Highly Carbon-doped TiO2 Derived from MXene Boosting the

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Highly Carbon-doped TiO2 Derived from MXene Boosting the Photocatalytic Hydrogen Evolution Guangri Jia, Ying Wang, Xiaoqiang Cui, and Weitao Zheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03406 • Publication Date (Web): 09 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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ACS Sustainable Chemistry & Engineering

Highly Carbon-doped TiO2 Derived from MXene Boosting the Photocatalytic Hydrogen Evolution Guangri Jia a, Ying Wang a, Xiaoqiang Cui, a, * and Weitao Zheng a, *

a

Key Laboratory of Automobile Materials of MOE and School of Materials Science and

Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China. *

Corresponding author: Xiaoqiang Cui and Weitao Zheng

E-mail address: [email protected]; [email protected] Tel & Fax: +86-431-85155279

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ABSTRACT The effective in-situ carbon-doping of titanium dioxide can greatly improve the lifetime of photogenerated carriers and widen the absorption range of light for photocatalytic reactions. Here, highly carbon-doped TiO2 (HC-TiO2) with hierarchical structure and good crystallization was synthesized by using the exfoliated MXene supernatant at low temperature. The resultant HC-TiO2 was fully characterized, and its catalytic performance for photocatalytic hydrogen evolution was investigated. It was found that the high content carbon-doping induced a valence band tail state that promoted photogenerated carriers’ effective separation and reduced bandgap, which greatly improved the utilization of light for photocatalytic reactions. This band tail was attributed to the strong electron withdrawing carboxylate groups from in-situ carbon doping. The photocatalytic hydrogen production rate of the hierarchical HC-TiO2 exhibited 9.7 times that of the commercial P25 without co-catalyst under simulated sunlight. This work enriched the synthesis method of C-doped TiO2 and the application of MXene based materials.

KEYWORDS: Photocatalyst; Hydrogen evolution; C-TiO2; MXene; Hierarchical structure

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INTRODUCTION As one of the most promising photocatalytic materials, titanium dioxide (TiO2) plays an important role in the environment, energy, and catalysis.1-5 A lot of photocatalysts have been developed up to now,6-9 but TiO2 is still the most promising photocatalyst10-11 because of its abundance, environmentally friendly, low cost, good thermal and chemical stabilities.12-13 However, it still suffers from the wide bandgap (3.2 eV) and the fast photo-generated carrier recombination velocity (1-10 ps) that hampers its large-scale application in photocatalysis field.14-18 Doping is efficient for changing the band position and the bandgap while enhancing the carrier concentration and lifetime. Transition metal cations (Fe, Cr, Ni, Cu, etc.) and anions (C, N, S, etc.) have been used as dopants,19-26 among which carbon doping provided critically narrow bandgap and high absorption coefficient.27-30 Carboxyl group provided by carbon-doping not only possessed a strong covalent binding ability to TiO2 but also acted as the anchoring group for photocatalysts.19, 31-33 Carbon-doping also broadened the utilization of light and sensitized TiO2 by influencing the electronic environments for Ti cations and O anions.34 Recent studies showed that C-TiO2 could further enhance carrier concentration and the lifetime.32, 35 Many synthetic methods for C-TiO2 materials have been reported, such as sol-gel process,36 heat oxidation of TiC,37 solvothermal synthesis,30 and electrospinning.38-39 These methods usually demand an annealing process at high temperature to remove organics and increase the crystallinity, during which the anatase TiO2 will gradually change to rutile TiO2. Developing a new strategy for preparing HC-TiO2 at a low temperature will be interesting for photocatalytic materials. Here, we chose an ultrathin structure Ti3C2Tx (Tx standing for the surface terminations) as a precursor for the synthesis of HC-TiO2.40-42 A large number of oxygen-containing functional groups (-O and -OH) facilitated the formation of TiO2 and carbon-doping in the strong alkaline environment.43 The stripped Ti3C2Tx dispersed in the organic alkali (tetramethylammonium hydroxide: TMAOH) was raw materials for hydrothermal treatment. The high carbon content changed the electronic structure and adjusted the bandgap of TiO2 for broadening the light absorption range of TiO2. It also increased the lifespan of the 3



