Fabrication of heterojunction MnTiO–TiO-decorated carbon nanofibers via electrospinning as an effective multifunctional photocatalyst

Environmental pollution and the energy crisis are considered the major challenges faced by mankind in the 21st century. Approximately 1%–20% of the worldwide dye production from industries is released into water as industrial effluent [1, 2, 3]. Industrial effluent—through oxidation, hydrolysis, or other chemical reactions—produces toxic metabolites in wastewater [4, 5, 6]. According to the UNESCO World Water Assessment Programme (2003), nearly 2 million tons of water per day are discharged from industries and agricultural fields [7]. Due to the limited freshwater availability and rapid population growth, an urgent need is encountered to pay greater attention to the recycling of wastewater effluent. Effective and safe treatment of wastewater incurs exorbitantly high investment. The ecofriendly and economical photocatalyzed water purification process that employs semiconductors has been considered the most suitable process to degrade and mineralize organic water pollutants under ultraviolet (UV) or visible rays. The most common photocatalyst is TiO₂, which is also used in various types of applications such as self-cleaning coatings, photoelectrochemical (PEC) water splitting, and dye-sensitized solar cells (DSSC) [4, 5]. Due to its wide bandgap (3.0 eV for rutile and 3.2 eV for anatase), it can only absorb light in the UV region. Moreover, it has drawback of rapid recombination of electron-hole pairs [6, 7]. Thus, the photolytic performance of TiO₂ is enhanced through the introduction of metallic and non-metallic ion doping, which expands the light absorption range of titania from UV to the visible range [8].

In recent years, ilmenite has been utilized as an efficient photocatalyst, owing to its ability to degrade organic pollutants when exposed to visible radiation [9, 10, 11]. Among them, manganese titanate (MnTiO₃) has attracted the most attention in solar energy applications due to its narrow bandgap (3.1 eV) in the visible region making it suitable for photocatalytic applications [12, 13, 14]. He et al. [15] reported the synthesis of MnTiO₃ powder and demonstrated its good photocatalytic activity toward aqueous methyl orange at various pH values. Nakhowong [7] synthesized MnTiO₃ nanofibers and demonstrated their applicability for utilization in DSSC. Alkaykh et al. [16] synthesized MnTiO₃ nanoparticles via sol–gel technique and studied their photocatalytic activity in methylene blue (MB) degradation. The as-prepared photocatalyst possesses high MB degrading ability for heterojunction MnTiO₃/TiO₂ and MnTiO₃ with the efficiency of 75% and 70%, respectively, under direct sunlight exposure for 240 min. Compared to TiO₂, MnTiO₃ has a low conduction band (CB) gap that inhibits the recombination rate of electron-hole pairs [17]. Due to certain unique properties of carbon materials such as graphene, interest in the study of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) has recently gained momentum, and accordingly, these are currently used in many applications due to these properties [18, 19]. Moradi et al. [20] incorporated FeTiO₃ with graphene oxide (GO) to reduce its bandgap to obtain a further shift toward the visible spectrum. The material was synthesized via an ultrasound-assisted technique and possessed high photocatalytic activity to degrade phenolic compounds when exposed to sunlight for 240 min. They observed that the visible light exhibited faster degradation compared with the UV light. Also, their study concluded that the phenol removal rate was increased with the increased addition of GO to FeTiO₃. GO acts as a good e⁻ receiver, which inhibits e⁻/h⁺ recombination. Moreover, GO improved phenol adsorption by GO/FeTiO₃ nanocomposite, which ultimately increases the contact of phenol to surface hydroxyl radicals and positive holes. Previous studies have shown that the CNFs have high electrical conductivity with facile capture and transfer of photoinduced charges during the photosynthesis process [21, 22]. Hydrogen is an ecological energy transporter, and is considered to be a clean fuel as compared to petroleum products. However, due to its wide application hydrogen storage in large quantities remains a major challenge. To overcome this issue, hydrogen is stored within solid materials such as boron hydride compounds. Among these compounds, ammonia–borane (AB) complex (NH₃BH₃) carries a sizeable hydrogen content (19.6 wt.%), and is also characterized by high solubility and high stability toward hydrolysis, with a 4-day aqueous solution capacity [23, 24]. Thus, an appropriate channel is vital to liberate hydrogen from AB. Various catalysts have been employed to produce hydrogen from AB. Recently, researchers have focused on the production of hydrogen using the photocatalytic hydrolysis of AB. In this paper, we introduce a hetero-junction MnTiO₃/TiO₂@CNFs photocatalyst to degrade MB dye and produce hydrogen from the hydrolysis of AB under direct sunlight. The photo-catalyst material has been prepared via electrospinning technique, where electrospun nanofiber mats comprised of manganese acetate tetra-hydrate, titanium (IV) isopropoxide, and polyvinylpyrrolidone (PVP) are calcined at a temperature of 900°C. The reaction kinetics and thermodynamics properties are also studied to ascertain the effects of various parameters on the photocatalytic activity.

