Among the mineral oxides, titanium dioxide (TiO2) is highly regarded due to its chemical stability, non-toxicity and low cost. The optical activity of titanium dioxide includes its photocatalytic, photovoltaic and super hydrophilic properties. The mechanism of action includes irradiating light with energy higher than the band gap energy of titanium dioxide and electron excitation from the valence level to the conduction level .
However, the use of titanium dioxide is limited for two reasons :
1- The titanium dioxide band gap is in the UV region, while only 3% of the sunlight spectrum is in this domain.
2- Rapid electron-hole coupling and consequent loss of photocatalytic activity.
Therefore, in recent years, various methods have been used to modify the band gap energy and increase the optical activity of titanium dioxide, such as reducing the particle size, increasing the specific surface area, and pairing titanium dioxide with other semiconductors. The modification of titanium dioxide with metals and non-metals and the sensitization of titanium dioxide has been mentioned previously .
There are many methods for depositing metal ions in TiO2 semiconductors, but the two main methods used are as follows :
2- Chemical modification method, which is mainly conducted through the sol-gel method. The Sol-gel method is one of the most widely used methods of preparing nanomaterials. In this method, combining the active metal in the cell allows the metal to interact directly with the preservative during the gelling step .
The optical activity of TiO2 modified with metals is significantly dependent on the method of preparation, the nature of the ion and its concentration. Some researchers claim that metal modification reduces the energy thresholds of TiO2. This produces two levels with a narrower energy difference, which leads to less energy consumption for the photocatalytic process. In addition, metals act as traps for electrons or holes preventing the electron–hole recombination. By increasing the lifespan of charge carriers, optical and photocatalytic activity may increase [8-6]. Various organic compounds such as safranin , eosin , rhodamine , thionine , macrocycles [12 and 11], etc. have been used as sensitizers for the sensitization of titanium dioxide.
The sensitizer is fixed on the surface of TiO2 particles. Stabilization of sensitizing molecules on the semiconductor surface is done in different ways such as covalent bonding, ion pair bonding, physical adsorption, trapping in pores, and hydrophobic interaction . Oxidation-reduction pairs are used to reproduce the sensitizing compound. If the process of reproducing the sensitizing compound is limited and prohibited, the sensitizing compound will undergo oxidative damage due to the attack of active radicals . Phthalocyanine (Pc) is an organic pigment consisting of a 16-membered ring  in which the ring of carbon and nitrogen atoms is intertwined, and it is a combination of four isoindole molecules. The family of phthalocyanine organic pigments has high thermal and chemical stability and high intensity of light absorption in the visible region .
In this study, in order to overcome the aforementioned limitations of titanium dioxide and to correct its band gap, a new hybrid nanocomposite containing titanium dioxide nanoparticles modified with copper metal and phthalocyanine organic pigment was synthesized and identified. For this purpose, in the first stage, modified titanium dioxide nanoparticles were prepared by the sol-gel method. Then hybrid nanocomposite using modified titanium dioxide nanoparticles has been synthesized by the chemical deposition method using non-toxic, inexpensive and available materials. The advantages of these methods include the ease of the methods, production of high purity products, available raw materials and their reproducibility.
Details of Precursors
Tetraisopropyl orthotitanate (Ti(OC3H7)4, purity: 98%, molecular weight: 255.284 g/mol, density: 0.96 kg/L), copper nitrate (II) hydrate (purity: 99%), and ethanol (purity: 99.95%, molecular weight: 0.488 g/mol, density: 0.3789 g/cm3) were purchased from Merck Co. Phthalocyanine (C32H18N8, Purity: 99%, molecular weight: 514.54 g/mol) was purchased from ACROS Co.
Synthesis of undoped TiO2 nanoparticles
TiO2 nanoparticles were prepared by the sol-gel procedure . 19 mL of Ti(OC3H7)4 was slowly added to 20 mL ethanol under vigorous stirring for 1.5 h. The solution was subsequently left for several days to form a gel. The gel was dried at 100 °C for 6 h and then milled by a mortar. The obtained powder was calcined at 400 °C for 4 h at the rate of 2 °C/min.
