Pt‐Decorated g‐C₃N₄/TiO₂ Nanotube Arrays with Enhanced Visible‐Light Photocatalytic Activity for H₂ Evolution

Abstract

Aligned TiO₂ nanotube layers (TiNTs) grown by self‐organizing anodization of a Ti‐substrate in a fluoride‐based electrolyte were decorated with graphitic‐phase C₃N₄ (g‐C₃N₄) via a facile chemical vapor deposition approach. In comparison with classical TiO₂ nanotubes (anatase), the g‐C₃N₄/TiNTs show an onset of the photocurrent at 2.4 eV (vs. 3.2 eV for anatase) with a considerably high photocurrent magnitude in the visible range. After further decoration with Pt nanoparticles, we obtained a visible‐light responsive platform that showed, compared with g‐C₃N₄‐free TiNTs, a strong enhancement for photoelectrochemical and bias‐free H₂ evolution (15.62 μLh⁻¹ cm⁻²), which was almost a 98‐fold increase in the H₂ production rate of TiNTs (0.16 μLh⁻¹ cm⁻²). In a wider context, the g‐C₃N₄‐combined 3 D nanoporous/nanotubular structure thus provides a platform with significant visible‐light response in photocatalytic applications.

Titanium dioxide (TiO₂) is the most studied photocatalyst in contemporary science and technology,1 namely due to its use in decomposing organic pollutants in the environment (e.g. in self‐cleaning coatings on buildings) and also due to its antimicrobial properties.2, 3, 4, 5 For photocatalysis, TiO₂ is irradiated with light of an energy equal to or higher than its band‐gap energy (3.0–3.2 eV), and electron‐hole pairs are generated in the conduction and valance band of TiO₂, respectively. The holes emitted from the valance band and electrons from the conduction band can be used for phtotocatalytic reactions, for example, to oxidize an organic compound or to “split” water into H₂ and O₂.6, 7 In order to optimize the performance of photocatalysts as suspension or photoelectrodes, a large number of TiO₂ morphologies have been explored. One of the most investigated morphologies are aligned TiO₂ nanotube layers (TiNTs). These TiNTs can be formed by self‐ordering electrochemical anodization of Ti. The wide research interest in these materials is due to a unique combination of their geometry and reactivity.8, 9, 10 Particularly, photoconversion efficiencies of nanotubular arrays of TiO₂ were reported to be very high due to a high charge transfer rate and orthogonal carrier separation.11, 12 An intrinsic drawback of TiO₂ is, however, its large band‐gap (3.2 eV for anatase‐type and 3.0 eV for rutile‐type) that makes it active only in the UV spectral range (λ<387 nm) which accounts for less than 5 % of solar light. Therefore, numerous approaches to achieve a visible response by band‐gap engineering (doping)4, 13 or sensitization (e.g., by dyes14 with other semiconductors15, 16, 17, 18, 19 or other photoactive compounds20, 21, 22, 23, 24) have been reported.

Recently, graphitic carbon nitride (g‐C₃N₄) has attracted considerable attention as a potential visible‐light photocatalyst owing to its visible‐light band‐gap at 2.69 eV and its comparably high stability.25, 26, 27, 28 However, the photocatalytic performance of g‐C₃N₄ is limited by the low quantum efficiency for the pristine semiconductor. To resolve this problem, many attempts have been carried out to improve the photocatalytic performance of g‐C₃N₄, by for example, nonmetal doping,29, 30, 31, 32 preparation of nano/porous C₃N₄,33, 34 and formation of heterojunction between C₃N₄ and other materials.35, 36, 37, 38, 39, 40 The highest occupied molecular orbital (HOMO) of the g‐C₃N₄ is located more negative than the conduction band (CB) of several common wide band‐gap semiconductor photocatalysts such as TiO₂, ZnO, and BiPO₄. Thus, by constructing heterojunctions with g‐C₃N₄, these wide band‐gap semiconductors can be sensitized by g‐C₃N₄, and a visible‐light response can be obtained.

In the present work, we explore the feasibility to decorate and activate TiO₂ nanotube arrays by graphitic carbon nitride (g‐C₃N₄) and explore them for visible‐light photocatalytic H₂ generation. We chose melamine as the precursor for a direct synthesis of g‐C₃N₄ and TiO₂ nanotube (g‐C₃N₄/TiNTs) decoration using a one‐step chemical vapor deposition (CVD) technique. The g‐C₃N₄ loaded anatase TiO₂ nanotubes were then investigated for photocatalytic H₂ evolution under solar illumination conditions, in a photoelectrochemical configuration as well as under open‐circuit potential (OCP) conditions. We show that a considerable enhancement for H₂ evolution under both conditions can be obtained.

