1 Introduction
Since the water splitting over a titania (TiO2) photoelectrode was reported for the first time [1], photocatalytic hydrogen (H2) production employing semiconducting photocatalysts has been considered as a promising and fascinating method to alleviate the current energy shortage [2–5]. Of various photocatalysts developed, TiO2 as an n-type semiconductor is the most extensively explored one owing to its merits of environmentally friendliness, low cost, and robust physicochemical stability. Nevertheless, TiO2 with a wide bandgap (approximately 3.20 eV) can only absorb the UV light in the solar spectrum, while the UV ray (λ<400 nm) just accounts for a small part (approximately 4%) of the solar spectrum [5]. To effectively utilize the approximately 53% of visible light and even approximately 43% of infrared rays in solar light and improve the photocatalytic performance, enormous approaches, such as doping with extra elements [6,7], exploiting narrow bandgap semiconductors [2,3], constructing semiconducting composites [7–12], and dye-sensitization systems [5,13], have been explored, of which, the construction of semiconductor-based heterojunctions is an efficient strategy to elevate the light capturing ability and to retard the photoexcited charge recombination at the same time, therefore making it attractive and challenging topic in the field of photocatalytic energy conversion [6–17].
Titanium nitride (TiN) with a typical face-centered cubic (fcc) structure has been extensively applied in the field of electrochemical energy conversion due to its excellent metallic characteristics and physicochemical stability [18–20]. In addition, TiN possesses a good plasmonic effect, even similar to gold, in the visible light and near-infrared (NIR) spectral range [20–22], and its work function (approximately 4.0 eV versus vacuum) is larger or equal to the electron affinity of most metal oxide semiconductors used in the field of photocatalysis [23], and thus it is expected to construct favorable energetic alignment for hot carrier-improved photocatalytic performance [21]. This inspires researchers to integrate the nonmetallic plasmonic TiN with TiO2 to attain synergistic benefits of efficient visible light capturing and electric conductivity for enhancing the photocatalytic energy conversion. Although there are some reports on plasmonic TiN boosting the photoelectrochemical oxygen evolution reaction [20,21], the application of TiN in photocatalytic H2 evolution reaction has not been reported to the best of the authors’ knowledge.
Herein, TiN was obtained via a facile and environmentally friendly nitridation process using a hydrothermally synthesized TiO2 and melamine (MA) as raw materials, whereby TiN was in situ grown on the formed N-doped TiO2 (N-TiO2), which was confirmed by a series of material characterization and analysis methods. By varying the TiO2/MA mass ratio, various TiN/N-TiO2 composites were synthesized. Under light irradiation, the photoinduced hot charge carriers can separate and transfer via the close contact interface of TiN/N-TiO2 composite, restraining the charge recombination, and the in situ formed plasmonic TiN and N-TiO2 can extend the spectral response range of TiO2 and promote the transfer and separation of the hot charge carrier of TiN. Therefore, a high H2 evolution activity was realized. In addition, the energy band structure and electrochemical behavior were investigated to probe the underlying photocatalytic mechanism.
2 Experimental
2.1 Material preparation
TiN/N-TiO2 composite was prepared via a two-step process, whereby TiO2 was pre-synthesized via a hydrothermal and calcination processes similar to the previously reported method [24]. Then the hydrothermally synthesized TiO2 was mixed with melamine (MA). After grinded evenly, the mixture was calcined at 750°C for 2 h in a tube furnace at a N2 flow of 50 mL/min. The resulting product was washed with ethanol and distilled water serval times, and dried at 100°C overnight to obtain the TiN/N-TiO2 composite.
For comparison, various composites were fabricated in a similar in situ nitridation process of the hydrothermally synthesized TiO2 by varying the TiO2/MA mass ratio to 1:5, 1:7, and 1:10, and the corresponding product was denoted as TiN/N-TiO2-5, TiN/N-TiO2-7, and TiN/N-TiO2-10, respectively. Moreover, a pure phase TiN product was prepared by enhancing the TiO2/MA mass ratio to 1:20 and denoted as TiN. In addition, the hydrothermally synthesized TiO2 was calcined at 750°C for 2 h in a tube furnace at N2 flow but without MA, and the relative sample was denoted as TiO2(c).
