VIETNAM JOURNAL OF CHEMISTRY VOL. 51(5) 632-636 OCTOBER 2013
PREPARATION AND PHOTOCATALYTIC ACTIVITY OF NITROGEN AND CARBON CO-DOPED TITANIUM DIOXIDE ON DEGRADATION OF
RHODAMINE B
Nguyen Thi Thien Kieu^ Nguyen Thi Lien', Le Thi Thanh Thuy', Dao Ngoc Nhiem^
Nguyen Thi Dieu Cam^, Nguyen Minh Phuong'', Nguyen Dinh Bang', Nguyen Van Noi", Le Thanh Son'
'Laboratory of Environmental Chemistry, Hanoi University of Science, Vietnam National University
^Institue of Materials Science, Vietnam Academy of Science and Technology
^Qui Nhon University {\J0 Received 15 September 2012
Abstract
The nanoparticles of nitrogen and carbon co-doped Ti02 were prepared by sol-gel method and dien followed by hydrothermal treatment. The amount of dopants and synthesize conditions were investigated and optimized. X-ray diffraction patterns demonstrated that anatase is of a major crystalline phase in the modified TiOi structure. SEM images indicated the formation of homogeneous nano particles. The intensity of Ught absorption in 400-600 ran of the modified Ti02 was increased con^armg to unmodified TiOj, suggesting a higher perfomiance of photocatalytic activity under visible light. The photocatalytic activity of the catalyst on Rhodamine B (RhB) degradation was investigated under visible light iiradiaUon. The degradation efficiency of N and C co-doped TiOi on RhB was signilicantly enhanced as compared to bare Ti02.
1. INTRODUCTION
Ti02 is a versatile material that finds applications in environmental field for destruction of undesirable chemical contaminants in water and afr.
Titanium dioxide (T\0^ is one of the most widely investigated photocatalytic materials due to its favorable physical, chemical, and optoelecfronic properties as well as its low cost, chemical stability, andnontoxicity. It is well understood that the photocataljlic reactions are initiated by the most important trigger of light absorption that relates directly to the band gap between the valence and the conduction of Ti02 crystals. Ti02 is known to be a semiconductor having a large band gap of 3.0 eV for rutile and 3.2 eV for anatase, which corresponds to wavelengths shorter than 387 nm. As a result, TiOz exhibits, however, photocatalytic activities only under ulfraviolet (UV) excitation source, which account for only small fraction of solar light (3-5%) [1-3]. In order to obtain visible-light-responsive titania photocatalysts, doping of transition metal cations such as Cr, V, and Fe or anions such as C, N, F, and S have been studied extensively [4-6],
In this study, nanoparticles of nifrogen and
carbon co-doped Ti02 were prepared by sol-gel method and then followed by hydrothermal freatment. The amount of dopants and synthesize conditions were investigated and optimized in order to obtain N and C co-doped TiOj (N-C-Ti02) material, which exhibits high efficiency on RhB removal.
2. EXPERIMENTAL 2.1. Chemicals
Tefraisopropyl orthotitanate (TIOT, 98%) were purchased from Merck. Nitric acid ^INOi, 68%), ethanol (C2H5OH, 99.7%), NH4CI and RhB (C2gH3]ClN203) were of analytical reagent grade.
2.2. Preparation of xN-C-TiOicataiyst 6 ml of TIOT was first dissolved m ethanol by 30 mm stirring at room temperature to obtain solution A. Solution B contains ethanol, 0.4 ml of HNO3 68%, distilled water and a desired amount of NH4CI (x - 5, 8, 10, 15wt.% of nifrogen in the titania powder, theoretically). The investigated
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alkoxide group of titania precursra' (OR):ethanoI: water ratios were 1:10:1; 1:17:1 and 1:25:1. Solution A was then added drop-wise into solution B, stimng at room temperature until a transparent sol was obtained. The sol was aged at room temperature for 2 days. After that, the obtained gel was transferred into a Teflon bottle and hydrothermally treated at 180X for 8, 10, 12 and 14 h. After the hydrothermal freatment, the precipitation was washed by distilled water followed by drying at 100°C for 24 hours to obtain flie N-C- Ti02 catalyst.
The bare TiOj and carbon doped TiO; (C-TiOi) were also synthesized for comparison. C- TiOzcatalyst was synthesized using a similar preparation method for N-C-TiOj, but in the absence of nitrogen precursor. The preparation procedure fw TiOihad no differences to that of the C-Ti02 catalyst However, after aging for 2 days ibc obtained gel was dried at lOO^C for 24 h, followed by calcination at 550°C for 3 h.
