Title Science and Technology Indonesia e-ISSN:2580-4391 p-ISSN:2580-4405 Vol. 7, No. 1, January 2022 Research Paper Effect of Synthesis Methods on Properties of Copper Oxide Doped Titanium Dioxide Photocatalyst in Dye Photodegradation of Rhodamine B Cheng Yee Leong1, Hao Lin Teh1, Man Ching Chen1, Siew Ling Lee1,2* 1Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia 2Centre for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia, 81300 Johor Bahru, Malaysia *Corresponding author: lsling@utm.my Abstract Copperoxidemodifiedtitaniumdioxidephotocatalystshavebeenwidelyreportedfortheirexcellentperformanceinthewastewater treatment. However, there is lack of information on the effect of different synthesis methods towards the properties and catalytic activity of the photocatalyst. In this research, a series of copper oxide doped titanium dioxide (Cu TiO2) photocatalysts were syn- thesized via three different methods of sonochemical, impregnation and physical mixing. Cu TiO2 of varied molar ratios of Cu dopant to TiO2 TR595 (1:99, 2:98, 3:97 and 4:96) were prepared. Comparison of physical-chemical properties and photocatalytic activity among the synthesized samples and unmodified TiO2 TR595 were made. X-ray diffraction analysis depicted the formation of TiO2 rutile phase in all samples. Besides, diffuse reflectance UV-visible analysis proved that the synthesized samples were active undervisible lightregion. AccordingtotheTaucplotandphotoluminescencespectra, thebandgapenergiesandrecombinationrate of electron-hole pairs of Cu TiO2 samples decreased upon loading of Cu. Moreover, EDX analysis confirmed the existence of Ti and Cu in all the samples. The photocatalytic efficiencies of the synthesized samples were discovered through photodegradation of RhodamineBorganicdye under6hoursofvisible light irradiation. Amongst, Cu TiO2 photocatalystssynthesizedviasonochemical method with molar ratio of 2:98 produced the highest photocatalytic activity of 65% which attributed to the lowest recombination rate of photogenerated charge carriers and availability of large number of reactive oxidative species. Keywords Copper Oxide Doped Titanium Dioxide, Photocatalyst, Sonochemical, Impregnation, Physical Mixing, Rhodamine B Received: 16 September 2021, Accepted: 3 December 2021 https://doi.org/10.26554/sti.2022.7.1.91-97 1. INTRODUCTION Titanium dioxide (TiO2) and its composites have excellent chemical stability and corrosion-resistance ability with favour able mechanical performance. There are three phases in TiO2 which are anatase, rutile, and brookite. The activation of TiO2 happens under ultraviolet light due to its wide bandgap of 3.2 eV for photoactivity. In other words, TiO2 has limited photo- catalytic activity under visible light. Apart from that, the high electron–hole pairs recombination rate of TiO2 has reduced its quantum e�ciency (Leong et al., 2021) . As a result, modi�- cation of TiO2 has been conducted using di�erent metals or metal oxides to broaden its absorption spectra to visible light region (Koh et al., 2017; Koh et al., 2020; Ooi et al., 2020; Ooi et al., 2016). Properties of TiO2 could be improved by doping of Cu compounds due to their high conductivity and low toxicity (Isa et al., 2020; Sabran et al., 2019). More im- portantly, Cu signi�cantly extends the light response of TiO2 into the visible region in solar energy area and the existence of Cu metal on TiO2 could obviously in�uence the particle size as well as oxygen number or intermediate species on TiO2 surface (Zuas and Budiman, 2013) . The suitable amount of the addition of Cu dopant was able to cause electron trapping and suppress the electron-hole recombination, thus improving the photocatalytic degradation rates greatly compared with bare TiO2 (Neena et al., 2018) . Besides, Cu-doping into/onto TiO2 decreased the band gap signi�cantly to increase the photoac- tivity rate in visible light region. As a result, the absorption capacity for aromatic organic pollutants enhanced drastically after the doping of Cu to TiO2. Various methods have been applied for the synthesis of Cu TiO2 