Exploiting the Bragg Mirror Effect of TiO2 Nanotube Photonic Crystals for Promoting Photoelectrochemical Water Splitting

Abstract

Exploiting the Bragg mirror effect of photonic crystal photoelectrode is desperately desired for photoelectrochemical water splitting. Herein, a novel TiO₂ nanotube photonic crystal bi-layer structure consisting of a top nanotube layer and a bottom nanotube photonic crystal layer is presented. In this architecture, the photonic bandgap of bottom TiO₂ nanotube photonic crystals can be precisely adjusted by modulating the anodization parameters. When the photonic bandgap of bottom TiO₂ nanotube photonic crystals overlaps with the electronic bandgap of TiO₂, the bottom TiO₂ nanotube photonic crystal layer will act as a Bragg mirror, leading to the boosted ultraviolet light absorption of the top TiO₂ nanotube layer. Benefiting from the promoted UV light absorption, the TiO₂ NT-115-NTPC yields a photocurrent density of 1.4 mA/cm² at 0.22 V vs. Ag/AgCl with a Faradic efficiency of 100%, nearly two times higher than that of conventional TiO₂ nanotube arrays. Furthermore, incident photon-to-current conversion efficiency is also promoted within ultraviolet light region. This research offers an effective strategy for improving the performance of photoelectrochemical water splitting through intensifying the light–matter interaction.

1. Introduction

Photoelectrochemical (PEC) water splitting has been regarded as a desirable avenue to exploit the abundant and sustainable solar energy by directly transforming the incident light into hydrogen fuel [1,2,3,4,5,6,7]. Among various PEC materials that can work as photoelectrodes, TiO₂ nanotube array (TiO₂ NTA), vertically grown on a conductive Ti foil by electrochemical anodization, distinguishes itself owing to its multifold unique merits including large internal and external surface areas, unidirectional electrical channel, and outstanding adhesion with Ti foil [8,9,10]. Unfortunately, the pristine TiO₂ NTA has a wide bandgap of 3.2 eV, which means that it still suffers from limited photoconversion efficiency even in the ultraviolet (UV) region [11,12,13]. Until now, element doping and narrow bandgap semiconductor coupling have been the two major strategies for extending the visible light absorption of TiO₂ [14,15,16,17]. However, these strategies bear many adverse effects, such as limited visible light response, increased carrier recombination centers, decreased incident photon-to-electron conversion efficiency ($IPCE$) in the UV region and the redox ability of the photogenerated charges [18,19,20,21]. Accordingly, gaining the utmost out of the UV light may be another promising approach to promote the PEC performance of TiO₂ NTA.

Recently, introducing a photonic crystal nanostructure into photocatalysts furnishes a new emerging route of strengthening light–matter interaction to improve light absorption [22,23,24]. The photonic crystal photocatalysts possess a periodic dielectric structure, which endows them with a photonic bandgap (PBG) for a certain frequency of photons [25,26,27]. To be specific, the group velocity of the photons with the frequency near the PBG edges can be significantly slowed, referred as the slow photon effect [28,29,30,31]. In addition, the photons with the frequency range of PBG are totally reflected and cannot propagate in the photonic crystal structure due to Bragg reflection (called the Bragg mirror effect) [32,33,34]. Obviously, the slow photon effect and Bragg mirror effect hold immense promise for intensifying light–material interaction, resulting in an amplified light absorption and photoelectrochemical reaction. Yet, to date, the existing investigations have mainly focused on the utilization of the slow photon effect in a single layer of three-dimensional (3D) TiO₂ inverse opal structures [13,28,35,36,37], which cannot use the reflected light at the PBG of TiO₂ to promote PEC performance in the UV region.

Apart from the 3D TiO₂ inverse opal structures, novel TiO₂ nanotube photonic crystals (TiO₂ NTPCs) with periodicities along the axial direction of nanotube have been successfully fabricated by a simple periodic current pulse anodization process [32,38,39]. Furthermore, the PBGs of TiO₂ NTPCs can be continuously adjusted through controlling the fabrication parameters [32,38,39]. Undoubtedly, after constructing the TiO₂ NTPC bi-layer structure consisting of a top nanotube (NT), which functions as an absorbing layer, and the bottom NTPC with PBG overlapping with an electronic bandgap of TiO₂ that acts as Bragg mirror layer, the interaction of top TiO₂ NT layer with reflected UV light should be greatly boosted, which could enhance its PEC performance in the UV region. Nevertheless, there is as yet no investigation available on the TiO₂ NTPC bi-layer structure focusing on the correlation between the Bragg mirror effect and PEC performance. This also implies that the underlying physical mechanism also remains unclear.

