control of selectivity in heterogeneous photocatalysis by tuning tio2 morphology for water treatment applications

ARTICLE Nanomaterials and Nanotechnology Control of Selectivity in Heterogeneous Photocatalysis by Tuning TiO2 Morphology for Water Treatment Applications Regular Paper Ahmed A Farghali1,2*, Ayman H Z[.]

Nanomaterials and Nanotechnology

Control of Selectivity in Heterogeneous

for Water Treatment Applications

Regular Paper

Ahmed A Farghali1,2*, Ayman H Zaki2 and Mohamed H Khedr1,2

1 Chemistry Department, Faculty of Science, Beni-Suef University, Egypt

2 Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt

*Corresponding author(s) E-mail: d_farghali@yahoo.com

Received 16 November 2015; Accepted 26 January 2016

DOI: 10.5772/62296

© 2016 Author(s) Licensee InTech This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the

original work is properly cited

Abstract

Heterogeneous photocatalysis using TiO2 is a non-selective

technique used for the degradation of organic molecules

Controlling the morphology of TiO2 has recently been

considered one of the important approaches for controlling

the selectivity of TiO2. In this work, TiO2 nanotubes and

nanosheets were synthesized from spherical TiO2 nanopar‐

ticles using the hydrothermal method The starting and

prepared samples were characterized by XRD, TEM and

FESEM The selectivity of the three morphologies towards

the photocatalytic degradation of three food dyes (colours

yellow sunset, red allura and red carmoisine) was tested

Importantly, changes in morphology led to each dye being

adsorbed preferentially by one of the three morphologies

and decomposing more rapidly, where the optimum rate

of degradation for sunset yellow, red allura and red

carmoisine was achieved by TiO2 nanosheets, spherical

TiO2 and TiO2 nanotubes, respectively

Keywords TiO2, Nanoparticles, Selectivity, Photocatalysis,

Food Dyes, Waste-water Treatment

1 Introduction

Photocatalytic activity of TiO2 is greatly influenced by a range of factors including surface area, crystal structure, crystallite size, morphology, shape, particle aggregation, phase composition, surface defects and surface hydroxyl group content, among others [19, 16; 13] These factors are strongly related to synthesis and processing routes [13] Tsai and Cheng (1997) [14] compared several commercial and lab-made TiO2 samples in the photodegradation of phenol and found that the lab-made rutile without thermal annealing showed the greatest activity in the complete oxidation of phenol to CO2 On systematically studying the photocatalytic activity of a variety of hydrothermally prepared TiO2 particles, Testino et al (Testino, Bellobono

et al 2007) [13] found that high crystallinity and high aspect ratio are key parameters for substantially improving the photocatalytic activity of rutile These findings prompted

us to prepare well-crystallized rutile TiO2 nanorods with high aspect ratio, where carriers can freely move along the length of the rods, which is expected to reduce electron-hole recombination Furthermore, nanorods with a high aspect ratio and large size are highly desirable, as these

1 Nanomater Nanotechnol, 2016, 6:12 | doi: 10.5772/62296

nanoparticles are easier to be separated in water treatment

[4; 6; 2] TiO2 nanocrystals with various morphologies and

shapes have been prepared using many methods, but

primarily electrospining, templating and hydrothermal

preparation [9; 3; 7; 11; 5; 16; 17] A distinct advantage of

hydrothermal preparation over electrospinning or templat‐

ing fabrication is that the as-made product is crystalline,

thus avoiding a thermal annealing step that consequently

decreases the surface hydroxyl group content However, it

is relatively difficult to achieve nanorods with high aspect

ratios like those obtained with electrospinning or templat‐

ing fabrication by means of hydrothermal preparation

Controlling of the reaction rate is crucial in obtaining TiO2

nanocrystals with the desired crystalline structure and/or

shapes [5]

In the process of using photocatalytic reactions to chemi‐

cally degrade a contaminant, the problem arises that such

reactions have low selectivity Selectivity controls a

reaction in such a way that one target molecule can be

adsorbed preferentially by the system in order to decom‐

pose more rapidly [8; 18] Research in the field of selective

photocatalysis is relatively novel; accordingly, this work

presents a different approach to selective heterogeneous

photocatalysis, based on the selectivity of TiO2 with

different morphologies regarding the degradation of a

group of food azo dyes

2 Experimental Section

2.1 Synthesis of TiO 2 nanotubes and TiO 2 nanosheets

All of the applied reactants and solvents were of analytical

grade and were used without further purification In a

typical procedure, 5g of pure anatase phase TiO2

bulk-powder (spherical morphology) was mixed with 250ml of

10N NaOH aqueous solution under constant stirring for

roughly 1 h A milky-white solution appeared, which was

then transferred to a Teflon-lined stainless steel autoclave

with 500 ml capacity and heat-treated at 160◦C for 4h and

16h, respectively, in order to prepare nanosheets and

nanotubes The autoclave chambers were air-cooled to

room temperature following the reaction The formed

white precipitates were recovered and washed several

times with distilled water A treatment of the products with

0.1N HCl solution was carried out and the precipitates were

finally calcinated at 500◦C for 4 h in air [1]

