Structural and photocatalytic properties of co-doped hybrid ZrO2–TiO2 photocatalysts
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In this study, pure TiO2, ZrO2, and hybrid ZrO2–TiO2 photocatalysts were synthesized through solgel process and calcined at three different temperatures. The synthesized photocatalysts were characterized using powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FESEM), Brunauer–Emmet–Teller (BET), ultraviolet–visible (UV–Vis) spectrometer, and photoluminescence (PL) spectrometer. The PXRD patterns show that the rutile phase of TiO2 was suppressed through co-doping with ZrO2 and produced small crystallite size. The hybrid photocatalysts with small crystallite size recorded the highest surface area of 114.7 m2/g compared to pure TiO2 and ZrO2 photocatalysts as confirmed by BET analysis. Irregular size and shape was observed in the hybrid photocatalysts compared to spherical shape and size in TiO2 and flaky shape in ZrO2 as shown by the FESEM images. The optical properties of the photocatalysts investigated using UV–Vis spectroscopy showed a decrease in band gap energy of pure TiO2 through linear extrapolation from the Tauc’s plot despite the slightly higher band gap energy of the hybrid photocatalysts. However, PL analysis showed that doping of ZrO2 into TiO2 increased the separation efficiency of the electron–hole pairs and enhanced the photocatalytic activity. The phenol degradation of the hybrid ZrO2–TiO2 photocatalysts was higher compared to those of the pure TiO2 and ZrO2.
KeywordsSolgel Hybrid TiO2–ZrO2 photocatalysts Phenol degradation
Traditional photocatalyst such as titanium dioxide or titania (TiO2)  with metastable state structure of anatase has been broadly used for the photodegradation of organic pollutants in water and air owing to its low cost , environmental friendliness , excellent oxidative properties, long-term stability without secondary pollution [4, 5], quick oxidation, high photocatalytic activity, chemical stability, and titania nontoxicity . Band gap energy around 3.2 eV for anatase TiO2 makes this photocatalyst can only be activated under UV light irradiation despite attempts made to study photocatalytic activity of TiO2 under irradiation of visible light. The anatase phase in TiO2 was reported to transfer to the rutile phase and reduced the band gap to around 3.0 eV at calcination temperature above 650 °C. This offers many advantages such as absorbing small quantity of the solar spectrum, from transparent to incoming light . Nevertheless, the intrinsic limitation of TiO2 in terms of the recombination of the large amount of the photoactivated electrons and holes is still a main challenge to be addressed to further improve the quantum yield of the photocatalytic activity . Among the advanced oxidation processes (AOP), heterogeneous photocatalysis is eminently used due to its environmentally friendly recognition and high oxidation efficiency [8, 9, 10].
Until now, the photocatalytic performance of TiO2 is still widely investigated. The cytotoxicity assessment using UV/TiO2-based degradation system for anthraquinone Reactive Blue 19 (RB-19) showed less toxic nature of the transformed by-products of RB-19 . Previously, the same researcher studied the TiO2-assisted Reactive Black 5 (RB-5) degradation and disclosed that the toxicity of RB-5 reduced significantly after photocatalytic treatment . The TiO2/UV-assisted Rhodamine B degradation was reported to eliminate the toxicity of recalcitrant compounds and textile wastewater effluents  and later tested on Rhodamine 6G . Urchin-like and yolk–shell TiO2 microspheres synthesized using solgel for degradation of methylthionine chloride displayed better photocatalytic activity than that of the commercial P25 . The optimization of old synthesis of TiO2 nanoparticles to degrade methyl orange and bromothymol blue resulted in the best performance of TiO2 nanoparticles of molar C12H28O4Ti/CO(NH2)2 in a ratio of 2:1 at 50 °C . Also, graphene–TiO2 (GT) nanocomposites for photocatalytic degradation of methylene blue (MB) showed that GT-8wt% exhibited the best photocatalytic activity toward the photocatalytic degradation of MB . Despite the improved photocatalytic activity reported, there are many other challenges such as controlling the particle size, homogeneity, and monodisperse ability of TiO2 which can affect the surface area of TiO2 and reduce the photocatalytic activity .
