- Email: [email protected]
Improved photocatalytic efficiency of titanium dioxide-hematite composite by air plasma
Chemical Physics Letters 730 (2019) 259–265
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Improved photocatalytic efficiency of titanium dioxide-hematite composite by air plasma Olaniyan Ibukuna, Hae Kyung Jeonga,b, a b
T
⁎
Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea Department of Materials-Energy Science and Engineering, Institute of Industry and Technique, Daegu University, Gyeongsan 712-714, Republic of Korea
H I GH L IG H T S
of the TiO -hematite composites by ambient plasma. • Modification decreased from 3.2 to 2.8 eV by the ambient plasma. • Bandgap resistance decreased from 16 to 11 Ω by the ambient plasma. • Electron-transfer response of the composites increased by 5.6 times compared to TiO . • Photocurrent • The composite degraded methylene blue in 80 min compared to 180 min of TiO . 2
2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium dioxide Iron oxide Photocatalysis Air plasma
Titanium dioxide (TiO2)-hematite composites are synthesized by the hydrothermal method and modified further by using air plasma for photocatalytic applications. High photocurrent response of the composite, 5.6 times better than TiO2, is observed, and high efficient degradation of methyl blue (MB) within 80 min is shown when TiO2 degraded MB in 180 min. It is found that the plasma treatment introduces electrochemical active surface area with formation of Ti3+ and oxygen vacancies and effectively reduces energy bandgap from 3.15 to 2.8 eV, electron-transfer resistance from 16 to 11 Ω, and particle size from 25 to 20 nm, compared to TiO2.
1. Introduction Rapid growth of industrialization and urbanization has adverse effect on the environment because the industries release large number of toxic waste into water-bodies [1]. Hence, there is an urgent need to degrade and remove the toxic materials in the water before they are discharged into the environment. Among various processes which are used for the degradation of toxic materials, photocatalysis has gained a vast attention because it can be carried out under ambient condition [2], and titanium dioxide (TiO2) is one of typical photocatalysts which has high availability, excellent optoelectronic properties, photo stability, and environmental friendliness [3,4]. However, the fast recombination of the photogenerated charges and wide energy bandgap (3.2 eV for anatase phase and 3.0 eV for rutile phase) have limited its use. Several strategies, such as doping with metallic/non-metallic elements [5,6] and various surface modifications [7,8], have been developed to reduce the energy bandgap and retard the recombination time
⁎
of the photogenerated charges in TiO2. The Fe3+ has been also considered as the most suitable among metallic dopants due to its halffilled electronic configuration and the similarity of its ionic radius (0.64 Å) with that of coordinated Ti4+ (0.68 Å) [9]. Many researches have claimed that the introduction of Fe3+ into TiO2 shifts the absorption spectrum of TiO2 from the ultraviolet (UV) to the visible region. M. Asilturk et al. [10] reported that Fe3+ doped TiO2 had absorption characteristics in the region between 450 and 650 nm. They argued that the energy shift of the doped TiO2 came from the electronic transition of the dopant energy level (Fe3+/Fe4+) to the conduction band of TiO2. W. Zhao et al. [11] also reported that 0.1% (molecular fraction) of Fe was the optimum loading for TiO2. They concluded that high loading of Fe could reduce the degradation efficiency of the composite because high concentration of Fe3+ can act as recombination centers. Plasma is an ionized gas which comprises of electrons, molecules, ions, radicals, and excited species. There are two types of plasma according to their energy level, and two types of plasma are high and low
Corresponding author at: Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea. E-mail address: [email protected] (H.K. Jeong).
https://doi.org/10.1016/j.cplett.2019.06.022 Received 10 May 2019; Received in revised form 5 June 2019; Accepted 7 June 2019 Available online 08 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
dispersive X-ray analysis (EDX) was combined to map the homogenous distribution of Fe. The crystalline structure of composites were analyzed by X-ray diffraction (XRD) at 40 kV with Cu Kα radiation (λ = 1.54 Å), using an automated X-ray diffractometer (D/MAX-2500/ PC, Rigaku, Japan). X-ray photoelectron spectroscopy (XPS, Thermo scientific, USA) with Al Kα X-ray source was used to investigate the chemical states of the samples. The photoluminescence (PL) emission spectra of the samples were measured using the spectrofluorometer (Jobin Yvon Flurolog-3-11) equipped with Xe lamp. Ultraviolet–visible spectroscopy (UV–VIS, CARY 5000, Varian) was also used to characterize the degradation of methyl blue (MB) dye and absorption spectra.
temperature plasma. The low temperature plasma can be further subdivided into the thermal and non-thermal plasma. If the gas temperature is close to room temperature, it is also called cold plasma. The electrons in the non-thermal plasma can be very hot but the heavy species (ions, atoms and molecules) remain cold. This feature of the cold plasma enables it to be used for catalyst preparation in various ways with enhanced reactivity at the surfaces [12,13]. The plasma method has been used to improve performance of TiO2 [14,15], and it is actually preferred because it creates highly stable Ti3+ and oxygen vacancies [16]. The oxygen vacancy is one of the most prevalent defects in metal oxides and is closely related to the photocatalytic properties of TiO2 [17]. Creation of the oxygen vacancy also leads to the creation of Ti3+ which could form donor states in the , electronic structure of TiO2. It has been known that the oxygen vacancy and Ti3+ further act as a sink for the photogenerated charge carriers, thus extending the lifetime of the charge carriers [18,19]. In this study, hematite (α-Fe2O3) was doped into TiO2 by using the hydrothermal method, and the synthesized composite was further modified with ambient plasma to improve the photocatalytic performance. Changes in morphology, surface area, bandgap energy, and charge-transfer resistance will be described, and the photocatalytic performance of the composites with respect to the degradation of MB is investigated.
2.3. Photocatalytic activities MB (M6900-50G, Acid blue 93, Sigma-Aldrich) was used to investigate the photocatalytic degradation activity of the samples. The photodegradation experiments were performed at ambient temperature. Each test sample (50 mg) was added to a dilute MB solution (1.6 × 10−4 M, 10 mg of MB in 200 mL of deionized water) and stirred magnetically in the dark for 30 min to reach an equilibrium of absorption and desorption. Analytikjena UV lamp (UVGL-55, 245 nm, US) was used, and the solution of 3 mL was taken out every 10 min during the irradiation followed by the centrifugation at 7000 rpm for 30 min in order to remove the residue of the photocatalyst. UV–VIS spectroscopy was then performed to detect the residue of MB in the centrifuged solution.
2. Materials and methods 2.1. Preparation of catalysts TiO2 (99.8%, 25 nm, Sigma-Aldrich) and hematite (α-Fe2O3, 99.8%, 2 um, Sigma-Aldrich) were used without pretreatment. First, TiO2 (50 mg) and hematite (10 mg) were suspended in deionized water (40 mL) under the magnetic stirring for 2 hrs. Thereafter, the solution was transferred into an autoclave and maintained at the temperature of 180 °C for 24 hrs for the hydrothermal synthesis of the TiO2-Fe2O3 composite. The solution was then filtered, and the sediment was dried for 12 hrs. The synthesized TiO2-hematite composite is denoted as “Composite”. Next, atmospheric pressure plasma was applied to the synthesized composite for 1 hrs. The discharge potential was 15 kV with the frequency of 25 kHz, resulting in the discharge current of 10 mA. The anode and cathode were the needle (diameter of 0.7 mm, kovaxneedle 22G) and the copper tape (3M electrical copper tape, width of 12 mm and length of 155 mm). The distance between anode and cathode was 10 mm. The plasma treated composite was denoted as “PComposite”.