Synthesis of Cu-TiO2 nanoparticles
19 mL of Ti(OC3H7)4 was gradually added to 20 mL ethanol under vigorous stirring (solution A). Then, 0.822 g of Cu(NO3)2 was dissolved in 5 mL of ethanol and added dropwise to solution A. After stirring for 1.5 h, the solution was left for several days to form a gel. The gel was dried at 100 °C for 6 h and then milled by a mortar. The obtained powder was calcined at 400 °C for 4 h at the rate of 2 °C/min.
Synthesis of Pc-Cu- TiO2 hybrid nanoparticles
1 g of Cu-TiO2 nanopowder was dispersed in 50 mL ethanol under ultrasonication for 60 min at 60 W. Then, 0.01 g of phthalocyanine was dissolved in 50 mL ethanol and added dropwise to the dispersant solution. The resulting solution was refluxed for 2 h at 70 °C. Subsequently, the formed nanocomposite was separated by high-speed centrifugation and dried at 70 °C for 1 h.
A PerkinElmer FT-IR spectrophotometer was used to identify the functional groups in the precursors. For sample preparation of FT-IR, about 1/8 of the solid sample on a microspatula and about 0.25-0.50 teaspoons of KBr are taken. Then, they are mixed thoroughly in a mortar while grinding with the pestle. If the sample is in large crystals, the sample is grinded separately before adding KBr. A CE7000-A spectrophotometer was employed to determine the absorption edge. The morphology of the samples was assessed by scanning electron microscopy (Philips XL 40SEM). A Zeiss EM 900 transmission electron microscopy was used to characterize nanoparticles and nanocomposites. For sample preparation of TEM, a small drop of colloidal suspension (usually about 5 µl) is pipetted onto a TEM grid and simply allowed to dry at room temperature. The grid can then be directly observed in a TEM once the medium is evaporated. TG/DTA analysis (Perkin Elmer Pyris Diamond model) was performed at a scanning rate of 5o/min. For sample preparation of TG/DTA, sample size should be between 2 and 50 mg. Many small pieces of sample are better than one large chunk. It is better to have a large surface area exposed to the sample purge. Crystalline structure of the samples was evaluated by a German-based Thermo-Noran X-ray diffraction analyzer. For sample preparation of XRD, nanopowders were prepared by hand grinding using a mortar and pestle. The mortar and pestle can be made out of a variety of materials such as agate, corundum, or mullite.
RESULTS AND DISCUSSION
Crystalline and Phase Structure
The X-ray diffraction patterns obtained from TiO2, Cu-TiO2, and Pc-Cu-TiO2 H-NCs calcinated at different temperatures are shown in Fig. 1. Results show that anatase is the dominant phase in all the samples. However, the rutile primary peak at 2θ= 27.3° was also observed in Cu-TiO2 sample calcined at 400 °C and 600 °C, and, in Pc-Cu-TiO2 H-NCs calcined at 600 °C . Copper species are believed to leave the lattice in small quantities and form nanoclusters which cannot be detected by the XRD analysis .
In addition, taking the Hume-Rothery law and the ionic radii of copper (86.2 pm) and titanium (74.6 pm) into closer consideration, if the ionic radius difference is less than 15%, no new phase is formed . Here, copper and titanium have nearly identical ionic radii. Hence, copper is most likely dissolved in the crystalline lattice of titania and does not form a new copper-based phase. The formation of copper solid solution in titania can be realized by comparing the X-ray diffraction patterns of the doped sample and pure titanium dioxide (Fig. 1a). As is seen, CuO peaks are visible at 2θ= 35.6o and 2θ=38.73o , and as the calcination temperature increases, the peak intensity of copper oxide increases as well .
