Role of heterojunction ZrTiO₄/ZrTi₂O₆/TiO₂ photocatalyst towards the degradation of paraquat dichloride and optimization study by Box–Behnken design

2.1 Synthesis of Zr/Ti photocatalyst

TiO₂ was synthesized using titanium isopropoxide (TTIP) obtained from Sigma Aldrich as precursor via modified sol gel method. Firstly, 1.76 g of PEG 2000 (Merck) was dissolved in 70 mL of ethanol (QRec). Next, 6.15 g of diethanolamine (Merck) was added to the solution followed by 12.00 g of TTIP. Subsequently, 1.60 mL of distilled water was added slowly and stirred continuously for another 2 h at room temperature (Ali and Hassan, 2008). Zirconium (IV) oxynitrate hydrate that has been dissolved in 10 mL of distilled water was then added into the clear solution. The mixture was stirred for 2 h to get homogenous photocatalyst. Then, it was aged at room temperature for 2 days and dried at 100 °C for 24 h before calcined at 750 °C for 5 h. Lastly, the sample was grinded to fine powder. The ratios were varied based on weight percentage from 10:90, 20:80 and 30:70.

2.2 Photocatalytic activity measurement

The photocatalytic degradation was conducted in a Pyrex beaker and was placed inside a box with 365 nm UV irradiation source (100 V, 6 W). Then, 0.1 g of Zr/Ti photocatalyst was added into 250 mL 15 ppm of paraquat dichloride solution. The suspension was stirred in the dark for the adsorption of paraquat dichloride for 30 min before irradiated for 240 min. The sample was collected every 60 min for 240 min irradiation. UV–Vis spectrophotometer Model UV-2501Pc Shimadzu was used to measure the samples. The percentage of photocatalytic activity was calculated using Eq. (1),

2.3 Characterization of the catalysts

X-ray diffraction analysis was done using Siemen D5000 diffractometer utilizing Cu Kα radiation (λ = 1.54060 Å). Data were collected in range of 2θ from 10° to 80° and analysed by software called Diffract Plus. XPS surface study was performed using Kratos surface analysis spectrometer XSAM HS instrument with Mg Kα (1253.6 eV) as the X-ray source. Transmission electron microscopy (TEM) was performed using JEOL-JEM 2100 operated at 200 kV. N₂ adsorption/desorption isotherm analysis was attained using Micromeritics ASAP 2010 volumetric adsorption analyser at −196 °C. Prior to the measurement, the calcined catalysts were degassed at 120 °C for 5 h. The DR UV–Vis spectra were measured by UV–vis spectroscopy model Perkin Elmer Lambda 550.

2.4 Response Surface Methodology (RSM)

The Response Surface Methodology (RSM) was used to validate the predicted value of the optimization parameter from RSM with actual value obtained from experiment. Design-Expert Version 7.1.6 software was used for generating and evaluating the statistical experimental design. To assess the interaction between parameters towards the degradation of paraquat dichloride (Y), three parameters were selected: calcination temperature (°C), ZrO₂/TiO₂ ratio and catalyst dosage (g). Total of 17 experiments were designed and performed in random order and levels of independent variables are displayed in Table 1.

3.1 Characterization of catalyst

Fig. 1 shows the diffractograms of XRD analysis for ZrO₂/TiO₂ photocatalyst at various ZrO₂/TiO₂ ratios. The XRD analysis showed the tetrahedral structure of anatase TiO₂ was displayed at diffraction peaks of 2θ = 25.3° (I₉₉), 48.0° (I₂₄), 37.6° (I₁₈), 53.9° (I₁₅) and 55.1 (I₁₅) in all samples. The percentage of anatase and rutile phases was calculated according to Eq. (2) and is tabulated in Table 2.

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Figure 1

XRD diffractogram patterns of (a) ZrO₂/TiO₂ photocatalyst and (b) zoom-in XRD diffractogram patterns of the anatase (1 0 1) diffraction peaks at various ZrO₂/TiO₂ ratios and were calcined at 750 °C for 5 h. The PDF referenced of Ti₂ZrO₆ (JCPDS 00-046-1265) and TiZrO₄ (JCPDS 00-007-0290).

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The presence of Zr as shown in Fig. 1(a) has delayed the phase transformation of TiO₂ at high temperature which was also reported by Kim et al. (2012). In addition, the new ternary species belong to orthorhombic structure of ZrTi₂O₆ (JCPDS 00-046-1265) at 2θ value of 30.71° (I₁₀₀) and second highest peak at 24.87° (I₇₂) was observed, while for ZrTiO₄ (JCPDS 00-007-0290) occurred at value 30.59° (I₁₀₀) and second highest peak at 24.71° (I₁₆) were also observed at ZrO₂/TiO₂ ratio of 20:80 and 30:70. Besides that, no diffraction peak due to ZrO₂ could be detected in all samples suggesting that the ZrO₂ might be formed in the bulk matrix of TiO₂ or was well dispersed in the catalyst surface and cannot be detected using XRD analysis.

