Nanowires on a Film for Photoelectrochemical Water Splitting

Nanowires-on-a-film structure (NFS) consisting of a dense array of semiconductor nano‐ wires on a conductive film has recently attracted much interest due to their wide range of applications in electronic, optoelectronic, electromechanic and electrochemical devices [15, 5, 25]. Thanks to their 3D geometry, NFSs provide various morphology-induced advantages resulting in improved devices performances. In addition, as the array of semiconductor NFSs can be grown on a conductive film, which realizes electrical connection between the nanowires, the integration of this nanostructured layer into devices is greatly facilitated.


Introduction
Nanowires-on-a-film structure (NFS) consisting of a dense array of semiconductor nanowires on a conductive film has recently attracted much interest due to their wide range of applications in electronic, optoelectronic, electromechanic and electrochemical devices [15,5,25]. Thanks to their 3D geometry, NFSs provide various morphology-induced advantages resulting in improved devices performances. In addition, as the array of semiconductor NFSs can be grown on a conductive film, which realizes electrical connection between the nanowires, the integration of this nanostructured layer into devices is greatly facilitated.
In this chapter, we review the applications of NFSs as efficient photoanodes in photoelectrochemical (PEC) water splitting cells. Recently, the efficiency of the PEC cells has been reported to be enhanced with the use of semiconductor NFSs [23,10,30,32]. The well-aligned geometry of a nanowire array serves as an anti-reflector in order to enhance light absorption in the structure. Also, an effective separation of the photo-generated carriers is achieved along the transverse direction of the nanowires due to the drastically reduced transport distance of minority carriers. Furthermore, the nanowire array possesses large surface areas that greatly enhance the chemical reaction of water splitting as compared to its smooth film counterpart. Lastly, the underneath film realizes electrical connection between the top nanowires. As a consequence, the semiconductor NFS offers a great potential for PEC water splitting applications. However, the fabrication of high quality NFS with functional semiconductor materials still remains a challenge. Single crystalline functional semiconduc-tor materials will ensure the photoanodes to have adequate semiconducting and electrochemical properties, leading to fulfill the PEC water splitting requirements [14,2], including: • Efficient light absorption • Effective carrier separation and transport • Suitable energy band position for redox reaction of water • Anti-photocorrosive property in the presence of water.
Furthermore, a controllable approach for the fabrication of the NFSs that enables production of large-area photoanodes at a low cost is not far from being achieved [34].
In the following sections, emphases are placed on the synthesis, property and PEC performance of the NFSs fabricated using various materials. The materials under review include TiO 2 , Fe 2 O 3 and ZnO, as they have the potential to achieve good performance in PEC water splitting. In addition, surface coating of the ZnO NFS with anti-photocorrosive ZnGa 2 O 4 photocatalyst layer is discussed as a mean to improve the stability of the ZnO based photoanodes. Finally, the remaining challenges for the NFS to be used as efficient photoelectrodes for PEC water splitting are presented. It should be noted that the NFS described here are based on arrays of one-dimensional (1D) nanowires, however, nanostructured photoanodes based on nanotubes, nanoplates and other nanostructures on a film should also provide improved PEC device performance.

TiO 2 NFSs for PEC water splitting
Since the phenomenon of water electrolysis on a titanium dioxide (TiO 2 ) electrode under UV light illumination was discovered by [9], TiO 2 has been considered as one of the most promising photocatalysts for energy conversion applications. Later in 1977, Frank and Bard achieved the decomposition of cyanide in water using TiO 2 material under UV light illumination [7]. This achievement further confirmed the photocatalytic behavior of TiO 2 material and leaded for the use of TiO 2 in environmental and pollution-degradation applications. Recently, extensive research has been conducted on the TiO 2 nanostructures in order to enhance device performances [3]. Versatile morphologies including TiO 2 nanowires, nanosheets, and nanotubes have been fabricated through different chemical and physical approaches. In many reports, the vertical alignment of the TiO 2 nano-architectures perpendicular to the substrates is found to be a crucial factor necessary to improve device performances [8,10]. In this section, we focus on the synthesis, characterizations and PEC performances of TiO 2 NFSs.

