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Construction of Direct Z-Scheme Photocatalyst by [Mg.sub.1.2][Ti.sub.1.8][O.sub.5] and g-[C.sub.3][N.sub.4] Nanosheets toward Photocatalytic [H.sub.2] Production and Disinfection.

1. Introduction

During the past decade, there is an increasing interest in addressing the problem related to the growing concentration of energy shortage and other related damaging environmental pollutions due to the combustion of the fossil fuels [1-3]. Under the circumstances, developing advanced technologies is very urgent, which are able to reduce the use of fossil fuels and exploit the renewable and environmentally friendly energy sources [4-6]. Photocatalytic technology is a promising approach towards solving the above problems, which can transform [H.sub.2]O into [H.sub.2] and relieve the environmental pollution under solar light irradiation. Thus, considerable research efforts have been made to develop efficient photocatalysts for the usage of photocatalytic hydrogen production and disin fection due to the technology advantage of convenient and environmentally friendly operation process and no harmful disinfection by-products, such as metal oxide and sulfide, as well as metal chloride semiconductors [7-11]. Lately, perovskite as a semiconductor photocatalyst for water splitting has become a research hotspot. Compared with the most studied metal oxides and sulfides, the perovskite shows a more suitable conduction band position and higher stability, as well as abundant light absorption. Magnesium titanate, as a member of the perovskite family, has been reported to be an efficient photocatalyst of water splitting to product hydrogen and the degradation of pollutants. However, pure perovskite has two major drawbacks, namely, low surface area and high recombination rate of photogenerated electron hole pairs [12-15]. Therefore, to enhance the photocatalytic efficiency, the strategy of designing effective artificial photosynthetic systems by mimicking the Z-scheme mechanism in the chloroplast has been developed. Graphitic carbon nitride (g-[C.sub.3][N.sub.4]), as a metal-free polymeric semiconductor with a narrow bandgap of about 2.7 eV, has been widely studied among various semiconductors in view of its low cost, nontoxicity, good thermal and chemical stability, and higher reduction potential position [16-22]. However, the poor separation efficiency of photogenerated charge carriers and insufficient light absorption of bulk g-[C.sub.3][N.sub.4] directly synthesized by the thermal polycondensation method would lead to inferior performance and limitation of application [23-28]. Given that of these shortcomings, constructing the Z-scheme heterostructure by recombining g-[C.sub.3][N.sub.4] with perovskite has been regarded as an ideal solution to expand the specific surface area and restrain the recombination of photogenerated charges.

Herein, we have successfully synthesized the Z-scheme heterojunction photocatalyst consisting of [Mg.sub.1.2][Ti.sub.1.8][O.sub.5] nanosheets (MT) and g-[C.sub.3][N.sub.4] nanosheets (CN) by the sol-gel method and calcination method. The photocatalytic performance was evaluated by photocatalytic water splitting and disinfection under simulated sunlight irradiations. Compared with pure [Mg.sub.1.2][Ti.sub.1.8][O.sub.5] and g-[C.sub.3][N.sub.4], the Z-scheme heterojunction photocatalyst exhibited excellent photocatalytic activity by taking the advantages of the more negative conduction band (CB) of CN, the more positive valence band (VB) of MT, the enhanced light absorption, and more efficient photogenerated charge carrier separation.

2. Experimental Section

2.1. The Preparation of Photocatalyst. All chemicals used in this work were analytical grade without any further purification. The heterojunction composite composed of g-[C.sub.3][N.sub.4] and [Mg.sub.1.2][Ti.sub.1.8][O.sub.5] was obtained by the sol-gel method and ionothermal method. In a typical sol-gel preparation procedure, 0.03 mol of acetic acid was poured into 0.16 mol absolute ethanol to form a transparent solution, and then 0.01 mol of Ti[(O[C.sub.4][H.sub.9]).sub.4] was dropped into the above solution with constantly stirring to form transparent solution. After that, 0.01 mol of Mg[(N[O.sub.3]).sub.2] * 6[H.sub.2]O was slowly added into the above solution to generate homogeneous yellow sol. Finally, the above yellow sol was dried in air to obtain MgTi gel. Additionally, in the process of ionothermal calcination method, a certain amount of melamine was mixed with the as-obtained Mg-Ti gel at molar ration of 5%, 10%, and 20%, respectively. Then, the above mixture (1.0 g) was mixed with KCl (2.75 g) and LiCl (2.25 g). After being mixed evenly, the resultant mixture was transferred into a corundum crucible and calcined at 550[degrees]C for 2 h with a heating rate of 5[degrees]C/min in a muffle furnace. The resultant product was washed with boiling distilled water and then followed by drying at 60[degrees]C overnight in a vacuum oven. By way of comparison, pure g-[C.sub.3][N.sub.4] and [Mg.sub.1.2][Ti.sub.1.8][O.sub.5] were also prepared using the similar procedure. The synthetic samples were labeled as CN, MT, MT/CN-5, MT/CN-10, and MT/CN-20, respectively.