Figure 1 A,B show scanning electron microscopy (SEM) images of the top view and the cross‐sectional view of the TiO₂ nanotube layer before loading with g‐C₃N₄. The tubes have an average diameter of 150 nm, and the layer has a thickness of approximately 1.5 μm. Figure 1 C and Figure S1 A (in the Supporting Information) show the nanotubes after loading with g‐C₃N₄ by using 15 mg melamine as a precursor. Compared with the neat nanotubes, distinct patches of g‐C₃N₄ nanofilm can be seen on the tube wall and entrance (Figure 1 C)—this appearance of g‐C₃N₄ is well in line with literature of g‐C₃N₄ deposition on other substrates.40 It should be noted that using our deposition process, a loading of the nanotubes from 15 mg melamine precursor represents an optimized treatment. In a set of preliminary experiments, a number of modification conditions (i.e. annealing temperature, temperature rise rate, Ar flow speed) were explored. As shown in Figure S2 in the Supporting Information, with further increase in the amount of precursor, large agglomerates of g‐C₃N₄ were observed on the surface of the tubes which can block the tube mouths. Figure 1 D and Figure S1 B in the Supporting Information show the g‐C₃N₄/TiNTs after depositing Pt by sputtering—the particles have a size of approximately 2 nm and preferably are loaded at the tube mouth.

Figure 1.

SEM images: A) top view and B) cross‐sectional view of TiO₂ nanotube arrays, C) top view of TiO₂ nanotube arrays after modification with g‐C₃N₄ by using 15 mg melamine as a precursor, and D) magnified top view of g‐C₃N₄/TiO₂ nanotube arrays after further decoration with Pt nanoparticles.

The successful decoration with g‐C₃N₄ was further verified by X‐ray photoelectron spectroscopy (XPS) analysis. The XPS survey spectra in Figure S3 (Supporting Information) characterize TiNT samples before and after the g‐C₃N₄ decoration. Figure 2 A,B shows the C 1s and N 1s spectra for g‐C₃N₄/TiNTs, and the spectra of bare TiNTs are also included as reference. The C 1s spectra of g‐C₃N₄/TiNTs in Figure 2 A exhibit three peaks located at 284.8, 286.6, and 288.0 eV. The peak at 284.6 eV (C1) is typically ascribed to sp² C=C bonds, the peak at 286.6 eV (C2) can be assigned to the sp²‐hybridized carbon atoms bonded to three nitrogen atoms in the g‐C₃N₄ layer, and the peak at 288.0 eV (C3) can be attributed to the sp² carbon atoms in the aromatic ring attached to the −NH₂ group.41 In Figure 2 B, the corresponding high‐resolution N 1s spectrum of the g‐C₃N₄/TiNTs can be fitted by three peaks centered at 398.3 (N1), 400.0 (N2), and 404.0 eV (N3), which correspond to sp²‐bonded N involved in the triazine rings (C−N=C), the bridging N atoms in N−(C)₃, and the amino functions carrying hydrogen (N−H),42 correspondingly. For the reference tubes, the appearance of the N 1s peak likely originates from the N‐containing electrolyte in the anodization process and N₂ absorbance from environment, which only exists on the top of tubes. To study the contribution of g‐C₃N₄ on the nanotubes, the higher energy peak of C 1s, O 1s, and Ti 2p is considered for sputtering profiles. As illustrated in Figure 2 C,D, a very clear indication of sputter removal of a g‐C₃N₄ and TiO₂ tube layer is obtained. The N 1s signal (398.3 eV) is lost with the O 1s (532.2 eV). In comparison with reference tubes without C₃N₄ coating, the N 1s peak is visible even at ∼1.3 μm, which is very close to the bottom of the tubes. The successful decoration with g‐C₃N₄ was also analyzed by X‐ray diffraction (XRD), as shown in Figure 2 E. For g‐C₃N₄, a typical peak was found at 27.6 °, which corresponds to the characteristic interplanar stacking peak of aromatic systems in the (002) diffraction plane.25 From g‐C₃N₄‐loaded TiO₂ nanotubes, only the diffraction peaks of the anatase phase were observed from the g‐C₃N₄/TiNTs XRD pattern due to comparably small amounts and lower crystallinity of C₃N_4. 40 However, the peak at 27.6 ° present in the spectrum of g‐C₃N₄/TiNTs could also be assigned to the presence of some rutile TiO₂. Finally, the successful loading of Pt was also confirmed by XPS (Figure 2 F).

