Synthesis, Features and Solar-Light-Driven Photocatalytic Activity of TiO2 Nanotube Arrays Loaded...

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ArticleJournal of

Nanoscience and NanotechnologyVol. 14, 7001–7009, 2014

www.aspbs.com/jnn

Synthesis, Features and Solar-Light-Driven

Photocatalytic Activity of TiO2 Nanotube

Arrays Loaded with SnO2

Lan Ching Sim1, Kai Wern Ng1, Shaliza Ibrahim1, and Pichiah Saravanan1�2�∗1Faculty of Engineering, Environmental Laboratory, Department of Civil Engineering,

University of Malaya, 50603 Kuala Lumpur, Malaysia2Centre for Research in Nanotechnology and Catalysis, University of Malaya, 50603 Kuala Lumpur, Malaysia

In the present study TiO2 nanotube arrays (TNTs) were loaded with a post-transition metal oxideparticles namely SnO2 via incipient wet impregnation method by varying its concentration (1.59 wt%,2.25 wt% and 2.84 wt%). The photocatalytic activity of the prepared photocatalyst was evaluatedfor the degradation of methylene blue (MB) in presence of natural solar light irradiation. The mor-phological analyses revealed that the prepared TNTs had average inner diameter of 109 nm, wallthickness of 15 nm and tube length of 7–10 �m, respectively, while the crystalline phase and ramanspectra confirmed the 100% anatase mineral form of TiO2. Further, the presence of SnO2 in TNTswas confirmed by high resolution transmission electron microscopy (HRTEM) and X-ray photoelec-tron spectroscopy (XPS). The visible light absorption properties of TNTs improved drastically withincreasing SnO2 loadings. The coupling effect of SnO2 and TiO2 significantly enhanced degradationefficiency of MB. An 84% degradation of MB was achieved in 6 h of irradiation under clear skycondition.

Keywords: TiO2 Nanotube Arrays, SnO2, Solar Light, Methylene Blue, Photodegradation.

1. INTRODUCTIONTiO2 is one of the most promising photocatalysts because

of its high photocatalytic activity, stability, low cost

and non toxic nature.1–3 Among TiO2 materials, self-

organized and vertically oriented TNTs are of great

interest due to their: (i) high surface-to-volume ratios,

high surface area (ii) good charge transport properties

and (iii) size dependent properties. These nanotubes are

most widely used in the quantum dots-sensitized solar

cells, carbon dioxide (CO2) reduction, dye degradation,

dye-sensitized solar cells, photoelectrolysis and various

biomedical applications.4–11

However, the photocatalytic activity is limited by its

visible light utilization and high recombination rate of

photogenerated electron–hole pairs due to its wide band

gap (3.2 eV for anatase and 3.0 eV for rutile). They can

be excited only with UV light irradiation (� < 380 nm),

which covers ∼5% of the total solar spectrum.12 The

∗Author to whom correspondence should be addressed.

researchers improved the photocatalytic activity of TiO2

in the visible light region by doping with metals (Cu,

Pt, Fe, Pd, Ag, Ni)7�8�13–16 and non-metal elements such

as nitrogen (N),17–19 boron (B),20 sulphur (S)21 and car-

bon (C)22�23 on its surface. The another alternative is to

combine TiO2 with semiconductors such as SnO2,24�25

NiO,26�27 In2O3,28 WO3,

29 Co3O4,30 Fe2O

153 and Cu2O.

31

The addition/inclusion of such semiconductors can facil-

itate charge separation and extend the photo-responding

range to the visible region.25�32�33

Among those semiconductors, SnO2 is most suitable to

couple with TiO2 due to its conduction band (ECB = 0 V

vs. NHE at pH 7) is lower than that of TiO2 (ECB =−0�5 Vvs. NHE at pH 7).33 The coupling allows the photoex-

cited electron transfer from TiO2 to conduction band of

SnO2, leaving behind a positive hole on TiO2 and thereby

hinder charge recombination and improve photocatalytic

efficiency. Vinodgopal and Kamat (1995) demonstrated the

principle of charge separation in a SnO2/TiO2 coupled

semiconductor.33 In addition, they also reported the elec-

trochemically assisted photocatalytic experiments using

J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 9 1533-4880/2014/14/7001/009 doi:10.1166/jnn.2014.8931 7001



