Enhancement of photocatalytic activities of nitrogen-doped titanium dioxide by ambient plasma

Journal Pre-proofs Research paper Enhancement of Photocatalytic Activities of Nitrogen-doped Titanium Dioxide by Ambient Plasma Olaniyan Ibukun, Hae Kyung Jeong PII: DOI: Reference:

S0009-2614(20)30149-4 https://doi.org/10.1016/j.cplett.2020.137234 CPLETT 137234

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Chemical Physics Letters

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2 October 2019 11 February 2020 14 February 2020

Please cite this article as: O. Ibukun, H. Kyung Jeong, Enhancement of Photocatalytic Activities of Nitrogendoped Titanium Dioxide by Ambient Plasma, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/ j.cplett.2020.137234

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Enhancement of Photocatalytic Activities of Nitrogen-doped Titanium Dioxide by Ambient Plasma Olaniyan Ibukun1 and Hae Kyung Jeong1,2* 1Department

of Physics, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea

2Department

of Materials-Energy Science and Engineering, Institute of Industry and

Technique, Daegu University, Gyeongsan 712-714, Republic of Korea

ABSTRACT Nitrogen doped titanium dioxide (TiO2) was prepared by the hydrothermal method and modified by air plasma for photocatalysis. The bandgap of TiO2 reduced from 3.1 eV to 2.7 eV, and the photocurrent response was enhanced by 3.6 times compared to pure TiO2. The photocatalytic activity was tested by degrading methyl blue (MB) under the ultraviolet irradiation, showing that the rate constant increased from 0.009 to 0.031 min-1 after the plasma treatment. The enhanced photocatalytic activity was attributed to the nitrogen doping as well as the ambient plasma treatment, changing the electronic structure, active surface area, and crystallite size of TiO2.

Keywords: titanium dioxide; nitrogen; photocatalysis; air plasma. * Corresponding author: [email protected], Tel: 82-53-850-6438, Fax: 82-53-850-6439.

1.

Introduction The rise in the global population has led to the rapid growth of industrialization and

urbanization. The discharge of hazardous dyes from textile, leather, and paper industries into the environment has led to serious environmental problems that affect both the aquatic and human life [1,2]. According to World Health Organization, many people rely on unsafe sources of water, which significantly increases the risk of waterborne diseases [3]. Therefore, there is an urgent need to degrade the toxic materials in the industry’s wastewater before they are discharged into the environment. Among the various methods that are used for wastewater treatment, photocatalysis has gained wider attention because it can be carried out under ambient conditions. It is less expensive, non-hazardous, and reusable, and the oxidant is strong enough for complete mineralization [4,5]. Photocatalysis uses a class of materials called semiconductors, and titanium dioxide (TiO2) has attracted lots of interest because of its high availability, high stability, low cost, and environmental friendliness [6,7]. However, the photocatalytic efficiency of TiO2 is adversely affected by its wide bandgap (3.0-3.2 eV) and the rapid recombination of the photogenerated charges [8,9]. One of the proven methods to overcome these challenges is to dope TiO2 with nonmetals such as carbon, nitrogen, boron, and sulphur [10,11]. Doping TiO2 with non-metal creates less recombination centers compared to doping with metals [12]. In addition, anion doping causes a red shift in the absorption threshold of TiO2, resulting reduction of the bandgap [13]. In comparison to other anion dopants, nitrogen doped TiO2 has been reported to be more effective for photocatalytic degradation of organic pollutants [14,15]. Up till date, there is no generally accepted fact about the dopant nature and electronic structure of nitrogen doped TiO2. From the report of Asahi et al [16], they reported that the red shift of the absorption edge and

electronic structure of N-doped TiO2 is due to the Ti-N bonding. They further concluded that that the N-atoms are capable of substituting the O-atoms in TiO2, and the mixing of the N 2p states and O 2p state causes the narrowing of bandgap. D. Oliver et al [17] also doped nitrogen into TiO2 via NH3 and reported that the increased photocatalytic activity in the N-doped TiO2 is due to a co-doping effect between nitrogen and hydrogen. The hydrothermal method has some advantages compared to the chemical vapor condensation and micro-emulsion, such as easy control of reaction of kinetics by varying the composition of the chemicals, high chemical homogeneity, and cost effectiveness [18]. Treating TiO2 with plasma has been used to improve performance of TiO2 [19,20], and it is actually preferred because it creates highly stable Ti3+ and oxygen vacancies [21]. The oxygen vacancy is one of the most prevalent defects in metal oxides and is closely related to the photocatalytic properties of TiO2 [22]. 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 [23,24]. In this work, using ammonium hydroxide (NH4OH), nitrogen was doped into TiO2 via the hydrothermal method, and the synthesized composite was further modified with ambient plasma to improve the photocatalytic performance. The modification of samples was characterized and the photocatalytic activities were evaluated. It was eventually found that the nitrogen doping and ambient plasma treatment introduced reduction of the bandgap, generation of defects and Ti3+, increase of the active surface area, and the combination of these effects led to enhancement of photocatalytic activities.

