Photo-catalytic degradation of brilliant green dye using recyclable ZnO/MWCNT@TiO₂ nano-composite

2.1.1. TiO₂ preparation

TiO₂ was initially prepared by a co-precipitation process. 8.5 mL of tetra-n-butyl orthotitanate, used as a precursor, is added drop by drop with continuous stirring to the 200 mL beaker, which contains a mixture of 15 mL isopropanol and 10 mL water. The 0.1 N conc. HNO₃ and NaOH solution (1:3) were added dropwise into the reaction mixture with continuous stirring to maintain the pH 11-12, and stirred for 24 h, followed by the aging of the reaction mixture for another 24 h. After that, the solution was heated at 100^oC for 4 h in an oil bath. It was heated in the oven for 6 h at 150^oC. The obtained white precipitate was kept in the muffle furnace for 5-6 h at 400^oC to yield pure TiO₂ (Hsu et al., 2024).

2.1.2. Activation of MWCNT

In this experiment, MWCNTs were activated (Harikrishna et al., 2022) before using for composite preparation, by using conc H₂SO₄/HNO₃ (3:1), stirred magnetically for 6 h, washed with distilled water several times to remove the impurities of carbon, then filtered, and dried at 80^oC for 15 h.

2.1.3. Preparation of ZnO/MWCNT@TiO₂ nanocomposite

ZnO/MWCNT@TiO₂ nanocatalyst was prepared by the wet impregnation method. 0.075 g of MWCNTs were added to a beaker containing 20 mL of distilled water, stirred magnetically for 1 h, and 0.9 g of TiO₂ and 0.025 g of ZnO were added. The mixture was stirred overnight till a homogenous MWCNTs-contained slurry was formed. The slurry was heated at 80^oC for 10 h to make ZnO/MWCNT@TiO₂ nanocomposite catalyst. By changing the weights of MWCNT and ZnO and keeping the weight of TiO₂ constant, different nanocomposite catalysts were obtained (SI-I).

3.1.1. XRD analysis

p-XRD assessed the purity, crystalline phase, structure, and average crystallite size of the prepared pure TiO₂ and ZnO/MWCNT@TiO₂ nanocomposite. The XRD patterns indicated the presence of the anatase phase, mainly along with the rutile phase, in the prepared titania. The presence of distinct crystalline phases within the material is shown by the numerous well-defined peaks in the powder XRD data at exact 2θ angles.

The peaks observed in the XRD pattern (Fig. 2) at 2θ angles of 29.36°, 38.93°, and 47.89° indicate pure TiO₂ (JCPDS No:21-1272). In the nanocomposite, the peaks observed at 2θ angles of 31.79°, 34.43°, 36.25°, and 47.50° represent the presence of ZnO within the composite matrix (JCPDS No: 36-1451). The peaks observed at 2θ angles 26.08° and 42.57° correspond to MWCNT (JCPDS No:01-0646) and the peaks observed at 2θ 29.43°, 38.93°, and 56.56° denote TiO₂ (JCPDS No:21-1272). In the XRD of ZnO/MWCNT@TiO₂, additional peaks are observed, and peaks appear sharper, indicative of better crystallinity. The XRD analysis suggests that the material under investigation is a composite made up of ZnO, MWCNT, and TiO₂. The material’s crystallite size (L) was calculated using the Debye-Scherrer formula, as represented below, and the average crystallite dimension of the synthesized TiO₂ was approximately 10 nm. In contrast, the ZnO/MWCNT@TiO₂ ​ nanocomposite exhibited an average crystalline size of around 25 nm.

$\text{L}=0.\text{89}\lambda /\beta \text{Cos}\theta$

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Fig. 2.

XRD patterns of pure TiO₂ and ZnO/MWCNT@TiO₂.

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Where, L - crystallite size (nm),

λ = wavelength (nm),

β = full width at half maximum (FWHM, in radians), and

θ = Bragg angle.

