PLD conditions and bandgap values of TiO2-based films.
Active under visible light, photocatalysts based on doped titania were obtained via pulsed laser deposition (PLD) method. To find out the crystalline structure, optical properties, and electronic structure, the following techniques such as X-ray diffraction, electronic spectroscopy, electrical conductivity measurements, and X-ray photoelectron spectroscopy (XPS) are used. Photocatalytic activity is monitored by applying the photoreduction of dichromate ions under UV and visible light. The influence of zirconium ions and its content and synthesis conditions on the efficiency of nitrogen incorporation into titania structure that, in turn, determines the electronic structure and photocatalytic ability of the semiconductive materials are discussed. A substitutional nitrogen (Ti–N) rather than an interstitial one (Ti–O–N) is mainly responsible for the observed photoactivity. It is pointed that substitutional nitrogen is responsible for bandgap narrowing or formation of intragap localized states within semiconductor bandgap. The bandgap energy values are sharply decreased, while the relative intensity of substitutional nitrogen XPS peaks is increased. Pulsed laser synthesis of TiO2 films in N2/CH4 atmosphere not only leads to nitrogen incorporation but also to the formation of defects including oxygen vacancies and Ti3+ states which are all contributing to light absorption. An appropriate ratio of gas mixture, optimum zirconia content, suitable pressure, and temperature during synthesis was found for the synthesis of highly active semiconductive films. The highest photocatalytic conversion yields are obtained for nitrogen-doped 10% ZrO2/TiO2 synthesized in N2:CH4 = 5:1 at 100 Pa and at 450°C under both UV and visible light.
- nitrogen-doped titania
- pulsed laser deposition
- substitutional and interstitial nitrogen
- X-ray photoelectron spectroscopy
Among the ways leading to the intensive climate change, the widespread contamination of air, soil, and water sources are to be considered. Titania is recognized as the most prospective photocatalyst used in air purification, surface self-cleaning, and wastewater treatment [1–7]. Since solar radiation includes light of wavelengths from 280 to 4000 nm, the use of titania that absorbs the edge (3–5%) of solar energy is not efficient. Many studies are performed to design and develop photocatalysts based on titania to allow for their use under visible-light irradiation [8, 9]. Nitrogen, the most promising among nonmetal dopants, facilitates visible light absorption by nanomaterials based on TiO2 as a result of its incorporation into titania matrix due to its comparable atomic size with oxygen, small ionization energy, and high stability. Another approach to change the physical, optical, structural, and photocatalytic properties of titania includes employment of d-block metal ions (
An incorporation of zirconium ions in TiO2 lattice was reported to enhance the specific surface area and the surface acidity and to modify the photo-electrochemical properties, leading to the improvement of the photocatalytic activity . Ti1−
Since Sato  reported the visible-light activity of N-doped titania in the oxidation of carbon monoxide and ethane, and later, Asahi  explored the visible-light activity of N-doped TiO2, there have been many efforts dealing with nitrogen doping of TiO2. Numerous physical techniques such as sputtering [19, 20] and ion implantation , atomic layer deposition , pulse laser deposition , gas phase reaction , and the sol–gel method  have been successfully applied to obtain nitrogen-modified TiO2.
It is reported  that the doping of nonmetals could narrow the bandgap and might drive the response to visible light due to the similar energies of oxygen and nitrogen resulting in the hybridization of N 2p with O 2p states in anatase TiO2. On the other hand, it is stated  that isolated impurity energy levels above the valence band are formed as a result of oxygen atoms substitution by nitrogen ones. Irradiation with UV light excites electrons in both the valence band (VB) and the impurity energy levels, but illumination with visible light only excites electrons in the impurity energy level. An alternative point of view, where the visible-light activation of anion-doped TiO2 originates from the defects associated with oxygen vacancies, was also reported. As a result, the color centers appeared displaying these absorption bands, and not to a narrowing of the bandgap of TiO2 . Whereas surface oxygen vacancies were reduced considerably after reheating at 400°C, the activity of N-doped TiO2 was enhanced fourfold suggesting no oxygen vacancies influence on visible-light activity . In addition, nitrogen-modified titania powder obtained by mixing a commercially manufactured titania powder and the different nitrogen containing organic substances were applied for photomineralization of formic acid under visible light. It is concluded that neither nitridic nor NOx species nor defect states are responsible for the visible-light photocatalytic activity of modified titania prepared from urea, but higher melamine condensation products acting as visible-light sensitizers .