2.2 Material characterization
A Miniflex 600 X-ray diffractometer (XRD) with CuKα radiation (λ = 1.54184 Å) was applied to acquire the crystal structure of the sample operated at 40 kV, 15 mA and a scanning rate of 8 °/min with a step of 0.02°. A Zeiss-Sigma field emission scanning electron microscope (FESEM) and a Laboratory6 JEM-2100 (HR) high-resolution transmission electron microscope (HRTEM) working at 200 kV were employed to observe the morphology and microstructure. A field-emission electron probe microanalyzer (EMPA) was applied to quantitatively analyze the contents of Ti, O, N, and C elements. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB 250 Xi photoelectron spectroscope (Thermo Fisher Co.) equipped with a standard and monochromatic source (Al Kα), and the binding energy was calibrated by C1s peak (284.6 eV). A Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere was used to obtain the UV-vis diffuse reflectance absorption spectrum (DRS) with BaSO4 as the reference.
The transient photocurrent response of the product was recorded on a CHI Model 618 C electrochemical workstation at a bias potential of 0.5 V using a typical three-electrode system, in which the Pt plate, the Pt wire, and the saturated calomel electrode (SCE) acted as the working electrode, the counter electrode, and the reference electrode, respectively. Typically, 20 mg of photocatalyst was dispersed in the Na2SO4 aqueous solution (1.0 mol/L). Before irradiation, the suspension was bubbled with the N2 flow for 0.5 h to eliminate the residual air. The electrochemical impedance spectrum (EIS) was measured over the frequency range of 0.01–10000 Hz using a three-electrode system, in which the Pt wire and the SCE electrode were used as the counter electrode and the reference electrode, respectively. The working electrode was prepared by depositing photocatalyst suspension containing Nafion and ethanol (0.20 mL, 2.5 g/L) on a fluorine-doped tin oxide (FTO) glass (1.0 cm−2), which was heated at 60°C for 1.0 h to volatilize the solvent and steady the sample. In a typical run, the above three electrodes were immersed into a Na2SO4 solution (1.0 mol/L), which was continuously purged by the N2 flow for 30 min before irradiation.
The flat-band potential (Efb) of the sample was measured using the same three-electrode system as the EIS spectrum. Efb is estimated by extrapolating each Mott-Schottky plot to the x-axis to obtain the intercept following Eq. (1) [25],
where C is the capacitance of the space charge layer, Nd is the number of donors and estimated by the slope of Eq. (1), e is the electron charge (1.602 × 10−19 C), ε is the dielectric constant, ε0 is the vacuum permittivity (8.85 × 10−14 F/cm), and K is the Boltzmann constant (1.38 × 10−23 J/K).
2.3 Photocatalytic performance measurement
The photocatalytic H2 evolution reaction was performed in a top-irradiated Pyrex glass reaction cell [26,27]. Typically, 30 mg of photocatalyst was suspended in a 50 mL of ascorbic acid (AA) solution (50 mmol/L), which was ultrasonically treated for 5 min, vacuumed thoroughly, and then exposed to the full spectrum or visible light (λ>400 nm) of a 300 W Xe-lamp (PLS-SXE, Beijing Perfectlight Co. Ltd.) under continuous stirring. The evolved H2 amount was analyzed by an online SP-6890 gas chromatograph (GC, TCD detector, 5 Å molecular sieve columns, and Ar as carrier gas).
