Photocatalytic degradation of organic dye using titanium dioxide modified with metal and non-metal deposition

Materials Science in Semiconductor Processing 41 (2016) 209–218

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Photocatalytic degradation of organic dye using titanium dioxide modified with metal and non-metal deposition Muhammad Anas a, Dong Suk Han a,n, Khaled Mahmoud b, Hyunwoong Park c, Ahmed Abdel-Wahab a,b,nn a

Chemical Engineering Program, Texas A&M University at Qatar, Education City, Doha 23874, Qatar Qatar Environment and Energy Research Institute (QEERI), Doha, Qatar c School of Energy Engineering, Kyungpook National University, Daegu 702-701, South Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 August 2015 Received in revised form 18 August 2015 Accepted 27 August 2015

In this study, photocatalytic degradation of methyl orange (MO) as an example of organic dye was investigated using different wt% Pd-loaded and N-doped P-25 titanium dioxide (TiO2) nanoparticles, as example of metal and nonmetal-doped TiO2, respectively. The Pd-loaded and N-doped TiO2 photocatalysts were prepared by post-incorporation method using K2PdCl4 and urea, respectively, as precursors. A variety of surface analysis techniques were used for characterization of surface and functional group while using ultraviolet/visible (UV–vis) analysis for monitoring photocatalytic degradation of MO. Kinetic parameters were obtained using Langmuir-Hinshelwood model to determine the degradation rate constants. It was found that the metal-loaded titanium dioxide degraded MO in water at a higher rate than did non-metal-loaded titanium dioxide fabricated by using the post-synthesis method. Also, the pure P25-TiO2 degraded MO more than N-doped TiO2 because of decreased surface area by particle agglomeration after being made by the post-incorporation method. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Methyl orange Titanium dioxide Palladium Nitrogen Langmuir-Hinshelwood kinetics

1. Introduction Water security is one of the major challenges of the 21st century. In order to conserve water resource from contamination, conventional water treatment technologies such as precipitation, adsorption, filtration, or stripping do not destroy pollutants but rather transfer them from one phase to another [1]. Therefore, advanced oxidation processes (AOPs) or advanced reduction processes (ARPs) combined with those conventional methods are gaining growing attention as effective water treatment processes because they can destroy pollutants and convert them to innocuous compounds [2,3]. Among AOPs, use of heterogeneous semiconductor photocatalysis (SP) is an attractive tool because of its high stability in biological and chemical environment, cost effectiveness, tangible redox potential, and efficient performance [4]. TiO2 has been widely used in many applications in the energy and environment fields because of its stability, low cost, nonn

Corresponding author. Fax: þ 974 4423 0065. Corresponding author at: Chemical Engineering Program, Texas A&M University at Qatar, Education City, Doha P.O. Box 23874, Qatar. Fax: þ 974 4423 0065. E-mail addresses: [email protected] (D.S. Han), [email protected] (A. Abdel-Wahab). nn

http://dx.doi.org/10.1016/j.mssp.2015.08.041 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

toxicity, feasible surface modification, and non-corrosivity [5–7]. For example, TiO2 semiconductor has been used for water and air purification [8], hydrogen production through water splitting [9], gas sensors [10], and smart materials [11]. However, its use in practical applications is hindered by its high band gap energy (Eg ¼3.2 eV) which allows the absorption of light only in the UV range, thus limiting the use of visible light and incurring high cost of operation. Also, the fast recombination of photogenerated electron–hole pairs leads to low quantum efficiency [12,13]. To overcome the limitations, several approaches have been employed including bulk doping or surface modification with organics (polymers, dyes), inorganics (metal, non-metal, semiconductor) [14], or carbon nanotubes materials [15]. Metal and nonmetal-doped TiO2 have been extensively used to remove a wide range of contaminants in air and water using sunlight [12]. Transition and noble metals are common modifiers enhancing the transfer of charge carriers of photoactive TiO2 to contaminants [16,17]. Nonmetals such as boron [18], nitrogen [19], fluorine [20], and carbon [21] have also been used to modify TiO2 in order to enhance its photocatalytic activity in the visible light range, but amongst them nitrogen has been found to be the most favorable dopant due to its stability and atomic size that is comparable to oxygen in TiO2 [12]. In addition to doping with metals and nonmetals, catalyst synthesis method was found to enhance

210

Table 1 Apparent rate constant for metal and non-metal-doped TiO2 Experimental conditions and method

Substrate

Rate constant, degradation time

Ref.

