2.2. Photocatalytic Decomposition of Acetaldehyde on N-TiO2 Samples
In Figure 2
, photocatalytic decomposition of acetaldehyde under fluorescent lamp irradiation is presented. This light contained visible light with a share of UV-A. The rate of acetaldehyde decomposition was dependent on the flow rate of acetaldehyde gas through the reactor; the slower the flow rate the higher the decomposition. The selected speed of gas flow was 5 mL/min.
All the N-doped TiO2
samples showed lower activity towards acetaldehyde decomposition by comparison to undoped TiO2
. Among all the prepared N-TiO2
, the one calcinated at 300 °C was the most active. Both reference TiO2
samples showed the same degree of acetaldehyde mineralisation, however, Reference 1 exposed higher conversion of acetaldehyde than Reference 2. Taking into account that the total decomposition of acetaldehyde in the concentration of 300 ppm gives 600 ppm of CO2
, it was calculated that the amount of acetaldehyde decomposition towards CO2
was equal to 77, 57, 37 and 24% for reference samples N-T300, N-T400, and N-T500, respectively. Although the fluorescent lamp contains a large region in the visible spectrum, the presence even of a small part of UV light caused TiO2
samples which did not have any significant visible light activity to be very active. The performed blank test of acetaldehyde decomposition in an empty reactor irradiated by a fluorescent lamp showed that 8.5% of acetaldehyde gas was totally decomposed. This test showed clearly that UV activity of TiO2
was much more powerful than the visible one. For comparison, activity of the prepared N-TiO2
and reference samples towards acetaldehyde decomposition under UV light irradiation was tested and the results are illustrated in Figure 3
From Figure 3
a it can be seen that samples prepared at 300 and 400 °C were the most active, whereas that prepared at 200 °C, which was not totally crystallized due to the low temperature of calcination, showed the lowest activity. Acetaldehyde decomposition on samples N-T300 and N-T400 was repeated but for the reduced surface (3 plates—12 cm2
) in order to determine which one was more active. The reference samples were also tested using three plates of sample coatings. The results are illustrated in Figure 3
c. This experiment showed clearly that the sample prepared at 300 °C was more active under UV light than that obtained at 400 °C. The Reference 2 sample had a comparable activity with N-T300, but both reference samples were more active than N-T400.
formation during acetaldehyde decomposition is illustrated in Figure 3
b,d. It can be seen that the amount of CO2
at the beginning of irradiation was increasing and then stabilized. This phenomenon was observed especially in the samples which showed the highest decomposition rate of acetaldehyde. It is assumed that the excess of formed CO2
at the beginning of the process was caused by the decomposition of preliminary adsorbed acetaldehyde (due to the heating of the UV lamp). So, it is deduced that samples which revealed the formation of a high quantity of CO2
at the beginning were good adsorbents for acetaldehyde. High BET surface area and high affinity of photocatalyst surface to adsorption of acetaldehyde could be the main factors determinating their photocatalytic performance.
2.3. Characteristics of N-TiO2 Photocatalysts
In Figure 4
, XRD patterns of N-TiO2
and reference samples were illustrated.
XRD analyses showed that the N-TiO2
sample calcinated at 200 °C was amorphous and those that were heat-treated at 300–500 °C had a single anatase phase. Crystallinity was increasing with increased temperature of calcination. The Reference 1 sample was a single anatase phase, whereas Reference 2 consisted of anatase and brookite. The reflex of brookite was visible at the 2 θ of around 30°. Both reference samples were obtained by a sol-gel preparation and further calcination at 400 °C, however, the N-T400 sample revealed higher crystallinity than the reference ones. The phase composition and crystallites size calculated from Scherrer equation are introduced in Table 1
together with other parameters of samples such as BET surface area and zeta potential.
All these samples showed acidic character of surface. The lowest value of negative potential zeta was noticed for amorphous TiO2 and sample doped with nitrogen, calcinated at 400 °C. Anatase crystallites were increasing with increase temperature of calcination. The lowest crystallites size of anatase had Reference 1 sample. The highest BET surface area among crystallised TiO2 was noticed for N-T300.
The activity of N-TiO2
samples under visible and UV light and the lifetime of charges after excitation with laser was measured by TRMC (Time Resolved Microwave Conductivity). For laser excitation, two wavelengths were selected, λ = 460 and 360 nm; the results are presented in Figure 5
Sample N-T200 was not included, because it was amorphous and did not show any TRMC profile. Some parameters such as maximal intensity of TRMC signal (Imax
), intensity after 40 ns (I40
) and I40
ratio are introduced in Table 2
The short-range decay was set to 40 ns after the maximum of the pulse was reached, and is expressed as the I40
ratio, which represents the fast processes occurring during and just after the pulse. The fast decay in this short range is caused mostly by an electron-hole recombination and scavenging of electrons, which can also occur. Similar analyses of TRMC signals were performed and published elsewhere [17
]. It can be observed, that under laser excitation at λ = 360 nm, N-TiO2
prepared at 500 °C showed the highest TRMC signal and at the same time the lowest one under excitation at λ = 460 nm. It can be deduced that this sample can have high photocatalytic activity under UV and low activity under visible light irradiation, by comparison with the others. TRMC short range decay (I40
) had the highest value for the N-T500 sample which means that this represents the longest lifetime of free charges, caused probably by its high size of anatase crystallites. Sample prepared at 300 °C showed longer lifetime of free charges than for the heat-treated one (at 400 °C) and also the highest TRMC signal under visible light excitation. This means that this sample (N-T300) can exhibit the highest activity under visible light among all the other samples and its activity under UV light can be higher than for N-T400.
shows the EPR spectra at T = 4 K performed for N-TiO2
nanocomposites obtained with different thermal treatment processes.
