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Article

Loading Nano-CuO on TiO2 Nanomeshes towards Efficient Photodegradation of Methylene Blue

by
Jingui Ma
1,
Zijin Tian
1,
Lei Li
2,
Yuan Lu
1,
Xiaoling Xu
1 and
Junwei Hou
1,*
1
State Key Laboratory of Heavy Oil Processing, College of Engineering, China University of Petroleum-Beijing at Karamay, Karamay 834000, China
2
Petrochina Kalamay Petrochemical Company, Karamay 834000, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(4), 383; https://doi.org/10.3390/catal12040383
Submission received: 7 March 2022 / Revised: 23 March 2022 / Accepted: 28 March 2022 / Published: 30 March 2022
(This article belongs to the Section Photocatalysis)
Browse Figures
Versions Notes

Abstract

:
In order to improve the photocatalytic activity of TiO2, we successfully loaded nano-CuO on the TiO2 nanomeshes as CuO-TiO2 nanocomposites through a facile electrodeposition method. The optimized calcined temperature after Cu electrodeposition is confirmed as 450 °C, which could furthest assist the crystallization of anatase TiO2 and guarantee the high photocatalytic activity of CuO-TiO2 nanocomposites. Comparing with pure TiO2 nanomeshes, CuO-TiO2 nanocomposites showed better degradability of methylene blue, and the degradation efficiency reached to 35% after 120 min irradiation. Additionally, CuO-TiO2 nanocomposites exhibit much stronger absorption intensity within the visible light scope, more than two times than that of pure TiO2 nanomeshes, which indicates that the loading of nano-CuO could promote photocatalytic efficiency by the strong visible light absorption. Additionally, CuO-TiO2 nanocomposites show faster photocurrent response and lower charge transfer resistance than that of pure TiO2 nanomeshes, which implies that the recombination rate of photogenerated electron-hole pairs was reduced after nano-CuO loading.

1. Introduction

In recent years, photocatalytic technology based on TiO2 has been attracting scientists’ attention after Fujishima and Honda’s research in 1972 [1]. Compared with conventional purification methods, such as ultraviolet, physical adsorption, filtration and sedimentation, TiO2 photocatalytic technology mainly uses the sunlight as radiation source by generating electron-hole pairs, which could react with oxygen and hydroxyl in contact with water to produce highly oxidizing hydroxyl radicals(OH) and superoxide radicals (O2−), and then oxidizing toxic organics or pollutants into H2O, CO2 and other small molecules to achieve purification [2,3,4,5]. As a prestigious photocatalyst, owing to its non-toxicity, high stability, low cost, simple preparation method and no secondary pollutants, TiO2 is widely applied in the air purification, wastewater purification, antimicrobial and catalytic aquatic hydrogen [6,7,8]. However, there are two main obstacles that hinder the industrial application of TiO2 photocatalyst. Firstly, the band gap of TiO2 is too wide (Eg = 3.0–3.2 eV), so only ultraviolet light could irradiate the electron-hole pairs, but ultraviolet light only accounts for 5% of the sun’s energy, whereas visible light accounts for 45%, meaning sunlight is not effectively utilized [9]. Secondly, more than 90% of TiO2 photoexcited electrons and holes recombine in a few nanoseconds after irradiation, which restrains the further oxidation–reduction reactions to degrade the pollutants [10,11]. In order to remove these two obstacles, different elements and oxides were loaded to optimize the TiO2, including metallic elements (Cu [12], Fe [13], Au [14], Ag [15], Pt [16], etc.), non-metallic elements (N [17], C [18,19], etc.), and semiconductors (ZnO [20,21], CuO [22,23], Cu2O [24,25], MnO2 [26], Al2O3 [22], etc.). Among these strategies, CuO is a p-type semiconductor with narrow band gap (Eg = 1.2 eV) and wide absorption range of sunlight, including ultraviolet and visible light regions. Additionally, it has superior photocatalytic activity, low price, abundant raw materials, contributes little pollution to the environment and is easy to prepare [27,28]. In addition, due to the large specific surface area of nano-CuO, it has more excellent physical and chemical properties than bulk CuO, which lays a foundation for the development of nano-CuO in the field of photocatalysis [29].
Compared with other studies, we creatively applied TiO2 nanomeshes as substrate to undertake the loading of nano-CuO through electrodeposition in this paper, which might inspire scientists to modify the conventional TiO2 photocatalyst with other elements or oxides. In addition, the morphology, microstructure and element composition features of CuO-TiO2 nanocomposites were systematically characterized by SEM, TEM, XRD and XPS. Especially, different calcined temperature after Cu electrodeposition was investigated to synthesize the best phase composition of CuO-TiO2 nanocomposites to degrade methylene blue (MB). Additionally, MB photo-degradation experiments were carefully processed on CuO-TiO2 nanocomposites and pure TiO2 nanomeshes to evaluate the effect of nano-CuO loading. In addition, ultraviolet–visible diffuse reflectance spectrum, transient photocurrent spectrum and electrochemical impedance spectroscopy were implemented on obtained samples to exploit the origin of enhanced photocatalytic activity of CuO-TiO2 nanocomposites.