Synthesis, Features and Solar-Light-Driven Photocatalytic Activity of TiO2 Nanotube Arrays Loaded with SnO2 Sim et al.

coupled SnO2/TiO2 semiconductor thin films in the degra-

dation of textile dye effluent. Hou et al. (2007) prepared

SnO2/TiO2 composite in hydrothermal condition with

proper amount of SnO2 loading that showed an increase

of photocatalytic activity for degradation of methylene

blue.25 A bicomponent TiO2–SnO2 nanofiber photocatalyst

prepared via electrospinning showed enhanced photocat-

alytic activity for the oxidation of RhB dye compared to

pure TiO2 under UV light.34 The studies on the natural

solar-light-driven photocatalytic activity are still limited,

although very few studies on SnO2/TNTs catalysts have

been reported. Most of them are widely used for lithium

storage.35�36 In recent years, self organized and vertically

oriented TNTs have been the focus of research8�15�21 due to

their easy recovery and large surface area available for the

photo reaction. Most suitably SnO2 could be the promising

materials for such surface functionalization.

The present finding emphasize on preparing TNTs with

varied weight percentage of SnO2 via simple incipient wet

impregnation method for higher visible light harvesting

from solar spectrum. The photocatalytic activity of the

SnO2/TNTs and pure TNTs were evaluated by degrading

methylene blue (MB) dye under natural solar-light irradia-

tion. The loading of nanosized SnO2 into TNTs possesses

the following advantages:

(1) the presence of Sn4+ species and surface defects

extend the photo-responding range.

(2) SnO2 nanoparticles play as an electron collector

to facilitate rapid separation of photogenerated charges

before recombination.

(3) aligned TNTs with a self-organized and self-supported

array structure lead to high physical stability.

2. EXPERIMENTAL DETAILS2.1. Preparation of SnO2/TNTsAll the chemical reagents were of analytical purity

and purchased from Sigma-Aldrich Chemical Co. Self-

organized TNTs layers were prepared directly on Ti foils

(99.7%, Sigma-Aldrich) via electrochemical anodic oxi-

dation in ethylene glycol (anhydrous, 99.8%) electrolyte

containing 0.3 M ammonium fluoride (NH4F, 98%) and

2 vol% water (H2O) with graphite rod as the counter elec-

trode under 50 V for 3 h. Ti foils (20 mm× 30 mm×0�25 mm) were ultrasonically cleaned with acetone and

ethanol prior to anodization. The anodized samples were

rinsed thoroughly with DI water and then annealed at

450 �C for 1 h. The annealed samples were sonicated

with ethanol for 30 min to remove the bundled impuri-

ties on top openings of the pores of the nanotubes. The

loading of SnO2 onto TNTs were achieved by incipi-

ent wet impregnation method. The preparation steps fol-

lowed were: The prepared TNTs samples were immersed

overnight in SnCl4 · 5H2O aqueous solution. The molar

concentration of SnCl4 ·5H2O aqueous solution was varied

in a range of 0.5–2.5 M. This resulted in different concen-

tration of SnO2/TNTs namely 1.59 Sn, 2.25 Sn and 2.84

Sn, respectively. The prepared samples were annealed at

450 �C for 2 h with the heating rate of 10 �C min−1 to

induce crystallization. Similarly, a pure TNTs sample was

prepared for comparison.

2.2. Characterization of SnO2/TNTsThe morphologies and composition of the samples

were examined using a field emission scanning elec-

tron microscope (FESEM, Auriga, Zeiss) equipped with

an EDS (energy dispersive X-ray spectroscopy) detec-

tor. The images were taken at an accelerating voltage of

20 kV. High resolution transmission electron microscope

(HRTEM, Tecnai 20, Philips) images were obtained at

200 kV. The crystalline phase was identified using X-ray

diffractometer (XRD, D8 Advance, Bruker). The target

used in the diffractometer is copper (Cu K� radiation, �=1�54 Å). The surface chemical composition of the sam-

ples was analyzed by X-ray photoelectron spectroscopy

(XPS, Axis Ultra DLD, Kratos) with Al K� radiation

source. The binding energies were normalized to the sig-

nal for adventitious carbon at 284.6 eV. Light absorption

properties were performed using UV-vis-NIR spectropho-

tometer (Lambda 750, Perkin Elmer) with a wavelength

range of 200–800 nm. Raman spectroscopy was recorded

on Raman spectrometer (RamanMicro 200, Perkin Elmer)

with Raman shift range of 100–700 cm−1.