photo-generated carriers and further inhibited carriers recombination rate. The HC-TiO2 showed a high-efficiency catalytic performance on photocatalytic hydrogen production. This work provided a promising way for preparing high carbon doping TiO2 and broadening the application of MXene. EXPERIMENTAL SECTION Preparation of Ti3C2Tx nanosheets Ti3C2Tx nanosheets precursors were prepared according to the literature.43 2 g Ti3AlC2 crystals were immersed in diluted 20 mL 10% HF for 30 minutes to remove surface oxidant. The above solution is centrifuged three times with deionized water, then attained crystals were dispersed in 20 mL 25% TMAOH aqueous for 24 h. The TMA-intercalated crystals were diluted into 300 mL using water. To achieve complete delamination, the solution was shaken by hand for 10 min repeatedly. And the supernatant was centrifuged at 3500 rpm. Synthesis of HC-TiO2 The attained supernate was encapsulated in the Teflon-lined autoclave and maintained at 160 C for 9 h. The obtained powders were centrifuged and ultrasonic and washed with water and ethanol three times, and then dried at 80 C under vacuum. Photocatalyst tests The experiments were conducted in a quartz container from top-irradiation containing 0.02 g of photocatalyst powder. The photocatalyst was dispersed in 50 mL (10 vol% TEOA, as a holes capture agent) aqueous solution. A 300 W Xe lamp as a full spectrum solar simulator was placed above quartz container. In a typical reaction, the reactor temperature was controlled using a circulating condensate system. The reactor was bubbled for 30 min to completely remove the dissolved oxygen using high purity Ar gas before illumination. The hydrogen produced was detected with GC2014C gas chromatography system equipped with a thermal conductivity detector.

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Photoelectrochemical (PEC) measurements The PEC performance of HC-TiO2 and P25 was measured. The 4 mg/mL samples slurry with 0.5 vol% Nafion was treated with ultrasound for 30 min and dropped onto a fluorine-doped tin dioxide (FTO). The areas of the photoanodes are kept in 1 cm2. After the sample drying, a standard three-electrode system using a Pt foil as the counter electrode, and saturated calomel electrode (SCE) as a reference electrode with 0.5 M Na2SO4 electrolyte was conducted. A 300 W Xe lamp was used. Electrochemical impedance spectra (EIS) were measured over a frequency range of 105 ~ 10-2 Hz with 10 mV amplitude under simulated sunlight. The potential was converted using the Nernst equation: ERHE = ESCE + 0.242 + 0.059 pH. RESULT AND DISCUSSION The Ti3C2Tx nanosheets were first synthesized by etching of the oxidation surface with HF acid and further etching of the Al layers in bulk Ti3AlC2 with TMAOH. The hierarchical structure of carbon doping TiO2 was attained by heating Ti3C2Tx nanosheets supernatant in an autoclave at a suitable temperature. The field emission scanning electron microscopy (FESEM) images showed the morphology of the obtained HC-TiO2. Figure 1a showed the typical SEM images of HC-TiO2 derived from two dimension Ti3C2Tx (Figure S1). The HC-TiO2 samples exhibited multangular flower-like morphology composed of some nanorods tightly aggregated together with diameter dimension of 35 ± 10 nm (Figure S2), which was well less than the diffusion length of photogenerated carriers of TiO2. This small nanorods feature was beneficial to carrier diffusion during the photocatalytic reaction.44 The hierarchical structure was dependent on the reaction temperature (Figure S3). The transmission electron microscope (TEM) image also showed multangular morphology composed of tightly aggregated nanorods in Figure 1b. The HAADF-STEM image and element mappings of the HC-TiO2 sample revealed that the Ti, O, and C distribute uniformly within the HC-TiO2 nanostructure in Figure 1c. The elemental analysis of the HC-TiO2 samples showed a higher carbon weight around 17.78 wt%, 20.93 wt% and 15.01 wt% at 140 C, 160 C, and 180 C, than most of the previously reported works (Table S1). The HRTEM 5



image analysis displayed the interplanar spacing of 0.347 nm corresponding to the (101) plane of anatase TiO2 in Figure 1d. The selected area electron diffraction (SAED, inset) revealed the polycrystalline nature and dominant (101), (112), (211), and (200) diffraction planes.

