Self-driven water purification and simultaneous hydrogen generation by all-nanowire photocatalytic fuel cell with enhanced mass and electron transfer

0.   Introduction

With the proposed sustainable development, there is an urgent need for new water treatment technologies with low energy consumption and even energy generation capability. By far, several techniques have been explored to recover energy from the wastewater treatment processes, such as fermentation methane production, bio-fuel cells (BFC), and photocatalytic fuel cells (PFC) ^[1-3]. Among them, light-driven processes, which use inexhaustible solar energy to simultaneously remove pollutants from water and generate new energy, are attracting more and more attention^[4-6]. In this protocol, the chemical energy of harmful contaminants is converted into electricity or hydrogen energy, which should be extremely important for reducing the energy consumption of water treatment. Unfortunately, the photoconversion efficiency of prevailing nanoparticle-based planar photoelectrodes is restricted by the limited contact between the contaminant and electrodes. The performance of a photoelectrode is also dependent on the light adsorption capability of semiconductor materials and the interfacial separation of charge carriers^[7]. To improve the reaction efficiency, a porous structure of photoelectrodes is preferred to ensure sufficient diffusion of reactants in the inner channels for mass transfer^[8]. However, the excessive accumulation of nanoparticles forms numerous grain boundaries, which inevitably induce the interfacial loss of photo-generated charges^[9]. A new material strategy is urgently desired to overcome the trade-off between mass diffusion and charge transfer around heterogeneous photoelectrodes.

Assembled nanoarchitecture with well-controlled locations, orientation, and spacing across multiple length scales has been confirmed as one of the ideal nanostructured materials for versatile applications^[10]. Particularly, one-dimensional (1D) nanowire arrays show several advantages as electrodes for photocatalytic energy conversion. The space between oriented nanowires can accommodate a large volume of reaction solution for mass diffusion^[11-12]. More importantly, since the ends of nanowires are intimately connected to the conductive substrate, the direct transportation of electrons along the 1D pathway enables efficient charge separation. Thus, assembled nanowires and their hierarchical structures provide a promising avenue to explore highperformance photoelectrodes with fast mass and charge transfer, which has been met with some success in photocatalytic reactions^[13-14]. For example, Zhang et al. employed an Au-decorated ordered Si nanowire array as photocathodes for nitrate reduction reaction, achieving a high Faraday efficiency of 95.6% to ammonia at 0.2 V (vs RHE)^[15]. Yang et al. found that TiO₂ nanowires showed an increase in photocurrent with length, and a maximum photocurrent density of 0.73 mA·cm^-2 was measured (1.5 V vs RHE) for 1.8 μm long nanowires under AM 1.5G simulated sunlight illumination. The atomic layer deposition of epitaxial rutile TiO₂ further enhanced the photocatalytic activity by 1.5 times^[16]. Recently, we advanced this hypothesis by constructing microfluidic-enhanced three-dimensional (3D) photoanodes and modulating their interfacial structure for photoelectrochemical applications. The flow of reaction solution through electrode channels via the microfluidic mode induced a localized turbulence effect, while the defect modulation of semiconductor materials significantly decreased the interfacial energy barrier for charge transfer, contributing to the efficient degradation of harmful contaminants in water^[17-18]. Despite these encouraging results, it is expected to develop a microfluidic PFC system with dual photoelectrodes for simultaneous water purification and energy production.

This paper constructed an all-nanowire photocatalytic fuel cell by integrating microfluidic TiO₂ nanowire photoanodes with Si nanowire photocathodes. We ensured efficient mass transfer in the electrode channels by depositing TiO₂ nanowires on 3D hierarchical carbon cloth substrates and in-situ etching silicon wafers into aligned nanowires, respectively. The facile modulation of oxygen vacancies in TiO₂ and the surface deposition of Pt cocatalysts on Si nanowires significantly enhanced the charge separation in photoelectrodes. Profiting from the improved mass and electron transfer, our all-nanowire microfluidic PFC device could simultaneously degrade the harmful contaminant molecules and generate hydrogen energy for self-sustaining water purification.

1.   Experimental

1.1   Materials and reagents

All chemicals were used as received, with details provided in the Supporting information. Carbon cloth (WOS 1009) used was provided by Taiwan CeTech. Deionized water (resistivity > 18.2 MΩ·cm) used for all experiments was purified by using Milli-Q Plus system.