2.4. Photoelectrochemical measurements The photoelectrochemical measurements were performed using a three electrode system with H2SO4 (0.1M) (7664-93-9, 99.99%, SigmaAldrich) electrolyte at ambient temperature. Platinum wire was used for the counter electrode, and an Ag/AgCl electrode was used as a reference electrode. 2 mg of the test sample was sonicated with 2 mL of isopropanol (190,764, ≥99.5%, Sigma-Aldrich) for 30 min. 5 μL of the mixture was then dropped on glass carbon electrode which was used as the working electrode. The photocurrent response of the working electrodes was carried out at a constant potential of + 1.0 V under onoff intervals (30 sec) of UV light. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 0.01 to 500,000 Hz with the voltage amplitude of 10 mV. Chronocoulometry (CC) was also performed to determine electrochemical active surface area and adsorption capacity by using 0.1M K3[Fe(CN)6] with the pulse width of 0.1 s.
2.2. Characterization techniques 3. Results and discussions Surface morphologies of the samples were studied by using scanning electron microscopy (SEM, S-4300, Hitachi, Japan) while energy
Fig. 1 shows the SEM results of TiO2, Composite, and P-Composite.
Fig. 1. SEM results of (a) TiO2, (b) Composite, and (c) P-Composite. 260
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 2. (a) XRD pattern and (b) Anson plot of TiO2, Composite, and P-Composite.
absorption edge was red shifted at two levels in Composite. The first level was due to the different chemical state of surround TiO2, and the edge was red shifted by almost 25 nm. The second level was red shifted by 226 nm, and the shift might be due to the substitution of Ti4+ by Fe3+ and/or the formation of a Fe3+/Fe4+ dopant energy level within the bandgap of TiO2. This substitution causes a change in the bandgap by forming mid gap energy levels along with the formation of Ti3+ and oxygen vacancies [23,24]. After the plasma treatment, similar phenomena were shown in P-Composite. The absorption edge was shifted to 429.02 and 636.86 nm. The absorbance of pristine hematite which was also subjected to same condition of hydrothermal treatment had almost same value as the absorbance of Composite. However, P-Composite exhibited a red shift compared to the pristine hematite. This is due to the increase of Oxygen vacancy and Ti3+ area, as shown in the XPS results. The optical bandgap energy of the samples was calculated by Tauc’s equation. Obtained bandgap energy of TiO2 was 3.15 eV, and the bandgap was red-shifted by 0.3 eV. In other words, two composites (Composite and P-Composite) had energy bandgap of 2.89 eV and 2.83 eV, respectively, as shown in Fig. 4b. In addition, additional bandgap of hematite near 2 eV was also found in the composites. The bandgap was 2.07 and 1.99 eV for Composite and P-Composite. PComposite exhibited lower bandgap energy compared to that of Composite, expecting that increase of Ti3+ and oxygen vacancies compared to Composite [25]. The optical results indicate that the incorporation of Fe into TiO2 can make TiO2 active in the visible region. In addition, the plasma treatment further increases optical absorbance in the visible region, resulting in the reduction of the energy bandgap. Fig. 5 shows the high resolution XPS spectra of O 1s, Ti 2p, and Fe 2p. All peaks were fitted with Gaussian function. As seen in Fig. 5a, the O 1s state of TiO2 was deconvoluted into two peaks at 530.43 eV and 531.67 eV. The peak at binding energy of 530.43 eV is attributed to lattice oxygen while the peak at 531.67 eV is attributed to non-lattice oxygen [26,27]. The lattice oxygen is part of the oxide catalyst while the non-lattice oxygen is the oxygen obtained from the reactions taking place [28]. The O 1s spectra of Composite and P-Composite were also deconvoluted into two peaks. Composite has peaks at 530.07 and 531.23 eV for lattice and non-lattice oxygen, respectively, and P-Composite has the binding energy of lattice oxygen at 529.52 eV and the binding energy of the non-lattice oxygen at 530.71 eV. The red shift of the peaks is due to the electronic interaction between TiO2 and Fe, confirming the formation of substitution between them [29]. The area of the non-lattice oxygen was calculated to estimate oxygen vacancy. The percentages of oxygen vacancy increased by 13% compared to TiO2, and further 20% increased after the plasma treatment compared to TiO2. This confirms that the oxygen vacancies were increased by
TiO2 has spherical particles bounded together, and the size of the spherical shape increased after loading of hematite as can be seen in Fig. 1b. The spherical particles were less densely packed in P-Composite due to the plasma treatment, compared to Composite, as shown in Fig. 1c. The XRD patterns of TiO2, Composite, and P-Composite are shown in Fig. 2a. TiO2 has characteristic peaks of the anatase phase (JCPDS no. 00-021-1272) at 24.85°, 37.57°, 47.56°, 53.50°, 54.40°, 62.27°, 68.36°, and 69.79°. The peaks were indexed to crystallographic planes of (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), and (2 0 0), respectively. Composite has additional peaks, and they were indexed to those of pure rhombohedral phase of hematite Fe2O3 which is in good agreement with the standard data from JCPDS card no. 33-0664. It is worthy to note that the (0 1 2), (1 1 6), (1 2 2) and (3 0 0) planes of hematite Fe2O3 were not detected in Composite and P-Composite, indicating that Fe3+ substitutes Ti4+ in the TiO2 lattice because Fe3+ has smaller ionic radius than Ti4+ (0.64 Å vs 0.68 Å) [20]. Scherrer’s equation was used to estimate the average crystalline size of the samples. Using the peak at 2θ = 24.85°, the average crystalline size of the samples obtained were 25, 24, and 20 nm for TiO2, Composite, and PComposite, respectively. It is evident that the plasma treatment reduced the crystalline size of TiO2. Chronocoulometry (CC) was used to investigate the adsorption capacity as well as electrochemical active surface area of electrodes. The electrochemical surface area can be calculated from the slope of Anson’s plot [21]. The Anson equation is as follows.
Q = 2nFACD1/2π −1/2t1/2, where Q, n, and A represent the charge (coulombs), the number of electrons, and real electrochemical surface area (cm2), respectively. F is the Faraday’s constant (96,500 C/mole). The concentration of the mediator is denoted by C while diffusion coefficient of the mediator (cm2/sec) and time (sec) are denoted by D and t [22]. As shown from Fig. 2b, the electrochemical active surface area was estimated by using the Anson equation, which was 1.99, 2.28, and 3.81 × 10−7 cm2 for TiO2, Composite and P-Composite, respectively. The surface area of Composite and P-Composite is 1.14 and 1.91 times larger than that of TiO2. The absorption capacity is also proportional to the surface area, based on Anson’s equation [21,22], concluding that P-Composite has the highest adsorption capacity and surface area compared to the others. To confirm the homogenous distribution of Fe, EDX mapping experiment was carried out. As seen in Fig. 3c and f, Fe was successfully loaded homogenously in both of Composite and P-Composite, respectively. The UV–VIS absorbance of TiO2, Composite, and P-Composite were shown in Fig. 4a. TiO2 had an absorption edge around 397 nm, and the 261
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 3. EDX mapping of (a-c) Composite and (d-f) P-Composite.
