Silver embedded C-TiO₂ exhibits improved photocatalytic properties with potential application in waste water treatment

2.1 Materials

Tween 80 [T80, polyoxyethylenesorbitanmonooleate], Isopropanol 99.8% ((CH3)₂CHOH), acetic acid (CH₃COOH), all from Guoyao Chemical Reagent Co. Ltd, Shanghai, China; Nitric acid (HNO₃) [0.05 mol L^-1], Silver nitrate (AgNO₃), Methyl orange (C₁₄H₁₄N₃NaO₃S) all from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China and titanium tetraisopropoxide (TTIP, 97%) from Sigma-Aldrich, St. Louis, Missouri, USA were procured commercially and used in the experiments.

2.2 Synthesis of C-TiO₂

C-TiO₂ nanoparticle photocatalysts were synthesized by using a sol–gel technique. Tween 80 (T80) was used as the carbon precursor and fixed in the modified sol-gel solution. T80 (5 ml) was mixed with 20 ml of isopropanol, and then 3 ml titanium tetraisopropoxide was added and was dissolved under continuous stirring condition for 24 h. Then, 3 ml of acetic acid was added into the total volume of solutions in order to achieve consistency of the admixture. The solution was kept for one day at room temperature and stirring condition to obtain a homogeneous and steady solution. Then the solution was incubated at 60 °C for 3 h, to get the sol–gel. Finally, the sol–gel was calcined in an advanced furnace under 400 °C for 3 h, and cooled down naturally and the C-TiO₂ nanoparticles thus obtained were collected.

2.3 Preparation of C-TiO₂/Ag

To prepare C-TiO₂/Ag nanoparticles, a slurry was prepared by mixing of C-TiO₂ powder with 50 ml of AgNO₃ alcoholic solution. The mixture was maintained under continuous stirring condition for 30 mins to obtain an adsorption equilibrium of the silver ions on the C-TiO₂ surface. Thereafter, the slurry was irradiated at room temperature using a commercial germicidal lamp (TecnoLite G15T8, Mexico, 214 nm, 17 W) for 60 min under vigorous stirring. The obtained solution was dried overnight in an hot air oven at 100 °C. The silver nanocomposites as (0.5, 1, 2, and 5 wt% Ag)/C-TiO₂ and 1%Ag/TiO₂ were prepared.

2.4 Characterization of C-TiO₂/Ag

X-ray diffraction (XRD) pattern of the synthesized nanomaterial was analysed in Rigaku Multiplex system, Rigaku Corporation, Tokyo, Japan) with the Cu Kα (λ = 1.5406 Å) radiation to ascertain crystallinity and phase of the samples. An X-ray photoelectron spectroscope (XPS, PerkinElmer Model 5300; Perkin Elmer, Waltham, Massachusetts, USA) was employed to determine fine elemental composition and electronic structure. Field-emission scanning electron microscope (FE-SEM) analysis was performed on a Hitachi S-4800 (Hitachi Hi-Technologies Corp., Tokyo, Japan). Fourier transform infrared (FT-IR) spectroscopy (FTIR Spectrometer Alpha II; Brucker, Massachusetts, USA) was conducted to detect chemical groups in the prepared catalysts. UV–vis absorption spectra of the nanomaterial was acquired using a UV–vis spectrophotometer (Shimadzu 2450 PC; Shimadzu Corporation, Kyoto, Japan), which was installed with an incorporating sphere supplement (ISR1200) using BaSO₄ as standard to examine the light absorption of the prepared nanoparticle catalysts (with the range of wavelength 240–800 nm). Photoluminescence spectra (PL) of the material was assessed at 25 °C on a Shimadzu RF-5301 PC spectrometer (Shimadzu Corporation, Kyoto, Japan) using excitation light EX = 240 nm and emission spectrometry range EM = 350–400 nm.

