10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

IMPROVED VISIBLE LIGHT RESPONSE OF METAL/NON-METAL DOPED TiO2

NANOPARTICLES UTILIZATION IN ORGANIC POLLUTANTS DEGRADATION

Maria E. Pappa1, Petros M. Sakkas

1, Tatjana Kosanovic Milickovic

1,2,

Alexandros Zoikis-Karathanasis2 and Ioanna Deligkiozi

*1

1 Center for Research and Technology Hellas, Institute for Research and Technology of Thessaly

2 Artia Nano Engineering & Consulting

ABSTRACT

Non-metal and metal modified TiO2 photocatalysts as well as pure TiO2 powders were synthesized via sol-gel

method, using titanium precursors with non-metal [N or S,N] and metal [Ag] doping elements. The synthesized

powders have been imposed to different heat treatment cycles at several temperatures above 400oC. Structural

and morphological characterizations have been accomplished through X-ray powder diffractometry (XRD),

scanning electron microscopy (SEM), DR-UV-Vis and IR spectroscopy. Band gap energy (Eg) measurements

have been performed via UV-Vis spectroscopy and accordingly calculated from a plot of modified Kubelka–

Munk function versus the energy of exciting light by converting the scanning wavelength (λ) into photon band

gap energies (Eg). According to XRD analysis, doped TiO2 powders consisted mainly of the TiO2 anatase phase,

especially in the case of calcination temperatures less than 600oC and the crystallite size ranges between 8–30

nm. Furthermore, the band gap energy was estimated to be less than 2.5 eV, therefore the photocatalytic

performance of doped TiO2 powders is extended in the visible range. Concerning the photocatalytic activity

determination, several experiments have been conducted in a photo-reactor, where the activity of the doped

nanostructured TiO2 catalysts were evaluated through the degradation of two model pollutants, namely

Methylene Blue (MB) and Methyl Orange (MO). The new powders exhibited significantly improved

photocatalytic performance in MO and MΒ in comparison to the non-doped TiO2 and reference compounds by

reducing up to 90% the model pollutants tested.

Keywords: photocatalysis, doped titania, pollutants degradation, building materials

INTRODUCTION

Titania (TiO2) based materials are well known semiconductors able to be activated under light irradiation and

therefore catalyze several chemical processes. This photocatalytic performance in combination with their

chemical stability, low toxicity and low cost of precursors, render such materials as potential candidates for

environmental friendly processes such as wastewater treatment and air purification [1,2,3]. In the present

research, these performance capabilities aspire to find application in novel multifunctional paintable systems for

façade applications acting as external surface coatings that will exhibit organic pollutants degradation through its

photocatalytic activity. Since TiO2 absorbs only a minor part of the active solar spectrum that constitutes only

the 5% of solar light spectrum, primary objective of this study is to improve light harvesting of these materials

making them absorb in the visible range of the electromagnetic spectrum and thereby improving their efficiency.

For this purpose TiO2 was modified via doping with metal (silver, Ag) and/or non-metal elements (nitrogen, N

and sulfur, S) that have the potential to stimulate the visible light response of the synthesized particles. The

origin of the shifted absorption and the therefrom derived photocatalytic activity mechanism of the [N]-doped

TiO2 remains a matter of controversial discussions [4] that is attributed among others to a delocalized mixing

between the O 2p and N 2p orbitals created from nitrogen doping [5,6] or to an isolated mid-gap level above the

valence gap generated from the dopant atom orbitals [5,7,8]. Similarly, in the case of the [S]-doped TiO2,

additional electronic states above the valence band edge of pure TiO2 have been reported [9]. On the other hand,

noble metals such as Ag, known for exhibiting unique electronic and catalytic properties, significantly promote

the electron-hole separation and therefore enhance the photocatalytic activity of doped TiO2 [10]. Furthermore,

in order the modified TiO2 particles to exhibit an improved photocatalytic activity, they need to meet some extra

criteria regarding their crystal structure as well as the crystallite and particle size. Anatase titania usually exhibits

higher photocatalytic activity than rutile and brookite, owing to its higher density of localized states, the surface-

adsorbed hydroxyl radicals and slower charge carrier recombination [11], while the small crystallite size

generally improve the phototoactivity of TiO2 as has been reported by several researches [12,13] For the

evaluation of the photocatalytic activity of the doped TiO2 powders several chemical compounds are used as

model pollutants in experimental protocols such as textile dyes, phenols, acetaldehyde etc. In the present

research two textile dyes, methyl orange and methylene blue have been chosen as model pollutants for the

photocatalytic experiments.