Composite. The presence of Fe 2p3/2 and Fe 2p1/2 demonstrates that Fe is in the Fe3+ ionic state [32]. P-Composite also showed very similar XPS Fe 2p spectra, concluding that both composites have the same Fe3+ ionic states. The amount of Fe was 3.3 and 2.6 at% in Composite and PComposite, obtained from the XPS results, which are corresponds to 0.08 and 0.06% of the molecular fraction and similar loading of Fe compared with the previous result [11]. The photocurrent response of the samples was plotted in Fig. 6a. Once the UV light is turned on, a photocurrent is immediately generated. When the light is turned off, the photocurrent decreases to its initial position. The photocurrent of TiO2 was the lowest due to the fast recombination of the photogenerated charge carriers [33]. The photocurrent response of Composite and P-Composite increased by 4.4 and 5.6 times more than that of TiO2 in the presence of Fe3+. In addition, the increment in the Ti3+ and oxygen vacancy, as sinks for the photogenerated charger carriers, of the composites plays important role in the high photocurrent response [34,11]. Impedance of the samples is shown in Fig. 6b, and the semicircle of
loading and plasma treatment [25,30]. Fig. 5b shows the Ti 2p spectra of the samples, and the spectra are deconvoluted to three peaks. TiO2 has Ti 2p3/2 and Ti 2p1/2 peaks at 459.31 eV and 465.14 eV, respectively, which indicates the presence of Ti4+ in the TiO2 lattice [31]. Additional peak at the binding energy of 460.53 eV is corresponds to Ti3+ of Ti2O3 [25]. After the introduction of Fe into TiO2, there was red shift in all peaks, indicating that there is interaction between Fe and Ti atoms which can influence on the electronic state of Ti. The area of Ti3+ increased by 18% in Composite. Ti3+ could introduce donor states in the energy gap which led to the red shift of the peaks. The Ti peaks of P-Composite were 458.37, 495.41, and 464.16 eV for Ti4+ 2p3/2, Ti3+ 2p3/2, and Ti4+ 2p1/2, respectively. There was also 63% increase in the area of the Ti3+ compared to TiO2. It is evident that the red shift of the binding energy and increase of Ti3+ percentages were observed in the XPS Ti 3d spectra. The XPS Fe 2p spectrum is shown in Fig. 5c. Four dominant peaks comprising of Fe 2p3/2 at 711.03 eV, Fe 2p1/2 at 724.51 eV, and satellite peaks of Fe 2p3/2 and Fe 2p1/2 at 717.25 and 730.93 eV, respectively, were observed in
Fig. 4. (a) UV–Visible absorption spectra and (b) optical bandgap of the samples. 262
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 5. XPS results of (a) O 1s, (b) Ti 2p, and (c) Fe 2p of the samples.
of MB after photo-irradiation at a given time t. Fig. 6d shows the degradation percentage as a function of time, and P-Composite exhibited the fastest MB degradation in 80 min. P-Composite had the largest active surface area, based on the CC results, expecting that a large amount of MB can be adsorbed on the surface leading to the fast degradation. In addition, efficient charge transfer and the retarded recombination time of the electron-hole pair made it better photocatalyst compared to Composite and TiO2. Fig. 6e shows a plot of logarithm of (Co/Ct) versus the irradiation time. According to Langmuir-Hinshelwood kinetic model [38] in which the rate equation In (Co/Ct) is equal to kt, where k is the rate constant, Co is the initial concentration, and Ct is the concentration at a time t, the rate constant was determined to be 0.229, 0.372, and 0.696 min−1 for TiO2, Composite and P-Composite, respectively, meaning that P-Composite has the fastest kinetics in the degradation of MB. The photoluminescence (PL) spectrum is used to investigate the separation and recombination of the photogenerated charge carriers as the primary process in the field of photocatalysis. Low PL intensity indicates low recombination rate of the electron-hole pair due to more electrons been trapped or being transferred. Fig. 7f shows the PL spectra of the sample by using an excitation wavelength of 325 nm. The PL intensity of P-Composite was the lowest followed by Composite and TiO2. The decrease of the PL intensity was due to the efficient charge migration between TiO2 and Fe as well as the increase of the surface
Nyquist plot provides the charge transfer resistance at the electrode interface [35] based on the equivalent circuit, as shown in the inset of Fig. 5b. Rb is the bulk resistance of the electrolyte, Ri is the chargetransfer resistance, CPE is constant phase element which also represents capacitance, and W is the Warburg impedance. The calculated chargetransfer resistance of TiO2, Composite, and P-Composite was 15.57, 13.33, and 11.02 Ω, respectively. It is clear that P-Composite had the smallest semicircle so that the smallest charge-transfer resistance among the samples. The photocatalytic activities of the samples were evaluated by degradation of MB under the UV irradiation. MB is one of the harmful organic dyes which cause serious environmental problems. Fig. 7 shows the UV–Vis spectra results of the samples after photocatalytic activity with MB in 20 min increments. The typical absorption peak of MB was observed near 600 nm [36], and the peak intensity decreased with the irradiation time. Before irradiation, the dark absorption of MB by PComposite was the best. This is due to the increased surface area of PComposite as shown in the CC results. TiO2 degraded MB completely in 180 min (Fig. 6a) while Composite and P-Composite degraded it within 120 min (Fig. 7b) and 80 min (Fig. 7c). The percentage of photo-degradation of MB was estimated by using the equation below [37]:
D(%) = {(Co − Ct )/Co} × 100, where Co is the initial concentration of MB, and Ct is the concentration
Fig. 6. (a) Transient photocurrent responses under the UV irradiation, and (b) EIS Nyquist plots under UV irradiation of the samples. (Inset is the equivalent circuit). 263
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 7. UV–Vis spectra as a function of time interval of 20 min for the degradation of MB in the presence of (a) TiO2, (b) Composite, and (c) P-Composite. (d) Photocatalytic degradation of MB with (e) the corresponding kinetic plot and (f) PL spectra of the samples are shown.
the generation of the oxygen vacancies and Ti3+ in the interaction between Fe3+ and TiO2. The loading of Fe3+ and modification by the plasma significantly reduced the recombination rate by the changes of electronic states of original TiO2, and the reduced bandgap from 3.15 to 1.99 eV was confirmed by the absorption spectra. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF2016R1D1A3B04931018). Fig. 8. Stability and reusability test of P-Composite.
References
oxygen vacancies and defects [18,37]. The reusability of P-Composite was investigated in Fig. 8. P-Composite was removed after the MB degradation experiment in 80 min by the centrifugation, and then the same MB degradation experiment was performed in 80 min. As a result, the degradation percentage was 96.7, 96.0, 95.4, and 94.3% for cycle 1, cycle 2, cycle 3, and cycle 4, respectively. This implies that P-Composite has good stability, sustainability, and reusability.
[1] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, Adsorption of methylene blue on low-cost adsorbents: a review, J. Harzard. Mater 177 (2010) 70–80. [2] M.N. Chong, B. Jin, C.W. Chow, C. Saint, Recent developments in Photocatalytic water treatment technology: A review, Water Res. 44 (2010) 2997. [3] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 144 (2001) 625–627. [4] A.A. Nada, M.F. Bekheet, R. Viter, et al., BN/GdxTi(1−x)O(4−x)/2 nanofibers for enhanced photocatalytic hydrogen production under visible light, Appl. Cata. B: Environmental 251 (2019) 76–86. [5] R. Asahi, T. Mikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible light photocatalysis in nitrogen doped titanium oxides, Science 293 (2001) 269–271. [6] W.J. Jiang, M. Zhang, J. Wang, Y.F. Liu, Y.F. Zhu, Photodegradation of phenol via C3N4-agar hybrid hydrogel 3D photocatalysts with free separation, Appl. Catal. B: Environ. 160 (2014) 44–50. [7] Y. Wang, Y. Zhang, G.H. Zhao, H. Tian, H. Shi, T. Zhou, Design of a novel Cu2O/ TiO2/Carbon aerogel electrode and its efficient electrosorption-assisted visible light photocatalytic degradation of 2,4,6-Trichlorophenol, ACS Appli. Mater. Interfaces 4 (2012) 3965–3972. [8] M. Nasr, C. Eid, R. Habchi, P. Miele, M. Bechelany, Recent progress on TiO2 nanomaterials for photocatalytic applications, ChemSusChem. 11 (2018) 3023–3047. [9] P. Pongwan, B. Inceesungvorn, J. Wetchakun, S. Phanichphant, et al., Highly efficient visible-light induced photocatalytic activity of Fe-doped TiO2 nanoparticles, Eng. J. 16 (2012) 143–152. [10] M. Asilturk, F. Sayilkan, E. Arpac, Effect of Fe3+ Ion doping to TiO2 on the photocatalytic degradation of malachite green dye under UV and Vis-Irradiation, Jour Of Photo. And Photo. A: Chemistry 203 (2009) 64–71.
