Jeffrey Baloyi Et Al., 2015

Materials Today: Proceedings 2 ( 2015 ) 3973 – 3987

Available online at www.sciencedirect.com

ScienceDirect

2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of the Conference Committee Members of 7th International Symposium on Macro- and Supramolecular Architectures and Materials.doi: 10.1016/j.matpr.2015.08.027

7th International Symposium On Macro- and Supramolecular Architectures and Materials

Preparation, characterization and growth mechanism of dandelion-like TiO2 nanostructures and their application in photocatalysis

towards reduction of Cr(VI)

Jeffrey Baloyi a, b*, Tumelo Seadira a, b, Mpfunzeni Raphulu b and Aoyi Ochieng a, †

a Centre for renewable energy and water, Vaal University of Technology, Private Bag X021, Andries Potgieter Boulevard, Vanderbijlpark, 1900; RSA

b Advanced Materials Division, Mintek, Private Bag X3015, Randburg, 2125; RSA

Abstract

Three-dimensional (3D) dandelion-like TiO2 nanostructures were successfully synthesized from TiCl4 and water via simple hydrothermal method. The samples were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET). The photocatalytic activity of the dandelion-like TiO2 nanostructures was evaluated by photoreduction of Cr(VI) under UV light irradiation. The results indicated that the dandelion-like rutile TiO2 nanostructures were composed of ordered nanorods with an average diameter of 17 nm. Furthermore the results indicated that the dandelion-like TiO2 can be easily scaled-up and reproducible. The growth mechanism of the dandelion-like TiO2 nanostructures has been proposed to occur in a four-step reaction, i.e., (i) nucleation and nanoparticle formation; (ii) formation of spheres through self-assembly growth; (iii) further growth and (iv) agglomeration of the dandelions to form flower-like rutile TiO2. The dandelion-like TiO2 structures exhibited higher photocatalytic efficiency compared to P25 TiO2 for the photoreduction of Cr(VI). The highest photocatalytic reduction rate was with 2 g of the catalyst in a 10 mg/L Cr(VI) solution with pH 2. The high photocatalytic activity of the dandelion-like TiO2 nanostructures was attributed to the flower-like morphology, highest light-harvesting efficiency resulted from multiple reflections of light, hierarchical mesoporous structure and large specific surface area (81 m2/g). © 2014 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +27 11 709 4182; fax: +27 16 950 9796. E-mail address: [email protected] * Corresponding author. Tel.: +27 16 950 9884; fax: +27 16 950 9796. E-mail address: [email protected]

© 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of the Conference Committee Members of 7th International Symposium on Macro- and Supramolecular Architectures and Materials.

3974 Jeffrey Baloyi et al. / Materials Today: Proceedings 2 ( 2015 ) 3973 – 3987

Keywords: Hydrothermal synthesis; nanoflower like-TiO2; scale-up; reproducibility; photocatalysis

1. Introduction

In recent years, TiO2 architectures with complex hierarchical-micro/nanostructured have received considerable interest for fundamental research and practical application due to their outstanding properties, such as low density, high specific surface area and large pores [1–3]. Particularly, three-dimensional (3D) dandelion like-TiO2 hierarchical superstructures, assembled by zero-dimensional (0D), one-dimensional (1D) and two-dimensional (2D) nanoscale building blocks such as nanowires [4], nanorods [5], nanotubes [6], and nanosheets [7], show diverse applications in various fields including microreactors [8], sensors [9], water-splitting [10], self-cleaning devices [11], environmental pollution [12], photovoltaic cells [13], Li-ion battery materials [14], optical emission [15], energy-conversion [16] and photocatalysis [17] owing to their unique structure and outstanding chemical/physical properties. It is generally known that the properties of TiO2 materials are related to their morphology and size, which will correspondingly affect their performances in the above-mentioned applications. Therefore, the controlled synthesis of TiO2 nanostructures with different shapes and particle sizes is currently regarded as the most attention-grabbing. In various morphologies, the hierarchical 3D dandelion like-TiO2 nanostructures have been regarded as the more attention-grabbing structures due to the greater number of active sites and good light absorption efficiency than other 2D or 1D nanostructures [18]. Therefore, it is of great scientific and industrial importance to explore novel 3D dandelion-like TiO2 nanostructures and find economical mass production methods which can keep the morphology of the 3D dandelion-like TiO2 nanostructures under effective control.

