Sunlight-driven efficient photocatalytic and antimicrobial studies of microwave-assisted Ir-doped TiO2 nanoparticles for environmental safety

INTRODUCTION

Recently, the use of solar irradiation-based semiconductors, dealing with the environmental problems, is drawing a lot of attention. Reducing the size of the semiconductors will modify their physical, chemical, or biological properties, especially under irradiation [1]. Among these various semiconductors at the nanoscale, TiO₂ NPs have been used widely to remove polluted organic chemicals and bacteria in the water because this material has high specific surface area and chemical stability. At present, antibiotics offered in the marketplace are primarily organic. Along with technical development, however, inorganic antibiotics have gradually replaced them. As a novel inorganic antibacterial material, TiO₂ NPs have received increasing attention for their stable, long-lasting properties, as well as their safety, low cost and broad-spectrum antibiosis [1-3]. As the production of TiO₂ and its use in consumer products increases, its release into the environment is increased, as a result of spills, use of products, or post-consumer degradation of the material. Predicted environmental concentrations of TiO₂ NP in water are on the order of parts per billion, with expected environmental loads as high as 10-60 lakh tons in the next 10 years [4-6]. The accumulation of NP in the environment is likely to be non-uniform [5], with higher concentrations in near-shore waters which are more impacted by run-off, wastewater discharge, and proximity to human populations. TiO₂ NP concentrations of 1-100 mg/L are known to have deleterious effects on the environment. Thus, there is a need to understand the environmental impact of these NP. The photocatalytic properties of metal oxide NP (e.g., TiO₂, CeO₂ and ZnO) are well known [7-8], and TiO₂ NP have been exploited in a wide range of photocatalytic applications including coated nanocomposites in sunscreens for ultraviolet (UV) protection and in ‘green’ technology cement that actively purify the air of pollutants [9]. In aqueous media, TiO₂ NP can produce radicals when activated by UV light. The radicals, in turn, can damage microbes and provide an antimicrobial property being utilized by medical, food and water-treatment industries [10].

The photocatalysts in photocatalytic process, especially those with high photocatalytic activity and strong stability under visible light have been regarded as capable materials for applications in solar energy conversion and treatment of wastewater [11-12]. Although TiO₂ is the most explored photocatalyst so far [13], the wide energy gap (3.2 eV) of TiO₂ has stalled its sufficient utilization of sunlight [14]. Therefore, the development of novel and more efficient visible-light-driven photocatalysts are still urgently needed from the viewpoint of solar energy conversion. TiO₂ is a wide band gap semiconductor corresponding to radiation in the near UV range (˂ 387 nm). Upon the absorption of UV energy, TiO₂ particles will form an electron-hole (e^–/h⁺) pairs in the conduction band and valence band, respectively. The use of high energy UV light is not only costly but also can be hazardous. Therefore, the use of visible light active materials has recently drawn the attention of the researchers. The sunlight is most abundantly available, renewable and clean source of energy, which is composed of less than ~50 % of visible radiation and less than ~5 % of UV radiation [15]. To extend the spectral response of TiO₂ to the visible region, many modification methods have been investigated, including surface modified using various stabilizers and capping agents. Preparation of TiO₂ NPs using green technologies is advantageous over chemical agents due to their less environmental consequences, cost-effectiveness.

Many organic pollutants (chlorinated phenols) have been found in different water resources from chemical industries. Chlorophenols (CPs) have found wide use in pesticides, herbicides, disinfectants, wood preservatives, personal care formulations, dyestuff intermediates, and many other products, and they are also substantial by-products of wood pulp bleaching with chlorine [16]. CPs is poorly biodegradable with a half-life that can exceed some years in organic sediments. However, growing knowledge about the toxicities and environmental fates of specific chlorinated phenols has made governments regulate these compounds [17-18]. CPs have been found in at least 166 of the 1467 National Priorities List sites, and 4 of them are listed as priority pollutants, including 2-CP, 4-CP, 2,4-DCP, and 2,4,6-TCP, which are present in the environment in significant quantities. Moreover, except for 2-CP, 2,4,6-TCP and 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP), have been classified as group 2B environmental carcinogens by the International Association for Research on Cancer (IARC) [19]. Since the intermediate products originating from CPs degradation might be more toxic and cause secondary pollution, it is crucial to determine the intermediates and their associated toxicity. Therefore, it is essential to develop highly efficient and reliable technologies for their removal.

