Effect of pH variation on Bandgap and Visible Light Photocatalytic Properties of TiO2 Nanoparticles

Document Type : Original Research Paper


1 Research Scholar, (Reg.No: 19111232132001) PG and Research Department of Physics, Sri Paramakalyani College Alwarkurichi Affiliated to Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli-627012, Tamilnadu, India

2 Principal, Sri Paramakalyani College Alwarkurichi-627412


Titanium-di-oxide nanoparticles are synthesized via a microwave-assisted solvothermal route for different pH values. The effect of the acidic and basic nature of the solvent due to the pH value is reflected in the crystalline size of the compound. The purpose of this work is to synthesize Titanium dioxide nanoparticles and to observe their application in degrading industrially contaminated water using normal tap water. The crystalline sizes are calculated using XRD analysis and confirmed with HRTEM. The chemical composition and oxidation state are confirmed with XPS studies. Optical properties are carried out with UV-Vis, FTIR, and PL spectra. Photocatalytic studies are carried out to degrade the dye in industrial water. The efficiency of degradation is calculated with the UV-Vis data and formula. The reduction in band gap and high permanence has greatly supported in making it acceptable for photocatalytic activity under visible light. Dependence of time, initial dye concentration, and pH of the dye solution on TiO2 as a catalyst is investigated under the illumination of a visible lamp, and degradation efficiency to the highest of 96.79% has been obtained. 