3 Results and discussion
3.1 Microstructure and composition analyses
Figure 1(a) presents the X-ray diffraction (XRD) patterns of the products derived from various TiO2/MA mass ratios. As seen, the TiO2(c) product obtained by calcining the hydrothermally synthesized TiO2 in the absence of MA is still TiO2 containing anatase and rutile phases, whereby the obvious diffraction peaks at 2θ = 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, and 62.7° can be ascribed to the (101), (004), (200), (105), (211), and (204) plane reflections of anatase TiO2 (JCPDS No. 21-1272) [25,28], respectively. The additional diffraction peaks at 2θ = 27.4°, 36.1°, 41.2°, and 54.3° can be attributed to the (110), (101), (111), and (211) plane reflections of rutile TiO2 (JCPDS No. 21-1276) [28,29], respectively. However, there is not any diffraction peak of anatase or rutile phase when the TiO2 to MA mass ratio is 1:20, and some new weak peaks are observed at 2θ = 36.8°, 42.8°, and 62.1°, which can be attributed to the (111), (200), and (220) plane reflections of face-centered cubic (fcc) TiN [18–20], respectively. This means that the hydrothermally synthesized TiO2 can be transformed into pure phase TiN, while the weak and broad diffraction peaks imply that the formed TiN has a moderate crystallinity.
When the TiO2 to MA mass ratio is enhanced to 1:5, 1:7, and 1:10, the corresponding products (TiN/N-TiO2-5, TiN/N-TiO2-7, and TiN/N-TiO2-10) still display the diffraction peaks of anatase TiO2, while those rutile peaks disappeared. Moreover, new peaks can be observed at 2θ = 36.8°, 42.8°, and 62.1°, which can be attributed to the (111), (200), and (220) plane reflections of fcc TiN [19,20], respectively. Besides, the diffraction peak intensities of anatase TiO2 decrease gradually, while those of TiN increase upon enhancing the MA addition amount, and those diffraction peaks such as (101) and (200) of anatase TiO2 slightly shift toward a higher 2θ angle (Fig. 1(b)), which might be ascribed to the substitution of O2‒ ions (with a radius of 1.40 Å) with N3‒ ones (with a radius of 1.46 Å) to form N-doped TiO2 (N-TiO2) [20]. These results demonstrate that N-TiO2 and TiN as well as their composites (TiN/N-TiO2) can be obtained by varying the TiO2 to MA mass ratio during the in situ nitridation process of the hydrothermally synthesized TiO2.
X-ray photoelectron spectroscopy (XPS) was performed to further explore the component and its surface chemical states of the products. The survey XPS spectra (Fig. 2(a)) suggest that the TiO2(c) product only contains Ti and O elements in addition to C 1s, while the TiN obtained from the TiO2 to MA mass ratio of 1:20 is mainly composed by Ti and N elements with very weak O 1s and C 1s peaks, indicating the formation of TiN, which is consistent with the above XRD result. As for the TiN/N-TiO2-7, there exist Ti, O, N, and C element signals in the survey spectrum, manifesting the existence of TiN and TiO2 (or N-TiO2). The high-resolution Ti 2p XPS spectrum (Fig. 2(b)) indicates that the TiO2(c) product exhibits a typical double peak with binding energy (BE) located at 458.6 (Ti 2p3/2) and 464.3 (Ti 2p1/2) eV, suggesting the valence state of Ti is+4 [20,30], while the TiN product derived from the TiO2 to MA mass ratio of 1:20 displays two BE peaks at 457.9 (Ti 2p3/2) and 463.6 (Ti 2p1/2) eV, denoting that the Ti valence state is+3 [31,32]. In contrast, the Ti 2p spectrum (Fig. 2b) of the TiN/N-TiO2-7 composite can be deconvolved into three kinds of BE peaks located at 458.9/464.6 eV for the Ti–O bond, 457.5/463.2 eV for the Ti–N bond, and 458.3/464.0 eV for the Ti–N–O bond [20], indicating the coexistence of TiN and N-TiO2 in the composite.
