In Situ Formation of Pyridine-Type Carbonitrides-Modified Disorder

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C: Energy Conversion and Storage; Energy and Charge Transport

In Situ Formation of Pyridine-Type CarbonitridesModified Disorder-Engineered C-TiO Used for Enhanced Visible-Light-Driven Photocatalytic Hydrogen Evolution 2

Xing Xu, Lei Lai, Tao Zeng, Yan Yu, Zhiqiao He, and Shuang Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06172 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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The Journal of Physical Chemistry

In Situ Formation of Pyridine-Type Carbonitrides-Modified Disorder-Engineered C-TiO2 Used for Enhanced Visible Light-Driven Photocatalytic Hydrogen Evolution

Xing XUa, Lei LAIb, Tao ZENGb,*, Yan YUb, Zhiqiao HEb, Jianmeng CHENa, Shuang SONGa,*

a

Collaborative Innovation Center of Yangtze River Delta Region Green

Pharmaceuticals, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China b

College of Environment, Zhejiang University of Technology, Hangzhou 310032,

People’s Republic of China

∗

Corresponding authors:

Shuang SONG E-mail: [email protected] Tel.: 86-571-88320331; Fax: 86-571-88320331. Tao ZENG E-mail: [email protected] Tel.: 86-571-88320726; Fax: 86-571-88320276.

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ABSTRACT The exploration of low cost, high performance, and stable photocatalysts for the highly efficient conversion and storage of solar energy in hydrogen is of great importance. This study presents a novel pyridine-type carbonitrides (CN)-modified surface-disordered C-doped TiO2 (PCN/CTiO2@TiO2-x) catalyst used for photocatalytic H2 evolution from water. The employed preparation method of hydrolysis-calcination has the advantages of low cost raw materials and easy scale-up. The optimized PCN/CTiO2@TiO2-x exhibited an impressive hydrogen evolution rate of ~3743 µmol h−1 g−1 under simulated solar light (AM 1.5) and remained stable after five cycles. The maximal quantum efficiency reached 37.5% at 370 nm and 7.0% at 400 nm, which was superior or comparable with several reported relevant TiO2-based catalysts. Benefitting from the pyridine-type CN modification, disordered surface layer (TiO2-x) and increased oxygen vacancies/Ti3+ species, the photogenerated electrons moved rapidly from the visible-response C-doped TiO2 to CN to participate in the photoreduction reaction, which led to a marked improvement in the catalytic activity.

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1. Introduction The photoconversion of H2O to valuable H2 using solar energy is one of the best solutions to the ongoing energy shortage problems.1-2 As an inexpensive, relatively nontoxic, abundant in nature, and chemically stable catalyst, TiO2 has attracted a large amount of attention in photocatalytic water splitting.3-4 However, the relatively wide band gap of TiO2 limits its practical application, because only ultraviolet light, which accounts for 5% of the solar spectrum, can be utilized to separate the electron hole (e-–h+) pairs. In addition, the short lifetime of the photo-generated carriers is another bottleneck in applying TiO2 as a photocatalyst. About 90% of the photo-excited e--h+ pairs recombine in the bulk or on the surface of TiO2. To enhance the photocatalytic activity of TiO2 towards H2 production, extending the photo response of TiO2 to the visible light region and prolonging the lifetime of the photo carriers are two primary strategies to improve the photocatalytic activity of TiO2. To develop visible light responsive TiO2, band gap engineering that allows the modification of the relative position of the electronic energy levels of the photocatalyst is necessary. Doping with various elements including transition metals and non-metals has been demonstrated to be an effective method to narrow the band gap of TiO2. However, doping transition metal cations (such as V, Cr, Mn, Fe, and Ni)5-7 into TiO2 at the Ti sites usually results in localized d-states deep in the band gap, which serve as recombination centers for the photo-generated e--h+ pairs.5 In addition, cationic doping may unfavorably shift the conduction band (CB) lower than the redox 3



potential of hydrogen evolution (H2/H2O), suppressing the possibility of photocatalytic H2 formation using water as an electron acceptor.8 Different from transition-metal doping, non-metal anion doping (including N, C, S, and B)8-10 to modify the optical properties of TiO2 generally via the electronic transitions from the dopant 2p or 3p orbitals to the Ti 3d orbitals scarcely affects the energy levels of the optically excited electrons at the CB. Among the various non-metal elements, the intentional and controlled introduction of carbon into the TiO2 lattice has been shown to result in a significant enhancement in the visible light-driven photocatalytic activity of TiO2.11-16 The doped carbon in TiO2 exists in the forms of C–O bonds via the substitution of carbon at the titanium sites, interstitial carbon, and Ti–C bonds via the substitution of carbon at the oxygen sites.14-16 To effectively separate the photo-induced charge carriers, various efforts have been made such as regulating the morphology, electronic structure, and interfacial characteristics of TiO2. Among these efforts, the loading of a secondary component onto TiO2 can induce synergetic effects that avoid the drawbacks of a single component.17 Carbonitrides (CN) with nitrogen atoms in the carbon framework have been proven to facilitate interfacial electron transfer from TiO2.18 Among the various CN composite structures, pyridine, as a typical π-deficient aromatic functional group, is prone to delocalize its π-electrons over the molecular framework due to the negative inductive effect of the nitrogen atom.19 In light of the fact that the transfer of electrons at the bottom of the CB of TiO2 to an additional strong electron-accepting 4