2.5 Photocatalytic degradation experiment

Photocatalytic performance of the synthesized nanomaterials was studied by degradation of Methyl orange, one of the organic water pollutants in water under visible light irradiation. First, a particle suspension of the catalyst (0.5 g) was dissolved in one liter of aqueous solution using a stirrer kept stirring for onvernight. Then, 400 µL of methyl orange solution (250 mg/L) was placed on 50 ml particle suspension. Finally, to ensure the acidity of the degradation system 50 μL of Nitric acid was added to the solution. The solution was placed under xenon light source, (xenon lamp current value was adjusted at 15 A, with UV filters to remove UV light to ensure all the light is within visible spectrum). To study its effect on photodegradation, 50 μL of hydrogen peroxide (H₂O₂) was added into the solution. A 5 ml sample was withdrawn at time 0, 30, 60, 90, 120, 150, and 180 min. The photocatalyst was promptly isolated from the sample specimens after centrifugation (5000) rpm for 10 min in room temperature. The development of photocatalytic breakdown was observed by the measurement of characteristic absorbance of the solution samples using a UV–vis spectrophotometer (Shimadzu 2450 PC; Shimadzu Corporation, Kyoto, Japan).

3.1 XRD reveals anatase TiO₂ phase and increased crystalline size of C-TiO₂/Ag

XRD were used to study crystal structure and crystal phase characterizations of the synthesized C-TiO₂/Ag. Since the photocatalytic efficiency of catalyst was highly affected by its crystallinity and the phase of the catalyst (Pelaez et al., 2009), TiO₂ anatase phase was reported with high photocatalytic activity compared with other crystalline forms (Ollis and Al-Ekabi, 1993; Kosowska et al., 2005). The XRD patterns of synthesised TiO₂, C-TiO₂, Ag-TiO₂ and C-TiO₂/Ag are shown in Fig. 1. The distinct diffraction peaks at 2θ of 25.3°, 37.8°,48°, 53.9°, 62.7°, 70.3° and 75° corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 0 4), (2 2 0) and (2 1 5) crystal faces of anatase TiO₂ were identified in all the samples which indicated that the presence of anatase TiO₂ phase in all the samples. Similar to our results, Huang et al. (2018) also reported that Ag nanoparticles uniformly embedded in the anatase TiO₂ nanotubes walls and brought large interfacial area between Ag particles and TiO₂ nanotubes indicating transformation of Ag-TiO₂ into anatase TiO₂. It can be observed that a major peak at 25.3° in C-TiO₂/Ag became narrow, which attributed to the increase in the crystallite size of titanium dioxide (TiO₂), whereas XRD pattern of C-TiO₂ does not displays any peak corresponding to silver crystal phases.

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Fig. 1

XRD patterns of C-TiO₂ (reference) and Ag/C-TiO₂.

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3.2 Scaning electron microscopy (SEM) shows improved specific surface area of C-TiO₂/Ag

It is visible that C-TiO₂/Ag nanoparticles are mainly presented as cube-like shape in scanning electron microscopy (SEM) as shown in Fig. 2(a & b). However, the surface Ag nanoparticles are unable to present the same feature (Fig. 2b); which may be due to less contents and small particle size of silver. It can be seen from Fig. 2a and b that the samples of C-TiO₂ and C-TiO₂/Ag showed fairly rough surface, on the other hand, little shining silver grains and very small punctures are observed on the surface of C-TiO₂/Ag (Fig. 2b). This indicates an improved specific surface area of the nanoparticles C-TiO₂ achieved by Ag embedding as evidenced in earlier studies where Ag compounds were used to embed TiO₂ (Huang et al., 2018). Higher specific surface area of photocatalyst could provide increased contact area with the organic pollutant such as methyl orange during photocatalytic reaction and may improve its activity. Transmission Electron Microscopy (TEM) analysis of Ag embedded TiO₂ had earlier revealed that the techinique is helpful in maintaining the tubular structure of nanomaterial because of the uniform distribution of Ag ions on the nanomaterial (Huang et al., 2018). Similar results were also noticed by Naraginti et al. (2015).

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Fig. 2

The SEM images of the (a) C-TiO₂ and (b) Ag/C-TiO₂.