The main technique proving the nitrogen incorporation into a semiconductor structure as substitutional nitrogen atom (Ns), where nitrogen substitutes oxygen lattice ions or interstitial one (Ni), and N binds to O lattice ions, is X-ray photoelectron spectroscopy (XPS). There is no definite opinion about the XPS measurements of N1s binding energy (BE). The peak at 395.7 eV is assignable to N3− bonded to three Ti atoms because the measured binding energy is very close to that of titanium nitrides (ca. 396.0 eV) [29, 30]; the values of 396–397 eV are assigned to the N–Ti–N  or O–Ti–N bonds . The formation of N–N or N–C, N–O groups or chemisorbed dinitrogen was suggested for higher energies of (400–402 eV) . Note that N1s binding energies of 399.1 and 400.5 eV measured for modified by urea commercially available TiO2 powder were assigned to carbon nitrides (399–400 eV, C=N–C), graphite-like phases (400.6 eV, N-Csp2), and to polycyanogen (399.0, 400.5 eV (–C=N–)x) . XPS and electron paramagnetic resonance (EPR) measurements showed that the interstitial nitrogen atoms corresponded to BE of 400–401 eV are the photoactive species and can be prepared from discharge in molecular nitrogen in the presence of pure anatase . However, the intensity of 400 eV peak belonged to the nitrogen atom from ammonium ions is reduced after rinsing as provided by XPS and solid-state nuclear magnetic resonance (NMR) .
The geometric structures of diamagnetic (Scheme 1
In light of numerous literature investigations, the applied synthesis conditions and the source of nitrogen are the turning points for the level of nitrogen doping and its states that change optical, electronic, and photocatalytic properties of a semiconductor.
The main principles of heterogeneous photocatalysis are presented as follows (Figure 1): (i) A semiconductor is characterized by an electronic band structure in which the highest occupied energy valence band (VB) and the lowest empty conduction band (CB) are separated by bandgap; (ii) only a photon of energy higher or equal to the bandgap energy (≥3.2 eV for anatase) promotes to a charge separation resulting in an electron (e−) transferring from the VB to the CB with simultaneous generation of a hole (h+) in the VB (pathway 1); (iii) an electron and a hole can be trapped in surface states and react with donor (D) and acceptor (A) species, respectively, adsorbed or close to the surface of the particle resulting in the simultaneous oxidation and reduction (pathways 1.1 and 1.2); or (iv) a photoformed electron–hole pair can be recombined (pathway 2) on the surface or in the bulk of the particle in a few nanoseconds. The energy level at the bottom of the CB is actually the reduction potential of photoelectrons and the energy level at the top of the VB determines the oxidizing ability of photoformed holes. The flat-band potential presents the energy of both charge carriers at the semiconductor–electrolyte interface and depends on the nature of the material and the system equilibrium.
From a thermodynamic point of view, the trapped electron reduces an electron acceptor only if its redox potential is more positive than the flat-band potential of the CB, while the trapped hole oxidizes the adsorbed species having the redox potential more negative than the flat-band potential of the VB. The visible-light photoactivity of metal-ions-doped TiO2 can be explained by a new energy level produced in the bandgap where an electron can be excited from the defect state to the TiO2 conduction band by photon with the lower energy (pathway 3). Inhibition of charge carrier recombination rate resulting in enhanced photoactivity is attributed to the transition metal ions doping of TiO2. Visible-light illumination can generate a hole in the mid-gap level above the top of the VB for the N-doped TiO2 (pathway 4).
The nitrogen-modified TiO2 thin films exhibited photocurrents in the visible up to 700 nm as a result of occupied nitrogen-centered surface states above the valence band edge as noted in Ref. . Moreover, the observed photocurrent under visible-light irradiation of N-doped TiO2 in the presence of reducing agent strongly depend on the reaction mechanism of oxidation rather than on the redox potential of the whole acceptors . Photo-electrochemical measurements showed that the visible-light photogenerated holes in TiO2 modified by urea are able to oxidize formic acid comparing with ammonia modified one. An enhanced stabilization of the photogenerated hole against recombination with conduction band electrons for the former sample is explained . In addition to the electrochemical investigations, preliminary computer simulations based on innovative cluster approaches demonstrated that nitrogen doping narrowed the optical bandgap of TiO2 generating N(2p) states . Density functional theory (DFT) was employed to demonstrate the interstitial nitrogen character within anatase TiO2. No significant shift in the CB or VB of the TiO2 was recognized. The energy bonding states associated below the valence band and anti-bonding states present above the valence band. The anti-bonding nitrogen orbital between the TiO2 VB and CB is suggested to be responsible for the visible light absorption by acting as a stepping stone for excited electrons between conduction and valence bands .
Our investigation was directed to the synthesis of nitrogen-doped photocatalysts based on titania and/or mixed semiconductive oxides films and understanding the relationship between synthesis conditions, electronic structures, and photocatalytic activity. The influence of zirconium ions and its content as well as pulsed laser synthesis conditions on the efficiency of nitrogen incorporation into titania structure that, in turn, determines the photocatalytic ability of the semiconductive materials are obtained.