2.3. Photocatalytic Degradation ofMethylene Blue (MB)

The solar-light-driven photocatalytic activity of SnO2/

TNTs was evaluated by adopting methylene blue (MB) as

model pollutant under solar light irradiation. The degra-

dation of MB was carried out in a 1000 mL beaker with

the working volume of 200 mL of MB solution (1�56×10−5 M). The prepared samples were loaded in MB solu-

tion with only one side faced to solar light irradiation.

Prior to the photocatalytic experiment, the dark reaction

was carried out for 30 min to achieve the adsorption

equilibrium. The samples were collected at regular time

interval and analyzed for residual MB concentration with

visible spectrometer (Spectroquant® Pharo 100, Merck) at

�max = 664 nm. The drawn samples were dropped back

into the beaker after the measurement to allow further

degradation. The experiment was carried out at clear sky

condition at University of Malaya, Kuala Lumpur (latitude

101� 39′ E and longitude 3� 7′ N) between 11.00 am and

5.00 pm in the months of May (2012). Solar light inten-

sity was measured using LT Lutron LX-101 Lux meter

of 1000×100 l× and the average light intensity over the

duration of clear sky weather condition is found to be 97

220 lux (For solar light (AM 1.5), 100 mW/cm2 corre-

sponds to approximately 120 000 lux).37

7002 J. Nanosci. Nanotechnol. 14, 7001–7009, 2014



Sim et al. Synthesis, Features and Solar-Light-Driven Photocatalytic Activity of TiO2 Nanotube Arrays Loaded with SnO2

3. RESULTS AND DISCUSSION3.1. Structural and Morphological CharacterizationFigures 1(A) and (B) show the surface and cross-sectional

FESEM image of synthesized TNTs. The top surface

image (Fig. 1(B)) shows that TNTs are well attached to

each other and one-dimensional vertically oriented. The

average inner diameter of these nanotubes is 109 nm,

nanotube wall thickness is 15 nm and the tube length is in

the range of 7–10 �m. The top view image of SnO2/TNTs

is shown in Figure 1(D). It is apparent that the mor-

phological structure of TNTs was not affected by SnO2

loading. Further, SnO2/TNTs sample did not show signif-

icant morphological changes with increasing SnO2 con-

centration (data not shown). The energy-dispersive X-ray

spectroscopy (EDS) analysis (Fig. 1(C)) confirms the pres-

ence of Sn, Ti and O. The uniformity of SnO2 disper-

sion was obtained via EDS analysis over different spots

on the nanotubes cross section area with very reproducible

peaks obtained, especially for Sn. Figure 2(A) shows that

average crystallite size of SnO2 nanoparticles less than

5 nm were distributed in TNTs surface. Direct evidence

of crystalline nature of TiO2 and SnO2 was observed and

is shown in Figures 2(A) (inset) and (B), respectively.

Two lattice planes with spacing of 0.35 nm and 0.33 nm,

Figure 1. FESEM images of sample (A) cross-sectional view of pure TNTs, (B) top view of TNTs, (C) EDX of 1.59 Sn and (D) top view of 1.59 Sn.

corresponding to the (101) plane of anatase TiO2 and (110)

plane of SnO2 was also observed clearly.38�39

Figure 3(A) shows the XRD pattern for TNTs and

SnO2/TNTs. The diffraction peaks of all prepared samples

are mainly ascribed to the peaks of TiO2 anatase phase

and Ti substrate. The Ti peak belongs to Ti foil substrate

underneath the oxide nanostructure layer. No peak cor-

responding to TiO2 rutile phase and SnO2 was detected.