Synthesis of the TiO 2 NFSs
Titanium (Ti) is a relatively refractory metal with a high melting temperature of about 1670°C under air condition. This high melting temperature results in a low Ti vapor pressure of 1 0 −3 Torr at 1577 °C, which hinders the TiO 2 nucleation in the direct Ti vapor-phase deposition processes such as the physical vapor deposition (PVD) and the thermal chemical vapor deposition (CVD). In order to increase the Ti precursor concentration for the growth of 1D TiO 2 nanostructures, hydrothermal growth, sol-gel growth and metal organic chemical vapor deposition (MOCVD) using Ti organic compounds as precursors are often employed [3].
The fabrication of TiO 2 NFS via the hydrothermal growth method has been proved to be efficient due to its low-temperature processing (which helps to protect the bottom conductive film during the growth of 1D nanowires), mass production of the nanowires, and their easily controllable alignment. As reported by [10], a densely packed, vertically aligned and single crystalline TiO 2 rutile nanowire array was synthesized on a transparent conductive layer consisting of fluorine-doped tin oxide (FTO) via the hydrothermal method. The fabrication process is described in the following.
The FTO coated glass substrate was first cleaned by sonication in a mixture of ethanol and water for 30 minutes in order to remove the impurities on the surface of the substrate. Then, a seed layer of TiO 2 was synthesized on the cleaned FTO coated glass substrate by a firststep hydrothermal growth. The FTO substrate was soaked in a 0.025 M TiCl 4 with n-hexane solution for 30 minutes. The sample was then rinsed with ethanol and finally annealed at 500°C for 30 minutes in air to form a thin layer of TiO 2 on the surface of the substrate. The formation of the TiO 2 -seeded layer on the FTO substrate improved the control and the integrity of the later TiO 2 nanowires growth. Moreover, the TiO 2 -seeded layer on the FTO layer prevented the formation of electrical short circuits between the electrode and the electrolyte when used as photoanode in the PEC water splitting cell, which guaranteed the photocatalytic function of the TiO 2 photoanode. In the next fabrication step, the TiO 2 nanowires were grown on the TiO 2 -seeded FTO substrate with a typical hydrothermal growth procedure. The seeded FTO substrate was placed in sealed Teflon-lined autoclave containing 50 mL of n-hexane, 5 mL of HCL and 2.5-5 mL of titanium isopropoxide. After the reaction was performed at 150°C for a certain amount of time, the autoclave was cooled down to room temperature and finally the TiO 2 NFS was synthesized.

Characterization of the TiO 2 NFSs
Cross-sectional and top view scanning electron microscope (SEM) images of the synthesized TiO 2 nanowires array are shown in Fig. 1 a and 1b [10]. The TiO 2 nanowires array consisting of vertically aligned and tetragonal shaped nanowires is highly uniform and densely packed on the FTO coated glass substrate. The average diameter of the nanowires is ~5 nm and their length is up to ~4.5 μm. The length of the nanowires can be tuned from ~0.5 to ~5 μm, by modifying the synthesis conditions including the precursors ratio (titanium isopropoxide/nhexane), the reaction time and the thickness of TiO 2 seed layer. [10]  The synthesized TiO 2 nanowires array was characterized by grazing incidence angle X-ray diffraction (GIXRD) technique as shown in Fig. 1 d. The TiO 2 nanowires structure was identified as a rutile crystal from the reference XRD data measured with standard rutile TiO 2 powder. Furthermore, the high resolution transmission electron (HRTEM) image in Fig. 1d showed that the observed rutile phase TiO 2 nanowire is of a single crystal quality, having clear (110) lattice fringes.

PEC performance of the TiO 2 NFSs
The PEC performance of the synthesized TiO 2 NFS (as previously described) was examined in a three-electrode PEC water splitting cell [10]. The TiO 2 NFS was used as the working electrode, a saturated Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode. The electrolyte consisted of one mol/liter KOH solution with pH = 13.5. For the PEC test, TiO 2 nanowires with an average diameter of ~5 nm and average length of ~1.5 μm [10] were studied. The linear sweep voltammetry measurement was performed under a 100 W Xenon lamp illumination (300 nm < wavelength λ < 800 nm) with a bias swept from -0.8 to 0.6 V (vs. Ag/AgCl), as shown in Fig. 2 a. A photocurrent density of 0.35 mA/cm 2 was obtained at a bias of 1.23 V RHE [10]. As reported by Tilley et al. [23,10], decoration of a photoelectrode with metal nanoparticles as co-catalysts exhibited an enhanced yield in hydrogen generation. [10], reported treatments by cobalt (Co) and silver (Ag) performed on the TiO 2 NFS samples. The photocurrent density of ~0.35 mA/cm 2 measured on the undoped TiO 2 NFS increased to ~0.45 mA/cm 2 for the Co and Ag coated TiO 2 NFSs, as shown in Fig. 2 b. The enhancement of the photocurrent after Co and Ag treatments was likely related to the improvement in the extraction of photogenerated holes from the surface of the TiO 2 NFS to the electrolyte, through the Co and Ag nanoparticles [23,10]