TiO2 properties can be enhanced by adding another metal oxide . The second metal oxide introduction, such as ZrO2, SiO2, La2O3, and Fe2O3, can generate new crystallographic stages with rather diverse properties than the original oxides and has proven to be a successful method to enhance the thermal stabilization of TiO2 [4, 5, 13] and UV light photocatalytic activity [14, 15]. The particle size of TiO2 can be reduced by adding a small quantity of ZrO2 into TiO2 owing to the different nuclei and coordination geometry. This has in turn increased the surface area of the photocatalyst  and acid–base properties . However, the properties of photocatalyst particularly rely on the synthesis methods plus the way of processing. Thus, selecting the most proper technique for photocatalyst preparation is vital to achieve the desired chemical purity, phase, and morphology .
To improve the photocatalytic properties of TiO2, a small quantity of ZrO2 can be used for co-doping purpose. Even though ZrO2 is considered as a poor photocatalyst due to its wide bad gap around 5 eV, doping of ZrO2 into TiO2 was reported to boost the photocatalytic practicality of advanced ZrO2–TiO2 mixed oxides . TiO2 doped by ZrO2 was studied by many researchers [4, 7, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27] in the form of photocatalysts or thin film. These hybrid photocatalysts were prepared using various methods including solgel, polymer gel templating, homogeneous precipitation, and hydrothermal. Due to its higher photocatalytic activity compared to pure TiO2 , the hybrid of ZrO2 and TiO2 has been widely investigated in the photocatalysis field by many researchers.
To the best of our knowledge, this paper is the first to study the nonporous or macroporous hybrid ZrO2–TiO2 photocatalysts for phenol degradation. Thus, this study attempts to produce hybrid ZrO2–TiO2 photocatalysts with enhanced photocatalytic activity. The influence of modified solgel method through sol evaporation at superheated temperature on the photocatalysts properties was analyzed using powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FESEM), Brunauer–Emmet–Teller (BET), ultraviolet–visible (UV–Vis) spectrometer, and photoluminescence (PL) spectrometer. The performance of the hybrid photocatalysts to enhance the photocatalytic activity was determined through phenol degradation and analyzed using high-performance liquid chromatography (HPLC).
2 Experimental procedures
Titanium(IV) isopropoxide (TTIP, 97%) and zirconium(IV) propoxide solution (TPZ, 70 wt% in 1-propanol) purchased from Sigma-Aldrich were used as the precursors for TiO2 and ZrO2. 2-Propanol anhydrous (99.5%), nitric acid (70%), and phenol (GR for analysis) were purchased from Sigma-Aldrich, RCl Labscan Limited, and Merck, respectively. All chemical reagents were used without further purification. Deionized (DI) water was used to prepare all photocatalysts as well as to dilute the phenol solution.
2.2 Synthesis of photocatalysts
TiO2 photocatalysts were synthesized using solgel method as described earlier . TiO2 was prepared by mixing 10 mL of TTIP with 90 mL of IPA. The mixture was then added dropwise into 900 mL of DI water that was maintained at pH 1.5 using nitric acid by means of portable pH meter (HQ11d, HACH), and the mixture was mechanically stirred using a magnetic stirrer. The reaction tub temperature was kept approximately at 2 °C during the mixing process using crushed ice. The mixture was stirred vigorously for 20 h at ambient temperature which later formed a colloidal suspension known as sol. The sol was later aged for 24 h before being evaporated using a hot plate at a superheated temperature of 200 °C until the sol transformed into gel. The gel layer continued to evaporate to form ZrO2–TiO2 powder (photocatalysts). The photocatalysts obtained were dried at 105 °C for 4 h followed by calcination at 500, 600, and 700 °C in a furnace (Nabertherm GmbH) with air flowing continuously for 3 h. Calcination at different temperatures was carried out to observe the stability of each photocatalyst especially on the effect of temperature on the transformation of TiO2 anatase phase into rutile phase as studied earlier . The dried photocatalysts were then ground into fine powder prior to testing. The hybrid ZrO2–TiO2 (1:1) photocatalysts were also prepared following the same experimental condition.
