Synthesis, characterisation, and effect of pH on degradation of dyes of copper-doped TiO₂

Abstract

Copper-doped TiO₂ nanoparticles were synthesised using an ultrasonic-assisted sol–gel method with various doping concentrations from 0 to 2.5 at.%. The samples were characterised by X-ray diffraction, UV–vis diffuse reflectance spectroscopy (UV–vis), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area determination, and zeta potential. The presence of copper in TiO₂ crystal structure was revealed by UV–vis spectra, and the TEM analysis showed that particles are mainly spherical around the size range of 15–20 nm. In addition, doping copper into TiO₂ lattice caused a decrease in the surface area due to the aggregation of nanoparticles and a shift of isoelectric point towards lower pH when the dopant concentration increased. The photocatalytic reactivity of these materials was evaluated by the degradation of methylene blue and methyl orange under the UV light. The effect of the initial solution pH on the adsorption capacity and the photocatalytic behaviour of the Cu-doped TiO₂ in the decolourisation of these dyes were also studied.

Keywords:

• adsorption capacity
• isoelectric point
• photocatalytic activity
• ultrasonic-assisted sol–gel method
• solution pH

1. Introduction

TiO₂ is considered as an efficient photocatalyst for decomposing a large variety of environmental contaminants, such as organic materials, bacteria, and viruses.[1,2] However, the most crucial problem in using TiO₂ as a catalyst is the low photoquantum yield in the photocatalysis process that arises from the fast recombination of photogenerated electrons and holes. Moreover, pure TiO₂ is inactive under illumination by visible light due to its wide band gap (3.18 eV for anatase form). Hence, it is extremely difficult to make use of the vast potential of the solar photocatalysis because in solar energy applications only ca. 3% of the sunlight is absorbed.[3] By doping metal and non-metal ions, the electronic structure and the optical properties of TiO₂ particles are altered in order to improve their response, slow down the recombination rate of the electron-hole pairs, and to enhance the interfacial charge transfer efficiency.[4–6]

Synthesis methods have a great influence on the properties of TiO₂ photocatalysts, such as the morphology, structure, surface properties, and phase transformation.[6] Ultrasound is an effective method to create the fine dispersion. The extremely fine-size dispersion increases the surface area available to the reactants, thus improving the chemical reaction by enhanced mass transfer. Using ultrasonic irradiation in synthesising materials resulted in smaller particles and also promoted greater homogeneity in terms of size of the particles.[7] The ultrasonic-assisted sol–gel technique is used during or after the precipitation of the titanium salt/alkoxide precursor in water or the mixture of alcohol and water to form TiO₂ nanoparticles within a short time frame. Under ultrasound irradiation, the micro-cavitation bubbles are formed in the liquid environment, develop and explode intensely in an extremely short duration, creating localised hot spots with high temperatures (∼5000 K), high pressures (∼1000 atm), and heating and cooling rates above 10¹⁰ K/s. During ultrasonic irradiation of liquid-powder suspensions, cavitation and the shockwaves can create high-speed interparticle collisions. The resultant collisions can cause dramatic changes in the surface morphology, composition, and reactivity.[8,9] Furthermore, the collapse conditions are dependent on the intensity and the frequency of the ultrasound. Operating variables, such as the ultrasonic power density and frequency, reactor size, etc., may have a great impact on the properties and morphology of TiO₂ nanoparticles. Thus, by using the ultrasonic-assisted sol–gel synthesis method, these properties of TiO₂ can be controlled.[10–12]

This work discusses the synthesis of copper-doped TiO₂ photocatalysts with different copper loadings using a sol–gel technique supported by ultrasound and the characterised physical properties of samples including crystalline structure, diffuse reflectance spectra, transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area, and zeta potential. The photocatalytic activity of prepared particles was evaluated based on the decolourisation of the anionic methyl orange (MO) dye and the cationic methylene blue (MB) dye under UV light irradiation. Furthermore, the effect of the solution pH on the adsorption behaviour and the photocatalytic degradation of these dyes was also considered.