There has been virtually no phase change in TiO2 after

the loading of SnO2 nanoparticles. The evident peaks of

tetragonal TiO2 anatase phase (JCPDS no. 21-1272) are

found at 25.3�, 36.9�, 37.8�, 48.0�, 53.9�, 55.1�, 62.7�,68.8� and 75.0�, corresponding to (1 0 1), (1 0 3), (0 0 4),

(2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6) and (2 1 5)

crystal planes, respectively. The crystalline phase of SnO2

could not be detected by XRD when TNTs were loaded

with different concentration of SnO2. It indicates that Sn

cations (0.69 Å) are well substituted into the Ti cations

(0.68 Å) in anatase TiO2 lattice due to their similar ionic

radii.40 In addition, this could be due to the presence of

SnO2 in low concentration and it is uniformly small and

well dispersed on TiO2 surface. Similar such observations

were reported by Sakthivel et al. (2004)41 and Ku et al.

(2011).42 The lattice parameters and crystallite size of all

J. Nanosci. Nanotechnol. 14, 7001–7009, 2014 7003




Figure 2. (A) HRTEM image of single TNT loaded with SnO2 (B) HRTEM image of the circle area in (A). Inset shows the HRTEM image of pure

TNTs.

samples are calculated using the formula stated below and

are summarized in Table I. The average crystallite sizes of

TiO2 anatase were calculated using Scherrer equation:

D = K�

� cos�(1)

where � is the full width half maximum (FWHM) for the

2� peak, K is the shape factor taken as 0.89 for calcula-

tions, � is the wavelength of X-ray (0.154 nm), and � is

the diffraction angle. The lattice parameters were measured

using (1 0 1) and (2 0 0) in anatase crystal planes by using

Bragg’s equations:

dh k l =�

2 sin �(2)

Formula for tetragonal system:

d−2h k l = h2a−2+k2b−2+ l2c−2 (3)

Figure 3. (A) X-ray diffraction pattern of photocatalysts ((a): TNTs; (b): 1.59 Sn; (c): 2.84 Sn). (B) enlarged region of 2 theta in a range of 24.8–26�.

Table I. Lattice parameters for TNTs and SnO2/TNTs samples.

Sample Crystallite size (nm)a Cell parameters a= b, c (Å)b

TNTs 34.50 3.7868, 9.5102

1.59 Sn 33.35 3.7863, 9.4308

2.84 Sn 25.00 3.7764, 9.3196

Notes: aMeasured by Scherrer’s equation; bEstimated according to Eqs. (2) and (3).

where dh k l is the distance between crystal planes of

(h k l), � is the X-ray wavelength, � is the diffraction angle

of crystal plane (h k l), h k l is the crystal index and a, band c are lattice parameters (in anatase form, a= b �= c).As depicted in Table I, the lattice parameters and crys-

tallite size of all samples decrease with increasing SnO2

concentration. It can also be observed from Figure 3(B)

that there is a shift of the (1 0 1) peak towards higher

angles with increase in SnO2 concentration, resulting in

changes in the lattice parameters. In addition, the decrease

in the (1 0 1) anatase peak intensity of SnO2/TNTs in





Figure 4. UV-visible absorption spectra of pure TNTs and SnO2/TNTs.

Inset shows the enlarged region of wavelength in a range of 340–700 nm.

contrast of pure TNTs indicate that the crystallinity of

TiO2 is reduced by SnO2 loading. It is feasible for Sn4+ to

substitute for Ti4+ since the ionic radius of Sn4+ (0.69 Å)

is similar to ionic radius of Ti4+ (0.68 Å). The interca-

lated SnO2 cause structural strain and disorder in the TiO2

crystalline lattice structure.43 Hence, the crystal growth of

TiO2 is inhibited and the crystallite size of TiO2 decreases.