Experimental

2.1

Materials

Titanium isopropoxide (97%), PVP, manganese acetate tetrahydrate (98%), and AB complex (97%) were purchased from Sigma Aldrich (MO 68178, USA). MB (95%) was procured from LobaChemie (mumbai 400 005, India). Ethanol and acetic acid were procured from Scharlau (08181 Sentmenat, BarcelonaSpain).

2.2

Synthesis of MnTiO₃/TiO₂-decorated CNFs

We employed the sol–gel method to synthesize pristine TiO₂-decorated CNFs. Initially, we added 4.4 mmol titanium isopropoxide to 10 wt.% PVP solution. PVP solution was prepared as reported previously in the literature [25, 26, 27, 28, 29, 30]. The solution was stirred until it turned into a yellow transparent sol–gel and the formed sol–gel was subjected to the electrospinning process. The positive electrode was connected to a copper pin immersed in sol–gel, and the cathode electrode connected with an aluminum foil covered steel drum. A high voltage of 20 kV magnitude was applied between the electrodes, which were held 15 cm apart. The formed NF mats were then detached from the aluminum foil, kept in a vacuum dryer overnight at 50°C, and then calcined at 900°C for 5 h in an argon atmosphere. The heating rate was maintained at 2.3°C/min. To prepare MnTiO₃/TiO₂-decorated CNFs, we added 4.5 mmol manganese acetate tetrahydrate to the previous solution, with continuous stirring. After obtaining a clear sol–gel, we subjected it to the same aforementioned conditions of electrospinning and calcination processes.

2.3

Characterization

We imaged the surface morphology of the MnTiO₃/TiO₂-decorated CNFs using a JSM-5900 SEM (JEOL Ltd., Tokyo, Japan) operational with energy-dispersive X-ray spectroscopy (EDX). The crystallinity of the NFs was analyzed by the XRD (Rigaku Corporation, Tokyo, Japan) with CuKα (λ = 1.54056 Å) radiation and 2θ scan in the angular range of 10°–80°. TEM images of high resolution were obtained using a JEOL JEM-2200FS (TEM) equipped with a 200 kV field emission gun (FEG).

2.4

Determination of MB photodegradation

The photodegradation of MB through the use of MnTiO₃/TiO₂-decorated CNFs was carried out in a simple batch photoreactor consisting of a 150 mL laboratory glass bottle. The light source used in the photocatalytic experiment was a visible fluorescent lamp with a specification of λ = 420–700 nm and I = 23–40 W·m⁻² (Philips Co., Amsterdam, The Netherlands). Stock solutions from different MB concentrations (5 mg/L, 7.5 mg/L, 10 mg/L, and 15 mg/L) have been prepared. The reactor was charged with a 100 mL aqueous mixture of MB and a predetermined amount of MnTiO₃/TiO₂-decorated CNFs photocatalyst. The solution was kept in darkness to determine the adsorption–desorption of photocatalyst NFs toward MB. Later on, the solution was stirred and exposed directly to visible light. The reaction temperature was controlled by a thermostat water bath. The 3 mL MB solution was withdrawn after specific time intervals of irradiation and solid NFs were separated out from the solution through centrifuging at 3,000 rpm for 5 min. Finally, the absorption of drawn solutions was measured using a UV-visible spectrophotometer (HP8453UV–visible spectroscopy system, Poway, CA, USA).

2.5

Photohydrolysis of AB complex

The photocatalytic activity of MnTiO₃/TiO₂@CNFs toward AB hydrolysis under visible light irradiation was analyzed according to our previous experiment [25]. The same light source that was used in the photodegradation of MB dye was used here also. The reaction mixture, carried in a flask with a Teflon coated magnetic bar, was placed on a magnetic stirrer and associated to a measuring cylinder full of water via a rubber piping to evaluate the volume of the H₂ gas released by the photocatalytic reaction. Then, a mixture of 1 mmol of AB and a predetermined amount of catalyst was added to the reaction flask, and the contents of the flask were exposed to visible light. The impacts of a range of variables such as AB concentration, catalyst concentration, reaction temperature, and light intensity were studied. The released H₂ gas volume was evaluated by measuring the displacement of the water level.

Results and discussion

3.1

Morphology study

Electrospinning is the most common technique to produce excellent nanofibers of inorganic and organic components [30, 31, 32, 33]. The calcination of produced electrospun nanofiber mats at high temperatures and inert atmosphere leads to a decrease in the decomposition rate of polymer while keeping the nanofibrous morphology intact [18, 19, 28, 34]. The low magnification (SEM) and high magnification (FESEM) images of electrospun nanofiber mats reveal the presence of MnAc, TIIP, and PVP after vacuum drying at 60°C, with smooth, continuous, and beads-free nanofibrous structure, as shown in Figures 1A and 1B. The NFs preserve a nanofibrous morphology even after the calcination process, as shown in Figures 1C and 1D. However, the size of the nanofibers is reduced, probably due to the partial decomposition of polymer from 245.28 nm (after drying process) to 156.91 nm (after calcination process), as shown in Figures 2A and 2B.