2. Experimental procedure

2.1. Materials

As starting precursors, the following reagents and apparatus were employed: titanium (IV) isopropoxide (TTIP, Sigma-Aldrich, 97%) as TiO₂ source; copper (II) nitrate trihydrate (Sigma-Aldrich, 99%) as dopant source; absolute ethanol (Sigma-Aldrich, ≥ 99,8%) as solvent; acetic acid (Sigma-Aldrich, ≥ 99,8%); diethanol amine (Sigma-Aldrich, 99%); triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers Pluronic P123 (EO₂₀PO₇₀EO₂₀, M = 5800, Sigma-Aldrich); deionised water and an ultrasonic horn (Cole-Palmer, USA, 500 W, 20 kHz).

2.2. Preparation of photocatalysts

Cu-doped TiO₂ catalysts were derived via the ultrasonic-assisted sol–gel method according to the procedure used by Khodadadi et al. [13] but P123 was used to replace PEG (poly ethylene glycol). First, TTIP was dissolved in absolute ethanol by stirring for 10 min. This solution was then added dropwise into another solution containing acetic acid, deionised water, and ethanol under magnetic stirring at room temperature. Pluronic P123 at 10 wt% was added into the above solution under vigorous stirring to obtain the TiO₂ sol. Second, the dopant solution was prepared by adding a mixture of diethanol amine and ethanol into a copper nitrate salt solution dissolved in ethanol. After that, the dopant solution was added dropwise into the above-obtained TiO₂ sol. The resulting transparent colloidal suspension was sonicated for 30 min, and aged at room temperature for 72 h to form the gel. Finally, the obtained gel was dried at 90 °C and ultimately was calcined at 500 °C for 2 h to obtain the final powder. In a typical procedure, the reaction was carried out with the following mole proportions: TTIP/ethanol/CH₃COOH/H₂O: 1:65:1.6:0.5. The concentration of Cu/Ti atom ratios varied from 0 to 2.5 at.%.

2.3. Characterisation of photocatalysts

The structural characterisation of the TiO₂ nanoparticles was investigated by the X-ray diffraction (XRD) (Siemen D5000 diffractrometer) analysis. The measurement conditions were as follows: CuKα radiation (λ = 1.5406 Å) at U = 30 kV and I = 25 mA, 0.03^o step size and 1 sec step time from 20^o to 70^o. The UV–vis diffuse reflectance spectroscopy was measured within the wavelength range of 200–800 nm using Jasco V-530 UV–vis spectrophotometer. The morphology and size of the catalyst samples was characterised by TEM using JEOL-JEM 1010. The specific surface area of the powder photocatalyst was measured using the BET method with a Micromeritics TriStar II 3020 (V1.03) for N₂ BET at 77.3 K with a 5 s equilibration interval. The surface charge of the catalyst was examined using Zetasizer Nano Series (Malvern).

2.4. Photocatalytic degradation experiments

The photocatalytic activity of synthesised particles was evaluated by the degradation of MO and MB in suspension form. The experiment was performed with the following conditions: concentration of photocatalyst 2 g/l; MO and MB concentration of 15 mg/l, under continuous stirring condition at room temperature.

At the first stage, the experimental system was placed in the dark for 30 min for the adsorption on the catalyst surface or to break the chain of dyes in the absence of light radiation. Afterwards, the system was irradiated by 365 nm UV lamp (UVP 90-0019-01 Pen-Ray, 4.8 W). Samples were taken periodically, centrifuged, and the UV–vis spectrum peaks at a maximum wavelength λ_max = 462 nm (MO) and λ_max = 663 nm (MB) to determine the concentration of the solution were measured.

The photocatalytic activity of the synthesised materials was assessed by the reaction rate constant k (min⁻¹). The degradation data showed a kinetic model of type 1: C/C₀ = exp (–kt).[14–16] C is the dye concentration (mg/l) in the radiation time t (min) and C₀ is the initial concentration of the dye (mg/l).