The high-resolution O 1s XPS spectrum (Fig. 2(c)) demonstrates that the TiO2(c) product has two main BE peaks at 529.8 and 532.1 eV, which can be attributed to the lattice oxygen atoms of TiO2 and the surface hydroxyl groups (–OH), respectively. The TiN/N-TiO2-7 composite displays two similar BE peaks at 530.0 eV and 532.1 eV. The BE peak at 532.1 eV for both TiO2(c) and TiN/N-TiO2-7 is an indicative of defect sites with a low oxygen coordination (i.e., oxygen vacancies (VO) or low-valence Ti3+), which would not only regulate the electron distribution to improve the electronic conductivity of TiO2 [33], but also narrow the bandgap of TiO2 to endow an enhanced visible-light-responsive activity [11,33,34]. The N 1s spectrum (Fig. 2(d)) of the TiN product delivers a BE peak at 398.1 eV, suggesting the existence of N3-, which can be ascribed to the lattice N of TiN [20], and the N 1s spectrum for TiN/N-TiO2-7 can be fitted by the two BE peaks at 398.2 and 398.9 eV, ascribable to the N–Ti bond in the TiN phase and the N–Ti–O bond in the N-TiO2 [35], respectively. The Ti 2p peak of TiN/N-TiO2 shifts approximately 0.3 eV toward the higher BE one compared with the TiO2(c), while the Ti 2p peak of TiN/N-TiO2 shift approximately 0.4 eV toward the lower BE one compared with the TiN (Fig. 2(b)). These BE peak shifts and the presences of N–Ti–O/Ti–N–O linkages suggest that the in situ formed TiN and N-TiO2 components have a strong electron donor-acceptor coupling. Although it was reported that the pyrolysis of MA at 450°C−500°C can produce g-C3N4 [9], the N 1s regional spectra of TiN and TiN/N-TiO2-7 (Fig. 2(d)) do not display any BE peak (at above 398.9 eV) that can be ascribable to the N atoms bonded to the C atom (C=N–C) and N-(C3) groups of g-C3N4 [9]. This implies that TiN and TiN/N-TiO2-7 have no coexisted g-C3N4, which is reasonable considering the fact that the present products were calcined at 750°C and the thermal decomposition temperature of g-C3N4 is in the range of 450°C−630°C [9].
According to the SEM and TEM images (Figs. 3(a) and 3(b)), the TiO2(c) product has nanorod-like morphology, and the lattice fringes (approximately 0.357 nm) in the HRTEM image with the corresponding fast Fourier transform (FFT) pattern (Fig. 3(c) and the inset) indicate that anatase TiO2 is the main component in the TiO2(c) product. From the SEM and TEM images (Figs. 3(d) and 3(e)) of TiN/N-TiO2-5 composite, it can be found that those newly formed nanoparticles closely contact on the nanorods, and the HRTEM image (Fig. 3(f) and the insets) displays two kind of lattice fringes of approximately 0.354 and approximately 0.207 nm, which can be attributed to the d-spacing of anatase TiO2 (101) and TiN (200) facets according to their respective FFT patterns, respectively. This observation combined with the above XRD and XPS analysis results demonstrate that TiN nanoparticles gradually formed on those nanorod surface.
Along with the increase of the TiO2 to MA mass ratio of 1:7 and 1:10, the corresponding composites (TiN/N-TiO2-7 and TiN/N-TiO2-10) changed into a more obvious mixed morphology with more TiN nanoparticles intimately attached on TiO2 nanorods (Figs. 3(g)−3(l)). When the TiO2 to MA mass ratio is 1:10, the resultant TiN/N-TiO2-10 displays an amorphous-like morphology (Figs. 3(j) and 3(k)), and the lattice fringes are mainly TiN with a d-spacing of 0.208 nm (Fig. 3(l)). Once the TiO2/MA mass ratio is enhanced to 1:20, the corresponding TiN product changes into amorphous-like nanoparticle agglomerates with not any nanorod-like morphology (Figs. 3(m) and 3(n)), and only lattice fringes (with a d-spacing of 0.208 nm) of TiN can be observed (Fig. 3(o)). This implies that the in situ nitridation process in the presence of MA leads to the change of the TiO2 morphology, and thus those nanoparticles attached on nanorods shown in Figs. 3(d)– 3(l) can be attributed to TiN. In addition, the elemental analysis results indicate that the TiN/N-TiO2-10 still contains certain amount of TiO2 (Table 1), and the O content in those TiN/TiO2 composites gradually reduce along with the increase of N contents upon enhancing the MA addition amount. This indicates that the oxygen atoms in TiO2 were successfully substituted with nitrogen atoms to form N-TiO2 and even TiN (whose amount is gradually increased). These results are well consistent with the above results on the XRD patterns and XPS spectra, from which the slight shifts of the diffraction peaks of anatase TiO2 (Fig. 1(b)) and the presences of N–Ti–O/Ti–N–O linkages (Figs. 2(b) and 2(d)) can be observed from those composites. Therefore, it can be concluded that the TiN decorated N-TiO2 composite with intimate interface contact can be fabricated via the in situ nitridation process, which will be beneficial for extending the spectral response range and accelerating the photoexcited charge transfer, and then enhancing the photocatalytic H2 evolution as discussed below.