2.

Materials and Methods 2.1 Preparation of catalysts TiO2 (99.8 %, 25 nm, CAS #: 1317-70-0, Sigma-Aldrich) and NH4OH (Sigma-Aldrich,

CAS #: 7664-41-7) were used without any further purification. 200 mg of TiO2 was dispersed in 40 ml of NH4OH and magnetically stirred for 3 hrs. The resulting mixture was transferred into an autoclave and maintained at the temperature of 100°C for 12 hrs. The mixture of TiO2 and NH4OH was then diluted with deionized water to attain a pH of 7, followed by typical filtering process and then drying at 90°C for 12 hrs. The obtained sample was grinded into powder and referred to as N-TiO2. Plasma treated N-TiO2 was synthesized as followings. 200 mg of TiO2 was dispersed in 40 ml of NH4OH and magnetically stirred for 3 hrs. The resulting mixture was transferred into an autoclave and maintained at a temperature of 100°C for 12 hrs. Atmospheric plasma was then applied to the mixture solution for 1 hr. 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, kovax-needle 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, and the needle also moved the whole area of the solution at a constant rate for the homogeneous plasma application for 1 hr. The schematic diagram of the experimental plasma setup is described in Fig. S1. The plasma treated sample was then diluted with deionized water to obtain a pH of 7. The mixture was then filtered, and the sediment was dried at 90°C for 12 hrs. The obtained sample was grinded into powder and named PN-TiO2. 2.2 Characterization Techniques Surface morphologies of the samples were studied by using scanning electron

microscopy (SEM, S-4300, Hitachi, Japan) while energy dispersive X-ray analysis (EDX) was combined to map the elements. The samples are attached on an aluminum stub by a selfadhesive carbon tape for the SEM and EDX measurements. The crystalline structure of composites were analyzed by powder 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). Xray photoelectron spectroscopy (XPS, Thermo scientific, USA) with Al Kα X-ray source was used to investigate the chemical states of the samples, and the samples were mounted individually onto a carbon tape. The photoluminescence (PL) emission spectra of the samples were measured by using the spectrofluorometer (Jobin Yvon Flurolog-3-11) equipped with Xe lamp. The powder sample was pressed onto the sample holder with a glass slide for the PL measurement. Ultraviolet-visible spectroscopy (UV-VIS, CARY 5000, Varian) was also used to characterize the degradation of methyl blue (MB) dye and absorption spectra. 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 x 10-4 M, 10 mg of MB in 200 mL of deionized water) and stirred magnetically in the dark for 30 min to reach equilibrium of absorption and desorption. Analytikjena UV lamp (UVGL55, 245 nm, 6W, US) was used as a UV source at the distance of 110 cm from the solution, and the solution of 3 mL was taken out every 15 min during the irradiation followed by the centrifugation at 7000 rpm for 30 min in order to remove the residue of the photocatalyst. The remaining MB, after the photocatalysis, was calculated by UV-VIS spectroscopy. 2.4 Photoelectrochemical Measurements

The photoelectrochemical measurements were performed using a three electrode system with H2SO4 (0.1M) (7664-93-9, 99.99%, Sigma-Aldrich) 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 (190764, ≥ 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 on-off intervals (30 sec) of UV light. Linear sweep voltammetry (LSV) was carried out under the irradiation of UV light with the scan rate of 10 mVs-1. Chronocoulometry (CC) was also performed to determine electrochemical active surface area and adsorption capacity by using 0.1 M K3[Fe(CN)6] with the pulse width of 0.1 s.

3.