1.2   Fabrication of TiO₂ nanowire-based microfluidic photoanodes

To construct the 3D microfluidic photoanodes, a hydrothermal method was adopted to grow TiO₂ nanowire arrays on carbon cloth. Briefly, the substrates with the size of 20 mm×40 mm×0.25 mm were pretreated by sonication in an acetone/ethanol mixture solution and thermal-activated at 500 ℃ for 2 h in a muffle furnace. A piece of carbon cloth was dipped into the 70 mmol·L^-1 tetrabutyl titanate seed solution in isopropanol for 30 s, which was then annealed at 400 ℃ for 2 h in the air. The seeded carbon cloth was held by a self-made PTFE (polytetrafluoroethylene) holder and placed vertically in a 50 mL autoclave containing 20 mL 6 mol·L^-1 HCl and 300 μL tetrabutyl titanate. The hydrothermal reaction was conducted at 150 ℃ for 8 h, and the final product was annealed in the air at 500 ℃ for 2 h^[3]. The 3D hierarchical channels of this sample would facilitate the diffusion of the microfluidic solution via flow-through mode; thus the corresponding sample was denoted as m-TiO₂. For comparison, TiO₂ nanowire arrays were also deposited onto FTO glass substrates to achieve the planar photoanodes with a flow-by diffusion mode, and the corresponding sample was denoted as p-TiO₂.

To improve the electron transfer capability of microfluidic photoanodes, a thermal reduction process was conducted to create oxygen vacancies in TiO₂. Briefly, an as - synthesized m - TiO₂ photoanode was annealed in a tube furnace at 500 ℃ for 2 h under an Ar/H₂ (9∶1) mixture gas with a flow rate of 50 mL·min^{-1 [9, 19]}. This photoanode with promoted mass and electron transfer was denoted as m-Vo-TiO₂.

1.3   Fabrication of Si nanowire photocathodes

A metal-catalyzed electroless etching method was used to fabricate Si nanowire-based photocathodes. After being pretreated by the H₂SO₄/H₂O₂ (3∶1, V/V) solution for 5 min and the mass fraction of 5% HF solution for 10 min, p-type silicon wafers with the size of 15 mm×30 mm×0.25 mm were successively placed in a mass fraction of 2% HF solution containing 20 mmol·L^-1 AgNO₃ for 30 s and an HF/H₂O₂/H₂O (3∶1∶10, V/V) solution for 40 s. Any Ag remaining on the surface of the etched sample was removed by soaking in a mass fraction of 40% HNO₃ solution for 5 min, forming the p-Si photocathodes with a nanowire array morphology^[20]. To facilitate the charge transfer, Pt nanoclusters were further deposited on the surface of p-Si nanowires by a photo-deposition method. In detail, as-synthesized p-Si photocathodes were put in an aqueous 10 mmol·L^-1 H₂PtCl₆ solution, which was irradiated by a UV lamp for 1 h to obtain Pt/p-Si^[21-22].

1.4   Characterization

The phase structure of the samples was identified by X-ray diffraction (XRD, Bruker D8, Germany). Cu Kα radiation was used as the X-ray source (wavelength: 0.154 06 nm), the voltage was 40 kV, the current was 40 mA), and the scanning range was 10°-70° with a scanning rate of 10 (°) ·min^-1. The morphology was observed by a scanning electron microscope (SEM, Hitachi S-4800, 15 kV) and a transmission electron microscope (TEM, JEOL-2010, 200 kV). Raman spectroscopy was performed with a Raman spectrometer (Labram Aramis model manufactured by Horiba Jobin Yvon T64000) using a 532 nm He-Ne laser. An X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB 220i-XL (ThermoFisher) system. The content of metals in the material was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES, 5110VDV Agilent). The electron spin resonance (ESR) measurement was conducted to confirm the formation of oxygen vacancies in photoanode samples. The UV-Vis diffuse reflectance spectra (DRS) were collected by a UV-Vis spectrophotometer (Hitachi U3900, Japan) equipped with an integration sphere (ISR 2600) in the interval ranging from 200 to 600 nm, using BaSO₄ as a reference.