photocatalysts. Sonochemical method could be con- ducted via an ultrasonic bath or by using a probe type ultrasonic homogenizer to obtain the desired e�ects from ultrasonication including homogenization, extraction, deagglomeration, dis- persing, emulsi�cation and disintegration (Sahrin et al., 2020) . On the other hand, impregnation method involved procedure https://crossmark.crossref.org/dialog/?doi=10.26554/sti.2022.7.1.91-97&domain=pdf https://doi.org/10.26554/sti.2022.7.1.91-97 Teh et. al. Science and Technology Indonesia, 7 (2022) 91-97 whereby active precursor that contained within a certain vol- ume of solution was contacted with the solid support, which then the mixture was dried to remove the imbibed solvent (De- raz, 2018) . The impregnation method was considered a fast and inexpensive approach. More importantly, it allowed de- termination of con�guration, crystallography and morphology of TiO2 �nal products to be controllable in advance (Deraz, 2018) . Physical mixing method is also known as mechanical mixing method. It is an alternate, simple and cost-e�ective syn- thesis method that enables formation of composites in better hybrid structures that would having chemical and mechanical advantageous properties for industrial production (Megha et al., 2018) . The utilization of this method in Cu TiO2 photocat- alyst synthesis for dye photodegradation aspect is yet to be known. In this work, di�erent synthesis methods of sonochemi- cal, impregnation and physical mixing methods were applied to synthesize Cu TiO2 photocatalysts. The physicochemi- cal properties and photocatalytic performance of the resulting photocatalysts were examined. The direct comparison would be important to understand the e�ect of the synthesis meth- ods on the properties and photocatalytic performance of these titania-based catalysts. 2. EXPERIMENTAL SECTION 2.1 Photocatalysts Preparation A series of CuO doped TiO2 photocatalysts were prepared via sonochemical method. For this purpose, appropriate amount of pure rutile TiO2 TR595 powder (≥ 99.0 %) was used as starting precursor. The TiO2 precursor was added into a beaker together with the presence of 2.5 mL propylene glycol (Merck, ≥ 99.0 %) and 26.5 mL water. After that, the mixture was stirred for 5 minutes. Sodium hydroxide (Merck, ≥ 99.0 %) was then added dropwise to the mixture until about pH 8-9 of solution was obtained. Again, the reaction mixture was stirred for another 5 minutes. Next, appropriate amount of copper(II) nitrate trihydrate (Merck, ≥ 99.0 %) was added to the mixture and stirred for 15 minutes. The molar ratio of TiO2 to Cu dopant was set at 98:2. After that, the ultrasonication process was carried out for a duration of 15 minutes at 91W. Eventually, the precipitate was dried in an oven to obtain the CuO doped TiO2 powder. Meanwhile, a solution of copper precursors was prepared for capillary impregnation method. This was done by dissolv- ing copper(II) nitrate trihydrate in 50 mL of distilled water. After that, the solution was stirred for 30 minutes at room temperature. Subsequently, appropriate amount of pure rutile TiO2 TR595 powder was added to the solution. It was noted that the molar ratio of TiO2 to Cu dopant was �xed at 98:2. The reaction mixture was then stirred for another 2 hours. At last, the sample obtained was dried in an oven. The physical mixing method was conducted with the direct application of copper(II) nitrate trihydrate and pure rutile TiO2 TR595 without any further treatment. The samples were mechanically mixed in a beaker for 2 hours. The molar ratio of TiO2 to copper(II) nitrate trihydrate was set as 98:2. After �nishing the mechanical mixing process using magnetic stirrer, the reaction mixture was dried in an oven. Samples of di�erent TiO2 to Cu molar ratios of 99:1, 97:3 and 96:4 were also prepared via the three methods mentioned above. All the samples prepared were denoted at x Cu:TiO2, x = molar ratio of Cu to TiO2. 