3.2. UV-Vis Diffuse Reflectance SpectraThe challenge in improving the properties of TiO2 is to

shift the absorption spectrum into the visible region to

enable more efficient solar light harvesting. The diffuse

reflectance spectra (DRS) of SnO2/TNTs and pure TNTs

samples are depicted in Figure 4. It can be seen that

1.59 Sn did not show a significant red-shifted absorption

edge towards visible region due to its marginal increase of

Sn concentration compared to that of TNTs. Meanwhile,

the further increasing Sn concentration up to 2.25 wt%

and 2.84 wt% results in a drastic shift towards the visible

region. It is known that the doping energy level of Sn4+

is located at 0.4 eV below the TiO2 conduction band.44

Hence, the visible light absorption of SnO2/TNTs can be

attributed to the charge-transfer transitions between Sn4+

electrons and the TiO2 conduction band.45 In addition, the

presence of Ti3+ species which is observed by XPS on

SnO2/TNTs (Fig. 6(D)) can induce oxygen vacancy states

between the valence and conduction band, which could

also contribute to the visible response46 and electron–hole

pairs separation.47 The obtained band gap energy through

Tauc plot (figure not shown) are 3.09 eV, 3.05 eV and

2.97 eV for TNTs, 1.59 Sn and 2.25 Sn, respectively. The

Tauc plot is a plot of (F (R) ·h�)1/2 against h�, where F (R)is the Kubelka-Munck function derived from F R= 1−R2/2R equation, R is diffuse reflectance, and h� is the

photon energy. The band gap energies can be determined

Figure 5. Raman spectra of (a) 1.59 Sn (b) pure TNTs. Insets show

the enlarged region of Raman shift in a range of 100–200 cm−1 and

350–650 cm−1.

by extrapolating the maximum slope of the curve to the

photon energy axis (x-axis).

3.3. Raman Shift SpectraFigure 5 depicts the Raman shift spectra of the pre-

pared samples. The Raman peaks for both pure TNTs

and SnO2/TNTs at 144, 197, 399, 519 and 639 cm−1

are assigned to Eg1, Eg2, B1g1, A1g B1g2 and Eg3

active modes of the anatase phase, respectively.48 This

result is consistent with crystallographic patterns and

confirms the formation of a bulk anatase phase after

annealing treatment. No peaks related to SnO2 or other

oxidation state of Sn was detected in the spectra of

SnO2/TNTs samples. This specifies the high dispersion

of SnO2 on TiO2 surface,49 which corresponds to the

XRD results. The SnO2/TNTs sample shows that Eg mode

of anatase (144 cm−1) (Fig. 5) decreases in intensity,

becomes broader and red shifted to higher frequency

region. Besides, few broader peaks also can be observed

at 399, 519 and 639 cm−1 in SnO2/TNTs sample. This

phenomenon is due to the breakdown of long-range trans-

lational crystal symmetry which is caused by the Sn4+

substitution defects.50

3.4. XPS AnalysisHigh-resolution XPS was performed to determine the

chemical composition and the oxidation state for SnO2/

TNTs samples. The fully scanned XPS spectra (Fig. 6(A))

indicate that Ti, O, C and Sn elements exist in SnO2/TNTs

samples. XPS of Sn 3d, O 1s and Ti 2p core levels of

the SnO2/TNTs exist as: (1) Ti 2p region (450–470 eV);

(2) Sn 3d region (480–500 eV); and (3) O 1s region

(520–540 eV). Peak fitting to the spectra was applied

using Gaussian-Lorentzian peak shape after subtraction





Figure 6. XPS spectra of 1.59 Sn (A) fully scanned spectra (B) O 1s peak (C) Sn 3d peak (D) Ti 2p peak. Inset in (D) shows the enlarged region of

binding energy in a range of 450–470 eV.

of Shirley background. The C element can be ascribed

to the adventitious carbon based contaminant from the

XPS instrument and the binding energy of C 1s peak at

284.6 eV is used as internal charge correction. As shown

in Figure 6(B), high-resolution XPS of O 1s core level

can be fitted to three kinds of chemical states. The peaks

at 529.9, 531.3 and 532.8 eV can be assigned to crystal

lattice oxygen of Ti–O, Sn–O and adsorbed water, respec-

tively. As shown in Figure 6(C), the Sn 3d5/2 peak is found

at 486.1 eV and the Sn 3d3/2 peak is located at 494.5 eV.

The splitting of Sn 3d doublet at 8.41 eV confirmed the

valence state of Sn is +4.