1.5   Photoelectrochemical performance of photoelectrodes

A three - electrode system was first employed to evaluate the photoelectrochemical characteristics of electrode materials, using the photoelectrodes, Ag/AgCl, and platinum foil as the working, reference, and counter electrodes, respectively. The electrode potential and current were controlled by an electrochemical workstation (CHI660E, Chenhua Shanghai). A 500 W Xe lamp (CEL-S500) equipped with an AM 1.5G filter was used as a light source for photoelectrochemical measurements. Mott-Schottky measurements were performed at a frequency of 1 000 Hz. Electrochemical impedance spectroscopy (EIS) was used between 100 kHz and 0.1 Hz, with the amplitude of the AC (alternating current) set at 10 mV. The applied bias-photonto-current conversion efficiency (ABPE) of photoelectrodes was calculated by the following equation^[23]:

Where J, V_b, and P_light are photocurrent density (mA·cm^-2), applied external bias (vs RHE), and light intensity (100 mW·cm^-2), respectively.

As - synthesized m - Vo - TiO₂ and p - Si nanowire arrays on substrates were separately employed as photoanodes and photocathodes to construct the dual-photoelectrode photocatalytic fuel cells. The simultaneous water purification and hydrogen generation were carried out in a glass H-cell with an ion-exchange membrane to separate the two compartments as shown in Fig. S1. Methyl orange, rhodamine B, and bisphenol A with a mass concentration of 10 mg·L^-1 were used as model organic pollutants. For each test, 20 mL of 0.1 mol·L^-1 Na₂SO₄ solution was used as the electrolyte. After an adsorption-desorption equilibrium in the dark for 30 min, 6 cm² of the photoelectrodes were exposed to AM 1.5G irradiation. The generation of H₂ per hour was determined using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD)^[24]. The maximum absorption peaks at 470, 554, and 276 nm were monitored by a UV - Vis spectrophotometer to estimate the changed concentrations of methyl orange^[25], rhodamine B^[26], and bisphenol A^[27], respectively. The degradation efficiency was calculated by the following formula:

where ρ₀ and ρ_t represent the instantaneous mass concentration of organic matter at the initial time and at time t, respectively.

2.   Results and discussion

2.1   Structure and properties of TiO₂ nanowire-based photoanodes

The structure and morphology of TiO₂-based photoanodes were studied by XRD and SEM. Fig. S2 suggests that all the deposited products on carbon cloth substrates are well indexed to pure TiO₂ (PDF No. 21-1276) ^[28]. The diffraction peaks at 2 θ of ca. 27.5°, 36.1°, 41.2°, and 54.3° correspond to the (110), (101), (111), and (211) planes of the rutile phase. Based on the SEM images, the blank carbon cloth had a 3D interdigitated structure composed of microfiber bundles (diameter of ca. 8 μm) (Fig. 1a). After the hydrothermal reaction, the surface of the carbon fibers was covered by abundantly aligned TiO₂ nanowires, forming a porous m - TiO₂ electrode with a 3D hierarchical structure (Fig. 1b). Although TiO₂ nanowires can also grow on FTO glass substrates, they are closely packed with little space on the sides of each nanowire (Fig. 1c), thus exhibiting a poor capability for mass transfer.

Figure 1

Figure 1.  SEM images of (a) carbon cloth (Inset: corresponding enlarged image), (b) m-TiO₂, and (c) p-TiO₂; TEM images of (d, e) m-TiO₂, and (f) m-Vo-TiO₂; (g) Raman spectra of m-TiO₂ and m-Vo-TiO₂; High-resolution (h) Ti2p and (i) O1s XPS spectra of m-TiO₂ and m-Vo-TiO₂

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TEM image of m - TiO₂ showed that the length of the TiO₂ nanowires was approximately 2 μm and the diameter was about 100 nm (Fig. 1d). High-resolution observation suggests the crystalline nature of these nanowires, and the lattice fringe of 0.32 nm corresponds to the (110) plane of rutile TiO₂. Compared to the pristine TiO₂ nanowires, hydrogen treatment leads to the formation of disordered profiles and abundant cavities in the m - Vo - TiO₂ nanowires (Fig. 1f). This could be the reaction of hydrogen with lattice oxygen resulting in the absence of oxygen atoms, which is consistent with the characteristics of defective TiO₂ materials.

