SYNTHESIS AND PHOTOCATALYTIC ACTIVITY OF Nb2O5

Int. J. Mat. Chem.

1

SYNTHESIS AND PHOTOCATALYTIC ACTIVITY OF Nb

2

O

5

–DOPED ANATASE TiO

2

Nurulhuda Abdullah¹*and Khairul Basyar Baharudin²

1Stesen Penyelidikan Sg. Buloh, Lembaga Getah Malaysia, 47000, Sg. Buloh, Selangor

2School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

*Corresponding author: nurulhuda.a@lgm.gov.my Abstract

This paper describes the synthesis procedure and photocatalytic activity of an active Niobium Pentoxide (Nb²O⁵) doped anatase phase Titanium Dioxide (TiO²) photocatalyst. The catalysts were prepared by impregnating ammonium niobate oxalate (V) hydrate ethanolic solutions (1-5%) with oxalic-acid- precipitated TiO2. XRD analysis of all synthesized TiO2 and Nb2O5 – doped TiO2 construct an anatase phase, however with reduced intensity. Evidently, a linear expansion on cell parameters and extreme reduction in crystallite size was observed as the Nb2O5 loading percentage increases. All prepared catalysts exhibit similar irregular to nearly spherical shape morphology with the size accumulated in the range of 10 – 70 nm using Transmission Electron Microscopy (TEM) analysis. The effect of doping Nb2O5 became more prominent when it’s assembled different shapes of hysteresis loops as displayed in N2 adsorption-desorption isotherms. Upon the addition of Nb2O5, the Brunauer-Emmett-Teller (BET) surface area showed incredible enhancement while pore size diameter as determined using Barrett, Joyner, and Halenda (BJH) method shifted to smaller pore size. The photocatalytic efficiency of synthesized Nb²O⁵ – doped TiO² was evaluated and results exhibited an appreciable improvement in comparison to the undoped TiO2. On the whole, 3% Nb2O5 doped TiO2 offered the highest activity with an optimum of 13.34 mg methyl orange are degraded per gram of UV illuminated photocatalyst. Kinetically, the rate of Methyl Orange (MO) photocatalytic degradation by Nb2O5 doped TiO2

followed first order.

Keywords: anatase phase titanium dioxide; niobium oxide; doped titanium dioxide; impregnation method;

photocatalytic

Introduction

Soon after the Fujishima and Honda phenomenon in 1972; the attention was dominantly pulled toward the efficacy of semiconductor TiO² (Valenzuela et al. 2002; Lin 2008; Magalhães et al. 2017). Since then, TiO² has been used extensively in many applications such as water treatment (Lazar M. A. et al. 2012; Mohamed M. A.

et al. 2019), self-cleaning materials (Farahmandjou M. and Khalili P. 2017; Leong et al. 2019), water splitting technology (Miyoshi A. et al. 2018) and many more. This is attributable to its high photocatalytic activity, strong oxidation power, chemical and photostability, non-toxic, chemical and biological inertness. However, there are some limitations drawn from TiO2 which is its high rate of e^-/h⁺ recombination and UV range activation (Ibrahim 2008). Nonetheless, the number of reports on TiO2 is continuously intensifying oblige by its efficiencies as well as its drawbacks.

Int. J. Mat. Chem.

2 Doping crystalline TiO2 lattice with a transition metal (Kanji S. et al. 2020), will alter the electronic structure thus led to the lessening of the above-mentioned drawbacks. The substitution of TiO2 with transition metal could induce electronic coupling effects with the Ti atom and creates an electron state within the oxide bandgap. These active localized states of the dopant can either shift towards the TiO² conduction band or the valence band edge. If the energy level of dopant shifts to the conduction band edge, the efficiency of trapping becomes higher, thus reducing the electron-hole recombination rate. Several dopants (Liang et al. 2008; Y.

