Graphene composite nanofibers as a high-performance photocatalyst for environmental remediation

Accepted Manuscript Graphene composite nanofibers as high-performance photocatalyst for environmental remediation Muzafar.A. Kanjwal, Kenneth Kin Shing Lo, Wallace Woon-Fong Leung PII: DOI: Reference:

S1383-5866(18)34467-8 https://doi.org/10.1016/j.seppur.2019.01.044 SEPPUR 15274

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

18 December 2018 16 January 2019 19 January 2019

Please cite this article as: Muzafar.A. Kanjwal, K. Kin Shing Lo, W. Woon-Fong Leung, Graphene composite nanofibers as high-performance photocatalyst for environmental remediation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.01.044

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Graphene composite nanofibers as high-performance photocatalyst for environmental remediation Muzafar. A. Kanjwal*, Kenneth Kin Shing Lo, Wallace Woon-Fong Leung **

Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong Corresponding author. *E-mail address: [email protected] (M. A. Kanjwal). **E-mail address: [email protected] (W.W.-F. Leung).

ABSTRACT: It is imperative to design and develop a technology which would completely remove pollutants from contaminated waters due to agricultural, industrial, and domestic uses. We discovered that graphene incorporated composite nanofibers (TZB-Gr) (Titanium dioxide-Zinc Oxide-Bismuth Oxide-Graphene) with band gap of (2.5eV) can effectively activate organic dyes under visiblelight and UV-light irradiation to produce active ·O2− and ·OH radicals. The produced radicals are powerful oxidizing species to degrade most of the organic pollutant to become CO2 and H2O. TZB-Gr demonstrated a higher activity than TZB (Titanium dioxide-Zinc Oxide-Bismuth Oxide) and P25 (Commercial TiO2 nanoparticles). Kinetic study of composite nanofibers (NFs) was carried out. Furthermore, a reasonable catalytic mechanism of the TZB-Gr (NFs) was proposed, based on electron-hole pairs and recombination of phtogenerated charges. It was shown that graphene based nanofiber photocatalysis is superior to that on transition metal oxide (TZB) and P25 in degradation of a dyes (methylene blue, MB) and (rhodamine B, RhB) in water, therefore providing a novel strategy for environmental remediation. Keywords: TiO2/ZnO/Bi2O3-graphene nanofibers; Visible Liight; Ultavoilet; Photocatalyst; Kinetics; methylene blue; and rhodamine B

1

Introduction: With the rise of population and the evolution of industrialization, the environment pollution has become major concern [1]. The pollution undermines the ecosystem and has harmful impacts on aquatic life [2]. The water and environment pollution could have negative impact on the human health and could result in several major health issues [3]. Organic pollutants are the primary contaminants in water and environment. Out of numerous organic pollutants available, dyes are extensively used synthetic chemicals that are mainly released from industries of cosmetic [4,5], textile [6,7], leather [8], paper [9], etc. It is well documented in literature that more than 1 × 105 commercial dyes are present, and above 7 × 105 ton of dye solutions are discharged yearly in different water bodies [10]. Organic dyes having extensive applications and high production rate, their discharge into water streams without quick fix has provoked public attention. It is a great challenge to environmental researchers and scientists; dyes even at low concentrations (less than 1ppm) are threat to ecosystem [11–15]. The wastewater from dye industries is very difficult to treat unfortunately. There are various technologies available to treat organic dyes such as biodegradation [12], coagulation [16], adsorption [17–20], photocatalysis [21], and ozonation. Among these technologies, photocatalysis is a critical technique for purification of waste water, which has attracted comprehensive consideration in scientific and industrial associations due to its capability to convert the harmful compounds to non-toxic compounds [22]. Therefore, it is imperative to develop, design and execute effective photocatalysts for dyes in wastewater analysis. It has been reported in the past decades that the breaking down of organic dyes by photocatalysts such as using TiO2 nanomaterials are favorable for organic wastewaters treatment [21,23,24]. It is well known; photocatalytic reactions depend upon the structure and active sites of catalyst, thus it is imperative to design the catalyst of the exceptional composition and arrangement [25]. The evolution of nanotechnology and the use of advanced carbon nanomaterials have grown tremendously to address these issues. Graphene is a “super star” on the forefront of photocatalysis. Graphene-semiconductor photocatalysis have been widely explored for the photocatalytic break down of organics and have been reported more competent than the solo semiconductor photocatalysis [26–30].