Pd

Sol–gel method for synthesis of TiO2 nanoparticles Pd/TiO2 using impregnation method with PdCl2 Molar ratio (Pd:TiO2 ¼ 0.03) 500 ml of 50 ppm substrate Catalyst used ¼ 0.25 g Gd/TiO2 by sol–gel method using tetra-n-butyl titanate and Gd(NO3)3  5H2O Catalyst Concentration ¼ 1 g dm  3 Initial concentration of substrate¼ 1.2  10  4 M pH¼ 6.5 Sm/TiO2 by sol–gel method using tetra-n-butyl titanate and Sm(NO3)3  5H2O Catalyst Concentration ¼ 1 g dm  3 Initial concentration of substrate¼ 1.2  10  4 M pH¼ 6.5 Nano-size Ag/TiO2 by sol–gel method involving a reduction agent. Titanium tetraisopropoxide, silver nitrate as precursors for titania and silver and sodium citrate tribasic dehydrate as a reduction agent Catalyst used ¼ 1 mmol Catalyst concentration ¼1 mg/L 500 ml of 50 mg/L substrate Calcination temperature ¼300 °C Cooled water hydrolysis Titanium butoxide and VCl3 as precursors for titania and vanadium V/Ti ratio ¼0.035 Catalyst used ¼ 0.05 g Initial concentration of substrate¼ 1.3  10  5 M Au/TiO2 using Photoreduction method P25 TiO2 mixed with tetrachloroauric acid, a gold precursor, and methanol, a hole scavenger Catalyst used ¼ 0.2 g 165 ml of 12 mg/L substrate 0.5% Au–TiO2 prepared TiO2 by sol–gel method with Ti(O–Bu)4 as Ti precursor Pt–TiO2 by photoreduction process using hexachloroplatinic acid as a Pt precursor Catalyst used ¼ 0.2 g 0.75% Pt/TiO2 photocatalyst 165 ml of 15 mg/L MB and 165 ml of 20 mg/L MO TiO2 nanotube using hydrothermal chemical process with TiO2 anatase powder. Zinc acetylacetonate as a Zn precursor. Zn–TiO2 by mixing TiO2 and Zn precursor with stirring for 6 h. Calcined at 400 °C for 1 h Catalyst used ¼ 0.2 g 100 ml of 20 mg/L substrate Oxidation of TiN at 450 °C for 2 h in air Nanoparticle catalyst used ¼ 0.20 g Substrate concentration ¼150 mg/m3 Incipient wet impregnation method TiO2 Degussa P25 mixed with urea Calcined at 773 K for 3 h Catalyst used ¼ 0.2 g/100 ml 600 ml of 1.0  10  2 M substrate pH¼ 5.9 0.50% N–TiO2 TiO2 nanotube using hydrothermal process with TiO2 rutile powder

Methyl blue

40  104 min  1

[23]

NO− 2

9.1  106 M min  1

[34]

NO− 2

2.2  106 M min  1

[34]

p-Nitrophenol

2.9  10  2 min  1

[35]

Crystal violet

3.07  10  7 M h  1

[36]

Gd

Sm

Ag

V

Au

Pt

Zn

N

N

N

Guanidine carbonate as N precursor N–TiO2 by mixing TiO2 and n precursor with stirring for 24 h Calcined at 350°C for 1 h

