Synthesis and photocatalytic performance of Ag-TiVOX nanocomposite

2.1 Materials synthesis

Chemicals used in this experimental, were analytical grade and supplied by Merck. TiO₂ based photocatalyst TiO₂, Ag-TiO₂, V-TiO₂ and Ag-TiVO_x composite, was fabricated by a sol–gel and hydrothermal method. To prepare TiO₂, 1 ml of Titanium (IV) butoxide was hydrolysed with 50 ml of deionised water (DI). The solution was then decanted after 10 mins. Thereafter 5 ml of hydrogen peroxide was added to the mixture along with 25 ml of deionised water, allowed for the formation of a gel like mixture. The gel was autoclaved in a stainless Teflon steel flask at 180 °C for 4 h. Followed by cooling at room temperature. It was then centrifuged and washed multiple times with ethanol and DI. The resulted precipitant was then kept in the oven at 80 °C to dry for 12 h. To synthesize Ag-TiO₂, 1 ml of Titanium (IV) butoxide was hydrolysed with 50 ml of DI. The solution was then decanted after 10mins and, 5 ml of hydrogen peroxide was added along with along with 5 ml of AgNO₃ (10 mg/l) and 25 ml of DI water. Continuous stirring was applied to the solution, until the formation of a gel, and then the solution was autoclaved at 180 °C for 4 h. The samples was centrifuged after cooing, and washed multiple times with ethanol and DI water. The resulted precipitant was oven dried at 80 °C for 12 h. For V-TiO₂ synthesis, the procedure above was repeated with 0.1 g of NH₄VO₃ instead of AgNO₃. While for Ag-TiVO_x photocatalyst composite synthesis, 5 ml of both AgNO₃ and NH₄VO₃ was added to the TiO₂ mixture to fabricate Ag-TiVO_x nanocomposite

2.2 Characterization

The crystal structure and phase purities were analysed, by (XRD) Diffractometer (X-Pert Philips), from 2θ 10° to 90° at 40 kV voltage and 40 μA current. The Brunauer-Emmett-Teller (BET) was utilized in analysing the surface area of the nanocomposite material, using micrometrics ASAP 2460. The ultraviolet visible diffuse reflection spectroscopy (UV–vis DRS) were recorded using Cary 5000, UV–Vis-NIR spectra-photo meter with a DRA inserter. The surface morphology were analysis using a SEM (ZEISS-AurigaCobra SEM, Oberkochen, Germany) and a TEM (JEM-2100 JEOL), connected with EDS CCD camera.

2.3 MB photo-degradation test

Photocatalytic activity test was conducted by analysing the changes in dye concentration after concentration reactions under visible light illumination utilising MB as a control pollutant in a photocatalytic reactor. MB (C₁₆H₁₈ClN₃S), is a heterocyclic aromatic chemical substance which has numerous usages in series of disciplines, such the chemical sciences (biology and chemistry). This occurs as an odourless, dark-green solid, material that produces a bluish solution when soluble in H₂O at room temperature. The colour is long-lasting and incompatible with reducing agents, heavy oxidizing agents and bases. In a biological or chemical reaction, cycle such dye compounds not just diminish dissolved oxygen in water sources, but also emit other hazardous compounds that endanger marine life (Dolgonos et al., 2016). In the experiment 10 mg/l of the MB solution was prepared and used as model pollutant. 0.1 g of the photocatalyst was dissolved in 50 ml of MB solution, were then irradiated with a lamp (λ ≥ 254 nm, eight 9 W compact Fluorescent Integrated lamp 220–240 V, 50–60 Hz mini white E14 cool daylight lamps Philips, Holland), as a visible source of illumination. The solution was mechanically stirred, while exposure to visible light illumination. Aliquots of 1.5 ml was collected from the solution and filtered through 0.2 µm PTFE syringes, at intervals of 15 mins for 1 h. The degradation efficacy for MB was calculated using Eq. (1)

Were, C_O is the initial absorption of MB preceding the photocatalytic operational testing. C is the MB final concentration after photocatalytic degradation (mg/L) at the appropriate time interval (Dolgonos et al., 2016).