Figure 1. (a) SEM, (b) TEM, (c) HAADF-STEM with Ti, O, C element distribution and (d) HRTEM (inset: SAED pattern) of as-prepared HC-TiO2.

The phase structure and composition of HC-TiO2 was shown in Figure 2a via using X-ray diffraction (XRD) patterns. The results revealed that the as-synthesized HC-TiO2 was mainly composed of anatase matching well with that of polycrystalline tetragonal TiO2 (PDF# 21-1272). There is no observable peak of Ti3C2Tx MXene.43 The facets of HC-TiO2 showed quite sharp strong intensities indicating good crystallinity. Diffraction peaks of HC-TiO2 were not shifted, confirming that carbon atoms did not simply exist as interstitial atoms. And the X-ray photoelectron spectroscopy (XPS) measurements of the C 1s spectra of HC-TiO2 were studied and fitted to three peaks at 284.6, 285.4, and 288.0 eV, which indicated that the carbon element in the HC-TiO2 had different chemical environments in Figure 2b. The peak at 284.6 eV was attributed to adventitious carbon.45 The two peaks at 285.4 and 288.0 eV were characteristic, corresponding to the existence of carbonate-like groups containing carbon and oxygen (C-OOR(H), C-OR(H), or C=O).12 There was no Ti-C bond centered at 281.5 eV in the XPS spectra (Figure S4a),46 indicating the absence of Ti3C2Tx and the lattice oxygen not replaced by the carbon element. The peak at 290.7 eV was not detected, suggesting no 6


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graphite-like carbon in the surface of HC-TiO2,47 which indicates that the C element in Ti3C2Tx had not formed graphite carbon. Similarly, XPS of Ti 2p and O1s were conducted to obtain the chemical states and binding energy of elemental composition of HC-TiO2 in Figure 2c and 2d. The high-resolution XPS (HRXPS) spectra of Ti 2p core levels in Figure 2c showed the binding energy of Ti 2p of HC-TiO2 assigned to Ti4+ ions,48 which was different from Ti 2p of Ti3C2Tx (Figure S4b). Compared with P25 (Figure S5), the Ti 2p core levels peak positions for the HC-TiO2 shifted to lower binding energies due to the charge redistribution between O and Ti atoms, respectively, which were resulted from non-stoichiometric TiO2. The change inferred a decrease of electron density in the Ti nucleus by the carbon doping.19 The high-resolution O 1s spectrum clearly evidenced the presence of two major peaks centered at around 529.6 eV and 531.4 eV in Figure 2d. The lower binding energy was assigned to the lattice oxygen of the TiO2 crystal lattice, while the higher was associated with the possible surface H-O bonds and/or C-O and C=O bonds of carbonate species.

Figure 2. (a) XRD patterns of the HC-TiO2 and Ti2C3, (b-d) Ti 2p, O 1s, and C 1s core levels from the XPS spectra of the HC-TiO2 respectively.