For the Pc-Cu-TiO2 H-NCs sample calcined at 500 °C, the diffraction peaks of the anatase phase remained unchanged, indicating that the size and crystalline phase of the titania matrix did not change with the addition of phthalocyanine to the Cu-TiO2 NPs. Peaks corresponding to the phthalocyanine have not been identified in XRD patterns, which may be due to the low concentration of Pc that is not detectable by XRD analysis . The crystallite size of undoped TiO2, Cu-TiO2, and Pc-Cu-TiO2 H-NCs at different calcination temperatures, calculated using the Scherer equation , are shown in Table 1.
The crystallite size of the samples was estimated from XRD peak broadening using the Scherrer equation .
where t is the crystallite size, the wavelength of X-ray radiation (CuK), the Bragg angle, and the full width at half maximum (FWHM) of the most intense diffraction peak.
The crystallite size of the Cu-TiO2 sample is smaller than that of Pc-Cu-TiO2 H-NCs. The presence of copper in the titania matrix as a solid solution prevents the growth of TiO2 crystals . However, in the case of Pc-Cu-TiO2 H-NCs, the addition of phthalocyanine results in the expansion of the titania lattice . According to Table 1, the particle size also increased with increasing the calcination temperature.
TG/DTA curves of the precursors of the Cu-doped titanium dioxide and Pc-Cu-TiO2 H-NC are shown in Figs. 2a and 2b, respectively. The endothermic peak at 100 °C is due to the water absorption and alcohol elimination that leads to a weight loss of 7.77% . The two exothermic peaks at 180 °C and 220 °C are related to the decomposition of organic matters and the removal of -OH groups. The broad exothermic peak at 468 °C can be attributed to the burning of organic compounds through changing the amorphous phase to the anatase phase .
The volume reduction is about 32%, which is demonstrated in the TG curve over the temperature range of 100-500 °C. The anatase to rutile phase transformation takes place at temperatures between 450 and 800 °C. The transition temperature depends on the type of the precursor and the conditions of the nanoparticles preparation .
The TGA curve in Fig. 2b demonstrates three weight losses of 14%, 20%, and 36.9% at 98, 250, and 372 °C, respectively. The DTA curve shows an exothermic peak at temperatures between 310 and 400 °C, which corresponds to the formation of the anatase phase. It should be noted that the exothermic peak of the anatase generally occurs at 460 °C; however, the transformation has occurred below 460 °C due to the presence of copper .
The exothermic peaks in the range of 250 to 400 °C are attributed to the oxidation and burning of organic compounds in the gel . The broad peak in the case of Pc-Cu-TiO2 H-NC is attributed to the crystallization of amorphous TiO2 and the release of energy over this temperature range. Copper doping also decreased the anatase transformation temperature . In general, exothermic peaks at low temperatures can be related to the decomposition and oxidation of acetate chelate, i.e. the cleavage of Ti-O bonds in (TiO2)n-O-CO-CH3, while those at high temperatures can be due to the oxidation of the alkoxide, i.e. the cleavage of Ti-O bonds in (TiO2)n-O-C4H9 . The peak at 510°C corresponds to the pyrolysis of the phthalocyanine .
SEM images of Cu-TiO2 and Pc-Cu-TiO2 H-NC are presented in Fig. 3. The Cu-TiO2 sample consists of a mixture of spherical and rod-shaped agglomerates (Fig. 3a). The Pc-Cu-TiO2 nanocomposite shows a rod-like morphology (Fig. 3b), but the rod agglomeration degree and its aspect ratio (length to diameter) have increased.
TEM micrographs of Cu-TiO2 and Pc-Cu-TiO2 H-NC samples are shown in Fig. 4. The Cu-TiO2 sample is composed of spherical particles with a diameter of 30 nm (Fig. 4a). Pc-Cu-TiO2 H-NC sample is made of nanorods with a diameter of 25 nm and a length of 100 nm (Fig. 4b).