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3.3 FT-IR spectroscopy indicates intense strength of O-H bonded group in C-TiO₂/Ag

The infrared spectra of TiO₂, Ag-TiO₂, C-TiO₂ and C-TiO₂/Ag samples are shown in Fig. 3. The bands at 1640 and 3380 cm⁻¹ are nascent from O—H stretching of adsorbed H₂O molecules. Many reports confirmed that O—H group on the catalyst surface may consider as a very important factor in enhancing photocatalysis efficiency of TiO₂ and inhibiting the recombination rate of charge carriers. From Fig. 3 it can be noticed that the strength of hydroxyl bonded group in C-TiO₂/Ag sample was more intense than that of other samples. From the result, the bands at 2330 cm⁻¹ are due to the stretching vibrations of C⚌O, indicating the residual C element in all the catalyst surface except the Ag-TiO₂, which might have come from Tween 80 used in the sol preparation. The same band also appeared in TiO₂ sample; which may be due to absorption of CO₂ from air during the preparation process (Kumar and Devi, 2011). Meanwhile, the peaks around 1380 cm⁻¹ in Ag-TiO₂ and C-TiO₂/Ag are due to NO₃⁻ bonds, which attributed to the AgNO₃ used in the preparation samples.

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Fig. 3

FT-IR spectra of TiO₂, Ag/TiO₂, C-TiO₂ and Ag/C-TiO₂.

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3.4 The UV–vis absorption spectra of C-TiO₂/Ag exhibits increased light absorption

UV–vis spectroscopy of 1% C-TiO₂/Ag and the C-TiO₂ (reference) are presented in Fig. 4. From the results, it is interesting to notice that C-TiO₂ embedded with silver (C-TiO₂/Ag) could significantly exhibit red shift to higher wave length. This result can be referred to the arrangement of Ag nanoparticles on the surface of TiO₂, which influenced insulation constant of the surrounding matrix and eventually promoting the visible light absorption of C-TiO₂ (Ung et al., 1998). Earlier studies wherein TiO₂ was embedded with Ag compounds also showed enhanced response in visible region of the light (Huang et al., 2018). While doping TiO₂ with Ag, Naraginti et al. (2015) calculated energy gap value of the composite to be 2.87 eV, indicating decrease in band energy and increase in wavelength value due to red shft. The authors also opined that this shift in the optical property of the composite had resulted in better photocatalytic abilities of the Ag embedded TiO₂.

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Fig. 4

The UV–vis absorption spectra of C-TiO₂ and Ag/C-TiO₂ samples.

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3.5 Photoluminescence spectra (PL) of C-TiO₂/Ag shows lower electron-hole recombination rate thus improving its photocatalytic efficiency

Photoluminescence (PL) emission spectra is produced from recombination of electron and hole (Li et al., 2005). PL results could be helpful in studying the efficiency of charge carrier trapping, its movement, and also to know the intensity of photo-generated electrons and holes and its fate. It is well known that the recombination rate of photogenerated electrons and holes is very important factor affecting the photocatalytic performance. Fig. 5 shows the PL spectra of C-TiO₂ and C-TiO₂/Ag nanoparticles. The emission intensity peaks of C-TiO₂/Ag and C-TiO₂ catalyst at 372 nm were about 85.95 and 130.39 arbitrary unit (au), respectively. When TiO₂-C-Ag composites are irradiated by visible light, shift in electron from valence band to conduction band creates a hole in valence band (Naraginti et al., 2015). Recombination of these holes reduces the photocatalytic activity of the composite material. Ag embedded in C-TiO₂ could trap the electron inhibiting hole recombination thus increasing the photocatalytic activity (Pantelides, 1978; Subash et al., 2012). In the present study, emission peak of C-TiO₂/Ag is lower than that of C-TiO₂, indicating that the addition of Ag improves the efficiency of C-TiO₂ and reduced the electron-hole recombination rate. This result can be attributed to the phenomenon that the electrons are excited from valence band to conduction band and then relocated to silver, due to the outstanding electronic conductivity of silver and prevented the recombination of electrons and holes. Huang et al. (2018) reported that Ag embedding of TiO₂ enhanced the separation effect of photogenerated electron hole pair in TiO₂.