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group will experience a facile charge separation process, it seems beneficial to deposit pyridine type CN onto TiO2 to permit the preferential electron transfer from the CB of TiO2 to the surface of CN. As a consequence, it sounds more attractive to engineer CN on C-doped TiO2 so as to achieve favorable thermodynamics and kinetics for photocatalytic H2 evolution. Nevertheless, little effort has been made to utilize pyridine type CN decorated C-doped TiO2 for the photo production of H2 under visible light irradiation. Additionally, disorder engineering of TiO2 nanocrystals has recently been demonstrated to promote the photocatalytic efficiency of H2 production.20-21 Therefore, it is envisioned that bringing a surface-disordered character to TiO2-based catalysts will be more advantageous. Nevertheless, the reported methods have a number of limitations for practical application (such as harsh reaction conditions, multiple steps, and/or expensive facilities), and the innovation of a disorder-engineering method for TiO2 is fundamentally important and urgent. With these in mind, we herein first report an in situ pyridine-assisted approach to synthesize novel C-doped black TiO2 with pyridine type carbonitride (PCN) coating. In this method, the atomic hydrogen formed during the carbonization of pyridine diffuses into the subsurface of CTiO2 to cause a surface-disordered TiO2-x layer, so that no additional reductants or high temperature treatment is needed. Meanwhile, the simultaneously formed PCN coating not only protected the Ti3+/oxygen vacancies from oxidation but also enhanced the photogenerated charge separation. More 5



importantly, this cost-effective method was capable of the scale-up production. Due to theses merits, the obtained PCN/CTiO2@TiO2-x exhibited significantly enhanced photocatalytic performance for H2 evolution.

2. Experimental 2.1. Materials Tetrabutyl titanate (Ti(OC4H9)4), pyridine, and sodium borohydride (NaBH4) were purchased from Aladdin Industrial Co., Ltd (Shanghai, China). Ar (99.999%) was obtained from Hangzhou Jingong Special Gas Co., Ltd. (Hangzhou, China). All chemicals used in the experiments were of analytical grade and used without any further purification. Deionized water was used for all the experiments. 2.2. Catalyst preparation All samples were prepared using a facile hydrolysis-precipitation-carbonization approach. Typically, 58 µL of pyridine was dissolved in 100 mL of deionized water and Ti(OC4H9)4 (25 mL) was then added dropwise to the pyridine aqueous solution with vigorous stirring. The resulting suspension solution was stirred continuously for 1 h and then dried at 80°C in open to air to obtain the xerogel precursor. Finally, the precursor was placed into a tube furnace under a flow of Ar gas and heated to 450°C for 3 h at a heating rate of 3°C min-1. After cooling to room temperature, a dark grey powder denoted as 1-PCN/CTiO2@TiO2-x was obtained. Different samples were prepared by varying the quantity of pyridine (58, 290, 6


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522, and 696 µL) dissolved in water. The resultant samples were labeled as y-PCN/CTiO2@TiO2-x (i.e. 1-PCN/CTiO2@TiO2-x, 5-PCN/CTiO2@TiO2-x, 9-PCN/CTiO2@TiO2-x, and 12-PCN/CTiO2@TiO2-x), where “y” indicated the various molar ratios of pyridine to Ti(OC4H9)4. For comparison, a sample of C-doped TiO2 (denoted as CTiO2) without the pyridine feedstock was also prepared using the same procedure. 2.3. Characterization The crystal phase evolved after carbonization of the powders was examined by X-ray diffraction (XRD) using a Thermal ARL X-ray diffractometer (Thermo, France) at room temperature with Cu Kα radiation (λ = 0.15418 nm) at a scan rate of 0.01° s-1 in the 2θ range from 10 to 80°. Raman spectroscopy was performed on a LabRam HR800 Raman system (Horiba Jobin Yvon, France) using a 531.95 nm excitation laser in the wavenumber region of 0–2000 cm-1. The Brunauer-Emmett-Teller (BET) surface area was determined using the nitrogen adsorption and desorption isotherms at 77 K recorded on an ASAP 2010 analyzer (Micromeritics, USA). Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed using a Tecnai G2 F30 S-Twin microscope (Philips-FEI, Netherlands) operated at 200 kV. The TEM samples were dispersed in ethanol and supported on a holey carbon film on a Cu grid. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI 5000C ESCA system (Perkin-Elmer, USA) with Mg Kα radiation (1253.6 eV). Data analysis was performed using XPSPEAK4.1 software. 7



Electron paramagnetic resonance (EPR) measurements were carried out at 77 K on a JES-FA200 spectrometer (JEOL Co., Tokyo, Japan) operating at a microwave frequency of 9.40 GHz, a modulation amplitude of 0.5 mT, a modulation frequency of 100 kHz, and a microwave power of 20 mW. The UV-Vis diffuse reflectance spectra (DRS) were recorded on a UV-Vis 2550 spectrophotometer (Shimadzu, Japan) fitted with an integrating sphere. The photoluminescence (PL) spectra were recorded at room temperature excited by incident light at 300 nm on a fluorescence spectrophotometer (FluoroMax-4P, Horiba Jobin Yvon, France). 2.4. Photoelectrochemical and electrochemical measurements The transient photocurrent responses were recorded using a three electrode quartz cell on a CHI 660D electrochemical workstation (CH Instrument, USA). The photocatalyst dispersed on indium tin oxide (ITO) glass acted as the working electrode. A Pt flake and Ag/AgCl (saturated KCl) were used as the counter and reference electrode, respectively. The electrolyte was 0.1 M Na2SO4 solution and the light source was a 250 W Xenon high brightness cold light source (XD-300) equipped with a UV cut-off filter (λ > 400 nm). Electrochemical impedance spectra (EIS) was recorded on a Solartron SI 1287 electrochemical interface and a SI 1260 impedance/gain-phase analyzer. The measurements were performed in a 0.1 M Na2SO4 solution with an AC voltage magnitude of 5 mV over a frequency range of 106–10−1 Hz in the dark. 2.5. Photocatalytic hydrogen evolution 8