4. Conclusion The TiO2-hematite composites have been prepared successfully by the hydrothermal method and modified further by using the ambient plasma. The electrochemical active surface area of the composite increased by twice, and the charge-transfer resistance decreased from 15.57 to 11.02 Ω compared to TiO2. The crystalline size of TiO2 also decreased from 25 to 20 nm after the plasma treatment. Finally, the photocurrent response of the plasma treated composite (P-Composite) showed 5.5 times higher compared to TiO2, and the MB degradation was the fastest among the samples. The good photocatalytic performance of the plasma modified composite (P-Composite) is attributed to 264
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
[11] W. Zhao, W. Fu, H. Yang, C. Tian, M. Li, et al., Synthesis and photocatalytic activity of Fe-doped TiO2 supported on hollow glass microbeads, Nano-micro Lett. 3 (2011) 34–42. [12] Z. Wang, Y. Zhang, E.C. Neyts, X. Cao, et al., Catalyst Preparation with Plasmas: How does it work? ACS Catal. 8 (2018) 2093–2110. [13] P. Lu, P.J. Cullen, K. Ostrikov, Atmospheric pressure nonthermal plasma sources, Cold Plasma in Food and Agri. (2016) 83–116. [14] I. Nakamura, N. Negishi, S. Kutsuna, K. Takeuchi, et al., Role of Oxygen vacancy in the plasma treated TiO2 photocatalyst with visible light for NO removal, Mol. Cata. 161 (2000) 205–212. [15] B.B. Adormaa, W.K. Darkwah, Y. Ao, Oxygen vacancies of the TiO2 nano-based composite photocatalysts in visible light responsive photocatalysis, RSC Adv. 8 (2018) 33551. [16] R. Zhou, R. Zhou, X. Zhang, J. Li, X. Wang, Q. Chen, et al., Synergistic effect of atmospheric-pressure plasma and TiO2 photocatalysis on inactivation of Escherichia coli cells in aqueous media, Sci. Reports 6 (2016) 39552. [17] J. Nowotny, Titanium dioxide-based semiconductors for solar driven environmentally friendly applications: impact of point defects on performance, Energy Environ. Sci. 1 (2008) 565. [18] T. Thompson, J. Yates, TiO2-based photocatalysis: Surface defects, Oxygen and charge transfer, Top. Catal. 23 (2005) 197. [19] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2008) 706–710. [20] K.P. Aryal, H.K. Jeong, Functionalization of thermally reduced graphite oxide and carbon nanotubes by p-sulfonatocalix[4]arene and supramolecular recognition of tyrosine, Chem. Phy. Letters 714 (2018) 69–73. [21] R. Schaub, P. Thostrup, N. Lopez, E. Laesgaard, I. Stensgaard, et al., Oxygen vacancies as active sites for water dissociation on Rutile TiO2 (110), Phys. Rev. Lett. 87 (2001) 266104. [22] D.V. Wellia, Q.C. Xu, M.A. Sk, K.H. Lim, et al., Experimental and theoretical studies of Fe-doped TiO2 films prepared by peroxo sol-gel method, App. Catal A: Gen 401 (2011) 98. [23] K. Nagaveni, M.S. Hegde, G. Madras, Structure and photocatalytic activity of Ti1xMxO2+δ(M = W, V, Ce, Zr, Fe, and Cu) synthesized by solution combustion method, J. Phys. Chem. B 108 (2004) 20204. [24] J. Zhu, F. Chen, J. Zhang, H. Chen, M. Anpo, Fe3+-TiO2 photocatalysts prepared by combinig sol-gel method with hydrothermal treatment and their characterization, J. Photochem. Photobio. A 180 (2006) 196. [25] B. Bharti, S. Kumar, H.N. Lee, R. Kumar, Formation of oxygen vacancies and Ti3+
[26]
[27]
[28]
[29]
[30]
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
265
state in TiO2 thin film and enhanced optical properties by air plasma treatment, Sci. Rep. 6 (2016) 32355. P.T. Hsieh, Y.C. Chen, K.S. Kao, C.M. Wang, Luminescence mechanism of ZnO thin film investigated by XPS measurement, Appl. Phys. A Matter. Sci. Process. 90 (2008) 317–321. J.F. Zhu, W. Zheng, B. He, J.L. Zhang, M. Anpo, Characterization of Fe-TiO2 photocatalysts synthesized by hydrothermal method and their photocatalytic reactivity for photodegradation of XRG dye diluted in water, J. Mol. Catal. A Chem. 216 (2004) 35–43. Y. Martynova, S. Shaikhutdinov, H.J. Freund, CO oxidation on metal-supported ultrathin oxide films: what makes them active? ChemCatChem 5 (2013) 2162–2166. M.N. Ghazzal, R. Wojcueszak, G. Raj, E.M. Gaigneaux, Study of mesoporous CdSquantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM, Beilstein Journal of nanotech. 5 (2014) 68–76. P. Xiaoyang, Y. Min-Quan, F. Xianzhi, Z. Nan, X. Yi-Jun, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013) 3601. W.K. Ho, J.C. Yu, J. Lin, J.G. Yu, P.S. Li, Preparation and photocatalytic behavior of MoS2 and WS2 nanocluster sensitized TiO2, Langmuir 20 (2004) 5865–5869. Z. Tian, C. Wang, Z. Si, L. Ma, et al., Fischer-Tropsch synthesis to light olefins over iron-based catalysts supported on KMnO4 modified activated carbon by facile method, Applied Catal. A 541 (2017) 50–59. Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene a cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578. C.Y. Wang, C. Bottcher, D.W. Bahneman, J.K. Dohrman, Bandgap engineering and modifying surface of TiO2 nanostructure by Fe2O3 for enhanced-performance of dye sensitized solar cell, J. Mater. Chem. 13 (2003) 2322. P. Muthirulan, C.N. Devi, M.M. Sundaram, TiO2 wrapped graphene as a high performance photocatalyst for acid orange 7 dye degradation under solar/uv light irradiation, Ceram. Int. 40 (2014) 5945–5957. A. Ahmad, X. Liu, L. Li, Y. Xu, Synthesis of Ag2O nanocatalyst in the spherical polyelectrolyte brushes and its application in visible photodriven degradation of dye, E-polymers 16 (2016) 57–63. T. Ali, P. Tripathi, A. Azam, W. Raza, et al., Photocatalytic performance of Fe-doped TiO2 nanoparticles under visible-light irradiation, Mater. Res. Express 4 (2017) 015–022. A.J. Frank, I.M. Gratzel, Energy Resources through Photochemistry and Catalysts, Academic Press, New York, 1983, pp. 99–122.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Improved photocatalytic efficiency of titanium dioxide-hematite composite by air plasma Olaniyan Ibukuna, Hae Kyung Jeonga,b, a b
T
⁎
Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea Department of Materials-Energy Science and Engineering, Institute of Industry and Technique, Daegu University, Gyeongsan 712-714, Republic of Korea
H I GH L IG H T S
of the TiO -hematite composites by ambient plasma. • Modification decreased from 3.2 to 2.8 eV by the ambient plasma. • Bandgap resistance decreased from 16 to 11 Ω by the ambient plasma. • Electron-transfer response of the composites increased by 5.6 times compared to TiO . • Photocurrent • The composite degraded methylene blue in 80 min compared to 180 min of TiO . 2
2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium dioxide Iron oxide Photocatalysis Air plasma
Titanium dioxide (TiO2)-hematite composites are synthesized by the hydrothermal method and modified further by using air plasma for photocatalytic applications. High photocurrent response of the composite, 5.6 times better than TiO2, is observed, and high efficient degradation of methyl blue (MB) within 80 min is shown when TiO2 degraded MB in 180 min. It is found that the plasma treatment introduces electrochemical active surface area with formation of Ti3+ and oxygen vacancies and effectively reduces energy bandgap from 3.15 to 2.8 eV, electron-transfer resistance from 16 to 11 Ω, and particle size from 25 to 20 nm, compared to TiO2.