Herein, the novel TiO₂ NTPC bi-layer structure consisting of a top NT layer and a bottom NTPC layer was designed and fabricated for PEC water splitting. As expected, the TiO₂ NTPC bi-layer structure, with the PBG of bottom NTPC overlapping with an electronic bandgap of TiO₂, yielded a photocurrent density of 1.4 mA/cm² at 0.22 V vs. Ag/AgCl with Faradic efficiency of 100%, nearly two times higher than that of conventional TiO₂ NTA. Furthermore, $IPCE$ was also promoted within the UV light region. Such remarkable enhancement of PEC water splitting activity was primarily derived from the fact that the bottom NTPC layer can function as a Bragg mirror that can promote the interaction of top TiO₂ NT layer with the reflected UV light, thus leading to the boosted UV light absorption of the top TiO₂ NT layer. This work offers an effective strategy for improving the performance of PEC water splitting through intensifying light–matter interaction.

2. Materials and Methods

2.1. Materials

Ammonium fluoride (NH₄F), Ethylene glycol and Sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ti foils (0.25 mm thick, 99.8% purity) were purchased from Anping Anheng Wire Mesh Co., Ltd., Hengshui, China. All the chemicals were utilized as received without any further purification.

2.2. Fabrication of the TiO₂ NTPC Bi-Layer Structure

The TiO₂ NTPC bi-layer structure consisting of a top nanotube (NT) layer and a bottom NTPC layer was prepared by successive two-step anodization. Specifically, the Ti foils with sizes of 1.5 cm × 1 cm were firstly pre-treated by anodization at 60 V for 1 h, using glycol aqueous solution containing 0.5 wt% NH₄F and 2 vol% DI H₂O. Then, the as-grown TiO₂ NT was ultrasonically removed in deionized (DI) H₂O. After that, the pre-treated Ti foils were subjected to a successive two-step anodization process composed of a constant current anodization part and subsequently a periodic current pulse anodization part. During the first step, the constant current was maintained at 7 mA/cm² for 10 min to form the top TiO₂ NT layer. During the second step, the periodic current pulse anodization with high current (HC, $J_{HC}=7{ \mathrm{mA}/\mathrm{cm}}^2$) and low current (LC, $J_{HC}=0{ \mathrm{mA}/\mathrm{cm}}^2$) was employed to fabricate the bottom TiO₂ NTPC layer. The time duration of the HC pulse was controlled from 120 to 180 s, while the time duration of the LC pulse was fixed at 180 s to tailor the lattice constant of NTPC. Finally, the bi-layer structures were annealed at 450 °C in air for 2 h to obtain anatase TiO₂. In addition, the single layer TiO₂ NTPC with the same thickness as the bottom NTPC layer of the bi-layer was also prepared by constant current anodization.

2.3. Characterization

The morphologies, microstructures and crystal structures of the as-prepared samples were inspected by field-emission scanning electron microscopy (FE-SEM, S4800, Hitachi Ltd., Tokyo, Japan), field-emission transmission electron microscopy (FE-TEM, JEM-2100, JEOL Ltd., Tokyo, Japan), and X-ray powder diffractometry (XRD, Xpert, Philips, Amsterdam, The Netherlands). The diffuse reflectance spectra were recorded by a VARIAN Cary5000 spectrophotometer (Varian, CA, USA). The X-ray photoelectron spectroscopy (XPS) data were collected by a PHI 5000 Versaprobe (Ulvac-Phi, Kanagawa, Japan).