2.2. Characterizations. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance instrument with Cu-K[alpha] as the radiation source. The morphologies of the as-obtained samples were characterized by a JSM-7500 field emission scanning electron microscope (FE-SEM). The specific surface area and pore structure were analyzed by [N.sub.2] adsorption-desorption isotherms using a Micromeritics ASAP 2020 Apparatus (Micromeritics Instruments, USA). The optical performance of samples was characterized by a UV-visible spectrophotometer (UV-2600, Shimadzu, Japan). Room temperature photoluminescence (PL) emission spectra of the samples were collected on a Hitachi F-700 fluorescence spectrophotometer at an excitation wavelength of 325 nm. Electrochemical measurements were carried out on an electrochemical workstation (CHI66E, China) with a standard three-electrode cell, where [Na.sub.2]S[O.sub.4] solution as the electrolyte, Pt foil acted as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode.

2.3. Evaluation of Photocatalytic Hydrogen Evolution. The photocatalytic hydrogen evolution was evaluated on a Labsolar-6A system (Pefect Light Company, Beijing China), where a 300 W Xenon arc lamp was used as simulated sunlight source. In the process of photocatalytic water splitting, circulating cooling water system is used to guarantee that the temperature of reaction system be kept at 25 [+ or -] 1[degrees]C. Firstly, 50 mg of the as-prepared photocatalyst was dispersed in 80 ml of triethanolamine (TEOA) aqueous solution (10 vol%). Then, a certain amount of [H.sub.2]Pt[Cl.sub.6] aqueous solution was dripped into the system to load 1.0 wt% Pt onto the surface of the photocatalyst by a photochemical reduction deposition method. After illumination for 1 h under magnetic stir, 0.4 ml of the produced gas was automatically withdrawn and injected into the gas chromatograph (GC-2014C, Shimazu, Japan) to determine the concentration of the produced [H.sub.2] gas.

2.4. Photocatalytic Disinfection Experiment. The photocatalytic performance of the obtained samples was evaluated by the inactivation of Escherichia coli (E. coli). Firstly, in order to obtain the inoculum, the bacteria was incubated in 100 ml Luria-Bertani (LB) nutrient solution and shaken at 37[degrees]C for 12 h, and then centrifuged to remove the metabolites. Subsequently, the bacterial cell pellet was washed twice with sterilized saline (0.9% NaCl) solution and then resuspended in the sterilized saline solution. The obtained bacteria cell density was about 5 x [10.sup.7] colony forming units per milliliter (CFU/ml). Finally, the photocatalytic disinfection experiment was carried out. Namely, 30 ml of E. coli suspension was mixed with 20 mg photocatalyst powder and stirred at 500 rpm for 5 min in the dark. A 300 W Xenon lamp with an AM1.5 filter was employed as the simulated sunlight source to irradiate the E. coli cells. At regular intervals, aliquots were taken from the suspension buffer and 0.1 ml of the suspension (undiluted, three replicates) was spread onto LB agar plates, and then incubated at 37[degrees]C for 18 h to form viable colony units. All inactivation experiments were performed in triplicate, and all glassware was heated at 120[degrees]C for 20 min in an autoclave to ensure the sterility.