Herein, we report a way for the microwave-assisted synthesis of Ir-doped TiO₂ NPs in the green synthetic route by using an aqueous solution of iridium (III) chloride monohydrate and TTIP as titanium precursor. The synthesized Ir-doped TiO₂ NPs are characterized by various physicochemical techniques. The remarkable photocatalytic activity was observed for the degradation of chlorinated pollutants such as 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCPs by the synthesized Ir-doped TiO₂ NPs. The degradation efficiency as a rate of conversion is also studied under sunlight irradiation. The rate of degradation is followed by the pseudo-first order kinetics through Langmuir-Hinshelwood model. The antimicrobial activity of the synthesized Ir-doped TiO₂ NPs shows prominent results.

EXPERIMENTALS

Methods and Materials

Titanium tetra isopropoxide (TTIP, 97%, Merck, India), iridium chloride monohydrate, isopropanol (99%, SD-Fine chemicals, India) and chlorophenols (2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP) were purchased from Merck, India. Double distilled water was used throughout the experiment.

Microwave-assisted preparation of Ir-doped TiO₂ NPs

The Ir-doped TiO₂ NPs were prepared using the simple microwave-assisted method. For the synthesis of Ir-doped TiO₂ NPs, the round bottom flask containing 10 mL of 0.25 M of titanium tetraisopropoxide (TTIP) in 90 mL isopropanol with constant stirring at room temperature to get a clear solution. 5 mL of 0.05 M of iridium chloride monohydrate solution was prepared and slowly added to the above solution with constant stirring for 20 min at 35 ^oC. These solutions were mixed together under an inert atmosphere and then heated to 50 °C for 1 h to get a clear solution. After 2 hours of continuous stirring, the precipitation of reaction mixture was formed. The precipitate was subjected to microwave irradiation for 5 min in a domestic microwave oven (Input 900 W, 250 MHz, LG Company) with on-off cycle (20 sec on to 40 sec off). Then, the formed Ir-doped TiO₂ NPs was acquired by centrifugation at 5000 rpm for 15 minutes. Then, the centrifuged particles were washed with ethanol and again subjected to centrifugation at 3000 rpm for 10 minutes. Separated Ir-doped TiO₂ NPs were dried, grinded, and calcinated at 500 ^oC in a muffle furnace for about 2 hours. The calcinated pure Ir-doped TiO₂ nanopowder was collected and used for the further analytical techniques.

Characterization techniques

The powder X-ray diffraction (XRD) spectra was recorded on a X’pert Pro powder X-ray diffractometer (Netherland) using the CuKα radiation (λ=٠.١٥٤٠٦ nm). The UV-Vis diffuse reflectance spectra (UV-Vis DRS) was recorded on a Shimadzu ٣٦٠٠ spectrophotometer in the spectral range of ٢٠٠-٨٠٠ nm. The morphology and particle size of the synthesized Ir-doped TiO₂ NPs were characterized by using a Transmission electron microscopy (TEM) on Tecnai G2 and the microscope was operated at 200 kV. Samples were prepared by dispersing the powder in double distilled water. Images were recorded by depositing few drops of suspension on a carbon coated 400 mesh Cu grid. The solvent was left to evaporate before imaging. SEM images of the prepared Ir-doped TiO₂ NPs were obtained using ZEISS EVO18 electron microscope. FTIR spectra on KBr pellet were measured on a Shimadzu spectrophotometer in the range of 4000-400 cm⁻¹. The fluorescence spectrum was measured with an RF-5301PC spectrofluorometer (Shimadzu, Japan). The thermal behavior and degradation of the Ir-doped TiO₂ NPs were investigated by thermogravimetric analysis in the range of 100 to 1000 °C at a heating rate of 10 °C min⁻¹ under the nitrogen atmosphere.