Wang et al. 2020) have been used to improve the anatase photocatalytic performance while simultaneously changing physical-chemistry of the surface of TiO2. Among others, Nb2O5 has also attracted a wide range of applications such as gas sensors, electrochromic, catalysis, photoelectrodes as well as in field-emission displays and microelectronics. Although bare Nb2O5 seems to demonstrate a low photocatalytic activity that this catalyst is often used as support or dopant (Xing et al. 2008). However, remarkable photocatalytic activity has been reported when Nb2O5 is incorporated into TiO2 (Kubacka et al. 2008; Castro et al. 2009;

Gardecka A. J. 2016; Silva et al. 2016).

It had been accounted for in many types of research that the preparative procedure is vital and can affect physicochemical and catalytic characteristics of doping material. Countless of contentious outcomes are descript in the literature as regards of the certainty that its catalytic enhancement is depended relatively on the method of doping. Throughout this work, a remarkable effect of doping a various amount of an amorphous Nb²O⁵ in the range of 1-5% into anatase TiO² via a simple ethanolic-aqueous impregnation method had somehow produced some synergistic effect that resulted to optimization of its physicochemical characteristic is presented. It is believed that the properties alteration (e.g: particle size, pore size, and surface area), caused by the introduction of Nb2O5 into TiO2 crystal lattice had assisted in enhancing the photocatalytic activity performance of doped TiO2.

Materials and Methods Synthesis

All reagents were used as received without any further purification. Homogenous 0.1 M of titanium (IV) butoxide (TiB) (Sigma-Aldrich, 99.9% purity) was prepared by mixing 3.5ml of TiB with 100ml analytical grade of absolute ethanol. 5ml of 0.1 M of oxalic acid was slowly added into the mixture until a thick white colloidal is formed. The white colloid was then collected via centrifugation (4000 rpm), washed 3 times with denatured ethanol to eliminate any presence of impurities and oven-dried at 70^oC overnight. The fine white powders produced were ground and heat-treated at 550^oC in the Carbolite Furnace under compressed airflow for 4 hours.

A series of various Nb2O5 percentages (1-5%) doped on TiO2 were prepared by a simple impregnation method.1g of synthesized TiO2 was suspended in 100 ml of ethanolic solution [water-ethanol (1:1 volume ratio)] containing a calculated amount of ammonium niobate oxalate (V) hydrate (ANOH) (Sigma-Aldrich, 99.9% purity). The mixture was continuously stirred for 8 hrs at room temperature and then, oven-dried at 70^oC overnight to ensure a complete removal of the ethanolic-aqueous phase. The impregnated samples powders were then calcined at 450^oC in Carbolite Furnace under compressed air flow for 4hr.

Characterization

The crystal structure and phase purity of the synthesized metal oxide were analyzed by X-Ray Diffractometer (XRD) (Shimadzu XRD-600) using CuKa radiation (λ = 1.5418^o and 40 kV). The crystallite size was estimated using the Debye-Scherrer equation (Dxrd). In order to study the effect of Nb2O5 doped on TiO2, the cell parameters of undoped and doped-TiO2 are calculated by using CheckCell software refinement on the XRD

Int. J. Mat. Chem.

3 peak list. Elemental analysis of the samples was performed using X-Ray Fluorescence (XRF) with the Easy- Air-Oxide method (EDX-720 Shimadzu).

The morphology and particle size distribution was characterized using a transmission electron microscope (Hitachi TEM model H-7100) that operated at an accelerated voltage of 200kV. The surface area and porosity of the sample were determined by N2 gas adsorption at -196^oC using the Autosorb-1 Quantachrome Instrument model. The sample was degassed and pre-heated at 250^oC prior to N² gas adsorption.

Photocatalytic Evaluation

To evaluate the photocatalytic ability, degradation of methyl orange (MO; molecular weight = 327 gmol^-1) (BDH) by the synthesized photocatalyst was performed in a batch reactor equipped with 6W UV lamp supplied by New York Spectronics having maximum intensity at 365 nm.