The anatase diffraction peak (1 0 1) was slightly shifted to lower angle suggested that the Zr was merged in the lattice of TiO₂ as shown in Fig. 1(b) and affects the lattice parameter and d-spacing value for ZrO₂/TiO₂ photocatalyst as tabulated in Table 3. The enhancement of distance between the crystal planes was due to the substitution of Zr ion which has larger ionic radius (0.072 nm) compared to Ti ion (0.065 nm) as has also been reported by Kim et al. (2008) and Venkatachalam et al. (2007).

XPS analysis was conducted to measure the chemical composition and electronic state of ZrO₂/TiO₂ (20:80) calcined at 750 °C which showed the best photodegradation of paraquat dichloride. Fig. 2 shows the XPS spectra of Ti 2p, Zr 3d and O 1s that has been deconvoluted into components. From the Ti 2p_3/2 three peaks of Ti⁴⁺ (457.21, 458.31 and 459.45 eV) were obtained belonging to ZrTi₂O₆, TiO₂ and ZrTiO₄ respectively (NIST XPS database). Meanwhile for the Zr 3d_5/2 spectra, there are two peaks for Zr⁴⁺ (181.54 and 182.01 eV) which can be assigned to the ZrTi₂O₆ and ZrTiO₄ respectively and also obtained by Gnatyuk et al. (2010).

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Figure 2

XPS spectra of ZrO₂/TiO₂ (20:80) photocatalyst (a) Ti 2p and (b) Zr 3d and O 1s.

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The shifting in binding energy might be due to the electron transfer from Zr⁴⁺ to Ti⁴⁺ and formation of Ti–O–Zr species. Moreover, the binding energy of XPS O 1s composed of four signals at 529–532 eV corresponded to the lattice oxygen (Ti/Zr–O) and surface hydroxyl groups (Ti/Zr–OH) (Sun et al., 2011). The results obtained show that zirconium had undergone solid state reaction and forms ternary species and the remaining Zr was homogenously dispersed on TiO₂ surface or incorporated in TiO₂ lattice.

Fig. 3(a) shows the nanoparticles of ZrO₂/TiO₂ (20:80) photocatalyst were agglomerated with particle sizes ranging from 9 to 21 nm. HR-TEM image in Fig. 3(b) shows that the lattice fingers of anatase TiO₂ (1 0 1) were located close to ZrTiO₄ (1 1 1). Meanwhile, at upper part the rutile TiO₂ was detected near ZrTi₂O₆ that corresponding with (1 1 0) and (1 1 1) planes respectively. This confirmed the existence of heterostructure in the catalyst interface that could retard the recombination of electron-hole.

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Figure 3

(a) TEM and (b) HR-TEM images of ZrO₂/TiO₂ (20:80) photocatalyst calcined at 750 °C.

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Fig. 4 illustrates the N₂ adsorption–desorption isotherms of ZrO₂/TiO₂ photocatalysts that show adsorption of Type III at ratio of 10:90. Meanwhile, at ratio of 20:80 and 30:70 it shows the mixture of Type III and IV isotherms with the presence of mesoporous and macroporous materials and exhibited hysteresis loop of type H2(b) which associated with capillary condensation in ink bottle pores that have larger neck radius (Thommes et al., 2015). At the ratio of 10:90, the hysteresis loop was appeared at higher relative pressure indicating the weak interaction between adsorbate and adsorbent.

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Figure 4

Isotherm plots of ZrO₂/TiO₂ photocatalysts at various ratios and were calcined at 750 °C for 5 h.

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The largest specific surface area 46.35 m²/g was obtained at ratio 20:80 with average pore diameter of 177.21 Å which indicates higher adsorptibility compared to ratios 10:90 and 30:70 as listed in Table 3. At ratio 30:70 of ZrO₂/TiO₂, the ternary species were possibly deposited and covered the pore on the surface of photocatalyst leading to average pore diameter reduction and diminish the pore volume as proven by Wronski et al. (2015). This observation was supported by XRD diffractogram (Fig. 1). ZrO₂/TiO₂ with ratio of 20:80 showed significant increment of catalyst surface area from ratio 10:90 due to more uniform particle distribution. It can be shown from the closing loop at P/P_o which was lower than the ratio of 10:90.