Tab.1 Composition and bandgap of TiO2(c), TiN, and their composites derived from different TiO2/MA mass ratios |
| Sample | Ti/mol% | O/mol% | N/mol% | Ti/O molar ratio | Ti/N molar ratio | TiN/TiO2 molar ratio | Bandgap/eV |
|---|---|---|---|---|---|---|---|
| TiO2(c) | 33.2 | 66.8 | – | 1:2.01 | – | – | 3.16 |
| TiN/N-TiO2-5 | 35.4 | 49.2 | 10.8 | 1:1.39 | 1:0.31 | 1:2.28 | 2.96 |
| TiN/N-TiO2-7 | 37.7 | 39.6 | 17.8 | 1:1.05 | 1:0.47 | 1:1.12 | 2.76 |
| TiN/N-TiO2-10 | 39.8 | 30.2 | 24.6 | 1:0.76 | 1:0.62 | 1:0.62 | 2.71 |
| TiN | 47.6 | 3.52 | 44.1 | 1:0.07 | 1:0.93 | 1:0.08 | – |
3.2 Photocatalytic H2 evolution performance analysis
By using the TiN/N-TiO2-7 composite as photocatalyst, control experiments were conducted to optimize the photoreaction condition. It was found that an optimal condition (30 mg of 2% (mass fraction) Pt-loaded photocatalyst dispersed in a 50 mL of sacrificial agent aqueous solution) can deliver the best photocatalytic performance under illumination of full spectrum of 300 W Xe-lamp. Among those used electron donors including 10% (volume fraction) triethanolamine (TEOA), 50 mmol/L ascorbic acid (AA), 10 mmol/L disodium ethylenediamine tetraacetic acid (EDTA), 10% (volume fraction) methanol (CH3OH) and Na2S (0.35 mol/L) + Na2SO3 (0.25 mol/L) solution (Fig. 4(a)), AA can enable the best H2 evolution activity (453 μmol/h). Besides, the activity of the reaction system containing AA can be further improved by adjusting the pH value using the diluent HCl or NaOH solution (Fig. 4(b)). When the pH value is enhanced from 1.0 to 3.0, the H2 evolution activity displays an increasing trend, and the activity reaches up to 703 μmol/h when the pH value is 3.0. Further enhancing the pH to higher than 3.0, the H2 evolution rate was remarkably reduced, which might be caused by the decreased electron-donor capacity of AA in a photoreaction system with pH>3.0 [36].
Based on the optimized condition, control experiments were also conducted to optimize the TiO2 to MA mass ratio during the in situ nitridation process of the hydrothermally synthesized TiO2. As depicted in Fig. 4(c), all those composites (TiN/N-TiO2-5, TiN/N-TiO2-7, and TiN/N-TiO2-10) exhibit a much higher H2 evolution activity than the single TiO2(c) and TiN illuminated with full spectrum of 300 W Xe-lamp, which can be ascribed to the close contacted TiN to N-TiO2 components to significantly retard the photogenerated charge carrier recombination. Especially, the TiN/N-TiO2-7 delivers the maximum H2 evolution activity (703 μmol/h), which is approximately 2.6 and 32.0 times more than that of TiO2 (270 μmol/h) and TiN (22 μmol/h) alone, respectively. Under visible light (λ>400 nm) illumination, the single TiO2(c) and TiN show a very limited photoactivity, but the TiN/N-TiO2-7 still shows the best H2 evolution activity (368 μmol/h), much higher than that (285 μmol/h) of TiN/N-TiO2-5. In addition, the recycling stability experiment for H2 generation over the TiN/N-TiO2-7 was conducted for three cycles, whereby the photoreaction system was evacuated thoroughly and then irradiated with the full spectrum of the 300 W Xe-lamp. After each cycle, the photocatalyst was repetitively washed with deionized water and alcohol for several times, then dried under vacuum. As seen, an average H2 evolution activity of 655 μmol/h is obtained in the first run of 3 h under 300 W Xe-lamp irradiation, and the activity remains 90% at the last run (Fig. 4(f)). The slight reduction in the activity may stem from the attachment of decomposed product of AA on the photocatalyst. The above results demonstrate that the TiN/N-TiO2-7 possesses a superior H2 evolution activity and a relatively good stability under the above optimized condition.