1. Introduction Rapid growth of industrialization and urbanization has adverse effect on the environment because the industries release large number of toxic waste into water-bodies [1]. Hence, there is an urgent need to degrade and remove the toxic materials in the water before they are discharged into the environment. Among various processes which are used for the degradation of toxic materials, photocatalysis has gained a vast attention because it can be carried out under ambient condition [2], and titanium dioxide (TiO2) is one of typical photocatalysts which has high availability, excellent optoelectronic properties, photo stability, and environmental friendliness [3,4]. However, the fast recombination of the photogenerated charges and wide energy bandgap (3.2 eV for anatase phase and 3.0 eV for rutile phase) have limited its use. Several strategies, such as doping with metallic/non-metallic elements [5,6] and various surface modifications [7,8], have been developed to reduce the energy bandgap and retard the recombination time
⁎
of the photogenerated charges in TiO2. The Fe3+ has been also considered as the most suitable among metallic dopants due to its halffilled electronic configuration and the similarity of its ionic radius (0.64 Å) with that of coordinated Ti4+ (0.68 Å) [9]. Many researches have claimed that the introduction of Fe3+ into TiO2 shifts the absorption spectrum of TiO2 from the ultraviolet (UV) to the visible region. M. Asilturk et al. [10] reported that Fe3+ doped TiO2 had absorption characteristics in the region between 450 and 650 nm. They argued that the energy shift of the doped TiO2 came from the electronic transition of the dopant energy level (Fe3+/Fe4+) to the conduction band of TiO2. W. Zhao et al. [11] also reported that 0.1% (molecular fraction) of Fe was the optimum loading for TiO2. They concluded that high loading of Fe could reduce the degradation efficiency of the composite because high concentration of Fe3+ can act as recombination centers. Plasma is an ionized gas which comprises of electrons, molecules, ions, radicals, and excited species. There are two types of plasma according to their energy level, and two types of plasma are high and low
Corresponding author at: Department of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea. E-mail address: [email protected] (H.K. Jeong).
https://doi.org/10.1016/j.cplett.2019.06.022 Received 10 May 2019; Received in revised form 5 June 2019; Accepted 7 June 2019 Available online 08 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
dispersive X-ray analysis (EDX) was combined to map the homogenous distribution of Fe. The crystalline structure of composites were analyzed by X-ray diffraction (XRD) at 40 kV with Cu Kα radiation (λ = 1.54 Å), using an automated X-ray diffractometer (D/MAX-2500/ PC, Rigaku, Japan). X-ray photoelectron spectroscopy (XPS, Thermo scientific, USA) with Al Kα X-ray source was used to investigate the chemical states of the samples. The photoluminescence (PL) emission spectra of the samples were measured using the spectrofluorometer (Jobin Yvon Flurolog-3-11) equipped with Xe lamp. Ultraviolet–visible spectroscopy (UV–VIS, CARY 5000, Varian) was also used to characterize the degradation of methyl blue (MB) dye and absorption spectra.
temperature plasma. The low temperature plasma can be further subdivided into the thermal and non-thermal plasma. If the gas temperature is close to room temperature, it is also called cold plasma. The electrons in the non-thermal plasma can be very hot but the heavy species (ions, atoms and molecules) remain cold. This feature of the cold plasma enables it to be used for catalyst preparation in various ways with enhanced reactivity at the surfaces [12,13]. The plasma method has been used to improve performance of TiO2 [14,15], and it is actually preferred because it creates highly stable Ti3+ and oxygen vacancies [16]. The oxygen vacancy is one of the most prevalent defects in metal oxides and is closely related to the photocatalytic properties of TiO2 [17]. Creation of the oxygen vacancy also leads to the creation of Ti3+ which could form donor states in the , electronic structure of TiO2. It has been known that the oxygen vacancy and Ti3+ further act as a sink for the photogenerated charge carriers, thus extending the lifetime of the charge carriers [18,19]. In this study, hematite (α-Fe2O3) was doped into TiO2 by using the hydrothermal method, and the synthesized composite was further modified with ambient plasma to improve the photocatalytic performance. Changes in morphology, surface area, bandgap energy, and charge-transfer resistance will be described, and the photocatalytic performance of the composites with respect to the degradation of MB is investigated.
2.3. Photocatalytic activities MB (M6900-50G, Acid blue 93, Sigma-Aldrich) was used to investigate the photocatalytic degradation activity of the samples. The photodegradation experiments were performed at ambient temperature. Each test sample (50 mg) was added to a dilute MB solution (1.6 × 10−4 M, 10 mg of MB in 200 mL of deionized water) and stirred magnetically in the dark for 30 min to reach an equilibrium of absorption and desorption. Analytikjena UV lamp (UVGL-55, 245 nm, US) was used, and the solution of 3 mL was taken out every 10 min during the irradiation followed by the centrifugation at 7000 rpm for 30 min in order to remove the residue of the photocatalyst. UV–VIS spectroscopy was then performed to detect the residue of MB in the centrifuged solution.
2. Materials and methods 2.1. Preparation of catalysts TiO2 (99.8%, 25 nm, Sigma-Aldrich) and hematite (α-Fe2O3, 99.8%, 2 um, Sigma-Aldrich) were used without pretreatment. First, TiO2 (50 mg) and hematite (10 mg) were suspended in deionized water (40 mL) under the magnetic stirring for 2 hrs. Thereafter, the solution was transferred into an autoclave and maintained at the temperature of 180 °C for 24 hrs for the hydrothermal synthesis of the TiO2-Fe2O3 composite. The solution was then filtered, and the sediment was dried for 12 hrs. The synthesized TiO2-hematite composite is denoted as “Composite”. Next, atmospheric pressure plasma was applied to the synthesized composite for 1 hrs. The discharge potential was 15 kV with the frequency of 25 kHz, resulting in the discharge current of 10 mA. The anode and cathode were the needle (diameter of 0.7 mm, kovaxneedle 22G) and the copper tape (3M electrical copper tape, width of 12 mm and length of 155 mm). The distance between anode and cathode was 10 mm. The plasma treated composite was denoted as “PComposite”.