Figure 1. Photocatalytic reactor

A known amount of catalyst was suspended in 1L of MO solution (10 mg/L) and was continuously stirred throughout this study. Air was bubbled into the reactor through an air pump for a continuous air supply. To maintain the reaction temperature, water was kept on flowing through the outer jacket of the reactor. The first-hour treatment of the reaction was conducted in the dark before the UV light is switched on. 5 ml sample was withdrawn and filtered from the treated suspension solution at predetermined time intervals. The MO concentration in the test samples was then, analyzed using UV-Vis Perkin Elmer Lambda 20 spectrophotometer at λ max of 465nm.

Result and Discussion Characterization

Phase and Elemental Analysis

TiO² that been calcined at 550^oC (Figure 2) constructed sharp anatase crystalline peaks which matched the JCPDS file no: 01-073-1764. As been illustrated, a small amount of Nb2O5 is doped on TiO2 (1-5 %), the Nb2O5- doped samples (b-e) adapted a single crystalline anatase lattice structure with no other additional diffraction

Int. J. Mat. Chem.

4 peaks assigned for other TiO2 polymorph or Nb2O5 crystalline phase. However, peak intensity decrease with the increase of Nb2O5 loadings by which this reduction in peaks intensity can be due to the increase of the Nb2O5 amorphous phase. The amount of Nb2O5 present, as determined using the XRF technique, is tabulated in Table 1. The slight difference observed can be due to adsorption of moisture by the niobium precursor that may have lead to a lesser amount of Nb added during synthesis.

Table 1. Effect of Nb²O⁵ doped onto TiO² average crystallite size calculated at 2θ = 25.3^o (101)

Figure 2. XRD diffraction pattern of (a) undoped TiO2 and (b) 1%, (c) 3%, (d) 4%, (e) 5% Nb2O5-doped TiO2

The average crystallite size of TiO2 was also observed to decline drastically upon the addition of Nb2O5 but remained constant with increasing Nb2O5 loading (Table 1). Conversion of anatase to rutile is also size dependant that the inhibition of rutile phase conversion behavior presented by this doping material is due to the mutual contact restriction between adjacent anatase crystallite (Zhang et al. 2007). The presence of Nb²O⁵ has also improved the anatase phase stability hence postponed the anatase to rutile phase transformation (Silva et al. 2016). The cell parameters of TiO2 (Table 1) however showed a linear expansion and elongation as the amount of dopant increased confirming the incorporation of Nb2O5 into TiO2. This is possible because

10 20 30 40 50 60

105211 004 200

100

(e)

(d)

(c)

(b)

(a)

2q / degree

Intensity / a.u

Sample Average

crystallite size (nm)

XRF (%) Cell parameters

(Å) BET specific surface area

(m²/g)

Pore size diameter

(nm) TiO2 Nb2O5 a = b c

TiO2 22.1 99.806 - 3.776 9.486 45.4 21.9

1% Nb2O5 doped-TiO2 9.7 97.472 1.119 3.787 9.499 113.5 4.3 3% Nb²O⁵ doped-TiO² 9.4 95.908 3.329 3.784 9.505 105.1 4.3 4% Nb2O5 doped-TiO2 8.7 95.535 3.682 3.802 9.533 100.9 3.1

5% Nb2O5 doped-TiO2 9.0 94.682 4.745 3.827 9.567 98.7 3.1

Int. J. Mat. Chem.

5 the ionic radius of Nb⁵⁺ (0.69Å) is almost equivalent to that of Ti⁴⁺ (0.68Å) (Tonejc et al. 2001; Ruiz A.M. et al.

2004; Valentin C. D. et al. 2009;). However, due to the extra charge for Nb⁵⁺, charge compensation must occur by decreasing the oxygen vacancies, hence stabilized the anatase phase. Therefore, this phenomenon explaining the presence of anatase TiO² only and the absence of other phases that consistent with rutile or even Nb2O5 as recorded in the XRD pattern (Figure 2).