3. Results and Discussion

The XRD patterns of the synthesized CN, MT, and MT/CN-10 composite are shown in Figure 1. It can be observed that the two typical diffraction peaks of CN appeared at 27.5[degrees] and 13.1[degrees] are classified to (002) and (100) diffraction planes, which are assigned to the interlayer stacking of aromatic segments and in-plane structural packing motif of tri-striazine units [29-31]. The pure MT exhibits five stronger diffraction peaks at 20 of 25.5[degrees], 32.1[degrees], 39.6[degrees], 45.4[degrees], and 48.2[degrees], corresponding to the (110), (023), (113), (043), and (200) facets, respectively [12]. After the introduction of CN into the MT, the heterojunction sample reveals characteristic diffraction peaks for both CN and MT without any position shifts, indicating the coexistence of these two components in the heterojunctions.

The field emission scanning electron microscope (FESEM) measurement was carried out to characterize the morphologies and structures of MT and MT/CN samples. As shown in Figure 2(a), the pristine MT displays the aggregated structure consisting of several two-dimensional (2D) sheets with a thickness of 10-20 nm. By contrast, the image of MT/CN composite shows that many CN nanoparticles grow on the surface of the 2D MT nanosheets (Figure 2(b)). The microstructure of the MT/CN composite was further explored by SEM-EDS elemental mapping measure. The elemental mapping images reveal C, N, Ti, and O, and Mg elements are well-distributed in the composite (Figure 2(c)). Those abovementioned results further proved that the MT/CN heterojunction was successfully synthesized.

The optical absorption properties of the obtained samples were investigated by UV-vis diffuse reflectance measurement. As shown in Figure 3(a), steep adsorption band edge of pure MT is observed in the ultraviolet region, whereas an obvious adsorption edge which can be assigned to the intrinsic bandgap absorption of pure CN is approximately located at 450 nm. Remarkably, the adsorption band edge of the MT/CN-10 heterojunction photocatalyst exhibits a red shift in comparison to that of pristine CN and MT, implying that the enhanced light absorption ability would be highly conducive to boost the photocatalytic performance. Moreover, the optical bandgap energy can be calculated by the following equation [32, 33]:

[alpha]h[upsilon] = A[(hv - [E.sub.g]).sup.n/2], (1)

where A, [alpha], h[upsilon], and [E.sub.g] represent a constant, the absorption coefficient, the photon energy, and the optical bandgap, respectively. In this equation, n is determined by the transition type. The bandgap energies of the MT and CN are determined from a plot of ([alpha] h[upsilon]) [2] versus h[upsilon] (n =1 for direct transition). Thus, the bandgaps of MT and CN are estimated to 3.45 eV and 2.71 eV, respectively (Figure 3(b)).

The adsorption-desorption curves and pore volume distribution curves of MT, CN, and MT/CN-10 composite are shown in Figure 4. And the corresponding calculated results of BET surface area are also listed in Table 1. Obviously, all samples exhibit the type IV isotherms with type H3 hysteresis loops [34, 35], which are characteristics of slit-like pores formed by the stacking of nanosheets. However, after combining CN with the MT, the specific surface area and pore volume of MT/CN composite distinctly decreased as compared to that of pure MT, which might attributed to the fact that the small CN nanosheets filled or blocked large numbers of micropores of MT.

The photocatalytic performance of the obtain catalysts was evaluated by photocatalytic hydrogen production under simulated solar light irradiation. Figure 5(a) depicts the photocatalytic [H.sub.2] evolution rate of various photocatalysts. The results showed that the [H.sub.2] evolution rate firstly increased and then decreased with the increasing loading amount of CN. The MT/CN-10 in particular outperforms (3128.3 [micro]mol/h/g) among all the photocatalysts, which was about 3.57 and 7.24 times higher than that of pure MT and CN, respectively. The stability and reusability of the photocatalyst is of great significance to its practical application. Figure 5(b) demonstrates no significant decrease in the photocatalytic [H.sub.2] production amount of MT/CN-10 after four repetitive cycles, indicating that MT/CN-10 possesses high photocatalytic stability and recyclability. Moreover, in order to assess the enhanced photocatalytic efficiency of MT/CN composites, the inactivation efficiencies of the E. coli over the as-prepared samples were also investigated in Figure 6. Compare the photocatalytic disinfection efficiencies of different samples under simulated solar light irradiation at room temperature and ambient pressure as well as the blank one with photocatalyst (MT/CN-10 sample alone) but under dark condition. In parallel, the inactivation efficiency of the E. coli without adding any photocatalysts was also evaluated. As it can be found in Figure 5(a), the inactivation of the bacteria barely happens in the absence of photocatalyst, and no disinfection efficiency of the E. coli was observed when the photocatalyst (MT/CN-10) is used without light irradiation. In addition, the MT/CN composites exhibit higher inactivation efficiencies than that of pure MT and CN by the simulated solar light illumination. Particularly, almost 100.0% of E. coli can be killed by MT/CN-10 over 100 min of simulated solar light exposure (Figures 6(b) and 6(c)), while the inactivation efficiencies only reach to 51.2% and 48.5% for pure MT and CN after 100 min, respectively. These results demonstrate that the photocatalytic E. coli inactivation activity can be significantly improved by constructing the heterojunction about CN and MT.