Photocatalytic degradation of chlorophenols

The photocatalytic activities of the prepared Ir-doped TiO₂ NPs were evaluated by the degradation of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP solutions under natural sunlight irradiation. All of the experiments were carried out under similar conditions on sunny days at times between 11.00 a.m. and 2.30 p.m. All photocatalytic experiments were applied at ambient temperature (28-38 °C). 0.05 g photocatalyst was added to 50 mL of 30 mg/L 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP solutions. Before the sunlight irradiation, the suspensions were magnetically stirred for 60 min to reach the adsorption-desorption equilibrium. At the given time interval, 3 mL aliquots were sampled and filtered with a micropore membrane. Simultaneously, the filtrates of phenol solutions at different conditions were analyzed by recording variations of the CPs peak area. The UV-Vis absorption spectrophotometer (UV-3600 spectrophotometer, Shimadzu) was adopted for analysis of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP degradation with irradiation time. The sample with a volume of 20 µL was used as injection volume and concentrations of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCPs were evaluated by UV detector at wavelengths 290, 295, 287 and 310 nm, respectively. The photocatalytic degradation was calculated as follows:

(1)

where D is the degradation efficiency, A₀ is the initial concentration of CPs and A_t is the concentration of CPs after irradiation in selected time interval.

Antibacterial activity

The synthesized Ir-doped TiO₂ NPs with different amounts (10 µL to 50 µL) were screened for in-vitro antibacterial activity against two Gram-negative bacteria such as Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and two Gram-positive bacteria such as Staphylococcus aureus (ATCC 25923) and Bacillus subtilis (MTCC 121) bacterial strains by paper disc method. Briefly, all cultures were routinely maintained on NA (nutrient agar) and incubated at 37 ^oC for overnight. The culture was centrifuged at 1000 rpm, pellets were re-suspended, and volume of 0.1 ml diluted bacterial culture suspension was spread uniformly with the help of spreader on NA plates. Wells of 6 mm diameter were punched into the agar medium and loaded with prepared NPs. Antibiotic disc; ampicillin (100 mg/disc) and solvent were used as positive and negative control, respectively. The plates were then incubated for 24 h at 37 ^oC and the resulting zones of inhibition (mm) were measured.

Antifungal activity

All cultures were routinely maintained on sabouraud dextrose agar (SDA) and incubated at 28 ^oC. The prepared Ir-doped TiO₂ NPs with different amounts (10 µL to 50 µL) were screened for their in-vitro antifungal activity against the fungi, A. niger, and C. albicans were performed by growing the culture in SDA broth at 37 ^oC for overnight. The volume of 0.1 ml of diluted fungal culture suspension was spread with the help of spreader on SDA plates uniformly. Sterile 6 mm discs were impregnated with the test samples and the wells of 6 mm size were cut and loaded with sample. Antibiotic disc, ketoconazole (10 µL/disc) was used as positive control. A. niger and C. albicans plates were incubated at 37 ^oC for 24–48 h and antifungal activity was determined by measuring the diameters of the inhibition zone (mm).

RESULT AND DISCUSSION

UV-Vis DRS analysis

UV-Vis DRS was used to determine particle size and the band gap of prepared Ir-doped TiO₂ NPs. UV-Vis DRS measurement is one of the most important methods to reveal the optical properties of TiO₂ samples. The UV-Vis DRS of the Ir-doped TiO₂ NPs is displayed in Fig. 1. The energy band gap of Ir-doped TiO₂ NPs was calculated using the equation: E_g = hc/λ, where h is Planck’s constant; c is the velocity of light and λ is wavelength (in nm). The band gap of 3.54 eV at wavelength 351 nm corresponds to absorption peak. It can be seen that there was a significant shift of the absorption edge towards visible region for Ir-doped TiO₂ NPs. The band gap of Ir-doped TiO₂ NPs shows as 3.54 eV, it is a large deviation from the bulk TiO₂. The increase in the band gap compared to that of TiO₂ bulk is attributed to the quantum size effect. Based on the DRS results, blue shift is indicative of the charge transfer between Ir³⁺ and TiO₂ conduction band and valence band when compared with the pure TiO₂ NPs. As the number of electron-hole pairs on the surface of TiO₂ increases, the photocatalytic activity increases [20]. The apparent large blue shift of the observed absorption indicates that the NPs are within the quantum confinement regime, in which a blue shift in absorption is often expected with decreasing particle size.