Figure 2.

XPS core‐level spectra of TiO₂ nanotube arrays before and after being modified with g‐C₃N₄ by using 15 mg melamine as a precursor for A) C 1s and B) N 1s. C) XPS depth‐profiling spectra of O 1s, N 1s, and Ti 2p peaks for g‐C3N4‐modified TiNTs, and D) the corresponding core‐level spectra of N 1s for g‐C3N4/TiNT and TiNT samples. E) XRD patterns of TiO₂ nanotube arrays annealed at 550 °C; g‐C3N4 and g‐C3N4‐modified TiO₂ nanotube arrays prepared at 550 °C. (F) The XPS score‐level spectra for Pt 4f of the Pt/g‐C₃N₄/TiNT sample.

To evaluate the photoelectrochemical properties of the different stages of g‐C₃N₄‐loaded TiO₂ nanotubes, incident photon‐to‐current conversion efficiency (IPCE) spectra were measured and compared with neat anatase TiO₂ (Figure 3 A,B). In the visible‐light region, g‐C₃N₄/TiNTs exhibited a considerably enhanced IPCE with an onset at about 2.4 eV (vs. pure anatase that showed the expected 3.2 eV optical gap, Figure S4 B in the Supporting Information). The magnitude of the visible response considerably increased with an increasing precursor amount up to the optimum loading of 15 mg (Figure S4 A in the Supporting Information). The enhancement of IPCE for g‐C₃N₄/TiNTs samples could be ascribed to the wider absorption spectrum region and the improved charge separation and transportation efficiency by the nanojunctions between the TiO₂ nanotubes and g‐C₃N₄ polymer. In the following study, 15 mg melamine was used for preparing g‐C₃N₄/TiNTs samples. However, a drop of IPCE was observed when 20 mg precursors were employed, which was likely due to the blocking of nanotube entrance by a thick g‐C₃N₄ layer (as illustrated in Figure S2 in the Supporting Information).

Figure 3.

A) IPCE spectra and B) enlarged IPCE spectra at a bias of 0.5 V in 0.1 m Na₂SO₄, C) water splitting performances in a 0.1 m Na₂SO₄ under AM 1.5 with a 400 nm filter, and D) photocatalytic bias‐free H₂ production in 20 vol % EtOH in H₂O under AM 1.5 for g‐C₃N₄/TiNT, Pt/g‐C₃N₄/TiNT, Pt‐TiNT, and TiNT samples.

Figure 3 A shows the photoelectrochemical properties of g‐C₃N₄/TiNTs before and after Pt grafting. Since Pt‐decorated anatase TiO₂ nanostructures (such as nanoparticles and nanotubes) have been studied extensively, we used anatase TiNTs before and after Pt decoration (Pt‐TiNT) for reference. In the visible range, the bare TiNTs exhibited only a very low response. Pt‐decorated g‐C₃N₄/TiNTs and TiNTs samples exhibited a slightly enhanced IPCE in the visible light range, which was likely due to the increased separation of photogenerated electrons and holes at the heterojunctions. It is noteworthy that the Pt loading on the TiNTs or g‐C₃N₄/TiNTs led to an obviously decreased UV response compared with the bare TiNTs and g‐C₃N₄/TiNTs. This is due to the “shading” effects of C₃N₄. C₃N₄ also absorbs in the UV range, and less UV light reaches the underlying TiO₂. Figure 3 C shows results for the photoelectrochemical water splitting performance under simulated sunlight AM 1.5 (100 mW cm⁻²) with a UV‐cutoff filter (λ<400 nm) in 0.1 m Na₂SO₄. From the photocurrent transient vs. potential curves, it is apparent that the Pt‐decorated g‐C₃N₄/TiNTs sample exhibits the highest photocurrents over the entire range. For comparison, a bare TiNTs layer loaded with a similar density of Pt nanoparticles was used. It was evident that the Pt/g‐C₃N₄/TiNTs heterojunctions exhibited a very good photocatalytic performance at open‐circuit condition under visible light. Moreover, the photocatalytic activity for bias‐free H₂ production was further investigated in water (containing 20 % ethanol as a hole capture agent) under AM 1.5 illumination conditions.Gas chromatography (GC) analyses (Figure 3 D) revealed an H₂ production rate of 0.78 μL h⁻¹ cm⁻² for g‐C₃N₄/TiNTs. These tubes, after grafting with Pt nanoparticles, showed an increase of H₂ production rate (15.62 μL h⁻¹ cm⁻²), which was almost a 20‐fold H₂ production rate of g‐C₃N₄/TiNTs, and a 98‐fold H₂ production rate of TiNTs (0.16 μL h⁻¹ cm⁻²).

