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Article

Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization

by
Yajun Wang
*,†,
Runhua Li
†,
Qiaohuan Wu
,
Zhuang Yang
,
Fan Fan
,
Yuming Li
and
Guiyuan Jiang
*
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Energies 2022, 15(12), 4180; https://doi.org/10.3390/en15124180
Submission received: 15 April 2022 / Revised: 9 May 2022 / Accepted: 14 May 2022 / Published: 7 June 2022
(This article belongs to the Special Issue Energy Storage and Conversion Based on Low-Dimensional Nanostructure)
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Abstract

:
Photoelectrocatalytic (PEC) oxidation is an advanced technology that combines photocatalytic oxidation (PC) and electrolytic oxidation (EC). PEC activity can be greatly enhanced by the PC and EC synergy effect. In this work, novel copolymerized g-C3N4 (denoted as CNx)/TiO2 core-shell nanowire arrays were prepared by chemical vapor deposition. CNx were deposited on the surface of TiO2 nanowire arrays using organic monomer 4,5-dicyanidazole and dicyandiamide as copolymerization precursor. TiO2 nanowire arrays provide a direct and fast electron transfer path, while CNx is a visible light responsive material. After CNx deposition, the light response range of TiO2 is broadened to 600 nm. The deposition of CNx shell effectively improves the PC efficiency and PEC efficiency of TiO2. Under visible light irradiation and 1 V bias potential, the rate constant k of PEC degradation of CNx/TiO2 core-shell nanowire arrays is 0.0069 min−1, which is 72% higher than that of pure TiO2 nanowires. The built-in electric field formed in the interface between TiO2 core and CNx shell would effectively promote photogenerated charge separation and PEC activity.

1. Introduction

Photocatalytic (PC) oxidation of organic pollutants by semiconductor is one of the most promising waste treatment methods, which can degrade organic compounds into non-toxic small molecules such as CO2 and H2O [1,2,3,4]. A series of photocatalysts with excellent performance have been developed, including metal oxide [5,6], sulfide [7] and nitride [8,9]. Photoelectrocatalytic (PEC) oxidation is an advanced technology that combines photocatalytic oxidation (PC) and electrolytic oxidation (EC), where the PC activity can be greatly enhanced by the synergy effect of PC and EC [10,11]. PEC can greatly enhance the pollutant degradation activity, due to the synergistic effect of PC and EC oxidation [12,13]. PEC can not only improve the efficiency of the reaction, but also solve the problem of difficult separation of powdery photocatalyst from reaction solution.
TiO2 is a kind of catalyst with extensive research and application prospects due to its relatively high photocatalytic activity, good chemical stability and nontoxicity [14,15]. TiO2 nanowires as one-dimensional nanomaterials have been demonstrated to have higher photocatalytic activity due to their large surface area, rapid charge transfer and short carrier diffusion distance [16,17]. One-dimensional TiO2 nanostructure immobilized on conductive substrate have stronger adsorption capacity, more reaction active centers and better reaction stability compared with film electrode [18,19]. However, the low quantum efficiency and poor visible light response of TiO2 greatly restrict its application. Many efforts have been devoted to enhancing the performance of TiO2, such as noble metal deposition [20,21], ion doping [22,23] and semiconductor coupling [24,25].
A semiconductor of narrow band gap combined with TiO2 can effectively improve the efficiency of charge separation and expand the range of light absorption. Numerous TiO2 composite semiconductors have been reported, such as CdS/TiO2 [26,27], CdSe/TiO2 [28,29], C3N4/TiO2 [18,30,31], CdSe/Al2O3/TiO2 [32], etc. However, sulfur compounds are limited in application in the environmental field, due to their photocorrosion and instability in photocatalytic reactions [33,34,35]. Graphitic carbon nitride (g-C3N4), as a visible light photocatalyst with a band gap of 2.7 eV, has become a hotspot in photocatalytic research in recent years. It is particularly stable at room temperature and can also maintain the stable character in the process of photocatalytic reaction [36,37]. Due to the excellent visible light absorption capacity and high conduction band position of g-C3N4, the coupling of g-C3N4 with wide-bandgap semiconductor is a promising system to achieve an enhanced charge separation and improved photocatalytic activity.
Our group prepared g-C3N4/TiO2 nanowire arrays by CVD (chemical vapor deposition) method, and the PC and PEC efficiency of TiO2 effectively improved by the deposition of g-C3N4 shell [18]. However, g-C3N4 composed of 3-s-triazine structural units exists with some disadvantages, such as insufficient light absorption (only light with a wavelength shorter than 460 nm can be absorbed) [38,39], which restrict the PC activity of g-C3N4. Wang et al. have synthesized some molecular doped g-C3N4 by copolymerization of dicyandiamide with organic small molecular [40,41]. Copolymerization of g-C3N4 can significantly narrow its bandgap and enhance its light-harvesting ability [41]. Compared with g-C3N4/TiO2 nanowire arrays, the combination of copolymerized g-C3N4 and TiO2 may further expand the light absorption range and greatly improve the PC activity of TiO2.
Herein, we prepared a copolymerized g-C3N4 (denoted as CNx)/TiO2 core-shell nanowire arrays by one-step CVD method (denoted as CNx/TiO2). CNx with extended light absorption range was achieved by using organic monomer 4,5-dicyanidazole containing cyano group and dicyandiamide as precursor. TiO2 nanowire arrays provide a direct rapid electron transfer path, while CNx serves as the visible light response material. The built-in electric field formed in the interface between TiO2 core and CNx shell would effectively promote photogenerated charge separation and transfer. PC, EC and PEC of CNx/TiO2 core-shell nanowire arrays were systematically studied in this paper, and the mechanism of catalytic activity improvement was also investigated.