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The photocatalytic hydrogen evolution reaction was performed in a cylindrical stainless steel reactor. The reactor consisted of an O-ring quartz window (6.5 cm in diameter) covering the top and three small openings sealed with silicone rubber stoppers. A 300 W Xe arc light source (Beijing Electric Light Sources Research Institute, China) equipped with a UV cutoff filter (λ > 400 nm) was used as the visible light source. The focused intensity on the reaction solution was measured as ~53.6 mW cm−2 using a visible light radiometer (FZ-A, Beijing Normal University, China). An AM 1.5G filter (CEAuLight, China) instead of the UV cutoff filter (λ > 400 nm) was used to simulate sunlight. In a typical reaction, the photocatalyst (0.1 g) was dispersed in 100 mL of an aqueous solution containing 15 vol% triethanolamine (TEOA). Prior to the photocatalytic H2 formation, the deposition of 1.5 wt% Pt (acting as a co-catalyst) was conducted by directly dissolving H2PtCl6 into the mixed solution. The suspension was then sonicated for 30 min followed by illumination for 30 min with magnetic stirring. Then, Ar gas was bubbled through the reaction solution for 30 min to remove the dissolved oxygen. During the whole photocatalytic H2 production process, the reaction solution temperature was kept at 25°C using a water bath system, and continuous agitation was maintained to ensure the uniform irradiation of the suspension. At preset intervals the gaseous products were collected using a 1 mL gastight syringe, and then manually injected into a 7890B gas chromatograph (GC, Agilent, USA) to quantitatively determine the amount of H2 formed. The GC was equipped with a thermal 9



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conductivity detector and a HP-MOLESIEVE capillary column (30 m × 0.32 mm × 12 µm). To evaluate the photocatalytic performance as a function of the wavelength of incident light, monochromatic light with various wavelengths were obtained upon inserting the appropriate band pass filter (370, 400, 430, 460, and 490 nm) in front of the 300 W Xe lamp. The quantum efficiency (QE) was defined using Eq. (1).

QE % =

number of H molecules generated × 2 × 100 number of incident photons

(1)

In which, the number of incident photons (Nphotons) can be calculated using Eq. (2).

N!"#$#%& =

'( hc

(2)

where E is the input optical power, λ is the wavelength of the monochromatic light, h is Planck’s constant and c is the speed of light.

3. Results and discussion 3.1. Characterization of the catalysts 3.1.1. XRD analysis XRD was first used to investigate the crystallinity and phase purity of the different samples. Figure 1 shows that anatase (JCPDS Card No. 21-1272) was the predominant phase and a small percentage of brookite (JCPDS Card No. 29-1360) also existed in all the samples, as calculated from the empirical relationship shown in Eq. (3).

)* =

k , -* k . -/ + k , -*

(3) 10


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where AB and AA are the peak intensity of the brookite (121) and anatase (101), respectively, while kA = 0.886 and kB = 2.721 are the correction coefficients,22 and the weight fraction of brookite (WB) in all the samples was ~13%. Since brookite TiO2 has a large band gap of 3.35 eV,23 making it only active in the UV region, the presence of brookite is scarcely responsible for the visible light photo-response. Notably, the XRD patterns of all samples remained almost unchanged regardless of the pyridine dosage, indicating its negligible effect on the crystallographic composition of TiO2 during Ti(OC4H9)4 hydrolysis. 3.1.2. Raman analysis Raman spectroscopy, which possesses high sensitivity for monitoring the variation of the microstructure, was used to reveal the structural characteristics of the PCN/CTiO2@TiO2-x and CTiO2 catalysts (Figure 2a and b). Five Raman active modes with frequencies at 136, 186, 387, 505, and 627 cm−1 were indexed as the anatase TiO2 phase. A closer observation found that the strongest peak at ~136 cm-1 was slightly up-shifted and broadened, which indicated the original symmetry of the TiO2 lattice was reduced.21, 24-25 Other researchers believe that the surface disorder and atomic rearrangement originate from the oxygen vacancies.26 Moreover, the spectra recorded for PCN/CTiO2@TiO2-x featured peaks at ~1350 cm−1 and ~1580 cm−1, corresponding to the D and G bands of carbon in CN as a result of the carbonization of pyridine.27 Similar results were obtained by Jiang et al. during the carbonization of pyridine under solvothermal conditions. The G band is related to the E2g vibration 11