Hematite NFS for PEC water splitting
Hematite (α-Fe 2 O 3 or iron oxide) has been widely investigated as a photocatalyst candidate for PEC water splitting [13,14,30] because of its abundance on Earth, its non-toxicity to environment and its stability in water under light illumination. In addition, a band gap between 2.1 and 2.2 eV of the hematite enables visible light absorption and thus makes this material promising for water splitting under solar light illumination. However, hematite has also some disadvantages such as: small light absorption coefficient, short lifetime of the excited-state carrier (10 -12 s), short hole diffusion length (2-4 nm), inefficient oxygen evolution reaction kinetics and improper position of the conduction band for hydrogen generation [23,16], limiting the hematite PEC performance. Different research groups made many efforts to address these issues [23,14,16,10]. To date, improved performances have been realized in the following areas: In the following section, we summarized the results obtained with the silicon-doped hematite nanowire-like array on a conductive film structure (Si-doped NFS-like hematite) decorated with IrO 2 nanoparticles [23] for PEC water splitting.

Synthesis of the Si-doped NFS-like hematite with IrO 2 surface decoration
Similarly to titanium, iron (Fe) has a relatively high melting temperature of ~1550 °C under air condition and thus hematite nanostructures growth by simple thermal CVD or PVD using solid-phase Fe-compound precursors remains difficult. Instead, solution-phase precursors, such as Fe(CO) 5 , Fe(AcAc) 3 and FeCl 3 , are used to provide sufficient amount of Fe precursors in the growth phase for the synthesis of hematite nanostructures via atmospheric pressure chemical vapor deposition (APCVD) or hydrothermal growth methods [23,14,16,10].
However, the undoped hematite nanostructure is inactive or very inefficient when used as a photoanode for PEC water splitting, because of its low conductivity and improper conduction band position for hydrogen generation. To improve the conductivity, hematite has been doped by different elements, such as Si, Ti, Sn, Al, Mg and Zn, leading to increase the electron density. To address the issue of improper conduction band position of hematite, hematite surface decoration treatments with co-catalysts nanoparticles, such as IrO 2 nanoparticles [18], were applied to increase the chemical reactions of water oxidation.
As reported by Tilley et al. [23], Si-doped NFS-like hematite sample coated with IrO 2 nanoparticles was fabricated and a large photocurrent density of ~3 mA/cm 2 was obtained at a bias of 1.23 V RHE under AM 1.5 illumination condition. In their synthesis process, a FTOcoated glass was used as the substrate for the NFS growth and put on an aluminum block heater. The substrate was maintained at 415 °C during the APCVD growth process. The precursors Fe(CO) 5 and tetraethoxysilane (TEOS) were carried to the FTO coated glass substrate by bubbling argon gas at fixed flow rates, calculated to be 3.5 mg/min for Fe(CO) 5 and 0.32 mg/min for TEOS. These two streams were mixed with an air flow of 2 L/min and vertically sprayed to the substrate. After 5 minutes, NFS-like Si-doped hematite was synthesized. According to Tilley's reports, the preparation of homogeneous films on large substrates required a continuously rotating substrate in a rotating belt APCVD reactor. Moreover, the improved PEC performance with Si-doped NFS-like hematite photoanode needed a first growth of an ultrathin SiO 2 interfacial layer of ~1 nm between hematite and FTO. It is very likely that this silica layer worked as an insulator layer to prevent the current leaking from the FTO layer to the electrolyte during the PEC experiments. Finally, the fabricated Si-doped NFS-like hematite was coated with ~2 nm IrO 2 nanoparticles using an electrophoresis deposition process. An increased photocurrent of ~3 mA/cm 2 was achieved with the IrO 2 nanoparticles coating, to be compared with a photocurrent of ~2 mA/cm 2 obtained for the Sidoped NFS-like hematite without co-catalysts [23].