Methylene blue 0.052 min  1

[24]

MB and MO

0.1042 min  1 for MB 0.0988 min  1 for MO

[37]

MO

Degraded 65% MO in 3 h

[38]

Toluene

10.5  103 min  1

[29]

4-Nitrophenol

18.388  103 min  1

[25]

Methylene blue Degrade 95.1% in 7 h under artificial solar light

[32]

M. Anas et al. / Materials Science in Semiconductor Processing 41 (2016) 209–218

Metal/nonmetaldoped TiO2

[25]

[25]

20.842  103 min  1

17.855  103 min  1

4-Nitrophenol

4-Nitrophenol

Methylene blue 0.278 h  1

[39]

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degradation kinetics of target contaminants [22]. Sol–gel method is the most common method to synthesize TiO2 and doped-TiO2 [23], however use of commercially available TiO2 nano-powder coupled with photodeposition or incipient wet impregnation methods are also employed to prepare modified photocatalysts [24,25]. Table 1 provides a summary of degradation rate of contaminants for different metal and nonmetal-doped TiO2 synthesized using diverse methods. This information could be a basis for comparison of kinetic effectiveness of various TiO2 types for target contaminants with respect to various experimental conditions. This study evaluated degradation rate of methyl orange (MO) as an example of dye pollutant using different type of surface-modified TiO2 as a function of loading concentration of dopant. In this study, palladium (Pd) was used for fabricating metal-loaded TiO2 while N was used for nonmetal-loaded TiO2. Effects of the type of loading material and synthesis method on the kinetics of MO degradation were evaluated together with surface and photoactivity characterization using surface analysis techniques.

2. Experimental

S

C

C

Catalyst used ¼ 30 mg 100 ml of 30 mg/L Substrate Sol–gel method using tetrabutyl orthotitanate and ethanol. Calcined at 200 °C for 5 h Nanostructured catalyst used ¼ 5 mg 15 ml of 1.6  10  5 M substrate Incipient wet impregnation method TiO2 Degussa P25 mixed with Glucose Calcined at 773 K for 3 h Catalyst used ¼ 0.2 g/100 ml 600 ml of 1.0  10  2 M substrate pH¼ 5.9 0.50% C–TiO2 Incipient wet impregnation method TiO2 Degussa P25 mixed with Thiourea Calcined at 773 K for 3 h Catalyst used ¼ 0.2 g/100 ml 600 ml of 1.0  10  2 M substrate pH¼ 5.9 0.50% S–TiO2

2.1. Materials and synthesis For the preparation of metal-loaded TiO2, palladium metal was loaded using photodeposition method [26,27]. Briefly, 2 g of P25 TiO2 (Sigma-Aldrich, anatase 80%, rutile 20%, 21 nm of particle size, 100 g) was mixed with specific amount of K2PdCl4 in 100 ml of deionized water and 5 ml of methanol to obtain the desired Pd loading. For example, 3 wt% of Pd-loaded TiO2 was prepared by adding approximately 0.19 g K2PdCl4 to 2 g of TiO2 suspended in the water–ethanol mixture. The mixture was stirred for 2 h under sunlight irradiation (AM 1.5 using solar simulator, ABET Tech.) to obtain gray colored Pd–TiO2. The solution was then filtered by a vacuum filtration device. Solids obtained were then dried using vacuum desiccator under anaerobic conditions and finally photocatalysts with 1 wt%, 3 wt%, and 5 wt% Pd were synthesized, respectively, and used in MO degradation experiments. For the preparation of nonmetal-loaded TiO2, urea (Amresco, 8 M) was used as a source of nitrogen with post-synthesis method [28]. Briefly, 0.6 g of P25 TiO2 was mixed with 10 ml of deionized water to produce a slurry. The desired volume of urea solution was mixed with the TiO2 slurry while stirring for 30 min to obtain the required mol% N in the TiO2. For example, 30 mol% of N-doped TiO2 was prepared by adding 2 ml of urea to 0.6 g of TiO2 water slurry. The mixture was then calcined at 500 °C in order to remove organic impurities from the TiO2 surface as well as to possibly incorporate N into the chemical structure of TiO2. After calcination, the yellow colored N-doped TiO2 was washed several times with deionized water and finally photocatalysts with 20 mol%, 30 mol%, and 40 mol% N were synthesized. 2.2. Materials characterization Scanning electron microscope (FEI Quanta 400 SEM) equipped with Energy Dispersive X-ray Spectrometry (EDS) was used to identify the shape and chemical structure of Pd-loaded and N-doped TiO2. For N-doped TiO2, sputtering using Leica EM SCD 050 instrument was employed to coat the gold on the powder in order to make it conductive. Fourier Transform Infrared Spectrometer (Varian 640-IR) was used to characterize the functional groups of N-doped TiO2 and X-ray Diffraction (XRD) patterns of Pd-loaded and N-doped TiO2 were obtained in order to identify phase changes in the samples. X-ray Photoelectron Spectroscopy (XPS, Kratos Axis Ultra DLD) was used to examine the binding energies of each element with 20 eV pass energy and 5 sweeps by