mode of sp2-bonded carbon atoms in a two-dimensional (2D) hexagonal lattice and the D band is associated with the vibrations of disordered carbon atoms with defects and heteroatoms in the lattice.28 3.1.3. XPS XPS was performed to investigate the surface composition and binding energy (BE) of the constituent elements in the as-prepared PCN/CTiO2@TiO2-x and CTiO2 catalysts. As shown in Figure S1, the wide-scan XPS survey spectra demonstrated the existence of Ti, O, C, and N in the samples. The deconvoluted high resolution C 1s XPS spectra are presented in Figure 3a. The three fitted peaks located at binding energies of 283.2, 284.6, and 287.2 eV represent three different chemical environments for the element carbon. The major peak at 284.6 eV was ascribed to adventitious hydrocarbon contamination, which could act as an internal reference to correct the shift of the BE. In contrast to CTiO2, a new peak assigned to the C–N bond was detected at 287.2 eV for PCN/CTiO2@TiO2-x, revealing the presence of CN,29 which was in good agreement with the Raman results. Different from other groups’ results,13-14 where the interstitial carbon-doped TiO2 was prepared through xerogel carbonization in a hypoxic atmosphere, the interstitial carbon in the crystal lattice could not be detected at ~288.9 eV. Instead, a progressively prominent shoulder peak at ~283.2 eV corresponding to the Ti–C bond emerged as a consequence of the gradual introduction of pyridine into the hydrolysis solution of Ti(OC4H9)4,30 which strongly suggested that some C atoms had 12


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successfully occupied the O sites in the TiO2 lattice. As demonstrated in the Raman analysis, the feedstock of pyridine facilitated the formation of oxygen vacancies. Richer oxygen vacancies will be beneficial to produce more Ti–C bonds by giving excess electrons to the substitutional carbon atoms.31 The Ti 2p spectra (Figure 3b) recorded for all the samples exhibited two peaks at ~457.3 and ~463 eV, corresponding to the Ti 2p3/2 and Ti 2p1/2 spin-orbital peaks of TiO2. The spin-orbital doublet splitting was 5.7 eV,32-33 implying an oxidation state of Ti4+ in TiO2. The absence of any shoulder peaks at the lower BE of Ti 2p3/2 and Ti 2p1/2 attributed to Ti3+ suggested that Ti3+ was not present on the surface. This result will be further proved by the EPR analysis described in the next section. In contrast to pure TiO2, a downward shift of the BE of Ti 2p was clearly observed, which originated from the strong interaction between the C anion and Ti cation.34 Figure 3c depicts the O 1s XPS peaks, which could be deconvoluted into three peaks. We assigned those at ~ 528.5 and ~529.6 eV to lattice oxygen in TiO2 and surface -OH species,35 respectively. The insignificant XPS peak near 531.4 eV could be ascribed to adsorbed H2O molecules. The lattice oxygen peak, situated at 528.5 eV, was shifted by ~1 eV towards a lower BE value relative to pure TiO2. This was connected with the presence of oxygen vacancies.36 The results were in accordance with the Raman results. The N 1s spectra of PCN/CTiO2@TiO2-x (Figure 3d) depicts that pyridinic-N (398.6 eV)37 was the main N species and its intensity was enhanced with an increased 13



pyridine dosage. This implied that the CN covered on the CTiO2 was pyridine-type carbonitrides (PCN). Also, the absence of a Ti–N bond (~397.0 eV) indicated that N did not substitute the O ions in the TiO2 lattice.9 3.1.4. EPR The presence of Ti3+ was further identified using low temperature EPR spectroscopy. It is known that an EPR signal featured at g = 2.02 if Ti3+ was present on the TiO2 surface because Ti3+ tends to adsorb O2 via trapping atmospheric oxygen molecules on the defect sites and reduces them to superoxide radical anions.38 However, a strong paramagnetic signal was observed at g = 1.99 rather than g = 2.02 in the as-prepared samples (Figure 4), implying that the Ti3+ mainly existed on the subsurface.39 Meanwhile, the relative intensity of the signals increased as the pyridine dosage was increased, suggesting that the surface modification of PCN promoted the creation of Ti3+ in CTiO2. 3.1.5. BET Generally, a large specific surface area for a photocatalyst is beneficial to promote the photocatalytic performance by providing effective surface active sites. The BET specific surface areas of our samples were examined using N2 adsorption/desorption measurements. The corresponding nitrogen adsorption-desorption curves are shown in Figure S2. As a control sample, CTiO2 showed a BET surface area of ∼15.5 m2 g−1. The increased dosage of the pyridine feedstock used for the preparation of PCN/CTiO2@TiO2-x led to a slight decreasing 14


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trend in the BET surface area; the BET surface areas of 1-PCN/CTiO2@TiO2-x, 5-PCN/CTiO2@TiO2-x, 9-PCN/CTiO2@TiO2-x, and 12-PCN/CTiO2@TiO2-x were found to be 115.3, 114.6, 113.5, and 100.5 m2 g−1, respectively. This could be attributed to the coverage of PCN on the TiO2 surface. Obviously, the BET data indicated that the decreasing surface area was not the determining factor for the different photocatalytic activity observed for our samples due to the small difference between the observed BET surface areas. 3.1.6. TEM analysis TEM analysis was conducted to investigate the morphology and structure of the CTiO2 and PCN/CTiO2@TiO2-x samples. As illustrated in Figure S3, the crystallite sizes were in the range of 10–15 nm and the addition of pyridine during the preparation process did not significantly change the morphology and crystal size of TiO2. Figure 5a and b show the HRTEM image of 9-PCN/CTiO2@TiO2-x. A disordered surface layer (TiO2-x) with a few atomic thick coating on CTiO2 was observed, as marked by the white dashed line. In order to gain more insight into the disordered shell, we selected two areas (red box) to perform the fast Fourier transform (FFT) and inverse fast Fourier transformation (IFFT). The ordered lattice areas in the IFFT showed well-resolved (101) and (004) lattice planes with typical anatase d-spacings of 0.35 nm and 0.23 nm, respectively. In contrast, the disordered layer displayed lattice distortions and dislocations, suggesting the modification or breaking of the symmetry of TiO2.40-41 Furthermore, the elemental mapping images of 15