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Fig. 5

Photoluminescence spectra of Ag/C-TiO₂ and C-TiO₂ (Reference).

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3.6 X-ray photoelectron spectroscope (XPS) indicates formation of metal Ag on C-TiO₂

To investigate and confirm the deposition of Ag on C-TiO₂ catalyst, XPS analysis was carried out and the results are shown in Fig. 6(a–d). Fig. 6(a) reflects the results of C 1s core levels. At the binding energy 284.8 eV, the C 1s peak of sample C-TiO₂/Ag formed a large migration than control sample (C-TiO₂). This result can be referred to the recent carbon species generated from heat processing. XPS peaks of Ti 2p were noticed at binding energies around 458.5 eV (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), as exhibited in Fig. 6(b). The XPS of O1s was revealed one peak at binding energy 528.8 eV in the control and C-TiO₂/Ag as demonstrated in Fig. 6(c). On the other hand, two peaks were noticed at the 367.5 eV and 373.5 eV binding energies in C-TiO₂/Ag, which may be attributed to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively. These results together indicate the formation of metal Ag in the sample.

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Fig. 6

XPS spectra of C1s, Ti2p, O1s, and Ag3d.

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3.7 Silver embedding of C-TiO₂ (C-TiO₂/Ag) improves photocatalytic degradation of organic water pollutant, methyl orange

Degradation of organic pollutant, methyl orange dye under visible light irradiation was performed to test visible-light photocatalytic performance of C-TiO₂/Ag. The highest wavelength absorption of methyl orange in acidic form under visible light was in the range of 503 nm. Hence the difference of methyl orange absorbance was determined at 503 nm. Figs. 7 and 8 reveals the photodegradation of methyl orange by TiO₂, C-TiO₂, 1% Ag/TiO₂ and (0.5, 1, 2, and 5 wt%Ag) of C-TiO₂/Ag nanomaterials. After irradiation for 2 h, the degradation rates of methyl orange by catalysts were (33.7, 87.5, 74.4, 90.1, 95.2, 93.6, and 83.0%) for TiO₂, C-TiO₂, 1% Ag/TiO₂ and (0.5, 1, 2, and 5 wt%Ag) of C-TiO₂/Ag, respectively. High degradation rate of C-TiO₂ should be attributed to doping of carbon on the surface of TiO₂ and photosensitization of TiO₂. In C-TiO₂/Ag samples, the results showed an increased C-TiO₂ activity in photocatalytic degradation of methyl orange after addition of silver to C-TiO₂ samples.

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Fig. 7

Degradation of methyl orange with C-TiO₂ and (0.5, 1, 2, and 5 wt%Ag)/C-TiO₂ samples.

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Fig. 8

Percentage degradation of methyl orange with C-TiO₂ and (0.5, 1, 2, and 5 wt%Ag)/C-TiO₂ samples.

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In a previous study (Huang et al., 2018), the authors reported that optimal dispersity (0.15 g/L) of Ag particles on TiO₂ gave the best photocatalytic activity on Rhodamine B, while higher concentration of Ag resulted in reduced photocatalytic activity. Metal ion doping on TiO₂ had enhanced effect on photocatalytic activity, and doping sites could act as recombination centres for electrons and holes, thus increasing the lifetime of photogenerated electron-hole pair which increased the photoactivity (Lopez et al., 2009). Additionally, Hernandez et al. (2017) also opined that in metal doping of TiO₂ nanocomposites, concentration of photogenerated charge carriers and life time of electron hole recombination plays a major role in deciding photocatalytic efficiency. From the characterization results in the present study, the improvement of the degradation rate of C-TiO₂ after addition Ag could be attributed to the smaller crystallite size that enhances the optical absorption edge. From the results of C-TiO₂/Ag samples it can be concluded that the addition of Ag in ratio less than 5 wt% enhanced the photocatalytic activity of C-TiO₂, and the best photodegradation rate was obtained by 1 wt% C-TiO₂/Ag.