MATERIALS AND METHODS

Synthesis

Pure and doped TiO2 powders were synthesized using the sol-gel method. 29.4mmol of Titanim(IV) butoxide

were hydrolysed in 50mL of acidified aqueous solution. After 3 hours of vigorous stirring, 109mmol of alcohol

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

were added and the mixture was mildly stirred for 24 hours. In the case of non-metal doped TiO2 powders ([N]-

doped and [S,N]-doped), various amounts of urea (16.7-666mmol) or thiourea (13.1-197mmol), respectively,

were added and the mixture was stirred for additional 2 hours followed by a sonication step around 15min. In the

case of the metal co-doped powders ([Ag-S,N]) various amounts of silver nitrate (0.382-1.531mmol) were

investigated and the mixture was sonicated and stirred under dark conditions for 24h. After the evaporation of

the solvents the crude amorphous powders were calcinated at 400oC for 2h and then purified using an acidified

aqueous solution or distilled water. For all the samples there have been followed additional calcination steps for

1h each at 500-800oC. Titanium(IV) butoxide, reagent grade, 97% was purchased from Aldrich, 1-butanol, Urea,

Thiourea, Silver nitrate, Methylene blue hydrate and Nitric acid 65% were all analytical grade (a.g.), purchased

from PENTA and Methyl orange from Panreac. All chemicals were used as received without further purification.

CHARACTERIZATIONS

X-ray powders diffraction (XRD)

The crystal structure of the synthesized powders was analyzed by X-ray diffraction using a Siemens D5000 X-

ray diffractometer (XRD) operated at 40 kV and 35 mA with a Cu-Kα radiation λ of 1.54 Ǻ. All the XRD data

were collected using a θ-2θ configuration and recording the diffraction intensity in the range of 2θ between 20–

100 degrees. The Difrac®Plus Eva software, version 2.0 (Siemens Energy and Automatization, Inc.) assisted to

identify the crystalline peaks. The crystalline phase of all samples has been identified by comparison with the

ICDD (International Centre for Diffraction Data) database and Joint Committee on Powder Diffraction Standards

(JCPDS) files. Anatase/rutile percentages have been calculated from the resulting diffracrograms using the Spurr

–Myers equation, while the average diameter of the crystallites has been estimated by using the Scherrer

equation.

Scanning Electron Microscopy (SEM)

The morphology of the powders has been investigated applying Scanning Electron Microscopy (SEM). Energy-

dispersive X-ray (EDX) spectroscopy analyses were obtained using a FEI Quanta 200 electron microscope.

Fourier transform infrared spectroscopy (FTIR)

This technique has been performed with a Perkin Elmer Spectrum 1 FTIR spectrophotometer using a KBr pellet

for sample preparation. All the samples were measured by using the attenuated total reflectance technique (ATR)

in the region 400-4000 cm-1

. The total numbers of scans for each measurement were 100 and the applied

resolution was about 4cm−1

. Under these specific test conditions, it has been feasible to evaluate the most of the

visible light sensitizing groups of the modified TiO2 nano particles.

Diffuse reflectance spectra

The optical reflectivity of the produced powders has been measured using a UV-Vis spectrometer (Agilent Cary

60) using a diffuse reflectance accessorize including remote diffused reflectance probe Harrick Barrelino, optical

fiber, optical coupler and computer control using the “Scan” software. The light source provided the light in the

range 250nm – 1100nm. UV-Vis scan rate was 300nm/min, with a data interval of 0.50 nm and an average time

0.1s. The baseline was corrected by using Agilent Halon plate as 100% reflectivity standard. The evaluation of

the band-gap (Eg) was performed by using Origin applications after the received R% vs. wavelength spectra were

modified to [F(R)hv]1/2

vs. hv spectra.

