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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 60, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Luca Di Palma, Elisabetta Petrucci, Marco Stoller
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608- 50-1; ISSN 2283-9216 

Improved Photocatalytic Properties of Doped Titanium–Based 
Nanometric Oxides 

Marco Scarsella*a, Maria P. Braccialea, Benedetta de Caprariisa, Paolo De 
Filippisa, Antonietta Petrulloa, Lucilla Prontib,  Maria L. Santarellia  
a
 Department of Chemical Engineering Materials Environment, University of Rome Sapienza, via Eudossiana 18, 00184, 

Rome, Italy  
b
 Department of Basic and Applied Sciences for Engineering, University of Rome Sapienza, via A. Scarpa 16, 00161, Rome, 

Italy  
marco.scarsella@uniroma1.it 

Photocatalysis is considered one of the most promising technologies for applications in the environmental field 
especially in the abatement of water-soluble organic pollutants. In this field, titanium dioxide nanoparticles 
have drawn much attention recently; however, the use of this oxide presents some limitation since it allows to 
obtain high photoresponse and degradation efficiency only under UV light irradiation, that represents the 3 to 
4% of the solar radiation, so preventing its environmental large-scale applications under diffuse daylight. In 
this work the photocatalytic efficiency of titanium-based oxides systems containing alkaline earth metals such 
as barium and strontium, prepared by a simple sol-gel method was investigated, evaluating the degradation of 
methylene blue as model compound under UV and visible light irradiation. The results were compared with 
those obtained with Degussa P25 titanium dioxide.  The achieved degradation percentage of methylene blue 
are very promising showing that under visible light irradiation it is possible to obtain a maximum dye removal 
percentage ~ 50 % higher than that obtained with the Degussa P25.  

1. Introduction 
The abatement of organic pollutants produced by several sectors of chemical industry is nowadays a big issue 
to be solved; different methods to face this problem have been proposed (de Caprariis et al., 2015; de 
Caprariis et al., 2014). Between them photocatalysis is one of the most promising technologies for applications 
in the environmental field. Photocatalysis is widely proposed for water disinfection (Rago et al., 2014), de-
coloration of industrial dyes, air pollutants removal and could represent an alternative to the traditional 
activated carbon treatment of water deriving from processes with high amount of soluble organic compounds 
(de Caprariis et al., 2017). Among this latter the decontamination of polluted water streams containing traces 
of pharmaceutical compounds is a major challenge (Iovino et al., 2016; Iovino et al., 2016). Photocatalytic 
remediation is based on redox reactions of electrons and holes generated from semiconductors by bandgap 
excitation under light irradiation with water, oxygen, and organic (or inorganic) species. The semiconductors 
photoresponse under light irradiation (and the consequent photooxidation process), is then primarily 
dependent on the band gap value and on the energy input from the radiation, which must be sufficient to 
produce energy-rich electron–hole pairs. 
Many efforts have been made by the scientific community for the improvement of the synthesis methods for 
the production of semiconductor oxide nanoparticles due to their unique properties in the photocatalysis. 
Among them, titanium dioxide (TiO2) nanoparticles have drawn much attention recently in the field of 
photocatalysis. Titania, in fact, is a wide bandgap semiconductor with many interesting properties such as, 
high light emission, low production cost, chemical stability, non-toxicity and tailorable electronics properties 
and morphology. These properties have contributed to the use of TiO2 nanoparticles in a variety of 
applications in different industries sectors. For example, TiO2 films grown on various substrates promise to 
have a high commercial potential in the environmental applications such as self-cleaning, anti-bacterial, and 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1760023

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Scarsella M., Bracciale M.P., De Caprariis B., De Filippis P., Petrullo A., Pronti L., Santarelli M.L., 2017, Improved 
photocatalytic properties of doped titanium–based nanometric oxides, Chemical Engineering Transactions, 60, 133-138   
DOI: 10.3303/CET1760023 