To reveal the significant effect of CN on the photocatalytic activity of MT, the photoluminescence (PL), photocurrent responses, and electrochemical impedance spectrum (EIS) analysis were conducted to explore the charge carrier transfer efficiencies of pure MT, CN, and MT/CN-10 composite. Figure 7(a) shows that the PL intensity of MT/CN10 is the weakest than that of pure MT and CN, and the CN displays the strongest PL intensity, indicating that the recombination of photogenerated charge carriers is effectively inhibited after loading CN on the surface of MT. Furthermore, the enhanced transport of charge carriers can be further evidenced by the lower electric resistance and higher photocurrent response. As shown in Figure 7(b), the MT/CN-10 composite exhibits the smallest hemicycle radius. And the MT/CN-10 hybrid can afford a much enhanced current density than that of the bare MT and CN (Figure 7(c)). These above results further corroborate that the heterojunction between MT and CN can enhance the separation and transfer efficiency of charge carriers, which is beneficial for improving the photocatalytic disinfection performance.

The electronic spin resonance (ESR) measurement was carried out to detect the active radicals [36, 37]. As displayed in Figure 8(a), the DMPO-OH signal can be observed for pure MT and MT/CN-10, while no DMPO-OH signal is found for pure CN; this is due to the weak oxidation potential of CN. The strongest DMPO-*OH signal for MT/CN-10 suggests that the photogenerated holes still stay in the valence band (VB). At the same time, the DMPO-*[O.sub.2.sup.-] signals can be also observed for pure CN and MT/CN-10, but weak DMPO-*[O.sub.2.sup.-] signal appears in the MT (Figure 8(b)). The results indicate that the photogenerated electrons and holes stay in the conduction band (CB) of CN and the valence band (VB) of MT, respectively. The conventional type II heterojunction mechanism is hard to explain the EPR results. Contrarily, the Z-scheme heterojunction mechanism is more reasonable to explain the enhancement of the photocatalytic [H.sub.2] production and disinfection performance.

According to the atom's Mulliken electronegativity definition formula (Equations (2) and (3)) [38, 39],

[E.sub.VB] = X - [E.sub.0] + 0.5[E.sub.g], (2)

[E.sub.CB] = [E.sub.VB] - [E.sub.g], (3)

where [E.sub.VB] is the valence band potential, [E.sub.CB] is the conduction band potential, [E.sub.g] is the semiconductor bandgap energy, [E.sub.0] is the electron free energy (in general 4.5 eV), and X is the geometrical mean of the absolute electronegativity of each atom in the semiconductor. Thus, the [E.sub.VB] and [E.sub.CB] of MT are 3.11 V and -0.34 V, whereas the [E.sub.VB] and [E.sub.CB] of CN are 1.59 V and -1.12 V, respectively. To the end, the mechanism of the enhanced disinfection performance for the MT/CN heterojunction composites under simulated solar light irradiation is shown in Figure 9. Under simulated solar light irradiation, the excited electrons transfer from the CB of CN to the CB of MT, and the excited holes transfer from the VB of MT to the VB of CN. Thus, an internal electric field is produced at the interface between CN and MT. Simultaneously, band edge of CN bends upward due to the loss of electrons, but band edge of MT bends downward due to the accumulation of electrons. Under the influence of the internal electric field, the electrons on the CB of MT transfer to the VB of CN and recombine with the holes on the VB of CN. Therefore, more electron hole pairs can be efficiently separated, and more electrons and active radicals are obtained to enhance the photocatalytic [H.sub.2] production and disinfection, respectively.