2

Graphene has earned substantial consideration in both engineering and scientific associations, ever since, the first reported through mechanical exfoliation [31]. Tremendous efforts have been devoted to graphene semiconductor photocatalysts to improve their photocatalytic performance. Different research groups have embraced different methods for photocatalytic applications [32–35]. However such methods and architecture has serious limitations as at the margins of graphene sheets electron and holes recombine, and resulting in erratic electron transport. These limitations mentioned above are addressed in our unique and novel structure by presenting graphene sheet in a dense, circular roll in the core of the TiO2/ZnO/Bi2O3 (NFs) (TZB-Gr (NFs) which acts as a shell) to speed up electron-hole pair separation and charge transport. The combination of graphene and TiO2, ZnO, Bi2O3, not only significantly amends light harvesting ability but also omits the border effect, and renders electrical charges a proper transport path along the nanofiber axis. In this work for the first time TZB and TZB-Gr (NFs) are reported in degradation of model dyes (methylene blue, MB) and (rhodamine B, RhB) representing organic pollutants.

Methodology Materials Zinc acetate dehydrate, bismuth (III) nitrate pentahydrate, Titanium tetra-isopropoxide (TIIP), polyvinyl pyrrolidone (PVP) (MW = 1,300,000), graphite powder (<20nm), isometric acetic acid and benchmark test TiO2 nanoparticles (Degussa P25), Methylene blue (CAS: 122965-43-9) and Rhodamine B (CAS: 81-88-9) were all purchased from Sigma–Aldrich, while ethanol was purchased from Advanced Technology & Industrial Company. All reagents were of analytical grade and used without any further purification.

Fabrication The graphene/PVP solution was typically blended for 20 min until a suspension of graphene and graphite with PVP in the solution is achieved. Commercially available graphite powder 10 wt.% was added to the 5% PVP (in ethanol) solution and blended with a Philips HR2096 blender at 21,000 rpm. The turbulence caused by blender lead to exfoliation and free floating graphene. To prevent re-aggregation of the graphene into graphite, graphene was subsequently 3

bound to the PVP in the solution. The suspension formed was then centrifuged at 9993g (1g = 9.81 m/s2) for 3–10 min for separation of large particles to obtain pure graphene suspension [36]. The content of graphene in the electrospun TZB-Gr (NFs) was achieved by controlling centrifugation time; the amount of graphene was found to be inversely proportional to the centrifugation time with less graphene in longer time centrifugation and vice versa. 3 ml Ti(OiPr)4 and isometric acetic acid, together with formulation of eg. (0.1g) Zn (CH3COO)2.2H2O and (0.2g) Bi(NO3)3.5H2O were then added to the suspension to form a precursor solution for electrospinning. Electrospinning using nozzle-less setup was utilized under the following parameters: supplied voltage, 70 kV; electrode-to-collector distance, 20 cm; and electrode rotating speed, 30 Hz. The resulting electrospun fibers were treated for 1 h at 600 oC to obtain the TZB and TZB-Gr (NFs).

Photocatalytic measurements The photocatalytic experiment reaction suspension was prepared by adding 0.5 g L-1 catalysts into MB and RhB aqueous solutions with the initial concentration of dyes 0.01 g L-1. Before the irradiation, the dye suspension loaded with catalysts was magnetically stirred in dark for 20 min to reach an adsorption and desorption equilibrium, which is known as dark adsorption. The test photoreactor was 400 mm high with a square cross-section of 300 × 450 mm2. The reactor barrel has a diameter of 250 mm and a depth of 375 mm. The photoreactor equipped with six top and eight side lamps provided with two complete sets of lamps. The light sources of 14 UVC (254 nm) lamps and 14 VIS (Cool white fluorescent tubes) lamps (LUZCHEM LZC-4Xb photoreactor) were symmetrically placed in the LZC-4X photochemical reactor LUZCHEM (Canadian Company). The light source can be changed with lamps of different wavelength while the intensity of irradiation can be modified by rearranging the number of lamps involved in the test. A UV–Vis spectrophotometer (Agilent Technologies Cary 8454) was used to determine the concentration of MB and RhB solutions. The photocatalytic activity of the photocatalyst can be quantitatively evaluated by the reaction rate constant. The experimental setup is shown in Fig. 1.

4

Fig. 1. Schematic diagram of test photoreactor.

Characterization The surface morphologies of (NFs) were investigated using scanning electron microscope (SEM) (JEOL Model JSM-6490). Phase analyses were carried out using XRD (Rigaku SmartLab) with Cu Ka radiation in the range of 20–90o (2ϴ) at room temperature. Raman spectrum was performed by Horiba HR 800. Photoluminescence (PL) spectra were recorded using an Edinburgh FLSP920 spectrophotometer. Electrochemical impedance spectroscopy (EIS) measurement was carried out with electrochemical workstation (CH Instruments CHI660c). The TZB and TZB-Gr (NFS) were peeled off from the glass slide after 1 h heat treatment and subsequently transferred to another FTO glass precoated with an ultra-thin adhesive layer of TiO2 paste. The device for the EIS testing was obtained after it was calcinated again at 450 oC for 2 h. The thickness was determined to be 20 µm from Surface Profilometer (VeekoDektak 8) and the active area was 0.15 cm2.

Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated

photocurrent spectroscopy (IMPS) were executed using a Zahner CIMPS photo-electrochemical workstation (Zahner-Elektrik) controlled by CIMPS and Thales Z software packages (Zahner5

Elektrik). The photo-anodes were backlit with a 370 nm LED, 20Wm-2. In the IMVS experiments, the ac frequency scanned from 500 Hz the AC frequency range is run from 100 Hz

1 kHz

10 kHz

100 mHz. In IMPS experiments, 1 Hz. In both measurements, 10

measuring steps (frequencies)/decade were carried out in the tests. Five measurement points were averaged at <66 Hz with 20 points averaged per measurement frequency at higher frequencies. The thermal decomposition behavior of TZB and TZB-Gr (NFs) were examined using a thermo gravimetric analyzer and differential scanning calorimeter (TGA–DSC) (Netzch) under ambient pressure in the temperature range between 30 oC and 900 oC at a controlled heating rate of 10 oC min-1. UV–vis diffuse reflectance spectra (DRS) were measured and recorded by a Varian Cary 100 Scan UV–Vis system equipped with a Lab sphere diffuse reflectance accessory to obtain the reflectance spectra of the catalysts over a range of 290–800 nm. BaSO4 (Lab sphere USRS-99-010) was used as a reference in the measurement. The measured spectra were converted from reflection to absorbance by the Kubelka–Munk equation. Photocatalytic tests were performed in photoreactor (LUZCHEM LZC-4Xb) and the spectra obtained were analyzed by UV–Vis spectrophotometer (Agilent Technologies Cary 8454).

6

Results and discussion Photocatalytic degradation of MB dye under Visible light

a

Fig. 2. Adsorption and photodegradation of MB (0.01 g/L-1) with P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1). Insert in (a) Rate constants of the adsorption of MB of different diameter of fibers under visible light irradiation intensity of (19780 lx).

The photocatalytic activity of P25, and TZB and TZB-Gr composite (NFs) were determined by the photocatalytic degradation of MB dye at (665 nm) wavelength under the irradiation of visible lights (19780 lx) intensity. MB has been used as a model dye molecule for photocatalytic degradation by a transition metal oxide. Figure 2. shows the degradation of the concentration of MB as a function of time during experiment with different photocatalysts in the precursor solution. The dark adsorption behavior of MB for TZB and TZB-Gr composite (NFs), each with different nanofiber diameter, is shown in the insert of Fig. 2(a). The adsorption rate constant (k) behaved inversely related to the diameter of the nanofiber. This is due to the fact that larger diameter nanofiber results in smaller specific surface area for adsorption. The dark adsorption can play an important role in reducing the MB in a two-step process (adsorption followed by oxidation) by furnishing small-diameter (NFs). It is important to note, that the wavelength of absorption peak of MB is 665 nm, therefore photosensitization at 352-nm irradiation should be ruled out, and the degradation of MB in the presence of P25, TZB and TZB-Gr are strictly 7

attributed to photocatalytic reaction. The degradation rate of MB was determined also from the rate constant based on the first order equation in Fig. 2(b). The larger is the degradation rate of MB, the higher is the photocatalytic activity.

0 -0.5 -1

ln(C/Co')

b

y = -0.0008x R² = 0.9233 y = -0.0088x R² = 0.9642

C/Co'=exp(-kt)

-1.5 -2

y = -0.0371x R² = 0.9898

-2.5

-3 0

20

40

50mg catalyst, MB 0.01mg/L

60

80

100

120

140

Time (min)

Fig. 2. (b) Rate constants of photocatalytic degradation of MB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) and visible light irradiation intensity of (19780 lx). Fig. 2(b) shows the different degradation rate constants of P25, and TZB, TZB-Gr composite (NFs). The visible light intensity to which the MB solution and photocatalyst were exposed was determined from the (UVA/ VIS power meter). When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0008), utilizing TZB photocatalyst the rate constant (k) increased 11 times to (0.0088) than P25. In case of TZB-Gr photocatalyst the rate constant (k) was found to (0.0371), 46 times faster than P25 and 11 times faster than TZB. To confirm the advantages of the new nano-photocatalysts, benchmark tests were carried out among commercial P25, as-prepared TZB (NFs) and TZB-Gr (NFs) under visible light irradiation. These three photocatalysts, all at the same dosage (50mg), were tested respectively with identical initial concentration of MB (0.01g/l); and the results are depicted in Fig. 2(c). The photocatalytic reactions took place after dark reaction when the adsorption and desorption equilibrium was achieved. The tests were carried out in closed chamber equipped with visible 8