As shown in Figure 6(D), there are two peaks observed

at 458.1 eV (Ti 2p3/2) and 463.9 eV (Ti 2p1/2), both cor-

respond to Ti4+. The nonstoichiometric nature of TiO2

(Ti4+) surface was proved by the presence of TiO (Ti2+)and Ti2O3 (Ti3+) peaks. The presence of Ti2+ can be

detected at peak 460 eV. The two shoulder like peaks in

the lower binding energy on Ti 2p3/2 (456.9 eV) and Ti

2p1/2 (462.2 eV) are assigned to Ti3+ which are generated

from the substitution of Ti4+ (0.68 Å) by Sn4+ (0.69 Å)

with similar ionic radii, leading to stoichiometry changes

in TiO2 lattice. Interestingly, the binding energies for Ti

2p3/2 and Ti 2p1/2 shifted towards lower side with the sub-

stitution of Sn4+ compared to pure TNTs (Fig. 6(D)). This

phenomenon can be correlated to the existence of Ti with

lower valence (Ti2+, Ti3+) due to formation of oxygen

vacancies after SnO2 loading.51 These XPS results also

in agreement with XRD results (Fig. 3(B)) which show

SnO2/TNTs samples shift to higher angle compared to that

of TNTs. Both results confirm that SnO2 is successfully

loaded in TiO2 lattice in substitution mode and accompa-

nied by oxygen vacancy formation.

3.5. Photocatalytic Activity SnO2/TNTsMethylene blue (MB) was used as the model pollutant

to evaluate the photocatalytic activity of the SnO2/TNTs.

Figure 7(B) shows the solar-light-driven photocatalytic

activity for the degradation of MB over all prepared pho-

tocatalysts at clear sky condition. The initial concentration

(C0) is the MB concentration after adsorption–desorption

equilibrium. Regardless of SnO2 concentration, 40.1 to

40.7% of the dye were removed under dark condition for

30 min before exposure to solar irradiation. Since the pho-

tocatalytic principles also depend on the surface phenom-

ena, this good adsorption property can further enhance the

photodegradation in the presence of light source.

The photocatalytic reactions for all samples followed

pseudo first-order reaction kinetics, which is expressed by

equation:

ln

(C0

C

)= kt (4)

where k is the first-order reaction constant, C0 and C are

the initial and the reaction concentrations of the MB dye,

respectively. The kinetic plot and photocatalytic results are

given in Figure 7(A) and Table II, respectively. Regardless





Figure 7. (A) kinetic plot of TNTs and SnO2/TNTs under solar light

irradiation. (B) fraction of MB left for TNTs and SnO2/TNTs under solar

light irradiation. MB (5 mg) dissolved in 200 mL of water is used as

initial solution (1�56× 10−5 M). The degradation of MB occurred in

360 min. Degradation is carried out at the average light intensity of 97

220 lux under solar light irradiation.

of SnO2 concentration, all SnO2/TNTs photocatalysts

showed higher degradation efficiency than pure TNTs.

In the initial reaction, the solar-light-driven activity of

SnO2/TNTs samples with lower concentration (1.59 wt%

and 2.25 wt%) increased with increase in SnO2 concen-

tration. After 210 min of reaction time, solar-light-driven

activity of 1.59 Sn and 2.25 Sn started to slow down

and reached identical MB degradation (84%) towards the

end. However, the photocatalytic activity decreased with

increasing SnO2 concentration up to 2.84 wt%. This is

due to SnO2 deposition on TNTs can form a space charge

layer, which can separate the photoinduced electron–hole

pairs. As the concentration of SnO2 increases, the surface

Table II. Photodegradation of MB dye for TNTs and SnO2/TNTs under

solar light irradiation.

Sample MB degradeda (C0 −C)/C0 (%) Kb (min−1)

TNTs 65.4 0.003

1.59 Sn 84.0 0.005

2.25 Sn 84.2 0.005

2.84 Sn 84.0 0.005

Notes: aAfter reaction for 6 h; bApparent rate constant deduced from linear fitting

of lnC0/C versus reaction time.

barriers become higher and the electron–hole pairs within

the region are efficiently separated by the large electric

field. But when in excess, the existence of SnO2 particles

on the TiO2 surface blocks the active sites of TiO2 and

lessens the adsorption of reactant. It can also become

the recombination centers of photoinduced electron–hole

pairs52 and thereby decrease the photocatalytic activity.

