Synthesis of Ag decorated TiO₂ nanoneedles for photocatalytic degradation of methylene blue dye

The following chemicals were used to prepare TiO₂ doped with Ag nanoparticles and to study its ability for degradation as photocatalysis. Titanium dioxide TiO₂ (Flucka, Switzerland) and Silver Nitrate AgNO₃ (Pub-Chem, United States) were utilized to prepare TiO₂ nanoneedles decorated with Ag nanoparticles. As well, Methylene Blue (MB) C₁₆H₁₈CIN₃C (Merck, United States) was used as simulated wastewater.

To synthesized TiO₂ nanoneedles decorated with Ag nanoparticles the following procedures were followed. The process involved to prepare two solutions. Solution A was prepared by dissolving 1 M of TiO₂ P25 Degussa in a mixture of 8 ml of HCl with concentration of (1 M) and 60 ml of Di-ionized water (DIW). Then, solution A was poured in a flask, rapped with plastic wrap and was heated in oven for 1 h at 90 °C. To create the solution B, 10 mg of AgNO₃ was liquified in 10 ml of DI water then both stirred together for 1 h to be homogenously mixed. Liquid C was formed by steadily adding liquid A to solution B and actively stirring it at room temperature for 10 min. Subsequently, the liquid C was irradiated with UV irradiation under continuous stirring. In addition, N₂ gas was flowed to the solution during the reaction. Following that, the precipitation was removed by filtration then rinsed three times with ethanol. The precipitated TiO₂ doped Ag (TiO₂- Ag) nanostructures were then filtrated and allowed to dry at 60 °C. Finally, the collected powder was annealed at 400 °C for 1 h to ensure the complete decomposition, remove all the organic residues and transformation of phases to a single anatas phase.

Several techniques were used to characterize the prepared sample. The crystal characteristics of the synthesized powder were examined via x-ray diffractometer (XRD) model Smart-lab using a Cu Ka radiation λ = 0.15418 nm and θ varied from 10 to100 degrees. Field Emission Scanning Electron Microscope (FE-SEM) brand Hitachi S-4800, Transmission Electron Microscope (TEM) brand JEOL JEM-2100 and a High-Resolution TEM (HRTEM) brand FEI Tecnai G2 F20 Super Twin TEM (accelerating voltage: 200 kV) were used to monitor the morphology and determine the size of the sample.

X-ray photoelectron spectroscopy (XPS) model Escalab 250XI with source of Al-Kα was used to determine the elemental composition. N₂ isotherm (desorption–adsorption) of the prepared powders, pore size and the surface area analyzed by Brunauer–Emmett–Teller (BET) assessed by means of a surface area and porosimetry analyzer (TriStar3000, GE). An ultraviolet and visible (UV/Vis) spectroscopy model PE lambda 750S was used to evaluate the optical absorption of materials.

The photocatalytic degradation of MB was investigated under irradiation of UV light (Xenon lamp, 300 W, 10 A) which was kept at RT via a stationary evacuator to prevent any thermal catalytic influence. A specific amount (30 mg) of the synthesized sample was stirred with a proper amount (30 mg l⁻¹) of dissolved MB in DIW. To obtain the adsorption- desorption symmetry for water, MB and TiO₂- Ag, all of these materials were stirred along 30 min in the shade. Later, the reacted materials were irradiated with ultra violet irradiation to evaluate the photodegradation behavior. During the irradiation processes a specific amount (3 ml) of the liquid was taken out regularly at each fifteen min. The UV/Vis spectrum analyzer was used to verify the optical absorption spectra of this solution.

The rate of degradation (K_app) and photodegradation efficiency (PDE) can be calculated using the following equations [55]:

$$\begin{eqnarray}&&\mathrm{ln}\,\displaystyle \frac{{C}_{o}}{{C}_{t}}={k}_{app}t\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&PDE\, \% =\displaystyle \frac{({C}_{o\,}-{C}_{t})}{{C}_{o}}\times 100\end{eqnarray} \tag{ 2 }$$

where C_o represents the MB concentration before irradiation while the C_t is the MB concentration under UV irradiation. Hence the calibration curve was plotted to calculate the C_t.