lights. After 120 min irradiation, 9.75%, 63.22%, and 99.86% of MB were degraded by the P25, TZB and TZB-Gr NFs, respectively. P25 degraded only 9.75%. P25 nanoparticles are single crystalline in structure, where as TZB and TZB-Gr (NFs) have a poly-nanocrystallite structure with each crystallite about 10 nm, packed on the (NFs) [25]. These nanocrystals have nanopores that promote the adsorbance of the target compound MB, thus enhancing the photocatalytic reaction. The benefit is more distinct as the diameter of the (NFs) is further reduced. Besides the band gap energy of TZB and TZB-Gr (NFs) is lower than P25, the excitation energy of the photocatalyst is also diminished leading to the red shift of the absorption range and increasingly utilizing visible light. This process also enhances the time period of the separation of electrons and holes, thereby decreasing the recombination rate [18]. All of these advantages enhance the photocatalytic reaction activity of TZB and TZB-Gr (NFs) in relation to P25 nanoparticles as evident in Fig. 2(c). This provides a fundamental reason on the fast kinetics of TZB-Gr (NFs), followed by TZB (NFs), and lastly P25 nanoparticles in spite of the diameter of these test nanophotocatalysts are in inverse trend. 1

c

0.9

P25 C/Co'=exp(-0.0008t)

0.8

C/Co'

0.7

TZB C/Co'=exp(-0.0088t)

0.6 0.5 0.4 0.3 0.2

TZB-Gr C/Co'=exp(-0.0371t)

0.1 0

0 20 50mg catalyst, MB 0.01mg/L

40

60

80

100

120

140

Time (min)

Fig. 2. (c) Photocatalytic degradation of MB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) under visible light irradiation intensity of (19780 lx). Photocatalytic degradation of MB dye under UV light

9

a

Fig. 3. Adsorption and photodegradation of MB (0.01 g/L-1) with P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1). Insert in (a) Rate constants of the adsorption of MB of different diameter of fibers under UV irradiation intensity of (1380 lx).

Methylene blue (MB), a cationic dye having molecular formula C16H18N3SCl, is also often chosen as a model contaminant in studies. It is a common commercial dye. In addition to the visible light Photocatalytic degradation, the photocatalytic activity of P25, and composite (NFs) TZB, TZB-Gr were determined by the photocatalytic degradation of MB dye (665 nm) under the UV lights. Figure 3. shows the degradation of the concentration of MB as a function of time during experiment with different photocatalysts in the precursor solution. The adsorption rate constant (k) behaved inversely related to the diameter of the nanofiber, is shown in the insert of Fig. 3(a). Due to small-diameter (NFs), the dark adsorption can play an important role in reducing the MB in a two-step process (adsorption followed by oxidation). Like in visible light degradation of MB, the UV light degradation of MB obeyed first order kinetics and larger the degradation rate of MB, the faster the photocatalytic activity of different photocatalysts.

10

0 -2

0

50

100

-6

-12

b

y = -0.0162x R² = 0.9543

-4

ln(C -8 /Co' ) -10

150

y = -0.0482x R² = 0.9378 y = -0.1557x R² = 0.9799

C/Co'=exp(-kt)

-14 -16 -18 -20

50mg catalyst, MB 0.011mg/L Time (min)

Fig. 3. (b) Rate constants of photocatalytic degradation of MB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) and UV light irradiation intensity of (1380 lx).

Figure 3(b), shows the different degradation rate constants of P25, and composite (NFs) TZB, TZB-Gr. When P25 photocatlyst was used, the rate constant (k) of MB was about (0.0162), utilizing TZB photocatalyst the rate constant (k) increased more than 2.9 times to (0.0482) than P25. In case of TZB-Gr photocatalyst the rate constant (k) was found to (0.1557), more than 9.6 times faster than P25 and around 3.2 times faster than TZB. To study the advantages of these nano-photocatalysts, different tests were carried out using commercial P25, as-prepared TZB (NFs) and TZB-Gr (NFs) under UV light irradiation. These three photocatalysts, all at the same dosage (50mg), were tested respectively with identical initial concentration of MB (0.01g/l); and the results are shown in Fig. 3(c). The photocatalytic reactions took place after dark reaction when the adsorption and desorption equilibrium was achieved. The tests were carried out in closed chamber equipped with UV lights. After 120 min irradiation, 51.41%, 83.75%, and 99.35% of MB were degraded by the P25, TZB and TZB-Gr NFs, respectively. If we compare UV light degradation with visible light degradation, as expected P25 nanoparticles showed better photocatalytic degradation of 51.41% in UV to 9.75% in Visible lights, TZB NFs showed 83.75% and 63.22% degradation in UV and visible lights respectively, whereas TZB-Gr behaved very 11

closely and exhibited more than 99% degradation in both cases. The many advantages of TZB and TZB-Gr (NFs) (already discussed) in relation to P25 nanoparticles provides a fundamental reason on the fast kinetics and the ultimate conversions of organics as evident in Fig. 3(c). 1

c

0.9 0.8

P25 C/Co'=exp(-0.0162t)