2. Experimental Section

Preparation of TiO2 nanowire arrays: TiO2 nanowire arrays were prepared via the method of our previous work [18]. The carbon cloth was cleaned with acetone and methanol, dried after rinsing with deionized water. It was placed in a reactor containing 100 mL of toluene, 10 mL of tetrabutyl titanate, 10 mL of titanium tetrachloride, and 10 mL of hydrochloric acid. It was then put in an oven at 180 °C for 22 h. After cooling down to room temperature, it was rinsed with ethanol and deionized water, and annealed in air at 500 °C for 2 h.
Preparation of copolymerized g-C3N4: 3 g of dicyandiamide and 0.05 g of 4,5-dicyanidazole were dissolved in 15 mL of deionized water, then dried in 80 °C water bath. The obtained powder sample was put into the tube furnace and heated at 550 °C for 4 h with a heating rate at 5 °C/min. CNx was obtained after natural cooling. For comparison, pure g-C3N4 without 4,5-dicyanimidazole was also synthesized in the same procedure.
Preparation of copolymerized CNx/TiO2 core-shell nanowire arrays: CNx was deposited on TiO2 nanowire arrays by one step CVD deposition method. First, 3 g of dicyandiamide and 0.05 g of 4,5-dicyanimidazole dissolved in 15 mL of deionized water, then dried in 80 °C water bath; the obtained powder was ground up and transferred into a porcelain boat with cover. Then, TiO2 nanowire arrays grown on carbon cloth were placed above the porcelain boat, and heated at 550 °C for 4 h with a heating rate at 5 °C/min. CNx/TiO2 core-shell nanowire arrays were obtained after natural cooling.

2.1. Characterization

The crystal structure of the samples was determined as 40 kV, 30 mA, λ = 1.5406 A. The optical response of the samples was analyzed by Hitachi U-4100 UV-visible spectrometer with solid barium sulfate as reference. The micromorphology of the samples was observed by scanning electron microscope (SEM, FEI Quanta 200F, FEI, Hillsboro, OR, USA) and transmission electron microscope (TEM, JEOL JEM 2100, JEOL, Akishima, Japan). The valence bond structure of the sample was analyzed by Thermo Fisher’s k-alpha X-ray photoelectron spectrometer (XPS, Thermo Fisher K-Alpha instrument, Waltham, MA, USA).

2.2. Photoelectrocatalysis Experiments

Methylene blue (MB) was used as the probe to evaluate the PEC activity of the sample. A 300 w Xe lamp with a cutoff filter (λ > 420 nm) was used as the light source, while the electrochemical workstation (CHI660E) applied a bias voltage of 0–2.5 V. About 200 mL of MB solution (the initial concentration of 1.5 × 10–5 mol/L) was added to a 5 × 5 × 10 cm quartz reactor with the light source at a distance of 10 cm from the reactor. After the dark reaction for 30 min, the light was turned on or bias was applied. Every 30 min, 2 mL solution was centrifuged and the absorbance of upper suspension at the wavelength of 664 nm was measured by UV-vis spectrophotometer, then the degradation rate of MB was calculated.