Nanoscience is a convergence of Physics, materials science, and biology, which deal with the manipulation of materials at atomic and molecular scales, while nanotechnology is the ability to observe, measure, manipulate, assemble, control, and manufacture matter at the nanometer scale. Nanotechnology deals with the use of nanomaterials for pragmatic applications for society-based projects and projects for sophisticated lifestyles. They are studied because of their size-dependent physical and chemical properties. Various industries like fabric, mat, wrapper, comestible, beauty-product, and pharmaceuticals industries use multiple ranges of colors for selling their materials to us [1]. These industries use colored dyes for their products.   Dyes are naturally available or synthetic compounds that make permanent color to fabric, reed mats, cosmetics, etc. [2] Based upon their composition, dyes are categorized as acid dyes, direct dyes, azo dyes, disperse dyes, sulfur dyes, fiber reactive dyes, basic dyes, oxidation dyes, mordant dyes, developed dyes, vat dyes, pigments, fluorescence or optical brighteners and solvent dyes [3]. These toxic dyes lead to severe health issues in living organisms. All these industries, after making use of the dyes, dispose of them as waste in nature. 
Nature never disturbs the qualities of water, light, and air.  But it may cause damage to the infrastructure as a whole.  Contamination of water bodies by human create a great disaster in our ecosystem and also for the living organisms. In most developing countries water contamination and the availability of clean water for human consumption is a leading problem [4]. An increase in population is also one of the reasons for water scarcity. In that view, the human need has made the water resources more tainted and the shortage of drinking water sources prevails [5]. Water resources for livestock production, all-year-round irrigation, and other agricultural activities are highly altered due to contamination by human activities like discharging industrial waste to water bodies [6]. The main component of this outflow of dyestuff is from the industries like food beverages, printing, plastics, paints, leather, and cosmetics [7]. They use a large quantity of fresh water and reverse it into an enormous amount of wastewater and make this poisonous and disagreeable waste to mix with the water bodies [8]. These dye wastes are adverse causing mucous membrane irritation, eye irritation, skin irritation, contact dermatitis, respiratory diseases and even carcinogenic to humans dyes on the surface of the water affect the living organism in water, photosynthesis in aquatic plants blocking penetration of sunlight [9]. Several areas of basic and applied research are concerned with removing dye from industrial waste [10]. Various methods like adsorption using dead/ living microbial biomass, degradation by white-rot fungi, using sodium hypochlorite, membrane filtration, electrokinetic coagulation, and adsorption by activated carbon. Though these methods can degrade toxic dyes they have some drawbacks such as high waste production, not applying to all dyes, very costly, short half-life, etc. [11]. Compared to all other methods photocatalysis is a prominent method for degrading dyes because its by-products i.e., mineral acids, carbon-di-oxide, and water are harmless. [12].
Some of the previous works for the same are as follows, Sayadi MH et al., (2022) have synthesized ZnO/SnO2 nanoparticles and studied its photocatalytic degradation of Congo red, Biphenyl A, and tetracycline with 5 ppm dye concentration and 0.5g of photocatalyst [13]. Poorsajadi et al., (2021) synthesized copper-supported Bismuth oxide nanoparticles and observed its decolorization effect on methyl orange dye under UV light and reported 93.76% efficiency [14]. Chamanehpour E et al., (2022) prepared g-C3N4@Cu/ZIF-8 nanocomposite and observed its photocatalytic property in removing carbon-di-oxide about 82.1% and reported it as stable material after 5 cycles [15].  Sayadi MH et al., (2022) fabricated nano adsorbent i.e. γ-Fe2O3/MWCNTs/Cellulose, and reported its absorption ability of malachite green dye as 99% [16]. The important thing to be noted in this work is solid dyes from the reed mat industries of veeravanallur are collected, mixed with the normal water from which they prepare the dye solution for industrial purposes, and tested for its degradation in TiO2 as photocatalyst. Malachite Green (MG), an extensively used cationic, water-soluble dye for the textile industry, known as N-methyl-di-amino-tri-phenyl-methane, is reported as more hazardous [17].   It is one of the most commonly used dyes in manufacturing industries [18] their presence in consuming water leads to extreme irritation, and adverse effects on the kidney, nervous system, respiratory system, and brain [19,20]. In viewing the quote “Sewage can lead to another disaster, which is the disease” by American sociologist Walter Maestri, treating those dye waste is one of the most required needs. Therefore researchers are keen on treating the dye-contaminated water by various techniques such as sedimentation, filtration, coagulation, and flocculation [21]. Because of the bio-persistent nature of dyes these above techniques are not able to treat them, photocatalysis using semiconductor catalyst is more advantageous owing to its complete mineralization of the dye into CO2 and H2O, eco-friendly and low requirement of catalyst [22,23]. Because of the sturdy degradation of toxic contaminants under light (photo) illumination titanium-di-oxide has been insistently scrutinized in sterilizing toxic materials [24,25]. 
Titanium dioxide (TiO2) is a promising semiconductor material with many excellent properties like innocuous, longevity, economical etc. [26] Rutile, brookite and anatase are the three different forms of titanium-di-oxide on nanoscale with bandgap 2.98eV-3.0eV, 3.26eV and 3.05eV-3.2eV [27, 28, 29]. TiO2 came out with excellent use in batteries, capacitors, solar cell fabrication, photocatalysis, etc., while there increase in deflection in the energy problem. Amidst these uses, TiO2 is employed in photocatalysis because of its charge movability, appropriate band gap, and photoreaction [30]. It has been captivating much attentiveness as a consequence of the splitting and removal of pollutants from water and air [31]. Especially, TiO2 in the anatase phase is utilized in solar energy conversion owing to its outrageous sensitivity when exposed to light (Photo) [32].  It can catalytically break down adulterants in water from household and commercial waste.
In this work, titanium-di-oxide nanoparticles (catalyst) under four different pH values were synthesized by exploiting the microwave irradiation technique. The samples were characterized by XRD, SEM, HRTEM, XPS, UV-Vis, FTIR, and PL. As an application of this work to society, photocatalytic degradation of the Malachite Green dye under visible light (photo) irradiation with different initial concentrations and pH values are investigated.

Synthesis of TiO2 with different pH values
All the required raw materials are purchased from Merck and it is of AR grade. Titanium tetrachloride was the precursor used as a metal (titanium) source, ethylene glycol and urea were also used for synthesis. Titanium tetrachloride and urea in the fixed ratio were mixed well in ethylene glycol as a solvent for about 1 hour. The entire solution was poured into a ceramic bowl and treated in a microwave oven at a regular interval of 60 sec/cycle. The substance which is left after the complete evaporation of the solvent is collected, cleaned with double distilled water about four to five times to remove the impurity materials present in the final product, filtered, and dried in a hot air oven at 70ºC for five hours. To revamp the crystallinity, the sample was annealed at 475ºC for two hours by placed in a muffle furnace [33]. 