3.3 Energy band and photocatalytic mechanism analysis
To explore the underlying photocatalytic mechanism of the TiN/N-TiO2 composite, its optical absorption property and energy band structure are investigated. Figure 5(a) displays the UV-vis diffuse reflectance absorption spectra (DRS). As shown, the TiO2(c) product exhibits a narrow UV-light absorption range with an onset absorption edge at 393.2 nm, corresponding to the bandgap of 3.16 eV, which is similar to that in Refs. [37,38]. On the contrary, the TiN obtained from the TiO2 to MA mass ratio of 1:20 exhibits a very broad spectral absorption range with an enhanced visible light absorption in the range of 400−800 nm, which is resulted from the plasmonic absorption of TiN [20,21]. Usually, TiN exhibits a broader plasmonic absorption peak ranging from 450 to 850 nm, further compared with the plasmonic noble metal (e.g. Au) nanoparticles [21]. As for those TiN/N-TiO2 composites, the absorption edges are extended to the visible region due to the nitrogen incorporation into the TiO2 lattice, whose absorption intensity elevates gradually with increasing the TiO2 to MA mass ratio, and then cause the corresponding bandgap to decrease (Table 1). Especially, the absorption edge of TiN/N-TiO2-10 can be extended to approximately 485.0 nm. This extended spectral absorption is in good consistent with the N-TiO2 reported previously [39]. More importantly, those TiN/N-TiO2 composites display an additional visible absorption in the range of 450–900 nm, which is very similar to that of the TiN/N-TiO2 reported previously [20], and thus can be ascribed to the plasmonic effect of the co-existed TiN in the present composite [20,21]. Due to the localized surface plasmon resonance of TiN, a strong electric field is formed at the TiN/N-TiO2 interface, which enhances the visible light absorption in the range of 600–900 nm and the photoelectrochemical oxygen evolution performance of the composite [20]. Similarly, it can be concluded that the coexistence of TiN will be conducive to the utilization of visible light, and resulting in the superior H2 evolution performance of the present TiN/N-TiO2 composite under UV–vis light.
