# Phtotocatalytic Degradation of Methylene Blue by Cu Doped TiO2 Thin Films under Visible Light Irradiation

Vidhya Rajendran, Gandhimathi Rajendran, Neyvasagam Karuppathevar

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Vidhya Rajendran, Gandhimathi Rajendran, Neyvasagam Karuppathevar. Phtotocatalytic Degradation of Methylene Blue by Cu Doped TiO2 Thin Films under Visible Light Irradiation. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.81.84.608. [https://doi.org/10.2412/mmse.81.84.608](https://doi.org/10.2412/mmse.81.84.608). [arXiv:2004.01504](https://arxiv.org/abs/2004.01504)

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# Phtotocatalytic Degradation of Methylene Blue by Cu Doped TiO₂ Thin Films under Visible Light Irradiation

Vidhya Rajendran¹,ª, Gandhimathi Rajendran², Neyvasagam Karuppathevar ³,ᵇ

1. Department of Physics, V.V.V College for women, Virudhunager-626001, India

2. Department of Physics, AMET University, Kanathur, Chennai-603112, India

3. Department of Physics, The Madura College, Madurai-625011, India

a. vidhya3vc@gmail.com

b. srineyvas@yahoo.co.in

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.81.84.608 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.81.84.608)

**Keywords:** Cu-TiO₂, thin films, XRD, SEM, UV-Visible, phtotocatalytic activity.

**ABSTRACT.** Pure TiO₂ and Copper doped TiO₂ thin films of various concentrations (0.02–0.05 mol %) have been successfully deposited onto glass substrate by sol-gel dip coating technique and annealed at 400°C for 3 hours. The XRD results showed the presence of anatase phase for all the films and the crystallite size decreases with increasing doping concentrations. The morphological observations and compositional analysis were recorded with SEM and EDX. The UV-Visible study showed that the optical band gap energy decreases with increasing Cu content. It was also found that copper doping shifted the absorption edge of TiO₂ toward a visible regime depicting the possible modification in the electronic band structure. The photocatalytic activity was evaluated by monitoring the degradation of methylene blue under visible light illumination. It was found that the percentage of degradation was higher in 0.05mol% Cu-TiO₂ when compared with other films. The efficiency of the Cu-TiO₂ thin film was preserved even after the extended usage of the films which sustains the reusability nature of Cu-TiO₂ films.

**Introduction.** Titanium dioxide (TiO₂) has been extensively studied because of its excellent phtotocatalytic activity, chemical stability, non toxicity, optoelectronic property and low cost. However, the wide band gap and intense electron-hole pair recombination limit its efficiency in photo electro chemical applications. Tailoring the band structure of TiO₂ is an effective method for widening its optical absorption range and improving its visible light phtotocatalytic activity [1, 2]. Catalytic activity of TiO₂ is dependent on its phase structure, crystallite size, specific surface area and pore structure. Doping additives were found successful in altering the photo electrochemical properties of TiO₂ [3, 4]. In particular, doping metal impurity in TiO₂ matrix has been found remarkable as the way it exploits the better photo catalytic activity [5, 6]. Cu is a potential metal dopant for phtotocatalytic mechanism since the visible light absorption of TiO₂ can be enhanced by the adding Cu [7]. Here the efforts have been made to prepare Cu doped TiO₂ film because the film shaped photocatalysts are convenient to use and recyclable. There are numerous methods to prepare Cu doped TiO₂ thin film but the sol-gel method shows more advantages over other methods [8]. By carrying out this method in solution, we can modify the few desired structural characteristics like compositional homogeneity, grain size, surface morphology and porosity is possible with this technique. Methylene blue is a harmful material existing in liquid coming from industries. Exposure to methylene blue causes various health issues such as increased heart beat rate, vomiting, shock, jaundice and tissue necrosis in human being. Cu doped TiO₂ thin film can be used to photo degrade the methylene blue in waste water. In this work, pure TiO₂ and Copper doped TiO₂ thin films with various Cu concentrations (0.02-0.05 mol %) were prepared by sol-gel technique. The structural and optical properties of the samples were reported. The phtotocatalytic activity of prepared films was evaluated by methylene blue degradation under visible light irradiation.
**Experimental methods.** The pure TiO₂ and Cu doped TiO₂ thin films were prepared by sol-gel method. Titanium tetra isopropoxide (Merk 95%) precursor was dissolved into ethanol and the mixture was stirred at room temperature for 15 minutes. To this mixture acetic acid was added as a chelating agent and stirred for 15 minutes. Titanium tetra isopropoxide, ethanol and acetic acid were used in the molar ratio of 1.5:10:0.3. In the process of copper doping, copper II nitrate trihydrate was dissolved in ethanol in the respective molar ratio (Cu/TiO₂ = 0.02, 0.03, 0.04 and 0.05 mol%) was mixed with TiO₂ gel and stirred in a magnetic stirrer for two hour for uniform dispersion. The Cu-TiO₂ sol were deposited on glass substrates by dip coating process at room temperature with the drawing speed of about 1.5 mm/s. The coated samples were dried at 100°C for 10 minutes and the process was repeated to obtain the desired thickness. The pure TiO₂ was also synthesized using the same procedure without the addition of copper precursor and all are annealed at 400°C for 3 hour.