Characterization of the Si-doped NFS-like hematite with IrO 2 decoration
The cross-sectional SEM images of the synthesized Si-doped NFS-like hematite are shown in Fig. 3 a and 3b [23]. The hematite nanowires with an average height of ~ 500 nm were densely packed on the FTO-coated glass substrate. The diameter of the hematite nanowires at the top area varied from 75 to 250 nm. It is observed that the surfaces of hematite nanowires were very rough and composed of nanoparticles with diameters as small as ~ 10-20 nm. As suggested by Tilley et al., the rough surfaces further increased the nanowire/electrolyte interface areas and also shortened the diffusion length of the photogenerated holes to the interface areas, contributing to the increase of PEC water splitting performance [23]. The microstructure and crystal quality of the NFS-like hematite samples with/without Si doping were further studied by XRD 2theta-omega scan technique. The XRD results are shown in Fig. 4. [13]. The undoped and Si-doped NFS-like hematite samples did not contain any other iron oxide phases than hematite, indicating a good experimental control over the hematite synthesis. The fact that the synthesized nanostructures are made of a singlephase hematite is important for the PEC water splitting applications, because other iron oxide phases such as Fe 3 O 4 always contain strong electron-hole recombination centers, thus leading to lifetime degradation of the holes used in the oxidation of water [13]. Another important feature shown in the XRD results is that the crystal orientation of the synthesized hematite varied with the synthesis conditions. In the case of the undoped NSF-like hematite sample, there is no dominant crystal orientation. The intensities of the different Xray diffraction peaks obtained with the undoped NSF-like hematite sample (see Fig. 4b) are similar to the XRD reference data measured on the randomly oriented hematite powders (see Fig. 4c). In contrast, the Si-doped NFS-like hematite sample showed a clear preferential [110] crystal orientation. These [110] oriented hematite nanowires likely have an enhanced conductivity compared to the randomly oriented hematite nanowires. In fact, the conductivity along the hematite [110] orientation was calculated to be of roughly four orders of magnitude higher than the conductivity along the other crystal orientations perpendicular to the [110] in hematite [12]. Therefore, the Si-doped NFS-like hematite with a preferential [110] orientation reported by Tilley et al. achieved a benchmark high photocurrent for PEC water splitting. Finally, the synthesized Si-doped NFS-like hematite was coated with IrO 2 nanoparticles (~2 nm in diameter) by the electrophoresis deposition process [23]. The X-ray photoelectron spectroscopy (XPS) results showed an iridium concentration of ~1% on the surface of the IrO 2 coated sample, confirming the presence of IrO 2 [23].

PEC performance of the Si-doped NFS-like Hematite with IrO 2 decoration
The PEC performance of the IrO 2 decorated Si-doped NFS-like hematite was studied using a three-electrode PEC cell [23]. In the reported experiment, the Si-doped NFS-like hematite sample was used as the working electrode, an Ag/AgCl in a saturated KCl solution electrode was used as the reference electrode and a platinum wire was used as the counter electrode. One mole/liter NaOH solution with pH = 13.6 was used as the electrolyte. The sample was irradiated with the standard AM1.5 G 100 mW/cm 2 solar simulator.
The chrono-amperometry measurement of the hematite photoanode at a bias of 1.23 V RHE with light on-off cycles shows that an on-off current behavior corresponding to the light onoff cycles is clearly observed (Fig. 5). The low dark current revealed that no/few chemical reactions occurred on the hematite photoanode without light illumination. A high photocurrent density of ~3 mA/cm 2 was obtained at a bias of 1.23 V RHE , as shown in Fig. 5. This photocurrent decreased within the 200-second chrono-amperometry scan. As suggested by Tilley et al., the photocurrent decrease was related to the detachment of the IrO 2 nanoparticles from the surfaces of the Si-doped NFS-like hematite due to a weak chemical bonding between the IrO 2 and the hematite. More robust method to attach the IrO 2 nanoparticles on hematite should further improve the PEC results, as was suggested by Tilley et al. [23]