The electronic structure of m-Vo-TiO₂ was investigated using Raman and XPS spectra. As shown in Fig. 1g, bands at 1 342 and 1 594 cm^-1 correspond to the D band and G band of carbon fibers, respectively. The appearance of peaks at 142, 442, and 610 cm^-1 are assigned to the B_1g, E_g, and A_1g vibration modes of rutile TiO₂^[29]. Compared to pristine TiO₂, no noticeable changes in the peak position or intensity were observed for the Raman spectrum of m-Vo-TiO₂. Thus, hydrogen treatment exhibits an ignorable impact on the dominant structure of TiO₂. In the XPS spectrum (Fig. 1e) of m-Vo-TiO₂, the Ti2p peaks shift toward lower binding energies compared to those of pristine TiO₂. This evidences the formation of oxygen vacancy defects caused by hydrogen annealing, thereby releasing free electrons to reduce Ti⁴⁺ into Ti³⁺. ESR analysis further confirms the formation of oxygen vacancies in m-Vo-TiO₂ (Fig.S3).

Based on the UV-Vis diffuse reflectance spectra (Fig. S4), the formation of oxygen vacancies in m - Vo - TiO₂ exhibits an ignorable influence on the light absorption capability. Consequently, electrical impedance measurements were carried out to reveal the contribution of oxygen vacancies to the charge transfer behaviors^[27]. The charge separation capability of photoanodes was evaluated using Mott - Schottky plots. In Fig. 2a, all samples exhibited positive slopes, signifying the n-type semiconductor feature of TiO₂. The flat potential (E_FB) and the donor density of photoanodes can be estimated by the x-intercept and slope in the linear region. Clearly, m-Vo-TiO₂ presented a much smaller slope compared to pristine m - TiO₂. This indicates that oxygen vacancies in TiO₂ can greatly increase the charge separation efficiency, thus inducing a higher concentration of charge donors. Moreover, the E_FB of m - Vo - TiO₂ shifted to a more negative potential compared to that of m-TiO₂, indicating a higher degree of band bending for charge separation. The charge transfer properties of TiO₂-based photoanodes were further evaluated by impedance (EIS) measurements. According to the Nyquist plots shown in Fig. 2b, m-Vo-TiO₂ displayed a much smaller Randle circuit than pristine m-TiO₂ without defects, signifying an improved charge transfer ability of hierarchical m-Vo-TiO₂ photoanodes.

Figure 2

Figure 2.  (a) Mott-Schottky plots, (b) EIS plots, (c) transient photocurrent curves, and (d) ABPE of m-TiO₂ and m-Vo-TiO₂

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The performance of the TiO₂ nanowire array photoanodes was evaluated using a three-electrode PEC system, where a Pt foil as the counter electrode and Ag/AgCl as the reference electrode. Based on the photocurrent-voltage curves in Fig. 2c, the PEC response of pristine m-TiO₂ was very low due to the poor charge separation. However, the introduction of oxygen vacancies into m-Vo-TiO₂ significantly improved the PEC property, inducing an 8-fold increase in the photocurrent value^[28]. Profiting from the hierarchical and defective structures, as - synthesized m - Vo - TiO₂ exhibited an ABPE efficiency of 0.34%, which was 8.5 times higher than that of pristine m - TiO₂. All these results confirm the construction of TiO₂ nanowire - based photoanodes with great mass and electron transfer capabilities for PFC applications.

2.2   Construction of Si nanowire - based photocathodes

We then attempted to construct nanowire - based photocathodes by the Ag - catalyzed etching of silicon wafers. As shown in Fig. 3a, a dense nanowire array was arranged vertically on the Si substrate after the etching procedure. According to the cross - sectional image (Fig. 3b), the average length of the nanowires was 300-400 nm. Similarly, the gaps between nanowires offer enough space for substance diffusion, which would surely improve the photoelectric response property. In the TEM image (Fig. 3c), the lattice spacing of 0.32 nm corresponds to the (111) crystal plane of p-type Si. To improve the charge separation, a photo deposition procedure was conducted to immobilize Pt cocatalysts onto Si nanowires. As confirmed by TEM (Fig. 3d), Pt nanoparticles with a size of 30 nm were successfully deposited onto the surface of nanowires and uniformly loaded on the upper part of Si nanowires. The lattice space of 0.23 nm corresponds to the (111) crystal plane of Pt (Fig. 3e). According to the ICP-OES analysis, the loading amount (mass fraction) of Pt on Si cathodes was determined to be 0.4%. The contribution of nanowire array morphology and Pt cocatalysts to the PEC performance of silicon was investigated. As shown in Fig. 3f, the pristine p-Si exhibited only a weak cathodic response under light irradiation, whereas the nanowire structure showed a significantly improved photoelectric response, which should be attributed to the increase in the specific surface area of the electrodes. Furthermore, the deposition of Pt cocatalysts further enhanced the PEC performance of nanowire - based photocathodes, inducing a current density of -3.86 mA·cm^-2 at -0.38 V (vs RHE).