Currently, methods used for the synthesis of 3D TiO2 hierarchical structures include chemical vapor decomposition [19], sol-gel [20], reversed micelle [21], Ostwald ripening [22], Kirkendall effect [23] and hydrothermal reactions [24]. However, the direct synthesis of 3D TiO2 hierarchical structures via simple and cost-effective techniques remains a significant challenge. The hydrothermal method seems to be an excellent choice for the synthesis of 3D TiO2 superstructure, particularly the dandelion-like structures. This method provides the advantages of controlling the stoichiometry, homogeneous products and allowing the formation of complex shapes and preparation of composite materials [25]. Moreover the hydrothermal method provides excellent homogeneity and possibility of deriving unique stable structures at low reaction temperatures [26]. Since the raw materials react in an enclosed system under a controlled temperature and pressure, the method using an aqueous solvent as reaction medium can be classified as environmentally friendly. Particles and rod-like TiO2 structures are easy to synthesize by this method, while the novel 3D TiO2 structures synthesized by this method are hardly ever reported in literature. Also, a systematic study on the synthesis of TiO2 by hydrothermal method has not been reported. Recent efforts have made this technique a new platform for synthesizing 3D TiO2 superstructures having controllable hierarchical features [27,28]. For example, Jin et al. [1] synthesized dandelion-like rutile TiO2 microspheres using rutile hollow SnO2 spheres as templates. Zhou et al. [2] synthesized a novel 3D sea-urchin-like hierarchical TiO2 microspheres with 1D nanostructure by hydrothermal method using Ti plate in a mixture of H2O2 and NaOH aqueous solution, and investigated the influence of reaction time, reaction temperature and calcination temperature.

Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 electrodes in 1972 [29], since then photocatalysis is known to be an important method to remove toxic heavy metals from wastewater. Among different semiconductor photocatalysts the 3D TiO2 superstructures exhibited excellent photocatalytic activities upon UV light irradiation. This was due to the high specific surface area which contributed to the formation and migration of photocarriers and the adsorption of reactants on the catalyst surface. The materials are also less likely to suffer from severe agglomeration and easier to be separated from the solution than traditional TiO2 powders. This makes them promising for potential applications in photocatalysis. To the best of our knowledge regarding Cr(VI) photoreduction by dandelion-like TiO2 nanostructures no report is available in the open literature.


In this study, 3D dandelion-like TiO2 nanostructures were synthesized from TiCl4 and water by a simple hydrothermal method, and Cr(VI) was selected as model compound. The morphology evolution and formation mechanism of the 3D dandelion-like TiO2 nanostructures were investigated in detail. The reproducibility, scale-up effects and photocatalytic activity were studied.

Nomenclature

e- Electron h+ Hole θ Bragg’s angle

2. Experimental methods

2.1. Materials

All chemicals used in this study were of analytical-grade and used directly without further purification. High purity water (resistivity 18.2 MΩcm) purified by a Milli-Q water system (Millipore) was used in all experiments. Degussa P25 (anatase: rutile 80:20, BET surface area of 49 m2/g, particle size 21 nm) was sourced from Merck Chemicals (Pty) Ltd., South Africa. Nitric acid (99.99% HNO3), sodium hydroxide (99% NaOH), potassium dichromate (99.99% K2Cr2O7) and titanium tetrachloride (99% TiCl4) were purchased from Merck Chemicals (Pty) Ltd., R.S.A.

2.2. Synthesis of dandelion-like rutile TiO2 photocatalyst

Dandelion-like rutile TiO2 structures were prepared using the hydrothermal method. Super-cooled high purity water (160 mL) at a temperature close to 0°C was added to a 2000 mL reaction flask. The flask was placed on a magnetic stirrer with a 2 cm magnetic stir bar inside. A desired amount (60 mL) of TiCl4 was added to the super cooled high purity water drop-wise using a separatory funnel while vigorously stirring at 500 rpm. The high purity water was super cooled to suppress vigorous reaction. After adding all the TiCl4 drop-wise into 160 mL of high purity water and the two substances had completely mixed and reacted, the heating mantle was turned on and the temperature was gradually increased to 100oC, the contents of the flask were then refluxed for the next 24 hours (h), while stirring at 100 rpm. After 24 h, a 5810-R Eppendorf centrifuge was used to centrifuge the suspension at 3000 rpm at 21˚C for a period of 99 minutes (min). The washing of the solids with high purity water was done five times for five minutes per wash in order to remove any chloride ions from the solid TiO2. The solids were extracted then from the tube into petri-dish and oven-dried using an oven for approximately 16 h overnight at 120˚C.