The UV-vis spectra were performed, followed by a tauc plot to estimate the bandgap as shown in Figure 3a. The observed bandgaps were 3.10 and 3.26 eV for HC-TiO2 and commercial P25, respectively. Compared with the commercial P25 (380 nm), the as-synthesized HC-TiO2 spectrum significantly had a certain redshift to 400 nm due to the 7



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doping of carbon.35 We also characterized the valence band (VB) for HC-TiO2 and P25 by VB-XPS in Figure 3b. The binding energy of HC-TiO2 was smaller than that of P25 likely because of doping carbon.36 The VB tail states occurred in HC-TiO2, but not in the P25. This band tail was assigned to the carboxylate groups with strong electron withdrawing ability, which induced additional transient electron states above the VB edge, further resulting in narrowed bandgap.12

Figure 3. (a) UV-vis spectra, (b) VB of the samples from XPS, (c) Mott-Schottky curves, and (d) FTIR spectra of as-prepared HC-TiO2 and P25.

To attain the Mott-Schottky (MS) plots, the impedance potential measurement was conducted in Figure 3c. The HC-TiO2 showed a positive slope similar to P25 manifesting an n-type semiconductor. The flat band (FB) potential approximates the conduction band (CB) edge from MS plot, which was generally accepted. The FB potential of HC-TiO2 shifted more negative than commercial P25. The negative FB potential revealed that photogenerated electrons in CB had strong reducibility.49 In other words, HC-TiO2 participated easily in water splitting reaction at the same condition. What’s more, the carrier concentration can also be expressed through the Mott-Schottky equation,50 that is: 1 / C2 = (2 / eεε0Nd) × [V − VFB − kBT / e]

(1)

From the Mott-Schottky plot, the carrier concentration Nd is a correlation with an inverse slope of the range of linearity. Where the electron charge e, relative dielectric constant ε, dielectric constant of vacuum (ε0), temperature (T), the flat-band potential (VFB) and 8


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Boltzmann constant (kB) were given and obtained. And the carrier concentration of HC-TiO2 is relatively higher than P25. FTIR spectroscopy of HC-TiO2 was further carried out to verify C-doping in TiO2 in the reaction process. As shown in Figure 3d, several new peaks appeared in the spectrum compared with that of P25. The peak at 1460 cm-1 could be assigned to COO symmetric stretching vibration,12, 51 and a shoulder peak at 1380 cm-1 could ascribe to C-H bending modes of the methyl groups derived from TMAOH. The peak at 1630 cm-1 was derived from the stretching vibrations of OH groups and absorbed water on the HC-TiO2 and P25 surface.52-53 The broad peak at 500 cm-1 was attributed to the bonds of Ti-O and Ti-O-Ti.54-55 The results of the FTIR spectrum confirmed that the carbon was doped in TiO2. The transfer behavior of photo-induced carriers of HC-TiO2 was investigated by the photoluminescence (PL) measurements. Compared with P25, the PL intensity was decreased drastically with C doping in TiO2 in Figure 4a, revealing that the recombination rate of photo-induced carriers was decreased significantly, which was hypothetically due to the enhanced charge transfer by the doping of the carbon in TiO2. Significantly, the emission peaks of HC-TiO2 were red-shifted from 367 nm to 384 nm, which was in agreement with the absorption edge 3.10 and 3.26eV for HC-TiO2 and P25, reflecting the band-to-band electron-hole recombination.56-58 To further understand the exciton separation behavior, the time-resolved transient PL (TRPL) spectra of HC-TiO2 and P25 were measured (Figure 4b). The HC-TiO2 exhibited biexponential decay profile.59-60 The average lifetime τave could be attained by integrating slow decay time τ1 and fast decay time τ2 and were summarized in Table S2. The τave for HC-TiO2 and P25 were 5.83 and 4.90 ns, respectively. The difference lifetime in HC-TiO2 and P25 indicated the photoexcited electron-hole of HC-TiO2 had a slower recombination rate than P25. The faster transfer rate of photogenerated electrons in the HC-TiO2 allowed to transport them to the surface and photoreduction process subsequently occurred.61-62

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Figure 4. (a) PL spectra, (b) TRPL spectra, (c) Electrochemical impedance spectra, (d) Transient photocurrent responses in 0.5 M Na2SO4 aqueous solutions under simulated sunlight irradiation of as-prepared HC-TiO2 and P25.

