Study of the photoactivation of titania Degussa P25 in ethanol–methanol suspensions using a piezoelectric sensor
Graphical abstract
Photoactivation of TiO2 Degussa P25 in ethanol–methanol suspensions was monitored in situ using a piezoelectric sensor. The process was studied in a specially designed cell in which the sample (in N2 flux) was illuminated by a continuous UV Hg lamp to induce photoactivation. It was found that the amplitude, the initial time and the velocity of activation signal are related to ethanol–methanol mixture ratio. The associated mechanisms are discussed.
Introduction
Titanium oxide (TiO2) is a promising material for photoelectrochemical energy production [1] and photocatalytic applications for oxidative degradation of environmental organic pollutants [2], [3], [4], [5], for remediation processes such as water and air purification [6], [7], [8], [9], [10], for prevention of stains and sterilization [11], [12], [13]. Titania have several advantages as a catalyst, it is an inexpensive material, commercially available in various crystalline forms, non-toxic and photochemically stable. Many approaches to improve titania photocatalytic activity have been tested, and many researchers have pointed out that it is dependent on its phase structure, morphology, crystallinity and surface area [14], [15].
Among the three crystalline phases (anatase, rutile and brookite) only anatase and rutile are catalytically active, with a semiconductor band gap located about 3.2 and 3.0 eV, respectively. Many studies have confirmed that anatase presents better photocatalytic properties due to the low recombination rate of its photogenerated electron–holes [15], [16]. Moreover, it has been found that the mixture of the two phases of titania was more beneficial for suppressing the recombination of photogenerated electrons and holes, and thus enhance photocatalytic activity [17], [18], [19], [20], [21]. The recombination process and the electron–hole formation are favored when OH groups increase.
Most of the literature reports on titania photoactivity mechanisms are related to n-type electrical properties. On the contrary, defect chemistry models are essentially based on the formation of oxygen vacancies and are considered to play an important role in extending the photoactivity of titania into the visible region of the solar spectrum [9], [22]. It is also known, that titanium vacancies contribute to p-type conductivity of dense polycrystalline rutile [23], [24]. Also, theoretical analysis, as first principles quantum mechanical calculations, has been applied to determine the formation energies of different native defects in anatase, as oxygen and titanium vacancies and intersticials [25]. The results showed that titanium vacancies had the lowest formation energies under oxygen-rich conditions. Furthermore, experimental evidence for titanium vacancies in TiO2 sol–gel, determined by Rietveld refinement of powder X-ray diffraction data, proposed that titanium vacancies are compensated for hydroxyl ions incorporated into the structure [26].
Degussa P25, a commercial powder, has been used frequently as a benchmark in titania photocatalysis studies. It has a relatively large surface area, about 53.2 m2/g and an average crystallite size of 25 nm. This powder consists of a mixture of anatase and rutile, with a ratio of 3–4:1 [27], and recent morphology studies by Ohno et al. showed that the particles of these polymorphs exist separately [20]. These authors have also reported that the presence of both phases is important in some photocatalytic reactions where oxygen is used as an electron acceptor [21].
Generally, titania photoactivation has been studied indirectly by exposing different organic compounds and monitoring their degradation rate. Few works have studied the photoreactivity of pure TiO2 powder by UV–vis, in a slurry-like phenol system [28], water [29] or ethanol [30].
In this work, a new technique is presented to study in situ the photoactivation process of alcohol–titania suspensions, using a piezoelectric sensor [31]. It consists on the simultaneous illumination of the liquid sample by a non-modulated UV lamp and a modulated red laser beam. The UV light induces the photoactivation process changing the optical properties of the sample, which are monitored by a red laser beam light reaching a piezoelectric sensor positioned at the bottom of the container with the suspension. The evolution of the dynamic processes occurring in the sample can be followed with high precision using this procedure. It was shown that the proportion of ethanol-to-methanol mixtures in titania suspensions affects strongly the dynamics of photoactivation.
Section snippets
Materials
Commercial titanium oxide in powered form, Degussa P25, was used. The solutions were prepared in plastic containers by mixing 0.04 g of TiO2 in 50 mL of solvent. Pure ethanol and methanol, and mixtures in the following molar percentage ratios 17–83, 33–67, 59–41, 82–18, 91–9 (MeOH–EtOH), were studied. After preparation, the solutions were homogenized in an ultrasonic bath during 30 min to obtain a suspension, and kept in a 60 mL plastic container closed with a plastic cap.
Cell design and measurement technique
The experimental
Results and discussion
The piezoelectric amplitude as a function of time, for pure alcohols and mixtures of different concentrations of ethanol–methanol suspensions, are shown in Fig. 3. As can be seen, the general behavior for all samples is very similar; however, the amplitude fall-off and the initial activation time are different.
In order to measure the piezoelectric signal fall-off (S) during stage 3, a sigmoidal fitting was performed [32] using the following equation in function time, t:where A1
Conclusions
The photoactivation of titanium oxide Degussa P25 in ethanol–methanol suspensions, with different molar ratios, was monitored at real time using a piezoelectric sensor. The photoactivation process generates a gradual decrement in the piezoelectric signal until it remains constant. In contrast, deactivation is observed as a gradual increase reaching its stabilization after a time interval. The analysis of the experimental data of the activation kinetics provides the parameters corresponding to
Acknowledgements
The authors wish to thank M.C. J. Bante and M.C. D.H. Aguilar for technical assistance.
References (40)
- et al.
Sol. Energy
(2005) - et al.
Catal. Today
(1999) - et al.
Appl. Catal. B
(2007) - et al.
Catal. Commun.
(2007) - et al.
Catal. Today
(2007) - et al.
Chem. Eng. J.
(2007) - et al.
Appl. Catal. B
(2007) - et al.
Appl. Catal. B
(2007) - et al.
J. Mol. Catal. A
(2000) - et al.
Water Res.
(2006)
Appl. Catal. B
J. Catal.
Catal. Today
Catal. Today
J. Catal.
J. Phys. Chem. Solids
Appl. Catal. A
J. Solid State Chem.
Nanostruct. Mater.
J. Non-Cryst. Sol.
Cited by (5)
Fully quantitative X-ray characterisation of Evonik Aeroxide TiO <inf>2</inf> P25<sup>®</sup>
2014, Materials LettersCitation Excerpt :Amongst the wide number of commercially available photocatalytic titanias, Evonik Aeroxide TiO2 P25® (formerly known as Degussa P25®, and hereafter referred to as P25), synthesised via flame pyrolysis of TiCl4, is widely used because of its high photocatalytic activity in many reaction systems, thus becoming a standard for photocatalytic reactions. From a mineralogical point of view, it is made of anatase and rutile phases, their ratio being typically reported as 70:30 or 80:20 [3–11]. Some authors have also reported that P25 contains some amorphous phase, from observations from TEM analysis [12,13].
Influence of Phase Structure of TiO2 Nanoparticles on Resistive Switching Devices
2024, Current Applied Science and TechnologySimple laboratory-made piezoelectric sensors for detection of selected gaseous and/or vapor sample
2016, Chiang Mai Journal of ScienceHeterostructures Based on TiO<inf>2</inf> and Silicon for Solar Hydrogen Generation
2015, Advanced Functional MaterialsDevelopment of UV-LED/TiO<inf>2</inf> device and their application for photocatalytic degradation of methylene blue
2013, Journal of Materials Engineering and Performance