Photocatalytic activity

The experiments for the evaluation of the photocatalytic activity of the powders have been conducted in a home

made photo-reactor, where all the samples were irradiated under visible light. The photo-reactor consisted of a

sealed and properly ventilated box to avoid any influence of the external radiation, equipped with a multiple

stirrer and a light system. Temperature was kept at 27±1ºC (room temperature). The irradiation system was

equipped by four parallel lamps, 380-790 nm, Daylight Sylvania, F18/54-765-T8 1050 Lumen with the average

irradiance measured at samples surface being 40±6W/m2 (plus 0.31±0.06 W/m

2 of UV-light irradiance). A series

of experiments has been accomplished under the presence of Methylene Blue (MB) [3,7-bis (Dimethylamino)-

phenothiazin-5-ium chloride] and/or Methyl orange (MO) (Sodium 4-[(4-

dimethylamino)phenyldiazenyl]benzenesulfonate) that have been used as model pollutants. The experimental

procedure includes the preparation of a fresh aqueous solution of MB or MO (2mg/L), kept in the dark before

use. Each powder (333mg/L) was dispersed into this solution in a 250mL beaker and stirred for 40 min in the

dark (t=0) before the experiment starts. The pollutant/catalyst system was magnetically stirred under dark

conditions in order to reach the adsorption/desorption equilibrium. The suspensions obtained were irradiated

under visible light for five or more hours and aliquots have been periodically collected. After being centrifuged

for 20 min in 13000 rpm in order the nanoparticles to be removed, the absorbance spectrum of each aliquot is

measured in a UV-Vis spectrometer.

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

RESULTS AND DISCUSSION

Different synthetic route combinations and conditions have been investigated by altering various parameters,

namely pH, water to Ti-precursor ratio, duration of hydrolysis, loading of doping sources, aging time and

calcination temperature. The non-metal elements that have been investigated were N, S and C and in the case of

metal doped TiO2 various amounts of Ag have been evaluated. Based on the best performed proportions of the

non-metal and metal doped powders, co doping with Ag and N or S,N took place. In Table 1 the compositions

as well as the characterizations of the TiO2 powders that have been chosen to be presented in this work are

shown.

X-ray powders diffraction (XRD)

In order to evaluate the morphology, crystallinity and crystal size of the as prepared powders, XRD

measurements have been implemented. These types of measurements provide qualitative and morphological

information about the doped TiO2 powders mostly based on the ratio percentage of anatase to rutile phases. In all

cases the patterns of the powders (doped and non-doped) prepared by the sol-gel method and calcinated at 400 oC for 2 h with a heating rate of 5

oC/min have been monitored in order to elucidate the accompanied structural

characteristics followed by 500, 600, 700, and 800 o

C. It should be noted that depending on the synthesis

parameters the powders showed an almost amorphous material at low temperatures that indicates a dominant

single anatase phase, while TiO2 that has been formed by a rapid hydrolysis shows a high rutile phase

transformation that has been indicated as a second phase. Figure 1 illustrates the X-ray diffraction spectra of

some [N]-doped, [S,N]-doped and [Ag-S,N]-co-doped TiO2 powders calcinated at 500oC. The spectra reveal

anatase as the predominant crystalline phase (2θ=25.25ο), that ranges from 66 to 84% in the case of [N]-doped

TiO2 powders, from 75 to 81% in the case of the [S,N]-doped powders and from 70 to 79% for the [Ag-S,N]-co-

doped powders. The calcination temperature increase results in the phase transformation from anatase to rutile,

where in the case of 800oC calcination the rutile phase dominates absolutely in all cases. The average crystallite

size of the synthesized powders (calculated by Scherrer’s equation), ranges from 8 to 12 nm both in the case of

the un-doped and doped TiO2 particles, regardless of the type of doping elements. Due to their small crystallite

size, all particles can be characterized as nanocrystalline. Moreover, as expected, the calcination resulted in

crystallite size increase. In Figure 1 it is clearly depicted that the doped TiO2 nanopowders are primarily in the

amorphous phase until 400 oC. Annealing at higher temperatures 500, 600, 700

oC leads to the formation of the

rutile and in most of the cases at 800 oC the anatase completely disappears, and rutile has been observed as the

dominate phase.