The optical properties were examined by using UV–Vis diffuse reflectance spectroscopy. Fig. 5 shows the band absorption edge was shifted to shorter wavelength when the Zr ratio was increased from 10:90 to 30:70. This might be due to the formation of ternary species as shown in XRD and XPS analyses. In contrast, Gao et al. (2010) stated that the formation of Ti₂ZrO₆ shifted the absorption band to longer wavelength at visible region with ratio 35:65.

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Figure 5

(a) DR UV–Vis spectra and (b) Tauc plots of ZrO₂/TiO₂, with various ZrO₂/TiO₂ ratios calcined at 750 °C for 5 h.

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Two absorption bands at 250 nm and 320 nm were observed indicating the presence of nano-sized of TiO₂ and assigned to ligand to metal charge transfer transition ${\text{O}}^{2-}\to {\text{Ti}}^{4+}({\text{or Zr}}^{4+})$ as also stated by Anzures et al. (2015) and Stefan et al. (2014). Besides that, overlapping at longer wavelength suggested the coexistence of heterojunction which is supported by the formation of hysteresis loop shown in Fig. 4. The extrapolated line fitted to the linear portions was carried out as shown in Fig. 5(b) to determine the band gap energy of the photocatalyst. Increasing of ZrO₂/TiO₂ ratio displayed bigger band gap energy might be due to the suppression of anatase to rutile phase transformation in the photocatalyst as shown in Table 3. Furthermore, it was concluded that the formation of heterojunction of ZrTiO₄/ZrTi₂O₆/TiO₂ has demonstrated larger band gap energy as illustrated from Tauc plot (Table 3).

3.2 Evaluation of photocatalytic activity

The control experiments were conducted to test the stability of paraquat dichloride before undergoing photodegradation reaction. Fig. 6 shows that 9.08% of paraquat dichloride was degraded without the presence of catalyst (photolysis), showing that paraquat dichloride is quite stable under UV irradiation (Cantavenera et al., 2007). Meanwhile adsorption/desorption experiment gave only 3.64% of paraquat dichloride degradation after 240 min reaction time. Interestingly, remarkable enhancement was obtained in the presence of UV irradiation and TiO₂ calcined at 450 °C as photocatalyst that gave 22.41% of paraquat dichloride degradation after 240 min irradiation.

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Figure 6

Control experiment under different reaction systems, [paraquat, PQ] = 15 ppm, λ_UV = 365 nm, reaction time = 240 min. Catalysts = TiO₂ (450 °C, 0.1 g).

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Fig. 7 shows kinetic study of paraquat dichloride using ZrO₂/TiO₂ photocatalyst at various ZrO₂/TiO₂ ratios and TiO₂ calcined at 450 °C for comparison study. The linear plots of ln(C/C_o) versus irradiation time (t) that parallels to a pseudo first-order reaction kinetics analysis, which its usual form as shown in Eq. (3). The relevant data are tabulated in Table 4.

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Figure 7

Pseudo first order kinetics of paraquat dichloride degradation.

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Further investigation was carried out using various catalyst dosages from 0.1 to 0.4 g of ZrO₂/TiO₂ (20:80) photocatalyst (Fig. 8) to gain the optimum photodegradation of paraquat dichloride. The order photocatalytic activity of 0.3 > 0.4 > 0.2 > 0.1 g was observed. From the results obtained, the optimum catalyst loading for ZrO₂/TiO₂ (20:80) was 0.3 g which gave 83.41% of paraquat dichloride degradation after 240 min reaction. Increasing the catalyst dosage increases the amount of active site and led to the increasing photodegradation of paraquat dichloride up to 0.3 g. In contrast, further increase of catalyst dosage up to 0.4 g lowers the photodegradation of paraquat dichloride. This might be due to scattering effect caused by increasing the turbidity of suspension that prevents UV light penetration into water as also observed by Bensaadi et al. (2014). In addition, Ahmed et al. (2011) had also reported that the agglomeration of catalyst might occur in the excess of catalyst dosage and this will result in the reduction of active site of the catalyst.

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Figure 8

Photodegradation of paraquat dichloride over ZrO₂/TiO₂ (20:80) photocatalyst calcined at 750 °C for 5 h at various catalyst dosages. [paraquat] = 15 ppm, λ_UV = 365 nm and reaction time = 240 min.

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From the parameters studied, it can be summarized that the optimum condition for degradation of paraquat using ZrO₂/TiO₂ photocatalyst was obtained at calcination temperature of 750 °C with ZrO₂/TiO₂ ratio of 20:80 and 0.3 g of catalyst dosage which gave 8.41% degradation of paraquat dichloride after 240 min irradiation.