3. Results and discussion

3.1. X-ray diffraction (XRD)

XRD was used to investigate the phase structure and the phase composition of TiO₂ samples. The powdered XRD patterns of copper-doped TiO₂ and pure TiO₂ were given in Figure 1. The XRD patterns showed broad peaks of anatase as the dominant phase. The distinctive peaks at 2θ = 25.3^o; 37.8^o; 47.7^o; 54^o, and 62.4^o, corresponding to the anatase crystal planes, were observed in all the samples.[17] In comparison to undoped TiO₂ samples, the intensity of peaks of Cu-doped TiO₂ photocatalysts was lower and broader, which suggested a decreasing degree of crystallinity. The possible reason is the substitution of Ti⁴⁺ by Cu²⁺ because of the similarity in ionic radius of Cu²⁺ (0.73 Å) to that of Ti⁴⁺ (0.64 Å). Nair et al. found that a dopant with an oxidation sate of 3+ or lower produced the oxygen vacancies in the lattice of TiO₂ to maintain the charge neutrality.[18] Therefore, the substitution of Cu²⁺ to Ti⁴⁺ increases the oxygen vacancies. As the dopant content increased, the diffusion of a few numbers of oxygen vacancies to the interstitial position or to the surface and grain boundary regions created the structural defects that affect the crystallinity of the material.[19] Consequently, the crystallisation of the TiO₂ anatase phase is lower and the peak intensity was weaker. Similar behaviours have also been observed in other transitional metals doping systems.[19,20]

Figure 1. XRD patterns of Cu-doped TiO₂ catalysts.

No characteristic peaks of the copper oxide crystalline phase appeared in the XRD pattern of Cu-doped TiO₂ photocatalyst samples. The reason may be that copper contained in the compound existed in the amorphous form, or the amount of copper was too low to detect by the XRD measurement or most copper ions were inserted into the crystalline structure of TiO₂ and located at interstices or occupied some of the Ti lattice sites forming a solid solution of copper–titanium oxides. The obtained results are in accordance with the results from the previous articles.[21]

3.2. UV–vis diffuse reflectance spectroscopy (UV-Vis)

Figure 2 provided the diffuse reflectance spectra of the Cu-doped TiO₂ series. According to Colon et al.,[22] the band at 210–270 nm would indicate the O²⁻(2p) →Cu²⁺(3d) ligand to the metal charge transfer transition. The absorption peaks shifted from 325 to 355 nm corresponding to the Cu content rise from 0% to 2.50% but there was no change in the position of the absorption edge of TiO₂ except for 2.50% Cu–TiO₂ sample. It is well known that CuO and Cu₂O have electronic band gaps that are 1.7 and 2.1 eV, respectively.[23,24] Therefore, the absorption in the visible region from 400 to 600 nm was attributed to the presence of Cu⁺ clusters in partially reduced CuO matrix as well as (Cu–O–Cu)²⁺ clusters. The absorption band at 600–800 nm indicated the crystalline and bulk CuO in octahedral symmetry.[22] This large absorption band increased with the increase in the dopant concentration level. When the copper ion was inserted into the bulk TiO₂ or on the surface sites, a rearrangement of the neighbouring atoms occurred to ensure charge balance, resulting in the lattice deformation. The lattice deformation caused by the substitution of Ti⁴⁺ by Cu²⁺ affects the electronic structure of TiO₂ crystalline lattice, thus leading to the change in the optical absorption of TiO₂.[25]

Figure 2. UV–vis diffuse reflectance spectra of Cu-doped TiO₂ catalysts.

3.3. Transmission electron microscopy (TEM)

Typical TEM images of the photocatalyst samples (Figure 3) depicted the particle structures to be spherical, and quite similar to each other. The size of the formed nanoparticles was within the range of 15–20 nm. Compared to the previous studies, the particle size of Cu–TiO₂ nanoparticles prepared by the ultrasonic-assisted sol-gel method was more homogenous and smaller than that of the doped TiO₂ ones using other synthesising methods.[26–29] From the TEM image, it could be observed that the TiO₂ nanoparticles were aggregated to form secondary particles with a relatively large size of 25–50 nm when the Cu dopant concentration rose from 0.0% to 0.15% and 2.5%. Moreover, the morphology obtained in TEM images did not indicate the presence of localised deposition of the added metal oxides. It was believed that the doped metal ions were evenly dispersed or incorporated into the crystal structure of TiO₂ during the preparation as indicated by the XRD patterns.

Figure 3. TEM micrographs of Cu-doped TiO₂ (a) 0% Cu–TiO₂; (b) 0.15% Cu–TiO₂ and (c) 2.5% Cu–TiO₂.

3.4. Zeta potential

Zeta potential measurements were used to elucidate the solid/liquid interfacial charge processes and interactions between particulate surfaces. Figure 4 presented the zeta potential of aqueous dispersion of Cu-doped TiO₂ photocatalysts. The pH at which the zeta potential shifted to zero (point of zero charge P_zc), was found at about 6–6.5. Around this pH, the nanoparticles favourably aggregated. The isoelectric point P_zc had a tendency to move towards lower pH as the Cu content increased. The zeta potential measurement revealed that all the Cu-doped TiO₂ nanoparticles exhibited the transition from the positive potential at low pH to the negative potential at high pH.