Furthermore, the charge-transfer transitions between Sn4+

electrons and TiO2 conduction band45 promotes absorp-

tion edge in visible region. Finally, substitution of Ti4+

by Sn4+ can cause a lattice deformation and produce

defects such as Ti3+ and oxygen vacancy in the crystal.

These defects can extend the absorption edge to visible

region.53�54 A competitive adsorption on the active sites

between the reactant and the intermediate products could

reduce the degradation rate towards the end of reaction.55

Hence, the accessibility of reactant to the active sites is

affected and resulting a non-significant difference of degra-

dation efficiency for all SnO2/TNTs when the reactions

end. But indeed, as shown in Figure 7(A), there is still a

difference in the kinetic rates for different concentration

of SnO2/TNTs.

3.6. Degradation MechanismThe degradation mechanism of MB over SnO2/TNTs is

illustrated in Figure 8. It is well known that conduction

band electrons (e−) and valence band holes (h+) are gen-

erated when TiO2 is irradiated with light energy greater

than its band gap (3.2 eV). As indicated in Raman and

XRD results, Sn4+ ions are incorporated in TiO2 lattice

in substitution mode. It creates Sn4+ doping energy level

which is located at 0.4 eV below the conduction band.44

The excited electrons transfer from valence band to Sn4+

doping energy level. The excited electron at the conduc-

tion band can also fall into Sn4+ doping energy level.56

The photogenerated electrons can also transfer from con-

duction band to SnO2 nanoparticles since the conduction

band of SnO2 (ECB = 0 V) is lower than that of the TiO2

(ECB =−0�5 V).33

Figure 8. Schematic diagram of charge transfer in SnO2/TNTs photo-

catalyst under solar light irradiation.





The photogenerated electrons at Sn4+ doping energy

level, TiO2 conduction band and SnO2 conduction band

react with O2 adsorbed on TiO2 surface or dissolved oxy-

gen in water to produce superoxide radical anion •O−2 .

55

While the photogenerated holes move from SnO2 valence

band to TiO2 valence band. It oxidize the organic molecule

in MB solution to form R+, or react with OH− or H2O then

oxidizing them into •OH radicals. Ti3+ which is the sur-

face defects as identified in XPS analysis can easily react

with O2 adsorbed on TiO2 surface, leading to the forma-

tion of radicals such as •O−2 , HO

•2, and

•OH radicals.46 The

resulting •OH radicals are strong oxidizing agent (standard

redox potential +2.8 V) can oxidize MB dye to mineral

end-products.

4. CONCLUSIONSSelf-organized and vertically oriented TNTs were loaded

with SnO2 particles using simple incipient wet impreg-

nation method. The characterization results indicated that

the morphology structure and crystalline phase of TNTs

was not affected by the loading of SnO2. The SnO2/TNTs

samples possess better visible light absorption property

compared to that of TNTs. Overall, SnO2/TNTs exhibited

higher solar-light-driven photocatalytic activity than TNTs

due to the contribution of substitutionally Sn4+, surfacedefects (Ti3+ and oxygen vacancy) and SnO2 nanopar-

ticles. The factor to enhance solar-light-driven photocat-

alytic activity does not always depend on the good visible

light absorption property. It also depends on the effec-

tive combination of the surface transfer of photoinduced

charge-carriers and further dosage optimizing of SnO2

nanoparticles to avoid electron–hole pairs recombination.

Acknowledgments: This work was supported by Uni-

versity of Malaya Research Grant (RG167/12SUS)

and Postgraduate Research Grant (PPP) (PV106/2012A)

respectively. The authors are grateful to Advance Materials

Research Centre (AMREC) and Low Dimensional Mate-

rials Research Centre (LDMRC), University of Malaya in

assisting characterization work.

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Received: 22 November 2012. Accepted: 29 December 2012.


Synthesis, Features and Solar-Light-Driven Photocatalytic Activity of TiO2 Nanotube Arrays Loaded...

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