2.4. Photoelectrochemical measurements The photoelectrochemical measurements were performed using a three electrode system with H2SO4 (0.1M) (7664-93-9, 99.99%, SigmaAldrich) electrolyte at ambient temperature. Platinum wire was used for the counter electrode, and an Ag/AgCl electrode was used as a reference electrode. 2 mg of the test sample was sonicated with 2 mL of isopropanol (190,764, ≥99.5%, Sigma-Aldrich) for 30 min. 5 μL of the mixture was then dropped on glass carbon electrode which was used as the working electrode. The photocurrent response of the working electrodes was carried out at a constant potential of + 1.0 V under onoff intervals (30 sec) of UV light. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 0.01 to 500,000 Hz with the voltage amplitude of 10 mV. Chronocoulometry (CC) was also performed to determine electrochemical active surface area and adsorption capacity by using 0.1M K3[Fe(CN)6] with the pulse width of 0.1 s.
2.2. Characterization techniques 3. Results and discussions Surface morphologies of the samples were studied by using scanning electron microscopy (SEM, S-4300, Hitachi, Japan) while energy
Fig. 1 shows the SEM results of TiO2, Composite, and P-Composite.
Fig. 1. SEM results of (a) TiO2, (b) Composite, and (c) P-Composite. 260
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 2. (a) XRD pattern and (b) Anson plot of TiO2, Composite, and P-Composite.
absorption edge was red shifted at two levels in Composite. The first level was due to the different chemical state of surround TiO2, and the edge was red shifted by almost 25 nm. The second level was red shifted by 226 nm, and the shift might be due to the substitution of Ti4+ by Fe3+ and/or the formation of a Fe3+/Fe4+ dopant energy level within the bandgap of TiO2. This substitution causes a change in the bandgap by forming mid gap energy levels along with the formation of Ti3+ and oxygen vacancies [23,24]. After the plasma treatment, similar phenomena were shown in P-Composite. The absorption edge was shifted to 429.02 and 636.86 nm. The absorbance of pristine hematite which was also subjected to same condition of hydrothermal treatment had almost same value as the absorbance of Composite. However, P-Composite exhibited a red shift compared to the pristine hematite. This is due to the increase of Oxygen vacancy and Ti3+ area, as shown in the XPS results. The optical bandgap energy of the samples was calculated by Tauc’s equation. Obtained bandgap energy of TiO2 was 3.15 eV, and the bandgap was red-shifted by 0.3 eV. In other words, two composites (Composite and P-Composite) had energy bandgap of 2.89 eV and 2.83 eV, respectively, as shown in Fig. 4b. In addition, additional bandgap of hematite near 2 eV was also found in the composites. The bandgap was 2.07 and 1.99 eV for Composite and P-Composite. PComposite exhibited lower bandgap energy compared to that of Composite, expecting that increase of Ti3+ and oxygen vacancies compared to Composite [25]. The optical results indicate that the incorporation of Fe into TiO2 can make TiO2 active in the visible region. In addition, the plasma treatment further increases optical absorbance in the visible region, resulting in the reduction of the energy bandgap. Fig. 5 shows the high resolution XPS spectra of O 1s, Ti 2p, and Fe 2p. All peaks were fitted with Gaussian function. As seen in Fig. 5a, the O 1s state of TiO2 was deconvoluted into two peaks at 530.43 eV and 531.67 eV. The peak at binding energy of 530.43 eV is attributed to lattice oxygen while the peak at 531.67 eV is attributed to non-lattice oxygen [26,27]. The lattice oxygen is part of the oxide catalyst while the non-lattice oxygen is the oxygen obtained from the reactions taking place [28]. The O 1s spectra of Composite and P-Composite were also deconvoluted into two peaks. Composite has peaks at 530.07 and 531.23 eV for lattice and non-lattice oxygen, respectively, and P-Composite has the binding energy of lattice oxygen at 529.52 eV and the binding energy of the non-lattice oxygen at 530.71 eV. The red shift of the peaks is due to the electronic interaction between TiO2 and Fe, confirming the formation of substitution between them [29]. The area of the non-lattice oxygen was calculated to estimate oxygen vacancy. The percentages of oxygen vacancy increased by 13% compared to TiO2, and further 20% increased after the plasma treatment compared to TiO2. This confirms that the oxygen vacancies were increased by
TiO2 has spherical particles bounded together, and the size of the spherical shape increased after loading of hematite as can be seen in Fig. 1b. The spherical particles were less densely packed in P-Composite due to the plasma treatment, compared to Composite, as shown in Fig. 1c. The XRD patterns of TiO2, Composite, and P-Composite are shown in Fig. 2a. TiO2 has characteristic peaks of the anatase phase (JCPDS no. 00-021-1272) at 24.85°, 37.57°, 47.56°, 53.50°, 54.40°, 62.27°, 68.36°, and 69.79°. The peaks were indexed to crystallographic planes of (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), and (2 0 0), respectively. Composite has additional peaks, and they were indexed to those of pure rhombohedral phase of hematite Fe2O3 which is in good agreement with the standard data from JCPDS card no. 33-0664. It is worthy to note that the (0 1 2), (1 1 6), (1 2 2) and (3 0 0) planes of hematite Fe2O3 were not detected in Composite and P-Composite, indicating that Fe3+ substitutes Ti4+ in the TiO2 lattice because Fe3+ has smaller ionic radius than Ti4+ (0.64 Å vs 0.68 Å) [20]. Scherrer’s equation was used to estimate the average crystalline size of the samples. Using the peak at 2θ = 24.85°, the average crystalline size of the samples obtained were 25, 24, and 20 nm for TiO2, Composite, and PComposite, respectively. It is evident that the plasma treatment reduced the crystalline size of TiO2. Chronocoulometry (CC) was used to investigate the adsorption capacity as well as electrochemical active surface area of electrodes. The electrochemical surface area can be calculated from the slope of Anson’s plot [21]. The Anson equation is as follows.
Q = 2nFACD1/2π −1/2t1/2, where Q, n, and A represent the charge (coulombs), the number of electrons, and real electrochemical surface area (cm2), respectively. F is the Faraday’s constant (96,500 C/mole). The concentration of the mediator is denoted by C while diffusion coefficient of the mediator (cm2/sec) and time (sec) are denoted by D and t [22]. As shown from Fig. 2b, the electrochemical active surface area was estimated by using the Anson equation, which was 1.99, 2.28, and 3.81 × 10−7 cm2 for TiO2, Composite and P-Composite, respectively. The surface area of Composite and P-Composite is 1.14 and 1.91 times larger than that of TiO2. The absorption capacity is also proportional to the surface area, based on Anson’s equation [21,22], concluding that P-Composite has the highest adsorption capacity and surface area compared to the others. To confirm the homogenous distribution of Fe, EDX mapping experiment was carried out. As seen in Fig. 3c and f, Fe was successfully loaded homogenously in both of Composite and P-Composite, respectively. The UV–VIS absorbance of TiO2, Composite, and P-Composite were shown in Fig. 4a. TiO2 had an absorption edge around 397 nm, and the 261
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 3. EDX mapping of (a-c) Composite and (d-f) P-Composite.