3.1 XRD analysis

Fig. 1(a) illustrates the phase diffraction peaks for TiO₂, Ag-TiO₂, V-TiO₂, Ag-TiVO_x. The diffraction peaks illustrate the combine crystallinity phases of anatase and rutile. The peak for anatase appears at a 2θ value of 25.3°. The correspondence is with agreement to crystal plane (1 0 1) consistent with JCPDS database # 89–4921. Ag-TiO₂ and co-doped Ag, V-TiO₂ exhibit higher anatase phase. It was observed, that the most intense peak for rutile phase appears at 2θ of 27. 4°. The 2θ values for 25.3 and 27.4°, appears in all the XRD patterns for each catalyst, which correspond to crystal planes (1 0 1) for anatase and (1 1 0) for rutile, respectively (Liu et al., 2019). The cubic phase for Ag-TiVO_x for anatase and rutile are with agreement with JCPDS database (#00-006-9240) and (#00-072-4812). Addition of the dopant causes an increase in the anatase phase, this agrees with results from other studies (Sanzone et al., 2018; Ruffino et al., 2011; Liu et al., 2019).

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Fig. 1

XRD (a) patterns, (b) Intensity ratios of TiO₂, Ag-TiO₂, V-TiO₂, Ag-TiVO_x samples.

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Fig. 1(b), exhibits the change in the (1 0 1)/ (1 1 0) plane intensity ratio for each catalyst. The plane intensity ratio for each catalyst upsurges as the structure of the photocatalyst changes due to doping, with Ag and V with Ag-TiVO_x shows the utmost plane intensity ratio (Liu et al., 2019). The crystalline size was determined using Scherrer rule (0.9 λ/β cos θ) of the line width (FWHM, β) of the most symmetrical then severe height (Imoisili et al., 2018). As shown in Table 1, Ag-TiVO_x oxide scale was estimated to be ≥ 18 nm, this was similar to 20 nm obtain by Tripathi et al., (2010).

3.2 Surface area (S_BET) analysis

The Specific Surface Area (S_BET) was record and tabulated in Table 2, while the Nitrogen adsorption–desorption isotherm are shown in Fig. 2(a–d). The specific areas record by S_BET for TiO₂, Ag-TiO₂, V-TiO₂ and Ag-TiVO_x, was 64.75 m²/g, 61.33 m²/g, 62.73 m²/g, and 46.01 m²/g. As seen, Ag-TiVO_x photocatalyst has the highest surface area owing to their smaller crystallite size. It was explained, that higher specific surface area improves the activity of photocatalyst as it provided more site for light absorption (Liu et al., 2019; Dolgonos et al., 2016; Ren et al., 2009; Reddy et al., 1998).

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Fig. 2

Nitrogen adsorption–desorption isotherm of (a) TiO₂, (b) V-TiO₂, (c) Ag-TiO₂ and (d) Ag-TiVO_x.

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3.3 UV –Visible spectroscopy (UV–Vis)

Fig. 3(a), demonstrate the absorbance spectrum of each synthesized photocatalyst The absorption edges for TiO₂, Ag-TiO₂ and V-TiO₂ occurs at wavelengths of 405, 404, 473 nm, while Ag-TiVO_x absorption edge shifted to higher wavelength with two additional peak around 550 nm and shoulder around 700 nm. This suggest that the Ag-TiVO_x photocatalyst successfully utilize the visible light spectrum (Liu et al., 2019). Band-gap transitions are indicative in the absorption edges. The effects of co-doing using Ag and V greatly enhances the absorption of light at longer wavelengths (>400 nm) and is the visible light regions. The co-doping with Ag and V indicates significant enhancements in the photocatalytic activity at visible light irradiation. With emphasis on synergistic effects of Ag and V, their effect has led to higher reaction kinetic due to shifting the light absorption to the visible light region therefore harvesting higher amount of energy.

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Fig. 3

(a) Absorbance spectrum (b) Energy band gap, of TiO₂, V-TiO₂, Ag-TiO₂ and Ag-TiVO_x.