Morphology

TEM analysis is not only used to define the detected shape of particles but also the exact size of a single particle. It is deemed that (Figure 3) the nano-sized particle is easily aggregated with each other prior to the drying process (Harris et al. 1988; Perego et al. 1997; and Kanade et al. 2006).

Figure 3. TEM image of a) undoped TiO2 b) 1%, c) 3%, d) 4%, e) 5% Nb2O5-doped TiO2

Specifically, all samples exhibit a blend of irregular to nearly spherical like shape without any remarkable changes to that of undoped TiO2 yet; its particles aggregation becomes prominent as the percentage of doped Nb2O5 increases. This morphological pattern had suggested that each individual Nb2O5-anatase particles aggregated to form bigger secondary particles as been projected in Table 2.

Table 2. Particle size determined from TEM image for anatase TiO2 and Nb2O5 –doped TiO2

a) b)

d) e)

c)

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6

Particle size range (nm) TiO2 Nb2O5 –doped TiO2

1% 3% 4% 5%

10 – 20 94 81 17 10 5

21 – 30 6 16 52 48 19

31 – 40 2 22 35 24

41 – 50 1 9 7 38

51 – 60

61 – 70 9

5

100% 100% 100% 100% 100%

Surface Area and Porosity

Figure 4 reveals adsorption-desorption isotherm and a single model BJH pore diameter for Nb2O5 doped TiO2

respectively. As illustrated, undoped TiO2 obeyed a type II isotherm which is classified for a non-porous or macroporous adsorbent while, all Nb2O5 doped TiO2, are characterized by a type IV isotherm which indicated for a mesoporous adsorbent according to IUPAC. This mesoporous structure is believed should be attributed due to the presence of amorphous Nb2O5 (Figure 2).

The different shapes of hysteresis loop which demonstrated for those Nb2O5 doped and undoped TiO2

revealed the possibility for inclusion of Nb2O5 onto TiO2 lattice. According to IUPAC, it can be categorized as H2 that represents capillaries with a narrow neck and wide-body like. Interestingly, as a percentage of Nb²O⁵ loaded onto TiO2 increased, the hysteresis loop which is in the range of 0.3 – 0.8 P/Po becomes slenderer presumably due to the shrinking effect on pore volume and pore size distributions. This characteristic behavioral may be connected with irreversible uptake of molecules in pores that have the same width as that of adsorbate molecule (Sing et al. 1984).

It is worth to notify that the shrinking effect in pore size, as well as volume uptake of each Nb2O5 doped TiO2, decreased as Nb2O5 loaded increases suggesting the pore-clogging behavior that occurs on the surface of the catalyst. The pore size and BET surface area of doped TiO2 were tabulated in Table 1 revealed the effect of diminished pore size onto their surface area. As the amount of Nb²O⁵ increased, the BET surface area and pore size were reduced too.

Int. J. Mat. Chem.

7 Figure 4. Adsorption-desorption isotherm and BJH method for pore diameter distribution of a) undoped TiO2,

b) 1%, c) 3%, d) 4%, ande) 5% Nb2O5-doped TiO2

The calculated BET specific surface areas (Table 1) revealed an almost double enhancement of surface area for all Nb2O5-doped TiO2 compared to undoped TiO2. This increment behavior could be attributed to the existence of the Nb2O5 amorphous phase on the anatase lattice due to the smaller calculated crystallite size for Nb2O5-doped TiO2 than undoped TiO2 (Table 1). However, further increment in Nb2O5 percentage loading had slightly reduced the surface area. This is perhaps due to the reduced pore size diameter, the shifting effect in particle size diameter and also the shifting of maximum size range as per tabulated in Table 1. It is truly accepted that bigger particle size has a lower surface area than smaller particle size. Nonetheless, the specific surface area of doped TiO2 is always higher than undoped TiO2.