Composite. The presence of Fe 2p3/2 and Fe 2p1/2 demonstrates that Fe is in the Fe3+ ionic state [32]. P-Composite also showed very similar XPS Fe 2p spectra, concluding that both composites have the same Fe3+ ionic states. The amount of Fe was 3.3 and 2.6 at% in Composite and PComposite, obtained from the XPS results, which are corresponds to 0.08 and 0.06% of the molecular fraction and similar loading of Fe compared with the previous result [11]. The photocurrent response of the samples was plotted in Fig. 6a. Once the UV light is turned on, a photocurrent is immediately generated. When the light is turned off, the photocurrent decreases to its initial position. The photocurrent of TiO2 was the lowest due to the fast recombination of the photogenerated charge carriers [33]. The photocurrent response of Composite and P-Composite increased by 4.4 and 5.6 times more than that of TiO2 in the presence of Fe3+. In addition, the increment in the Ti3+ and oxygen vacancy, as sinks for the photogenerated charger carriers, of the composites plays important role in the high photocurrent response [34,11]. Impedance of the samples is shown in Fig. 6b, and the semicircle of
loading and plasma treatment [25,30]. Fig. 5b shows the Ti 2p spectra of the samples, and the spectra are deconvoluted to three peaks. TiO2 has Ti 2p3/2 and Ti 2p1/2 peaks at 459.31 eV and 465.14 eV, respectively, which indicates the presence of Ti4+ in the TiO2 lattice [31]. Additional peak at the binding energy of 460.53 eV is corresponds to Ti3+ of Ti2O3 [25]. After the introduction of Fe into TiO2, there was red shift in all peaks, indicating that there is interaction between Fe and Ti atoms which can influence on the electronic state of Ti. The area of Ti3+ increased by 18% in Composite. Ti3+ could introduce donor states in the energy gap which led to the red shift of the peaks. The Ti peaks of P-Composite were 458.37, 495.41, and 464.16 eV for Ti4+ 2p3/2, Ti3+ 2p3/2, and Ti4+ 2p1/2, respectively. There was also 63% increase in the area of the Ti3+ compared to TiO2. It is evident that the red shift of the binding energy and increase of Ti3+ percentages were observed in the XPS Ti 3d spectra. The XPS Fe 2p spectrum is shown in Fig. 5c. Four dominant peaks comprising of Fe 2p3/2 at 711.03 eV, Fe 2p1/2 at 724.51 eV, and satellite peaks of Fe 2p3/2 and Fe 2p1/2 at 717.25 and 730.93 eV, respectively, were observed in
Fig. 4. (a) UV–Visible absorption spectra and (b) optical bandgap of the samples. 262
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 5. XPS results of (a) O 1s, (b) Ti 2p, and (c) Fe 2p of the samples.
of MB after photo-irradiation at a given time t. Fig. 6d shows the degradation percentage as a function of time, and P-Composite exhibited the fastest MB degradation in 80 min. P-Composite had the largest active surface area, based on the CC results, expecting that a large amount of MB can be adsorbed on the surface leading to the fast degradation. In addition, efficient charge transfer and the retarded recombination time of the electron-hole pair made it better photocatalyst compared to Composite and TiO2. Fig. 6e shows a plot of logarithm of (Co/Ct) versus the irradiation time. According to Langmuir-Hinshelwood kinetic model [38] in which the rate equation In (Co/Ct) is equal to kt, where k is the rate constant, Co is the initial concentration, and Ct is the concentration at a time t, the rate constant was determined to be 0.229, 0.372, and 0.696 min−1 for TiO2, Composite and P-Composite, respectively, meaning that P-Composite has the fastest kinetics in the degradation of MB. The photoluminescence (PL) spectrum is used to investigate the separation and recombination of the photogenerated charge carriers as the primary process in the field of photocatalysis. Low PL intensity indicates low recombination rate of the electron-hole pair due to more electrons been trapped or being transferred. Fig. 7f shows the PL spectra of the sample by using an excitation wavelength of 325 nm. The PL intensity of P-Composite was the lowest followed by Composite and TiO2. The decrease of the PL intensity was due to the efficient charge migration between TiO2 and Fe as well as the increase of the surface
Nyquist plot provides the charge transfer resistance at the electrode interface [35] based on the equivalent circuit, as shown in the inset of Fig. 5b. Rb is the bulk resistance of the electrolyte, Ri is the chargetransfer resistance, CPE is constant phase element which also represents capacitance, and W is the Warburg impedance. The calculated chargetransfer resistance of TiO2, Composite, and P-Composite was 15.57, 13.33, and 11.02 Ω, respectively. It is clear that P-Composite had the smallest semicircle so that the smallest charge-transfer resistance among the samples. The photocatalytic activities of the samples were evaluated by degradation of MB under the UV irradiation. MB is one of the harmful organic dyes which cause serious environmental problems. Fig. 7 shows the UV–Vis spectra results of the samples after photocatalytic activity with MB in 20 min increments. The typical absorption peak of MB was observed near 600 nm [36], and the peak intensity decreased with the irradiation time. Before irradiation, the dark absorption of MB by PComposite was the best. This is due to the increased surface area of PComposite as shown in the CC results. TiO2 degraded MB completely in 180 min (Fig. 6a) while Composite and P-Composite degraded it within 120 min (Fig. 7b) and 80 min (Fig. 7c). The percentage of photo-degradation of MB was estimated by using the equation below [37]:
D(%) = {(Co − Ct )/Co} × 100, where Co is the initial concentration of MB, and Ct is the concentration
Fig. 6. (a) Transient photocurrent responses under the UV irradiation, and (b) EIS Nyquist plots under UV irradiation of the samples. (Inset is the equivalent circuit). 263
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
Fig. 7. UV–Vis spectra as a function of time interval of 20 min for the degradation of MB in the presence of (a) TiO2, (b) Composite, and (c) P-Composite. (d) Photocatalytic degradation of MB with (e) the corresponding kinetic plot and (f) PL spectra of the samples are shown.
the generation of the oxygen vacancies and Ti3+ in the interaction between Fe3+ and TiO2. The loading of Fe3+ and modification by the plasma significantly reduced the recombination rate by the changes of electronic states of original TiO2, and the reduced bandgap from 3.15 to 1.99 eV was confirmed by the absorption spectra. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF2016R1D1A3B04931018). Fig. 8. Stability and reusability test of P-Composite.
References
oxygen vacancies and defects [18,37]. The reusability of P-Composite was investigated in Fig. 8. P-Composite was removed after the MB degradation experiment in 80 min by the centrifugation, and then the same MB degradation experiment was performed in 80 min. As a result, the degradation percentage was 96.7, 96.0, 95.4, and 94.3% for cycle 1, cycle 2, cycle 3, and cycle 4, respectively. This implies that P-Composite has good stability, sustainability, and reusability.
[1] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, Adsorption of methylene blue on low-cost adsorbents: a review, J. Harzard. Mater 177 (2010) 70–80. [2] M.N. Chong, B. Jin, C.W. Chow, C. Saint, Recent developments in Photocatalytic water treatment technology: A review, Water Res. 44 (2010) 2997. [3] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 144 (2001) 625–627. [4] A.A. Nada, M.F. Bekheet, R. Viter, et al., BN/GdxTi(1−x)O(4−x)/2 nanofibers for enhanced photocatalytic hydrogen production under visible light, Appl. Cata. B: Environmental 251 (2019) 76–86. [5] R. Asahi, T. Mikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible light photocatalysis in nitrogen doped titanium oxides, Science 293 (2001) 269–271. [6] W.J. Jiang, M. Zhang, J. Wang, Y.F. Liu, Y.F. Zhu, Photodegradation of phenol via C3N4-agar hybrid hydrogel 3D photocatalysts with free separation, Appl. Catal. B: Environ. 160 (2014) 44–50. [7] Y. Wang, Y. Zhang, G.H. Zhao, H. Tian, H. Shi, T. Zhou, Design of a novel Cu2O/ TiO2/Carbon aerogel electrode and its efficient electrosorption-assisted visible light photocatalytic degradation of 2,4,6-Trichlorophenol, ACS Appli. Mater. Interfaces 4 (2012) 3965–3972. [8] M. Nasr, C. Eid, R. Habchi, P. Miele, M. Bechelany, Recent progress on TiO2 nanomaterials for photocatalytic applications, ChemSusChem. 11 (2018) 3023–3047. [9] P. Pongwan, B. Inceesungvorn, J. Wetchakun, S. Phanichphant, et al., Highly efficient visible-light induced photocatalytic activity of Fe-doped TiO2 nanoparticles, Eng. J. 16 (2012) 143–152. [10] M. Asilturk, F. Sayilkan, E. Arpac, Effect of Fe3+ Ion doping to TiO2 on the photocatalytic degradation of malachite green dye under UV and Vis-Irradiation, Jour Of Photo. And Photo. A: Chemistry 203 (2009) 64–71.