3.1.2. SEM - EDX analysis

Pure TiO₂ and ZnO/MWCNT@TiO₂ were analyzed using field emission-SEM (Figs. 3a and b). The SEM image shows the manner in which both TiO₂ and ZnO, as near-spherical fluffy nanoparticles, are closely aligned with one another, and MWCNTs, as tangled cylindrical hollow tubes, are distributed in the matrix. This confirms the homogenous and even distribution of ZnO and MWCNT in the TiO₂ matrix. The energy-dispersive X-ray (EDX) analysis (Fig. 3c) of ZnO/MWCNT@TiO₂ ​nanocomposite shows the presence of titanium (Ti), carbon (C), oxygen (O), and zinc (Zn), which apparently indicates the successful preparation of the composite.

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Fig. 3(a and b)

. SEM image of (a) pure TiO₂, (b) ZnO/MWCNT@TiO₂.

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Fig. 3(c).

SEM-EDX images of ZnO/MWCNT@TiO₂

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3.1.3. TEM analysis

The TEM image of TiO₂ and ZnO/MWCNT@TiO₂ nanocomposite has been shown in Figs. 4a & b. The micrograph reveals that the average crystallite size of the nanocatalyst is ∼28 nm and is also in good concordance with the powder XRD analysis. The TEM micrograph suggests that ZnO and MWCNT are consistently well distributed on the TiO₂ matrix. The dark area represents TiO₂ and ZnO clusters with MWCNT distribution evenly. The ZnO and TiO₂ nanoparticles reveal an irregular spherical shape, and MWCNT is seen as fairly dispersed, which in turn increases active sites and, hence, the catalytic activity.

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Fig. 4.

TEM image of (a) pure TiO₂ (b) ZnO/MWCNT@TiO₂.

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3.1.4. IR analysis

The FTIR spectrum of ZnO/MWCNT@TiO₂ (Fig. 5) was recorded between 4000-500 cm⁻¹. The broad peak at 3420 cm^-1 corresponds to the O-H stretching and denotes the presence of surface hydroxyl (–OH) groups, which might have been due to TiO₂, ZnO, or adsorbed moisture. The presence of hydroxyl groups enhances hydrophilicity and, thus, the photocatalytic activity. The peak at 1618 cm^-1 can be attributed to bending vibrations in water molecules (H-O-H), suggesting moisture adsorbed on the nanocomposite surface. The peaks at 1032-1000 cm^-1 may be respectively indicative of C–O stretching (MWCNT oxidation), which can lead to improved dispersibility, and Ti–O–C bond, which is likely from Ti–O stretching in TiO₂ when interacting with carbon. Strong peaks in 624-520 cm^-1 confirm the presence of metal-oxygen bonds. The peak at 540 cm^-1 is indicative of Zn–O, and the peak at 620 cm^-1 is attributed to Ti–O–Ti stretching. The presence of hydroxyl groups and adsorbed moisture implies likely surface modification, and the peaks corresponding to Zn–O and Ti–O confirm the formation of a metal oxide framework. Hence, the FTIR spectrum confirms the different functionalities in ZnO, TiO₂, and MWCNTs and confirms the successful incorporation of ZnO and MWCNT onto TiO₂.

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Fig. 5.

IR spectra of ZnO/MWCNT@TiO₂.

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3.1.5. UV-DRS

A nanophotocatalyst’s UV-DRS graph shows important characteristics pertaining to the electrical structure and optical characteristics of the composite material. ZnO exhibits considerable absorption in the UV region with a bandgap of around 3.37 eV, whereas TiO₂ absorbs in the UV range with a bandgap of about 3.2 eV (Figs. 6a and b) (Anantha et al., 2024). The UV-DRS spectrum of ZnO/MWCNT@TiO₂ demonstrates a discernible increase in absorption in the visible range upon incorporation of MWCNTs into the ZnO/TiO₂ combination around 3.4 eV (Fig. 7a and b). This is explained by the MWCNTs’ capacity to mediate electrons, allowing ZnO and TiO₂ to separate and transfer charges more easily. Additionally, the composite shows a widened absorption edge. Since improved light absorption and charge transfer dynamics encourage more effective use of solar energy and speed up photocatalytic reactions, these modifications might be linked to higher photocatalytic performance.

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Fig. 6.

UV DRS Spectrum of (a) TiO₂ (b) ZMT-3.

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Fig. 7.

Tauc Plot of (a) TiO₂ (b) ZMT 3.