2.2 Characterizations The crystalline phases and crystal structures of the samples were examined with the technique X-ray Di�ractometry (XRD). Bruker Advance D8 was applied with Cu Kα irradiation (λ = 0.15406 nm, 40 KV, 40 mA) and the samples were scanned in the range 2θ = 2 – 80o with step size 0.05o/s to obtain the XRD patterns. UV-Vis spectrometer (Perkin- Elmer Lambda 35) equipped with integration sphere (BaSO4 coated 76 mm) was applied to study the optical properties of the samples. Ap- propriate amount of sample was placed on the sample holder and scanned at wavelength ranged from 200 to 800 nm to collect the spectrum. Tauc plot was used to determine the band gap energy of the samples. It was plotted as (αhν)2 against hν, where α was the di�use re�ectance spectra absorbance, h was the Planck constant and ν was the frequency of light. Photoluminescence spectroscopy ( JASCO, FG-8500) with an excitation of 310 nm was used to study the rate of electron-hole recombination of the samples. The morphology of the selected samples was con�rmed with Field Emission Scanning Electron Microscopy (FESEM). Field emission scanning electron mi- croscope (Hitachi SU8020) coupled with EDX analyser was utilized to scan the samples in order to produce high quality images and to con�rm the elements present in the selected samples. 2.3 Photocatalytic Testing Photocatalytic testing was conducted by using 50 mL 15 ppm Rhodamine B and 0.05 g samples and irradiated under 15 W LED light source for 6 hours. Prior to reaction, adsorption was carried out to ensure the decreased concentration of Rho- damine B was attributed solely to photodegradation. The pho- tocatalytic performance of each synthesized Cu TiO2 sample was calculated according to the formula below: Photodegradation e�ciency(%)= C0 −Ct C0 x100% (1) where, C0 = Concentration of Rhodamine B before reaction (ppm), Ct = Concentration of Rhodamine B after reaction (ppm) 3. RESULTS AND DISCUSSION 3.1 Structural Properties Figure 1 displays the XRD di�raction patterns of Cu TiO2 samples with varied molar ratios which were synthesized via © 2022 The Authors. Page 92 of 97 Teh et. al. Science and Technology Indonesia, 7 (2022) 91-97 Figure 1. XRD Patterns of TiO2 and Synthesized Cu TiO2 Samples (1 to 4 mol% of Cu in TiO2) Via (a) Sonochemical Method, (b) Impregnation Method and (c) Physical Mixing Method; R = Rutile Phase di�erent methods. The XRD patterns were �tting well with the standard data of rutile TiO2 which referenced from the Joint Committee on Powder Di�raction Standard (JCPDS Card No. 21-1276). This could be due to the small amount of Cu dopant in the prepared samples. With the doping of Cu to the titania, it was noticed that either strong or weak peaks of the rutile phase still could be identi�ed easily as there were total 11 peaks originated from each Cu TiO2 synthesized sam- ple. The di�raction peaks at 27.40°, 36.10°, 39.20°, 41.33°, 44.00°, 54.30°, 56.70°, 62.70°, 64.00°, 69.00° and 69.80°, corresponding to (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), and (112) planes respectively, which de- noted the tetragonal structure of titania rutile phase (Phromma et al., 2020) . Deviation of peak position was not observed be- cause the Ti4+ substitutional sites were replaced and occupied by Cu2+ dopant ions instead of causing distortion of the lattice structure (Raguram and Rajni, 2019) . Furthermore, the ionic radius of Cu2+ (0.73 Å) was similar to Ti4+ ion (0.74 Å) which allowed the entry of Cu2+ ion to replace Ti on TiO2 (Isa et al., 2020) . The crystallite size of the synthesized Cu TiO2 samples was calculated using Scherrer equation at (1 1 0) plane and the results are listed in Table 1. Overall, the crystallite size of Cu doped samples was smaller than that of TiO2. Upon modi�ca- tion with addition of Cu dopant, the crystallite size of samples decreased to 47.55 nm for Cu TiO2 with molar ratio 2:98 but slightly increased after more Cu doping. The Cu doping successfully reduced the crystallite size of TiO2 by inhibiting the TiO2 crystallite growth. Apart from that, the increase in the Cu doping ratio enabled the increase in peak intensity which enhanced the crystallinity of doped TiO2 TR595. The state- Figure 2. DR-UV-Vis Spectra