4. Conclusion The TiO2-hematite composites have been prepared successfully by the hydrothermal method and modified further by using the ambient plasma. The electrochemical active surface area of the composite increased by twice, and the charge-transfer resistance decreased from 15.57 to 11.02 Ω compared to TiO2. The crystalline size of TiO2 also decreased from 25 to 20 nm after the plasma treatment. Finally, the photocurrent response of the plasma treated composite (P-Composite) showed 5.5 times higher compared to TiO2, and the MB degradation was the fastest among the samples. The good photocatalytic performance of the plasma modified composite (P-Composite) is attributed to 264
Chemical Physics Letters 730 (2019) 259–265
O. Ibukun and H.K. Jeong
[11] W. Zhao, W. Fu, H. Yang, C. Tian, M. Li, et al., Synthesis and photocatalytic activity of Fe-doped TiO2 supported on hollow glass microbeads, Nano-micro Lett. 3 (2011) 34–42. [12] Z. Wang, Y. Zhang, E.C. Neyts, X. Cao, et al., Catalyst Preparation with Plasmas: How does it work? ACS Catal. 8 (2018) 2093–2110. [13] P. Lu, P.J. Cullen, K. Ostrikov, Atmospheric pressure nonthermal plasma sources, Cold Plasma in Food and Agri. (2016) 83–116. [14] I. Nakamura, N. Negishi, S. Kutsuna, K. Takeuchi, et al., Role of Oxygen vacancy in the plasma treated TiO2 photocatalyst with visible light for NO removal, Mol. Cata. 161 (2000) 205–212. [15] B.B. Adormaa, W.K. Darkwah, Y. Ao, Oxygen vacancies of the TiO2 nano-based composite photocatalysts in visible light responsive photocatalysis, RSC Adv. 8 (2018) 33551. [16] R. Zhou, R. Zhou, X. Zhang, J. Li, X. Wang, Q. Chen, et al., Synergistic effect of atmospheric-pressure plasma and TiO2 photocatalysis on inactivation of Escherichia coli cells in aqueous media, Sci. Reports 6 (2016) 39552. [17] J. Nowotny, Titanium dioxide-based semiconductors for solar driven environmentally friendly applications: impact of point defects on performance, Energy Environ. Sci. 1 (2008) 565. [18] T. Thompson, J. Yates, TiO2-based photocatalysis: Surface defects, Oxygen and charge transfer, Top. Catal. 23 (2005) 197. [19] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2008) 706–710. [20] K.P. Aryal, H.K. Jeong, Functionalization of thermally reduced graphite oxide and carbon nanotubes by p-sulfonatocalix[4]arene and supramolecular recognition of tyrosine, Chem. Phy. Letters 714 (2018) 69–73. [21] R. Schaub, P. Thostrup, N. Lopez, E. Laesgaard, I. Stensgaard, et al., Oxygen vacancies as active sites for water dissociation on Rutile TiO2 (110), Phys. Rev. Lett. 87 (2001) 266104. [22] D.V. Wellia, Q.C. Xu, M.A. Sk, K.H. Lim, et al., Experimental and theoretical studies of Fe-doped TiO2 films prepared by peroxo sol-gel method, App. Catal A: Gen 401 (2011) 98. [23] K. Nagaveni, M.S. Hegde, G. Madras, Structure and photocatalytic activity of Ti1xMxO2+δ(M = W, V, Ce, Zr, Fe, and Cu) synthesized by solution combustion method, J. Phys. Chem. B 108 (2004) 20204. [24] J. Zhu, F. Chen, J. Zhang, H. Chen, M. Anpo, Fe3+-TiO2 photocatalysts prepared by combinig sol-gel method with hydrothermal treatment and their characterization, J. Photochem. Photobio. A 180 (2006) 196. [25] B. Bharti, S. Kumar, H.N. Lee, R. Kumar, Formation of oxygen vacancies and Ti3+
[26]
[27]
[28]
[29]
[30]
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
265
state in TiO2 thin film and enhanced optical properties by air plasma treatment, Sci. Rep. 6 (2016) 32355. P.T. Hsieh, Y.C. Chen, K.S. Kao, C.M. Wang, Luminescence mechanism of ZnO thin film investigated by XPS measurement, Appl. Phys. A Matter. Sci. Process. 90 (2008) 317–321. J.F. Zhu, W. Zheng, B. He, J.L. Zhang, M. Anpo, Characterization of Fe-TiO2 photocatalysts synthesized by hydrothermal method and their photocatalytic reactivity for photodegradation of XRG dye diluted in water, J. Mol. Catal. A Chem. 216 (2004) 35–43. Y. Martynova, S. Shaikhutdinov, H.J. Freund, CO oxidation on metal-supported ultrathin oxide films: what makes them active? ChemCatChem 5 (2013) 2162–2166. M.N. Ghazzal, R. Wojcueszak, G. Raj, E.M. Gaigneaux, Study of mesoporous CdSquantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM, Beilstein Journal of nanotech. 5 (2014) 68–76. P. Xiaoyang, Y. Min-Quan, F. Xianzhi, Z. Nan, X. Yi-Jun, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013) 3601. W.K. Ho, J.C. Yu, J. Lin, J.G. Yu, P.S. Li, Preparation and photocatalytic behavior of MoS2 and WS2 nanocluster sensitized TiO2, Langmuir 20 (2004) 5865–5869. Z. Tian, C. Wang, Z. Si, L. Ma, et al., Fischer-Tropsch synthesis to light olefins over iron-based catalysts supported on KMnO4 modified activated carbon by facile method, Applied Catal. A 541 (2017) 50–59. Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene a cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575–6578. C.Y. Wang, C. Bottcher, D.W. Bahneman, J.K. Dohrman, Bandgap engineering and modifying surface of TiO2 nanostructure by Fe2O3 for enhanced-performance of dye sensitized solar cell, J. Mater. Chem. 13 (2003) 2322. P. Muthirulan, C.N. Devi, M.M. Sundaram, TiO2 wrapped graphene as a high performance photocatalyst for acid orange 7 dye degradation under solar/uv light irradiation, Ceram. Int. 40 (2014) 5945–5957. A. Ahmad, X. Liu, L. Li, Y. Xu, Synthesis of Ag2O nanocatalyst in the spherical polyelectrolyte brushes and its application in visible photodriven degradation of dye, E-polymers 16 (2016) 57–63. T. Ali, P. Tripathi, A. Azam, W. Raza, et al., Photocatalytic performance of Fe-doped TiO2 nanoparticles under visible-light irradiation, Mater. Res. Express 4 (2017) 015–022. A.J. Frank, I.M. Gratzel, Energy Resources through Photochemistry and Catalysts, Academic Press, New York, 1983, pp. 99–122.