23. Photocatalytic activity measurements The photocatalytic activities were evaluated by measuring the decomposition of 20 mg/L RhB under visible light irradiation (X = 400-600 nm). A 36W compact lamp was used as visible light source. For a typical photocatalytic experiment, an amount of catalyst with proportion of 1.2 g/L was dis[>ersed in 100 ml of the above RhB solution. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium. The above suspensions were kept under constant air- equilibrated conditions before and during the inadiation. At given time intovals, about 5 mL aliquots were sampled, centrifuged, and filtered to remove the particles. The absorption intensity of flie titrates was measured by spectrophotometer at 553 nm for RhB determination.
3. RESUTLS AND DISCUSSION 3.1. Optimization of preparation method 3.1.1. Effect of dopants
It is suggested that the substitution of doping anions for oxygen in the Ti02 lattice was responsible for the band gap narrowing of TiO? and consequently leading to an enhancement in the photodegradation efficiency under visible light [5].
Doping anions, like N and C alters the conductivity and optical properties as they introduce new surface
Nguyen Minh Phuong, et al.
states that may lie close to the conduction band or valence band of Ti02. While doping cations can trap part of photogenerated elecfron, doping anions play important role in trapping part of photogenerated hole, inhibiting the recombination of photogenerated electron and hole [5, 6]. Therefore, the existence of dopants would be able to improve flie photocatalytic activity in flie visible hght region.
The photodegradation efficiency of bare Ti02, C doped Ti02 as well as N and C co-doped TiOi catalysts were demonstrated in Fig, 1. The results indicated that bare Ti02 exhibited almost no photocatalytic activity under visible light irradiation (Fig. 1). However, in the presence of carbon, the photocatalytic efficiency was significantly enhanced.
Moreover, Ti02 modified by doping both C and N showed greater degradation efficiency of RhB than single C doped TiO;.
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- • - T K t t - • - C - 1 I 0 2
^ M - C - T K M
^
^ ^
^
0 » l 60 M 120 ISO ISO 210 Irradiation ttane (min)
Fig. 1: Effect of dopants on the photocatalytic activity
The synerpstic effect of co-doping with C and N was also reported [8, 9]. Therefore, C and N co- doped T1O2 catalyst was chosen for further study.
The optimal amount of dopants is one of the most imfMrtant factor affect the photocatalytic efficiency. Within a suitable concentration range of dcq>ant, the dopant can serve as a trap for elecfron or hole retarding the elecfron-hole combination rate and enhancing the interfacial charge transfer.
However, when the dopant concentration is too high, the recombination rate will mcrease because the distance between tr^iping sits in a particle decreases [6]. Thus, a suitable adding amount of nitrogen precursOT should be investigated. Various dopant:
titanium dioxide ratios (5, 8, 10, 15wL%) were examined for RhB degradation efficiency under visible Ught illumination. The results were shown in Fig. 2.
The results mdicated that the 8% N-C-TiOi shows highest efficiency on RhB removal, which accounted fw 97.8% after 90 min of visible light
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irradiation. The higher amount of nifrogen resulted m the decrease in RhB degradation rate. The results were in accordance with UV-Vis diffrjse reflectance spectra (Fig. 3).
m
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efficiency Remova
«—• v^z
-*-5%N.C-Ti02 -•-a%N.c-no2 -*-10-iN.C-nO2 -•-1S%N-C-Ti02 0 30 60 90 12Q 150 180 21
i r r a d i a t i o n t i m e ( m i n )
Fig. 2: Effect of dopant amount on the photocatalytic activity
Preparation and photocatalytic activity of...
2. Hydrolysis:
M(OR)n+ oHiO ^ M(OH)„ + nROH (2) 3. Condensation reactions: are competitive with
alkoxolation, oxolation and olation Alkoxolation:
O - M - O R ^ ^ H'
Oxolation:
Olation:
H
= M - n - M — a )
o: — H + R-OH
M - D H + M ^ M : ) =^^ O l — H 4 H ; 0 H M
Therefore, the formation of the final three- dimensional skeletons structure of titania depends on the relatively contribution of the above reactions [7].
In other words, the molar ratio between OR (precursor):ethanol:H20 is an important factor, which is responsible for the formation of nano structure of a material.