PL analysis

The PL spectroscopy is the direct method for probing the electronic transition of nanomaterials. The study of transfer behavior of photo excited electron-hole pairs in the semiconductors and the rate of recombination is carried out by PL spectra [21]. The PL spectrum of the synthesized Ir-doped TiO₂ NPs is shown in Fig. 2. It shows the emission peak in the range of 425-465 nm after doping of Ir³⁺ ions into the TiO₂ NPs. The peak at 438 nm appeared in the PL spectrum. The appeared peaks correspond to a radiative transition of the excited electrons from occupied d-band to the higher state of Fermi level of Ir-doped TiO₂ NPs. The PL intensities of Ir-doped TiO₂ NPs were lower in comparison to those of reported TiO₂ NPs. It is observed due to changes in the electronic structure of Ir³⁺containing TiO₂ NPs. Lower PL intensity indicates the delay in the rate of recombination and thus obtained photocatalytic activity is higher for Ir-doped TiO₂ NPs [22].

FTIR analysis

The FTIR spectra of the synthesized Ir-doped TiO₂ NPs were analyzed in the wave number range of 4000-600 cm^-1 and are given in Fig. 3. Many absorption bands belong to the organic groups such as OH and C-C were appeared. In FTIR spectrum of Ir-doped TiO₂ NPs, the broad peak appeared between 3800 to 3000 cm^-1 was attributed to stretching hydroxyl (O-H), representing the water as moisture. The other peaks at 1658 cm^-1 were indicated to stretching of titanium carboxylate, formed from TTIP and ethanol as precursors [23]. The peak between 724 and 645 cm^-1 was assigned to the Ti-O stretching bands. In FTIR spectrum of Ir-doped TiO₂ NPs in the range of about 680 cm⁻¹ is the Ti-O bending mode of vibrations which confirms the formation of metal-oxygen bonding [24]. Similarly, the broad band centered at 3286 and 1524 cm⁻¹ was respectively attributed to stretching and bending vibrations of O-H of adsorbed water on the surface of TiO₂ which plays an important role in photocatalytic activity [25]. The peaks between 2941 and 2324 cm^-1 are assigned to C-H stretching vibrations of alkene groups. A broad absorption band between 600 and 1000 cm^-1 is attributed to the vibration of Ti-O-Ti association in TiO₂ NPs. In general, the conformation of the formation of doped TiO₂ NPs when compared to the pure TiO₂ NPs is the changes in intensity and position of the bands shifted towards the lower wave number after doping [26]. Only the strong absorption between 800 and 650 cm^-1 was remained, which was attributed to the formed Ir-doped TiO₂ NPs.

XRD analysis

The formation of Ir-doped TiO₂ NPs was analysed by the powder X-ray diffraction measurements and are shown in Fig. 4. In the XRD analysis, five diffraction peaks at 24.8º, 37.4º 47.6º, 53.8º, and 62.2º obtained, have planes (101), (004), (200), (105) and (204), respectively, of the tetragonal body-centred titanium dioxide (JCPDS No. 86-1156). Very high anatase content is observed for the as-synthesized nanoparticles compared to the reference sample (99.7% of anatase, Aldrich). Indeed, no rutile peaks were found. This is due to the heat treatment at 450 °C which is known to produce high anatase content. Zhang et al. chose this calcination temperature because anatase has a better photocatalytic activity due to the higher electron-hole pair lifetime compared to rutile. It attributes the difference of activity, between anatase and rutile, to the direct/indirect band gap behavior of the phases [27]. Indeed, it was shown that anatase behaves as an indirect semiconductor; thus, electron de-excitation from the conduction band to the valence band is forbidden by selection rules. Hence, electron-hole lifetime is higher with anatase while rutile acts as a direct semiconductor with a lower carrier lifetime. The average particle size of Ir-doped TiO₂ NPs was estimated 10-15 nm which was calculated by Scherer’s formula. The broadening of diffraction peak gradually increases, suggesting a systematic decrease in the grain size. Smaller crystallite size redounds to photogenerated carriers shifting to the surface of photocatalyst and prevents the recombination of the charge carriers, which, in turn improves the photocatalytic activity [28].