ZnO NFS for PEC water splitting
Due to their unique optoelectronic and semiconducting properties, single crystal ZnO densely packed nanowire arrays have been intensively studied in the past few decades for their potential use in electronic, electromechanical, and photonic devices [29,20,5,26,4,32]. Recently, much research effort has been devoted to the fabrication of a high quality ZnO nanowire arrays and their application in PEC water splitting with the aim of taking advantage of ZnO material properties such as: 1. suitable band edge positions for PEC water splitting and 2. an increased conductivity when compared to TiO 2 and hematite for efficient charge transport [14,30,32].
For PEC water splitting applications, an efficient means permitting a good electrical connection of the nanowires through a conductive film, namely a ZnO NFS, is required. Yang [24]. However promising these results may be, further efforts to increase the stability of the ZnO NFSs in water under light illumination are required.
Since ZnO is chemically unstable in water under light irradiation, surface coating of the ZnO NFS with anti-photocorrosive and photocatalytic shell layers is one of the strategies to improve its chemical stability. However, the search for suitable anti-photocorrosive photocatalysts, which can grow easily on the ZnO nanowire surfaces with a high crystal quality, is a major concern. One possible candidate for the shell material is ZnGa 2 O 4 . Indeed, ZnGa 2 O 4 is conductive, has anti-photocorrosive ability in water and photocatalytic property for PEC water splitting [11]. In addition, single crystal quality ZnGa 2 O 4 can be synthesized via hightemperature chemical reactions using ZnO as the precursor material. Thus, surface coating of the ZnO NFS with conductive and anti-photocorrosive ZnGa 2 O 4 shells to form ZnO-ZnGa 2 O 4 core-shell NFS has been investigated [33]. With this structure, stable and efficient water splitting was achieved [32]. Further improvements are expected, including enhanced visible light absorption with doping, and efficient charge transfer to the electrolyte with the co-catalyst decoration of the ZnO nanowires.
In this section, we first review the synthesis of high crystal quality ZnO NFSs using different approaches. The realization of high quality ZnO nanowire arrays with efficient electrical connection is crucial for their subsequent use in high performance PEC cells. Then, the surface coating of the anti-photocorrosive ZnGa 2 O 4 shells on the ZnO NFS is presented. Finally, the electronic properties and a stable PEC performance of the single crystal ZnO-ZnGa 2 O 4 core-shell NFS are reported in detail.

Synthesis of the ZnO NFSs
A variety of physical and chemical approaches have been used to synthesize dense and single crystal ZnO nanowire arrays, including laser interference lithography [27], template-assisted wet-chemical growth [30,4], MOCVD [21] and CVD [29,5,34]. As evidenced by these reports, the choice of a suitable substrate is of highest importance to synthesize single crystal ZnO nanowires with an aligned geometry.
Sapphire is a chemically stable and relatively cheap ceramic material and thus has been widely used as substrates for the growth of ZnO nanowire arrays in various fabrication processes. Erdélyi, et al. reported the synthesis of a single crystal quality and mono-oriented ZnO nanowire array on a c-plane sapphire by the pulsed laser deposition [4]. In their experiment, the pre-growth of a high quality and size controlled ZnO nanoseeds is needed for the growth of the nanowires. The top-view SEM image of the ZnO nanowire array is shown in Fig. 6 a. Because sapphire is an insulator, the as-grown ZnO nanowires on the sapphire substrate are not electrically connected, preventing their efficient use in optoelectronic applications. Thus, high quality ZnO nanowire arrays grown on conductive films are a prerequisite to the development of advanced nanowire-based materials.
One strategy is to form ZnO NFS on metal substrates. However, the growth of nanowires on metal substrate presents many difficulties. Three major issues should be addressed to realize the functionality of the ZnO NFS using metal substrates. They are as follows: 1. In order to avoid the formation of a Schottky barrier at the interfaces between the ZnO nanowires and the metal substrate, the work function of the metal should be lower than the Fermi level of the ZnO. This requirement severely limits the possible candidates for the metal substrates.

2.
Metal substrates usually have large lattice mismatches with ZnO and, therefore, a pregrowth of lattice matched ZnO nanoseeds on the metal substrate is necessary to obtain vertically aligned ZnO nanowires. As a result, the fabrication process becomes intricate, hard to control and expensive.

3.
Another difficult issue related to the metal/ZnO interface is the existence of interface defects between the ZnO nanowires and the metal substrates, or even the formation of an insulator metal oxides layer at the nanowire and substrate interface that will block electron transport.