2. Results

2.1. Characterization of CuO-TiO2 Nanocomposites

As shown in Figure 1a,d, the color of TiO2 nanomeshes was transformed from light blue to dusty blue after loading of nano-CuO, which could improve the absorption of sunlight. SEM and EDS were implemented systematically to investigate the surface topography and microstructure of the CuO-TiO2 nanocomposites. Comparing with TiO2 nanomeshes (Figure 1b,c), the CuO-TiO2 nanocomposites are darker, and the surface of CuO-TiO2 nanocomposites are coarser, which is due to the filling of CuO nanoparticles in the interspace of TiO2 nanoparticles. Additionally, high-magnification SEM images (Figure 1g–h) further confirm that these nanocomposites are composed of elliptical nanoparticles, with a diameter range of about 50~90 nm, which are stacked with each other and arranged loosely. The EDS spectrum (Figure 1i) indicates that the typical region in Figure 1h is composed of Cu, O and Ti, which indicates the formation of CuO-TiO2 nanocomposites. The peak at 2 keV should be defined to Pt element, which comes from the metal spraying during SEM and EDX tests to enhance the conductivity for better imaging [30].
The crystal phase of CuO-TiO2 nanocomposites was further confirmed by X-ray diffraction (Figure 2a). Peaks appearing in both CuO-TiO2 nanocomposites and pure TiO2 nanomeshes could be well indexed to mixed phase of anatase TiO2 and rutile TiO2. Additionally, the peak at 40.2° corresponds to titanium, which should be attribute to the incomplete anodization, and the peak intensity is decreased after nano-CuO loading. More importantly, there is a typical peak at 35.5°, corresponding to the (111) plane of CuO in CuO-TiO2 nanocomposites. Additionally, the peak at 37.8° is overlapped by (111) plane of CuO and (004) plane of anatase TiO2. [31,32] Further, the average crystallite size was calculated as around 50 nm according to the Scherrer Formula, which also accorded with the results of SEM and TEM. The above XRD results convincingly suggested that nano-CuO was successfully loaded on the mixed phase of anatase TiO2 and rutile TiO2. High-resolution transmission electron microscopy (HRTEM) was also used to characterize the CuO-TiO2 nanocomposites. Figure 2b shows the characteristic spacing of around 0.23 nm and 0.32 nm for the (111) lattice plane of CuO and the (110) lattice plane of TiO2, respectively, which accords with the results in XRD [23]. Elemental mapping under TEM mode is another powerful technique to characterize the element distribution of CuO-TiO2 nanocomposites. An overlay of Cu, Ti and O mapping is shown in Figure 2c–f, indicating a clear separation of the three elements and confirming the nanocomposites structure.
X-ray photoelectron spectroscopy was carried out to investigate the chemical composition and element valence state of CuO-TiO2 nanocomposites. The characteristic peaks of Cu 2p, O 1s, Ti 2p and C 1s can be clearly observed in the full XPS spectrum (Figure 3a), which indicates the nanocomposites contain elements of Cu, O, Ti and C, the C element mainly comes from instrument pollution. As for the high-resolution spectrum, the peaks in the Ti 2p (Figure 3b) are centered at about 464.2 eV and 458.3 eV, which are attributed to Ti 2p3/2 and Ti 2p1/2, respectively, demonstrating the existence of the oxidation state of Ti4+ [13]. Figure 3c is the high-resolution spectrum of Cu 2p: the main peaks at about 933.8 eV and 953.8 eV, and the oscillating satellite peaks at about 943.4 eV and 962.2 eV confirm the existence of CuO [31]. Figure 3d shows the high-resolution peak fitting spectrum of O 1s: the peaks with binding energies of about 529.7 eV and 529.5 eV are distributed to Ti-O and Cu-O species, whereas the peak with higher binding energy of 531.6 eV is distributed to hydroxide or carbonate in the atmosphere, which indicates that O mainly comes from metal oxides [33].