0.7 0.6

C/C 0.5 o'

TZB C/Co'=exp(-0.0482t)

0.4 0.3

TZB-Gr C/Co'=exp(-0.1557t)

0.2 0.1 0 0

20

40

50mg catalyst, MB 0.011mg/L

60

80

100

120

140

Time (min)

Fig. 3. (c) Photocatalytic degradation of MB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) under UV light irradiation intensity of (1380 lx). Photocatalytic degradation of RhB dye under Visible light

a

12

Fig. 4. (a). Adsorption and photodegradation of RhB (0.01 g/L-1) with P25, TZB and TZBGr (NFs) in suspension of catalyst (0.5 g/L-1). Insert (a) Rate constants of the adsorption of MB of different diameter of fibers under visible light irradiation intensity of (19780 lx).

RhB is a toxic dye having molecular formula (C28H31N2O3Cl) used extensively in different industries, which must be remediated before its release to environment. (RhB) is one of the famous dyes and is widely used as a colorant agent in foodstuffs and textiles due to its high stability. It is dangerous to human beings and animals and causes irritation of the eyes, skin, and respiratory tract. The photocatalytic activity of P25, TZB and TZB-Gr composite (NFs) were determined by the photocatalytic degradation of RhB dye at a wavelength of (555 nm) under the irradiation of visible lights intensity of (19780 lx). RhB has been used as a model dye molecule for photocatalytic degradation by these nano-photocatalysts. Fig. 4. shows the degradation of the concentration of RhB as a function of time during experiment with P25, TZB and TZB-Gr composite (NFs) in the precursor solution. The dark adsorption behavior of RhB for TZB and TZB-Gr photocatalysts, each with different nanofiber diameter, is shown in the insert of Fig. 4(a). The adsorption rate constant (k) is inversely related to the diameter of the (NFs). The dark adsorption plays an important important role in reducing the RhB in a two-step process adsorption followed by oxidation. The photodegradation rate of RhB was determined from the rate constant based on the first order equation in Fig. 4(b). The larger is the degradation rate of RhB, the higher is the photocatalytic activity.

13

b

0 -0.2

0

50

100

-0.4

150 y = -0.0009x R² = 0.8589

y = -0.0105x R² = 0.9875

-0.6 -0.8 -1

ln( C/C -1.2 o')

C/Co'=exp(-kt)

-1.4 -1.6

y = -0.0139x R² = 0.9814

-1.8

50mg catalyst, RhB 0.011mg/L

Time (min)

Fig. 4. (b) Rate constants of photocatalytic degradation of RhB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) and visible light irradiation intensity of (19780 lx). Figure 4(b) shows the different degradation rate constants of P25, and composite (NFs) TZB, TZB-Gr. The visible light intensity to which the RhB solution and photocatalyst were exposed was determined from the (UVA/ VIS power meter). When P25 photocatlyst was used, the rate constant (k) of RhB was about (0.0009), utilizing TZB photocatalyst the rate constant (k) increased 11 times to (0.0105) than P25. In case of TZB-Gr photocatalyst the rate constant (k) was found to (0.0139), 15 times faster than P25 and 1.3 times faster than TZB. These three photocatalysts, P25, TZB and TZB-Gr, all at the same dosage (50mg), were tested respectively with identical initial concentration of RhB (0.01g/l); and the results are displayed in Fig. 4(c). The photocatalytic degradation reactions took place after dark reaction when the adsorption and desorption equilibrium was achieved. After 120 min of visible light irradiation, 12.58%, 53.99%, and 73.86% of RhB were degraded by the P25, TZB and TZB-Gr NFs, respectively. P25 degraded only 12.58%.

14

The nanocrystals of TZB and TZB-Gr have nanopores that promote the adsorbance of the target compound RhB, thus enhancing the photocatalytic reaction. Lower band gap, higher separation of electrons and holes, lower recombination rates and fast kinetics enhance the photocatalytic reaction activity of TZB and TZB-Gr (NFs) in relation to P25 nanoparticles as evident in Fig. 4(c).

1

c

0.9

P25 C/Co'=exp(-0.0009t)

0.8

0.7

TZB C/Co'=exp(-0.0105t)

C/Co'

0.6 0.5 0.4 0.3 0.2

TZB-Gr C/Co'=exp(-0.0139t)

0.1 0 0

20

40

50mg catalyst, RhB 0.011mg/L

60

80

100

120

140

Time (min)

Fig. 4. (c) Photocatalytic degradation of RhB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) under visible light irradiation intensity of (19780 lx).

Photocatalytic degradation of RhB dye under UV light

15

a

Fig. 5. (a). Adsorption and photodegradation of RhB (0.01 g/L-1) with P25, TZB and TZBGr (NFs) in suspension of catalyst (0.5 g/L-1) under UV irradiation intensity of (1380 lx).