2.3. Characterization

The crystal structure and morphology of the prepared photocatalysts were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) analysis. The PXRD spectrum was obtained from a Bruker AXS D8 X-ray diffractometer machine with Ni-filter Co Kα at 40 kV and 35 mA with a wavelength alpha as 1.78897 Å. The samples were scanned in the 2Ө range from 20˚ to 70˚ with a step size of 0.02˚ and a count time of 1s each point. The SEM images for determining surface morphology were done on a FEI Nova NanoSem 200 scanning electron microscope from FEI Company. The scanning electron micrographs were obtained at an operating voltage of 3kV. The TEM images for the determination of internal morphology and particle size were carried out in a JEM2100F Electron Microscope from JEOL Ltd, using an accelerating voltage of 120 kV. The samples for TEM analysis were prepared by dispersing some of the powder products into methanol and then sonicated for about 3s. A few drop of the suspension were deposited on the copper grid, which was then put into the desiccator. Furthermore,


the BET and BJH analyses were carried out in a Micromeritics ASAP 2020 unit by using the adsorption-desorption isotherm at 77K. The samples were placed in the sample holder and first degassed at 150˚C for 3 h prior to analysis. After degassing, the samples were re-weighed and analysed under liquid nitrogen. The BET surface area was determined by multipoint BET method. A desorption isotherm was used to determine the pore size and pore volume distributions via the BJH method.

2.4. Photocatalytic activity test

The photocatalytic activity of dandelion-like TiO2 nanostructures was investigated by photocatalytic reduction of Cr(VI) in aqueous solution. Cr(VI) was chosen as a model pollutant, as is listed as priority and hazardous pollutant (WHO, 2008) which is very harmful to the environment and human. Photocatalytic experiments were carried out in a batch type photoreactor at room temperature (25 ± 2oC). Prior to UV light irradiation, the suspension was stirred in the dark for 60 min to reach a complete adsorption-desorption equilibrium. The light source used in photoreactions was a HPR 125 W UV-C lamp developed by Philips lighting and was preheated for 30 min to obtain a constant light intensity. Photocatalytic experiments were carried out to investigate the effects of photocatalyst dosage, initial pH and initial Cr(VI) concentration. A desired amount of photocatalyst was added to a reaction flask containing 250 mL of an aqueous solution of K2Cr2O7 with defined concentrations (10 to 50 mg/L). The Cr(VI) stock solution (1000 mg/L) was prepared by dissolving 2.835 g of K2Cr2O7 in 1 L high purity water. The stock solution was diluted with high purity water to obtain the desired concentration range of Cr(VI) solutions. The initial pH of the solution was adjusted by HNO3 or NaOH aqueous solutions. At different time intervals during the runs, 5 mL samples of the suspension were withdrawn from the batch reactors and centrifuged employing a 5810-R Eppendorf centrifuge before the absorption spectrum was taken. The absorbance of Cr(VI) was measured spectrophotometrically at 360 nm, by UV-vis spectrophotometer using American Public Health Association (APHA), standard methods for examination of water and wastewater [30]. The photocatalytic activity was evaluated by analyzing the Cr(VI) reduction after a fixed contact time. The photocatalytic activity of Degussa P-25 was also measured as a reference to compare with that of the synthesized dandelion-like TiO2 catalyst.

2.5. Kinetic studies

Many researchers [31, 32] have observed that the reduction rates of photocatalytic reduction of various metal ions over irradiated TiO2 follows the Langmuir – Hinshelwood (L-H) kinetics model. The Langmuir–Hinshelwood kinetic relates the rate of surface catalysed reactions to the surface covered by the substrate, being the kinetic equation expressed as

where is the reduction rate, is the reactant concentration, is the time, H-L is the rate constant, and ad is the adsorption equilibrium constant.

This model assumes that, if the adsorption of Cr(VI) onto the surface of the photocatalysts is very low, ads can be neglected in the denominator simplifying the equation to a pseudo-first-order equation as given by [33].