Table 1: Synthesized powders, synthesis conditions and characterizations.

Sample Urea

(mmol)

Thiourea

(mmol)

AgNO3

(mmol) Eg (eV) Anatase % Rutile %

Crystallite

size (nm)

1 - - - 3.0 82 18 11

2 16.7 - - 3.0 66 34 9

3 83.3 - - 2.3 82 18 -

4 166.5 - - 2.2 78 22 11

5 249.8 - - 2.3 73 27 9

6 333.0 - - 2.3 77 23 10

7 499.5 - - 2.2 84 16 9

8 666.0 - - 2.3 64 36 8

9 - 13.1 - 3.0 - - 12

10 - 65.7 - 2.4 75 25 -

11 - 131.4 - 2.2 78 22 10

12 - 197.0 - 2.3 81 19 12

13 - 131.4 0.382 3.0 79 21 8

14 - 131.4 0.765 2.3 75 25 12

15 - 131.4 1.531 3.0 70 30 -

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

Figure 1: XRD spectra of the doped TiO2 powders. A and R referred to the characteristic peaks of anatase and

rutile, respectively.

Scanning electron microscopy (SEM)

SEM and EDAX techniques were used to study the morphology and composition of the doped materials. SEM

images revealed in all cases a relative homogeneity regarding the shape and size, which is attributed to the

milling process applied. The EDAX spectrum was recorded in the binding energy region of 0-10keV. EDAX

analysis of the synthesized powders confirmed the presence of high Titanium and Oxygen content and at the

same time some relatively smaller amounts of Nitrogen and Sulphur (0.6-0.9%, atomic percentage) attributed to

the doping sources (urea and thiourea). The atomic percentage of Silver ranges from 0.14% to 0.30%. In Figure

2 and

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

Figure 3 the SEM images and EDAX spectra of a [S,N]-doped and [S,N-Ag]-co-doped TiO2 powders are

indicatively presented.

Figure 2: SEM image (mix mode image) (left) and EDAX spectrum (right) of the [S,N]-doped TiO2 (sample

11).

Figure 3: SEM image (mix mode image) (left) and EDAX spectrum (right) of the [Ag-S,N]-co-doped TiO2

(sample 14).

Fourier transform infrared spectroscopy (FTIR)

Σhe FTIR spectra revealed in all cases a broad peak in the range of 3700-2900cm-1

corresponding to the strongly

absorbing hydroxyl stretching vibrations and a peak at around 1630cm-1

, attributed to the bending vibration of

hydroxyl groups. In the case of the doped TiO2 powders several bands are detected in the 1500-1000cm-1

region.

These bands can be partly ascribed to the vibrations of N-Ti bond, as well as to Ti-O-N and Ti-O-S bonds. The

strong band located in the range of 400-800cm-1

is attributed to Ti-O stretching and Ti-O-Ti bridging stretching

modes. In the case of the [Ag-S,N]-co-doped TiO2 powders, a small shift of the characteristic bands towards

lower wavenumbers has been observed.

Band gap (Eg)

Considering that the synthesized TiO2 powders have an indirect energy band gap, the Eg was obtained by using

the modified Kubelka–Munk function, following by a Tauc plot (F(R).hν)1/2

vs. E(=hc/λ) [14].The Kubelka-

Munk function F(R), enables the optical absorbance of the samples to be estimated from its reflectance. The

band gap for optical absorption is defined as the minimum energy required to excite an electron from the ground

state (HOMO, at the top of the valence band) to the exciting state (LUMO, at the bottom of the conduction

band). As shown in Figure 4, in the case of the undoped TiO2 powder (sample 1), the energy band gap (Eg) was

found 3.0eV, whereas in most of the cases of the [N]- and [S,N]-doped TiO2 powders the band gap has been

significantly reduced beyond 2.5eV in comparison to the undoped TiO2. These results demonstrate that the

doping with urea and thiourea causes a narrowing of the band gap suggesting that the obtained powders could be

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

active in the visible light range. In the case of the [Ag-S,N]-co-doped TiO2 powder (sample 14) there is also a

significantly reduced Eg, but this stands only for a certain thiourea/silver nitrate amount combination. As it can