2.2. Photocatalytic Performance of CuO-TiO2 Nanocomposites

In order to investigate the optimized calcined temperature after Cu electrodeposition, the concentration of MB degradation of samples at 450, 550, 650 and 750 °C for 2 h were measured by using a UV-visible spectrophotometer. The effects of different calcined temperatures on the photocatalytic performance of CuO-TiO2 nanocomposites were evaluated by calculating the degradation efficiency. The anatase TiO2 would transform to rutile TiO2 at around 450 °C, and the Cu would convert to CuO at 350 °C [34,35]. According to the results of photocatalytic degradation efficiencies of MB at different calcined temperatures after Cu electrodeposition, the degradation ability of samples calcined at 450 °C is obviously better than that of other temperatures (Figure 4a). The reasons may be as follows: (1) too low calcined temperature (<450 °C) could not facilitate the conversion from Cu to nano-CuO, leading to unsuccessful loading of nano-CuO on the TiO2 nanomeshes; (2) too high calcined temperature (550–750 °C) would trigger more anatase transform to rutile, and because the photocatalytic activity of rutile is lower than that of anatase, a high proportion of rutile in mixed phase TiO2 would inhibit the photocatalytic activity of CuO-TiO2 nanocomposites as a result. To gain more insight about the photocatalytic activity of CuO-TiO2 nanocomposites, we monitored the photocatalytic degradation efficiencies of CuO-TiO2 nanocomposites, pure TiO2 nanomeshes and without catalysts (Figure 4b). Apparently, the photocatalytic degradation efficiency of CuO-TiO2 nanocomposites is higher than those of pure TiO2 nanomeshes and reaches 35% after 120 min irradiation. Additionally, only around 10% of the MB could be degraded without any catalysts. The COD removal rate shared the same growth trend as the degradation efficiency; the COD removal rate reached around 34% after 120 min irradiation (Figure 4c). The above results suggest that CuO-TiO2 nanocomposites show higher degradation efficiency and better photocatalytic decomposition ability. As for the unexpected minor effect on degradation efficiency after nano-CuO loading, we think Cu electrodeposition and further calcination create uniform nano-CuO particle “clothes” on the TiO2 nanomeshes, which might hinder the ability of light to irradate the eletron-hole pairs in TiO2 and, following oxidation–reduction reactions, to produce highly oxidizing hydroxyl radicals(·OH) and superoxide radicals (O2−) to a certain degree, and MB cannot be degraded completely, so the degradation efficiency was unsatisfactory.
To achieve a better understanding of the origin of enhanced photocatalytic activity of CuO-TiO2 nanocomposites, ultraviolet–visible diffuse reflectance spectrum (UV–vis DRS), transient photocurrent spectrum and electrochemical impedance spectroscopy were implemented on obtained samples. The optical absorption properties of CuO-TiO2 nanocomposites and pure TiO2 nanomeshes are recorded in Figure 5a,b. Pure TiO2 nanomeshes and CuO-TiO2 nanocomposites exhibit stronger absorption intensity within the visible-light scope, more than 10 times than that of pure TiO2 nanomeshes, which indicates that the loading of nano-CuO could promote photocatalytic efficiency by its strong visible light absorption. Additionally, the band gap has been roughly calculated based on the formula: Eg = 1240/λg. The Eg of TiO2 nanomeshes and CuO-TiO2 nanocomposites were confirmed as 3.5 eV and 3.1 eV. Although the values were a little overestimated, the decreasing tendency of Eg accorded with corresponding research [23,31,32].
As presented in Figure 5c, both CuO-TiO2 nanocomposites and pure TiO2 nanomeshes display fast and uniform photocurrent response, and the photocurrent of CuO-TiO2 nanocomposites reach a peak at 45 μA/cm2, more than two times that of pure TiO2 nanomeshes, which indicates that the produced composites might be also suited for photodetector applications. As presented in Figure 5d, semicircle diameters of CuO-TiO2 nanocomposites are much smaller than those of TiO2 nanomeshes, which indicates that CuO-TiO2 nanocomposites possess much lower charge transfer resistance than TiO2 nanomeshes. Thus, the recombination rate of photogenerated electron-hole pairs was reduced after nano-CuO loading on TiO2 nanomeshes.
Based on the relevant bibliographic reports [23,32,33], the possible photocatalytic decomposition mechanism of CuO-TiO2 nanocomposites is summarized in Figure 5e. Before loading the nano-CuO on TiO2 nanomeshes, only ultraviolet light could induce electron-hole pairs in TiO2, and these separated electron-hole pairs would recombine in a few nanoseconds after irradiation, which restrains the further oxidation–reduction reactions to degrade the pollutants. After loading nano-CuO, the electrons in the valence band (VB) of TiO2 and CuO would absorb visible light energy and transfer to the conduction band (CB) of TiO2 and CuO. Because the CB of CuO is lower than that of TiO2, photogenerated electrons would transferred from TiO2 to CuO and accumulate holes in the VB of TiO2 and CuO. Therefore, the electron transfer between TiO2 and CuO could reduce the recombination rate of photogenerated electron-hole pairs in TiO2, enlarge the photo-response range of the catalyst and enhance the photocatalytic ability of the catalyst. Then, the photogenerated electrons react with O2 absorbed on the material surface to form ·O2−, while the photogenerated holes react with H2O or OH− to form highly oxidizing ·OH; these two intermediate products could degrade the pollutants into H2O, CO2 and other small molecules to achieve the purification effect.