Figure 3

Figure 3.  (a) Plane and (b) cross-section SEM images of p-Si; TEM images of (c) p-Si and (d, e) Pt/p-Si; (f) Photocurrent-voltage curves of different photocathodes

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2.3   Performance of photoelectrodes for PEC reactions

The prepared TiO₂ and Si nanowire photoelectrodes were evaluated for their activities in photocatalytic pollutant degradation and hydrogen generation, respectively (Fig. 4a). When employing the etched Si nanowires as photocathodes, the PEC device can continuously catalyze the splitting of water into hydrogen gas, with a hydrogen generation rate of 26 μmol·h^-1. In the above discussion, the deposition of Pt co-catalysts enhanced the water-splitting capability of Si nanowire-based photocathodes, inducing a 0.8-fold (47 μmol·h^-1) improvement in the hydrogen generation rate. We then combined the Si nanowire-based photocathodes with TiO₂ nanowire-based photoanodes to construct a sustainable PFC system for the simultaneous degradation of organic contaminants and hydrogen generation. To demonstrate the synergistic effect, Pt-plated and platinized Si nanowire photocathodes (m-Vo-TiO₂‖Pt and m-Vo-TiO₂‖Pt/p-Si, respectively) were integrated with TiO₂-based photoanodes to construct the single and dual photoelectrode systems, respectively. In both cases, an applied potential of 0.8 V (vs RHE) was used to degrade the methyl orange solution with a concentration of 10 mg·L^-1 (Fig. 4b). Obviously, the dual photoelectrode system exhibited a 1.5 times higher pollutant degradation rate compared to the single photoanode protocol. Based on the current density-time curves in Fig. 4c, our PFC system can work for a long time, thus offering a sustainable way to purify water with low energy consumption. We then used the nanowire-based dual-photoelectrodes for the synergistic degradation of various organic contaminants, such as methyl orange, rhodamine B, and bisphenol A. As shown in Fig. 4d-4f, the integration of m-Vo-TiO₂ photoanodes with Pt-catalyzed Si photocathodes always exhibited the best catalytic degradation rates compared with planar TiO₂ photoanodes and the hierarchical system without oxygen vacancies. This suggests that interfacial mass and charge transfer play a significant role in enhancing the performance of photoelectrodes.

Figure 4

Figure 4.  (a) Hydrogen generation rate over p-Si and Pt/p-Si photocathodes; (b) Photodegradation of methyl orange over m-Vo-TiO₂‖Pt and m-Vo-TiO₂‖Pt/p-Si photocathodes; (c) Change of current density over time in the m-Vo-TiO₂‖Pt and m-Vo-TiO₂‖Pt/p-Si photoelectrode systems; Performance of photocatalytic fuel cell for degrading (d) methyl orange, (e) rhodamine B, and (f) bisphenol A under a bias potential of 0.8 V (vs RHE)