The results of FTIR analysis of Cu-TiO2 and Pc-Cu-TiO2 H-NC samples are shown in Figs. 5a and 5b, respectively. The absorption peaks at 1628 cm-1 and 3431 cm-1 are attributed to the bending and stretching vibrations of O-H, respectively . The adsorption peak at 1384 cm-1 for Cu-TiO2 and Pc-Cu- TiO2 H-NC may be related to the formation of copper oxide during calcination. Nitrate groups are removed during the calcination process, so this absorption cannot be due to the presence of nitrate groups. The small absorption band at 2390 cm-1 is related to the CO2 from the atmosphere . The absorption bands in the range of 900 to 400 cm-1 are attributed to Ti-O stretching vibration . The absorption peak at 3860 cm-1 is due to the chemisorbed vibrations of water . Fig. 5c shows the results of FTIR of the phthalocyanine sample. The absorption peaks in the range of 1600 to 1300 cm-1 are related to the stretching and bending vibrations of the phthalocyanine ligands . The absorption peaks at 1495, 1443, and 1326 cm-1 are due to the C=C and C=N skeletal vibrations of the porphyrin rings, while other bands are attributed to CH deformation as well as C=C stretching and bending modes in the pyrrole ring .
UV-visible spectroscopy curves for Cu-TiO2, Pc-Cu-TiO2 H-NC, and phthalocyanine are shown in Fig. 6. The results show that Cu-TiO2 nanoparticles are capable of absorbing light at longer wavelengths and have a broader absorption spectrum in comparison to pure titanium dioxide. The adsorption edge of the Pc-Cu-TiO2 H-NC is significantly shifted to the visible region, which is higher than that of the pure Cu- TiO2 and TiO2 NPs. The UV-Vis absorption spectroscopy for phthalocyanine shows π-π electron transfer of the aromatic ring and orbitals linked to the central atom of the metal .
The pristine TiO2 at anatase phase shows an intense absorption in the UV region(around 350 nm). While the Cu-doped sample indicated a red shift extending up to 600 nm. This is attributed to the characteristic surface plasmon resonance of Cu nanoparticles. The successful doping of Cu is also evident from the change in the color observed in the samples, shifting from pure white to light yellow. The band gap values of pristine TiO2 at anatase and rutile phases were observed to be 3.17 and 3.03 eV, respectively. The Cu-doped sample showed a significant dip in the band gap value up to 2.8 eV. The Tauc method was used to calculate the band gap of the samples .
Fig. 6 (c) shows an absorption in the UV region known as the Soret band (B), which is composed of three absorption peaks and two shoulders. A broad absorption band is located in the ultraviolet region before the UV absorption of the phthalocyanine molecule. Other well-known bands of the phthalocyanine molecule are the Q-band that appears in the 550–570 nm range. The high-energy Q-band peak shows the first π-π transition and the low-energy Q-band peak represents the second π-π transition of phthalocyanine. In the energetic region of the Soret band, which is close to 288 nm, there are many differences (could not understand) in the absorption spectrum of phthalocyanine. It can be attributed to the presence of a “d- band” attached to the central metal atom .
The hybrid nanocompound of phthalocyanine-copper-titania (Pc-Cu-TiO2 H-NC) was synthesized by the sol-gel method. Their phase structure, morphology, thermal and optical properties were studied and compared to those of pure and copper-doped titania. It was found that Cu formed a solid solution in titania while Pc-Cu-TiO2 formed an alloy. The anatase-rutile phase transformation was shown to be dependent upon the calcination temperature and the addition of dopants or alloying elements. A rod-like morphology with a particle size of 15-20 nm and different agglomeration tendency in Cu-TiO2 and Pc-Cu-TiO2 samples was illustrated. A red-shift in the absorption edge was observed in Pc-Cu-TiO2 sample as compared to pure TiO2 nanoparticles.
Authors acknowledge the support by the Institute for Color Science and Technology.
CONFLICTS OF INTEREST
The authors announce that there are no conflicts of interest.