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The bang gap energy (eV) calculation was used to evaluate the energy required to activate the photocatalyst. The energy band gap (eV) of each photocatalyst was calcluate by the Tauc equation, as shown in equation 1 (Reddy et al., 1998). where h is Planck constant, ν is photon’s frequency, and B is a constant. The γ factor is equal to 1/2 or 2 for the indirect and direct transition band gaps, respectively and depends on the nature of the electron transition

As shown in Fig. 3(b), the bands gap energies for TiO₂, Ag-TiO₂, V-TiO₂ and Ag-TiVO_x samples were interpolated to be 2.9, 2.8, 2.3, 2.2 (eV) respectively. The doped TiO₂ photocatalyst is seen to have achieve a desirable smaller band energy value which is consistence with previous reported (Liu, et al., 2019; Weng and Han, 2017; Dolgonos et al., 2016). This smaller band gap enables the photocatalyst to be triggered with lesser energy in the visible light region.

3.4 SEM analysis

Ag-TiVO_x been the most reactive photocatalyst as shown in Fig. 4(a), was used as representative sample for SEM surface morphology. As revealed in the SEM, image shows the formation of a regular cluster composed of spherical aggregates particles of non-uniform diameter. The EDX findings as exhibited in Fig. 4(b), of the representative sample (Ag-TiVO_x) photocatalyst indicated the existence of Ti, V, Ag, and O elements and the tests were compatible with the XPS findings.

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Fig. 4

SEM image (a) Ag-TiVO_x nanocomposite (b) EDX spectral.

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3.5 Transmissions electron microscopy (TEM)

HRTEM images and bright field patterns was record and shown in Fig. 5. The synthesis particles were non-uniform roughly rod-like shaped, with a particle size distribution range between 10 and 50 nm. The selected area electron diffraction (SAED) pattern (Fig. 5(b) inset) supports the polycrystalline existence in the fabricated sample. The HRTEM micrograph (Fig. 5(b) reveals the 0.416 and 0.590 nm distance fringes corresponding to (1 1 0) TiVO_x plane and (1 1 1) Ag plane, respectively (Reddy et al., 1998). The TEM findings of this analysis are compatible with those stated by previous studies (Shi et al., 2019; Liu et al., 2019). Nonetheless, separate synthetic methods, surfactant and precursor to titanium, Vanadium and silver were used in this present study to synthesize the Ag-TiVO_x samples

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Fig. 5

TEM illustration (a) HRTEM images and SAED template inset (b) of Ag-TiVO_x.

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3.6 X-ray photoelectron spectroscopy (XPS)

The chemical structure and valance state of various species was determined and analysed on the surface region of Ag-TiVO_x by XPS. The full XPS scan survey spectrum is show in Fig. 6. The XPS study, shows the binding energy peaks C 1 s core level peaks at 283.09 eV. While the binding energy peaks for Ti 2p_3/2 and Ti 2p_1/2 occurs at peak values of 458.30 and 464.16 eV, signifying the formation of Ti⁴⁺ in TiO₂ (Liu et al., 2019, 2011; Zhang et al., 2008; Whang et al., 2009). The peaks values of 469.51 and 513.14 eV corresponds to binding energies for V 2p_3/2 and V 2p_1/, illustrating the occurrence of V, on the catalyst surface in the IV state (Reddy et al., 1998). The chemical state of Ag shows large to low binding energy levels at 3d_5/2 and 3d_3/2 correspond to peaks values of 367.4 eV and 373.4 eV (Zhang et al., 2008; Liu et al., 2011). A significant interaction of the Ag nano-sized particles with TiO₂ and V can be seen to have transferred excited electrons, depleting a significant interaction between the TiO₂ and V particles, resulting in excited electrons transfers on the catalyst surface area. Fig. 6, also indicates a significant signal at 528.03 eV, which agrees to the photo transition of O1s; ultimately, the signal found at 283.09 eV relates to the electronic transformation of C 1 s. This signal is characteristic of the ambient CO₂ absorbed on the clear air (Whang et al., 2009; Liu et al., 2011).

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Fig. 6

Full Scan Survey of the Ag-TiVO_x samples.