The reduction rate for the reduction of Cr(VI) for the pseudo first order reaction was calculated in terms of mg. L−1 . min−1. The integration of Eq. (2) with boundary conditions of C = Co at t = 0 is represented by


3. Results and discussion

3.1. Structure and morphology

Figure 1 show typical PXRD diffraction peaks for the different quantities (10 to 100 g) of the dandelion-like TiO2 hydrothermally treated at 100˚C for 24 h. As shown in Figure 1, all the diffraction peaks in the PXRD pattern of the dandelion-like TiO2 can be indexed to a pure rutile TiO2 crystal structure (JCPDS No. 21-1276). No other diffraction peaks of anatase or brookite phase are detected, indicating the formation of dandelion-like TiO2 samples with high purity. The Bragg diffraction peaks at 2Ө = 32˚, 42.2˚, 48.2˚ and 64˚ are the main peaks observed in the diffraction analysis. It is well accepted that the broadening of the diffraction peaks reflects a decrease in the particle size and the intensity of the diffraction peaks reflects the crystallization of the samples [1]. The results suggest that the effect of increasing the preparation scale of the dandelion quantity has no significant effect on the crystalline phase. This confirms that scale-up and reproducibility of these materials may not a challenge.

20 30 40 50 60 70

a

c

b

Inte

nsity

(a.u

)

2-theta (degree)

Fig. 1. PXRD diffractograms for different preparation scales (a. 10 g; b. 50 g; c. 100 g) of the rutile dandelion-like TiO2

The morphologies and structures of the as-prepared dandelion-like TiO2 nanostructures with 24 h reaction time

were characterized by SEM and TEM. Figures 2a-c show the SEM images of the dandelion-like TiO2 nanostructures synthesized for 24 h and hydrothermally treated at 100˚C. Enlarged individual 3D dandelion-like TiO2 nanospheres with numerous densely packed TiO2 nanostructures can be observed in Figure 2a and Figure 2b. The top surface reveals that the spheres were composed of many loosely packed nanometer-scale crystals in Fig. 2c. Similar results were obtain by Bai et al.[24], where they synthesized dandelion-like TiO2 structure by facile hydrothermal method using TiCl3 and NaCl as the main starting materials. The TEM images of the dandelion-like TiO2 further provided

(110) (110) (110)

(110)

(110) (110) (110) (110)

(110) (110) (110) (110)


information regarding the interior structure of the dandelion-like TiO2 nanostructures. The corresponding TEM images of the dandelion-like TiO2 in Figures 3a-b show a typical well-crystallized and half dandelion-like rutile TiO2, respectively. The TEM images demonstrated that each dandelion-like nanostructure is composed of ordered nanorods with an average diameter of 17 nm. A similar nanorod-structured TiO2 surface was observed by Jin et al. [1], who synthesized dandelion-like microspheres as anode materials for lithium ion batteries with enhanced rate capacity and cycling performances. The full rutile dandelion-like TiO2 nanostructures were formed as shown in Figure 3a-b. This was also supported by the XRD patterns of the dandelion-like TiO2 which was identified as pure rutile phase (Figure 1). The morphological results obtained using SEM and TEM images indicated that the dandelion-like TiO2 nanostructures are comprised of highly close-packed nanorods.

Fig. 2. (a) SEM images of the rutile TiO2 synthesized for 24 h and hydrothermally treated at 100˚C; Inset: an enlarged individual dandelion-like

TiO2 sphere; (b) typical well-crystallized dandelion-like TiO2 microsphere; (c) top-image of a dandelion.

Fig. 3. TEM images of the dandelion-like rutile TiO2 synthesized for 24 h and hydrothermally treated at 100˚C: (a) typical well-crystallized dandelion-like TiO2 nanosphere; (b) dandelion-like TiO2 half nanostructure.

Generally, the photocatalysts with higher specific surface area and porous structures are known to enhance

photocatalytic activity of the photocatalysts. This could be attributed to more surface active sites for the adsorption of reactant molecules, ease transportation of reactant molecules and product through the interconnected porous networks, and enhanced harvesting of exciting light by multiple scattering within the porous frame work. Table 1 shows the properties of as-prepared dandelion-like TiO2 and commercially available Degussa P25. There is little information reported in the literature on the BET surface area of the dandelion-like TiO2 structure. Jin et al. [1] synthesized dandelion-like rutile TiO2 microspheres using rutile hollow SnO2 spheres as template. They obtained the dandelion-like rutile TiO2 which contained pores of about 4 nm, and the BET surface area was 46 m2.g-1. Zhou et al. [2] synthesized dandelion-like rutile TiO2 via a one-step solvothermal approach using titanium n-butoxide in a mixture of HCl and n-hexane, they obtained dandelion-like TiO2 with the BET surface area of 40 m2.g-1. As shown in Table 1, as-prepared dandelion-like TiO2 kept a high surface area of 81 m2.g-1 with pores of about 3.2 nm, which contributes to the excellent adsorption capacity. This indicates the importance of obtained results as a positive


scientific and technological contribution.