Figure 1 depicts the XRD study of the prepared thin film. The XRD pattern of the sample exhibited peaks which were found at 2θ° = 25.33°, 36.9°, 37.8°, 38.6°, 48.0°, 53.9°, 55.0°, 62.1°, 62.6°, 68.7, 70.3°, 75.0°, 76.0°, 82.6°, 83.1°, 94.2°, 95.1° and 98.3°. These values equate to planes (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (215), (301), (224), (312), (305), (321) and (109). These indications are characteristic of the TiO₂ anatase tetragonal phase (JCPDS # 00–021–1272) with a = b = 8.94 Å and c = 3.56 Å. Using the Scherrer equation, according to the domain peak (101) the average anatase crystalline size was 68.6 Å.

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The diffraction peaks even reveal additional diffraction patterns at 2 θ° = 31.7°, 45.4, 56.4, and 66.2. These reflections may be aligned with the Bromargyrite (Ag) cubic crystal structure planes (002), (022), (220) and (400) (JCPDS # 96–901–1685), hence the lattice parameters were a = b = c = 5.3 Å. The crystallite size was calculated based on (002) plane and it was found to be 39.45339 Å.

These results indicated that the prepared sample was Ag doped TiO₂. No other peaks were detected indicating the high purity of the prepared sample.

Moreover, since the amplitude of the preferred peak (located at 2 θ° = 25.3°) related to TiO₂ was not decreased meaning that the Ag was deposited on the surface sites. Therefore, the attaching mechanism did not affect the TiO₂'s characteristic crystalline phase; these findings will be supported by SEM analysis.

The morphology, distribution and size of the TiO₂–Ag structures were observed by FESEM as shown in figure 2. As can be observed, the samples exhibited semi-spherical structures with size around 300 nm. The high magnification micrographs (c and d) indicated the growth of nanoneedles which had the dimensions of 200 nm length and 20 nm diameters. These nanoneedles are agglomerated to form semi-spherical structures. The morphology of the structures revealed the presence of a thin layer covered the nanoneedles as in figure 2, profile (d). However, the dimensions of the layer are not clear yet. This finding was identical to that obtained from XRD analysis.

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Energy dispersive x-ray (EDX) spectroscopy coupled to a FE-SEM instrument was used to determine the elemental composition of the TiO₂–Ag nanorods. The nanocomposite contains titanium, oxygen, silver and gold as predicted in figure 3, which confirmed the doping of silver in TiO₂. The atomic ratio of Ti: O₂ is equal to 40:60% while the ratio of Ag: TiO₂ is about 9.3%. Those it can be predicated the sample as (Ti₂ O₃)–Ag. Since the silver nanoparticle accepts electrons from TiO₂, it becomes charged. To maintain charge equilibrium, oxygen vacancies occur on the anatase surface, that facilitates the atom reordering.

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The EDX elemental mapping strategy was followed to trace the arrangement of the elements in the matrix of the nanostructures as shown in figure 4. The collected analytical mapping confirmed the existence of well-dispersed Ag nanoparticles in the synthesized nanocomposite, which conformed the presences of the diminutive amount of Ag in TiO₂. Limited locations exhibited nanostructures agglomeration, owing to high surface energy. The EDX pattern also indicates the existence of a small amount of Au particles as a result of the use of a gold sputtering in the FE-SEM study.