4.
Finally for the purpose of PEC applications, an insulator layer should be deposited over the metal substrate surfaces that are not covered by nanowires in order to prevent a short circuit between the metal substrate and the electrolyte when this structure is used as photoanode.
In summary, the ZnO nanowire array on a metal substrate is still difficult to fabricate with adequate functionalities for use as a photoanode in a PEC cell. A different strategy is to grow ZnO nanowire arrays on conductive semiconductor substrates. GaN matches the in-plane geometry and lattice constants with ZnO and, therefore, has been employed as a high-quality epitaxial substrate for the growth of ZnO nanowire arrays [5,27]. Fan et al. and Wei et al. synthesized orderly aligned, size-controlled and single crystalline quality ZnO nanowire arrays using single crystal GaN as substrates via a template-assisted growth process. In Fan's method, a porous anodic alumina membrane was used as a template mask to control the gold catalysts deposition on the GaN substrate, whereas in Wei's method, laser interference lithography technique was used for a pregrowth of high quality ZnO nanoseeds on the GaN substrate. According to their reports, the diameters and distributions of the synthesized ZnO nanowires are governed by the parameters of the porous alumina membrane and the laser interference lithography techniques, respectively. The top-view SEM images of the synthesized ZnO nanowires are shown in Fig 6  b and 6c. Advanced piezo-electric performances have been reported for the fabricated ZnO NFSs. For the use in PEC applications, special attention should be paid to the interface defects and the barrier formation between the ZnO and the GaN in order to realize an efficient electrical charge collection.
The fabrication of a high quality ZnO nanowire array on a ZnO film (ZnO NFS) using a single-step CVD process has been recently reported by our group [34]. The synthesis process enables the growth of a dense ZnO nanowires array on a ZnO film. The nanowires, as well as the film, have a single crystal domain texture quality and the nanowires are perfectly aligned over a large area, which comes as the result of the single crystal domain texture. The top-view SEM image of the fabricated array is shown in Fig 6 d. Efficient electrical connection is realized because nanowires and the underneath film layer are made of the same material ZnO [34]. Compared to other growth processes of ZnO NFSs, our one-step CVD process is easily controlled and cheap. However, the diameters and distributions of the ZnO nanowires cannot be as precisely controlled as in the template-assisted growth processes. The synthesized ZnO NFS offers an ideal template for the subsequent synthesis of a single crystal ZnO-ZnGa 2 O 4 core-shell NFS, which provides anti-photocorrosive property for the PEC applications.

Synthesis of the ZnO-ZnGa 2 O 4 core-shell NFSs
ZnGa 2 O 4 is a ternary metal oxide compound with a complicated crystal structure: one conventional ZnGa 2 O 4 unit cell is composed of 8 Zn atoms, 16 Ga atoms and 32 O atoms, arranged into a spinel structure [6]. The direct fabrication of ZnGa 2 O 4 nanowire array with a single crystal quality is very difficult and has not been reported yet. Thus, a two-step process to grow dense ZnO-ZnGa 2 O 4 core-shell NFS has been proposed recently [33]: 1. synthesis of the high quality ZnO NFS and 2. surface coating of the ZnO NFS with the ZnGa 2 O 4 shells (Fig. 7). A high quality ZnO NFS consisting of dense and vertically aligned nanowires on the ZnO film was first fabricated using an a-plane sapphire substrate through a simple thermal CVD process [32]. In this process, Au-coated a-plane sapphire was selected as the growth substrate. ZnO powder and graphite powder mixed with a 2:1 weight ratio were used as source material and placed at 1 cm upstream position away from the substrate in a tubular furnace. The furnace was operated at 1000 ºC for 30 minutes with the argon and oxygen gas mixture (5:1 in volume ratio) flowing through the tube at the working pressure of 50 mbar. At the end of the synthesis, the furnace cooled down naturally to the room temperature. The synthesized product consisted of a dense nanowires array on a ZnO film (ZnO NFS). Such a structure offers several advantages, namely,

1.
the ZnO nanowires grown on the ZnO film are less prone to the formation of defects at the interface between the nanowires and the film and thus an efficient electrical connection of the ZnO nanowires is realized; 2. the underneath ZnO film is stable at high temperature, which enables the subsequent growth of the ZnGa 2 O 4 shells in the second CVD step, without damaging the NFS.
In a second step, a CVD growth was performed on the ZnO NFS sample. The sample was put in a furnace at a temperature of 900ºC. Mixed precursors of Ga 2 O 3 , ZnO and graphite were placed at 11 cm upstream position from the sample. The temperature of the precursor zone was set to 1180 ºC. The carrier gases consisted of the argon and oxygen (10:1 in volume ratio) at the working pressure of 50 mbar. After a 30 minutes-long growth, the ZnO-ZnGa 2 O 4 core-shell NFS was obtained [32].