2.4. Photoelectrochemical Measurements

Photoelectrochemical measurements were performed in a three-electrode system connected to a CHI 660E electrochemical workstation (CH Instrument, Chenhua Ltd., Shanghai, China) utilizing the as-prepared samples with an exposed area of 1 cm² as the working electrode, the Pt mesh as the counter electrode, and the Ag/AgCl (3 mol/L KCl-filled) as the reference electrode. The 1 M NaOH (pH = 13.6) solution was electrolyte, which was purged with N₂ (99.999%) flow for 1 h to remove dissolved oxygen. The illumination source was a 500 W Xe lamp (Solar 500, NBet Group Corp. Beijing, China) with a calibrated intensity of 100 mW/cm², and a water filter was placed between the lamp and the electrochemical cell to eliminate the infrared heating of the electrolyte. The incident photon-to-current conversion efficiency ($IPCE$) measurements were conducted at an applied potential of 0.22 V vs. Ag/AgCl by means of a monochromatic system. During the PEC stability measurement, the photoelectrodes were biased at 0.22 V vs. Ag/AgCl. The amount of evolved oxygen was quantified by an Ocean Optics oxygen sensor system equipped with a FOXY probe (NeoFox Phase Measurement System), which was measured together with PEC stability.

3. Results and Discussion

3.1. Morphological Characterization of the TiO₂ NTPC

Figure 1a,b are a schematic illustration of the current–time curve of anodization and TiO₂ nanotube photonic crystal (NTPC) bi-layer structure consisting of a top NT and a bottom NTPC. In brief, the single constant current anodization was first utilized to form the top smooth-walled TiO₂ NT layer on the Ti foil. The as-grown sample was then subjected to periodic current pule anodization with high current and low current to form a TiO₂ NTPC layer with periodicities along the axial direction beneath the TiO₂ NT layer. The most crucial step in this work was to accurately modulate the lattice constant of the TiO₂ NTPC for obtaining the desired featured. This could be realized by adjusting the duration of high current pulse anodization, since the lattice constant of the NTPC layer was almost linearly increased with it.

Figure 1c–e display the FE-SEM images of single-layer TiO₂ NTPC fabricated by the periodic current pulse anodization with different durations of the HC pulse of 7 mA/cm⁻². Obviously, a TiO₂ NTPC presents a bamboo-shaped periodic structure in the axial direction of the NT. Such a periodic structure with alternating protrusive bamboo node layers and smooth-walled tube layers can result in a periodical refractive index change in the longitudinal direction, indicating that it can exhibit a structural modulated photonic bandgap (PBG). The length of the node and smooth-walled layer is the lattice constant of TiO₂ NTPC, which increases from 115 nm to 180 nm for HC pulse durations of 120 s and 180 s, respectively. The corresponding samples are denoted as 115-NTPC and 180-NTPC, respectively. Additionally, these TiO₂ NTPC have well-ordered and hexagonally arranged tubular structures with an average diameter of about 100 nm and a wall thickness of about 10 nm. More importantly, this allows the PBG of TiO₂ NTPC to be adjusted at will to match the electronic bandgap of anatase TiO₂.

3.2. Optical Obsorption Properties of the TiO₂ NTPC Structure

The reflectance spectra of TiO₂ NTPCs with different lattice constants in air, ethanol and electrolytes are measured under normal incidence and are presented in Figure 2a. As the lattice constant increases from 115 nm to 180 nm, the refection peak of the TiO₂ NTPC shifts to longer wavelengths. The positions of PBG of 115-NTPC with 15 periods and 180-NTPC with 15 periods are located at around 378 and 462 nm, respectively. The reflectance spectra of TiO₂ NTPC are strongly influenced by the refractive index contrast [40,41]. When the TiO₂ NTPC was put in ethanol, a remarkable red-shift of the reflection peaks could be observed, compared with that sample in air. The result is further reflected from the colors of the TiO₂ NTPCs (Figure 2b,c and Figure S1). Specifically, after being infiltrated with ethanol, its color changes from purple to green. It should be noted that no color change in TiO₂ 115-NTPC can be found, since its PBG is in the ultraviolet region (below 400 nm). When the refractive index contrast is further reduced by infiltration with liquid electrolyte (1 M NaOH), the reflection peaks of TiO₂ NTPC shift to an even longer wavelength. Taking TiO₂ 115-NTPC as an example, when the sample is immersed in electrolyte, the position of PBG shifts from 378 nm (air) to 384 (1 M NaOH) nm, which is very close to the electronic bandgap of TiO₂.