Figure 4. Variation of zeta potential versus pH for selected Cu-doped TiO₂ catalysts.

3.5. BET

A specific surface area of the undoped TiO₂ catalyst was 79 m²/g, whereas the surface area of the copper-doped TiO₂ samples declined from 65 to 38 m²/g when the copper content increased to 0.05% and 2.5%, respectively (Table 1). It could be inferred that the copper doping on the TiO₂ surface changes the surface area of the catalyst. This decrease might be caused by the aggregation of nanoparticles as observed in TEM images. A similar finding was reported by Ganesh et al.[26]

Table 1. BET surface area of selected Cu-doped TiO₂ catalysts.

Sample	SBET (m^2/g)
0% Cu–TiO2	79
0.05% Cu–TiO2	65
0.15% Cu–TiO2	49
2.50% Cu–TiO2	38

3.6. Photocatalytic activity

3.6.1. Dark adsorption of MO and MB on the Cu-doped TiO₂ surface

The solution pH is the key factor for the photocatalytic reaction and can have an impact on the adsorption of dyes on the photocatalyst surface. Table 2 showed the relationship between the pH value and the adsorption capacity of MO and MB when using various Cu-doped TiO₂. The adsorption capacity of MO on the Cu-doped TiO₂ catalyst surface was very high at acidic condition (pH = 3), but decreased when the solution pH went up from 3 to 6, and no adsorption occurred at pH = 9. Adsorption capacity decreased when pH rose due to MO containing negatively charged sulfonate groups. Thus, at acidic conditions, the interaction between a positively charged catalyst surface and negatively charged dyes favours adsorption.[30,31] The results also indicated that the adsorption capacity of MO reduced as the volume of the copper content increased. The reason was that the substitution of Ti⁴⁺ by Cu²⁺ in the TiO₂ structure resulted in a more negatively charged surface.[29] Therefore, the adsorption capacity of MO on the Cu-doped TiO₂ catalyst decreased because the charge of the catalyst surface was more negatively when the copper dopant concentration was higher.

Table 2. Adsorption amount of methyl orange and methylene blue by using various Cu-doped TiO₂.

	Adsorption amount of dyes in 30 min (mg/g)
	MO			MB
Sample	pH = 3	pH = 6	pH = 9	pH = 3	pH = 6	pH = 9
0% Cu–TiO2	0.57	0.15	NA	NA	0.89	5.05
0.05% Cu–TiO2	0.23	0.06	NA	NA	0.92	5.56
0.10% Cu–TiO2	0.09	0.04	NA	NA	1.05	6.82
0.15% Cu–TiO2	0.06	0.02	NA	NA	1.16	7.71
0.20% Cu–TiO2	0.03	NA	NA	NA	1.33	7.85
0.25% Cu–TiO2	NA	NA	NA	NA	1.53	7.89
0.50% Cu–TiO2	NA	NA	NA	NA	2.36	7.92
2.50% Cu–TiO2	NA	NA	NA	NA	4.48	7.95

Note: NA: no adsorption

In contrast, MB is a cationic dye when dissolved in water. Its adsorptive behaviour was opposite to that of MO. At low pH value (pH = 3), the adsorption was not recognised. However, when the solution pH increased from 3 to 9, the larger amount of MB was adsorbed on the catalyst surface and the maximum dye uptake was observed in the pH range of 9. In the acidic medium, positively charged surfaces of the catalyst tended to oppose the adsorption of cationic adsorbate species. When the pH of the dye solution became more basic, the surface tends to acquire negative charge, thereby resulting in an increased adsorption of dyes because of the rising electrostatic attraction between the positively charged dye and the negatively charged catalyst, this finding is in agreement with previously reported results.[32,33] Similar to the above explanation, the charge of the catalyst surface was more negative as the quantity of the copper content went up, thus enhancing the adsorption capacity of the cationic dye, such as MB on Cu-doped TiO₂ catalysts.