3. Results and Discussion

3.1. Catalyst Characterization

The synthesis process of CNx/TiO2 core-shell nanowire arrays is shown in Figure 1. Firstly, the TiO2 nanowire arrays grew on the carbon cloth by hydrothermal method. Then, CNx were deposited on the surface of TiO2 nanowire arrays by CVD method using organic monomer 4,5-dicyanidazole and dicyandiamide as copolymerization precursor. Figure 2a shows the XRD patterns of TiO2, CNx, and CNx/TiO2 core-shell nanowire arrays. The CNx sample shows the characteristic peak of g-C3N4. The peaks at 13.1° and 27.4° can be indexed to interplanar (100) crystal plane and the stacking of the conjugated aromatic system (002) of C3N4 [42]. Characteristic peaks appearing in CNx/TiO2 core-shell nanowire arrays were corresponded to CNx and TiO2, respectively. The peaks at 27.45°, 36.0°, 41.2° and 54.3° belonged to the (110), (101), (111) and (211) crystal plane of the TiO2 rutile phase, respectively [43]. The characteristic peak of the (002) crystal plane of g-C3N4 cannot be observed because it overlaps with that of the (110) crystal plane of TiO2. Nevertheless, the peak at 13.1° corresponding to the (100) crystal plane of CNx can be obviously seen, indicating the successful deposition of CNx on the TiO2 nanowire. This result suggests that the CNx/TiO2 composite nanowire arrays were successfully synthesized by CVD method, and the crystal structures of TiO2 and CNx were unchanged. Figure 2b shows the UV-vis diffuse reflectance spectrum (DRS) of the prepared samples. As can be seen, TiO2 mainly absorbs light less than 400 nm due to its wide bandgap, while both g-C3N4 and CNx can absorb visible light. Compared with pure g-C3N4, the absorption band edge of CNx shows an obvious red shift to 700 nm (Figure S1). Compared with g-C3N4, the bandgap of CNx is narrowed to 2.2 eV, indicating that the ability of visible light absorption is greatly improved. Compared with pure TiO2 nanowire arrays, all composite samples (g-C3N4/TiO2 and CNx/TiO2) present enhanced light absorption capacity. Moreover, compared with g-C3N4/TiO2 in our previous work [18], CNx/TiO2 has a significantly improved light absorption capacity at 400–600 nm, which is favorable for enhancing the visible light photocatalytic activity.
Figure 3a–c shows the SEM images of pure TiO2 nanowire arrays and the CNx/TiO2 nanowire arrays. As can be seen from Figure 3a, the surface of the TiO2 nanowire arrays is relatively smooth, and the diameter of TiO2 nanowire is about 25 to 40 nm. After deposited with CNx, the surfaces of the TiO2 nanowire become rough, which may be related to the tendency of CNx to form layered structure in the process of CVD (Figure 3b,c). The diameter of CNx/TiO2 core-shell nanowire arrays is about 45–65 nm, indicating the formation of CNx/TiO2 core-shell nanowire composite. Figure 3d shows the HRTEM images of the prepared CNx/TiO2 core-shell nanowire arrays. The HRTEM images of CNx/TiO2 indicates that TiO2 nanowire is covered by a thin layer of CNx. The diameter of CNx/TiO2 measured by HRTEM is about 50 nm, which is consistent with the SEM results. The measured lattice fringe of TiO2 is 0.325 nm, which is related to the rutile TiO2 (110) plane [44]. The measured lattice fringe of 0.325 nm in the deposition layer is correlated with the (002) crystal planes of CNx [45].
X-ray photoelectron spectroscopy was used to analyze the chemical states of g-C3N4/TiO2 and CNx/TiO2 core-shell nanowire arrays (Figure 4a and Figure S2). The peak position correction is based on 283.7 eV of C1 s (Figure S2b). Figure 4 shows the XPS survey spectra and magnified spectra of C1 and N1s. As can be seen from Figure 4a, the peak at 283.7 eV represents the characteristic peak of C–C bond of the carbon cloth. The peak at 287.3 eV is attributed to the characteristic peak of sp2 hybridized carbon [46]. The spectra of CNx/TiO2 core-shell nanowire array remains unchanged relative to that of g-C3N4/TiO2 core-shell nanowire array, indicating that they have similar chemical structures. From the N1s spectrum of g-C3N4/TiO2 and CNx/TiO2 core-shell nanowire arrays (Figure 4b), three peaks can be distinguished to be centered at 403.4 eV, 399.9 eV and 397.9 eV, respectively. The main peak at 397.9 eV can be attributed to the C–N=C of heterocycle. The other two peaks at 403.4 eV, 399.9 eV can be assigned to the positive charge localization effect and N atoms of N–H and N–(C)3. Compared with the g-C3N4/TiO2, the N1s peak of CNx/TiO2 core-shell nanowire array at 399.9 eV shifted 0.5 eV towards the direction of low binding energy, and the peak area and the peak strength were significantly enhanced. It can be indicated that copolymerization can effectively enhance the conjugation effect and extend the π conjugate system, conducive to the transmission and separation of photogenerated charges [47].