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collecting the high resolution XPS spectra of C 1s, O 1s, N 1s, Ti 2p, and Pd 3d, respectively.

terms of normalized concentration, defined as C/C0 where C and C0 are concentrations of MO at time t and t¼0, respectively.

2.3. Photocatalytic activity measurements 3. Results and discussion Methyl orange (MO) dye was used as a model pollutant and its degradation using the Pd-loaded and N-doped TiO2 was carried out under irradiation of one sun (AM 1.5G) using solar simulator (ABET Tech.). A weight of 50 mg Pd-loaded or N-doped TiO2 catalyst was suspended in 200 ml of 40 μM MO solution and mixed using a magnetic stirring during the reaction. Samples were taken at the desired reaction time, filtered through 0.45 μm pore sized membrane filter (Whatmans), and the concentration of MO was measured using UV–vis Spectrometer (Perkin Elmer Lambda 950) at 465 nm. The degradation concentrations were expressed in

3.1. Characterization Fig. 1a and b shows the XRD patterns for photocatalysts of Pd–TiO2 and N–TiO2, respectively and the patterns for pure TiO2 are also presented for comparison. Pure TiO2 shows peaks for both rutile (2θ ¼27.8°, 34°, 44°) and anatase phase (2θ ¼ 25.2°, 37.9°, 48°) (JCPDS nos.: 88-1175 and 84-1286). However, the decrease of rutile phase peaks is not significant in both Pd–TiO2 and N–TiO2 photocatalysts by observing the calculated fraction value (f) of the

Fig. 1. XRD patterns for (a) Pd–doped TiO2 and (b) N–doped TiO2.

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213

Fig. 2. SEM/EDS analysis of (a) TiO2, (b) Pd(1 wt%)–TiO2, (c) N(40 mol%)–TiO2.

rutile less than  7%. [29]. So there is no substantial change in the intrinsic structure of TiO2 even after surface modification and heating. Fig. 2 shows the SEM image and EDS spectra for TiO2, Pd(1 wt%)–TiO2, and N(40 mol%)–TiO2, respectively. The EDS spectra show that all types of catalysts exhibit titanium, carbon, and oxygen as major peaks. For Pd–TiO2, the EDS spectra shows very small peak for palladium, confirming that Pd deposited onto TiO2 surface with relatively low percentage of Pd compared to other elements. Likewise, the nitrogen peak for N–TiO2 confirms the deposition of nonmetal onto the powder.