The crystalline phases of the products were detected by X-ray diffraction The morphologies of the samples were studied using a transmission electron microscope (TEM)

2.2 Photocatalytic experiments

Solutions of yellow sunset (YS), red allura (RA) and red carmoisine (RC) were prepared by dissolving the coloured powder in distilled water to obtain a solution of 1×10−6M concentration The photocatalysis experiments were carried out in a 100mL beaker containing about 25mL of dye aqueous solution and about 0.05g of the catalyst The irradiation was carried out using a 12W UV lamp as a source of UV radiation, which was placed vertically on the reaction vessel at a distance of 12cm At specific time intervals, a certain amount of the sample solution was withdrawn and the changes in concentration of the dye were observed from its characteristic absorption at 480, 508 and 502nm for SY, RA and RC, respectively, using a UV-vis spectrophotometer model (Thermo Scientific, Evolution 600) The structural formulae of the three dyes are shown

in Figure 1

3 Results and Discussion

3.1 Physical characterization of the photocatalysts

The commercial TiO2 (spherical particles) and prepared TiO2 nanosheets and nanotubes powder were character‐ ized by X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM) The XRD patterns of the investigated TiO2 samples with different morphologies are shown in Figure 2(a-c) In Figure 2a, the bulk TiO2 reveals that anatase formed with excellent crystallinity, obvious from the peak intensities It crystallized in the well-known tetragonal symmetrical manner, with four molecules per unit cell The data were compared and indexed with ICDD card no 21-1272

For the synthesized nanotubes: Figure 2(b) shows that planes (004) and (112) disappeared, while (105) and (211) decreased sharply in intensity An observed improvement for the intensity of the (200) plane is very clear, which is a preferred orientation in the case of the tubular (1-D) shape,

as clarified and seen in the TEM micrographs

Figure 1 Structural formula of the colour dyes yellow sunset, red carmoisine and red allura

The nanosheets exhibit different features; the (101) plane

became sharper, while all other planes were broader

Planes (004), (105) and (211) reappear again, owing to the

2D sheet morphology, but always remained broad and with

low intensities, compared with the bulk sample (see Figure

2(a)).The appearance of a broad peak at roughly 30o can be

assigned to the possible transformation of some anatase

crystals into rutile, as was previously reported in nanore‐

gime a possible transition can take place at about 550o [10]

The main differences between the three diffractograms are

the intensities of the main peaks, as well as the existence of

a preferred orientation (200) for nanotubes and for nano‐

sheets (101) Moreover, the broad peaks point to the small

crystallite size, as reported in Table 1

Catalyst K (min -1 )

K (min -1 ) for

Yellow sunset

K (min -1 )

Red Allura

K (min -1 ) for

Red Carmoisine Morphology Crystal size

(nm)

Spherical TiO 2 95 nm 0.0065 0.0098 0.0082

TiO 2 nanosheets 67.9 nm 0.0513 0.0059 0.0103

TiO 2 nanotubes 27.1 nm 0.0324 0.0060 0.0189

Table 1 Structural and kinetic parameters of TiO2 nanostructures

Figure 2 XRD patterns of (a) spherical TiO2 ; (b) TiO 2 nanotubes; (c) TiO 2

nanosheets A = anatase; B = TiO 2 (B).

The TEM images of commercial TiO2 (spherical particles)

are shown in Figure 3a It can be seen that the TiO2 powder

consists of nanosized grains with the presence of agglom‐

erated particles Figure 3b shows the TEM image of the

prepared multilayered TiO2 nanosheets, while Figure 3c

shows the TEM images of the prepared TiO2 nanotubes

The tubes have a diameter range of 16-70 nm and are

arranged parallel to one another with nearly homogenous

dimensions, with some intercalated tubes pointing in a

different direction The FESEM images of the three mor‐ phologies – spherical, nanosheets and nanotubes – are shown in Figure 4(a-c), respectively In Figure 4a, TiO2grains appear to have homogenous distribution with

a small degree of coalescence This was primarily due to electrostatic attraction between grains Figure 4b clarifies that nanosheets overlap with one another, with no prefer‐ red direction of orientation The sheets possess nearly similar dimensions, i.e., narrow distribution The agglom‐ erated appearance of grains originates from the absence of surfactant during preparation In Figure 4c it is clear that the nanotubes are randomly oriented and appear to have uniform dimensions in terms of cross-section and length, and form a cluster-like network

a

b

c

TiO 2 nanosheets; (c) TEM image of TiO 2 nanotubes

3 Ahmed A Farghali, Ayman H Zaki and Mohamed H Khedr: Control of Selectivity in Heterogeneous Photocatalysis by Tuning TiO2 Morphology for Water Treatment Applications