Removal of Methyl Orange Adsorption

Removal of MO (~10 – 20%) by Nb²O⁵ - doped TiO² via adsorption is higher than that of TiO² alone (Figure 5) is most likely facilitated by the existence of amorphous Nb2O5 and also to the high surface area of the Nb2O5

doped TiO2. While a slight reduction in MO removal percentage which is later observed as the amount of Nb2O5 doped increases is attributed to the pore-clogging effect. It is obvious that the decreases of pore volume (Figure 4) as the percentage of Nb2O5 increases could also reduce the percentage of MO adsorbed onto the surface of Nb2O5 – doped TiO2.

0 50 100 150

0.2 0.4 0.6 0.8 1.0

undoped TiO₂ 1% Nb₂O₅- doped TiO₂ 3% Nb₂O₅- doped TiO₂ 4% Nb₂O₅- doped TiO₂ 5% Nb₂O₅- doped TiO₂

Relative Pressure (P/Po)

Volume (cc/g)

0 2 4 6 8

5 10 20 50 100

(a) (b) (c) (d) (e)

Pore diameter (nm) Pore volume (cm3 /g)

Int. J. Mat. Chem.

8 Figure 5. Removal of MO using a) 1%, b) 3%, c) 4%, d) 5% Nb2O5-doped TiO2. Mass loading = 1.0gL^-1, [MO] =

10mgL^-1 Photocatalytic process

Photodegradation of MO is enhanced remarkably when Nb2O5 – doped TiO2 is used as photocatalyst. It is believed that this excellent performance is due to its ability to efficiently reduce the charge recombination rate possibly by enriching trap sites to scavenge electrons and promote charge transfer to adsorbed species.

The charge compensation due to substitution of Ti⁴⁺ by Nb⁵⁺ could create cation vacancies or reduction of Ti⁴⁺

to Ti³⁺ per Nb incorporated (Castro et al. 2009). Charge defects (Ti³⁺ species and oxygen vacancies) will act as holes traps and combined with the photogenerated holes to become charged species thus boosted the photodegradation activity. This charge defects can recover to their original state and continuously repeat the previous reactions.

It is also suggested that the hydroxyl groups on the surface of Nb2O5 – doped TiO2 may act as active sites for MO adsorption, thus facilitate the photodegradation process (Huang et al. 1999). It is worth mentioning that, higher content of OH groups could lead to high photocatalytic activity in doped photocatalyst (Mitadera et al., 2002). Moreover, defect structure due to oxygen vacancies could also affect the adsorption of water and dissociation of water to hydroxyl radicals. Intrinsically, these hydroxyl radicals can further produce H2O2, a very powerful oxidizing agent (Hirakawa et al., 2007). On the whole, the high surface area will definitely improve the photocatalytic activity as it allowed more MO molecules to be adsorbed and decomposed simultaneously on the photocatalyst surfaces.

Effect of Operational Parameters Nb2O5 loading into TiO2

Clearly as in Figure 5, 1% and 3% Nb2O5 showed almost equivalent performance whilst exceeding 3% of Nb2O5 a drastic reduction was observed. As the concentration of dopant increases, the recombination rate also increases as the space-charge region became narrower (Akpan et al. 2009). Moreover, beyond an optimal percentage, the dopant itself can also act as a charge recombination center, thus hampered the photodegradation rate.

C/Co

Time/min

(a) (b) (c) (d) UV irradiation

Adsorption

Int. J. Mat. Chem.

9 In order to select the optimum amount of Nb2O5 loaded into TiO2, further elucidation is needed by observing their photocatalytic activity at a higher concentration of MO (Figure 6 (a)). Above all, 3% Nb2O5 – doped TiO2

offered the highest photocatalytic activity as compared to 1% of Nb²O⁵ loading. The emergence of optimum Nb2O5 loading suggesting an equilibrium condition confined between trap sites and trapped charge carriers which sufficiently lead to expanded lifetime for interfacial charge transfer (Carp et al. 2004 and Han et al.

2009).

Mass loading

By varying the amount of 3% Nb2O5 doped TiO2 in degrading 30mgL^-1 MO, the effect of photocatalyst loading could further scrutinize (Figure 6 (a)).