4. Conclusions

In this work, we successfully synthesized a Z-scheme 2D/2D MT/CN heterostructured photocatalyst by a simple sol-gel method followed by calcination method to grow CN nanosheets on the surface of MT nanoflakes. And the MT/CN-10 exhibited much more excellent photocatalytic [H.sub.2] production performance and disinfection efficiency than that of pure MT and CN. The establishment of heterojunction between MT and CN can lead a superior interfacial charge transfer under simulated solar light irradiation so as to generate more electrons and active radicals for [H.sub.2] production and bacteria inactivation, respectively. In addition, the efficient harvesting of light and the large specific surface area of 2D MT/CN nanosheets are of great importance of the enhancement of the photocatalytic efficiency. These results could provide a new view for the design of heterojunction photocatalysts for applications in solar-to-chemical energy conversion and environmental remediation.

https://doi.org/10.1155/2019/6307858

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there is no conflict of interests regarding the publication of this paper.

Authors' Contributions

Dan Zhang and Shirong Luo contributed equally to this work.

Acknowledgments

This work was supported by the Science and Technology Planning Project of Huizhou (2018Y322).

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Sijia Gu, (1) Dan Zhang, (1) Shirong Luo, (1) and Heng Yang [ID] (2)

(1) Affiliated Huidong Hospital of Guangdong Medical University, Guangdong Medical University, Huizhou 516300, China

(2) Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430070, China

Correspondence should be addressed to Heng Yang; ccnu_yh@sina.com

Received 15 September 2019; Accepted 11 November 2019; Published 1 December 2019

Guest Editor: Muhammad S. Javed

Caption: Figure 1: XRD patterns of pure CN, MT, and MT/CN-10 Z-scheme heterojunction photocatalyst.

Caption: Figure 2: (a) FE-SEM images of pure MT. (b) MT/CN heterojunction. (c) SEM-EDS element mapping analysis of the MT/CN heterojunction.

Caption: Figure 3: (a) UV-Vis absorption spectra and (b) the bandgap evaluation from the ([alpha]h[upsilon]) [2] photon energy plots for CN, MT/CN-10, and MT samples.

Caption: Figure 4: The [N.sub.2] adsorption-desorption isotherms and pore size distributions (inset of the figure).

Caption: Figure 5: (a) Comparison of [H.sub.2] generation rate of as-prepared samples under Xenon lamp irradiation (full spectrum), with photodeposition of 1% Pt as cocatalyst. (b) Time courses of photocatalytic [H.sub.2] generation amount of MT/CN-10.

Caption: Figure 6: (a) Inactivation efficiency against E. coli by photocatalyst in the dark and without photocatalyst under simulated solar light. (b) The inactivation of E. coli by as-prepared photocatalysts under simulated solar light. (c) Photographs of colonies formed by E. coli cells in water samples after treated by MT/CN-10 suspensions with simulated solar light for 0 (c1), 20 (c2), 60 (c3), and 80 (c4) min.

Caption: Figure 7: (a) Steady-state photoluminescence spectrum (PL) of samples (excitation wavelength of 325 nm). (b) Nyquist plots of electrochemical impedance spectra (EIS) for samples. (c) Transient photocurrent-time profiles for samples.

Caption: Figure 8: The DMPO spin-trapping ESR spectrum for (a) DMPO-*OH and (b) DMPO-[O.sub.2.sup.-] under simulated solar light irradiation.

Caption: Figure 9: A possible photocatalytic mechanism for [H.sub.2] production based on MT/CN heterostructure.
Table 1: Physicochemical properties of MT, CN, and MT/CN-10
composite.

Sample     [S.sub.BET]    Cumulative volume     Average pore
           ([m.sup.2]    of pores ([cm.sup.3]   diameter (nm)
           [g.sup.-1])       [g.sup.-1])

MT            34.7               0.21               24.1
CN            12.1               0.10               34.4
MT/CN-10      20.9               0.15               31.5
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Title Annotation:Research Article
Author:Gu, Sijia; Zhang, Dan; Luo, Shirong; Yang, Heng
Publication:International Journal of Photoenergy
Date:Dec 1, 2019
Words:4960
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