(A, B) SEM and FESEM images of the electrospun MnAc/TIIP/PVP nanofibers mat after drying at 60°C for 24 h. (C, D) SEM and FESEM images of the calcined nanofibers at 800°C in Ar atmosphere. PVP, polyvinylpyrrolidone

Size distribution of the nanofibers before calcination (A) and after calcination (B)

3.2

Phase study

XRD analysis is considered the most reliable technique to analyze the crystal structure of materials. Figure 3 shows the XRD patterns of TiO₂ NFs containing anatase and rutile phases, and the XRD patterns of the synthesized NFs after calcination process. The obtained results reveal the formation of a mixed phase comprising tetragonal TiO₂ rutile phase (JCPDS Card No. 21-1276) and rhombohedral pyrophanite phase MnTiO₃ (JCPDS Card No. 29-0902). Additionally, low intensity peak is observed at 2θ value of 23.1° corresponding to the (002) plane, suggesting the formation of carbon-like graphite because of the partial decomposition of polymer for the duration of the calcination process [35, 36].

XRD patterns of the powders obtained from the calcination of MnAc/TIIP/PVP at 800°C in Ar atmosphere. CNFs, carbon nanofibers; PVP, polyvinylpyrrolidone

TEM EDX has been performed to ascertain the chemical composition of the prepared NFs, as shown in Figure 4. Figure 4A indicates the STEM image of a selected nanofiber, wherein the NF surface is observed to be rough, perhaps due to the escape of gases during the calcination process. EDX analysis of the line in Figure 4A is shown in Figures 4B–4E. It is evident from Figures 4B–4D that Ti, Mn, and O have the same distributions, which confirms the formation of MnTiO₃ and TiO₂. Figure 4D suggests that carbon is the outermost element in the synthesized NFs composite. In other words, MnTiO₃ and TiO₂ composites are covered with carbon. The presence of carbon can enhance the overall photocatalytic process due to enhancement in photoinduced e⁻ and h⁺ separation, increased exposure to the photocatalyst, and ease of adsorption process.

(A) TEM image of a single calcined nanofiber along with the TEM EDX line analysis, (B–E) corresponding to Ti, C, O, and Mn TEM EDX line analyses. EDX, energy-dispersive X-ray spectroscopy

3.3

Photocatalytic degradation of MB

To understand the photodegradation of MB dye employing MnTiO₃/TiO₂@CNFs, we examined the effect of the following parameters on the photodegradation process: (i) MB dye concentration (C_i), (ii) reaction temperature (T), (iii) CNFs dosage, and (iv) light intensity (I). The mathematical model employed to evaluate the photocatalytic reaction as a function of operational parameters is as follows [37]: (1) $\begin{array}{l} K_{app} & = K^{'} (\frac{K_{R}}{1 + K_{R} C_{i}}) \\ \cdot (\frac{K_{NFs} C_{NFs}}{1 + K_{NFs} C_{NFs}}) (\exp \frac{- E_{a}}{RT}) (mI) \end{array}$ \matrix{{{K_{app}}} \hfill & {= {K^\prime}\left({{{{K_R}} \over {1 + {K_R}{C_i}}}} \right)} \hfill \cr {} \hfill & {\cdot \left({{{{K_{NFs}}{C_{NFs}}} \over {1 + {K_{NFs}}{C_{NFs}}}}} \right)\left({\exp {{- {E_a}} \over {RT}}} \right)\left({mI} \right)} \hfill} {\rm{}}

Where:

K_app – represents the reaction rate constant (expressed in min⁻¹),

K′ – the equation constant (mg·L⁻¹),

K_R – the adsorption equilibrium constant (L·mg⁻¹),

E_a – the activation energy (J·mol⁻¹),

R – the universal gas constant (J·K⁻¹·mol⁻¹),

C_i – the initial MB dye concentration (mg·L⁻¹),

T – the reaction temperature (K),

I – the light intensity (W·m⁻²),

m – the coefficient for light intensity (m²·W⁻¹·min⁻¹),

K_NFs – the adsorption constant for catalyst dosage (L·mg⁻¹),

and C_NFs – the catalyst dosage (mg·L⁻¹).

Figure 5 shows the change of MB concentration vs. irradiation time in the absence of photocatalyst NFs, and with TiO₂@CNFs and MnTiO₃/TiO₂@CNFs. It can be inferred from the figure that the MB decolorization efficiency utilizing MnTiO₃/TiO₂@CNFs (82.4%) is higher than that of TiO₂@CNFs (62.5%) after 120 min visible light irradiation.