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waste water purification containment (Wanga et al.,1998). To extend its applicability, the ease of production 
and reproducibility has to be secured in terms of production cost and product quality. 
One serious limit of this oxide is the wide band gap of its two main forms, corresponding to 3.18 eV and 3.02 
eV for anatase and rutile respectively. Therefore, with pure TiO2 it is possible to obtain a high photoresponse 
and degradation efficiency only under UV light irradiation, that is only 3 to 4% of the solar radiation, so limiting 
its environmental large-scale applications under diffuse daylight. Consequently, different strategies, such as 
metal and non-metal doping and co-doping, sensitizing the TiO2 surface, and their combination by means of 
different synthesis routes are being studied. 
Another important approach is based on the synergistic combination of TiO2 with a different semiconductor 
metal oxide, in the form of mixed oxides composites or as binary/ternary oxides systems (such as perovskites, 
or pyrochlores) that can lead to an improved photocatalytic efficiency under visible light (λ >400nm), 
presenting a narrow bad gap, broadening the band absorbance or modifying the light-mediated surface redox 
reactions. Furthermore, to improve the photocatalytic activity of a material it is fundamental to take into 
account morphological parameters such as specific surface-area, average pore size, pore volume and 
distribution, crystallites average size and the amount of different phases. Accordingly, nanocrystalline 
materials with high surface area is imperative for application with high photocatalytic activity under visible light 
irradiation. In this work the photocatalytic efficiency of titanium-based oxides systems containing bivalent 
metals and prepared by a simple sol-gel method was investigated, evaluating the degradation of methylene 
blue as model compound under UV and visible light irradiation. The obtained results were also correlated with 
the morphological and optical properties of the developed products by X-ray diffraction (XRD), scanning 
electron microscopy (SEM) and diffuse reflectance UV–vis spectroscopy. 

2. Materials and methods 

2.1 Photocatalyst preparation 

Titania-doped nanopowders were produced using a sol-gel method. Two photocatalysts were synthesized Ba-
TiO2 and Sr-TiO2 were the metal load is for both 1 % by weight. The starting materials were Titanium(IV) 
butoxide (reagent grade, 97 %), Strontium nitrate (99 %) and Barium nitrate (99 %), all the products were 
purchased from Sigma-Aldrich. To a solution of the selected metal nitrate in methanol (1.5 mmol/L) kept under 
continuous stirring was added dropwise a solution of titanium butoxide in ethanol (volumetric ratio: 0.25) in 
order to obtain a metal concentration in the titanium doped-oxide of 1 % by weight. The formation of the gel 
phase takes place keeping the precursor solution under continuous stirring at ambient temperature for 24 
hours. Then the obtained gel was dried at 80 °C overnight, grinded and then calcined in air at 350 °C. Three 
different calcination times were tested for the same sample, 3, 6 and 9 hours in order to evaluate the influence 
of this parameter on the TiO2 structure and thus on the photocatalytic activity.  

2.2 Photocatalyst characterization 

The anatase and rutile contents and crystallite sizes of the materials prepared in this study were determined 
via quantitative X-ray diffraction (XRD) using a Philips Analytical PW1830 X-ray diffractometer, equipped with 
Cu Kα (1.54056 Å) radiation, in the 2θ range from 2.5 to 70° with a step size of 0.02° and a time for step of 
1.25 min. The data were collected with an acceleration voltage and applied current of 40 kV and 30 mA, 
respectively. The average crystallite sizes were calculated with the Scherrer’s equation and the fraction of 
phases in the mixture by method reported by Di Paola et al. (Di Paola et al., 2014).  
Diffuse reflectance UV–Vis spectra (UV-vis-DRS) to determine the band gap of the photocatalysts were 
carried out on a dry pressed disk sample using a spectrophotometer (AvaSpec-2048, Avantes) equipped with 
a halogen lamp with a tungsten filament (HL-2000 FHSA, Avantes) as light source. The spectrometer is 
composed by a diffraction grating of 300 lines/mm, blazed at 500 nm, and a CCD linear sensor (2048 pixels). 
The reflectance measurements were collected with a spectral resolution of 0.8 nm between 300 and 1100 nm 
with an integration time of 40 ms and 100 scans. The samples were illuminated at 45° with a bifurcated fiber 
(diameter of 600 μm), connected to the halogen lamp, and the reflected light was collected with a 200 μm 
diameter fiber at 90° respect to the surface (45°/0° geometry). A Spectralon standard (Labshere SRS-99-010, 
99 % reflectance) was taken as reference for the reflectance spectra. 
BET-surface area was calculated from the BET plot acquired using Quantasorb surface area analyzer 
(Quantachrome Corporation) and helium-nitrogen gas mixture. 
High Resolution-Field Emission Scanning Electron Microscopy (HR-FESEM, AURIGA Zeiss) images of the 
most active TiO2 among the synthesized catalysts was recorded at 150 kV.  