For samples heat treated at 200 and 300 °C, some of the resonance lines centered at g = 1.985, g = 2.003, g = 2.023 are seen. They can be related to the paramagnetic nitrogen centers (g = 2.003) and oxygen vacancies (Ti3+
in hydrated titania), g = 2.023 and g = 1.985. They are more intensive in N-T300 than in N-T200. In the sample calcinated at 400 °C, there are still signals from paramagnetic nitrogen and oxygen vacancies, however, less intensive than in the sample prepared at 300 °C and additional resonance lines appear at g = 2.02, g = 2.001, g = 1.98 and g = 1.93. Sample N-T500 showed resonance lines at g = 2.02, g = 2.001, g = 1.98 and g = 1.93, but these were less intensive than sample N-T400. At higher temperatures of calcination, oxygen vacancies disappear. Samples prepared at 500 °C showed less defected structure than the others. The presence of oxygen vacancies for primary particle sizes below ca. 20 nm is typical and have been already reported in the literature [9
]. High intensity of resonance lines related to paramagnetic nitrogen in N-T300 sample can explain why this sample generated the highest amount of free charges under visible light. Meroni et al. [2
] noticed the highest activity of N-doped sample under solar light irradiation, which showed the largest amount of paramagnetic nitrogen species.
The amounts of nitrogen and carbon in the bulk titania samples were measured in CN628 elemental analyzer; the results are listed in Table 3
The sample prepared at 200 °C showed the highest quantity of both, nitrogen and carbon. For higher temperature of calcination, lower amounts of nitrogen and carbon were present in the samples. In N-T500 sample, both nitrogen and carbon were out of the detection limit. The sample calcinated at 200 °C showed higher content of nitrogen, but lower EPR signal related to the paramagnetic nitrogen. It is suggested that in this sample, some of the nitrogen species were physically adsorbed on the titania surface. The reference samples did not contain any carbon and nitrogen.
FTIR measurements were performed to determine the presence of functional groups on the nitrogen-modified TiO2
surface. In Figure 7
, FTIR spectra of the prepared N-TiO2
samples are presented.
All the FTIR spectra showed characteristic bands for OH groups at: 1630–1620 cm−1
, 3700–2600 cm−1
, and 3700 cm−1
. The broad band at 3700–2600 cm−1
is due to the stretching vibrations of adsorbed water and hydrogen-bonded hydroxyl groups. The broad band at 3700–2600 cm−1
is assigned to the stretching vibrations of adsorbed water and hydrogen-bonded hydroxyl groups. The band at 1630–1620 cm−1
is characteristic of the bending mode of adsorbed water on titania surface and the band at 3700 cm−1
is assigned to hydroxyl groups bound to the surface of one Ti atom [19
]. OH bands were significantly reduced upon calcination at higher temperatures due to the dehydratation process. The broad and high intensive band at 1450 cm−1
is visible for sample prepared at 200 °C and can be assigned to vibrations of the Ti-N bond [20
]. The intensity of this band was reduced after calcination at 300 °C and disappeared in samples heat-treated at higher temperatures. The band at 1532 cm−1
is assigned to the stretching vibrations of titanium carboxylate. Carboxylate groups on the TiO2
surface could be formed from ethanol and TIP used during titania preparation.
The presence of nitrogen groups in TiO2
can give response of titania in the visible region, therefore, some of UV-Vis/DR measurements were performed and recorded spectra are illustrated in Figure 8
Samples prepared at 200 °C showed absorption in all the range of visible light, but samples heat-treated at 300–500 °C showed significant absorption at the range of around 400–520 nm, which was attributed to the building of nitrogen to the titania lattice. It is worth noting that the N-T300 sample, which revealed the highest intensity of resonance line related to the paramagnetic nitrogen, showed also the highest intensity of absorption peak at the range of 400–520 nm. In order to determine the energy of the band gap, the function of Kubelka–Munk was applied for semiconductors with indirect allowed transition. This method was described in detail elsewhere [21
]. The plots of Kubelka–Munk transformation versus Eg
samples together with calculated Eg
values are introduced in Figure 9
For sample N-T500, one value of Eg was determined. Sample N-T200, which consisted of small size particles had a higher value of Eg than the other samples, which is typical in case of nanoparticles. For samples N-T200, N-T300 and N-T400, two values of Eg were calculated, one in the UV and the second in the visible region. This suggests formation of a midgap inside of the Eg of titania sample.