The photodegradation profile of MB dye (200 mg·L⁻¹, C_i = 5 mg·L⁻¹, T = 298 K, and I = 25 W·m⁻²). CNFs, carbon nanofibers; MB, methylene blue

3.3.1

Effect of initial dye concentration (C_i)

The effect of initial MB dye concentration on the photodegradation reaction under visible light irradiation and in the presence of MnTiO₃/TiO₂@CNFs is shown in Figure 6A. It can be inferred from the figure that the MB photodegradation decreases with increasing MB concentrations due to reduced active species at higher MB concentration during the photocatalytic process [38, 39]. Also, the increase in MB concentration may cause the dye molecules to soak up light, preventing the photons from reaching the surface of the photocatalyst and thereby reducing the efficiency of the photodegradation process [40, 41, 42]. Thus, photodegradation process is efficient at lower MB concentrations due to the accessibility of additional active sites for MB molecules, which enhance their adsorption on the photocatalyst surface and increase their removal at low concentrations. The photodegradation of the organic compounds can be modeled via the Langmuir Hinshel-wood (LH) pseudo-first order reaction according to Eq. (2). (2) $r_{MB} = - \frac{d C_{f}}{dt} = K_{1} C_{f} = \frac{K_{LH} K_{R} C_{f}}{1 + K_{R} C_{i}}$ r_{MB}=-\frac{dC_f}{dt}=K_{1}C_{f}=\frac{K_{LH}K_{R}C_f}{1+K_{R}C_i} (3) $K_{1} = \frac{K_{LH} K_{R}}{1 + K_{R} C_{i}}$ K_1=\frac{K_{LH}K_R}{1+K_RC_i}

Where:

r_MB – represents the reaction rate of MB (expressed in mg·L⁻¹·min⁻¹),

K₁ – the first order rate constant for C_i (min⁻¹),

K_LH – the reaction rate constant (mg·L⁻¹·min⁻¹),

t – the photocatalysis time (min),

K_R – the adsorption equilibrium constant (L·mg⁻¹),

C_i – the initial MB dye concentration (mg·L⁻¹),

and C_f – the final MB dye concentration (mg·L⁻¹).

(A) Effect of MB on the photodegradation of MB, (B) plot of ln C_i/Cf vs. time, and (C) modified LH plot for MB photodegradation (200 mg·L⁻¹, T = 298 K, and I = 25 W·m⁻²). LH, Langmuir Hinshelwood; MB, methylene blue

Figure 6B shows the kinetics of MB degradation in the company of various concentrations of MnTiO₃/TiO₂@CNFs photocatalyst. The values of rate constant (K₁) can be obtained by the slope of straight lines obtained in Figure 6B. The value of K₁ diminishes from 0.0153 min⁻¹ to 0.0044 min⁻¹ as the MB concentration raises from 5 mg·L⁻¹ to 15 mg·L⁻¹, as shown in Table 1. The reason behind this may be the restricted formation of active species at higher MB concentrations.

Table 1

Reaction rate constants of MB photodegradation at various MB concentrations, reaction temperatures, light intensities, and MnTiO₃/TiO₂@CNFs dosages

Rate constant (min⁻¹)	MB dye concentration (C_i) (mg · L⁻¹)

0.0153	5
0.0088	7.5
0.0068	10
0.0044	15

Rate constant (min⁻¹)	Reaction temperature (T) (°C)

0.0153	25 T
0.0166	30
0.0188	35
0.0222	40

Rate constant (min⁻¹)	Light intensity (I) (W·m⁻²)

0.0153	25 I
0.0166	30
0.019	35
0.0228	40

Rate constant (min⁻¹)	Catalyst dosage (C_NFs) (mg·L⁻¹)

0.0153	200
0.0216	400
0.027	600
0.0324	800

CNFs, carbon nanofibers; MB, methylene blue

The relation between K₁ and C_i is obtained by plotting 1/K₁ vs. C_i (refer Figure 6C) and modifying the LH model by transforming Eq. (4) into a straight-line equation (y = ax + b) as follows: (4) $\frac{1}{K_{1}} = \frac{1}{K_{LH}} C_{i} + \frac{1}{K_{LH} K_{R}}$ \frac{1}{K_{1}} = \frac{1}{K_{LH}} C_{i} + \frac{1}{K_{LH}K_{R}} here: $a = \frac{1}{K_{LH}}$ a=\frac{1}{K_{LH}} , $b = \frac{1}{K_{R} K_{LH}}$ b=\frac{1}{K_{R}K_{LH}} , K_LH = 6.75 × 10⁻²·mg·L⁻¹ and K_R = 4.99 L·mg⁻¹