3.3. Microstructure, Crystalline Phase, and Chemical Composition Analysis of the TiO₂ NTPC

The microstructure, crystalline phase, and chemical composition of the TiO₂ NTPCs are also analyzed by FE-TEM, XRD, and XPS. The low-magnification FE-TEM images further confirm that the TiO₂ NTPC samples show a hexagonally arranged and bamboo-shaped periodic structure in the axial direction, which is consistent with FE-SEM results (Figure 3a,b and Figure S2). The HR-TEM image reveals that the well-resolved lattice spacing of 0.35 nm matches the d-spacing of the (101) plane of the anatase TiO₂, which is further proved by the corresponding Fast-Fourier Transform diffraction pattern (inset of Figure 3b) [42,43]. Figure 3c presents the XRD pattern of the TiO₂ NTPC, suggesting that all the diffraction peaks can be indexed to the anatase TiO₂ (JCPDS 21-1276) except those from the Ti substrate [44,45,46]. The XPS spectra also demonstrate that the TiO₂ NTPC samples are pure anatase with some oxygen deficiencies (Figure S3) [47].

3.4. Morphological Characterizaiton and Optical Obsorption Properties of the TiO₂ NTPC Bi-Layer Structure

To confirm that the PBG reflection effect can lead to a significant enhancement in light absorption, we further fabricated the TiO₂ NTPC bi-layer structure by successive two-step anodization. Figure S4 depicts the cross-sectional FE-SEM image of the TiO₂ 115-NTPC bi-layer structure (referred as TiO₂ NT-115-NTPC). The TiO₂ NT-115-NTPCs with 15 periods can be clearly seen to seamlessly grow beneath the smooth-walled NT layer with a thickness of approximately 500 nm, ensuring excellent connection between the two layers and easy electrolyte infiltration. For comparison, we also fabricated a TiO₂ 180-NTPC bi-layer structure (referred as TiO₂ NT-180-NTPC), another TiO₂ 115-NTPC bi-layer structure consisting of top NTPC layer and bottom NTs (TiO₂ 115-NTPC-NT), and a TiO₂ NT without a photonic crystal layer. To gain more realistic insights into the optical properties of TiO₂ NT-115-NTPC, TiO₂ NT-180-NTPC, TiO₂ 115-NTPC-NT, and TiO₂ NT, the reflectance spectra of the four samples in electrolytes were examined. As shown in Figure 3d, the TiO₂ NT-115-NTPC exhibits the strongest UV light harvesting capacity among the aforementioned four samples. This could be mainly attributed to the Bragg mirror effect of the bottom TiO₂ 115-NTPC with the PBG (3.2 eV) coinciding with the electronic bandgap of anatase TiO₂ (3.2 eV) that can reflect the UV light back to the absorbing NT layer, thus leading to the boosted UV light absorption of the top TiO₂ NT layer. Enhanced light absorption has been found in the TiO₂ NTPC bi-layer structure-based dye-sensitized solar cells [48].

3.5. PEC Water Splitting Activity of the TiO₂ NTPC Structure

To determine the promoted PEC performance of TiO₂ NT-115-NTPC, a set of PEC measurements were carried out in a three-electrode configuration using the as-prepared samples, Pt mesh, and Ag/AgCl (3 mol L⁻¹ KCl-filled) as the working, counter, and reference electrodes, respectively. The electrolytes for the PEC water splitting reaction were an aqueous solution of 1M NaOH (pH = 13.6). Figure 4a displays the linear-sweep voltammogram (LSV) sweeps for TiO₂ NT-115-NTPC, TiO₂ NT-180-NTPC, TiO₂ 115-NTPC, and TiO₂ NT under light irradiation and dark conditions. All the samples produced almost negligible dark current in comparison with their photocurrent, suggesting no occurrence of electrocatalytic water splitting. Under irradiation, the photocurrent density of TiO₂ NT-115-NTPC sharply increased and largely surpassed those of TiO₂ NT-180-NTPC, TiO₂ 115-NTPC, and TiO₂ NT, which signifies that the TiO₂ NT-115-NTPC had the highest PEC performance among the four samples. To elucidate this phenomenon more distinctly, their transient photocurrent responses were also measured under illumination with several 10 s light on/off cycles at 0.22 V vs. Ag/AgCl [1.23 V vs. RHE (reversible hydrogen electrode)], and the results are presented in Figure 4b.