In addition to the visible light Photocatalytic degradation, the photocatalytic activity of P25, and composite (NFs) TZB, TZB-Gr were determined by the photocatalytic degradation of RhB dye at a wavelength of (555 nm) under the UV lights of (1380 lx) intensity. Fig. 5(a) shows the photocatalytic degradation of RhB with different photocatalysts in the precursor solution. The dark adsorption plays a crucial role in reducing the RhB in a two-step process (adsorption followed by oxidation). Like in visible light degradation of RhB, the UV light degradation of RhB obeyed first order kinetics and larger the degradation rate of RhB, the faster the photocatalytic activity of different photocatalysts.

16

0

0

20

40

60

80

-1

100 120 y = -0.0181x R² = 0.9786

b

-2

ln( C/C o')

-3

-4

y = -0.0331x R² = 0.9723

C/Co'=exp(-kt) -5 -6 -7

50mg catalyst, RhB 0.011mg/L

y = -0.0624x R² = 0.982

Time (min)

Fig. 5. (b) Rate constants of photocatalytic degradation of RhB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) and UV irradiation intensity of (1380 lx).

Figure 5(b) shows the different degradation rate constants of P25, and composite (NFs) TZB, TZB-Gr. When P25 photocatlyst was used, the rate constant (k) of RhB was about (0.0181), utilizing TZB photocatalyst the rate constant (k) increased 1.8 times to (0.0331) than P25. In case of TZB-Gr photocatalyst the rate constant (k) was found to (0.0624), 3.4 times faster than P25 and 1.8 times faster than TZB. These three photocatalysts, P25, TZB and TZB-Gr, at the same dosage (50mg), were tested respectively with similar initial concentration of RhB (0.01g/l); and the results are displayed in Fig. 5(c). The photocatalytic degradation reactions took place after dark reaction when the adsorption and desorption equilibrium was achieved. After 120 min of UV light irradiation, 46.44%, 73.47%, and 95.49% of RhB were degraded by the P25, TZB and TZB-Gr NFs, respectively. If we compare UV light degradation of RhB with visible light degradation, as expected P25 nanoparticles showed better photocatalytic degradation of 46.44% in UV to 12.58% in Visible lights, TZB NFs showed 73.47% and 53.99% degradation in UV and visible lights

17

respectively, whereas TZB-Gr NFs exhibited more than 95.49% degradation in UV and 73.86% in visible lights. The nanopores of TZB and TZB-Gr NFs assist the adsorbance of the RhB molecules, thus enhancing the photocatalytic reaction. The synergetic effects of kinetic parameters and thermodynamic properties enhance the photocatalytic reaction activity of TZB and TZB-Gr (NFs) in comparison to commercial P25 nanoparticles as displayed in Fig. 5(c). 1

c

0.9

0.8

P25 C/Co'=exp(-0.0181t)

0.7

C/Co'

0.6 0.5

TZB C/Co'=exp(-0.0331t)

0.4 0.3

TZB-Gr C/Co'=exp(-0.0624t)

0.2 0.1 0 0

20

40

50mg catalyst, RhB 0.011mg/L

60

80

100

120

140

Time (min)

Fig. 5. (c) Photocatalytic degradation of RhB solution using P25, TZB and TZB-Gr (NFs) in suspension of catalyst (0.5 g/L-1) under UV light irradiation intensity of (1380 lx).

Organic Pollutant Degradation Dyestuffs are extensively widely used in textiles, food, coloring paper, medicines, cosmetics, and other applied materials to meet the demands of everyday and better human life. Moreover, most pigments and synthetic dyes are usually highly toxic with very low biocompatibility, and are generally released into the water without any pretreatment, which poses a great threat to the ecosystems.

18

B

A

Fig.6. (a). Molecular Structure of Methyle Blue (MB) (A) and Rhodamine B (RhB) (B). Figure 6(a). shows the molecule structures of RhB, and MB, two representative organic dyes. Both of them have stable structure (lot of benzene rings) and very difficult to degrade under normal conditions. Among great number of composite semiconductors, TZB (NFs) has been proven to be an active photocatalyst [37,38] for the environmental remediation because of its narrow bandgap and suitable band edge position. The reaction mechanism are given by TZB + hv → TZB (h+) + TZB (e−)

(1)

H2O → H+ + OH−

(2)

e− + O2 → ⋅O2−

(3)

⋅O2− + H+ → ⋅HO2

(4)

2 ⋅HO2 → O2 + H2O2

(5)

⋅HO2 + e− + H+ → H2O2

(6)

H2O2 + e− → ⋅OH + OH−

(7)

H2O2 + ⋅O2− → ⋅OH + OH− + O2

(8)

dye + ⋅OH+O2− + h+ → CO2 +H2O + small molecules

(9)