It is interesting to note that increasing the copper level decreased the adsorbed amount of MO in the acidic and neutral medium but caused a rise in that of MB in the neutral and basic medium, while BET measurements presented a reduction in the surface area of photocatalysts since the dopant concentration increased. These findings led to the conclusion that a reduction in the BET surface area has an insignificant effect on the adsorption capacity of dyes onto the Cu-doped TiO₂ catalysts surface.

3.6.2. Photodegradation of MO and MB on the Cu-doped TiO₂ surface

The activity of the prepared particles was measured based on the rate constant k of the photocatalytic degradation reaction of MO and MB. Photodegradation kinetic dependence on the dopant content and pH was given in Table 3.

Table 3. Rate constant of the decolourisation reaction of methyl orange and methylene blue by using various Cu-doped TiO₂.

	First-order rate constant of the photocatalytic reaction, kapp (×10^−3 min^−1)
	MO			MB
Sample	pH = 3	pH = 6	pH = 9	pH = 3	pH = 6	pH = 9
0% Cu–TiO2	12.5 ± 0.6	5.9 ± 2.1	3.5 ± 0.5	7.9 ± 0.7	10.1 ± 0.6	13.2 ± 0.6
0.05% Cu–TiO2	16.7 ± 0.4	8.4 ± 0.8	3.6 ± 0.7	11.1 ± 0.3	21.0 ± 2.3	26.7 ± 1.2
0.10% Cu–TiO2	15.9 ± 1.5	8.3 ± 0.9	7.0 ± 0.5	15.2 ± 2.2	25.3 ± 1.9	28.3 ± 1.9
0.15% Cu–TiO2	15.0 ± 2.1	8.1 ± 0.4	4.1 ± 0.4	16.9 ± 1.4	28.9 ± 2.6	88.0 ± 4.3
0.20% Cu–TiO2	13.5 ± 0.3	6.8 ± 0.6	3.9 ± 0.2	13.5 ± 0.9	19.3 ± 0.9	67.2 ± 3.5
0.25% Cu–TiO2	10.2 ± 0.8	5.0 ± 0.3	3.0 ± 0.4	8.9 ± 0.3	14.1 ± 0.5	25.1 ± 1.7
0.50% Cu–TiO2	8.5 ± 0.5	2.9 ± 0.6	2.1 ± 0.1	3.2 ± 0.1	7.2 ± 0.3	19.2 ± 0.4
2.50% Cu–TiO2	5.6 ± 0.2	1.1 ± 0.1	1.9 ± 0.2	0.9 ± 0.4	1.1 ± 0.1	15.6 ± 0.9

Results present that the photocatalytic activity of synthesised particles in the decolourisation reaction of MB was significantly higher than that in the decolourisation reaction of MO. As can be seen from the results, the optimal dopant concentration was different for each dye. For MO, the optimal concentration of dopant over Cu was 0.05% and for MB, it was 0.15% in the same experimental conditions.

According to the presented data, at pH = 6, in the degradation of MO, the photocatalytic activity of Cu-doped TiO₂ nanoparticles increased when the amount of Cu went up from 0% to 0.05% under the same experimental conditions. However, when the Cu content was raised to 0.15%, the photocatalytic activity declined nearly to that of the undoped sample. In the degradation of MB, the optimal amount of dopants was 0.15% and at this content, the rate constant of the degradation reaction doubled in comparison to the rate of the undoped sample. Beyond this optimal value, the photocatalytic activity declined rapidly if the content of dopant is further increased. When the dopant concentration reached 0.5%, the photocatalytic activity was lower than that of pure TiO₂ samples. Choi et al. reported that there appears to be an optimum content of doping ion in TiO₂. A metal ion dopant serves not only as a direct mediator of the interfacial charge transfer but also as a recombination centre.[34] At optimal dopant concentration levels, the degradation rate of dyes was highest, which could be attributed to the following factors.