9-PCN/CTiO2@TiO2-x in Figure 5c showed that Ti (green) and O (yellow) were homogeneously distributed, whereas N (orange) was mostly focused on the C signal (deep red). These results further confirmed the presence of PCN and the surface disordered layer of TiO2 in addition to the Raman and XPS analyses. 3.1.7. EIS Electrical conductivity is an important property for a photocatalyst. To explore the effect of the PCN on the electron-transfer capacity of CTiO2, the EIS of 9-PCN/CTiO2@TiO2-x and CTiO2 were measured in the dark under open circuit conditions. From Figure 6, the Nyquist plots of both catalysts include semicircle and linear line regions, which reflected the charge transfer process and diffusion-controlled step, respectively. After simulating the equivalent electrical circuit shown in the inset of Figure 6, it can be seen that 9-PCN/CTiO2@TiO2-x exhibited a smaller semicircle radius when compared to CTiO2. The results indicated that the PCN improved the electronic conductivity of CTiO2 in its non-photoexcited state. This was attributed to the introduction of nitrogen atoms giving an additional electron to the delocalized π-system, regardless of the distortion of the carbon skeleton.42-44 As a consequence, the presence of the PCN boosted the photoexcited charge separation during photocatalysis. 3.1.8. UV-Vis DRS The optical absorption features of CTiO2 and PCN/CTiO2@TiO2-x are shown in Figure 7. It can be seen that the CTiO2 responds to visible light with an absorption 16


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edge of ~403 nm (3.07 eV). XPS analysis confirmed that substitutional carbon rather than interstitial carbon could be detected. We therefore concluded that the band gap narrowing originates from the hybridization of the O 2p orbital with the C 2p orbital of substitutional carbon, leading to an impurity energy level on the top of the valence band.9 When pyridine was used to modify the TiO2, the absorption edge varied scarcely upon initially increasing the pyridine concentration, suggesting that the N atoms in the pyridine rings were not incorporated into the TiO2 lattice. In addition, for the CTiO2 and PCN/CTiO2@TiO2-x samples, a considerably large background absorption in the visible light region was observed and the absorption intensity increased with the initial pyridine content. The results were consistent with the change in color observed for the powders from light grey to deep grey. These absorptions should be mainly caused by the surface modified PCN, demonstrating that the PCN was present on the surface of CTiO2@TiO2-x. 3.2. The formation mechanism of PCN/CTiO2@TiO2-x A possible formation mechanism of the PCN/CTiO2@TiO2-x can be elucidated based on the results of the physical chemical characterization of the photocatalysts. In the present work, the hydrolysis of Ti(OC4H9)4 should proceed insufﬁciently at a molar ratio of H2O/Ti ≤100. Thus, the Ti groups in the hydrolyzed products contain Ti–O–C in addition to Ti–OH.45 Because Ti–O–C has been demonstrated to serve as a structural impurity and inhibits crystallization, the carbon-doped TiO2 is formed comprised of a mixed crystallin phase in terms of anatase and brookite upon 17



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calcination under an Ar atmosphere, as verified by the XRD and XPS measurements. No obvious difference in the absorption edge between CTiO2 and PCN/CTiO2@TiO2-x could be observed, further proving the above proposition. Accompanied with carbon-doping, the PCN was deposited on the surface of CTiO2 followed by the carbonization of pyridine, which could be confirmed by the Raman, XPS, and UV-Vis DRS results. It should be noted that pyridine carbonization is usually accompanied with a dehydrogenation process, as shown in Eqs. (4) and (5).46-47 The atomic hydrogen formed will interrupt the bonds between the lattice oxygen and Ti, leading to the generation of oxygen vacancies.21, 40 In other words, the hydrogenation of TiO2 leads to the formation of a surface-disordered layer and defect-rich TiO2-X. Because the adsorption energy of hydrogen in the subsurface sites was ~0.4 eV, which is smaller than that at surface sites, the atomic hydrogen can diffuse into the subsurface to reduce some Ti4+ to Ti3+ (as detected by EPR analysis) during the calcination process.48 C5H5N (pyridine) → o-C5H4N (o-pyridyl) + H

(4)

H + C5H5N (pyridine) → o-, m-, p-C5H4N (pyridyl) + H2

(5)

3.3. Photocatalytic hydrogen evolution Photocatalytic H2 evolution over the CTiO2 and PCN/CTiO2@TiO2-x catalysts was carried out in a TEOA aqueous solution containing 1.5 wt% Pt under visible light illumination (λ > 400 nm) at 25°C. As shown in Figure 8a, CTiO2 exhibited a photocatalytic H2 evolution rate of 121 µmol h−1 g−1. In contrast, all of the 18