3.2. Enhancement of PC and PEC Activity

MB was used as probe to investigate the PEC activity of CNx/TiO2 core-shell nanowire arrays under different test conditions. The PC and PEC degradation processes obey pseudo-first-order kinetics [48]. PC and PEC performance of CNx/TiO2 core-shell nanowire arrays and TiO2 were evaluated under the conditions of visible light irradiation (λ ≥ 420 nm) and bias potential of 1 V (Figure 5a,b). Both PC and PEC degradation rate constant of MB with CNx/TiO2 core-shell nanowire arrays are obviously higher than that of pure TiO2 sample. It can be speculated that the CNx/TiO2 core-shell nanowire arrays heterojunction structure can promote the separation of photogenerated charge carriers and prolong the lifetime of photogenerated carriers, thus the performance of PEC was enhanced. It can be seen from the results that when only PC or only the potential of 1 V is applied, the reaction rate of MB degradation is comparatively slow. However, when the potential of 1 V was applied to both CNx/TiO2 core-shell nanowire arrays and TiO2 samples under visible light irradiation, the degradation rate of MB was significantly increased, indicating that the synergistic effect of PC and EC was beneficial to accelerate the migration of photogenerated carriers and increase the reaction rate. Under visible light irradiation and 1 V bias potential, the rate constant k of PEC degradation of CNx/TiO2 core-shell nanowire arrays is 0.0069 min–1, which is 72% higher than that of pure TiO2 nanowires.
The effect of bias potential on the PEC degradation of MB by CNx/TiO2 core-shell nanowire arrays was explored, and the results are shown in Figure 5c,d, indicating the self-degradation rate of MB under the visible light irradiation without catalyst. The PEC activity of CNx/TiO2 first increases and then decreases with the increasing bias potential. When the potential is reached at 2 V, CNx/TiO2 core-shell nanowire arrays show an optimal PEC activity (the reaction rate constant k is 0.01268 min–1), which is 3.36 times that of PC degradation reaction rate constant. The effect of bias is not only powerful to the formation of active species with stronger oxidation ability, but also provides a driving force for the directional migration of photogenerated electrons, which can greatly improve charge separation efficiency. When the potential increased to 2.5 V, the degradation effect of MB decreased significantly. This is possibly due to MB rapidly decomposed; the decomposition products were accumulated on the surface of the working electrode, thus passivating the PEC reaction. Therefore, it can be determined that the optimal potential for PEC degradation of MB by CNx/TiO2 core-shell nanowire arrays is 2 V.

3.3. Mechanism of Enhancement of PEC Activity

The possible condensation steps of forming the copolymerized g-C3N4 via the precursors of dicyandiamide and 4,5-dicyocyanidazole are shown in Figure S3. It can be seen that the basic structural unit 3-s-triazine ring of g-C3N4 does not change during the condensation process, but only changes the terminal group. On the one hand, the change of the terminal group will bring different π electron delocalization effect, which is beneficial to the separation of photogenerated carriers [49]. On the other hand, by adding 4,5-diabenzimidazole, the bandgap of the as-prepared sample is reduced from 2.70 eV to 2.20 eV, illustrating more visible light could be absorbed. Based on the above characterization and activity tests, the possible mechanism of photoelectric synergistic effect on degradation of MB was discussed (Figure 6). The bandgap of CNx was narrowed compared with pure g-C3N4. The conduction band (CB) of CNx is higher than that of TiO2, and CNx can absorb visible light to generate electrons and holes. Under the visible light excitation, the exited electrons of CNx are transferred to the CB of TiO2, leaving the holes on the CNx, resulting in the reduced photogenerated charge carrier recombination efficiency. Forcing the bias potential, the electrons of TiO2 would transfer to the counter electrode, further enhancing the separation efficiency of photogenerated charge carriers and PEC activity.