Results and Discussions The SEM images of TiO2, N-TiO2, and PN-TiO2 are shown in Fig. 1. TiO2 had an

image of irregularly shaped particles, bounded together. After the doping of nitrogen, it can be seen that particles became more agglomerated as shown in Fig. 1b. Plasma is an ionized gas which comprises of electrons, molecules, excited species, and radicals whose energies are high enough to cause dispersion when they hit particles. Therefore, the particles became less agglomerated after the plasma treatment, as shown in Fig. 1c. However, there was no significant morphological change found after the hydrothermal and plasma treatment. It is worth noting that the particle sizes are estimated to be less than 30 nm from the SEM results. The XRD spectra of the samples are displayed in Fig. 2a. From the XRD study, the phase of TiO2 could be assigned to the anatase structure [25]. No additional peaks related to

the doped nitrogen were observed in the XRD pattern. It is remarkable to note that the intensity of the peaks reduced in N-TiO2, and there was further reduction in the peak intensity of PNTiO2. Scherrer’s equation was used to estimate the average crystalline size of the samples. Particle size plays an important role in photocatalytic activity. Smaller crystals give grater surface area, thus increasing the surface absorbability [26]. Using the peak at 24.6°, the average crystalline size of the samples obtained were 24.7, 23.6, and 20.9 nm for TiO2, N-TiO2 and PN-TiO2, respectively. It is evident that the plasma treatment reduced the crystalline size of TiO2, implying that the surface area is enhanced. 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 [27]. 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, diffusion coefficient of the mediator is denoted by D (cm2/sec), and time is denoted by t (sec) [28]. The diffusivity of K3[Fe(CN)6] was determined to be 7.4 x 10-6 cm2/sec, and using the range of 0.8 ≤ t ≤ 1.4, the slope of the equation was estimated. As shown from Fig. 2b, the electrochemical active surface area was estimated by using the Anson equation, which was, 7.52 x10-7, 6.04 x10-6, and 1.02 x10-5 cm2 for TiO2, N-TiO2, and PN-TiO2, respectively. The increase in the active surface area is an important factor for the photocatalytic performance because it assists in the adsorption of the organic pollutants [29]. The active surface area of N-TiO2 and PN-TiO2 is 8 and 14 times larger than that of TiO2. The absorption capacity is also proportional to the active surface area [27,28], concluding that PN-TiO2 has the highest adsorption capacity and active surface area compared

to the others. To confirm the homogenous distribution of nitrogen, EDX elemental mapping was carried out. As seen in Fig. 3, the map shows that nitrogen was successfully and homogenously loaded in both of N-TiO2 and PN-TiO2. The UV-Vis spectra of the samples are shown in Fig. 4a. The pure TiO2 displayed an absorbance in the UV region at a wavelength of around 397 nm. After the nitrogen doping, the absorbance of N-TiO2 is red shifted into the visible region. This shift can be due to the presence of nitrogen in N-TiO2 which causes the bandgap narrowing [30]. The red shift in the absorbance edge occurred again after the plasma treatment, and the absorbance edge of PN-TiO2 was 460 nm. This shift is attributed to Ti3+ and oxygen vacancy which will be discussed later with XPS results. The bandgap energies of the samples were estimated using Tauc’s equation with indirect transition. The bandgap energy of N-TiO2 is 2.9 eV which is lower than 3.1 eV which was obtained for the pure TiO2. PN-TiO2 exhibited the lowest bandgap energy of 2.7 eV (Fig. 4b). The optical results revealed that the doping of nitrogen in TiO2 shifted the absorbance edge of TiO2 into the visible region, and the plasma treatment further shifted the absorbance edge, resulting in the reduction of the bandgap. XPS was used to investigate the surface composition and chemical states of the samples. The elemental composition results are presented in Table 1. TiO2 has the atomic percentage of Ti, O, and C was 24.5%, 54.9%, and 20.6%, respectively. No nitrogen was detected. While N-TiO2 has atomic percentage of Ti, O, N, and C was 23.5%, 54.3%, 1.1%, and 21.1%, respectively, and the atomic percentage of Ti, O, N, and C in PN-TiO2 was 23.4%, 53.9%, 1.3%, and 21.4%, respectively. The results showed that nitrogen was introduced into N-TiO2 and PN-TiO2. Carbon which was found from the XPS results is due to the carbon tape used for the XPS instrument.