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PCN/CTiO2@TiO2-x samples consistently yielded larger amounts of H2 within 3 h of photocatalysis. The activities of the PCN/CTiO2@TiO2-x catalysts were in the order 9-PCN/CTiO2@TiO2-x > 5-PCN/CTiO2@TiO2-x > 1-PCN/CTiO2@TiO2-x > 12-PCN/CTiO2@TiO2-x. The 9-PCN/CTiO2@TiO2-x sample showed the highest photocatalytic activity with a H2 production rate of ~384 µmol h−1 g−1 (1152 µmol g−1 after 3 h of irradiation), which was 3.17-fold higher than that of CTiO2. As well-known, semiconductor photocatalysis involves two crucial processes, semiconductor photoexcitation and the transfer and separation of photoinduced charge carriers.4 The photoexcitation arising from photon energy depends on the band gap of the semiconductor. However, no obvious difference in the band gap was observed for the different photocatalysts, as shown in Figure 7. Therefore, the improved photocatalytic activity of PCN/CTiO2@TiO2-x towards H2 formation does not stem from the electronic excitation of the visible light absorbing units. Because charge separation together with charge-carrier mobility play key roles in determining the photoactivity of a photocatalyst, we concluded that the efficient photocatalytic activity of PCN/CTiO2@TiO2-x should be mainly ascribed to the enhanced charge separation efficiency. PL measurements were performed to disclose the dynamics of the photogenerated species.49 As shown in Figure 8b, the PL emission intensity of CTiO2 was consistently higher than PCN/CTiO2@TiO2-x. The PL intensity became weaker with the stepwise increase in the dosage of pyridine used in the catalyst preparation 19



process. Among the various PCN/CTiO2@TiO2-x samples, 12-PCN/CTiO2@TiO2-x exhibited the weakest emission intensity, which could be mainly attributed to the light shielding effect of the surface PCN. The light shielding effect prevented TiO2 from absorbing light, thereby reducing the generation of photoexcited e-–h+ pairs. This result was inconsistent with that of photocatalytic hydrogen evolution experiments, where 9-PCN/CTiO2@TiO2-x presented the highest photoactivity, as will be discussed below. The transient photocurrent response was also recorded to further characterize the photoexcitation and the transfer and separation of photoinduced charge carriers in the various catalysts. Figure 8c shows the photocurrent-time (i-t) curves for several on-off cycles of irradiation. Generally, the photocurrent value increased sharply when the light was turned on and quickly decreased to its initial state once the light was turned off; this photoresponsive phenomenon was entirely reversible. The photocurrent density of CTiO2 under visible light irradiation was about 4.3 µA cm−2, whereas those of the 1-PCN/CTiO2@TiO2-x, 5-PCN/CTiO2@TiO2-x, 9-PCN/CTiO2@TiO2-x, and 12-PCN/CTiO2@TiO2-x samples were ~6.0, 7.7, 11.8, and 4.4 µA cm−2, respectively. These values were in accordance with the photocatalytic H2 generation results. As demonstrated, the surface disordered shell of PCN/CTiO2@TiO2-x is beneficial for photo-generated electron transfer by eliminating the original energy barrier.41 Furthermore, the PCN/CTiO2@TiO2-x samples possess more Ti3+/oxygen vacancies than CTiO2. The oxygen vacancies are shallow donors in TiO2 with a 20


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relatively low formation energy, which decreases the number of surface recombination centers.50-51 The increased surface oxygen vacancies (donor density) of PCN/CTiO2@TiO2-x improve charge transportation. Moreover, the PCN with conjugated functional groups of π-deficient aromatic systems act as an effective electron transfer channel for the excited electrons.19 The electron transfer was then boosted and the recombination rate of the photogenerated charge carriers was enormously restrained. However, the amount of introduced C in the TiO2 lattice increased upon increasing the initial pyridine content, as determined by XPS. Excessive C-doping results in charge carrier recombination because the localized C 2p narrow band in the band-gap acts as a recombination centers for photo-generated holes and electrons.52-53 In addition, as mentioned above, the PCN covering the surface of TiO2 can cause a light shielding effect to prevent TiO2 from absorbing light, resulting in the weakest PL response observed for the 12-PCN/CTiO2@TiO2-x photocatalyst. As a result, an optimal 9-PCN/CTiO2@TiO2-x catalyst with respect to the photocatalytic H2 formation is required to balance the positive and negative effect of pyridine addition during the preparation process. To confirm the role of PCN in separating the photogenerated carriers, we reduced CTiO2 using a strong reducing reagent, NaBH4 (0.1 M), at 30 °C in a hydrothermal environment for 1 h (CTiO2-NaBH4).54-55 From the EPR spectrums, the CTiO2-NaBH4 showed a similar Ti3+ signal intensity with 9-PCN/CTiO2@TiO2-x. However, the relevant i–t curve of CTiO2-NaBH4 presented a weaker photocurrent 21



than 9-PCN/CTiO2@TiO2-x. The photocurrent of the reduced CTiO2 was 5.4 µA cm−2 at 1.23 V vs. RHE, which is approximately half of that of 9-PCN/CTiO2@TiO2-x. Therefore, the enhanced photocatalytic activity of PCN/CTiO2@TiO2-x towards H2 evolution was partially associated with the PCN. The π-electrons in the pyridine ring are prone to delocalize over the ring due to the negative inductive effect of the nitrogen atom, accelerating the separation of photogenerated e-–h+ pairs. Combining the abovementioned findings, carbon doping was responsible for the visible photoactivity of the PCN/CTiO2@TiO2-x catalysts through mixing the C 2p orbitals with O 2p orbitals. The superior performance of the PCN/CTiO2@TiO2-x samples when compared with CTiO2 could be ascribed to the synergistic effect of the high concentration Ti3+ in TiO2, surface disordered shell, and surface covered CN. Their cooperating function favored the separation and migration of photogenerated carriers and thus enhanced the photocatalytic H2-evolution reaction. To compare the photocatalytic activities of the prepared catalysts with other reported TiO2-based materials, we measured the QE for H2 production from water using 9-PCN/CTiO2@TiO2-x as a function of the wavelength of incident light used. The QE values were approximately 37.5% at 370 nm, 7.0% at 400 nm, 0.068% at 430 nm, 0.041% at 460 nm, and 0.032% at 490 nm (Figure 8d). Clearly, the variation trend of the QE matched well with that of the UV-Vis absorption spectra recorded using irradiation wavelengths < 400 nm. This result suggested that the reaction was initiated by the photo-absorption of the catalyst. The broad background absorption at 22