Thereafter, the ·O2− and ·OH radicals produced from TZB (NFs) under visible-light irradiation have strong oxidizing capacity to degrade most of the organic pollutant to become CO 2, H2O or other mineralization products. However, TZB (NFs) suffers from short electron–hole lifetime (ɽn 19

= 0.17 s at 20Wcm-2) due to its narrow and direct bandgap, causing the decrease of radical output and photocatalytic degradation activity. When a suitable amount of graphene is introduced into a TZB (NFs), it could transport electrons away from the TZB and effectively inhibit the recombination of photogenerated electron-hole pairs. The TZB-Gr (NFs) have longer electron– hole lifetime (ɽn = 0.51 s at 20Wcm-2). Thus, more electrons are available to participate in the photocatalytic reactions to produce more oxidizing radicals. In addition, graphene could also increase the adsorption of organic dyes on the nanofiber surface, because inter-plane conjugated π bonds can be easily established between graphene and dye molecules through their benzene ring structures. Therefore, higher loads of dyes on the nanofibrous surface expedite the degradation reactions, resulting in better photocatalytic degradation activity.

Fig. 6. (b), an illustrative scheme of the MB and RhB photodegradation for TZB-Gr (NFs) under Visible light radiation. An illustrative scheme is shown in Figure 6(b) to provide further clarification on the mechanism of the MB and RhB photodegradation for TZB-Gr (NFs) under Visible light radiation. The TZBGr NFs overlap and form a porous network structure that can increase the contact area between TZB-Gr NFs and dyes and adsorb dye molecules effectively.

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Photocatalytic Degradation of MB using TZB-Gr composites (NFs): The energy levels of LUMO and HOMO of MB are –0.25 and 1.61 V vs. NHE, respectively. [39,40] MB can also be self-sensitized by visible light irradiation to form the excited MB*. However, the photogenerated electrons in the LUMO of MB cannot transfer to the CB of TZB (Figure 7), which is different from the cases of RhB degradation. Moreover, as displayed in Figure 7, the photogenerated electrons from the CB of TZB can transfer to MB* and the graphene, leading to longer lifetime of charge carriers. Moreover, the electrons from MB* can transfer to the graphene due to the less negative level and the photogenerated holes from the VB of TZB can be injected into the HOMO of MB to oxide MB molecule. Therefore, efficient photodegradation of MB could be achieved on the TZB-Gr composites (NFs).

Fig.7. Schematic illustration for the MB degradation over TZB-Gr composites photocatalyst under visible-light irradiation. Photocatalytic Degradation of RhB using TZB-Gr composites (NFs): The energy level of the unoccupied molecular orbital (LUMO) and HOMO of RhB are –1.00 and 1.10 V vs. NHE, respectively. It suggests that RhB can be self-sensitized by visible light irradiation to form the excited RhB* molecule. Based on the relative potential position among RhB, TZB, and graphene (Figure 8), the photogenerated electrons from RhB* can transfer to the CB of TZB, forming 21

oxidized dye (RhB+·) and endorsing the charge separation. After that, these electrons, together with the ones from the CB of TZB, transfer to the graphene, further suppress the charge recombination (Figure 8). The electron derived oxidizing radicals (including ·OH and ·O2−) can oxidize RhB into CO2, H2O and other non-hazardous small molecules. [41] In synergy, the photogenerated holes from the VB of TZB can also oxidize RhB into RhB+· by injecting into the HOMO of RhB [42]. Therefore, better photocatalytic degradation of RhB can be accomplished through TZB-Gr composite (NFs) under visible light irradiation.

Fig. 8. Schematic illustration for the RhB degradation over TZB-Gr composites photocatalyst under visible-light irradiation.

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Fig. 9. SEM images of pristine TIP-ZnAc-BiNa and TIP-ZnAc-BiNa-Gr (NFs) (A and B) and calcined TZB and TZB-Gr (NFs) (C and d). TEM image of the prepared (NFs) and SAED in inset TZB and TZB-Gr (NFs) (E and F). The morphology of the (NFs) was investigated by SEM and TEM as shown in Fig.9. Asprepared pristine TIP-ZnAc-BiNa (Titanium Isopropoxide-Zinc acetate-Bismuth nitrate) and TIP-ZnAc-BiNa-Gr (Titanium Isopropoxide-Zinc acetate-Bismuth nitrate-Graphene) hybrid fibers are shown in Fig. (9A and B). As can be seen, fine fibers are dispensed randomly in layers indicating that all of the samples are continuous one-dimensional (1D) fibrous structure with 23