Characterisation Technique
A digital pH meter with ATC Glass Electrode, MP1 plus Sushima is used to measure the pH value of Ethylene Glycol (solvent) during synthesis. The synthesized samples are characterized for the structural properties through powder X-ray diffraction (XRD) using XPERT-PRO spectrometer with CuKα radiation (λ = 1.54 Å) in powder form of sample, Scanning Electron Microscope (SEM) Micrograph is recorded using VEGA3 TESCAN, High-Resolution Transmission Electron Microscope (HRTEM), SAED and EDAX using JOEL (JEM-2100) Electron Microscope by coating the sample in a copper grid, elements electronic state using PHI - VERSAPROBE III – X-ray Photoelectron Spectroscopy (XPS) in powder form of sample, optical properties through Fourier Transform Infrared Spectroscopy (FTIR) using FT-IR Nicolet IS5R FTIR, KBR Windows with AR Diamond Crystal Plate with samples in powder form, absorbance spectra using UV-Visible Spectrophotometer, Double beam, make UH-5300, Photoluminescence (PL) using Varian Cary Eclipse Photoluminescence spectrophotometer. The photocatalytic application of the synthesized sample is studied by using a temperature-controlled condenser setup.2.3 Photocatalytic Experiment
Photocatalytic Degradation of the malachite green dye in the presence of Titanium-di-oxide as a catalyst under irradiation of visible lamp with respect to time is observed first. Initially, the four pH varied catalyst (TiO2) of 50 mg is measured and made to stir in dark along with the dye solution made out of the freshwater used in the reed mat industries in the veeravanallur area for about 1 hour. As a next step, the entire solution was placed in the dark chamber, illuminated with a visible lamp, and stirred continuously using a magnetic stirrer [34]. Then the photocatalytically treated dye water is at a regular interval of 15 min and tested for its degradation efficiency. The nanocatalysts are also tested for malachite green dye degradation with 50mg catalyst, and dye solution at pH-7, the temperature of the photocatalytic chamber is maintained at room temperature and by varying the initial concentration of dye solution ranging from 100ppm to 400ppm. pH value of the dye solution is varied for both acidic and basic nature. The same amount of nanocatalyst (50mg) is added to the dye solution subjected to various pH conditions. 1M of HCl and NaOH solution is utilized to diverge the pH value of the dye solution. Other parameters like initial dye concentration (100 ppm), photocatalytic chamber temperature (room temperature), and visible light source are kept constant all over the examination of dye degradation using TiO2 as a catalyst. A cyclic test for the catalyst synthesized under pH-12, which is our best photocatalyst, is analyzed. The same procedure is repeated as mentioned before, with the catalyst load of 50mg in 100ppm of dye solution having a neutral pH i.e. pH-7 under room temperature in 90 min. For each cycle, samples are washed in double-distilled water and acetone 10 times and dried for 5 hours to detach the dye adsorbed in the catalyst [35]. The reusability performance is carried out five times with the same catalyst resulting in a mild decrease in its photocatalytic activity.

XRD Analysis of TiO2 nanoparticles 
XRD patterns of the pH-varied TiO2 nanoparticles are shown in Fig. 1(a). The anatase phase of the synthesized TiO2, peak position, and their intensities agree with the Joint Committee on Powder Diffraction Studies (JCPDS) card using X’pert high score software. The dominant peak arises at 25.5º with the corresponding plane at (101) [36]. Other peaks were spotted at 38º, 48º, 53º.55º, 62º, 70º,75º, and 82º [37] with their respective Miller indices at (004), (112), (200), (105), (211), (204), (220), (215) and (224) ascribing the anatase phase, tetragonal crystal system with the space group of 141/amd [38]. The achievement of better electron transfer and adsorption of dye is due to this pure anatase form [39]. There was an increase in peak intensity with the increase in pH value (Fig.1 (b)) ascribing the increase in crystalline size. The average crystalline size for 2, 5, 9, and 12 pH-varied synthesized TiO2 is calculated as 23.87nm, 24.26nm, 26.54nm, and 27.62nm from the Debye-Scherrer equation with the Scherrer constant of 0.94 [40]. During chemical synthesis, pH may alter the synthesis time and size of the nanoparticles, agglomeration of particles occurs with an increase in reaction time leading to increasing in crystalline size. Table 2 provides the JCPDS file, lattice parameters a, b, and c, volume of the cell, and crystalline size. The lattice parameters a, b, c, and volume of the cell is calculated by using equation 1(a) and 1(b),          