of TiO2 and The Synthesized Cu TiO2 Samples Via (a) Sonochemical Method, (b) Impregnation Method and (c) Physical Mixing Method in The Range of 250 – 450 nm ment was in accordance with the slight increase in crystallite size obtained when the concentration ratio of Cu dopant in- creased and surpassed the optimum level (molar ratio 2:98). Similar results can be referred from another previous report that mentioned that the crystallite size of the samples increased slightly upon addition of Cu (Rajamannan et al., 2014) . In short, the smaller crystallite size of TiO2 TR595 after doping with Cu in optimum level could increase the surface area as well as the number of active sites which in turn enhanced the transfer rate of surface charge carrier in the photocatalytic ac- tivity (Carrera-López and Castillo-Cervantes, 2012; Ooi et al., 2020). 3.2 Optical Properties The optical absorption characteristics of the synthesized sam- ples were performed by DR-UV-Vis analysis. The optical re�ectance spectra of Cu TiO2 at di�erent molar ratios which © 2022 The Authors. Page 93 of 97 Teh et. al. Science and Technology Indonesia, 7 (2022) 91-97 Table 1. Crystallite Size of TiO2 and The Synthesized Cu TiO2 Samples Methods Synthesized Sample Crystallite Size/nm TiO2 TR595 53.89 Sonochemical 1:99 Cu TiO2 50.52 2:98 Cu TiO2 47.55 3:97 Cu TiO2 50.52 4:96 Cu TiO2 53.89 Impregnation 1:99 Cu-TiO2 50.52 2:98 Cu TiO2 47.55 3:97 Cu TiO2 50.52 4:96 Cu TiO2 50.52 Physical Mixing 1:99 Cu TiO2 53.89 2:98 Cu TiO2 47.55 3:97 Cu TiO2 50.52 4:96 Cu TiO2 50.53 synthesized via di�erent methods are depicted in Figure 2 in the range of 250 – 450 nm. Based on Figure 2, there was a similar absorption peak at 345 nm for all the synthesized Cu TiO2 samples. Generally, the DR-UV-Vis spectra of typical TiO2 has strong absorption where its peak ranging from 200 - 400 nm. A broad peak can be observed from range of 550 - 800 nm except for TiO2. The broad absorption peak was due to the presence of Cu+ or Cu2+ ions which accompanied by the surface plasmon resonance e�ect (Gondal et al., 2013) . The optical absorption proper- ties were enhanced after Cu doping which caused red shift to happen as the absorption spectra of TiO2 were shifted towards longer wavelengths (Biru et al., 2021) . This could be explained as the shifting of absorption band towards visible light region after the incorporation of Cu into TiO2 TR595. The doping of Cu slightly decreased the band gap energy of TiO2 (2.98 eV) to around 2.94 eV for Cu TiO2 with molar ratios 1:99, 2:98, 3:97 and 4:96. The results obtained were matched with a previous report that claimed the greater reduc- tion in band gap energy of TiO2 with increasing concentration of dopant which caused by the reason of Cu-O interaction with covalent characteristic (Mathew et al., 2018) . Moreover, it can be deduced that the addition of Cu dopant caused the reduction of band gap energy from UV light region to visible light region. The visible light absorption can be attributed to the replacement of Ti4+ from TiO2 lattice by Cu 2+/Cu+ and caused the formation of mid gap energy levels in the synthe- sized samples along with formation of oxygen vacancies, hence narrowing the band gap value (Bharti et al., 2016) . 3.3 Photoluminescence Study Figure 3 illustrates photoluminescence spectra of the selected samples at the excitation wavelength of 310 nm. The lower intensity of photoluminescence spectrum, the lower density of the recombination centres in a material, leading to low recom- bination rate of electron-hole pairs. As noticed from Figure 3, Cu TiO2 with molar ratio 2:98 synthesized via sonochemical method has the lowest emission intensity while TiO2 TR595 has the highest emission intensity. A reduction of 48.59% in emission intensity was observed in Cu TiO2 synthesized by sonochemical method as compared with TiO2 TR595. This could be due to the improvement of charge separation by ultra- sonication through generation of more hydroxyl radicals which thus reducing the electron-hole recombination rate (Sivakumar et al., 2010) . A broad peak in spectrum was appeared