4. Conclusions

In summary, we prepared a copolymerized CNx/TiO2 core-shell nanowire arrays by one-step CVD method. CNx serves as the visible light response material, and TiO2 nanowire arrays provide a direct rapid electron transfer path. Compared with pure TiO2 nanowires, the CNx/TiO2 core-shell nanowire arrays present enhanced PC and PEC activity under visible light irradiation. The PEC activity enhancement of the CNx/TiO2 core-shell nanowire arrays is mainly due to extended π conjugation system of C3N4 and matched band energy alignment between the CNx shell and TiO2 core, which can facilitate the separation of photogenerated electrons and holes. The CNx/TiO2 core-shell nanowire arrays synthesized in this work are not only easy to recycle for further use, but also possess the excellent optical properties and PEC degradation ability, making them excellent catalysts for environmental applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en15124180/s1, Figure S1: (a) UV-Vis diffuse reflectance spectra of g-C3N4 and CNx. (b) Kubelka-Munk transformed reflectance spectra and estimated optical absorption bandgap, Figure S2: (a) XPS spectra of CNx/TiO2 core-shell nanowire arrays. (b) C1s spectra of carbon cloth, Figure S3: The copolymerization of dicyandiamide and 4,5-dicyanoimidazole.

Author Contributions

Investigation, Q.W., F.F. and Y.L.; methodology, Z.Y.; resources, Y.W.; supervision, Y.W. and G.J.; writing—review & editing, Y.W., R.L. and G.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (2019YFC1904500), Science Foundation of China University of Petroleum, Beijing (2462019QNXZ05, 2462020YXZZ018), and State Key Laboratory of Heavy Oil Processing, China University of Petroleum.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic illustration of CNx/TiO2 core-shell nanowire arrays.
Figure 1. Schematic illustration of CNx/TiO2 core-shell nanowire arrays.
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Figure 2. (a) XRD patterns of TiO2 nanowire arrays, CNx, CNx /TiO2 core-shell nanowire arrays. (b) UV-Vis diffuse reflectance absorption of TiO2 nanowire arrays, g-C3N4, CNx, g-C3N4/TiO2 nanowire arrays and CNx /TiO2 core-shell nanowire arrays.
Figure 2. (a) XRD patterns of TiO2 nanowire arrays, CNx, CNx /TiO2 core-shell nanowire arrays. (b) UV-Vis diffuse reflectance absorption of TiO2 nanowire arrays, g-C3N4, CNx, g-C3N4/TiO2 nanowire arrays and CNx /TiO2 core-shell nanowire arrays.
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Figure 3. SEM images of (a) TiO2 nanowire arrays and (b,c) CNx/TiO2 core-shell nanowire arrays, (d) HRTEM image of CNx/TiO2 ore-shell nanowire arrays.
Figure 3. SEM images of (a) TiO2 nanowire arrays and (b,c) CNx/TiO2 core-shell nanowire arrays, (d) HRTEM image of CNx/TiO2 ore-shell nanowire arrays.
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Figure 4. XPS spectra of (a) C1s and (b) N1s peaks of g-C3N4/TiO2 and CNx/TiO2 core-shell nanowire arrays.
Figure 4. XPS spectra of (a) C1s and (b) N1s peaks of g-C3N4/TiO2 and CNx/TiO2 core-shell nanowire arrays.
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Figure 5. (a,b) Comparison of EC and PEC degradation rate of MB with CNx/TiO2 and TiO2. (c,d) Comparison of PC and PEC degradation rate of MB with CNx/TiO2 core-shell nanowire arrays by different bias voltage under visible light (λ ≥ 420 nm).
Figure 5. (a,b) Comparison of EC and PEC degradation rate of MB with CNx/TiO2 and TiO2. (c,d) Comparison of PC and PEC degradation rate of MB with CNx/TiO2 core-shell nanowire arrays by different bias voltage under visible light (λ ≥ 420 nm).
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Figure 6. Charge transfer mechanism of CNx/TiO2 core-shell nanowire arrays.
Figure 6. Charge transfer mechanism of CNx/TiO2 core-shell nanowire arrays.
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Wang, Y.; Li, R.; Wu, Q.; Yang, Z.; Fan, F.; Li, Y.; Jiang, G. Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization. Energies 2022, 15, 4180. https://doi.org/10.3390/en15124180

AMA Style

Wang Y, Li R, Wu Q, Yang Z, Fan F, Li Y, Jiang G. Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization. Energies. 2022; 15(12):4180. https://doi.org/10.3390/en15124180

Chicago/Turabian Style

Wang, Yajun, Runhua Li, Qiaohuan Wu, Zhuang Yang, Fan Fan, Yuming Li, and Guiyuan Jiang. 2022. "Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization" Energies 15, no. 12: 4180. https://doi.org/10.3390/en15124180

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