=  +       1(a)

V= a2c                                1(b)

SEM and EDAX Analysis
For the application in dye degradation, pure TiO2 nanoparticles were tested for morphological analysis. Fig. 2(a) gives the Scanning Electron Microscope image of the pH-varied synthesized TiO2 at pH 12. It provides an SEM image of TiO2 with a granular size of about 27nm [44]. Plate-shaped grains are found from SEM analysis. Their high surface area in this plate form helps in promoting the photocatalytic activity of the catalyst. The presence of titanium and oxygen is confirmed by the X-ray Electron Diffraction Analysis. Since the study is made by coating synthesized nanopowders in a copper grid there are some peaks representing copper are present. Table 3 shows the weight percent and atomic weight percent of TiO2 using EDAX spectra. Fig. 3(a) provides the HRTEM morphological analysis of Titanium-di-oxide nanoparticles. Following the histogram from Fig. 3 (b), 25.97nm was the particle size obtained with the standard deviation value of about 4.65nm which is similar to the crystalline size obtained from XRD analysis. The distribution of particles in the nanometre range is attested by this HRTEM analysis [45]. In the SAED portrait (Fig. 3 (c)), the diffraction patterns are designated to their respective crystalline planes (101), (200), (105), (204), (220), and (215) of anatase state [46]. The interplanar spacing dhkl matches well with the JCPDS file: 01-071-1166 from XRD analysis. Both the HRTEM and SAED analysis are in tune with the XRD report.

Chemical States Analysis
To validate the existence of an electronic state of elements and binding energy XPS analysis is studied. Fig. 4(a) portrays the survey scan of the TiO2, corresponding peaks of Ti 2p and O 1s are revealed in this figure. The anatase titania is shown by the Ti 2p plot with two peaks around 457.2 eV, 462.95 eV of 3/2, 1/2 can be ascribed to Ti 2p3/2 and Ti 2p½ respectively labeling the Ti4+ in TiO2. [47, 48]. The peaks Ti 2p3/2 and Ti 2p½ are at a distance of 5.75 eV which is in agreement with the standard distance in the binding energy of Ti 2p (Fig. 4 (b)) [49].  The pure Titania anatase phase is well agreed with the previous report [50]. The O1s peak (Fig. 4 (c)) is spotted at 528.4 eV which is attained to oxygen bonded with titania ions and 531.1eV is assigned to the vacancy of oxygen boosting up the semiconductor oxide’s photocatalytic action [51]. The atomic percentage from this analysis is reported for Ti 2p and O 1s as 22% and 78%. This reveals oxygen-rich TiO2, which aids in elevating photocatalytic activity under visible light [52].

Fourier Transform Infrared Analysis
FTIR analysis of TiO2 nanoparticles is observed in the range 4000 cm−1– 400 cm−1. Fig. 5 depicts the FTIR spectra of the four pH-varied synthesized nanoparticles. In the four pH variation, a broad transmittance band in the region of 900 cm-1 to 400 cm-1 is assigned to vibrations of Ti-O-Ti interconnection and the peak at 432 cm-1 might be due to the TiO2  (anatase) vibration mode of Ti-O linkage [53].