diameters in the range of 300 nm and lengths of several micrometers. The Bi concentration at 0.2%, gives best fibers, which appear smooth and uniform. For higher Bi concentration the morphology is destroyed, which is due to the difference in the coefficients of thermal expansion among these three which are 9×10−6 K−1 (TiO2), 4.75×10−6 K−1 (ZnO), and 18×10−6 K−1 (Bi2O3), respectively [43,44]. The mixing of graphene solution and acetates has not affected the efficiency of electrospinning. SEM images of the calcined TIP-ZnAc-BiNa and TIP-ZnAc-BiNaGr hybrid fibers after calcination at 600 oC for 1 hr are shown in (Fig.9C and D). The decrease in diameter is because of calcination process due to evaporation of water and removal of PVP, or decomposition of ZnAc, BiAc or Ti(OiPr) 4 after calcination, respectively, at 600 oC as shown in (Fig.9C and D). As seen in all these figures, the electrospun (NFs) are haphazardly distributed. Graphene flakes are not visible from the entire SEM image which implies that graphene, are all well incorporated in the TZB (NFs). The beam energy used in SEM analysis was 20 kV. These calcined (NFs) are having one-dimensional (1D) fibrous structure with Avg. diameter of 139nm and 65nm for TZB and TZB-Gr 3min respectively and lengths of several micrometers and are supported by the low magnification TEM images (Fig.9E and F). The TEM picture also displays the internal structure of the composite (NFs), which shows numerous nanocrystals. This unusual structure furnishes nanopores between the nanocrystals, which are highly advantageous for photocatalytic reactions.

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Fig. 10. XRD patterns of TZB, TZB-Gr composite (NFs) and commercial P25 TiO2 nanoparticles. The XRD spectrum of calcinated TZB and TZB-Gr NFs and commercial P25 TiO2 nanoparticles are shown in Fig. 10. It indicates the presence of anatase (JCPDS card No. 21-1272), rutile (JCPDS card No.21-1276), zincite (JCPDS card No.65-682) and bismuth oxide (JCPDS card No. 41-1449) in the composite nanofiber. TZB-Gr composite fibers (Fig.10 C) exhibit similar XRD spectra to pure TZB NFs (Fig.10B), graphene diffraction peaks were not observed in the composite (NFs). This might be due to the low content of graphene used and relatively low diffraction intensity of the graphene. In a word, the peaks for graphene might be shielded by the strong peak of anatase TiO2 at 25.3° [45]. In addition, it is observed that the peak width broadened slightly with the introduction of graphene. Fig. (10A) shows the typical P25 XRD spectra.

25

Conclusion In summary, TZB and TZB-Gr composite (NFs) have been successfully prepared via a sol-gel based nozzle-less electrospinning process. The calcined electrospun (NFs) has been explored for the removal of various organic dyes. The coupling of 2D graphene with photoactive semiconductors (NFs) improves the removal capacity of organic dyes. Additionally, 2D graphene can act as stabilizer, enhancing the durability and stability of photocatalysts. The incorporation of graphene in semiconductor (NFs) not only reduces band gap and electron hole recombination, it also enhances charge transport and specific surface area. Photocatalysts were used to degrade organic dyes (MB and RhB) in visible light and UV light. TZB-Gr exhibited a higher degradation rate (k) of (0.0371 min-1) than the P25 and TZB (k = 0.0008 min-1, 0.008 min1

) under visible light degradation of MB and higher degradation rate (k) of (0.1557 min-1) than

the P25 and TZB (k = 0.0162 min-1, 0.0482 min-1) under ultraviolet degradation of MB. Similarly, TZB-Gr demonstrates a higher degradation rate (k) of (0.0139 min -1) than the P25 and TZB (k = 0.0009 min-1 and 0.0105 min-1) under visible light degradation of RhB and higher degradation rate (k) of (0.0624 min-1) than the P25 and TZB (k = 0.0181 min-1 and 0.0331 min-1) under ultraviolet degradation of RhB. The consequential enhancement in photodegradation rate of MB and RhB by TZB-Gr than P25 and TZB corresponds to the excellent charge separation and transport of photogenerated charge and the joint effect of TiO2, ZnO and Bi2 O3. Conflicts of interest There are no conflicts to declare. Acknowledgement The authors make acknowledgement to the funding source Hong Kong innovation Technology Commission on the University Industrial Collaboration Fund (UICP) on the Project No. UIM/280 and also the Hong Kong Research Grant Council, on the General Research Fund, Project No. 15207917.

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properties

and

Highlights • TZB and TZB-Gr composite (NFs) have been prepared via a sol-gel based nozzle-less electrospinning process. • TZB-Gr composite (NFs) have low band gap, reduced charge recombination and greater charge separation. • Degradation of Organic dyes Methylene Blue and Rhodamine B. • TZB-Gr exhibited a higher degradation rate (k) in both dyes and under Visible and UV light than TZB and commercial TiO2 P25 nanoparticles. • Catalytic mechanism of the TZB-Gr NFs was proposed.

31