data in a reasonable request.

Declarations

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Competing interests

The authors declare no competing interests.