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To verify the application of nanowire-based PFC in solar-driven sustainable water treatment, the simultaneous pollutant degradation and hydrogen generation over a dual-photoelectrode system was conducted without external bias. Fig. 5a and 5b show that the PFC device with improved mass and charge transfer can effectively degrade methyl orange and rhodamine B pollutants under illumination, especially for rhodamine B with a current value up to 0.6 mA·cm^-2. Due to differences in oxidizability, the hydrogen generation rate of photocathodes was dependent on the types of organic matter. Combining with Fig. S5 the continuous decomposition of RhB contaminants into small molecule intermediates ensured a mineralization ratio of ca. 20% after 2 h and a highest hydrogen generation rate of 88 μmoL·h^-1, which was 2.8 and 1.4 times higher than the pure water splitting and BPA-promoted reaction, respectively (Fig. 5c). More importantly, the degradation of RhB contaminants enables the continuous generation of hydrogen energy. As can be seen from Table S1, the activity of our all-nanowire system even exceeds that of several reported protocols. After five cycles of the synergistic reaction, we observed only a slight decrease in performance, which was consistent with the morphology and structure characterizations of the recycled photoelectrodes (Fig.S6 and S7). This indicates that our all-nanowire system exhibited good stability for photoelectrochemical applications (Fig. S8). The effect of photoanode structure on the synergistic hydrogen generation over dual photoelectrodes was investigated in Fig. 5d. Owning to the fast mass and charge transfer, the hierarchical structure of m-Vo-TiO₂ photoanodes contributes to the twice enhanced hydrogen generation activity compared to that of m-TiO₂ photoanodes. For a better understanding, a similar strategy was used to construct hierarchical WO₃-based photoanodes (Fig. S9), which were integrated with Si nanowire-based photocathodes for synergistic applications. Although the charge transfer at the electrode interface could be promoted by creating oxygen vacancies, the hydrogen generation capability of the WO₃-based dual-photoelectrode system was much lower than that of the TiO₂-based one. Considering the differences in the electronic structure of WO₃ and TiO₂, this should be attributed to the more positive onset potential of WO₃, which cannot match silicon photocathodes. Overall, simultaneous and efficient water decontamination and water splitting were realized by designing the hierarchical structure and modulating the charge transfer pathways of photoelectrodes, which can offer a sustainable way for environmental remediation and energy production.

Figure 5

Figure 5.  (a) Degradation curves of methyl orange and rhodamine B, (b) photocurrent response curves, and (c) hydrogen generation rates in the m-Vo-TiO₂‖Pt/p-Si system without external bias; (d) Performance of PFC for hydrogen generation by employing different types of photoanodes

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Based on the above results, the working principle of the simultaneous degradation of pollutants and hydrogen generation by a full nanowire photocatalytic fuel cell is analyzed in Fig. 6. The dual photoelectrode PFC system consists of two photocatalytic semiconductor electrodes, including an n-type photoanode (m-Vo-TiO₂) and a p-type photocathode (Pt/p-Si). When the system is illuminated with light, both photoelectrodes generate electron-hole pairs. The driving force of the PFC is the difference in the Fermi levels between the two photoelectrodes, with the Fermi level of the photoanode being more negative than that of the photocathode. Electrons are transferred through the external circuit to the photocathode for hydrogen generation, and the loading of platinum as a co-catalyst on the surface of Si nanowires further enhances electron enrichment. The preserved holes at the photoanodes are consumed by the in-situ degradation of pollutants. Oxygen vacancies in TiO₂ significantly improve charge mobility, promoting the charge separation efficiency of photoelectrodes. Meanwhile, fast mass transfer among the nanowire matrix of dual-photoelectrodes enhances the interfacial exchange between reactants and electrodes, improving the utilization efficiency of charges. As a result, for our all-nanowire PFC, the TiO₂ photoanodes efficiently degrade the pollutants, and the Si photocathodes produce hydrogen energy.

Figure 6

Figure 6.  Working principle of an all-nanowire photocatalytic fuel cell for simultaneous water purification and hydrogen generation

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The formation of aligned TiO₂ and Si nanowires on the surface of carbon cloth and silicon wafer ensures sufficient spaces between the nanowires for the diffusion of pollutant molecules into the electrodes. With a hierarchical structure, fast mass transfer in TiO₂-based photoanodes induced a turbulent flow field, significantly improving the degradation efficiency via the microfluidic effect. The effective consumption of photo holes by oxidative reactions enables efficient charge separation, allowing great enhancement in the hydrogen generation performance of photocathodes. The band alignment between TiO₂ photoanodes and Si photocathodes finally contributed to self-driven water purification and simultaneous hydrogen generation for sustainable development.

3.   Conclusions

In summary, we have demonstrated a dual photoelectrode photocatalytic fuel cell comprising TiO₂ nanowire arrays on 3D hierarchical carbon cloth as photoanodes and aligned Si nanowires on a silicon wafer as photocathodes. Thanks to the channels between oriented nanowires and the modulated defect/cocatalyst active sites, both the photoanode and the photocathode showed excellent mass diffusion and electron transfer for photoelectrochemical applications. Different from conventional planar photoelectrodes, our PFC could efficiently degrade several harmful organic compounds in water via a microfluidic flow - through mode. The consumption of photoelectrons by oxidation synchronously promoted the photocatalytic reduction of water into hydrogen gas, thus the all-nanowire PFC presented in this paper could provide a new prototype to efficiently degrade contaminants in the water while performing energy recovery for sustainable development.

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