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3.1.6. BET analysis

The BET analysis is pivotal in determining the pore size distribution, porosity, and specific surface area of the ZnO/MWCNT@TiO₂ catalyst. Integrating TiO₂-ZnO and MWCNT promotes the development of an interconnected porous network. The nitrogen adsorption isotherm of the catalyst corresponds to type-IV, characteristic of mesoporous materials (Fig. 8), with a distinct hysteresis loop appearing in the relative pressure range of 0.5 to 0.9 (P/P₀). The pore size distribution curve highlights that the material predominantly exhibits mesoporous features, with pore sizes ranging between 2 and 50 nm, significantly contributing to its overall porosity. The average pore diameter is approximately 37.7 nm, reflecting a well-defined mesoporous structure. Additionally, the BET analysis indicates that the ZnO/MWCNT@TiO₂ catalyst has a high specific surface area of about 133.4624 m²/g. This substantial surface area is attributed to the porosity of MWCNTs and the porous nature of the TiO₂ matrix. The catalyst also exhibits a pore volume of roughly 0.57 cm¹/g.

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Fig. 8.

BET of ZnO/MWCNT@TiO₂

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3.3.1. pH 3

At pH 3, mineralization in sunlight alone is minimal. The systems ZMT 1 (TiO₂+MWCNT) and ZMT 2 (TiO₂+ZnO) show much lower mineralization (12-14%). ZMT 3 (TiO₂: MWCNT: ZnO) (0.9: 0.075: 0.025) shows moderate effectiveness in promoting mineralization (20.59% at 100 mins). The ZMT 4 (TiO₂: MWCNT: ZnO) (0.9: 0.05: 0.05) and ZMT 5 (TiO₂: MWCNT: ZnO) (0.9: 0.025: 0.075) systems seem more effective, reaching up to 50% and 63% mineralization at 120 min, respectively. However, mineralization at acidic pH is irregular (SI-III).

3.3.2. pH 5.5

The degradation of BG at its natural pH showed low efficiency, reaching a maximum mineralization of 35.97% at 120 min under sunlight. At pH 5.5, sustained and moderate degradation was registered with ZMT 1 alone, while moderate performance peaking at 48.01%, evidence of improved assisted degradation, was achieved using ZMT 1. ZMT 2 performed poorly, attaining a maximum mineralization of only 11.59% with very low overall activity. ZMT 3 outperformed the other systems at pH 5.5, with the maximum being 78.24%. Although ZMT 4 tended to be effective at consistently moderate performance at a maximum of 49.96% after 120 min, ZMT 5 had minimal performance at a maximum of 30.34%, which, although somewhat better than ZMT 2, was far less efficient overall. ZMT 3 performed best at pH 5.5, with ZMT 4 next in line (SI-III).

3.3.2. pH 7

In the absence of a catalyst, a moderate performance, with mineralization peaking at 55.18%, was observed, which was consistent but not the highest. ZMT 1 showed strong performance, peaking at 69.63%, comparatively better than prior. ZMT 2 performed excellently, peaking at 86.58% and outperforming all other systems at pH 7. ZMT 3 illustrated excellent performance, peaking at 86.25%, comparable to ZMT 2. ZMT 4 demonstrated moderate performance, peaking at 56.57%. ZMT 5 also displayed strong performance, peaking at 76.15%, second-best after ZMT 2 and ZMT 3. In conclusion, ZMT 2 and ZMT 3 perform best at pH 7, followed by ZMT 5, while others are less efficient (SI-III).

3.3.3. pH 9

Photodecomposition carried out without a catalyst showed good efficiency, with mineralization reaching a maximum of 79.20%. It was a significant improvement over other pH levels. ZMT 1 performance was moderate, peaking at 48.95%-less efficient than previously mentioned. ZMT 2 performed excellently and consistently, achieving a maximum of 87.04%. ZMT 3 did outstandingly well, with a maximum of 94.50%, and dominated all other systems at pH 9. In the second instance, ZMT 4 performed moderately, at a maximum of 65.33%- better than ZMT 1, but otherwise less effective. ZMT 5 performed well, with a maximum of 76.44%. In conclusion, ZMT 3 is the best performer at pH 9, followed by ZMT 2, and ZMT 1 is the least effective (SI-III).