At first, the photodegradation performance improved linearly according to mass loading as the increment of mass provided a larger surface area thus more active site surface for an efficient photodegradation activity.

However, beyond 2.5 g, the ability to degrade had reduced tremendously. This reducing trend is definitely associated with several factors:

a) The shielding effect of light irradiation that affected the photon absorbance of the system (Akpan et al.

2009; Ibrahim et al. 2008)

b) Terminal reactions that formed less reactive hydroperoxyl radicals (Carp et al. 2004) H^. + HO^. H2O2 (1) H2O2 + HO^. H2O + HO2. (2)

0 20 40 60 80 100

1.0 g 1%

Nb2O5 doped TiO2

1.0 g 3%

Nb2O5 doped TiO2

1.5 g 3%

Nb2O5 doped TiO2

2.0 g 3%

Nb2O5 doped TiO2

2.5 g 3%

Nb2O5 doped TiO2

3.0 g 3%

Nb2O5 doped TiO2

Removal %

0 0.2 0.4 0.6 0.8 1

0 100 200

C/Co

Time (s)

30 mg/L

a) b)

Int. J. Mat. Chem.

10 Figure 6. Effect of different a) Nb2O5 loaded into TiO2 and its mass loading, [MO] = 30mgL^-1 b) Initial concentrations, photocatalyst mass loading = 2.5 gL^-1 c) Amount of MO removed according to its initial

concentration.

Initial concentrations

Obviously, as concentration increases, photodegradation activity reduces significantly as illustrated in Figure 6 (b). A perfect explanation is that this aqueous dyes solution started to behave as filters that shielding the irradiation light from penetrating the photocatalyst surface (Gupta et al. 2006) caused dye solution to become more impermeable to the UV light and restricted the amount of photon absorbed. Besides, there are not enough catalysts surface that available to react with the excess amount of dye. It is also believed that higher MO concentration generates intermediates that could possibly be adsorbed onto the photocatalyst surface, hence reduced its ability to photodegrade. In general, an optimum amount of MO being removed by 2.5 g 3%

Nb²O⁵ doped TiO² is 13.34 mgg^-1 (Figure 6 (c)).

Photocatalytic mechanism of Nb2O5 doped TiO2

It is suggested that when Nb doped onto TiO2, the niobium dopant will acts as donor centers due to the charge transfer to TiO2 conduction band (Castro et al. 2009). Intrinsically, the charge compensation due to the substitution of Ti⁴⁺ by Nb⁵⁺ would creates cation vacancies or stoichiometric reduction of Ti⁴⁺ to Ti³⁺ per niobium incorporated. It is believed that Ti³⁺ species and oxygen vacancies act as holes traps and combined with the photogenerated holes to become charged species. Simultaneously the oxygen will trap electron, and the trapped holes transferred to the adsorbed organic pollutants that lead to enhance degradation. These charged defects (Ti³⁺ and oxygen vacancies) will recover to their original state and continuously repeat the previous reactions (Carp et al. 2004).

Another important factor that could lead to higher photocatalytic activity as Nb2O5 doped onto TiO2 is the amount of surface hydroxyl group. Higher content of OH groups could be achieved by impregnating sample with water (Carp et al. 2004). The surface hydroxyl group on the Nb2O5 – doped TiO2 may acts as active sites for MO adsorption and covering the electron trapping site by adsorbed O2 which resulting in the production

10.5 11 11.5 12 12.5 13 13.5

30 mg/L 40mg/L 50mg/L 60mg/L

Amount of MO removed (mgg-1) c)

Int. J. Mat. Chem.

11 of oxygen radicals and thus inhibited the recombination of photoinduced charged. Moreover, defect structure due to the oxygen vacancies, could also affected the adsorption of water and the rate of water dissociation into hydroxyl radicals. These hydroxyl radicals can further produce H2O2 which led to production of more super radicals. Hence, lead to high photocatalytic activity.