3. Experimental Section

3.1. Raw Materials and Reagents

Titanium meshes (200 mesh, purity 99.9%) were purchased from Hengshui Kangwei Company (Hengshui, China). The main analytical pure reagents such as methylene blue (MB), N-propanol (C3H7OH), isopropanol (C3H8O), Ethylene Glycol ((CH2OH)2), Ammonium fluoride (NH4F), Hydrofluoric acid (HF), acetic acid (CH3COOH), copper chloride dihydrate (CuCl2·2H2O), methylene blue (MB), anhydrous ethanol (CH3CH2OH), sodium hydroxide (NaOH) and ammonium hydroxide (NH3·H2O) were purchased from China National Pharmaceutical Group Co., Ltd. (Beijing, China). without purification throughout the experiments.

3.2. Fabrication of TiO2 Nanomeshes

According to our previous research [30], Ti meshes were first cut into 2.2 cm × 2.2 cm and then dipped in a propanol, methanol and isopropanol solution and deionized water in turn for 10 min ultrasonic cleaning to remove the impurities. Then, the mixed acid (HF: CH3COOH = 1:8, in volume total 18 mL) was used to chemically etch the aforementioned Ti meshes for 2 min. After cleaning with deionized water, the etched Ti meshes were applied as anode and a platinum sheet worked as cathode to prepare the amorphous TiO2 nanomeshes; the anodic oxidation reaction was carried out at 60 V for about 2 h in an electrolyte containing an aqueous solution of ethylene glycol (5.5% vol) and ammonium fluoride (0.5 wt%). Finally, crystalline TiO2 nanomeshes were fabricated by calcining the amorphous TiO2 nanmeshes in a furnace and then cooled to room temperature (Figure 2a).

3.3. Fabrication of CuO-TiO2 Nanocomposites

As shown in Scheme 1, the CuO-TiO2 nanocomposites were fabricated through a facile electrochemical technology. Firstly, the crystalline TiO2 nanomeshes, platinum and Ag/AgCl were used as the working electrode, counter electrode and reference electrode, respectively, for electrodeposition. The electrolyte consisted of 50 mL 0.003 mol/L copper chloride dihydrate, 2 mL 0.05 mol/L ammonium hydroxide and 0.5 mL 2 mol/L sodium hydroxide solution. The medium pH during electrodeposition was 11, and the step current was set at 0.04 A, and it was stopped for 5 s after working every 10 s. After 10 cycles of electrodeposition, the nanocomposites were heated to 450 °C in a furnace for 2 h and then cooled to room temperature (Figure 2d).