XPS measurements were performed to identify the structure of nitrogen doped to TiO2
. XPS spectra of the prepared N-TiO2
samples are presented in Figure 10
and the elemental composition of the surface in Table 4
The restrictions for parameters of the Ti 2p fitting model were set [22
]. The energy difference of Ti3+
signals was 1.5 eV. The intensity ratio of the doublet components i.e., Ti 2 p3/2
and Ti 2p1/2
was 2:1, respectively. The separation of Ti 2p doublet signal was set to be 5.73 eV. The transitions of the same kind were forced to have the same full width at half maximum (FWHM) e.g., Ti 2p3/2
had the same FWHM for Ti4+
but different for Ti 2p1/2
. The sample charge was calibrated according to Ti4+
transition at 458.6 eV. The O 1s signal was deconvoluted for two components at 530.3 eV and at about 532.0 eV. These components were attributed to titania phase and OH groups on titania surface, respectively [23
]. The FWHM of oxygen components was forced to be the same due to the same kind of transition. In Figure 10
a, the Ti 2p XPS spectrum of the sample obtained at 300 °C is presented. The deconvolution of the spectrum enables the determination of Ti4+
contents which are given in Table 5
In all samples, the content of Ti3+ is below 2% of all titanium atoms, therefore, all Ti 2p spectra look very similar. The same situation is in case of O 1s signals. This is why only one spectrum of the most photocatalytically active sample is presented.
In case of N 1s signal, it can be observed that surface nitrogen content is highest for the N-T200 sample (Table 4
). The concentration values are consistent with works of other researchers where the nitrogen concentration is in the range of 0.15–2.0 at.% and decreases with increase of temperature of preparation [24
]. In our experiments, the nitrogen concentration also decreases at higher temperatures which can be explained by thermal desorption of nitrogen species. It is consistent with elemental analysis. Interestingly, the sample prepared at 400 °C has higher surface nitrogen concentration compared to these obtained at 300 °C and 500 °C. Note that XPS delivers information about surface concentration. Therefore, a higher nitrogen concentration at 400 °C compared to that at 300 °C may be a result of thermal segregation of nitrogen from the bulk to the surface. At 500 °C, the bulk of the material is depleted from nitrogen as confirmed by elemental analysis (Table 3
) which correlates with decrease of nitrogen surface concentration. Summarizing, there are two processeses, i.e., segregation of nitrogen from the bulk to the surface followed by desorption of nitrogen species. Depending on the rate of these processes, which in turn are dependent on temperature, there is difference in the surface concentration of nitrogen.
Interestingly, the N 1s signal consists of two components at least. The one located at 400.2 eV is attributed to bulk nitrogen located in interstitial positions, thus, commonly named interstitial nitrogen [24
]. The literature data gives the binding energy of such nitrogen at about 400 eV [24
]. Nitrogen may also substitute oxygen in the lattice. However, in such position, the binding energy is lower, i.e., 396 eV [24
]. In our measurements, we have not detected such nitrogen. There is lot of work indicating that interstitial nitrogen enhances the photocatalytic activity in visible light while the substitutional does not [24
Nitrogen may be present also over the very surface as atomic nitrogen or in the form of a compound, e.g., NHx
]. These species exhibit banding energies of 400, 407.3 and 401.7 eV, respectively [25
]. In our measurements, the N 1s signal was not symmetric and deconvolution reveals the presence of 402.0 eV component. We attribute this component to either atomic nitrogen over the titania surface or the species like NO or even N-C. Interestingly, in studies [27
] and [28
], the 400 eV N 1s signal is also asymmetric, however, the presence of higher binding energy component is not commented upon. In [26
], the position of N 1s signal is 401 eV. Thermal treatment leads to significant narrowing of the signal FWHM (Full Width at Half Maximum). This indicates that in [26
], there may be both 400 and 402 eV components interpreted erroneously as 401 eV signal. The literature data does not allow to uniquely determine the chemical nature of 402 eV nitrogen. Especially the oversimplification of the N-TiO2
systems is observed, i.e., small numbers of possible bulk and surface species are assumed. However, the sample of the highest photocatalytic activity exhibited the lowest surface concentration of the 402.0 eV nitrogen.
shows ERDT (Energy-Resolved Distribution of electron Traps)/CBB (Conduction Band Bottom) patterns of N-TiO2
samples prepared at 300 and 400 °C.
The CBB positions, reflecting the bulk (crystalline) structure, was slightly different; CBB positions of N-T300 and N-T400 were ca. 3.2 and 3.1 eV, respectively. The lower value of CBB for the latter sample could be caused by the formation of reduced titania below the conductive band. ERDT patterns of N-T400 contained deep (low energy) ETs at 2.7–2.8 eV in addition to the shallow (high energy = around CBB) ETs, while N-T300 did only shallow ETs. Although it is still speculative and there is no direct evidence obtained until now, shallow and deep ETs are beneficial and detrimental, respectively, due to promoted transfer of photoexcited electrons and enhanced recombination of an electron and a positive hole [29
]. Higher temperature calcination at 400°C might lead to formation of deep ETs leading to slight reduction of photocatalytic activity.