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wavelengths > 400 nm scarcely contributed to the photoactivity. The photocatalytic H2 synthesis from water using 9-PCN/CTiO2@TiO2-x was also performed in the same experimental apparatus equipped with an AM 1.5G filter. During 3 h of illumination, the 9-PCN/CTiO2@TiO2-x could steadily produce hydrogen at about 3743 µmol h−1 g−1, as illustrated in Figure 9a. A comparison of the photocatalytic hydrogen production with other relevant TiO2-based catalysts is provided in Table S1. It demonstrated that the performance of PCN/CTiO2@TiO2-x was superior or comparable to similar reported materials. In order to further investigate the potential application of the 9-PCN/CTiO2@TiO2-x catalyst under natural sunlight irradiation, photocatalytic H2 production from water was performed between 11:30 am and 2:30 pm on the roof of the laboratory building at Zhaohui Campus of Zhejiang University of Technology (Hangzhou, China) on August 13, 2017. The operating conditions were identical to those proceeded in the visible light irradiation experiments. As shown in Figure 9b, H2 was continuously produced from aqueous TEOA solutions under direct sunlight illumination. The generation of H2 reached ~4275 µmol g−1 after 3 h of reaction, which is half of that produced in the laboratory using a solar simulator (AM 1.5) as the light source. This was because the maximal incident solar intensity of the outdoor experiment was lower than 100 mW (light intensity for simulated sunlight experiment with an AM 1.5 filter). In addition, the light intensity of natural sunlight (Figure 9b) varied during the course of the day and was affected by the atmospheric conditions 23



including clouds and wind. Together with the good reusability of the catalyst, we believe that PCN/CTiO2@TiO2-x can act as an alternative catalyst for realizing practical H2 production under sunlight. To evaluate the photo-stability of our photocatalysts, a recycling test was performed for the representative 9-PCN/CTiO2@TiO2-x catalyst. The reaction was allowed to run accumulatively for 40 h under visible light irradiation with intermittent evacuation every 8 h. As shown in Figure 10, 9-PCN/CTiO2@TiO2-x exhibited a catalytic ability for the continuous and stable production of H2 over five cycles.

Conclusions In conclusion, a novel PCN/CTiO2@TiO2-x catalyst has been successfully fabricated via the hydrolysis of Ti(OC4H9)4 in an aqueous pyridine solution followed by calcination under a flow of Ar. The material functions as a highly efficient and stable photocatalyst for hydrogen generation from water using solar energy. The 9-PCN/CTiO2@TiO2-x catalyst showed the best catalytic activity with a hydrogen evolution rate of 384 µmol h−1 g−1 under visible light irradiation (λ > 400 nm). The QE for H2 formation over PCN/CTiO2@TiO2-x at 370 nm and 400 nm reached ~37.5% and 7.0%, respectively. The joint action of Ti3+ in the surface-disordered CTiO2 and pyridine-type CN in the separation of photo-excited electron hole pairs was demonstrated to be the origin of the enhanced photocatalytic activity. We anticipate that this low cost catalyst prepared using a facile method can render versatile 24


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applications in solar energy conversion.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.0000000. Figure S1. The wide-scan XPS spectra. Figure S2. The nitrogen adsorption-desorption curves. Figure S3. The TEM images. Table S1. A comparison of the hydrogen evolution performance of 9-PCN/CTiO2@TiO2-x with other relevant TiO2-based materials.

Acknowledgements This work was surpported by the National Science Foundation of China (21477117 and 21607130) , the Zhejiang Provincial Natural Science Foundation of China (LZ18B070001, LGF18E080017, and LR14E080001), and the China Postdoctoral Science Foundation (2017T100436).

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Figure Captions Figure 1. The XRD patterns recorded for CTiO2 and PCN/CTiO2@TiO2-x. Figure 2. The Raman spectra recorded for CTiO2 and PCN/CTiO2@TiO2-x. Figure 3. The high resolution scanning XPS spectra recorded for (a) C 1s, (b) Ti 3d, (c) O 1s, and (d) N 1s. Figure 4. The EPR spectra recorded for CTiO2, PCN/CTiO2@TiO2-x, and CTiO2-NaBH4. Figure 5. (a, b) The HRTEM and (c) the elemental mapping images obtained for 9-PCN/CTiO2@TiO2-x. The insets are the corresponding IFFT images. Figure 6. The EIS recorded for CTiO2 and 9-PCN/CTiO2@TiO2-x in the dark. Figure 7. The UV-Vis DRS recorded for CTiO2 and PCN/CTiO2@TiO2-x. Figure 8. (a) The photocatalytic hydrogen evolution of CTiO2 and PCN/CTiO2@TiO2-x under visible light irradiation (λ > 400 nm). (b) The PL spectra of CTiO2 and PCN/CTiO2@TiO2-x. (c) The time-based photocurrent response of CTiO2, PCN/CTiO2@TiO2-x, and CTiO2-NaBH4 at 1.23 V vs. RHE under visible light irradiation. (d) The wavelength-dependent QE (red dots) of H2 production from water using 9-PCN/CTiO2@TiO2-x and the UV-Vis absorption spectrum (black) of 9-PCN/CTiO2@TiO2-x. Figure 9. (a) The photocatalytic hydrogen evolution of 9-PCN/CTiO2@TiO2-x under simulated solar light irradiation. (b) The H2 production rate of 9-PCN/CTiO2@TiO2-x under natural sunlight (August 13, 2017; at Zhejiang University of Technology, 34