The molar ratios OR:ethanol:H20 of 1:10:1, 1:17:1 and 1:25:1 were applied for the sol-gel synthesization of 8%N-C-Ti02 catalyst. The optimal ratio was evaluated based on the photodegradation efficiency of RhB. The obtained results were shown in Fig. 4.
2D0 300
Fig. 3: The UV-Vis diffiise reflectance specfra of xN-C-Ti02
It is apparent that the diffuse specfra of Ti02 modified by C and N have extended to a red shift and increased absorbance in the visible range (X. = 400-600 nm). The reasons for the extension to visible light region of non-metal ions should be attnbuting to the fact that these doped ions lead to a significant reduction of the energy cost to form oxygen vacancies in bulk titanium dioxide [5], 3.1.2. Effect ofOR:ethanol:H20 molar ratio
In general, the sol-gel synthesis can be described by an alcoholic permutation reaction, hydrolysis, and condensation reactions as follows:
I. Alcoholic permutation:
M(OR)z + xR'(OH) « M(OR)^x(OR')^+ xROH(l)
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of RhB
,1 *o
—
emoval
jT ^ , . ^ ^
/ ^
y/
< F ^ ,^
-•-1:10 01 -*-i:lT.oi - • - r i s i o i
Irradiation time (min)
Fig. 4: Effect of OR:ethanol:H20 molar ratio on the photocatalytic activity of the 8%N-C-Ti02 catalyst According to the results, the optimal OR:ethanol:H20 molar ratio for the synthesization of the N-C-Ti02 nanoparticle was 1:25:1.
5.1.3 Effect of hydrothermal treatment condition Hydrothermal freatment is a crucial process in accelerating the crystallization of nano material.
Efficiency of hydrothermal freatment process
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depends on both heating duration and heating temperature. The results of RhB removal percentage using the 8%N-C-Ti02 catalyst prepared imder different hydrothermal freatment conditions were illustrated in Figs. 5 and 6.
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r"^
^
- * - l 6 0 o C - • I S O o C
GO 90 120 150 160 210 irradiation time (mm)
Fig. 5: Effect of heating temperature on the photocatalytic activity of the 8%N-C-Ti02 catalyst
Irradiation time (min)
Fig. 6- Effect of heating duration on the photocatalytic activity of the 8%N-C-Ti02 catalyst
The results indicated that the highest photocatalytic activity N-C-Ti02 was obtained after 12 hours hydrotiienmal freatment at 180°C of the aging gel.
3.2. Characterization of the N-C-TiOi photocatalyst
The XRD patterns of 8%N-C-Ti02that was synthesized by optimal preparation method were demonstrated in the Fig. 7. The typical peak at ca. 20
= 25.4° corresponding to (101) plane of anatase phase IS clearly observed, suggesting that anatase is dominant in the N-C-Ti02 catalyst.
The homogeneous distribution of the nano- crystallines with sizes of about 20 nm was observed
Nguyen Minh Phuong, et al.
in the SEM image of the 8%N-C-Ti02 catalyst (Fig. "^
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Fig. 7: XRD pattern of the 8%N-C-Ti02 catalyst
Fig 8: SEM image of the 8%N-C-Ti02 catalyst
The 8%N-C-Ti02 catalyst prepared by sol-gel method follow by hydrothermal treatment under optimum conditions performed a great photocatalytic activity in visible light region, which accounts for 98% RhB removal after 90 min of illumination.
4, CONCLUSION
In this research, C and N co-doped Ti02 catalysts are successfully synthesized with merely anatase titanium dioxide phase. The photocatalyst showed best photodegradation efficiency as amount of doping N was 8wt.%. The synthesization isoptimal at the OR:ethanol:H20 molar ratio of 1:25:1 and hydrothermal hearing duration is 12h at 180°C. The photocatalytic activity of the catalyst in visible light range was significantly enhanced in the presence of C and N in the Ti02 nano-crytallines.
Acknowledgpients: This research was financially supported by Project KC02.TN08/11-15 under
V J C , V o l . 5 1 ( 5 ) , 2 0 1 3
Potential Program for Science and Technology.
Ministry of Science and Technology, Vietnam.
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Corresponding author. N g u y e n IVlinh P h u o n g
L a b o r a t o r y o f E n v i r o n m e n t a l C h e m i s t r y ,
H a n o i U n i v e r s i t y o f Science, V i e t n a m N a t i o n a l U n i v e r s i l y 19 L e T h a n h T o n g , H o a n K i e m , H a n o i , V i e t n a m E m a i l : n g u y c n n i i n h p h u o n g @ h u s . e d u . v n .