Characterization of ZnO-ZnGa 2 O 4 core-shell NFSs
The morphologies of the synthesized ZnO and ZnO-ZnGa 2 O 4 core-shell NFSs are presented in Fig. 8. The tilted-angle SEM image of Fig. 8a shows a highly uniform and densely ZnO nanowire array grown over a large area on the Au-coated a-plane sapphire substrate. The inset in Fig. 8a is an enlarged SEM image revealing the vertically aligned nanowires. The vertical alignment and smooth sidewalls of the nanowires can be clearly observed. The average diameter of the nanowires is ~70 nm and the average length is about 1 μm. Energy-dispersive X-ray analyses (EDX) performed on the nanowire array gave a Zn/O atomic ratio of 1/1, indicating the formation of the ZnO nanowires [34].
The tilted-angle SEM image of Fig. 8b shows a densely packed array of ZnO-ZnGa 2 O 4 coreshell nanowires homogeneously arranged over large areas. The inset in Fig. 8b is an enlarged SEM image of the vertically aligned ZnO-ZnGa 2 O 4 core-shell nanowires. The initial vertical alignment of the ZnO nanowires is well maintained in the ZnO-ZnGa 2 O 4 core-shell nanowire sample. The average diameter of the ZnO-ZnGa 2 O 4 core-shell nanowires is ~100 nm. EDX analysis performed on the surfaces of the core-shell nanowire gave a Zn/Ga/O atomic ratio of ~14/28/58 (~1/2/4), indicating the formation of the ZnGa 2 O 4 shells. [32] In this work, a nanowires-on-a-film structure  This layer provides an improved in-plane geometry and lattice constant matching over the a plane of sapphire and, therefore, helps sustaining the growth of the high quality (0001) ZnO [34].

Electronic properties of ZnO-ZnGa 2 O 4 core-shell NFSs
It is of first importance for the development of PEC water splitting cells to understand the electronic properties of the ZnO-ZnGa 2 O 4 core-shell NFS photoanode in the electrolyte solution. The carrier density and the flatband potential of the ZnO-ZnGa 2 O 4 core-shell NFS were measured by electrochemical impedance spectroscopy [32]. For this purpose, a solution of 0.5 M NaClO 4 buffered (pH of 7.0) was used as an electrolyte, Ag/AgCl electrode in saturated KCl solution as the reference electrode, a Pt wire as the counter electrode and the ZnO-ZnGa 2 O 4 core-shell NFS sample as a working electrode. Prior to the electrochemical impedance measurement, N 2 gas was bubbled for 10 minutes to get rid of O 2 in the electrolyte solution. The electrochemical impedance analysis was then performed for a bias ranging from -1 to 1 V (versus Ag/AgCl), at the frequency of 1 kHz under dark condition. The results of this experiment are presented in a 1/C 2 vs. V plot (Fig. 10a).
The carrier density and the flatband potential of the cylindrical nanowires were quantified using the model proposed by [17]. Solving the Poisson equation with a cylindrical capacitance approximation at the nanowire/electrolyte interfaces, we obtain: where R is the radius of the nanowires, x the central radius of the quasi-neutral region in the nanowires, C S the capacitance per unit area of the nanowires, V the applied bias between the electrodes, V FB the flatband potential of the nanowire at the nanowire/electrolyte interface, q the unit electron charge, ε 0 the permittivity of vacuum and ε the relative dielectric constant of ZnO. The results of the fitting of the measured 1/C 2 vs. V curve according to the Mora-Seró model are shown in Fig. 10  The flatband potential of the ZnO-ZnGa 2 O 4 NFS sample at the nanowire/electrolyte interface is another important parameter when analyzing the photoanodes properties for water splitting in a PEC cell. Typically, a negative flatband potential and an upward band bending occur at the interface between an n-type semiconductor and an electrolyte, due to their Fermilevel difference [22]. A schematic illustration of the band bending with a contact energy barrier at an idealized interface between a n-type semiconductor in equilibrium with an electrolyte is shown in Fig. 10 c. The flat-band potential of the ZnO-ZnGa 2 O 4 core-shell NFS sample was estimated to be about -0.4 V (versus NHE). The cathodic flatband potential of the ZnO-ZnGa 2 O 4 NFS sample provides a suitable energy band position for PEC water splitting.