The XPS O 1s of TiO2 was deconvoluted into two peaks using the Gaussian function (Fig. 5a). The O 1s peak at the binding energy of 530.4 eV can be attributed to oxygen bonded to Ti (Ti-O-Ti peak), and the peak at 531.5 eV is corresponded to the hydroxyl (OH-) groups on the surface [31]. After the doping of nitrogen, the peak at 530.4 eV was shifted to 529.3 eV, and the OH peak was also shifted to 530.3 eV. The Red shifts of approximately 1 eV has been observed after the nitrogen doping [32,33]. The red shift similarly occurred in the PN-TiO2 sample in which the binding energy of the OH peak was at 530.3 eV, and the lattice oxygen was at 529.2 eV for PN-TiO2. The area of the non-lattice oxygen for N-TiO2 was 26% greater than the non-lattice area of TiO2, and PN-TiO2 was 31% greater than that of TiO2. The increased non-lattice oxygen is related with defects generated in TiO2, causing a red shift in the absorbance edge of TiO2 and leading to better photocatalytic performance [34-36]. Therefore, PN-TiO2 is expected to have the best photocatalytic performance. The XPS Ti 2p spectra of the samples were deconvoluted into three peaks as shown in Fig. 5b. The highest intensity located at 459.27 eV could be easily attributed Ti4+ 2p3/2 and the peak at 465.14 eV was assigned to Ti4+ 2p1/2 in TiO2, demonstrating the presence of Ti4+ in the TiO2 lattice [37]. Additional peak at the binding energy of 460.53 eV corresponds to Ti3+ of Ti2O3 [36]. The doping of nitrogen into TiO2 replaces O2- with N-, which gives rise to an increase in the electron density on Ti because the N atom is less electronegative than the O atom. Thus, the binding energy in both N-TiO2 and PN-TiO2 decreased compared to pure TiO2. The Ti peaks of N-TiO2 were 458.12, 459.23, and 463.93 eV for Ti4+ 2p3/2, Ti3+ 2p3/2, and Ti4+ 2p1/2, respectively. The area of the Ti3+ peak increased by 11% in N-TiO2 and 49% in PN-TiO2 compared to TiO2, expected its positive effect on the photocatalytic performance. The N 1s spectra in Fig. 5c were deconvoluted into two peaks. The peaks located at

399.18 eV can be attributed to N- in the O-Ti-N bond in N-TiO2 [37]. The small bump at 401.25 eV can be assigned to N atoms located at the interstitial sites of TiO2 lattices as Ti-O-N or TiN-O [38,39]. The interstitial insertion of nitrogen into TiO2 lattice is very efficient in the photocatalytic performance [40], and the area of the interstitial nitrogen in PN-TiO2 was three times more than that of N-TiO2, implying that the plasma treatment can promote the photocatalytic activities. Fig. 6a shows a comparison of the photocurrent responses of the samples. 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, indicating that the current was due to the activities of the photoelectrons. The photocurrent of TiO2 was the lowest due to the fast recombination of the photogenerated charge carriers. The drop in the photocurrent indicates that the holes on the surface of TiO2 either recombines with electrons in the conduction band rapidly or just gathered at the surface instead of capturing electrons from the electrolyte. The photocurrent response of N-TiO2 and PN-TiO2 increased by 2.9 and 3.6 times more than that of TiO2, respectively. It is evident that the introduction of nitrogen into TiO2 elongated the lifespan of the holes and electrons. In addition, the increment in the Ti3+, oxygen vacancy, and interstitial nitrogen atoms plays important role for the high photocurrent response [33,37]. It was concluded that N-TiO2 and PN-TiO2 had better photoelectrochemical performances compared to TiO2. Fig. 6b clearly shows that PN-TiO2 exhibited the best photocurrent enhancement, implying that the recombination of photo induced holes and electrons is retarded, and the transfer of electrons to the external circuit can be accelerated [38]. The photocatalytic degradation of MB using the samples was conducted under UV irradiation. Fig. S2 shows the dark adsorption and photodegradation of MB at 15 mins intervals. The intensity peak of MB (~600nm) decreased with the irradiation time. Before irradiation, the