SEM and TEM analysis

To know the morphological information of Ir-doped TiO₂ NPs, SEM and TEM techniques were carried out. Figs. 5 and 6 represent the SEM and TEM images of the synthesized Ir-doped TiO₂ NPs. SEM image of the sample indicates that the particles are nearly uniform size nano-domain and the particles vary in their shape with some agglomeration. The SEM with EDX images confirmed the spherical and bubble morphology of the prepared Ir-doped TiO₂ NPs. From the TEM image, the particle size of Ir-doped TiO₂ NPs is in the range of 10-20 nm which are in good agreement with XRD results. The decrease in the particle size of the TiO₂ is related to the nucleation mechanism in the growth process. To find out the elemental composition present in Ir-doped TiO₂ NPs, EDX recorded in the binding energy region of 0-10 keV. The number of well-defined peaks in the spectrum clearly supports the presence of Ti, O and Ir in the NPs. In the spectrum, no other peaks were detected which confirm the purity of nanomaterial. The atomic % of Ti, O and Ir are 32.51, 62.47 and 5.02, respectively. This indicates that the material formed is non-stoichiometric, with oxygen deficiency, which leads to the best performance for photocatalytic activity [29].

Thermal analysis

Fig. 7 shows TGA curve of Ir-doped TiO₂ NPs after drying at 100 ^oC for 4 h. Three main stages of TGA curve of TiO₂ sample according to the heat profile were observed in the range of 100-1000 ⁰C. The temperature increases from 100 to 250 °C as the first stage, which is assigned to remove the remaining ethanol, water and about 32% weight loss occurred. Temperature range of 310 to 420 °C, is attributed to the decomposition of the organic compounds completely with about 9.02 % weight was lost [30]. The amorphous precursor was converted to anatase phase as the temperature increases from 425 to 500 ^oC. The TiO₂ anatase was transferred to rutile phase between 550 to 600 ^oC [31]. There is gradually weight loss up to 78.27% on TGA curve up to 1000 °C indicating that decomposition of TiO₂ precursor is completed.

Photocatalytic activity

Photocatalytic degradation of chlorinated phenols

The presence of CPs in aquatic environments has caused severe environmental pollution problems. In CPs, 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCPs are typical phenolic substances that have been used as an intermediate in making insecticides, herbicides, preservatives, antiseptics, disinfectants and other organic compounds [32] and 2,4,6-TCP is particularly of environmental interest owing to its mutagenicity and carcinogenicity [33]. Traditional wastewater treatment techniques consist of activated carbon adsorption and biological digestion. Each of these techniques has limitations and disadvantages. For activated carbon, adsorption only involves the phase transfer of pollutants without decomposition thus introduces another pollution problem [34]. On the other hand, in biological treatment, the decomposition of many CPs has proven inefficient since CPs resist to biodegradation within an acceptable time period and tend to accumulate in sediments [35]. The CPs do not undergo direct sunlight photolysis in the natural environment since they only absorb light below 290 nm [36]. Therefore, the advanced oxidation processes (AOP) appears to be a good method for destruction the toxic pollutants into nontoxic substances. Photocatalytic oxidation reactions are initiated when a photon of higher energy level or equal to the band gap energy is absorbed by a TiO₂ catalyst promoting an electron (e‾) from the valence band to the conduction band with simultaneous generation of a positive hole (h⁺) in the valence band. One possible mechanism for this change in the photodegradation efficiency is as follows; by irradiation of the Ir-doped TiO₂, Ir³⁺ ions work as electron scavengers which may react with the superoxide species and prevent the holes-electrons (h⁺/e^-) recombination and consequently increase the efficiency of the photooxidation. The mechanism of radical generation (^•OH and ^•O₂^-) is presented as follows [37]:

TiO₂ + hν → TiO₂ (e‾ + h⁺) (2)

H₂O + TiO₂ (h⁺) → TiO₂ + ^•OH + H⁺ (3)

O₂ + TiO₂ (e‾) → TiO₂ + ^•O₂¯ (4)

Ir³⁺ + O₂⁻ (ads) → Ir²⁺ + O₂ (ads) (5)