for each Cu TiO2 sample which was similar as the peak location discovered in TiO2 TR595. The broad peaks were located at the range around 500 to 650 nm with the peaks of Cu TiO2 were situ- ated at approximately 550 nm. This was due to the occurrence of electrons self-trapping by octahedral shape of TiO6 (Alotaibi et al., 2020) . The results obtained were similar to another study using di�erent Cu-doping concentrations with TiO2 to get the photoluminescence emission spectra (Alotaibi et al., 2020) . Figure 3. Photoluminescence Spectra of (a) TiO2 TR595, (b) 2:98 Cu TiO2 Sonochemical, (c) 2:98 Cu TiO2 Impregnation and (d) 2:98 Cu TiO2 Physical Mixing Moreover, the addition of Cu dopant decreased the pho- toluminescence intensity e�ciently as compared with pure TiO2 TR595. It can be explained as e�ective inhibition of the photogenerated electron to be recombined from conduc- tion band to valance band of TiO2 (Reda et al., 2020) . Since Cu2+ ions were well doped into TiO2 structure, some unde- sired Cu–Cu interactions were acted as luminescent quencher to make the emission intensity to decline (Raguram and Ra- jni, 2019) . Cu TiO2 with molar ratio 2:98 synthesized via sonochemical method has the slowest electron-hole recombi- nation speed which revealed its top e�ectiveness in the highest reduction in photoluminescence peak intensity. 3.4 Morphology Study Figure 4 shows the FESEM images of the selected sample of 2:98 Cu TiO2 synthesized via sonochemical method. As observed in Figure 4 (a) and (b), it can be deduced that the © 2022 The Authors. Page 94 of 97 Teh et. al. Science and Technology Indonesia, 7 (2022) 91-97 Cu doping did not change the morphology much although the Cu2+ ions were incorporated into the TiO2 matrix (Raguram and Rajni, 2019) . The size range of TiO2 TR595 was between 130 nm and 250 nm. Meanwhile, the size of Cu TiO2 sample observed from the FESEM image was estimated in the range from 120 to 240 nm. Figure 4. FESEM Images of (a) TiO2 TR595, (b) 2:98 Cu TiO2 Sonochemical and (c) EDX Spectrum of 2:98 Cu TiO2 Sonochemical The size variation was possibly caused by the doping of Cu into some TiO2 surfaces which made the suppression of TiO2 by grain boundaries resistivity (Biru et al., 2021) . As a result, it was clearly seen that some Cu TiO2 particles were smaller than those did not covered by Cu dopant. Based on the EDX spectrum in Figure 4 (c), there was an obvious strong peak for element Ti of Cu TiO2 with molar ratio 98:2 which located at 4.5 keV as well as some weak peaks witnessed at 0.4 and 5.0 keV. Furthermore, the element O was found at peak 0.5 keV while the element Cu was noticed peaks approximately at 1.0, 8.0 and 9.0 keV for the respective sample. There was no peak of nitrate found in the spectrum where the nitrate came from the precursor Cu(NO3) 2.3H2O. Besides, there were three signi�cant compositions including 59.5 wt% of Ti, 38.8 wt% of O and 1.7 wt% of Cu that originated from the quantitative results of elements which made up total 100 wt%. 3.5 Photocatalytic Testing Photodegradation of Rhodamine B under visible light irra- diation was conducted to evaluate the photocatalytic perfor- mance of the synthesized Cu TiO2 samples. The obtained absorbance at 553 nm was used to calculate the photodegrada- tion e�ciency of the synthesized samples. As depicted in Table 2, Cu TiO2 with molar ratios of 1:99, 2:98, 3:97 and 4:96 have higher photodegradation per- formances as compared to unmodi�ed TiO2 TR595. The lowest photocatalytic activity of TiO2 TR595 was due to its large band gap values (2.98 eV) as shown in the DR UV-Vis analysis. As a result, the visible light uptake by TiO2TR595 par- ticles was very limited, leading to its low photocatalytic activity under visible light irradiation (Reda et al., 2020) . Therefore, the addition of Cu transition metal could en- hance the photocatalytic activity of TiO2. It can be explained as Cu dopant served as a trapping site for photogenerated electrons to increase the electron-hole pairs’ lifetime as well as raised the chance of reactions to produce reactive oxygen species (Kerkez and Boz, 2014) . In addition, the Cu dopant also served as an intermediate level of visible light electron excitation for the electron movement from valence band to conduction band after getting su�cient energy from visible light (Biru et al., 2021) . Table 2. Photodegradation E�ciency of TiO2 and The Synthe- sized Samples Methods Samples Photodegradation e�ciency (%) TiO2 TR595 54 Sonochemical 1:99 Cu TiO2 61 2:98 Cu TiO2 65 3:97 Cu TiO2 58 4:96 Cu TiO2 58 Impregnation 1:99 Cu TiO2 61 2:98 Cu TiO2 63 3:97 Cu TiO2 58 4:96 Cu TiO2 57 Physical Mixing 1:99 Cu TiO2 60 2:98 Cu TiO2 62 3:97 Cu TiO2 58 4:96 Cu TiO2 58 Noticeably, there was subsequent increase in the photocat- alytic activity when the amount of Cu dopant increased from 1 to 2 molar ratio in the samples. Interestingly, all Cu TiO2 samples with molar ratio of 2:98 showed the best photocatalytic activity of 65%, 63% and 62% in their series which synthesized via sonochemical, impregnation and physical mixing methods, respectively. This could be attributed to the smaller crystallite size and particle size of these samples as evidenced by the XRD and FESEM analyses. It is widely accepted that smaller particle size is crucial for a larger surface area of the sample in order to increase the number of active sites, thus leading to the highest photocatalytic e�ciency. The current �ndings suggested that more reactive oxygen species such as OH radicals would be produced through the sonochemical synthesis method. It is ex- pected that the ultrasonication process has increased e�ciency of charge separation and decreased the speed of electron-hole © 2022 The Authors. Page 95 of 97 Teh et. al. Science and Technology Indonesia, 7 (2022) 91-97 recombination (Sivakumar et al., 2010) . When the molar ratio of Cu increased above the optimum level (2 mol%), the continuous addition of Cu did not bene�cial to the photocatalytic activity of TiO2. The excess amount of Cu2+ ions were unable to di�use into lattice structure of TiO2, but only deposited on the surface area (Biru et al., 2021) . This led to the blockage of active sites of TiO2 which hindered the penetration of light reaching on TiO2 surface (Riaz et al., 2014) . The inhibition of photocatalytic activity happened since the number of photogenerated electrons and holes also declined. Apart from that, another report stated that when the metal loading kept on increasing, it caused the occurrence of metal particles to agglomerate and hence reduced the photocatalytic performance of the photocatalyst (Koh et al., 2017) . Thus, the reasons above explained the decrease in photodegradation percentages for Cu TiO2 of molar ratios 3:97 and 4:96. In short, the modi�cation of Cu dopant on TiO2 was able to enhance to photocatalytic activity but if only doped with the optimum amount without excess. 4. CONCLUSIONS CuO doped TiO2 photocatalysts of di�erent molar ratios were successfully synthesized via three di�erent methods includ- ing sonochemical, impregnation and physical mixing methods. The XRD analysis proved the presence of rutile TiO2 in the samples and smaller crystallite size of Cu TiO2 were obtained due to broadening e�ect after the incorporation of Cu into TiO2 matrix. Reduction of band gap energy with Cu dopants were observed under Tauc plots, lower recombination of elec- tron hole pairs was discovered under photoluminescence anal- ysis and reduction of particle size of Cu TiO2 were justi�ed via FESEM analysis. The formation of new energy level in Cu TiO2 allowed these CuO doped TiO2 samples to be active in visible region. Among all the samples, Cu TiO2 of molar ratio of 2:98 synthesized via sonochemical method achieved the best photocatalytic performance of 65% photodegradation of Rhodamine B under visible light irradiation. 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Page 97 of 97 INTRODUCTION EXPERIMENTAL SECTION Photocatalysts Preparation Characterizations Photocatalytic Testing RESULTS AND DISCUSSION Structural Properties Optical Properties Photoluminescence Study Morphology Study Photocatalytic Testing CONCLUSIONS ACKNOWLEDGEMENT