Kinetics of the reactions

There are various kinetic models that been used and proposed in order to study and calculate the rate of organic pollutants removal for photodecomposition. Predicting the rate of reaction is the most important factor to study the mechanism of the reaction. As been proposed by Langmuir – Hinshelwood (L-H) model (Baran et al. 2008), the rate of unimolecular surface reaction, R is proportional to the surface coverage, θ, where the reactants are more strongly adsorbed on the surface than the products (Eqn.3 and 4):

R = -dC/dT = Krθ = KrKC0/(1+KC0+KsCs) (3)

R = -dC/dT = Krθ = KrKC0/(1+ KC0) (4)

Where; Kr is the reaction rate constant, while θ is the fraction of the surface covered by the reactant, K is the adsorption coefficient of the reactant, K^s is the adsorption coefficient of the solvent and C^s is the concentration of the solvent. Eqn.3 can only be applied when both the reactant and the solvent compete for the same active site while Eqn.4 is only applicable when both the reactant and the solvent are adsorbed on the surface without competing for the same active sites. When the concentration is too low, the term of KC^o is often negligible, thus, integration of these equations under assumption that C=Co at t=0 yields:

ln (C0/C) = kt (5)

By plotting graph ln C0/C vs t, the first-order rate constant, k can be determined from the slope of the straight-line graph. A useful indication of the rate of a first-order chemical reaction is the half-life, t1/2, of a substance, the time for the concentration of reactant to fall to half of its initial value. Therefore, when C/C0 = 0.5,

t1/2 = ln 2/k = 0.693/k (6)

Figure 7 apparently shows the linear plot of logarithmic scales on photodecomposition of different MO initial concentrations using 2.5g 3% Nb2O5 doped TiO2 via UV irradiation; that proves this photocatalytic MO degradation follows first-order kinetics. Hence, Table 3 tabulated the correlation factor, half time t1/2 and rate constant which was derived from Figure 7.

Int. J. Mat. Chem.

12 Figure 7. Graph ln C⁰/C versus irradiation time for kinetic rate study with initial MO concentration of 30, 40,

50 and 60 mg/L. Mass loading = 2.5gL^-1

Table 3. The first-order rate constant, k correlation factor, R² and half lifetime t1/2 for different initial dye concentration

The initial concentration of

dye (mg/L)

First-order

(Photocatalytic decomposition) The first-order

rate constant, k (min^-1)

Correlation

factor, R² Half lifetime (t1/2)

30 0.0206 0.9938 4.58

40 0.0092 0.999 5.38

50 0.0045 0.9979 6.1

60 0.0037 0.9986 6.3

Conclusions

A series of active Nb2O5 doped TiO2 were successfully prepared through the ethanolic-aqueous impregnation method. The effect of doping Nb2O5 into TiO2, despite does not alter the anatase phase construction however, had reduced its peak intensity and crystallite size as the percentage of Nb2O5 loading increases. Attentively through TEM analysis, as a percentage of Nb2O5 doped increases, even so, its spherical shape remained unaffected, but the high degree of aggregation with gradual increases in particle sizes was observed. The inclusions of amorphous Nb²O⁵ were obviously transpired as perceived by the shifting effect on the adsorption-desorption isotherm slope and reduction trend on the calculated specific surface area which is

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

0 30 60 90 120 150 180 210

Irradiation Time (min) ln Co/C

30 mg/L 40 mg/L 50 mg/L 60 mg/L

Int. J. Mat. Chem.

13 due to the increment of Nb2O5 dosage. As anticipated, these intangibles circumstances incur due to the insertion of Nb2O5 onto TiO2 facilitate a better photocatalytic activity in comparison to that of bare TiO2 with 3% Nb2O5 doped TiO2 is by far the best in which its kinetic reaction was observed to obey first order.

Ultimately, a gram of 3% Nb²O⁵ doped TiO² can permanently remove an optimum amount of 12.8mg MO.

Acknowledgment

The authors would like to thank all the assistance rendered throughout this study. The authors declare that they have no conflict of interest.

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