In conclusion, g‐C₃N₄‐loaded TiNTs were successfully fabricated. The as‐prepared g‐C₃N₄/TiNTs exhibited a clearly higher photoelectrochemical activity under visible light conditions. Even more, the Pt‐decorated g‐C₃N₄/TiNTs showed significantly enhanced photoelectrochemical water splitting activity and had a high potential for energy conversion. Obviously, the g‐C₃N₄/TiNTs nanocomposite showed considerable photocatalytic activity in the visible‐light range and excellent stability. Moreover, this study provides a simple and fast approach to grafting g‐C₃N₄ on 3 D nanoporous/nanotubular structures, which then provides a platform with a considerable photocatalytic performance under visible light.

Experimental Section

Preparation of TiO ₂ nanotube arrays: Ti foils (0.1 mm thickness, 99.6 % purity, Advent) were first degreased by sonication in acetone, EtOH, and deionized H₂O (DI), and then dried in a nitrogen stream. Self‐organized TiO₂ nanotube layers with approximately 1.5 μm thickness were grown in glycerol (50 vol %)‐H₂O electrolyte containing 0.27 m NH₄F at 30 V for 4 h.43 The layers were annealed in air at 550 °C for 1 h.

Decoration of g‐C ₃ N ₄ layer: Deposition of g‐C₃N₄ onto the TiNTs was performed by a chemical vapor deposition (CVD) approach by using melamine as a precursor.44 Briefly, melamine (5.0–20.0 mg) was added in a cleaned alumina crucible with a cover, and the TiNTs was placed (top‐down) several centimeters above the melamine powders, and then the crucible was heated at 550 °C for 3 h in a tube furnace. After the process, the g‐C₃N₄ polymer was successfully depositied onto the TiO₂ nanotubes, while some g‐C₃N₄ powder was also obtained in the crucible.

Decoration with Pt nanoparticles: Pt‐decorated g‐C₃N₄/TiNTs samples were prepared by sputtering Pt nanopaticles via a plasma sputter equipment (EM SCD500, Leica, Wetzlar, Germany) at 15 mA at 1 bar of Ar condition (at a normal loading of 1 nm).

Apparatus: The morphologies of the g‐C₃N₄/TiNTs were characterized using a field‐emission SEM (Hitachi FE‐SEM S4800, Tokyo, Japan). X‐ray diffraction (XRD) patterns were acquired on an X′Pert X‐ray diffraction spectrometer (Philips, Andover, MA, USA) with a Cu Kα X‐ray source. The composition of the layers was analyzed using an X‐ray photoelectron spectrometer (XPS, PHI 5600, Physical Electronics, Chanhassen, MN, USA) using Al Kα radiation at 13 kV as excitation source with a takeoff angle of 45° for the emitted photoelectrons. All the peaks are shifted based on a standard of C 1s at 284.8 eV. A three‐electrode configuration containing the TiNTs as photoanode, Pt foil as cathode, and Ag/AgCl (3 m KCl) as reference electrode was employed for all photoelectrochemical experiments. Photocurrent measurements were performed with a setup consisting of a 150 W Xenon lamp as the light source. Photocurrent spectra were acquired in 10 nm steps in a range of 300–700 nm in an aqueous solution of 0.1 m Na₂SO₄ at an applied bias of +0.5 V. The value of photocurrent density was measured as a difference between current densities acquired under irradiation and in the dark, and then IPCE was calculated from the measured photocurrent densities. The water‐splitting performance experiments were carried out by applying an external bias to the cell with a scan rate of 1 mV s⁻¹ at rt. The light source was a 300 W Xenon lamp (100 mW cm⁻²) with a cutoff filter λ<400 nm. The photocatalytic H₂ production tests were carried under simulated AM 1.5 illumination provided by a solar simulator (300 W Xe with optical filter, Solarlight, Glenside, PA, USA).

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Z.-D. Gao, Y.-F. Qu, X. Zhou, L. Wang, Y.-Y. Song, P. Schmuki, ChemistryOpen 2016, 5, 197.