The Mott-Schottky plots (Fig. 5(b)) were employed to determine the Efb of TiO2(c), and the positive slope denotes that TiO2(c) is an n-type semiconductor. All Mott-Schottky plots of the TiO2(c) film have the same intercept with the x-axis at −0.66 V versus SCE (Eθ = 0.24 V versus NHE) [40,41], and thus its flat-band potential (Efb) can be calculated to be −0.66 V+ 0.24 V= −0.42 V versus NHE. Since the conduction band (CB) bottom is more negative by approximately −0.1 V than Efb for a certain n-type semiconductor [42], the CB level (ECB) of TiO2(c) can be estimated to be approximately −0.52 V versus NHE. According to Fig. 5(a) and Table 1, the Eg value of TiO2(c) is 3.16 eV, thus, its valence band (VB) level (EVB) can be calculated to be 2.64 V versus NHE according to the formula (EVB = Eg + ECB). Similarly, the Mott-Schottky plots (Fig. 5(c)) of the TiN film indicate that the Efb is −1.05+ 0.24= –0.81 V versus NHE, corresponding a work function of 3.93 (= –0.81+ 4.74) eV versus vacuum, similar to the reported value (approximately 4.0 eV) [23]. According to the above results, the energetic alignment of the TiN/N-TiO2 composite (derived from the TiO2 to MA mass ratio of 1:7) can be drown as shown in Fig. 5(d). Under full spectrum illumination of the Xe-lamp, the TiN/N-TiO2 composite enables a broad UV-vis light photon capturing by the nitrogen-doping and the plasmonic effect of the in situ formed TiN. The ECB level of N-TiO2 is negative compared to the H+/H2 reduction potential (E = –0.0592pH= − 0.18 V versus NHE) since the pH value of photoreaction suspension is 3.0 (Fig. 4(b)), while the Efb (−0.81 V) of TiN is negative compared to the ECB (–0.52 V) of N-TiO2. This energetic alignment is thermodynamically favorable for hot carrier injecting to the CB of N-TiO2 [21], and those injected electrons and the photoexcited ones of N-TiO2 are further transferred to the Pt cocatalyst to participate in the H2 evolution reaction [43,44]. Meanwhile, the photogenerated holes remain in N-TiO2 transfer to the photocatalyst for the oxidation reaction of hole scavenger [45].
To further confirm the above mechanism, the transient photocurrent response and the electrochemical impedance spectra (EIS) of the TiO2(c), TiN, and TiN/N-TiO2 composite (derived from the TiO2 to MA mass ratio of 1:7) were also conducted. As shown in Fig. 5(e), the TiN/N-TiO2 composite displays the largest photocurrent among the three materials, demonstrating that the composite has more efficient charge transfer/separation processes. Additionally, the smaller radius (Fig. 5(f)) of the TiN/N-TiO2 composite than the TiN and TiO2(c) alone indicates the lower interfacial resistance, which is consistent with the superior photocatalytic performance. As mentioned above, TiN/N-TiO2-7 still shows the best H2 evolution activity (368 μmol/h) under visible light illumination (Fig. 4(c)). However, there is no obvious H2 evolved over the single TiN and TiO2(c). It is reasonable to consider that the TiO2(c) has a wide bandgap as shown in Fig. 5(a). As for the single plasmonic TiN, it covers a wider wavelength range of the solar spectrum (Fig. 5(a)), enabling a more efficient and broadband hot carrier generation [21], and its limited activity (Fig. 4(c)) may be caused by the rapid recombination of those broadband-derived hot carriers of TiN, which can be demonstrated by the very low photocurrent and larger interfacial resistance (Fig. 5(e) and 5(f)). The reason for the obviously enhanced activity of those composites is that the coexisting N-TiO2 can accept the hot electrons of TiN and then transfer them to Pt for H2 evolution reaction. Based on the above results and discussion, it can be concluded that the high visible light utilization stemming from both the N-doping and the plasmonic effect of TiN, the well-aligned energy band structure and intimate interfacial contact between TiN and N-TiO2 facilitate the generation, transfer, and separation of photoinduced charge (and hot charge) carriers, thus boosting the photocatalytic H2 generation performance of TiN/N-TiO2 composite.
4 Conclusions
In summary, a nonmetallic plasmonic TiN decorated N-doped titania (N-TiO2) composite (TiN/N-TiO2) was fabricated via an in situ nitridation of hydrothermally synthesized TiO2, and the TiN was in situ grown on the formed N-TiO2 with an intimate contact interface. The resultant TiN/N-TiO2 composite was characterized with various techniques to explore the crystal phase, morphology, microstructure, chemical composition/states, and energy band structure. The in situ formed plasmonic TiN and N-TiO2 can extend the spectral response range of TiO2 and promote the hot charge carrier separation of TiN via the close contact interface, therefore delivering a highly efficient H2 evolution activity (703 μmol/h), which is approximately 2.6 and 32.0 times more than that of TiO2 and TiN alone under the irradiation of full spectrum of the 300 W Xe-lamp, respectively. This work provides a fascinating approach to in situ formation of nonmetallic plasmonic material/N-TiO2 composite photocatalysts for high-efficiency photocatalytic H2 evolution.