2.3 Characterization of photocatalysts
PXRD (D/max rB 12 kW, Rigaku @ D5000, Siemens) equipped with nickel-filtered copper Kα radiation (λ = 1.54056 Å) operated at 30 mA and 40 kV was used to confirm the crystal structure of the photocatalysts. The measurement was executed by monitoring the diffraction angle 2θ in the range of 5°–60° with a step increment of 0.05°. The photocatalysts powder was fitted into 20 × 20 × 0.5 mm sample holder for testing purpose.
The micrographs of the photocatalysts were obtained using Zeiss FESEM Crossbeam 340 instrument to investigate the surface morphology of the photocatalysts. The powder samples were spread evenly over carbon tape used as substrate and coated with a thin layer of gold prior to analysis. The surface area, pore volume, and pore size of the photocatalysts were determined using Thermo Scientific surface analyzer. The BET method was used to obtain the surface area and pore volume. Meanwhile, Barrett–Joyner–Halenda (BJH) model was used to determine the pore size of the photocatalysts derived from the adsorption branch of the isotherm. The measurement was performed by the N2 adsorption isotherm at 77 K. Prior to analysis, the samples were degassed at 200 °C for 2 h.
The optical band gap energy Eg was obtained from the (F(R) · hv)n versus hv plot which is also known as Tauc’s plot. The values of n = 2 for direct allowed transition and n = 1/2 for an indirect allowed transition were employed in this study for comparison purpose. The fluorescence emission spectrum of the photocatalysts was obtained using Perkin Elmer LS55 fluorescence spectrometer. The phenol concentration and the intermediate compounds produced during its degradation were quantitatively analyzed using HPLC system (Agilent Technologies 1220 Infinity LC).
2.4 Photocatalytic activity evaluation
3 Results and discussion
3.1 Effect on crystal structure
For photocatalysts calcined at 600 °C as shown in Fig. 1b, pure TiO2 showed mixed anatase–rutile phase with anatase dominated. However, slightly significant rutile peaks emerged when the calcination temperature was increased. The hybrid ZrO2–TiO2 presented a mixed zirconia–anatase phase with diffraction peaks of ZrO2 and TiO2 indexed in ICCD DB card no. 01-079-1796 and 01-075-2553, respectively, with dominant orthorhombic zirconia phase surpassed over anatase phase. No rutile phase was observed. This shows that the addition of ZrO2 stabilized the anatase titania phase . Pure ZrO2 showed mixed diffraction peaks of tetragonal–monoclinic phase with ICCD DB card no. 01-079-1765 and 01-078-0047, respectively. The tetragonal phase showed an intense peak at peak (101) compared to the monoclinic phase although the RIR values showed that 95 wt% of the content was dominated by monoclinic phase. The hybrid ZrO2–TiO2 displayed a mixed orthorhombic–anatase phase with ICCD DB card no. 01-079-1796 and 01-075-2553, respectively. An intense orthorhombic peak was observed at peak 2θ = (111).