**Results and discussions.**

**Structural analysis.**

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD pattern of Cu-TiO₂ films with various Cu doping concentrations.</figcaption></figure>

The XRD pattern of pure and Cu doped TiO₂ thin films with different concentrations are shown in Fig.1. Both undoped and Cu-doped TiO₂ films show pure anatase phase with strong peak (1 0 1) at 2θ=25.5° which is in agreement with JCPDS card no 89-4203 [9]. However, no peaks related to Cu impurities are found due to the low doping concentrations of Cu or the Cu metal ions have been well dispersed into the TiO₂ matrix in the form of small cluster [10]. The crystallite size of as-synthesized samples were estimated from Scherrer's equation using the full width at half maximum of the (1 0 1) peak of anatase TiO₂. It was perceived that the crystallite size decreases with the increased Cu proportions [11] as the stress created by the difference in ionic radii of Ti⁴⁺ (0.75Å) and Cu²⁺ (0.87Å). Furthermore a large number of dislocations were initiated when Cu ions occupying interstitial sites within Ti lattice which led to decrease in crystallize size [12].

**Surface morphological analysis.** The microstructure and chemical composition of Cu-TiO₂ thin films were characterized using SEM and EDX. Fig.2 shows the SEM and EDX images of 0.04mol% Cu-TiO₂ thin films. The SEM image illustrates a few localized agglomerations with cracked patterns [13]. The crack formation was possibly by the repeated process done to make thicker coatings of films
(7-layered) [14]. The energy dispersive spectrum (EDX) exposes the presence of titanium, copper and oxygen in appropriate proportions.

<figure><img src="image_4.png" /><figcaption>Fig. 2. SEM and EDX spectrum of Cu-TiO₂ (0.04mol %) thin film.</figcaption></figure>

Optical Analysis

<figure><img src="image_5.png" /><figcaption>Fig. 3. Absorption spectra of Cu-TiO₂ films with various Cu doping concentrations.</figcaption></figure>

The UV-visible absorption spectra of pure TiO₂ and Cu-TiO₂ films are shown in Fig.3. It was viewed that the pure TiO₂ shows UV light absorption as it requires maximum energy for the transition of electrons from the top of the valence band to the bottom of the conduction band (wide band gap). Conversely Cu-TiO₂ demonstrates enhancement in visible light absorption as Cu doping creates new electronic levels and reduces the band gap energy requirement. Accordingly the absorption edge of
TiO₂ was shifted towards visible light region [15]. Widening the absorption edge to the visible region can upraise the phtotocatalytic activity reasonably [16].