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Hangzhou, China) and the intensity of natural sunlight observed during the reaction process. Figure 10. The stability of 9-PCN/CTiO2@TiO2-x under visible light irradiation (λ > 400 nm).

35



Figure 1.

A(101)

12-PCN/CTiO2@TiO2-x 9-PCN/CTiO2@TiO2-x 5-PCN/CTiO2@TiO2-x

20

30

40

50

2θ (degree)

36


60

A(204)

CTiO2

A(211)

A(105)

A(004)

B(121) 10

A(200)

1-PCN/CTiO2@TiO2-x

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 46

70

80

Page 37 of 46

Figure 2.

(a)

(b) 12-PCN/CTiO2@TiO2-x

Intensuty (a.u.)

9-PCN/CTiO2@TiO2-x

CTiO2

5-PCN/CTiO2@TiO2-x

1-PCN/CTiO2@TiO2-x

Intensity/ (a.u.)

136

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5-PCN/CTiO2@TiO2-x 9-PCN/CTiO2@TiO2-x 100

120

140

12-PCN/CTiO2@TiO2-x

160 −1

Raman shift (cm )

387

186

0

100

200

300

400

505

500

1-PCN/CTiO2@TiO2-x CTiO2

627

600

700

800

1000

−1

1200

1400

1600 −1

Raman shift (cm )

Raman shift (cm )

37


1800


Figure 3. (a) C 1s

(b) Ti 3d CTiO2

1-PCN/CTiO2@TiO2-x

5-PCN/CTiO2@TiO2-x

Intensity (a.u.)

Intensity (a.u.)

CTiO2

1-PCN/CTiO2@TiO2-x

5-PCN/CTiO2@TiO2-x

9-PCN/CTiO2@TiO2-x

9-PCN/CTiO2@TiO2-x

12-PCN/CTiO2@TiO2-x

12-PCN/CTiO2@TiO2-x 282

284

286

288

456

290

458

460

462

464

466

Binding energy (eV)

Binding energy (eV)

(d) N 1s

(c) O 1s CTiO2

1-PCN/CTiO2@TiO2-x

5-PCN/CTiO2@TiO2-x

1-PCN/CTiO2@TiO2-x

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 46

5-PCN/CTiO2@TiO2-x

9-PCN/CTiO2@TiO2-x 9-PCN/CTiO2@TiO2-x

12-PCN/CTiO2@TiO2-x

527

528

529

530

531

532

12-PCN/CTiO2@TiO2-x

533 392

394

396

398

400

Binding energy (eV)

Binding energy (eV)

38


402

404

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4.

39



Figure 5.

40


Page 40 of 46

Page 41 of 46

Figure 6. 120

CTiO2 9-PCN/CTiO2@TiO2-x

100

Rs

Rt

80

Zim (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

CPE

40

20

0 0

20

40

60

80

Zre (ohm)

41


100


Figure 7.

42


Page 42 of 46

Page 43 of 46

Figure 8. (a)

1200

(b) 1-PCN/CTiO2@TiO2-x

1000

−1

5-PCN/CTiO2@TiO2-x

800

CTiO2

Intensity (a.u.)

9-PCN/CTiO2@TiO2-x 12-PCN/CTiO2@TiO2-x

600 400

1-PCN/CTiO2@TiO2-x 5-PCN/CTiO2@TiO2-x 9-PCN/CTiO2@TiO2-x 12-PCN/CTiO2@TiO2-x

200 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

400

450

Time (h)

550

600

650

50

(d)

5-PCN/CTiO2@TiO2-x

−2

1-PCN/CTiO2@TiO2-x

40

12-PCN/CTiO2@TiO2-x CTiO2

light off

15 10

Absorbance (a.u.)

CTiO2-NaBH4

20 light on

30

20

10

5 0 0

50

100

150

200

700

Wavelength (nm) 9-PCN/CTiO2@TiO2-x

(c)

25

500

250

300

350

200

Time (s)

300

400

500

600

Wavelength (nm)

43


700

0 800

Quantum efficiency (%)

H2 evolution (µmol g )

CTiO2

Current density (µA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



70

(a)

5000

(b)

60

−1


−1

10000 8000 6000 4000 2000 0 0.0

4000

50

3000

40 30

2000 20 1000

10

0 0.5

1.0

1.5

2.0

2.5

3.0

0 11:30 12:00 12:30 13:00 13:30 14:00 14:30

Time (h)

Operation time

44


−2

12000

Intensity of natural sunligh (mW cm )

Figure 9.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 46

Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10.

45



TOC Graphic

46


Page 46 of 46

In Situ Formation of Pyridine-Type Carbonitrides-Modified Disorder

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