Table 1. BET specific surface area, pore volume, pore diameter for the dandelion-like TiO2 structure and commercial available Degussa P25.

TiO2 SBET (m2g-1) PV (cm3/g) PD (nm) References

Dandelion 81 0.071 3.2 [present]

Dandelion 46 4 [1]

Dandelion 40 4 [2]

Degussa P25 49 [present]

SBET, PV, PD represent BET surface area, pore volume, pore diameter, respectively

The nitrogen adsorption–desorption isotherm and Barret-Joyner-Halenda (BJH) pore size distributions derived

from the desorption branch of the adsorption isotherm of the as-prepared dandelion-like TiO2 nanostructures are shown in Figure 4. The N2 adsorption–desorption isotherm of the dandelion-like TiO2 nanostructures can be ascribed to a type-IV, which is typical of mesoporous solid materials (2-50 nm) according to the International Union of Pure and Applied Chemistry (IUPAC) classification [34]. The hysteresis loop of as-prepared dandelion-like TiO2 fit well to type H2, which is characteristic for mesoporous materials with ink-bottle pores [34]. The pore size distribution plot (inset of Figure 4) calculated using the BJH equation from the adsorption branch of the isotherm shows an average pore size of 3.2 nm. The materials exhibited a narrow pore size distribution suggesting good homogeneity of the pores.

Fig. 4. Nitrogen adsorption-desorption isotherms and BJH pore size distributions (inset) plots of as-prepared dandelions-like TiO2.


3.2. Possible growth mechanism of 3D dandelion-like TiO2 nanostructures

To study the growth mechanism of dandelion-like TiO2 nanostructures, the morphology evolution was investigated from the SEM images collected at different growth stages presented in Figure 6a–c. Based on the obtained results it is concluded that the growth of the dandelion-like TiO2 assembled through nanorods is governed by a nucleation and nuclei growth-dissolution-recrystallization growth mechanism a from time dependent morphology formation. The effect of reaction time on the growth mechanism was studied in order to understand the formation mechanism of dandelion-like TiO2 nanostructures. The mechanism adopted for the purpose of this work was similar to the one used by Ani et al. [35], where a TiO2 was produced by vapour phase hydrolysis of TiCl4 at low temperatures. Low temperatures close to the boiling point of the precursor, in this case H2O and TiCl4. In this study the reaction was carried out in a liquid phase at 100˚C a temperature close to the boiling point of the precursors, and in this case the boiling points are 136.4˚C and 100˚C for TiCl4 and H2O, respectively. From the results, a possible mechanism for the formation of dandelion-like TiO2 is illustrated in Figure 5. As can be seen in Figure 5, if there is no lattice match between the TiO2 and the substrate, the TiO2 is firstly nucleated as islands (i) and then nanowires grow (ii) from these islands to form dandelion-like (iii) morphologies.

Fig. 5. Schematic illustration of a possible growth mechanism for the dandelion-like TiO2 nanostructures.

3.3. Influence of reaction parameters on the morphology of products

Figure 6 shows the SEM images of the dandelion-like TiO2 samples hydrothermally reacted for 12 h, 24 h, and 48 h. The SEM image presented in Figure 6a represents the initial growth stage of the dandelion structure at a hydrothermal reaction time of 12 h. In this stage, many immature aggregated solid TiO2 structure and irregular spheres were observed. Prolonging the reaction time to 24 h (Figure 6b), resulted in the denser structure without irregular spheres. Figure 6c shows a collapsed dandelion-like TiO2 structure, as the individual dandelion-like TiO2 recombined to form undesirable sheet-like rutile TiO2. It can be concluded that prolonging the reaction time makes the particles grow and agglomerate. Zhang et al. [36] observed the agglomeration of shuttle-like particles forming

(I)

TiCl4 + H2O

(II)

(III)

TiO2

islands Growth of nano wires

(IIIDandelion-like TiO2

nanostructures


petal-like bundles of particles, with prolonged reaction time. Based on the results and analysis a complete morphology evolution process of the 3D TiO2 nanostructures was proposed as follows: (i) nucleation and nanoparticle formation; (ii) formation of spheres through self-assembly growth; (iii) further growth and (iv) agglomeration of the dandelions to form sheet-like rutile TiO2. Time was found to be the most important controlling factor.