Fig. 3 shows the FTIR spectra of TiO2 and N–doped TiO2. No functional groups corresponding to nitrogen species were observed, even though N peak was detected in the EDS. Since the synthesized photocatalysts were annealed thoroughly, no peaks for NH2 (1600–1640 cm  1) or CONH2 (1650–1690 cm  1) were observed confirming that there is no urea remaining in the photocatalysts. This confirms that nitrogen is incorporated into the interstitial site of TiO2 structure rather than depositing on the surface. In addition, the peak at  2350 cm  1 is due to CO2 contamination. The UV–vis spectra of TiO2 and Pd–TiO2 are presented in Fig. 4a

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Fig. 3. FTIR spectra for synthesized N(20 mol%)–, N(30 mol%)–, and N(40 mol%)–TiO2.

and show a shift of the Pd–TiO2 light absorption spectra to wider range of wavelength compared to pure TiO2. High absorbance in the visible light region is due to the presence of metal ion energy levels in the TiO2 band gap [27]. It is also seen that as the amount of Pd increases, the absorbance in the visible light region increases. Fig. 4b shows the UV–vis spectra of N–TiO2 compared with TiO2. It can be observed that the spectra of N–TiO2 shift slightly toward the visible light region. However, the extent of light absorption decreased with increasing mol% of N showing that N(20 mol%)–TiO2 exhibits the highest absorption. The shift in the spectra is not affected by the amount of urea because the observed absorption shift is probably due to the electron transfer from urea decomposition product polytriazine to TiO2 rather than the local excitation of the decomposition product [28]. Fig. 5 shows the XPS analysis for TiO2, Pd–TiO2, and N–TiO2 photocatalysts. For all photocatalysts, the Ti 2p spectrum is composed of a doublet having two symmetric peaks. This doublet corresponds to the Ti(IV) oxidation state. For the pure TiO2, the difference in binding energy between the two peaks is about 5.7 eV which agrees with previously reported values in the literature [30]. However, for Pd–TiO2 and N–TiO2 photocatalysts, the binding energy values shift to lower values, indicating strong interaction between Pd and TiO2 and between N and TiO2. In addition, the magnitude of the shift was highest in Pd(1 wt%)–TiO2 as compared to other Pd–TiO2 photocatalysts. The difference between Pd(3 wt%)–TiO2 and Pd(5 wt%)–TiO2 was minor with Pd(3 wt%)–TiO2 being lower than Pd(5 wt%)–TiO2. This indicates that as the weight percentage of Pd increases, the interaction between Pd and TiO2 decreases [30]. The binding energy and strength of interaction did not change by changing the percentages of N in TiO2 between 20 mol% and 40 mol%. Similar behavior is observed from the O 1s spectrum. Further information about the properties of synthesized photocatalysts can be obtained from Pd 3d and N 1s XPS spectra. The peaks at 342.0 eV and 337.0 eV for Pd–TiO2 photocatalysts correspond to Pd2 þ [30]. Pd(1 wt%)–TiO2 binding energy shifts to a lower value meaning Pd has lower oxidation state as compared to Pd in Pd(3 wt%)–TiO2 and Pd(5 wt%)–TiO2 photocatalysts which is consistent with the results from Ti 2p and O 1s spectra. In the N 1s spectra, a broad peak ranging from