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3.7 Photocatalytic evaluation

3.7.1

3.7.1 Degradation of MB

Photocatalytic evaluation of synthesised photocatalyst was performed by investigating the kinetic degradation of MB solution during visible light irradiation. The MB solutions was exposed to photocatalytic activity beneath visible light by a model reactor for a duration of 60mins with sample extraction at 15 min intervals. The hole and electron are produced in the in the valence band and conduction band of the nanocomposite photocatalyst by UV irradiation, respectively, as shown in (X1) (Wu et al., 2010). The hydroxide ions (or water molecule) are oxidized by the positive hole adsorbed on the surface of Ag-TiVO_x particles to yield hydroxyl radical (Wu et al., 2010), as presented in (X2). For the photosensitized oxidation mechanism, in the existence of the photocatalysts nanocomposite, an electron is inserted into the conducting band by the excited state of MB (X3). The hydroxyl radical present on the photocatalyst nanocomposite surface, fast-tracked the degradation of MB (X5). The transformed of the MB dye to a cationic radical that is degraded to yield products demonstrated by (X4) to (X7) (Le et al., 2012). The variations in absorption reading of the MB solution are presented in Fig. 7. TiO₂ showed poor photocatalytic degradation effect within the first hour under visible light irradiation, with an average removal rate of 73.94% meanwhile the Ag-TiO₂ and V-TiO₂ photocatalyst removed MB with an average removal rate of 82.54% and 82.10%, respectively. In contrast the Ag-TiVO_x photocatalyst exhibited improved activity, with a successful removal rate of 99.33% within 15 min, which is significant in this study.

(1)

$$\left(\text{Ag}-\text{TiVOx}\right)+\text{hv}\to {\text{e}}_{\mathit{CB}}^{-}+h_{\mathit{VB}}^{+}$$

(2)

$$\left(\mathit{Ag}-TiVOx{h}_{\mathit{VB}}^{+}\right)+H_{2}O\to Ag-TiVOx+H+OH°$$

(3)

$$\text{MB}\ast +{\text{TiO}}_2\to \text{MB}°+{\mathrm{e}}_{\mathrm{CB}}^{-}\left(\text{Ag}-\text{TiVOx}\right)$$

(4)

$$O_2+e^{-}\to O_2^{°-}$$

(5)

$${\text{MB}}^{°+}+{\text{OH}}^{-}\to \text{MB}+{\text{OH}}^{°}$$

(6)

$$M{B}^{°+}+O{H}^{°}\to products$$

(7)

$${\text{MB}}^{°+}+{\text{O}}_2^{°-}\to \text{products}$$

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Fig. 7

visible light degradation of MB for (a) TiO₂, (b) Ag-TiO₂, (c) V-TiO₂, and (d) Ag-TiVO_x.

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The Ag-TiVO_x catalyst has a fairly high rate of MB degradation relative to TiO₂, Ag-TiO₂, and V-TiO₂. Synthesised Ag-TiVO_x, photocatalyst achieves 99.83% degradation within the first 15 min. This is a remarkable observation for the activity test indicating that a high active photocatalyst has been synthesised. The findings revealed that the photocatalytic function of titanium dioxideTiO₂ has been significantly enhanced, with the existence of transition metal ions and Ag-TiVO_x was the utmost photocatalyst (Liu et al., 2019, 2011; Whang et al., 2009; Wilke and Breuer, 1999). This observation is in support with the reduce band gap found by the Tauc plot, the highest surface area analysis by S_BET, and the XRD plane intensity ratio (101/110). The escalation in photocatalytic behaviour of the TiO₂ transition metal was ascribed to narrower crystalline dimension, larger surface area, decreased band gap capacity, lesser electron-hole recombination rate and greater electron-hole generation performance than pure TiO₂ (Liu et al, 2011; Whang et al., 2009). As shown in Fig. 8, Ag-TiVO_x photocatalytic nanocomposites did not show any significant decrease in its activity, after the experiments was repeated three times using the same sample after the first 15 min. This suggest the economy viability of the synthesised photocatalyst.

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Fig. 8

Usability of synthesized photocatalyst nanocomposite after three utilization.

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