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TEM images at various magnification of TiO₂ decorated with silver nanoparticles were presented in figure 5. The figures deliver strong indicators for size and distribution of the prepared sample. From figures 5(a) and (b) it's clear that the bigger particles are assigned to be nanoneedles TiO₂ composed to form larger semispherical particles and the smaller ones are Ag NPs. As can be seen in figures 5(c) and (d), Ag NPs appeared as black particles with average size 5–10 nm. This is due to higher atomic number of Ag, 47 compared to Ti, 22 'bright field'. It is clear that Ag was well distributed on the surface of TiO₂ nanoneedles. This indicated that Ag NP is decorated on the surface of TiO₂ nanoneedles. It is worth to mention that the preparation strategies were controlled to achieve the stated dimensions of the nanostructures. Furthermore, larger Ag–TiO₂ anatase phase is due to the large ionic radius of silver. The findings of the TEM study of TiO₂–Ag were found same as described elsewhere in previous work.

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High resolution transmission electron microscopy (HRTEM) image was shown in figure (5(e)), which revealed a well distribution of silver into the TiO₂ surface. From these micrographs, the atomic arrangement, and grain size of Ag–TiO₂ were detected. The lattice fringes of d spacing 0.16 nm can be appointed to (211) (JCPDS#0000–021–1272). This result is consistent with the findings of XRD observations.

The selected area electron diffraction (SAED) pattern of TiO₂ shown in figure 5(f) which displays separated diffraction rings matching TiO₂ anatase phase. It also verifies the growth of tetragonal anatase TiO₂ by displaying more than four diffraction rings of the TiO₂ crystal planes (101), (112), (211), and (220). Whereas, the brighter lattice fringes denoted to Ag dopant. Moreover, the sharp lattice fringes in patterns of SAED was formed by combination of the separate spots. It is worth to make comparison with some important previous work. As example in 2005 Lee et al described that the Ag doped TiO₂ nanoparticles were synthesized via the sol gel technique via a reduction agent. The structure of the synthesized TiO₂-Ag nanoparticles was an anatase phase. The synthesized TiO₂-Ag nanoparticles exhibited a spherical shape. By calcination TiO₂ for 300 °C, the crystal size was around 5–6 nm while for Ag nanoparticles raised from 10 to 15 nm by increasing AgNO₃ ratio [56].

X-ray photoelectronic spectroscopy (XPS) spectrum was shown in figure 6. The obtained XPS spectra were evaluated with SmartSoft program then handled with Multipack PHI. The values of binding energy were compared to the standard carbon adventitious C 1s signal (284.8 eV) which is usually occurred due to the exposure to environment. Spectra are often equipped with Gauss–Lorentz curves. The spectrometer's energy wide scan was measured using photoelectron lines of O 1s, Ti 2p, Ag 3d and C 1s at 530 eV, 465 eV, 370 eV and 285 eV, respectively. Since the XPS studies were performed for powder sample, the C 1s core level spectrum displayed the attendance of adventitious carbon species. They match to C–O and C–C bonds, at binding energies ∼284.7, and 286.5 eV, respectively.

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The spectrum shape of Ti 2p core level proposes the existence of an important part of TiO₂, beside the predictable Ti₂O₃ (binding energy of Ti 2p1 ∼ 472 eV, Ti 2p 1/2 ∼ 465 eV and Ti 2p 3/2 ∼ 458 eV). Because XPS is a surface profiling method, the existence of oxygen in TiO₂ may result from the powders' fractional surface oxidation prior to examination. These results are in good agreement with previous work [57].

The findings, besides the TiO₂ species, demonstrate the existence of Ag 3d 3/2 and 3d 5/2 at BE of 374 eV and 367.5 eV in the XPS spectrum of the Ag–TiO₂ sample, showing that Ag has been effectively grown on the surface of TiO₂, which is comparable to FESEM photographs. These results in consistence with earlier reported results [58].