At 0.22 V vs. Ag/AgCl, the TiO₂ NT-115-NTPC delivered a maximal photocurrent density of 1.4 mA/cm², and it was about 2.05, 2.15 and 3.5 times those of TiO₂ NT TiO₂ NT-180-NTPC and TiO₂ 115-NTPC, respectively. The low photocurrent of the TiO₂ NT-180-NTPC can be ascribed to the fact that its PBG position of the bottom TiO₂ 180-NTPC was outside of the electronic bandgap of anatase TiO₂ (3.2 eV), meaning that it could not reflect the UV light back to the top NT layer. That is to say, the bottom TiO₂ 180-NTPC had no impact on the PEC performance of TiO₂ NT-180-NTPC. Compared with TiO₂ NT, the low photocurrent of the TiO₂ 115-NTPC was mainly due to the single NTPC layer with a PGB position of 384 nm, which resulted in the decrease in UV light absorption.

To visualize the photocurrent enhancement owing to the promoted UV light absorption, incident photon-to-current conversion efficiency ($IPCE$) measurements were conducted on TiO₂ NT-115-NTPC and TiO₂ NT at 0.22 V vs. Ag/AgCl. The $IPCE$ could be calculated as a percentage according to the following equation [49]:

(1)$IPCE={\displaystyle \frac{1240I}{\lambda J_{light}}}$

where $I$ is the measured photocurrent density at a specific wavelength, $\lambda$ is the wavelength of the incident light, and $J_{light}$ is the light intensity of a specific wavelength. As shown in Figure 4c, the TiO₂ NT-115-NTPC has greatly boosted $IPCE$ values only in the UV region and reaches its highest value of 96% at 380 nm compared with TiO₂ NT. The result provides a clue suggesting that the bottom TiO₂ 115-NTPC makes a great contribution to PEC performance in the UV light region.

PEC stability and Faradic efficiency are two important parameters for the practical application of photoelectrode. Figure 4d presents the photocurrent–time curves of TiO₂ NT-115-NTPC and TiO₂ NT measured at 0.22 V vs. Ag/AgCl and continuous light illumination. The photocurrent densities of both samples are very stable, and there is no indication of deterioration during the entirely measured 3 h. To clarify whether the observed photocurrent originates from the oxygen evolution reaction, the fluorescence sensor is employed to determine the amount of oxygen evolved from the TiO₂ NT-115-NTPC. The amount of evolved oxygen increases linearly with the illumination time with unity Faradic efficiency. In addition, the surface morphology and crystal phase of the TiO₂ NT-115-NTPC remain intact after PEC water splitting for 3 h (Figure S5), illustrating that the TiO2 NT-115-NTPC possess prominent stability in the oxygen evolution reaction.

3.6. PEC Water Splitting Activity Mechanism

Based on the above experimental results, the promoted PEC performance of TiO₂ NT-115-NTPC can be mainly attributed to the significant enhancement of UV light absorption induced by the its bi-layer structure consisting of a top NT layer and a bottom NTPC layer (Figure 5a). As shown in Figure S6, the PBG (3.2 eV) of the bottom TiO₂ 115-NTPC overlaps with the electronic bandgap of anatase TiO₂ (3.2 eV). When the UV light strikes the TiO₂ NT-115-NTPC, a portion of UV light is absorbed by the top TiO₂ NT, producing the photoexcited electrons and holes, whereas another portion of UV light penetrates the top TiO₂ NT layer and is reflected by the bottom the 115-NTPC layer serving as the Bragg mirror. In such cases, the reflected light can be absorbed again by the top TiO₂ NT, hence promoting UV light absorption by the top TiO₂ NT. Additionally, optical interference occurs when UV light is being transmitted and reflected, which leads to strong UV photon resonance modes in the top NT absorbing layer, thus also boosting UV light absorption by the top TiO₂ NT (Figure 5b,c). Similar phenomena have been confirmed for other opal photonic crystal photocatalysis [50,51,52]. Accordingly, more photoexcited electrons and holes are generated, and remarkable promotion of PEC performance is achieved for the TiO₂ NT-115-NTPC.