3.4. Characterization

The morphology and microstructure of the samples were systematically investigated by scanning electron microscopy (SEM, Hitachi SU4800) and transmission electron microscopy (TEM, JEOL JEM 2100F). Additionally, X-ray powder diffraction (XRD, Ultimo IV) was carried out using Cu Kα radiation over the range of 5~90° measurement. X-ray photoelectron spectra (XPS) were recorded by a Kratos AXIS SUPRA XPS system.

3.5. Photocatalytic Performance Test

The photocatalytic activity of CuO-TiO2 nanocomposites was evaluated by degrading the MB (12 mg/L, denoted as C0) under the irradiation of a 500 W xenon lamp (CEL-S500) without filter to simulate sunlight, and the full wavelength was 320–2500 nm. The MB aqueous solution was regarded as the organic contaminant to be degraded. The distance between the lamp edge and liquid surface was 15 cm. Before the photocatalytic test, the MB solution and CuO-TiO2 nanocomposites were mixed in darkness for 30 min. During photoreaction, the MB solution was taken out every 30 min intervals. The absorbance of MB at the maximum absorption wavelength of 664 nm was measured by UV-Vis spectrophotometer and converted to the corresponding concentration. F factor was used to correct the concentration deviation of the solution (maintained at about 8), and the degradation efficiency was calculated by the equation Degradation efficiency = [(C0 − Ct)/C0] × 100%, where C0 is the initial concentration of MB, and Ct is the concentration of MB after degradation. We also monitored the COD of the CuO-TiO2 nanocomposites through rapid digestion spectrophotometry. COD removal (%) = (COD0 − CODt)/COD0 × 100%, where COD0 is the initial chemical oxygen demand (mg/L), and CODt is the chemical oxygen demand at t min in the reaction process (mg/L).
The I-T current mode of the electrochemical workstation was used to record the changing trend of the photocurrent of the composite material, and impedance was measured under the same lamp for the photocatalytic test. The photocurrent densities of the samples were calculated according to the formula J = I/A, where J is the photocurrent density, I is the photocurrent, A is the sample area and the applied potential for the photocurrent was the open circuit potential, without bias potential.
Electrochemical impedance spectroscopy (EIS) was performed in 0.1 mol/L Na2SO4 solution with open circuit potential. The amplitude was 5 mV, and the frequency range was 1 × 106 to 1 × 10−2 Hz.

4. Conclusions

In summary, nano-CuO was successfully loaded on the TiO2 nanomeshes as CuO-TiO2 nanocomposites through a facile electrodeposition method, and their morphology, microstructure and element composition features were systematically characterized by SEM, TEM, XRD and XPS. Especially, the optimized calcined temperature after Cu electrodeposition is confirmed as 450 °C, which could furthest assist the crystallization of anatase TiO2 and guarantee the high photocatalytic activity of CuO-TiO2 nanocomposites. More importantly, the photocatalytic degradation efficiency of CuO-TiO2 nanocomposites is higher than those of pure TiO2 nanomeshes and reached to the 35% after 120 min irradiation, which suggests that CuO-TiO2 nanocomposites show enhanced degradation efficiency and photocatalytic decomposition ability. Additionally, CuO-TiO2 nanocomposites exhibit much stronger absorption intensity within the visible-light scope, which indicates that the loading of nano-CuO could promote the photocatalytic efficiency by the strong visible-light absorption. Additionally, CuO-TiO2 nanocomposites display faster photocurrent response and lower charge transfer resistance than those of pure TiO2 nanomeshes. The enhanced photocatalytic activity may originate from the nano-CuO loading, which could scavenge the photogenerated electrons from TiO2, reduce the recombination rate of photogenerated electron-hole pairs in TiO2, enlarge the photoresponse range of the catalyst and enhance the photocatalytic ability of the catalyst. We hope this study could provide a new way for the preparation of CuO-TiO2 nanocomposite materials and pave the way to their application in the photocatalytic field.