3.2 Photocatalytic activity of the three morphologies

The three colours – yellow sunset, red allura and red

carmoisine – were completely decolorized as shown in

Figure 5a and changes in concentration were recorded as a

function of UV-irradiation exposure time for different

TiO2 nanostructures; this is illustrated in Figure 5b and

Figure 6(a-c) It is clear that photocatalytic degradation was

strongly dependent on TiO2 morphology The photocata‐

lytic activity of the three morphologies was tested as a

model first on YS and it was found that as it moved from a

spherical to a tubular structure through the sheets’ struc‐

ture, the time of degradation reduced from 400 min for

spherical, 75 min for nanotubes and 55 min for nanosheets

When we tested the catalytic activity in the degradation of

the other two colours – RA and RC – it was found that the

previous trend did not occur again As shown in Figure

7(a-c) and as summarized in Table 1, it is clear that the best rate

of degradation for sunset yellow was achieved by TiO2

nanosheets, by spherical TiO2 for allura and by TiO2

nanotubes for carmoisine These results are extremely

promising and allow us to tune the morphology of TiO2 based on the targeted dye From these observations, it is clear that the preferred orientation of each morphology rendered it more specific in terms of action; each dye is adsorbed preferentially by one of the three morphologies and decomposes more rapidly In compliance with these results, Sofianou et al [12] found that calcined TiO2 anatase nanoplates exhibited the best photocatalytic activity for oxidizing the NO gas to NO2 and NO3, whereas the washed TiO2 anatase nanoplates, which preserved the initial morphology, exhibited the best photocatalytic activity in terms of decomposing acetaldehyde They concluded the dominant exposed {1 0 1} or {0 0 1} crystal facets of the TiO2 anatase nanoplates to be the key factor in tuning the adsorption selectivity of air pollutants Xiang et al [15] also found that TiO2 films composed of flower-like TiO2 microspheres with exposed {001} facets exhibited tuneable photocatalytic selectivity towards the decomposition of azo dyes in water by modifying the surface of TiO2 micro‐ spheres, as well as by varying the degree of the etching of {001} facets

Figure 5 (a) Optical image of colour yellow sunset, colour red carmoisine and colour red allura before and after degradation; (b) Changes in UV-vis spectra

of colour yellow sunset, red carmoisine and red allura following irradiation with UV light in the presence of TiO 2 nanoparticles with different morphologies

18

0.0

0.2

0.4

0.6

0.8

1.0

Yellow sunset Red allura Red Carmoisine

0.0

0.2

0.4

0.6

0.8

1.0

Yellow sunset Red allura Red Carmoisine

Irradiation time (Min.)

0.0

0.2

0.4

0.6

0.8

1.0

Yellow sunset Red allura Red Carmoisine

a b c

Figure 6: (a) Photocatalytic activity of Spherical TiO 2 ; (b) TiO 2 nanosheets; (c)

TiO 2 nanotubes

Comment [LJ12]: Please check thata a, b and c

have been denoted correctly here

Figure 6 (a) Photocatalytic activity of Spherical TiO2 ; (b) TiO 2 nanosheets; (c)

TiO 2 nanotubes

19

Figure 7: Linear transform Ln (C o /C) = f(t) of kinetic curves

Figure 7 Linear transform Ln (Co /C) = f(t) of kinetic curves

To understand this phenomenon, we have to know that the adsorption of reactants on the catalyst surface is one of the prerequisites in any heterogeneous catalytic chemical reaction Selectivity in the adsorption stage can be achieved

by changing the amount, size, morphology and surface of the catalyst used, as well as the type or size of the target compounds As the adsorption phase becomes faster, the degradation time also decreases In this work, experiments were designed to alter the exposed surfaces of TiO2 nanoparticles by controlling the morphologies of these particles When the reaction time increased from four to 16 hrs, the nanoparticles showed different morphologies with preferred orientations A preferred orientation (200) for nanotubes and for nanosheets (101), as well as the broad peaks, point to the small crystallite size as reported in Table

1, caused the variation in degradation rates and controlled selectivity

A good correlation between crystal structure, morphology and photocatalytic activity will be established in order to recommend the use of such TiO2 in the selective photode‐ gradation of organic dyes

4 Conclusion

TiO2 nanotubes and nanosheets were successfully synthe‐ sized from commercial TiO2 (spherical particles) using a hydrothermal method The photocatalytic activity of the three morphologies was tested for the degradation of yellow sunset, red allura and red carmoisine It was concluded that nanoparticle morphology serves as the primary influence in the selectivity of catalysts The optimum degradation of sunset yellow was achieved by TiO2 nanosheets, by spherical TiO2 for allura and by TiO2 nanotubes for carmoisine These results are extremely promising and present the potential for tuning the mor‐ phology of TiO2 based on the targeted dye, thereby issuing

in a new era in the field of selective heterogeneous photo‐ catalysis

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