3.3.2

Effect of reaction temperature

Figure 7A shows the variation of MB concentration vs. irradiation time at various reaction temperatures for MnTiO₃/TiO₂@CNFs photocatalyst. It can be inferred from the figure that the photodegradation of MB enlarged with the raise in the reaction temperature from 25°C to 40°C. The high temperature assists the mobility of charge carrier and interfacial charge transfer. As the reaction temperature increases, the photoelectron-hole pairs become more mobile, due to which electrons rapidly unite with the adsorbed oxygen and holes can produce OH radicals more rapidly [43, 44, 45], thus obliging the MB photodegradation process. Temperature ranging from 20°C to 80°C is considered to be the most suitable for effective photodegradation of organic compounds [43, 46]. Further increase in the reaction temperature reduces the photocatalytic activity since the electron-hole recombination rate increases [43], while low reaction temperature reduces the MB solubility in water, causing partial condensation of MB in water [43, 47]. Figure 7B shows the kinetics of the MB photodegradation given the existence of MnTiO₃/TiO₂@CNFs at various temperatures. The value of reaction rate (K₂) is increased from 0.0153 min⁻¹ to 0.0222 min⁻¹ with the increase in reaction temperature from 25°C to 40°C, as shown in Table 1. This implies an enhancement in the MB photodegradation efficiency with the temperature increase due to high charge mobility. The rate constant (K₂) can be firm, as observed from the slope of straight lines obtained in Figure 7B. The plot of K₂ vs. 1/T shows a straight-line relation, as shown in Figure 7C. To obtain the activation energy (E_a) of the photocatalyst, we employ the Arrhenius Eq. (5). (5) $K_{2} = A exp (\frac{E_{a}}{RT})$ K_{2}=A\exp \left(\frac{E_a}{RT}\right) where K₂ represents the first order rate constant for T (expressed in min⁻¹) and A the frequency factor (min⁻¹).

Applying natural log to both sides of Eq. (5) yields Eq. (6), as follows: (6) $\ln K_{2} = ln A + (\frac{E_{a}}{R}) (\frac{1}{T})$ \ln K_{2}=\ln A+\left(\frac{E_a}{R}\right)\left(\frac{1}{T}\right) which is a straight line curve y = ax+b with $a = \frac{E_{a}}{R}$ a = \frac{E_a}{R} and E_a = 1.9204 × 10⁴ J·mol⁻¹.

3.3.3

Effect of light intensity

The photodegradation rate of organic compounds relies on the wavelength and intensity of light [40, 48, 49, 50]. Figure 8A represents the influence of intensity of light on the MB photodegradation. It suggests that the rate of MB photodegradation is boosted as the intensity of light is raised. An augment in the intensity of light implies enhancement in the number of photons, due to which the MB photodegradation increases. The rate constant (K₃) can be determined from the slope of straight lines obtained from the plot of ln (C_i/C_f) against time under various light intensities, as revealed in Figure 8B. The value of K₃ is amplified from 0.0153 min⁻¹ to 0.0228 min⁻¹ with the raise in light intensity from 25 W·m⁻² to 40 W·m⁻², as shown in Table 1. This suggests an enhancement in the MB photodegradation competence with the increase in light intensity. We obtained the relation between K₃ and I by plotting 1/K₃ vs. I (see Figure 8C) and found K₃ to be directly proportional to I [37], in the form y = ax, where a = m, x = I, and m = 6 × 10⁻⁴. (7) $K_{3} = mI$ K_{3} = mI where K₃ represents the first order rate constant for I, and is expressed in min⁻¹.

3.3.4

Effect of MnTiO₃/TiO₂@CNFs photo-catalyst dosage on MB degradation

Figure 9A shows the concentration of MB vs. irradiation time at various MnTiO₃/TiO₂@CNFs dosages. The efficiency of MB photodegradation increases with the increase in MnTiO₃/TiO₂@CNFs dosage in anticipation of the available active sites that improve the MB photodegradation. It is also observed that the MB photodegradation at 400 mg·L⁻¹ is very close to that at the dosage of 500 mg·L⁻¹. The reason underlying this result is reduction of the available active sites due to the aggregation phenomenon at high MnTiO₃/TiO₂@CNFs dosage. It may be also due to the light scattering and screening effect that decrease the photoreactions and active radicals [39, 51, 52, 53]. Figure 9B represents the kinetics of the MB photodegradation given the existence of MnTiO₃/TiO₂@CNFs at various catalyst dosages.

(A) Effect of catalyst amount on photodegradation of MB, (B) plot of ln C_i/C_f vs. time, and (C) Langmuir-type plot for photodegradation of MB (C_i = 5 mg·L⁻¹, T = 298 K, and I = 25 W·m⁻²). CNFs, carbon nanofibers; MB, methylene blue

The value of the reaction constant (K₄) is increased from 0.0126 min⁻¹ to 0.0245 min⁻¹ with the increase in the photocatalyst dosage from 200 mg·L⁻¹ to 500 mg·L⁻¹, as shown in Table 1. The improvement in the MB photodegradation efficiency due to the increase in MnTiO₃/TiO₂@CNFs dosage facilitates the availability of active sites on the surface of the catalyst. The influence of the MnTiO₃/TiO₂@CNFs dosage on K₄ is determined by employing a Langmuir-type relation [37, 54] according to Figure 9C and Eq. (8). (8) $K_{4} = \frac{K_{0} K_{NFs} C_{NFs}}{1 + K_{NFs} C_{NFs}}$ K_4=\frac{K_{0}K_{NFs}C_{NFs}}{{1+K}_{NFs}C_{NFs}}\

Eq. (8) can be transformed to a straight-line equation (y = ax + b) as shown in Eq. (9). (9) $\frac{1}{K_{4}} = \frac{1}{K_{0} K_{NFs}} (\frac{1}{C_{NFs}}) + \frac{1}{K_{0}}$ \frac{1}{K_{4}} = \frac{1}{K_{0}K_{NFs}} \left(\frac{1}{C_{NFs}}\right) + \frac{1}{K_{0}} where

K₄ represents the first order rate constant for C_NFs (expressed in min⁻¹),

K₀ – the reaction rate constant for C_NFs (min⁻¹),

a = 1/K₀K_NFs,

K₀ = 4.6 × 10⁻²·min⁻¹

and K_NFs = 2.444 × 10⁻³·L·mg⁻¹.