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2.3 Photodegradation experiments 

The photocatalytic activity of the catalysts was evaluated by measuring the decrease in concentration of 
methylene blue dye in a water reaction solution under UV and visible irradiation. The performances of the 
prepared catalysts were compared with the performances of Degussa P25 titanium dioxide purchased by 
Sigma-Aldrich (nanopowder with primary particle size of 21 nm and surface area of 35-65 m2/g).  
The experimental tests were performed in pyrex reactors opened to air having a capacity of 100 mL. the 
irradiation was provided by UV (365 nm and irradiancemax = 2.5 W/m

2 at 6 cm to the surface; VL215-L, 
Consort) and visible light Led lamp (400 < λ < 800 nm and irradiancemax = 5.5 W/m

2 at 6 cm to the surface; 
Superstar Classic-A100, Osram). An amount of 50 mL of 13 mg/l of methylene blue aqueous solution was 
kept in constant agitation, insured by magnetic stirrer, with 0.045 g of the prepared catalysts and 0.015 g of 
Degussa TiO2 due to the much higher surface area of this material. Before starting the illumination phase the 
suspensions were stirred continuously for 30 min in the dark in order to evaluate the adsorption of the 
materials. The irradiation time was for all the tests 1 hour. After the irradiation, the catalyst was separated by 
centrifugation from the aqueous solution prior to analysis. 
The concentration of methylene blue dye in the aqueous solution was measured at λmax = 664 nm with an UV-
visible spectrophotometer (Sequoia-Turner, 340) equipped with a quartz cell having a path length of 1 cm. The 
concentration of methylene blue in the solution was determined using a calibration curve of methylene blue 
(concentration vs absorbance) obtained with known concentrations. 

3.  Results and discussion 
The characteristics of catalysts including anatase/rutile phases, crystallite size, surface area and optical band 
gap are summarized in Table 1. 

Table 1:  Catalyst characteristics. 

 UV-vis band gap XRD BET 
 Eg direct 

eV 
Eg indirect

eV 
 Crystallite size

nm 
Phasea 
wt % 

Surface Area 
m2/g 

Ba-TiO2_350 °C -3h 2.45 3.10 A: 16 A: 47.11; Am: 52.87 60.94 

Ba-TiO2_350 °C -6h 2.72 3.13 A: 16 A: 66.96; Am: 33.04 73.36 

Ba-TiO2_350 °C -9h 2.81 3.16 A: 14 A: 97.60; Am: 2.40 77.49 

Sr-TiO2_350 °C -3h 2.91 3.22 A: 13 A: 60.39; Am: 39.61 63.45 

Sr-TiO2_350 °C -6h 2.91 3.22 A: 11 A: 60.68; Am: 39.32 58.40 

Sr-TiO2_350 °C -9h 2.85 3.20 A: 10 A: 63.07; Am: 36.93 56.57 

P25 2.99 3.15 A: 19; R: 28 A: 70.80; R: 17.40; Am: 11.80 52.66 
 a
determined as reported by Di Paola et al. [2]: A, R and Am denote anatase, rutile, and amorphous, respectively. 

 
The average crystallite size of nanopowders (d) was determined from the XRD patterns, according to the 
Scherrer’s equation: 
 
d=kλ/βcosθ                                                                                                                                                         (1)
  
where k is a constant (shape factor, about 0.9), λ the X-ray wavelength (0.154 nm), β the full width at half 
maximum (FWHM) of the most intense diffraction peak and θ is the diffraction angle. The values of β and θ of 
anatase and rutile phases were taken from anatase (101) and rutile (110) planes diffraction lines, respectively 
(Figure 1) (Alijani et al., 2017).  
Figure 1 reports the X-ray diffraction patterns of the catalysts synthesized in this work. All the catalysts 
samples consisted of only anatase, whereas Degussa P25 consisted of both anatase and rutile. Phases 
related to Ba and Sr were not significantly detected in any of the samples due to their low concentration levels 
and hence their doping did not result in any significant change in the phase crystallinity. Ba and Sr did not 
show any phase changes suggesting that aggregates might have been formed on the crystal borders and on 

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the surface of the photocatalyst, thus promoting visible light absorption, as discussed earlier (Naraginti et al., 
2015). As shown in Table 1, the anatase crystallite size decrease with calcination time for all catalyst. 
In order to evaluate the crystallinity of the samples XRD diffractograms were recorded for mixtures of TiO2 and 
CaF2 (50 wt %). By determining the ratio between the areas of anatase (101) or rutile and CaF2 (220) peaks 
the amount of crystalline and amorphous phases present in the samples was determined (Di Paola et al., 
2014). 
Degussa P25 is close to being fully crystalline consisting of ~ 71 % anatase and ~ 17 % rutile with the amount 
of amorphous part in agreement with the value reported by Di Paola et al. (Di Paola et al., 2014). Among the 
synthesized samples, a higher crystallinity percentage (~ 98 %) was found for the sample Ba-TiO2 calcined at 
350 °C for 9 hours whereas only 47 % was the crystallinity achieved for Ba-TiO2 warmed at the same 
temperature for 3 hours. The Sr-TiO2 series show a constant crystallinity around 60 % with a slight variation 
due to the calcination time. 
The bandgap energy (Eg) of semiconductors can be evaluated from the diffuse reflectance (UV-vis-DRS) 
spectrum. It is established that TiO2 has direct and indirect band gaps (Janitabar-Darzi et al., 2009). The two 
different kinds of transitions can be distinguished by their energy dependence of the optical absorption 
coefficient (α) near the absorption edge. The optical gap value is estimated relying on the Kubelka–Munk 
method combined with the Tauc relation (Alijani et al., 2017). 
  