To summarize, the best systems in each pH range are ZMT 3 at pH 5.5, ZMT 3 and ZMT 4 at pH 7, and ZMT 3 at pH 9. ZMT 2 and ZMT 3 are versatile systems performing well across all pH ranges, but ZMT 3 was the highest at pH 5.5, 7, and 9 (Fig. 9). Likewise, the photodegradation plots on ln(C/C_o) vs. time (SI-IV) for the three photocatalysts suggest that ZMT 3 (ZnO/MWCNT@TiO₂) performed better in degrading BG dye.

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Fig. 9.

Photodegradation of BG dye (% Mineralization) in Sun+ ZMT 3 @ pH 3, 5.5, 7, and 9.

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3.4.1. Optimal pH selection

The higher efficacy of the sunlight + ZTM 3 system compared to other conditions can be attributed to its stability and reactivity under such conditions. At neutral pH 7, the surface charge of ZMT 3 allows optimal interactions with dye adsorbate that yield good catalytic activity. Indeed, TiO₂ and ZnO have higher photocatalytic efficiency in neutral to slightly alkaline conditions, as the surface charge of the solid particles attracts the cationic dye molecules enough, thus leading to better degradation.

At pH 9, still slightly alkaline, it helps to generate some hydroxyl radicals (OH·) from water, which are important to decompose the dye into non-toxic byproducts. It also allows better charge separation at that pH and reduces recombination rates of the photoexcited electron-hole pairs on the catalyst surface. On the other hand, at low pH (pH 3), the excessive surface protonation of the catalyst probably inhibits dye adsorption and thereby influences the decrease in photocatalytic efficiency. A steep rise in the recombination rates at acidic pH will also give rise to erratic results, as the zigzag graphs depict. Hence, the optimal pH range for this system is pH 7 and 9.

Moreover, under alkaline conditions, the surfaces of TiO₂ and ZnO become negatively charged, resulting in considerable enhancement in the adsorption of cationic BG dye molecules owing to stronger electrostatic interactions. This increased adsorption results in a better effective interaction between the dye and the catalyst surface, maximizing the utilization of photo-generated charge carriers. In contrast, while pH 7 still performs relatively well, the generation of OH radicals and the electrostatic interactions are somewhat lower. Conversely, due to the protonation of the catalyst surface at pH 5.5, being the natural pH of the dye, the adsorption of dye on the catalyst decreases, rendering an average degradation of BG. Thus, pH 9 provides the most favorable conditions for ZMT 3, with the greatest degradation and mineralization efficiency.

3.4.2. Optimal catalyst load

When 5 mg catalyst loading was taken, average mineralization was evidenced. The catalyst loading of 10 mg enabled efficient degradation (94%) of BG dye. On the other hand, when the catalyst loading was doubled, there was no improvement in the mineralization percentage. Taking higher amounts of catalyst (30 or 40 mg) resulted in regression in the mineralization, probably due to the agglomeration of catalyst, absorption of ROS, and an increase in turbidity, leading to light scattering and blockage. Further increase in catalyst amounts (50 mg) resulted in very poor degradation (Table 1). Hence, a catalyst load of 20 mg was fixed for all systems.

3.5.1. Analysis of value-added products

After completing the photodegradation of BG dye for 100 min, solution samples (each 1-2 mL) were withdrawn and tested to detect the presence of the residual products.

Test for carboxylic acids: When the neutral ferric chloride solution was added dropwise to the sample solution, a reddish-brown coloration was observed, confirming the presence of the carboxylic acid group. Secondly, when a pinch of sodium bicarbonate was added to the test sample, effervescence was observed, confirming the presence of carboxylic acid.

Test for alcohol: A small piece of sodium metal, when dropped in the sample solution, gave effervescence with the release of tiny gas bubbles, indicating the presence of alcohol functionality.

Test for CO₂: To freshly prepared limewater taken in a test tube, the sample solution (at pH 3) was added dropwise. The limewater turned milky, showing the presence of carbon dioxide. However, the samples subjected to photodegradation at pH 7 and 9 gave this test only after adequate acidification due to the presence of CO₂ as HCO₃^- ion at higher pH.