396.0 eV to 402.0 eV and centered at 400.0 eV was observed for only N(40 mol%)–TiO2. This peak does not correspond to TiN but it is typical of the substitutional N state in N-doped TiO2 [29]. 3.2. Photocatalytic degradation of MO 3.2.1. Pd–TiO2 Fig. 6 shows the rate of MO degradation with time for different wt% of Pd-loaded TiO2 photocatalysts, in which the MO degradation rate increased by increasing the wt% of Pd which indicates that increasing the loading of Pd onto TiO2 led to increasing the photocatalytic activity of the modified catalyst under sunlight. Regardless of Pd loading ratios, the MO was completely degraded in less than 45 min and all degradation rates were faster than the corresponding rates with pure TiO2 even though the particle size of Pd–TiO2 was not nano-sized like TiO2 (0.5–200 μm in Fig. 2). These results are consistent with the UV–vis light absorption results (Fig. 4), which showed that the Pd–TiO2 photocatalysts absorb light over a wide range of the sunlight spectra resulting in increased photocatalytic activity. The Pd(1 wt%) TiO2 catalyst was recovered after use in one degradation experiment, filtered, and reused again for MO degradation in order to investigate the continuity of photocatalytic activity of the fabricated Pd–TiO2. However, the recycled Pd–TiO2 required 90 min to completely degrade MO unlike the fresh catalyst which required only 40 min to completely degrade MO. This indicates that the activity of the catalyst decreased after it was used once and this decrease in photocatalytic activity could be due to release of Pd from the solid during the regeneration process. Also, it could be due to the accumulation of organic intermediates on the surface of the catalyst thus affecting its adsorption capacity [31]. However, there was no significant decrease (o 2% of initial concentration) of MO with time for experiment without any photocatalyst under only one sun illumination. Several studies have reported that photocatalytic degradation of MO with low concentration (o20 μM) often follows LangmuirHinshelwood (L-H) kinetics. The L-H kinetics can be governed by many parameters,

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215

0.14

0.13

Absorbance

0.12 TiO2

0.11

Pd(1wt%)-TiO2 Pd(3wt%)-TiO2

0.10

Pd(5wt%)-TiO2

0.09

0.08

0.07 200

300

400

500

600

700

Wavelength, nm 0.18

TiO2 0.16

N(20mol%)-TiO 2 N(30mol%)-TiO 2

Absorbance

N(40mol%)-TiO 2 0.14

0.12

0.10

0.08 200

300

400

500

600

700

800

Wavelength, nm Fig. 4. UV–vis spectra for (a) synthesized Pd-doped TiO2: Pd(1 wt%), Pd(3 wt%), Pd(5 wt%), and for (b) synthesized N-doped TiO2: N(20 mol%), N(30 mol%), and N(40 mol%).

r MO =

−

−dCMO = f (I , s , CMO, K ) dt

(1)

where rMO is observed degradation rate of MO, I is the light intensity, s is the surface area of the catalysts, CMO is concentration of MO, and K is the adsorption coefficient of MO. Since the rate of photocatalytic degradation of MO under sunlight follows pseudofirst-order reaction rate (L-H mechanism) driven by a unimolecular reactant on the photocatalyst, Eq. (2) can be represented by

r MO =

dCMO KCMO − = k dt 1 + KCMO

(2)

where k is the L-H rate constant and t is the time. The L-H rate constants (k) and adsorption coefficient (K) for different photocatalysts were optimized by nonlinear regression with “nlinfit”

function using MATLAB and their values are shown in Table 2. Apparently, the L-H rate constants represent how fast MO is degraded, which highly depend on various photocatalytic factors such as available active species (  OH, O2∙−, h+vb ), lattice oxygen defect, bandgap energy of photocatalyst, and so on. Once the light higher than bandgap energy illuminates TiO2 surface, the active species can be generated by the following reactions, which can be involved in oxidative degradation of MO.

TiO2 + hv → h+vb + e−cb

(3)

e−cb + O2 → O•− 2

(4)

h+vb + H2 O → • OH + H+

(5)

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Fig. 5. High resolution XPS spectra of Ti 2p, O 1s, Pd 3d, and N 1s for synthesized Pd(1 wt%), Pd(3 wt%), Pd(5 wt%), N(20 mol%), N(30 mol%), N(40 mol%), and TiO2.

TiO

1.0

Pd(3wt%)-TiO

Concentration, C/C

O•− + MO → degraded product 2

(6)

•OH + MO → degraded product

(7)

Pd(1wt%)-TiO Pd(5wt%)-TiO

0.8

Used Pd(1wt%)-TiO

0.6

0.4

0.2

0.0 0

20

40

60

80

100

Irradiation time, min

Fig. 6. Degradation kinetics of MO by TiO2, Pd (1 wt%)-, Pd(3 wt%)-, Pd(5 wt%)doped TiO2, and reused Pd(1 wt%)–TiO2: 0.25 g/L photocatalyst, 40 μmol/L MO, no pH control, and AM 1.5G irradiation.