To study charge transfer ability, electrochemical impedance analysis was operated. The EIS Nyquist plots were shown in Figure 4c, and the inset showed simulated equivalent circuit. The Rt was charge-transfer resistance which can be represented as the diameters of the semicircle. The diameter of HC-TiO2 was much smaller than that of P25, indicating that the HC-TiO2 had superior charge transfer capacity. And the Rs stood for electrolyte solution resistance. Figure 4d showed the on/off photocurrent under simulated sunlight irradiation. It was clear that the HC-TiO2 had remarkably enhanced photocurrents under illumination. The change of the photocurrents was observed during on-off cycles of the irradiation owing to the transient effect of the photon-generated carrier, and a slight drop was found for HC-TiO2 in the 400s, while it was always higher than that of P25. The reason was that the recombination of photogenerated electron-hole carriers was inhibited due to the increase of lifespan of the carriers for HC-TiO2. And the current density increased gradually to 3.20 μA cm-2 when illumination was on for 100s, which was eight times as much as P25. It shows the amounts of photo-induced electrons for HC-TiO2 was more than that of P25. The photocatalytic H2 evolution was investigated under simulated sunlight with 10% TEOA without co-catalysts. As shown in Figure 5, it was found that the steady hydrogen evolution at a 33.04 μmol·h-1·g-1 rate of HC-TiO2 was higher than that of other sample prepared at different temperatures (Figure S6) and 9.7 times of that of commercial P25 10


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(Table S3). The different H2 evolution performances of these samples were mainly attributed to the variations of the C concentration that affected the electron-holes recombination and the band structure based on their different properties of MS plots (Figure S7) and transient photocurrent (Figure S8). The XRD remained consistent before and after the photocatalyst reaction that showed a good stability of the sample (Figure S9). The N2 adsorption-desorption isotherm was shown in Figure S10, and the BET specific surface area of the sample was attained. The BET specific surface area of HC-TiO2 (1.50 m2/g) was much smaller than that of P25 (46.94 m2/g). The improvement of the photocatalytic property of the HC-TiO2 was not from the specific surface area of TiO2. For the improved photocatalytic property of HC-TiO2, there were two main reasons. First, the C-doping was able to enhance the light absorption range by VB shifting up. Second, the C-doping extended the lifespan of the photo-generated carriers that inhibits carrier recombination rate by trail sensitization.

Figure 5. Typical time course of H2 evolution rates of HC-TiO2 and P25 from water with 10% TEOA under simulated sunlight without co-catalysts over various samples.

Based on the results above, we could briefly summarize that the major processes of the higher hydrogen evolution rate of HC-TiO2 were as follows. Firstly, the size of the nanorods of HC-TiO2 with hierarchical structure was less than the diffusion length of photogenerated carriers of TiO2, which is beneficial to carrier diffusion during the photocatalytic reaction. Secondly, the tail state induced by carbon-doped TiO2 widened the absorption range of light. When the sample was irradiated by light, the photons induced the generation of electron-hole pairs in Figure 6. The electrons in the VB transited to the CB after photons exciting. Due to 11



the carbon doping, the new impurity was generated between the CB and the VB, which could further narrow the band gap. Finally, the impurity which tended to induce acceptor level above the VB maximum of TiO2 could be used as an efficient hole transfer medium,63-64 which further achieved the effective separation of electron-holes. Due to enough energy, electrons that were excited to CB could easily reduce protons to produce hydrogen.

Figure 6. Photocatalytic mechanism of HC-TiO2 materials.