The BET studies including N₂ adsorption-desorption isotherm curves of the prepared sample are shown in figure 7. It is clearly observed that at low relative pressure below 0.1 P Po⁻¹ (region i), the isotherm adsorption constantly raised due to the presences of micropores. By increasing the relative pressure higher than 0.1 until 0.53 P Po⁻¹ (region ii), weak hysteresis loop could be observed due to the existences of mesopores. Further increment in relative pressure (region iii) demonstrated that the sample exhibited hysteresis loop in the range of 0.53 to 0.84 P Po⁻¹ which could be assigned that the sample exhibited hollow structure. At higher relative pressure more than 0.84 (region iv), small hysteresis loop could be noticed, which could be attributed to the large mesopores [59]. These mesopores structure may enhance the photocatalytic behavior of the sample.

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The Barrett–Joyner–Halenda (BJH) method represented in figure 8. The curve of pore size distribution (PSD) demonstrating that the adsorption average pore size is equal to 5.2410 nm. It is obviously noticed in figure 8 that the pore volume is equal to 0.159716 cm³ g⁻¹. The results demonstrated that the micropore surface area is equal to 88.5738 m² g⁻¹. The inset figure revealed that the surface area (BET) of the TiO₂- Ag nanoparticles is equal to107.1327 m² g⁻¹. The high surface area could be ascribed to the presence of Ag which is in agreement with earlier work, hence it was found that the surface area increased from 51 to 110.5 m² g⁻¹ due to doping TiO₂ with Ag. Therefore, doping with silver caused a significant increment in the surface area compared to bare TiO₂. They mentioned that higher specific surface area of TiO₂ doped with silver would be advantageous for greater adsorption of MO in water solution, as well improved degradation activity may be predicted [60]. Other researchers studied the pore size of TiO₂ and found it to be between 2–5 nm [61].

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The sample Ag–TiO₂ has a high specific surface area, which might be attributed to the exposure to N₂ during the preparation process. At high temperatures, AgNO₃ decomposes in the presence of DI-water to produce Ag, thinning the nanoneedles wall and leaving holes, causing the needles wall to shatter under the impact of a significant volume of gas.

An ultraviolet–visible spectrophotometer was utilized to analyze the optical absorption spectra. Figure 9 depicts the UV/Vis spectrum of Ag–TiO₂. The low absorption edge in visible area was evident in the absorption spectrum of Ag-doped TiO₂. Ag–TiO₂ nanoneedels exhibited an absorption edge at 303 nm and a peak at 358.6 nm which indicated that the absorbance red shifted toward longer wavelength caused by the electron transition from the semiconductor's valence band to the conduction band. Tauc model was used to analyze the UV/Vis results. These results were plotted and are shown inset in figure 9. It could be noticed that the Ag–TiO₂ exhibited energy gap of 3.2 eV.

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It is well known that the nanomaterials exhibited larger electron band gap (Eg) than the bulk size of the same materials. It was reported that the Eg of TiO₂ is comparatively large and it depends on the phase hence it is around 3.2 eV for anatas [62], 3.0 eV for rutile [63] and 3.4 eV for brookite [64]. However, the nanosize of these phases exhibited higher energy gap as well, hence the TiO₂ nanoparticles in anatas phase exhibited 3.4 and 3.5 eV [65].

For comparison, previous work reported that the pure TiO₂ nanotubes exhibited an energy gap of 3.3 eV [66]. Therefore, our results showed a slight reduction of bandgap energy by doping TiO₂ with Ag. In addition, previous work investigated the effect of doping Ag to TiO₂ on the optical properties. They found that pure TiO₂ exhibited absorbance edge at 340 nm with a bandgap of 3.29 eV [67]. Other researchers reported that the presence of Ag in TiO₂ displayed a redshift toward the visible region. Indeed, they predicted that the addition of silver ions is often used to create transitional energy states resulting from the high electronic interaction between the orbitals of TiO₂ and silver [68].