Optical Absorption and Bandgap Analysis
To find the optical bandgap, UV-Vis absorbance spectra are measured in the ranges from 200nm to 800nm. Fig. 6 (a) shows the UV-Visible absorption spectra of pH- modified TiO2 nanoparticles.  The optical absorption edge of pH-2 modified TiO2 is 426nm while with pH -12 modified TiO2 is 434nm. A minute redshift is noted in this case which might be ascribed to the surface modification of TiO2. [54] Which in turn can boost the photocatalytic nature of TiO2 below visible light, and more charge carriers can be produced. This shift in the UV spectrum might be the reason for the difference in the optical bandgap. The bandgap of the synthesized nanoparticles is deduced by sketching (αhυ)2 versus (hυ). On extending the plot, (Fig. 6 (b, c, d, e)) direct bandgap of the material is computed as 3.11eV, 3.10eV, 3.10eV and 3.09eV [55]. It is noted that there is a decrease in bandgap with an increase in the pH of the solvent.  By comparing all the samples, it is distinguished that TiO2 synthesized under pH-12 showed higher absorbance and a small optical bandgap. The tapering of bandgap energy also affirms the oxygen-rich nature of the sample enhancing its photocatalytic properties [56]. The value of absorbance (A), absorption coefficient (α), and absorbance depth (δ) are computed and cataloged in Table 4 using equation 2 (a) and (b) the relation [57] 

α =  cm-1,     2(a)

δ =  cm              2 (b)

Photoluminescence Analysis
In semiconductor particles, to acknowledge the destiny of hole/electron pair photoluminescence spectra have been intensively used. PL spectrum depends on precursor materials, the solvent used, time taken for synthesis, annealing temperature, and surrounding conditions [58].  For determining the exact excitation wavelength in PL spectra absorption spectra from UV-Vis analysis corresponding to the wavelength of excitation is used. In the current work, the photoluminescence emission analysis spectrum of various pH-controlled synthesized titanium-di-oxide nanopowders with an excitation wavelength of 300nm is sported in Fig. 7. Generally, lower intensity of PL spectra corresponds to higher photocatalytic activity. The absorption spectrum is shifted to shorter wavelengths relating it to the maximum emission spectrum. This shift is named as stokes shift. There was a decrease in peak intensity is observed when the pH of the solvent during synthesis gets increased, this results in demoting the ratio of electron-hole pair recombination, by overcoming the drawback of rapid recombination of electron and hole pairs promoting the charge carrier separation which results in enhancing dye degradation process in photocatalytic treatment [59]. The visible emission peaks at 489nm, 490nm, and 492nm are attributable to electrons caught in sites of oxygen vacancy. The broad emission peak from 360nm to 430nm rises from exciton recombination.
Photocatalytic Degradation
Effect of Contact Time 
Under the illumination of a visible lamp, the dye samples are collected at regular time intervals and tested for dye absorption by the catalyst using UV-Vis Spectrometer. Fig. 8 depicts the UV absorbance curve on degrading the malachite green dye by adding TiO2 as a catalyst. To determine the degradation efficiency of the malachite green dye using equation 3 following relation 

Degradation efficiency (%) = (A0 − At) /A0 × 100 (3)

Where A0 and At are the initial and final absorbance of the malachite Green dye is used [60]. 
Dye solution in the presence of TiO2 as a catalyst on visible lamp illumination was collected under a regular interval of time. The degradation of malachite green dye goes on increasing as time goes by. The presence of dye molecules gets reduced with an increase in time.   It takes about 240 min on degrading the dye with a concentration of 400 ppm.  The dependence of time on degrading the malachite green dye for the four samples is sketched in Fig. 9(a). The catalyst synthesized under pH-12 condition turns up with the best result of 93.27% within 240 min. Some of the previous literature work on degrading malachite green dye with TiO2 as a catalyst is compared with current work and tabulated.

Effect of Initial Dye Concentration
The reliance of dye concentration at the initial stage on photocatalytic degradation for all four samples is represented in Fig. 9(b). It is observed that with the increase in the concentration of dye the degradation efficiency by the catalyst drops a little amount. When the concentration of dye increases from 100ppm to 400ppm the degradation efficiency is noted from 96.79% to 93.27%, a decrease in efficiency. The time required for degrading dye solution with varying initial concentrations also gets prolonged. An increase in time was observed on degrading the dye with higher concentration. Electron and hole pair originated upon visible lamp illumination gets lower in count resulting in diminishing the efficiency of degradation [62]. 