Fig. 6. SEM images of TiO2 samples selected at different intervals (a) 12 h; (b) 24 h; (c) 48 h.

In the present study, it was found that the reaction temperature plays an important role in the nucleation and

growth of TiO2 crystallites which determine the final dandelion morphology of the products. Figure 7 illustrates the SEM images of dandelion-like TiO2 hydrothermally treated at 80 °C, 100 °C, and 190 °C for 24 h. At 80˚C reaction temperature (Figure 7a), the dandelion-like TiO2 were formed by loosely packed nanorods and TiO2 dandelions or strands grow slowly and freely from the nucleus in all directions. When the reaction temperature was raised to 100˚C (Figure 7b), compact dandelion were obtained with rods originating from the centre. At a high temperature (190˚C), the dandelion-like TiO2 structures were destroyed, resulting in sectors composed of nanorods and sheet-like TiO2 structure (Figure 7c).

In order to investigate the effects of scale-up and reproducibility of the dandelion-like TiO2 structures samples were prepared at different quantities namely 10 g, 50 g and 100 g at 100 °C for 24 h as shown in Figure 8. In Figure 8a-c, the overall morphology indicates the existence of many uniform, dandelion-like TiO2 nanostructures. These results suggest that increasing the dandelion-like TiO2 preparation quantity has no significant change on the structural and chemical characteristics of the materials. Furthermore, this is in agreement with the results obtained from the XRD data obtained in Figure 1. Therefore, it is of great scientific and industrial importance to note that the dandelion-like TiO2 nanostructures can be reproduced and scaled-up while keeping its morphology under effective control.


Fig. 7. SEM images of the rutile TiO2 synthesized for 24 h and hydrothermal treated at various temperatures; (a) 80˚C; (b) 100˚C; (c) 190˚C.

Fig. 8. SEM images of the rutile TiO2 synthesized for 24 h and hydrothermal treated at 100˚C; individual dandelion-like TiO2 spheres of different

quantities, namely; (a) 10 g; (b) 50 g and (c) 100 g.

3.4. The photocatalytic activity

To show the potential environmental application of the dandelion-like TiO2 nanostructures for reduction of Cr(VI) wastewater in aqueous solutions, photocatalytic activity of the synthesized dandelion-like TiO2 nanostructures were performed under UV light irradiation at room temperature. Background experiments were carried out in the absence of catalyst under UV light and with the catalyst in the dark. The control experiments were designed to exhibit possible effects of photolysis and adsorption in the system. Also a control experiment was done with the commercial catalyst Degussa P25 TiO2 under the same conditions, which was used as the basis for comparison with as-prepared dandelion-like TiO2 nanostructures. Finally, the influence of experimental parameters on the reduction rate of Cr(VI) was investigated.

3.4.1 Background experiments

To investigate the extent of Cr(VI) reduction by photolysis, a solution containing 20 mg/L of Cr(VI) was irradiated by UV lamp for 120 min at pH 2. On the other hand, in order to investigate the adsorption effect of Cr(VI) onto dandelion-like TiO2, experiments were conducted in the presence of dandelion-like TiO2 in dark. Briefly, 2 g of dandelion-like TiO2 were introduced into a Cr(VI) solution (20 mg/L) and stirred in the dark for 120 min at pH 2. As shown in Figure 9, no reduction of Cr(VI) was observed without catalyst under UV light irradiation and with catalyst in darkness. The reduction efficiency of adsorption in the dark and the photolysis of Cr(VI) within 120 min


were 10% and 1%, respectively, indicating that Cr(VI) is stable under UV light irradiation and in the dark. Both photolysis and adsorption in the dark can be considered to have negligible effect on the reduction of Cr(VI).