However, in this study, O2 was not purged into the system for saturation of oxygen but naturally dissolved in the solution. Table 2 indicates that the rate constant of 3 wt% Pd–TiO2 for MO degradation was greater than that of 1 wt% Pd–TiO2 photocatalyst, meaning that as the Pd loading increases, activity of the catalyst increases caused by enhanced electron–hole separation. However, when Pd loading amount was increased to 5 wt% Pd, the rate constant was lower than that for 3 wt% Pd–TiO2. It was reported in the literature that low percentage of Pd in TiO2 interacts strongly with Ti and results in higher photocatalytic activities as compared to higher percentages of Pd [30]. However, in this study Pd(1 wt%)–TiO2 shows slower degradation compared to Pd(3 wt%)–TiO2. This can be attributed to the different oxidation state which lowers the photocatalytic activity which was also confirmed

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Table 2 The L-H rate constants of MO degradation by Pd-doped TiO2 and N-doped TiO2 Pd-doped TiO2

L-H rate constant, k (μmol  1 min  1)

Adsorption coefficient, K (l μmol  1)

N-doped TiO2 L-H rate constant, k (μmol  1 min  1)

Adsorption coefficient, K (l μmol  1)

P25–TiO2 Pd(1 wt%)–TiO2 Pd(3 wt%)-TiO2 Pd(5 wt%)–TiO2 Used Pd(1 wt%)–TiO2

0.06 17.2 21.3 15.0 0.032

0.88 0.006 0.005 0.17 2.21

N(20 mol%) N(30 mol%) N(40 mol%)

25.0 0.57 11.3

TiO

1.0

0.04 0.024 0.017

TiO , one sun

1.0

N(20%mol)-TiO

TiO , UV cutoff N(20mol%)-TiO , one sun

N(40%mol)-TiO

0.8

Concentration, C/C

Concentration, C/C

N(30%mol)-TiO

0.6

0.4

0.2

N(20mol%)-TiO , UV cutoff

0.8

0.6

0.4

0.2

0.0

0.0

0

20

40

60

80

100

120

140

0

Irradiation time, min

20

40

60

80

100

120

140

Irradiation time, min

Fig. 7. Degradation kinetics of MO by N(20 mol%), N(30 mol%), and N(40 mol%)doped TiO2: 0.25 g/L photocatalyst, 40 μmol/L MO, no pH control, and AM 1.5G irradiation.

Fig. 8. Degradation kinetics of MO by TiO2 and N(20 mol%)–TiO2 under irradiation of one sun (AM 1.5G) and visible light (UV cutoff): 0.25 g/L photocatalyst, 40 μmol/L MO, and no pH control.

by XPS peaks that show different oxidation state for Pd(1 wt%)–TiO2 photocatalyst.

degradation rate under sunlight. From Table 2, it is seen that as the amount of N in the photocatalyst increase, the rate of MO degradation decreases. It is because there is an optimum amount for nitrogen on TiO2 and the amount greater than optimum leads to decrease in activity due to decrease in absorption and higher electron–hole recombination [33]. Also, higher percentage of N results in larger particle assemblies due to post-synthesis method employed in this study, in contrast with sol–gel method. Even though N 1s XPS peak in N(40 mol%)–TiO2 shows highest detection of N, the kinetic degradation is lowest. This confirms that particle-size effect is more rate-determining factor rather than deposited-amount of N on the TiO2 in the system made with postsynthesis method. To obtain more information about the effect of particle size versus the effect of visible light in both the UV and the visible light ranges when N-doped TiO2 photocatalysts was used for degradation of MO, UV cut-off filter that is able to screen light less than 400 nm was used in the solar simulator. Fig. 8 shows the degradation rate of MO using TiO2 and N–TiO2 under illumination of one Sun with and without UV light. Without UV light, MO degradation rates using TiO2 and N–TiO2 are slower than the rates using the whole sunlight spectra, confirming that the photocatalytic performance of both photocatalysts shows similar behavior under sunlight with and without UV light. The fact that TiO2 degrades MO faster than N-doped TiO2 both under sunlight and UV light denotes that particle size plays a crucial role in the degradation rate because TiO2 is nano-sized while N-doped TiO2 agglomerated during synthesis thus reducing the number of active sites.