[(αhν) = B (hν-Eg)n]                                                                                                                                            (2) 
 
 

Figure 1: Sx: XRD patterns of the used photocatalysts; Dx: Indirect and direct band gap of catalysts. 

where B is a constant, α is absorption derived by the remission function of Kubelka-Munk F(R∞) (α = k(λ)/s(λ) 
= (1- R∞)2/2R∞. Here, R∞ is defined as R∞ = Rsample/Rreference with Rreference, the diffuse reflectance measured 
for the Spectralon standard), Eg is the average band gap of the material and n depends on the type of 
transition (n = 1/2 for direct transmission and n = 2 for indirect transmission) (Lopez et al., 2012), h is the 
Plank’s constant (6.626 10-34 J s), and ν is the frequency of photons. The direct and indirect average bandgap 
transition energies were estimated by the intercepts at α = 0 of the linear portion of the (αhν)2 or (αhν)1/2 
versus hν of plots, respectively, as shown in Figure 1. 

It was observed that the doping of TiO2 with alkaline earth metals (Ba and Sr) resulted in a decrease in the 
band energy (see also Table 1) and an increase in the wavelength (red shift). These results indicate that Sr 

 

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and Ba doped TiO2 nanoparticles have greater possibility to exhibit a higher photocatalytic activity in the 
visible region. 
 

Ba-TiO2_350C-6h Sr-TiO2_350C-3h 
  

Figure 2: FESEM images of the catalysts. 

A representative SEM images in Figure 2 shows an uneven distribution of agglomerated particles with 
spherical shape and an average size in agreement with those determined from XRD.  
The results of the methylene blue removal are reported in Table 2 for UV and visible light irradiation, the 
obtained values are compared with the results obtained by the Degussa P25 in the same conditions. The 
removal percentage was calculated as: 
 
% removal= (Cin - Cout)/Cin x 100   (3) 
 
where Cin and Cout are the concentration calculated by the adsorption data measured with the 
spectrophotometer at the beginning and the end of the tests. All the powders were active for the 
photodegradation of the substrate under UV light although the efficiency of P25 was still higher. 

Table 2: Photocatalytic activity on % degradation of methylene blue dye solution in the presence of UV and 
visible light 

 
UV 
% 

Visible 
% 

Ba-TiO2_350C-3h 70.10 9.67 
Ba-TiO2_350C-6h 86.25 14.60 
Ba-TiO2_350C-9h 91.12 5.88 
Sr-TiO2_350C-3h 97.56 12.90 
Sr-TiO2_350C-6h 92.62 9.78 
Sr-TiO2_350C-9h 90.48 9.70 

P25 98.90 9.91 

 
Under visible light irradiation, the prepared Ba and Sr doped TiO2 calcined at 350 °C for 6 and 3 hours 
respectively, exhibit higher degradation efficiency as compared to Degussa P25. This can be due to the 
reduced bandgap and to the prevention of electron–hole recombination owing to TiO2 doping. Indeed, as 
previously reported, alkaline earth metal ions (Sr and Ba) doping in TiO2 acts as an electron trap and leads to 
suppression of electron and hole recombination (Sood et al., 2015). 
It is known that the photocatalytic activity of doped TiO2 generally increases with higher crystallization degree, 
lower crystallite size and lower band gap. The combination of these parameters gives to the Ba-TiO2 350C-6h 
the best performances under visible light irradiation. 

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4. Conclusions 
TiO2 nanoparticles doped with alkaline earth metal ions (Sr and Ba) have been synthesized by a simple sol-
gel method and were calcined at 350 °C in air for 1, 3 and 9 h. The TiO2 nanopowders obtained consist of 
anatase and the size of the crystallites, depending on the calcination time, is always smaller than that of 
Degussa P25. The prepared catalysts show excellent crystalline, morphological and optical properties.  
The sunlight assisted photocatalytic experiments have been performed for the degradation of methylene blue 
dye. Photo-degradation of about 15 % within 60 min was observed for Barium doped TiO2 calcined at 350 °C 
for 6 hours due to its band-gap, crystallinity and particle size, all contributing to the enhanced photocatalytic 
activity. The obtained results are an important first step for the development of alkaline earth metal doped TiO2 
nanocatalysts to be used in the photocatalytic degradation of organic contaminants present in wastewater 
under solar light irradiation. 

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