3.3.5

Development of reaction rate model

The aforementioned results obtained in Eqs (6), (7), and (9) suggest that the rate constant is a function of the MB concentration, light intensity, reaction temperature, and MnTiO₃/TiO₂@CNFs dosages. The values of K_R, E_a, m, and K_NFS obtained from multiple regression analysis are shown in Table 2. The equation constant K′ is calculated from Eq. (1) by substituting the previously obtained values of C_i, T, I, and C_NFS and constant values from Table 2. (10) $\begin{array}{l} K_{app} & = K^{'} (\frac{4.99}{1 + 4.99 \times 5}) \\ \cdot (\frac{2.444 \times 10^{- 3} \times 200}{1 + 2.444 \times 10^{- 3} \times 200}) \\ \cdot (\exp \frac{- 1.9204 \times 10^{4}}{8.314 \times 298}) (6 \times 10^{- 4} \times 25) \end{array}$ \matrix{{K_{app}}\hfill &{=K^{\prime}\left(\frac{4.99}{1+4.99\times{}5}\right)}\hfill \cr {} \hfill & \cdot \left(\frac{2.444\times{}10^{-3}\times{}200}{1+2.444\times{}10^{-3}\times{}200}\right) {} \hfill \cr &\cdot \left(\exp{\frac{-1.9204\times{}10^4}{8.314\times{}298}}\right)(6\times{}10^{-4}\times{}25)}

Table 2

Values of various constants obtained by employing multiple regression analysis in model equation

Parameter	k′	K_R (L mg⁻¹)	E_a (J mol⁻¹)	R (J K⁻¹ mol⁻¹)	m (m² W⁻¹ min⁻¹)	K_NFs (L mg⁻¹)
Value	3.7551 × 10⁴	4.99	1.9204 × 10⁴	8.314	6 × 10⁻⁴	2.444 × 10⁻³

The reaction rate constant (K_app) can be rewritten as follows: (11) $\begin{array}{l} K_{app} & = 3.7551 \times 10^{4} (\frac{4.99}{1 + 4.99 \times C_{i}}) \\ \cdot (\frac{2.444 \times 10^{- 3} \times C_{NFs}}{1 + 2.444 \times 10^{- 3} \times C_{NFs}}) \\ \cdot (\exp \frac{- 1.9204 \times 10^{4}}{8.314 \times T}) (6 \times 10^{- 4} \times I) \end{array}$ \matrix{{K_{app}}\hfill &=3.7551\times{}10^4\left(\frac{4.99}{1+4.99\times{}C_i}\right)\hfill \cr {} \hfill &\cdot \left(\frac{2.444\times{}10^{-3}\times{}C_{NFs}}{1+2.444\times{}10^{-3}\times{}C_{NFs}}\right)\hfill \cr {} \hfill &\cdot \left(\exp{\frac{-1.9204\times{}10^4}{8.314\times{}T}}\right)\left(6\times{}10^{-4}\times{}I\right)}

A comparison between the experimental and calculated values of K_app for the MB photodegradation under various conditions is shown in Figure 10. The obtained plot shows that the experimental data are in good agreement with the calculated data from the model. This demonstrates that our model can successfully predict the reaction rate constant at various operational conditions.

Comparison between experimental and predicted K_app values

3.4

Photohydrolysis of H₂ from AB

The photocatalytic activity of TiO₂@CNFs and MnTiO₃/TiO₂@CNFs toward the H₂ release from the hydrolysis of AB (1 mmol AB) is measured under the visible light irradiation (I = 25 W·m⁻²), as shown in Figure 11. The H₂ evolution over TiO₂@CNFs and MnTiO₃/TiO₂@CNFs for 12 min exposure is determined to be 0.35 mmol and 2.97 mmol, respectively. The higher photohydrolysis activity of MnTiO₃/TiO₂@CNFs was observed than TiO₂@CNFs. This may be due to the fast separation of e⁻/h⁺ on MnTiO₃/TiO₂@CNFs, as compared with TiO₂@CNFs, which improves the efficiency of H₂ generation from AB photohydrolysis. A significant H₂ production enhancement was observed under visible light as compared to darkness. Moreover, no release of H₂ was detected in the absence of a catalyst even under visible light irradiation, which suggests that AB is quite stable in water.