3.2.2. N–TiO2 The N-doped TiO2 photocatalysts were prepared with three different mole ratios of nitrogen in urea to TiO2, i.e., 20 mol%, 30 mol%, and 40 mol%. Fig. 7 shows the MO degradation rate for different N-modified photocatalysts with initial MO concentration of 40 mM. As the nitrogen loading increases, the rate of photocatalytic degradation of MO decreased. Pure TiO2 shows faster degradation than N-doped photocatalysts, possibly due to the large particle size of the photocatalyst compared to TiO2. As shown in the SEM image (Fig. 2), pure TiO2 is nano-sized powder (21 nm) while the N-doped photocatalysts do not have the same nano-size even though the same TiO2 was used for doping. After annealing the N-doped TiO2 photocatalysts, the particles were crystallized and aggregated, making their size larger (50–200 μm) resulting in decreasing the number of active sites. Furthermore, there has been contradiction in the literature regarding the nitrogen-included chemical species contributing to visible light absorption in the urea modified TiO2 for specific synthesis method. For the method used in this research, i.e. calcining the mixture of urea and TiO2, it was reported that substituted nitrogen and molecularly chemisorbed N2 contribute to visible light absorption [32]. On the other hand, Mitoraj and Kisch [28] have confirmed that calcining TiO2 and urea mixture at 400 °C produces condensed aromatic s-triazine compounds attached to TiO2 which are active in the visible light range. In this study, no functional groups were observed in the FT-IR spectra for N–TiO2 photocatalysts (Fig. 3) and UV–vis spectra (Fig. 4) together with XPS (Fig. 5) results confirm the presence of nitrogen in the photocatalysts. Therefore it can be suggested that visible light absorption by the synthesized N–TiO2 photocatalysts is due to the substituted nitrogen. Regardless of synthesis methods, N-modified TiO2 is able to absorb visible light due to lowered bandgap energy. This is also supported by UV–vis spectra but the particle size effect of N–TiO2 governs the MO

4. Conclusion This study evaluated kinetics of MO dye degradation using different dopant-deposited TiO2 photocatalysts synthesized by a facile post-synthesis method in order to investigate which dopant

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can degrade rapidly organic dye. Palladium metal was used to synthesize metal-loaded TiO2 and urea was used as precursor of nitrogen to synthesize non-metal TiO2. It was determined that as the Pd loading concentration on TiO2 increased, the photocatalytic activity increased until it reached a maximum and then decreased again due to saturation of the TiO2 surface. In the case of nonmetal loaded TiO2, as the nitrogen loading increased, the photocatalytic activity decreased because higher percentage of nitrogen by post-synthesis using urea led to unexpected particle aggregates with larger particle size that are not mostly observed in the incipient synthesis, and limited the rate of degradation as compared to band gap narrowing. In addition, synthesis method also played a key role in the photocatalytic properties. Since loading of metal or non-metal onto the surface of TiO2 was carried out using postsynthesis methods, the control of particle size was found to be very important although the band-gap shift was observed, leading to lower rates of degradation.

Acknowledgment This publication was made possible by the National Priorities Research Program (NPRP) award (NPRP 7-865-2-320) from the Qatar National Research Fund (QNRF) (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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