First, Xin et al. suggested that the copper ion deposited on the TiO₂ particles could create oxygen vacancies on the surface layer of TiO₂ which could effectively trap the photogenerate electrons. While at high concentrations of dopant ions, the excessive oxygen vacancies and Cu species could become the recombination centres of photoinduced electrons and holes, which are detrimental to the photocatalytic activity of the catalyst.[35]

Second, due to the difference in the electron negativity between Ti and Cu, the Ti–O–Cu formed via Cu²⁺ entering into the shallow surface of TiO₂ could promote the charges to transfer, resulting in an increase of the photocatalytic activity.[36]

Third, copper ion doping can cause a lattice deformation and produce defects in the crystal. The defects can inhibit the recombination of electron-hole pairs and enhances the activity.[36] However, when the content of copper ion was higher than its optimal level, an excess amount of copper ion could not diffuse into the TiO₂ lattice, but deposits on the surface of TiO₂ particles, leading to the hindrance of UV light penetration reaching the surface of TiO_2, which could block the active sites of TiO₂ and therefore inhibits the photocatalytic activity.[37] On the other hand, it was also reported in the literature that as the metal loading increases, agglomeration of metal particles is believed to occur and decrease the photocatalytic activity of photocatalyst.[27]

Replacement of Cu ions for Ti ions within the TiO₂ crystalline matrix resulted in a more negatively charged surface. Consequently, the anionic dyes adsorption capacity reduced and cationic dyes adsorption capacity rose when the dopant content increased. The optimal copper concentration for the photodegradation of MO was very small, just 0.05% due to the low adsorption of MO on the surface of the photocatalyst as Cu was doped into the TiO₂ lattice. In this case, an increase in the photoreaction rate was attributed to the effective charge separation of photogenerated electrons and holes pairs. On the other hand, for the removal of MB from aqueous solutions, the optimal dopant content reached 0.15% and the reaction rate constant doubled as compared to that of pristine TiO₂, thanks to not only an improvement of the charge separation within the excited photocatalyst but also an increased MB adsorption as the copper content went up.

Figures 5 and 6 showed the photocatalytic activity of catalyst with the optimal copper dopant concentration in the degradation of MO and MB at different kinds of medium (acid, neutral, and base). The adsorptions of MO and MB on Cu-doped TiO₂ powders were carried out in dark conditions for 30 min to reach the adsorption equilibrium. By using 0.05% Cu–TiO₂, the amount of adsorbed MO after 30 min in dark was 0.23, 0.06, and 0 mg/g at pH 3, 6, and 9, respectively. Besides, the amount of MO was removed in 360 reaction minutes under Ultraviolet A (UVA) irradiation was 100%, 95%, and 71% separately and the k values was 0.0167, 0.0084, and 0.0036 min⁻¹ corresponding to the above pH values. In the case of the MB object, the MB adsorption amount on 0.15% Cu–TiO₂ catalyst after 30 min was 0, 1.16, and 7.71 mg/g. At pH 9, MB was completely removed after 30 reaction minutes. At pH 3, it took 300 reaction minutes to treat MB entirely. The k values of photodegradation of MB were 0.0169, 0.0289, and 0.088 min⁻¹ at pH 3, 6, and 9, respectively. The results confirmed that the photocatalytic activity of as-synthesised samples strongly depend on the adsorption of MO and MB molecules on the catalyst surface which is caused by the solution pH.

Figure 5. Photodegradation of methyl orange by 0.05% Cu–TiO₂ catalyst as a function of irradiation time: (a) disappearance of MO by adsorption and photocatalysis and (b) rate constant of the decolourisation reaction.

Figure 6. Photodegradation of methylene blue by 0.15% Cu–TiO₂ catalyst as a function of irradiation time: (a) disappearance of MB by adsorption and photocatalysis and (b) rate constant of the decolourisation reaction.

4. Conclusion

In summary, copper-doped titanium dioxide photocatalysts were prepared by the combination of the sol–gel process via an ultrasonic technique. Influence of initial pH on the photocatalytic degradation of the anionic dye (MO) and the cationic dye (MB) in the presence of Cu–TiO₂ under UVA irradiation was investigated. The results suggested that the adsorption capacity of MB increased with rising pH and copper content, whereas that of MO was favoured at low pH and the dopant concentration. Moreover, the efficiency of dyes photocatalytic reaction went up corresponding to the increase of adsorption capacity because the preliminary adsorption process of dye molecules onto the catalyst surface is an important step for the photocatalytic oxidation to take place. Doping of copper greatly improves the photocatalytic activity of TiO₂ in the decomposition of MO and MB under visible light irradiation with an optimal doping concentration of 0.05% and 0.15%. However, when the copper content is higher than the optimal value, the dye removal efficiency declines.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Vietnam–Germany Cooperation Research Project [04/2012/HĐ-NĐT] funded by the Ministry of Science and Technology (MOST) of Vietnam.