When calcined at 700 °C, as shown in Fig. 1c, TiO2 exhibited a mixed rutile–anatase phase with intensity of rutile peaks observed throughout the XRD pattern indicating the transfer of anatase phase to rutile phase at temperature above 650 °C . A highly intense peak (110) of rutile phase was observed compared to peak (101) of anatase phase. The results obtained are in line with the RIR values which recorded that 59.8 wt% of the content is in rutile phase. Pure ZrO2 exhibited a mixed baddeleyite–tazheranite phase as indexed in ICCD DB card no. 01-075-9454 and 01-072-7115, respectively, with intensity in the baddeleyite peak observed at peak (− 111). The baddeleyite phase recorded 81.2 wt% of the RIR value. The hybrid ZrO2–TiO2 showed a zirconia-rich composition with intense ZrO2 diffraction peaks in zirconia (nanocrystalline) at peak (101) indexed in ICCD DB card no. 01-070-6627 and strong peak (− 111) of monoclinic phase indexed in ICCD DB card no. 01-078-0047, together with TiO2 in anatase phase indexed in ICCD DB card no. 01-071-1168. The RIR values recorded that 40.4 wt% of the contents is in the zirconia (nanocrystalline) phase.
Crystallite size of photocatalysts (based on most intense peak) calcined at 500–700 °C
Crystallite size (nm)
Surface area (m2/g)
Pore volume (cm3/g)
Pore radius (nm)
3.2 Effect on optical band gap
The small changes in the Eg values of the hybrid ZrO2–TiO2 compared to that of pure TiO2 for both transitions at different calcination temperatures suggest that the thermal stability of TiO2 improved through ZrO2 doping. Based on the Eg values, both photocatalysts calcined at 500 °C showed better results. Therefore, the present work was further tested for oxidative photodegradation of phenol. The doping of ZrO2 into TiO2 could increase the separation efficiency of the electron–hole pairs and enhance the photocatalytic activity. The fluorescence intensity with higher value from the PL spectra indicates more recombination of electron–hole pairs which leads to lower photocatalytic activity .
3.3 Effect on photocatalytic activity
Careful observation on phenol degradation by pure ZrO2 showed that ZrO2 can also act as a photocatalysts following the reduction of the intermediate compounds. The same result was observed elsewhere for phenol degradation at 25 ppm . This was probably due to the lower Eg values of the prepared ZrO2 at around 3.15–3.17 eV. ZrO2 was reported to have the Eg values between 3.25 and 5.1 eV, depending on the sample preparation technique . The observed photodegradation activity for phenol follows this order: hybrid ZrO2–TiO2 photocatalysts > pure TiO2 photocatalysts > pure ZrO2 photocatalysts. The increased photocatalytic activity was probably due to the presence of more hydroxyl groups on the photocatalysts surface that inhibited the recombination of electron–hole pairs by trapping holes and generated powerful oxidants such as OH· radicals [7, 22].
Similarly, the separation of electron and hole between TiO2 and ZrO2 in the hybrid photocatalysts may also take place due to the energy level in both valence band (VB) and conduction band (CB) of the pure TiO2 that corresponds well within the band gap of pure ZrO2. During the excitation of electrons from the hybrid photocatalysts, most electrons from the CB of ZrO2 easily transfer to the CB of TiO2 from thermodynamic considerations, which inhibits the electron–hole pairs recombination . The results did not represent total degradation, but serve as an indicator of potentially effective photocatalysts.
In this work, hybrid ZrO2–TiO2 photocatalysts synthesized using solgel method showed higher phenol degradation than those of the pure TiO2 and ZrO2 after being exposed to UV light. This is due to the small crystallite size and higher surface area exhibited by the hybrid ZrO2–TiO2 photocatalysts. The shape and size of the hybrid ZrO2–TiO2 photocatalysts also impacted the photocatalytic activity. The reduction of intermediate compounds at the end of the degradation process indicates the hybrid ZrO2–TiO2 photocatalysts potential to remove phenol in wastewater. In general, co-doping of ZrO2 into the TiO2 lattice could address the low photocatalytic activity of a single metal oxide photocatalyst such as TiO2 and suppressed the electron–hole pairs recombination to improve the photocatalysts performance.
This work was financially supported by the Universiti Teknologi Malaysia (Project No. R.J090301.7809.4J195) under the Research University Grant and Ministry of Higher Education Malaysia.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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