**Phtotocatalytic activity.** Phtotocatalytic activity of TiO₂ and Cu doped TiO₂ thin films was analyzed by examining the degradation of aqueous solution of methyelene blue (MB) under visible light irradiation using 500-W halogen lamp. In order to carry out the process, 3µml of the methyelene blue was mixed with 30ml water in a 50-ml beaker. The thin films were soaked into this and kept under visible light irradiation for 0,1,2,3 and 4 hour. After that the 5ml of the degradation solution was taken to measure the absorbance using Schimadzu-1800 UV-Vis spectrometer.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Optical absorption spectra of degradation of MB dye for Cu-TiO₂ thin films.</figcaption></figure>

Fig. 4 show the absorbance spectra of degraded MB with different time periods (3 hr and 4hr) of pure TiO₂ and Cu-TiO₂ thin films. It was observed that the doped TiO₂ films depict enhanced catalytic activity than undoped film. The interpretation is that due to illumination, the generated electron-hole pair is transferred to the surface of photo catalyst which excites electrons of the pollutant to the conduction band of TiO₂ [17, 18]. Here Cu doping delays electron-hole recombination and thus by promotes the photo induced interfacial charge-transfer between Cu ion and TiO₂ energy levels. This leads to the production of more reactive species.

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Percentage of degradation of MB (b) Reusability of the film.</figcaption></figure>
The produced OH and $O_2^-$ radicals oxidize and decompose the molecules of MB photo reactor. As a result, the characteristic color of MB dye was degraded with time. The photo catalytic mechanism is influenced by the degradation time too. Hence the absorption peaks corresponding to the MB decreases further to a minimum value with increase in time. The percentage of degradation of MB is shown in Fig. 5a. The 0.05 mol% Cu doped $TiO_2$ thin films show the highest percentage of degradation (92%) than that of pure $TiO_2$ (68%). This is attributed to that the smaller particle size of 0.05 mol% Cu-TiO$_2$ compared to that of pure $TiO_2$ as the photo bleaching process rely upon particle size, crystallinity and surface area. Also, the larger surface area facilitates the films to have more adsorption site and enhances the surface contact between photo reactor MB and Cu-TiO$_2$ catalyst. Consequently fast degradation process has taken place under visible light irradiation. Thus with 0.02 to 0.05 mol% stoichiometric range, Cu dopant promotes the reaction rate and increases decolorisation efficiency appreciably.

One of the important features of thin film catalyst is its reusability. This catalyst could be easily recovered by cleaning with water and reused without loss of its catalytic activity [19]. Hence to calculate Cu-TiO$_2$ catalyst stability, the film was undergone with 6 cycles of photo degradation in MB dye under visible light irradiation. Here, we report the reusability of 0.05 mol% of Cu-TiO$_2$ thin film. After each cycle, the film was washed with distilled water, dried in air and reused in same conditions. The catalytic efficiency was evaluated in each cycle. It was noticed that it maintains its excellent catalytic efficiency even after 6 cycles with a small loss which is shown in Fig. 5b. The Cu-TiO$_2$ thin film (0.05 mol %) was preferred for testing the reusability since it showed the improved catalytic performance than all other prepared films in degradation of dyes.

**Summary.** Pure $TiO_2$ and Cu doped $TiO_2$ with different concentrations were deposited on glass substrates by sol-gel dip coating technique. The presence of anatase phase and decreasing crystallite size with the increasing Cu ratio were spotted from the recorded XRD pattern. The morphology study reveals that the surface of the film was comprised with agglomerated flakes like structure and the existence of Cu, Ti and O elements are confirmed from EDX measurements. Copper doping shifted the absorption edge of $TiO_2$ towards the visible light region. The $TiO_2$ thin film doped with 0.05 mol % Cu exhibits strong photo catalytic activity thus by drawing higher percentage of methyelene blue degradation. The aptness of the Cu-TiO$_2$ thin film as a photo catalyst with the repeated usages was found to be excellent.

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Cite the paper

Vidhya Rajendran, Gandhimathi Rajendran, Neyvasagam Karuppathevar (2017). Phtotocatalytic Degradation of Methylene Blue by Cu Doped TiO2 Thin Films under Visible Light Irradiation. Mechanics, Materials Science & Engineering, Vol 9. doi:10.2412/mmse.81.84.608