H₂ generation in the presence of various photocatalysts (200 mg·L⁻¹, C_i = 0.1 M, T = 298 K, and I = 25 W·m⁻²). CNFs, carbon nanofibers

3.4.1

Effect of MnTiO₃/TiO₂@CNFs photocatalyst dosage on H₂ production from AB

Figure 12A shows the H₂ generation rate vs. irradiation time at various MnTiO₃/TiO₂@CNFs dosages. It can be inferred from Figure 12A that the rate of H₂ production raises with the rise in MnTiO₃/TiO₂@CNFs dosage. This may be owing to the enhancement of the available surface area of the MnTiO₃/TiO₂@CNFs, which ultimately enhances the AB photohydrolysis. Figure 12B shows the variation of reaction rate (ln K) with respect to the ln (MnTiO₃/TiO₂@CNFs dosage). The slope of the best-fitted line is determined to be 1.028, which suggests that the generation of H₂ gas can be predicted well with the pseudo-first order kinetics in terms of catalyst dosage. The dependence of the MnTiO₃/TiO₂@CNFs dosage on K is determined by employing a Langmuir-type relation as in Eqs (11) and (12) [37, 54]. The value of K is found to be 0.361 min⁻¹.

(A) Effect of photocatalyst dosage on H₂ generation, and (B) plot of logarithmic value of the hydrogen production rate vs. logarithmic value of the catalyst amount (C_i = 0.1 M, T = 298 K, and I = 25 W·m⁻²). CNFs, carbon nanofibers

3.4.2

Effect of initial AB concentration (C_i)

We studied the outcome of the original AB concentration on the H₂ production from AB photohydrolysis in the existence of MnTiO₃/TiO₂@CNFs upon light irradiation. Figure 13A shows that the AB concentration does not affect the H₂ generation rate, and the initial H₂ production rate almost remains unchanged even with increasing AB. The photoproduction of H₂ using various AB concentrations can be predicted via the pseudo-zero order reaction in accordance with the LH model of Eq. (3), as evidenced in Figure 13B. The incline of the best-fit line is evaluated to be 0.114, which suggests that pseudo-zero order kinetics reaction with reverence to AB was followed in H₂ production.

(A) Effect of AB concentration on H₂ production, and (B) plot of the logarithmic value of the H₂ production rate vs. logarithmic value of the AB amount (200 mg·L⁻¹, T = 298 K, and I = 25 W·m⁻²). AB, ammonia–borane

3.4.3

Effect of reaction temperature (T)

Figure 14A shows the H₂ production rate vs. irradiation time in the presence of MnTiO₃/TiO₂@CNFs at various reaction temperatures. As shown in the figure, the photohydrolysis of AB undergoes enhancement as the temperature increases from 25°C to 40°C. This is due to the fact that at high temperature, interfacial charge transfer and mobility of charge carrier have increased. The electron-hole pair becomes active and mobile at high temperature, where electrons combine rapidly with absorbed oxygen and holes generate OH radicals after combining with ⁻OH ions [43, 44, 45], which results in humanizing the AB photohydrolysis. The plot of K₂ vs. 1/T gives a straight-line relation, as shown in Figure 14B. We have observed the activation energy (E_a) as 35.36 KJ · mol⁻¹ of photocatalytic reaction via the Arrhenius Eq (8) [55].

(A) Effect of the reaction temperature on H₂ production, and (B) plot of logarithmic value of hydrogen production rate constant vs. (1/T) (200 mg·L⁻¹, C_i = 0.1 M, and I = 25 W·m⁻²)

3.4.4

Effect of light intensity (I)

The hydrogen generation rate from AB photo-hydrolysis is affected by the light intensity, as indicated by Figures 15A and 15B [56, 57, 58]. It is evident from the figure that the rate of AB photohydrolysis has increased concomitant with the rise in light intensity. A rise in the intensity of the light promotes more photon-generation, which ultimately improves the AB photohydrolysis.

Effect of light intensity on H₂ production (A) and logarithmic value of the H₂ generation rate vs. logarithmic value of light intensity (B) (200 mg·L⁻¹, C_i = 0.1 M, and T = 298 K)