On the contrary, when the system was exposed to UV light in the presence of both dandelion-like TiO2 and P25 TiO2 photocatalysts, a rapid decrease in Cr(VI) concentration was observed. The obtained results indicate that UV light energy in the presence of catalyst is of fundamental significance in Cr(VI) reduction in aqueous solution and proposed that photocatalytic activity of both dandelion-like TiO2 and P25 TiO2 for the reduction of Cr(VI) ions may occur through direct reduction by the photo-generated electrons. The as-prepared 3D dandelion-like TiO2 exhibits the highest Cr(VI) reduction efficiency, and about 82% of Cr(VI) was reduced, which was higher than 70% photocatalytic activity of Degussa P25. The high photocatalytic activity of dandelion-like TiO2 sample can be attributed two factors. Firstly, the dandelion-like TiO2 has a larger surface area due to the higher destiny of flower-like nanostructures, which leads to the strong absorption capacity on the surface of dandelion-like TiO2 nanostructures. Secondly, the 3D flower-like morphologies that were observed by SEM, can utilize their radiation light more efficiently due to the multi-reflections on the high density surface of the flower patterns. As a result, the absorption of photons and production of photo carriers are improved significantly. Therefore, enhancing the photocatalytic activity of dandelion-like TiO2 for the reduction of Cr(VI) in aqueous solution.

Fig. 9. Photoreduction curves of Cr(VI) using 3D dandelion-like TiO2 and the P25 powder. The initial composition of the solution is: Cr (VI),

20 mg/L; catalyst dosage, 2 g/L and pH, 2.

3.4.2 Influence of experimental parameters on the reduction rate

3.4.2.1 Effect of initial pH

A series of experiments were conducted at different pH values ranging from 2 to 12, containing 20 mg/L Cr(VI) and 2 g/L of catalyst in order to investigate the influence of initial pH on the photocatalytic reactions of Cr(VI). The influence of initial pH on the Cr(VI) photoreduction was studied and the results are presented in Figure 10. It can be seen that the efficient photoreduction is reached in the solution at pH 2, and the photoreduction drastically decreases with the increasing pH up to pH 12. The decrease in reduction rate with increasing pH of the Cr(VI) solution. This was attributed to the neutralization of the anionic sites and negative potential of the TiO2 surface at lower pH. Furthermore such tendency can be explained based on the speciation of Cr(VI) as a function of pH alteration. Cr(VI) occurs in oxy anions as HCrO4

-, Cr2O72- and CrO2

4- depending on the pH. In acidic medium (pH > 1), HCrO4-

predominates and converts to CrO24- with increasing solution pH, owing to the acidity constant (pKa = 6.49 at

00.10.20.30.40.50.60.70.80.9

1

0 20 40 60 80 100 120

C/C

o

Irradiation time (min)

Without catalystDarkP25Dandelion-like TiO2


25ºC). At pH 2 to 4, Cr(VI) ion in the solution exists as HCrO4- and Cr2O7

2- [37], that act as strong oxidizing agents, therefore they are easily reduced.

At higher pH, pH 6 to pH 12, the fraction of HCrO4- and Cr2O7

2- decreases gradually, but the fraction of CrO42-

species which is a weaker oxidizing agent, increases. Moreover, Cr(III) in the solution resulted from Cr(VI) photoreduction may precipitate as Cr(OH)3 [38]. This can scatter the entering light leading to the low photoreduction rate. The trend is consistent with that reported by Prairie et al. [39].

Fig. 10. Influence of initial pH on the photocatalytic reduction rate of Cr(VI). Experimental conditions: [initial pH (2-12); catalyst dosage, 2 g/L and initial concentrations, 20 mg/L)].

3.4.2.2 Effect of Cr(VI) concentration initial

From both mechanistic and application points of view, the study of dependence of the photoreduction rate on the substrate concentrations is important [40]. Hence, the effect of the initial concentration of Cr(VI) on the photoreduction efficiency was studied and the obtained results are illustrated in Figure 11. The influence of Cr(VI) initial concentration on the reduction rate was determined by varying the initial Cr(VI) concentration within the range 10–50 mg/L at 2 g/L catalyst loading. The Cr(VI) reduction rate gradually decreased upon increasing the initial concentration of Cr(VI) in the solution from 10 to 50 mg/L. With increasing initial concentration of Cr(VI), a large amount of UV radiations was absorbed by the substrate before it reaches the surface of the dandelion-like TiO2 nanoparticles and then the reduction decreased [41]. Moreover at a fixed dosage of dandelion-like TiO2, the total available electron–hole pairs are limited to obtain a higher reduction efficiency of Cr(VI) at higher concentration [42]. By observing the colour change of the dandelion-like TiO2 nanostructures, the white-coloured nanostructures became yellow after adsorption of Cr(VI), and then gradually turned into pale green during the photocatalytic reduction of Cr(VI) to Cr(III). Therefore, for high initial Cr(VI) concentrations of Cr(VI) a yellow deposit on dandelion-like TiO2 nanostructures was observed, which corresponded to the adsorption of anionic Cr(VI) species [43].