Author Contributions

Conceptualization, J.M.; Data curation, Z.T.; Funding acquisition, J.H; Investigation, Z.T.; Methodology, J.M.; Project administration, X.X.; Software, L.L.; Supervision, L.L.; Validation, Y.L. and J.H.; Visualization, Z.T.; Writing—review and editing, J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [Tianshan Young Scholars] grant number [2018Q031] and [Educational Foundation of Xinjiang Province] grant number [XJEDU2018Y060].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Digital image of TiO2 nanomeshes; (b,c) SEM images of TiO2 nanomeshes at different magnification; (d) digital image of CuO-TiO2 nanocomposites; (e–h) SEM images of CuO-TiO2 nanocomposites at different magnification; (i) EDS spectrum of the typical region in Figure 1h.
Figure 1. (a) Digital image of TiO2 nanomeshes; (b,c) SEM images of TiO2 nanomeshes at different magnification; (d) digital image of CuO-TiO2 nanocomposites; (e–h) SEM images of CuO-TiO2 nanocomposites at different magnification; (i) EDS spectrum of the typical region in Figure 1h.
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Figure 2. (a) X-ray diffraction pattern of CuO-TiO2 nanocomposites and pure TiO2 nanomeshes; (b) HRTEM image of CuO-TiO2 nanocomposites; (c–f) TEM image and corresponding element maps of CuO-TiO2 nanocomposites.
Figure 2. (a) X-ray diffraction pattern of CuO-TiO2 nanocomposites and pure TiO2 nanomeshes; (b) HRTEM image of CuO-TiO2 nanocomposites; (c–f) TEM image and corresponding element maps of CuO-TiO2 nanocomposites.
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Figure 3. XPS spectra of (a) sample surface, (b) Ti 2p, (c) Cu 2p and (d) O 1s of CuO-TiO2 nanocomposites.
Figure 3. XPS spectra of (a) sample surface, (b) Ti 2p, (c) Cu 2p and (d) O 1s of CuO-TiO2 nanocomposites.
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Figure 4. (a) Photocatalytic degradation efficiencies of MB at different calcined temperatures after Cu electrodeposition; (b) photocatalytic degradation efficiencies of MB of CuO-TiO2 nanocomposites, TiO2 nanomeshes and without catalysts.; (c) COD removal rate of CuO-TiO2 nanocomposites.
Figure 4. (a) Photocatalytic degradation efficiencies of MB at different calcined temperatures after Cu electrodeposition; (b) photocatalytic degradation efficiencies of MB of CuO-TiO2 nanocomposites, TiO2 nanomeshes and without catalysts.; (c) COD removal rate of CuO-TiO2 nanocomposites.
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Figure 5. (a,b) UV–vis DRS absorption curves of TiO2 nanomeshes and CuO-TiO2 nanocomposites; (c) photocurrent-time image of CuO-TiO2 nanocomposites and TiO2 nanomeshes; (d) Nyquist plots of CuO-TiO2 nanocomposites and TiO2 nanomeshes; (e) possible photocatalytic mechanism of CuO-TiO2 nanocomposites.
Figure 5. (a,b) UV–vis DRS absorption curves of TiO2 nanomeshes and CuO-TiO2 nanocomposites; (c) photocurrent-time image of CuO-TiO2 nanocomposites and TiO2 nanomeshes; (d) Nyquist plots of CuO-TiO2 nanocomposites and TiO2 nanomeshes; (e) possible photocatalytic mechanism of CuO-TiO2 nanocomposites.
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Scheme 1. Synthesis scheme of the CuO-TiO2 nanocomposites.
Scheme 1. Synthesis scheme of the CuO-TiO2 nanocomposites.
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Ma, J.; Tian, Z.; Li, L.; Lu, Y.; Xu, X.; Hou, J. Loading Nano-CuO on TiO2 Nanomeshes towards Efficient Photodegradation of Methylene Blue. Catalysts 2022, 12, 383. https://doi.org/10.3390/catal12040383

AMA Style

Ma J, Tian Z, Li L, Lu Y, Xu X, Hou J. Loading Nano-CuO on TiO2 Nanomeshes towards Efficient Photodegradation of Methylene Blue. Catalysts. 2022; 12(4):383. https://doi.org/10.3390/catal12040383

Chicago/Turabian Style

Ma, Jingui, Zijin Tian, Lei Li, Yuan Lu, Xiaoling Xu, and Junwei Hou. 2022. "Loading Nano-CuO on TiO2 Nanomeshes towards Efficient Photodegradation of Methylene Blue" Catalysts 12, no. 4: 383. https://doi.org/10.3390/catal12040383

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