To confirm the degradation of CPs due to photocatalysis, the sample of Ir-doped TiO₂ NPs tested as an adsorbent of these pollutants. Fig. 8 shows the UV-Vis absorption spectra of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP in the absence of Ir-doped TiO₂ NPs as adsorbent at the maximum wavelength is 290, 295, 287 and 310 nm, respectively. The removal of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP slightly decreased the absorption at the corresponding wavelengths in 60 min of sunlight irradiation. On the other side, Ir-doped TiO₂ NPs acted as an adsorbent for the removal of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP. In this process, it is observed that the absorption intensity of the band at the corresponding wavelengths produced by the solution after adsorption with Ir-doped TiO₂ NPs is high in comparison with the band of only chlorinated phenols, indicating that under these conditions Ir-doped TiO₂ NPs adsorb molecules of CPs; however, the solution shows a light yellow colour, indicating that the adsorption process may produce some by-products, since it cannot degrade the benzene ring of the CPs. The interaction between the two compounds in water may lead to derivatives and by-products of CPs, corroborated by the yellow color of the solution and the increment in the absorption of the corresponding band at 225 nm for the benzene ring. Thus, this effect indicates that CPs are adsorbed via a link in the Ir-doped TiO₂ NPs by C–Cl linkages, with more than 80% of the CPs ultimately removed.

As mentioned before, a common characteristic of the presence of by-products generated by CPs, as the result of degradation process, is the yellow coloration of the solution, which indicates that intermediates are generated; however, if the final solution is transparent, it indicates that by-products generated are not excessive. Compounds such as hydroquinone and benzoquinone give a yellow color to the final solution of a treated CPs. The characteristic bands of CPs (290, 295, 287 and 310 nm) disappear independently in the spectra of Ir-doped TiO₂ NPs in the photocatalyst tests; it is standard to calculate the removal percentage with the band at 225 nm, which is the distinguishing band for this quantification [38-39]. The enhanced photocatalytic activity for Ir-doped TiO₂ NPs could be attributed to the ability to capture and transport electrons and to promote charge separation. It is known that the higher separation efficiency of electron-hole pairs will enhance photocatalytic activity and results in a large number of holes participating in the photocatalytic process [40]. Degradation of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCPs, using TiO₂ as photocatalysts activated in UV zone supported in different materials such as activated carbon, silica, and zeolite have been reported [41-42]. Based on this data we developed a new synthetic route of Ir-doped TiO₂ NPs for the photocatalytic degradation of CPs in under sunlight irradiation. It is an industrial application of this method recommended because of the simple modification, high-efficiency removal and prevention of the environment pollution.

Degradation mechanism and efficiency

In order to avoid excess use of photocatalyst, it is essential to find out the optimum quantity of photocatalyst for better photodegradation of CPs. The effect of catalyst loading on the degradation of CPs was checked under sunlight irradiation by using optimal parameters of the concentration of CPs and amount of Ir-doped TiO₂ NPs are identical. It is seen that, initially, degradation efficiency increases with increase in the irradiation time. The photodegradation mechanism of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP involves the photodissociation of the C-Cl bond in CPs has been reported using UV laser excitation [39]. The photochemical reaction is the first stage in the C-Cl bond cleavage of CPs, with the subsequent decomposition of the intermediate ion or radical [40]. It is important to consider the band at the corresponding wavelengths which define the generation of intermediate compounds in the progression of CPs degradation. Figs. 9 and 10 show the 3D-bar diagrams of photocatalytic degradation efficiencies of 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP in the absence and presence of Ir-doped TiO₂ NPs under 60 min of sunlight irradiation. It clearly indicates more than 12% and 80% of photodegradation of CPs in the absence and presence of Ir-doped TiO₂ NPs, respectively. It indicates that the wavelength emitted by the used light is capable of activating OH groups in the solution, causing oxidation due to air trapped in the reactor where the reaction takes place and breaking the C-Cl bonds present in CPs. In principle, the photodegradation rate of CPs is affected by not only the active sites but also the photoabsorption of the catalyst used [43]. It is important to remember that the by-products in many cases are more toxic so that the photolysis process alone is not a sufficient method to completely degrade CPs.