In this work the UV irradiation was used due to the large band gap which it needs high photon energy. Although Ag was doped in TiO₂ structure but the Eg still large this is due to the small amount of Ag, hence the reduction of Eg depends on dopants ratio in the host material. To evaluate the photocatalytic activity of the synthesized nanoparticles, the decolorization of the methylene blue -as a wasted water- was investigated as shown in figure 10. It has been established that the photocatalytic reaction is strongly influenced by essential factors like the crystallization of the substance, the dimension, the surface area the ability of the absorption. MB was utilized as an indication of free radical removal and hence photoreduction. The photo-reduction of the MB of all Ag–TiO₂ powders over 50 min period was found to be greater than 40%. The greatest reduction of the MB dye occurred between 30 and 50 min in the majority of the solutions. Overall, the 4 mg of Ag–TiO₂ nanoneedles realized the maximum and faster MB degradation. As predicted, the control solutions of MB and MB with Ag–TiO₂ exhibited no degradation because Ag–TiO₂ particles mainly display photocatalytic activities during UV light environments. Visible light retains the energy to cross the energy gap and initiate a photocatalytic process. Unlike the pure TiO₂ powder, which is solubilized easily in the MB, the residual Ag–TiO₂ particles did not disperse well, given the significant processing. However, part of the powder would settle at the bottom of the container.

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The values of the calibration curve for MB, expressed by the dye concentration as a function of absorbance, are shown below in figure 11. The Lambert-Behr law states that the relationship between concentration and absorbance is linear, thus increasing the concentration will cause the absorbance to increase. Finally, it was found from the figure that the value of the slope = 0.2336.

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In figure 12, (4 mg) of the photocatalyst represented by silver decorated titanium dioxide was used, where the graph represents the relationship between (ln C_o/C_t) with time (min) with correspond matching line (red line). The highest rate of reaction velocity, k, and PDE% were calculated to be 0.0788 min⁻¹ and 98.7%, respectively for dose 4 mg.

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Figure 13 represents the PDE % of various catalyst dose ca, 3, 4, 5 and 6 mg. The results revealed that the lowest PDE % was recorded using 3 mg of Ag–TiO₂ hence about 40% of MB removal was obtained after 50 min UV irradiation. Meanwhile the highest PDE% (98.7%) was realized using 4 mg of the prepared sample. By increasing the dose of Ag–TiO₂ to 5 mg the removal was reduced to 72%. Further increment in dose up to 6 mg reduced the ability of MB degradation to 57%. These results were tabulated in table 1.

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The pseudo first-order degradation rate constant is denoted by k as depicted in figure 14. The degradation rate constant improved from 0.012 to 0.788 min⁻¹ as the dose of Ag–TiO₂ increased from 3 to 4 mg. Further increments in Ag–TiO₂ reduced the degradation rate to 0.294 and 0.176 min⁻¹ corresponds to 5 and 6 mg, respectively. This could be attributed to the fact that when Ag–TiO₂ dose increase may block the light penetration and therefore inhibit the degradation process [69, 70].

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Several studies were conducted to investigate the ability of pure and doped TiO₂ for photocatalytic application. As example, by using an ion-exchange technique, Ag₃PO₄ was combined with Bi₄Ti₃O₁₂ to create Bi₄Ti₃O₁₂/Ag₃PO₄ heterojunction nanocomposites. The composites showed better performance compared to bare Bi₄Ti₃O₁₂ and Ag₃PO₄ nanoparticles in terms of photocatalytic activity [71]. At air pressure, Ti vapor annealing was used to create B-TiO₂, with Ag surface modification. After 60 min of visible light irradiation, the degradation rate of RhB reached 97.76 percent [72]. Other work reported that the anatase phase of TiO₂ was prepared to investigate the effect of grain size on photocatalytic efficiency. The findings showed that the photocatalytic degradation rate of TiO₂ nanoparticles in the anatase process rose dramatically as the grain size increased from 6.6 to 26.6 nm. They assume that increasing the grain size of the TiO₂ anatase process will inhibit the undesired redox reaction of hydroquinone-benzoquinone, thereby facilitating the full decomposition of phenol and phenolic compounds [73]. By comparing our results with previously published work, we can conclude that our photocatalyst has higher efficiency for degradation with a shorter time.