0

0.2

0.4

0.6

0.8

1

1.2

1.4

2 4 6 8 10 12

Cr(V

I) re

duct

ion

rate

(mg.

L-1.m

in-1

)

pH


00.20.40.60.8

11.21.41.61.8

2

10 20 30 40 50

Cr(V

I) re

duct

ion

rate

(mg.

L-1.m

in-1

)

Initial concentration (mg.L-1)

Fig.11. Influence of initial Cr(VI) concentration on the photocatalytic reduction rate of Cr(VI). Experimental conditions: [initial Cr(VI) concentration (10-50 mg/L); catalyst dosage, 2 g/L and initial pH, 2)].

3.4.2.3 Effect of catalyst dosage

To optimize the degradation conditions of Cr(VI), the effect of catalyst dosage on the photocatalytic reduction rate of Cr(VI) was examined by varying the dandelion-like TiO2 concentration in the suspension from 0.5 g/L to 3 g/L. Figure 12 shows the dependence of the reduction rate constant on dandelion-like TiO2 concentrations on the percentage of reduction of Cr(VI) in a solution. The reduction rate initially increased and then decreased with the increase of the dandelion-like TiO2 catalyst amount. It was observed that an increase in a catalyst dosage up to 2 g/L the reduction rate increased and there after it decreased. This may be due to the fact that with increase in catalyst dosage the number of photons absorbed by dandelion-like TiO2 nanostructures and number of reacting molecules adsorbed on dandelion-like TiO2 surface are increased. However at high catalyst dosage (2 g/L) there was no further increase in the reduction rate, thus might be due to the fact that no reacting molecules are available for adsorption and light scattering of dandelion-like TiO2.

From this observation it was presumed that increasing the amount of catalyst exhibited two opposing contributions to the photocatalytic reduction process. On one hand, with increase in catalyst dosage there was an increase in the surface area of the dandelion-like TiO2 catalyst available for adsorption and hence photoreduction. On the other hand, scattering phenomena appeared leading to a decrease in the penetration of the light and the generation of e- – h+ pairs on the surface of the catalyst, thereby affecting the reduction rate. Similar results were previously reported by Munoz and Domenech, [44] & Chaudhary and Thakur [45].


Fig. 12. Influence of catalyst concentration on the photocatalytic reduction rate of Cr(VI). Experimental conditions: [catalyst dosage (0.5-3 g/L)

initial Cr(VI) concentration (20 mg/L) and initial pH, 2)].

4. Conclusions

In summary, morphology and structural-controlled dandelion-like TiO2 structures have been successfully synthesized by a simple and cost effective hydrothermal method. It was found that the reaction time and reaction temperature can critically affect the formation of the dandelion-like TiO2 structures. The results reported in this work also suggest that the dandelion-like TiO2 can be easily scaled-up and reproduced. The growth mechanism of the 3D dandelion-like TiO2 structures was reasonably deduced according to the SEM images at different reaction time. The formation mechanism of the 3D dandelion-like TiO2 structures was proposed as follows; (i) nucleation and nanoparticles formation; (ii) formation of spheres through self-assembly growth; (iii) further growth and (iv) agglomeration of the dandelions to form flower-like rutile TiO2. Due to the larger specific surface area (81 m2/g) and the light capture of the flower-like structure, the dandelion-like TiO2 structures exhibited excellent photocatalytic activity, which was high than Degussa P25 TiO2. The reduction rate of Cr(VI) depended on several experimental parameters, including catalyst dosage, initial Cr(VI) concentration, and solution pH. Dandelion-like TiO2 appears as promising materials for the reduction of Cr(VI) from aqueous solutions using UV/TiO2 photocatalysis process.

Acknowledgements

This work was carried out and financially supported by Mintek. Jeffrey Baloyi and Tumelo Seadira are M.tech students funded in part by the National Research Foundation (NRF)/ Department of Science and Technology (DST) of South Africa.

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