PEC performances of ZnO-ZnGa 2 O 4 core-shell NFSs
The PEC performance of the ZnO-ZnGa 2 O 4 NFS sample used as a photoanode was studied in a 0.1 M Na 2 SO 4 electrolyte solution at pH = 6 under illumination with a 300 W Xenon lamp [32]. An Ag/AgCl electrode in a saturated KCl solution was used as the reference electrode and a Pt wire used as the counter electrode. Fig. 11a shows the current-voltage curve for the ZnO-ZnGa 2 O 4 nanowire array at a bias ranging from -0.8 V to 1 V (versus Ag/AgCl) with light on-off cycles, indicating a good photoresponsibility of the NFS sample used as photoanode.
The flatband potential of the NFS sample is equivalent to the onset potential of the anodic photocurrent, as shown to be ~ -0.35 V vs. NHE with an estimated error of ±0.1 V in the inset of Fig. 11a [32]. This value of the onset potential agreed well with the flatband potential of -0.4 V estimated from the fit of the electrochemical impedance data in Fig. 10 b.
The amperometric study of the ZnO-ZnGa 2 O 4 core-shell nanowire array was performed at a fixed bias of 1.23 V RHE under on-off illumination cycles. The obtained data are plotted in the form of current-time curve (Fig. 11b) The on-off current cycles presented a very good reproducibility and anti-photocorrosion stability as shown in Fig. 11 b. Therefore, the core-shell NFS demonstrated an improved stability and a large photocurrent of 1.2 mA/cm 2 when used as the photoanode in the PEC water splitting cell [32].

Conclusions
Recent advances in the development of densely packed and single crystalline quality semiconductor NFSs with a controllable doping level offer promising opportunities for the future PEC water splitting applications. In this type of nanostructured photoanodes, an improved light absorption, enhanced charge separation and an increased surface area for chemical reaction are obtained due to the shell layer of the semiconductor nanowire array of the NFS. Further, the film underneath the nanowires realizes an efficient electrical connection of the semiconductor nanowire array, thus facilitating the direct use of the NFS as the photoanode in PEC water splitting applications. However, many difficulties still remain to be resolved. Two essential issues related to the reliability and the efficiency needs to be addressed for all the PEC water splitting systems. In this Chapter, the NFS-based photoanodes made of different semiconductor photocatalysts have been reviewed and their advantages and the remaining challenges have been discussed in detail. The TiO 2 NFS photoanode has an excellent chemical stability in water, but its large bandgap and inefficient doping limit the efficiency in the PEC applications under solar illumination. The hematite NFS photoanodes have a bandgap which permits a good photoresponsivity under the illumination with the visible part of the solar spectrum and a good chemical stability in water. However, the large resistance and inadequate energy band position of the hematite render this NFS inefficient for the PEC applications without any further efficient doping and the co-catalysts dec-oration. An efficient doping and the co-catalyst decoration of the hematite NFS have not yet been achieved to improve its performances and obtain a stable operation in water splitting PEC. The ZnO-ZnGa 2 O 4 core-shell NFS photoanode offers an enhanced conductivity over TiO 2 and hematite, as well as anti-photocorrosive property in water. Thus this material offers stable and efficient operation of water splitting PEC. However, the ZnO-ZnGa 2 O 4 coreshell NFS only responds to the UV light, which results in a low efficiency under solar light illumination. Further work is now in progress to achieve an efficient and stable solar water splitting with doped ZnO-ZnGa 2 O 4 core-shell NFSs. Besides the photocatalyst materials already fabricated in the form of NFSs reviewed in this Chapter, it is worth noting that a large variety of alternative photocatalysts exist, which can permit to grow nanostructures for high efficiency PEC applications. Finally, still more progress is expected to be made to realize an efficient and stable solar water splitting system permitting to achieve a new, sustainable and environmentally friendly hydrogen production method, contributing to a better daily life of human beings in the near future.