Figure 4: Plots of [F(R)hv]1/2

vs hv for indirect transition of an undoped (sample 1), [N]-doped (sample 4),

[S,N]-doped (sample 11) and [Ag-S,N]-co-doped (sample 14) powder.

be seen from Table 1, the low dopant amounts of urea and thiourea (16.7 and 13.1mmols, respectively, per

29.4mmol of Ti-precursor) do not cause any improvement in the Eg, which in both cases has been estimated at

3.0eV (samples 2 and 9) like in the case of the undoped TiO2. By increasing the dopant amounts, the Eg has been

significantly improved, with the best Eg value accomplished at 166.5mmols of urea and 131.4 mmols of thiourea

(sample 4 and 11). Further increase of dopant amounts resulted in good though not further improved Eg . On the

other hand, in the case of the [Ag-S,N]-co-doped TiO2 powders only a certain amount of silver nitrate resulted in

Eg narrowing and the decrease or increase of this certain amount not only does not reduce or increase the Eg of

the new particles but also prevents the Eg reduction due to thiourea doping that has been well confirmed until

now.

Photocatalytic activity

The powders exhibiting the best morphological, structural as well as functional properties have been further

evaluated concerning their photocatalytic activity under visible light irradiation.

Figure 5 presents the photocatalytic performance of a [S,N]-doped and a [Ag-S,N]-co-doped TiO2 powder

(sample 11 and 14, respectively) under visible light irradiation, using Methyl orange and Methylene blue as

model pollutants (Graphs of normalized degradation C and D, respectively). In both experiments the

photocatalytic performance of the two powders was quite satisfactory after 5 hours of constant visible light

irradiation. More specifically, the photocatalytic degradation activity of the [S,N]-doped and [Ag-S,N]-co-doped

powders, expressed as the removal% of methylene blue in the aqueous solution, was 84% and 78%, respectively,

after five hours of visible light irradiation. In a similar experiment using methyl orange as model pollutant, the

photocatalytic activity of the two powders was also significant, with the [S,N]-doped and [Ag-S,N]-co-doped

powders exhibiting 64 and 67% removal of methyl orange in the aqueous solution, respectively. In order to

estimate the effectiveness of the photocatalytic performance a benchmark material, Degussa P25 was used where

the degradation has been estimated to be around 23 %. The characteristic absorbance spectra (Images A and B)

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

show the photocatalytic activity of Degussa P25 and sample 14 during five hours of constant visible light

irradiation.

Figure 5: Photocatalytic activity of [S,N]-doped (sample 11) and [Ag-S,N]-co-doped (sample 14) powders in

methylene blue and methyl orange degradation under visible light irradiation.

CONCLUSIONS

In the present experimental study an attempt has been made to improve the photocatalytic activity of TiO2

particles by doping with metal and non-metal elements, namely nitrogen, sulfur and silver. All the compositions

were characterized using XRD, SEM, EDX and DR-UV-Vis spectrophotometry. The most representative results

were presented and discussed. The improvement of TiO2 photocatalytic activity was focused in producing doped

TiO2 particles with specific structural and morphological characteristics as well as functional properties. The

synthesized doped TiO2 particles were anatase based nanocrystalline materials and exhibited significantly

reduced bang gap energy (Eg) beyond 2.5eV. Moreover, the new doped TiO2 particles demonstrated a quite

10ο ΠΑΝΔΛΛΗΝΙΟ ΔΠΙΣΗΜΟΝΙΚΟ ΤΝΔΓΡΙΟ ΥΗΜΙΚΗ ΜΗΥΑΝΙΚΗ, ΠΑΣΡΑ, 4-6 ΙΟΤΝΙΟΤ, 2015.

satisfactory photocatalytic activity in the model pollutants tested, by removing the pollutants up to 84%. This

first evaluation renders these catalysts as promising doped TiO2 based photocatalytic materials ready to be

exploited in various applications, among others in the environmental pollutants degradation.

ACKNOWLEDGEMENTS

This work has partially received funding from the European Union’s Seventh Framework Programme for

research, technological development and demonstration under grant agreement No. 609345 (MF-Retrofit:

“Multifunctional facades of reduced thickness for fast and cost-effective retrofitting”).

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