Effect of pH
Initially, the dye solution is made with their appropriate pH values, stirred after adding the catalyst for about one hour in dark. The appearance of the dye solution is dark in acidic nature upon addition of HCl to it. The appearance of the dye solution looks paled upon the addition of NaOH for its basic nature. The reason is that the excess amount of OH free radicals boosted the degradation efficiency of TiO2 nanocatalysts. The concentration of MG on the TiO2 surface is reduced owing to the positive surface of both TiO2 and dye molecule leading to poor degradation of dye in an acidic state whereas vice-versa happens in the basic state reporting better degradation efficiency. The time factor also gets reduced when the dye solution moves from acid to base state [71].  Fig. 9 (c) delineates the pH vs. degradation efficiency of malachite green dye for the four photocatalysts. The dye waste under neutral pH value exhibited better results when compared to other pH states. Fig. 9 (d) shows the variation of particle size, band gap, and degradation efficiency in degrading malachite green dye. The catalyst with a longer band gap has increased the degradation of dye reducing the rate of recombination of electrons and holes. 

Degradation Kinetics
By the photocatalytic reaction principle, a high adsorption range of dye on the surface of the nanocatalyst promotes the degradation process. On taking a look at the degradation rate of MG dye with an initial concentration of 400ppm for the nanocatalyst TiO2 (50mg), pH-7 in visible lamp irradiation for the reaction time of 240min, Pseudo-first-order reaction kinetics relation ln(A0/A)=kt where initial absorbance of dye solution A0, the concentration of dye collected after regular time interval A, time t, rate constant of Pseudo first order k, is used [70]. Pseudo-firs order kinetic plot for ln(A0/A) Vs. time is plotted in Fig. 10(a). The straight slope from this plot makes us know the first-order rate constant. Through the linear plot, pseudo-first-order rate constants for the four samples are computed as 0.00391min-1, 0.00468min-1, 0.00683min-1, and 0.00963min-1 and it is plotted, shown in Fig. 10 (b). The nanocatalyst with fine degradation efficiency i.e. nanoparticle synthesized at pH-12 condition reported an increase in reaction rate [72].

Reusability of catalyst
The recyclability and steadiness of the nanocatalyst are key factors responsible for an economical and achievable catalyst. Recycling possibilities of pH-12 controlled synthesized TiO2 for photocatalytic degradation property are displayed in Fig. 11.  Catalyst, after the recycling process, exhibited a fall in efficiency from 96.79% (1st Run) to 85.78% (5th Run). Considerable stability of the sample is recommended by the result implying a mild fall in degradation during the second cycle. A small depletion in surface photocatalytic activity might be due to the loss of the material during the cleaning process [73]. It may also be due to a decrease in bond strength uniting dye (Malachite Green) and immobilized TiO2 which can be ascribed to immobilized TiO2 active sites occupation.

Titanium-di-oxide nanoparticles are successfully synthesized under controlled pH conditions via the smart microwave-assisted solvothermal method. The size of the particle is found to be ranging from 23.87 nm to 27.62 nm using XRD analysis.  FESEM and HRTEM also satisfy the particle size from the XRD result. In the pure anatase phase, the elemental composition is confirmed from XPS analysis. The optical bandgap of the four samples is 3.11eV, 3.10eV, and 3.09eV by the tauc plot method. The significance of this work i.e. studying the photocatalytic degradation efficiency of TiO2 in degrading malachite green dye using the normal tap water from the reed matt industrial areas of veeravanallur is observed by varying dye’s initial concentration and the result is outputted that with the increase in concentration there was a decrease in degradation efficiency and the time taken for degrading the dye also increases. In the photodegradation studies taken by varying the pH value of the dye solution, from acidic to alkaline the degradation is speed enough in the base region. Though the neutral pH exhibited a good efficiency. pH-controlled condition of the solvent during synthesis has affected particle size in turn bandgap also have varied, which has enhanced the photocatalytic activity of TiO2 under visible lamp irradiation reporting an excellent degradation efficiency of 96.79%.

R. Sarathi- Processed the samples, collected the samples, Data Analysis. Compiled and analyzed the data, writing –original draft, S. Meenakshi Sundar- Conceptualization, supervision, validation. All authors have read the final manuscript and confirmed its content.

The authors declare no conflict of interest.

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