4. Conclusions

In summary, we designed and fabricated a novel TiO₂ NTPC bi-layer structure photoanode consisting of a top NT layer and a bottom NTPC layer. In this architecture, when the PBG of bottom NTPC overlapped the with electronic bandgap of TiO₂, the bottom TiO₂ NTPC produced the Bragg mirror effect, leading to boosted UV light harvesting of top TiO₂ NT layer. Benefiting from promoted UV light absorption, the TiO₂ NT-115-NTPC yielded a photocurrent density of 1.4 mA/cm² at 0.22 V vs. Ag/AgCl with a Faradic efficiency of 100%, nearly two times higher than that of conventional TiO₂ NT. Furthermore, $IPCE$ was also promoted within UV light region. This work provides an effective strategy for improving PEC water splitting through intensifying light–matter interaction.

Author Contributions

Conceptualization, M.M., Y.H. and Z.G.; methodology, H.Z. and J.Y.; investigation, L.W.; writing-original draft preparation, H.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are available from the authors on request.

Conflicts of Interest

The authors declare no conflicts of interests.

Footnotes

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Figures

Figure 1. (a) A schematic illustration of the current–time curve of anodization and the TiO2 nanotube/nanotube photonic crystal bi-layer structure. (b) Schematic illustration of the TiO2 nanotube/nanotube photonic crystal bi-layer structure. (c–e) FE-SEM images of the TiO2 180-NTPC. (f–h) FE-SEM images of the TiO2 115-NTPC.

Figure 2. (a) The reflectance spectra of the TiO2 180-NTPC and TiO2 115-NTPC samples in air and infiltrated with ethanol and electrolytes, respectively. (b,c) Photographs of the TiO2 180-NTPC samples in air and infiltrated with ethanol, respectively.

Figure 3. (a) A low-magnification FE-TEM image of the TiO2 115 NTPC. (b) A HR-TEM image of the area highlighted by the white dashed hexagon in (a). Inset: Fast-Fourier Transform diffraction patterns of the areas bounded by the white dashed box in (b). (c) Corresponding XRD pattern. (d) Reflectance spectra of the TiO2 NT-115-NTPC, TiO2 NT-180-NTPC, TiO2 115-NTPC-NT, and TiO2 NT infiltrated with electrolytes, respectively.

Figure 4. (a) Current vs. voltage (J−V) curves acquired from TiO2 NT-115-NTPC, TiO2 NT-180-NTPC, TiO2 115-NTPC, TiO2 NT, respectively. (b) The corresponding transient photocurrent responses performed at 0.22 vs. Ag/AgCl. (c) The [Forumla omitted. See PDF.] spectra of TiO2 NT-115-NTPC and TiO2 NT measured at an incident wavelength range from 300 nm to 550 nm at a potential 0.22 V vs. Ag/AgCl. (d) Photocurrent versus time (J−t) curves of TiO2 NT-115-NTPC and TiO2 NT obtained at 0.22 V vs. RHE. The dashed line and colorful spheres show the amount of evolved O2 calculated theoretically and detected experimentally for TiO2 NT-115-NTPC, respectively.

Figure 5. (a) A schematic drawing of mechanism of boosted UV light absorption of TiO2 NT-115 NTPC. Down arrow and up arrow represent the incident light and reflected light, respectively. (b) Schematic optical band structure of TiO2 nanotube photonic crystal. (c) Photons reflected within bandgap for further absorption.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/nano14211695/s1, Figure S1: Photographs of the TiO₂ 115-NTPC in air and infiltrated with ethanol, respectively. Figure S2: (a) A low-magnification FE-TEM image of the TiO₂ 180-NTPC. (b) An HR-TEM image of the area highlighted by the white dashed box in (a). (c) Fast-Fourier Transform diffraction patterns of the areas bounded by the white dashed box in (b). (d) Projected atomic models in [101] directions. Figure S3: (a) The XPS survey spectra of the TiO₂ NT-115-NTPC. (b) The corresponding normalized Ti 2p XPS spectra and normal O 1s XPS spectra. Figure S4: FE-SEM images of the TiO₂ NT-115-NTPC. Figure S5: (a) A FE-SEM image of the TiO₂ NT-115-NTPC after undergoing the PEC water splitting reaction for 180 min. (b,c) The corresponding FE-TEM image.

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