dark adsorption of MB by PN-TiO2 was the best. This is due to the increased surface area as shown in the CC measurements. Pure TiO2 degraded MB completely within 135 mins (Fig. S2a), while N-TiO2 and PN-TiO2 degraded it within 90 mins (Fig. S2b) and 60 mins (Fig. S2c), respectively. Using the formula below, the percentage photo-degradation of MB was estimated: (Co – Ct) / Co x 100% Where Co is the initial concentration of MB, and Ct is the concentration of MB after photoirradiation at a given time t. MB degradation percentage versus time is displayed in Fig. 7a, and PN-TiO2 had the fastest degradation of MB. The introduction of Ti3+ levels, interstitial insertion of nitrogen and oxygen vacancies promoted the lifespan of the photogenerated charge pairs [39,40], thus making PN-TiO2 to achieve the highest photocatalytic performance. In addition, PN-TiO2 has the largest active surface area, implying that a large amount of MB can be adsorbed on the surface leading to rapid degradation. Langmuir-Hinshelwood kinetic model was used to estimate the rate constant of the degradation of MB by the samples, and the results are presented in Fig. 7b. The rate constant was 0.009, 0.023, and 0.031 min-1 for TiO2, N-TiO2, and PN-TiO2, respectively. Previous results where MB was also degraded are summarized in Table 2 for the comparison. The PL spectrum was used to investigate the separation and recombination of the photogenerated charge carriers. Low intensity peaks are obtained if there is a higher separation rate of the electrons and holes [41]. Fig. 7c shows the PL spectra of the sample by using an excitation wavelength of 325 nm. It is clear that PN-TiO2 exhibits the weakest PL intensity compared with N-TiO2 and pure TiO2. This implies that the recombination of the electrons-holes pair was significantly retarded, and the separation of the photogenerated charge carriers is accomplished.

The reusability of PN-TiO2 was investigated in Fig. 8. PN-TiO2 was removed after the MB degradation experiment in 60 mins by centrifugation, and then the same MB degradation experiment was performed in 60 min using the centrifuged PN-TiO2. The corresponding degradation percentage was 97.1, 96.4, 95.8, 94.9, and 94.2% for cycle 1, cycle 2, cycle 3, cycle 4, and cycle 5, respectively. This implies that PN-TiO2 has good stability, sustainability, and reusability.

4

Conclusion Nitrogen doped TiO2 have been successfully prepared by the hydrothermal method

and further modified by ambient plasma. XRD results showed that the crystalline size of the plasma treated TiO2 decreased by 4 nm compared to precursor TiO2. The electrochemical active surface area of the plasma treated TiO2 increased by 14 times compared to TiO2, thus allowing more absorption of MB to its surface which aided the photocatalytic degradation. Additionally, the ambient plasma treatment caused narrowing of the bandgap by creation of Ti3+ levels and oxygen vacancies and increase of the interstitial doping of nitrogen into the lattice of TiO2. The plasma treated TiO2 finally degraded MB within 60 mins, while TiO2 degraded MB within 135 mins, because of the synergetic effect of all factors.

Acknowledgement This work was supported by the Daegu University Research Grant, 2018.

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Fig. 1: SEM image of (a) TiO2, (b) N-TiO2, and (c) PN-TiO2

Fig. 2: (a) XRD pattern and (b) Anson plot of TiO2, N-TiO2, and PN-TiO2.

Fig 3: EDX elemental mapping of (a-c) N-TiO2 and (d-f) PN-TiO2.

Fig. 4: (a) UV-Visible absorption spectra and (b) the bandgap energy of the samples.

Fig. 5: XPS results of (a) O 1s, (b) Ti 2p, and (c) N 1s of the samples.

Fig. 6: (a) Transient photocurrent responses and (b) LSV curves of the samples under the UV irradiation.

Fig. 7: (a) Photocatalytic degradation of MB, (b) kinetic plot of the degradation of MB by the samples, and (c) PL spectrum of the samples.

Fig 8: Stability test of the PN-TiO2 sample.

Table 1: Elemental Composition data from XPS results. Sample

Ti (at%)

O (at%)

N (at%)

C (at%)

TiO2

24.5

54.9

0.0

20.6

N-TiO2

23.5

54.3

1.1

21.1

PN-TiO2

23.4

53.9

1.3

21.4

Table 2: Kapp values from previous results Material

Light Source

Kapp (min-1) Ref.

PAni-TiO2

Visible light

7.8 x 10-3

42

LDH-VB9-TiO2

UV light

0.85 x 10-2

2

7.93 x 10-2

5

Vacuum

Ultraviolet- Vacuum Ultraviolet light

TiO2 MoS2-TiO2

UV light

4.0 x 10-2

31

PN-TiO2

UV light

3.1 x 10-2

This work

Graphical abstract

Highlights  Introduction of ambient plasma for the enhanced photocatalytic activity of TiO2.  Resultant photocurrent response increased by 3.6 times.  The bandgap of TiO2 reduced from 3.1 eV to 2.7 eV after the plasma treatment.

Declaration of interests

☒

The authors declare that they have no known competing financial interests or personal

relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Olaniyan Ibukun: Experiment and writing draft manuscript. Hae Kyung Jeong: Corresponding author, Reviewing and Editing.