Mechanism of photocatalysis

The scheme of the reaction mechanism for MnTiO₃/TiO₂@CNFs catalyst is shown in Figure 16. Determining the CB and valence band (VB) potentials of the components is essential for understanding the separation of photogenerated electron-hole pairs over the MnTiO₃/TiO₂@CNFs. The following empirical formulae were used to determine these energies: (12) $E_{CB} = χ - E^{e} - 0.5 E_{g}$ E_{CB} = \chi{} - E^{e} - 0.5E_{g} (13) $E_{VB} = E_{CB} + E_{g}$ E_{VB} = E_{CB} + E_{g} where the VB potential is denoted by E_VB, and the CB potential by E_CB. In addition, E^e is the energy of free electrons vs. NHE, which is 4.5 eV [13, 14]. E_g is the band gap energy of the semiconductor. Finally, the electronegativity of the semiconductor is denoted by χ. In the case of TiO₂, E_g is 3.20 eV and χ is 5.81 eV, while in case of MnTiO₃, E_g is 3.10 eV and χ is 5.59 eV. The calculated E_VB values for MnTiO₃ and TiO₂ are 2.64 eV and 2.91 eV, respectively, vs. NHE, with corresponding E_CB values of −0.46 eV and −0.29 eV [13]. When compared to MnTiO₃, TiO₂ has a higher positive VB, and yet its CB is substantially more negative. Due to the adsorption capacity of MnTiO₃/TiO₂@CNFs, the first MB molecules were able to be adsorbed on the surface of the hybrid. When the MnTiO₃/TiO₂@CNFs and MB reaction system was illuminated with visible light, the MB dye molecules that were adsorbed on the surface of the MnTiO₃/TiO₂@CNFs were first excited and generated photoelectrons (e⁻), and then the excited MB* injected electrons into the CB of MnTiO₃ [13]. Since MnTiO₃ and TiO₂ have different band gaps, injected electrons in the CB of MnTiO₃ have a tendency to move to the CB of TiO₂, which is driven by the potential energy. The redox potential has been reported to be $E_{0} (O_{2} / O_{2}^{-} = - 0.046 eV vs . NHE)$ {\rm E}_{0}({\rm O}_{2}/{\rm O}_{2}^{-} = -0.046~{\rm eV \ vs. \ NHE}) [48], which is a value that is much more positive than the CB potential of TiO₂ (−0.46 V vs. NHE). It is plausible to suppose that the $O_{2}^{• -}$ {\rm O}^{\bullet{}-}_{2} radical species might have been readily produced by the adsorbed oxygen through the process of capturing electrons from the CB of TiO₂. As a result, the cation radical of MB was able to be effectively degraded with the assistance of the strong oxidizing power of the $O_{2}^{• -}$ {\rm O}^{\bullet{}-}_{2} . On the other hand, CNFs have the potential to operate as an electron acceptor, which not only helps to enhance the separation of electron charges, but also protects TiO₂ and MnTiO₃ nanoparticles from photocorrosion. Additionally, the VB of TiO₂ has a higher anodic energy level than the valence band (E_VB) of MnTiO₃. This result in holes moving from the VB of TiO₂ to the VB of MnTiO₃. As a result of the reaction of holes in the VB of MnTiO₃, OH radicals were produced. This occurred because the MnTiO₃ is more positively charged than the typical redox potentials of OH/•OH (1.99 eV) and H₂O/•OH (2.38 eV). Following this, the MB molecules were degraded with the assistance of the high oxidizing power of the $O_{2}^{• -}$ {\rm O}^{\bullet{}-}_{2} and OH•, which may be described as follows:

In the CB—

MB + hv → MB• + + → e⁻

$e^{-} + O_{2} \to O_{2}^{• -}$ {\rm e}^{-} + {\rm O}_{2}\to {\rm O}^{\bullet{}-}_{2}

${MB}^{• +} + O_{2} / O_{2}^{• -} \to {CO}_{2} + H_{2} O$ {\rm MB}^{\bullet{}+}+ {\rm O}_{2}/{\rm O}^{\bullet{}-}_{2} \to {\rm CO}_{2} + {\rm H}_{2}{\rm O}

In the VB—

h⁺ + OH⁻ → OH•

OH• + MB dye → Degradation products (CO₂ + H₂O)

Scheme for the creation and influence of electrons and holes in the photocatalytic degradation of MB and photohydrolysis of AB. AB, ammonia–borane; CB, conduction band; CNFs, carbon nanofibers; MB, methylene blue; VB, valence band

Conclusion

We successfully prepared a heterojunction MnTiO₃/TiO₂@CNFs composite via electrospinning and calcination techniques. The synthesized composite demonstrated rapid release of hydrogen by photohydrolysis of the AB complex and the photodegradation of MB dye under visible light. We also proposed a mathematical model that successfully predicts the photocatalytic activity of composite NFs in producing H₂ via AB photo-hydrolysis in a batch reactor. The mathematical model effectively predicts the reaction rate constant (K_app) by determining the effect of various parameters employed in this study. We conclude that the experimental and theoretical data are in agreement with each other. Finally, we observe that the production of H₂ considerably increases under visible light irradiation compared to darkness.

eISSN:: 2083-134X
Langue:: Anglais

Périodicité:: 4 fois par an
Sujets de la revue:: Materials Sciences, other, Nanomaterials, Functional and Smart Materials, Materials Characterization and Properties

RSS Feed de la revue

Fabrication of heterojunction MnTiO₃–TiO₂-decorated carbon nanofibers via electrospinning as an effective multifunctional photocatalyst

Publié en ligne: 25 déc. 2022

Pages: 289 - 305

Reçu: 13 juin 2022

Accepté: 24 oct. 2022

DOI: https://doi.org/10.2478/msp-2022-0028

Mots clés
manganese titanate nanoparticles, dye removal, ammonia–borane complex, hydrogen release, electrospinning, visible light

© 2022 Ayman Yousef, published by Sciendo

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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Fabrication of heterojunction MnTiO3–TiO2-decorated carbon nanofibers via electrospinning as an effective multifunctional photocatalyst