CONCLUSIONS In summary, the HC-TiO2 with hierarchical structure was prepared by in situ growth strategy using MXene as the precursor. The HC-TiO2 with high carbon content changed the electronic structure of TiO2 and adjusted the band structure of TiO2, which further inhibited photogenerated carriers recombination rate and broadened the range of TiO2 on the absorption of light. HC-TiO2 exhibited excellent light-driven catalytic activity in photocatalytic water splitting, being better than the commercially available P25. This work paved a new way to fabricate functional materials from two dimensions MXene for extensive photocatalysis and electrocatalysis application. ASSOCIATED CONTENT

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Supporting Information Chemicals and materials, characterization methods, TEM of Ti3C2Tx, SEM after the stripping, size distribution of HC-TiO2 nanorods, SEM of HC-TiO2 at different temperature, XPS spectra of the Ti3C2Tx and the P25, H2 production of HC-TiO2 of different temperature, Mott-Schottky curves and transient photocurrent responses of samples, XRD patterns before and after the photocatalysis, N2 adsorption-desorption isotherm, summary the carbon species in C-doped TiO2, the values estimated PL lifetime τi and corresponding constant Ai and enhancement ratio of H2 production of HC-TiO2 at 140 C, 160 C, 180 C relative to P25 under simulated solar without co-catalyst. ACKNOWLEDGMENTS We greatly acknowledge Natural Science Foundation of China (51702116, 51602305, and 51571100), National Key Research and Development Program of China (2016YFFA0200401), the Fundamental Research Funds for the Central Universities, and Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09). REFERENCES (1). Zhou, W.; Li, W.; Wang, J. Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D., Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst, J. Am. Chem. Soc. 2014, 136 (26), 9280-9283. DOI 1021/ja504802q. (2). Tian, J.; Zhao, Z.; Kumar, A.; Boughton, R. I.; Liu, H., Recent progress in design, synthesis,

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Figures Captions: Figure 1. (a) SEM, (b) TEM, (c) HAADF-STEM with Ti, O, C element distribution, and (d) HRTEM (inset: SAED pattern) of as-prepared HC-TiO2. Figure 2. (a) XRD patterns of the HC-TiO2 and Ti2C3, (b-d) Ti 2p, O 1s, and C 1s core levels from the XPS spectra of the as-prepared HC-TiO2 respectively. Figure 3. (a) UV-vis spectra, (b) VB of the samples from XPS, (c) Mott-Schottky curves, and (d) FTIR spectra of as-prepared HC-TiO2 and P25. Figure 4. (a) PL spectra, (b) TRPL spectra, (c) Electrochemical impedance spectra, (d) Transient photocurrent responses in 0.5 M Na2SO4 aqueous solutions under simulated sunlight irradiation of as-prepared HC-TiO2 and P25. Figure 5. Typical time course of H2 evolution rates of HC-TiO2 and P25 from water with 10% TEOA under simulated sunlight without co-catalysts over various samples. Figure 6. Photocatalytic mechanism of HC-TiO2 materials.

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Figure 1. (a) SEM, (b) TEM, (c) HAADF-STEM with Ti, O, C element distribution, and (d) HRTEM (inset: SAED pattern) of as-prepared HC-TiO2.

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Figure 2. (a) XRD patterns of the HC-TiO2 and Ti2C3, (b-d) Ti 2p, O 1s, and C 1s core levels from the XPS spectra of the as-prepared HC-TiO2 respectively.

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Figure 3. (a) UV-vis spectra, (b) VB of the samples from XPS, (c) Mott-Schottky curves, and (d) FTIR spectra of as-prepared HC-TiO2 and P25.

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Figure 4. (a) PL spectra, (b) TRPL spectra, (c) Electrochemical impedance spectra, (d) Transient photocurrent responses in 0.5 M Na2SO4 aqueous solutions under simulated sunlight irradiation of as-prepared HC-TiO2 and P25.

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Figure 5. Typical time course of H2 evolution rates of HC-TiO2 and P25 from water with 10% TEOA under simulated sunlight without co-catalysts over various samples.

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Figure 6. Photocatalytic mechanism of HC-TiO2 materials.

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Graphical Abstract

The hierarchical HC-TiO2 induced a valence band tail state that promoted photogenerated carriers’ effective separation and reduced bandgap, which exhibited excellent photocatalytic water splitting.

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Highly Carbon-doped TiO2 Derived from MXene Boosting the

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