Kinetic analysis

The kinetics of photocatalytic reactions of CPs using Ir-doped TiO₂ NPs in aqueous solution follows Langmuir-Hinshelwood model:

Rate = -dC/dt = K_app C, (K_app = k_rK_a) (6)

where k_r is the rate constant and K_a is the adsorption equilibrium constant. When the adsorption of CPs on Ir-doped TiO₂ catalyst is relatively weak, the K_a and C are insignificant, and the equation can be simplified to the first-order kinetics with an apparent rate constant (K_app = k_r K_a).

ln [C₀/C_t) = k_app t (7)

The apparent rate constant (k_app) evaluated by using above-equation and pseudo-first-order kinetics of CPs degradation was obtained. The degradation efficiencies, apparent rate constants and half-life period values are reported in Table 1 to show the wide-ranging comparison. The kinetic plots were drawn for the photocatalytic degradation of CPs in the absence and presence of Ir-doped TiO₂NPs and it follows the pseudo-first order kinetics (Figs. 11 and 12). The overall photodegradation rate decreases on order of 2-CP > 4-CP > 2,4-DCP > 2,4,6-TCP. These results indicate that the rate constant reduces in sequence from 2-CP to 2,4,6-TCP, which illustrates the difficult attack on the 2,4,6-TCP ring by HO^• radical because it is an electrophilic reagent. The 2,4,6-TCP ring has lower solubility property (higher hydrophobicity) than that of 2-CP, 4-CP and 2,4-DCP. From the previous results, the degradation rate of CPs is faster when chlorophenol hydrophobic property is decreased and increases the solubility [44].

Antimicrobial activity

The synthesized Ir-doped TO₂ NPs with various amounts (10 µL to 50 µL) were screened for the in-vitro antibacterial activity against Gram- positive B. subtilis, S. aureus and Gram-negative E. coli, P.aeruginosa, and antifungal activity against A. niger and C. albicans and the relevant data are presented in Table 2 and Fig. 13. The diameter of the zone of inhibition (mm) was used to compare the antimicrobial activity of the tested sample with the standards (Ampicillin and Ketaconazole were used as standard drugs for antibacterial and anti-fungal activity, respectively). The results revealed that nanoparticles have varying degrees of inhibitory effects on the growth of bacterial and fungal strains that may be due to the effect of the metal ion on the cell metabolism. The 50 µL amount of Ir-doped TiO₂ NPs showed remarkably good anti-bacterial activity against B. subtilis and E. coli with zone diameter 14 and 11 mm, respectively, while moderate activity against S. aureus and P. aeruginosa with zone diameter 10 and 8 mm, respectively. The synthesized Ir-doped TiO₂ NPs (10 µL to 50 µL) were also screened for their antifungal activity against A. niger, and C. albicans. Interestingly the prepared sample exhibited prominent antifungal activity against the all fungi with significant inhibition zone diameter. The significant antimicrobial activity of the nanoparticles can be attributed to the fact that the lipid membranes that surround the cell favor the passage of only the lipid-soluble materials which makes liposolubility as an important factor that controls the antibacterial activity.

CONCLUSION

In summary, the microwave-assisted method was used for the preparation of Ir-doped TiO₂ NPs and characterized using different techniques such as XRD, SEM and TEM for structure and morphology studies. Optical properties of the synthesized Ir-doped TiO₂ NPs were characterized using UV-Vis DRS and PL. XRD analysis showed that the anatase TiO₂ sample has a tetragonal structure. The energy band gap of the synthesized TiO₂ NPs was calculated 3.54 eV which was greater than that of the TiO₂ bulk with a band gap of 3.23 eV. The present work proves that Ir-doped TiO₂ NPs is a new technique using cheap precursors for photocatalytic degradation of CPs. This study provides a versatile approach for high-efficiency method for the degradation of toxic compounds of CPs such as 2-CP, 4-CP, 2,4-DCP and 2,4,6-TCP under solar irradiation in the presence of Ir-doped TiO₂ NPs as an outstanding photocatalyst. The overall degradation efficiencies for CPs under sunlight irradiation were found to be more than 70% within 60 min, that significantly enhanced by doping with Ir³⁺ ions. The increase in photocatalytic activity was due to the higher absorption and the 5d-electron transition of metal ions. Therefore Ir doping is effective for improving photocatalytic activity of TiO₂ for degradation of chlorinated pollutants under sunlight. In addition, the prepared Ir-doped TiO₂ NPs with various amounts (10µL to 50 µL) were screened for antimicrobial activity and revealed that the tested sample has an elevated antimicrobial activity.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the Head, Department of Chemistry, Osmania University for providing the necessary facilities. The authors would like to thank DST-FIST, New Delhi, India for providing necessary analytical facilities in the department.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this paper.