2. Correlation of electronic structure, optical, structural, and photocatalytic properties of TiO2-doped films
Pulsed laser deposition method used for the film synthesis allows obtaining high purity thin films of single or multicomponent materials .
The PLD presents a series of advantages [47–50], among which we mention: almost any material or materials’ combination may be congruently transferred as thin film on a suitable substrate after the identification of a selected deposition regime, a fine control of thickness is obtained by the number of the applied laser pulses (and the appropriate setting of various parameters), it is a clean, non-contaminating method, also allowing for an easy doping or multilayer structures. These and others made PLD a tool of remarkable performance, versatility, and simplicity for the growth of various types of thin films: complex oxides, high-temperature superconducting, protective or ultrahard coatings, polymer or organic thin films [50–52].
|0||N2:O2 = 1:1||10||450||2.8|
|0||N2:СН4 = 5:1||100||450||3.3|
|0||N2:СН4 = 5:1||3||450||–|
|2.5||N2:СН4 = 9:1||100||450||3.0|
|5||N2:СН4 = 9:1||100||450||2.9|
|2.5||N2:СН4 = 5:1||100||450||3.4|
|5||N2:СН4 = 5:1||100||450||3.3|
|10||N2:СН4 = 5:1||100||450||3.1|
|50||N2:O2 = 1:1||5||600||3.5|
|50||N2:CH4 = 10:1||3||600||–|
|50||N2:CH4 = 5:1||10||600||3.6|
PLD experiments were performed using a KrF* laser source (COMPexPro 205 Coherent, τFWHM ≤ 25 ns). Pristine TiO2 or ZrO2-mixed TiO2 targets with different content of zirconium oxide were laser ablated to grow coatings on glass plates. The films were synthesized at substrate temperatures of 450 or 600°C. The samples were deposited at different pressure of pure oxygen, nitrogen, methane, and mixed N2:CH4 atmosphere with 1:1, 5:1, 9:1, and 10:1 ratios.
Photocatalytic activity of the films was assessed via dichromate ions reduction reaction. The film was immersed in 15 ml of an aqueous solution of potassium dichromate (in all experiments, the initial concentration of dichromate ions was 2*10−4 M) and the reducing agent (disodium salt of ethylenediaminetetraacetic acid) in the molar ratio 1:1 adjusted to pH ≥ 2 by perchloric acid. The mass of the films was found to be 0.40 ± 0.02 mg (the concentration of the catalyst in aqueous solution of redox couple was 2.7 × 10−2 g/L, 37 times lower than usually used suspension 1 g/L). The reaction temperature was kept constant (25°C) during the experimental procedure. The change of Cr(VI) ions concentration was monitored with a Lambda 35 UV–Vis spectrophotometer (PerkinElmer) every 20 min. The film was immersed in the solution until complete adsorption in the dark occurred and then irradiated by 1000 W middle-pressure mercury lamp for 120 min. The distance lamp reactor was set at 90 cm. Two blank experiments were carried out: the catalytic reduction of dichromate ions (dark condition) and photoreduction reaction (a bare glass was used instead of film). The conversion percentage of every photocatalytic run is given after subtraction of the corresponding value of the last blank. For testing the visible-light sensitivity, a filter transmitting light with λ > 380 nm was introduced in the photocatalytic setup.
The sample codes that will appear over the chapter can be easily understand as
2.1. Nitrogen-doped TiO2 films
Pulsed laser synthesis of TiO2 films in oxygen, nitrogen, methane and their mixtures was performed (Table 1) . Excluding the UV part of spectrum by cut-of-filter, the films obtained at different atmospheres exhibited the visible light absorption (Figure 3). Compared to bare TiO2, nonmetal-doped films show a change in optical absorption that was originally associated to a bandgap narrowing. Comparing the values of bandgap energy calculated by extrapolating the linear parts of the (α
As revealed by X-ray diffraction (XRD) spectra (Figure 4), anatase phase formation has been obtained for all films excepting TiC/5. Moreover, the samples TiNO/10 and TiO/20 present a preferential texture along the <004> plane. On the other hand, the peaks at 37.80°, 43.30°, and 62.80° visible in the diffractogram of the TiO2 film deposited in CH4 (TiC/5 film) could be related to <004>, <002>, and <202> planes of TiO phase indicating the formation of a sub-stoichiometric compound  or even Ti crystalline phase [29, 54, 55]. It is obvious that pure methane used in PLD prevents anatase formation providing oxygen atoms deficiency.
Intensity belonging to substitutional N
Replacing dinitrogen or the mixture of dinitrogen/oxygen by the mixture of dinitrogen/methane leads to the change not only the line intensities but also some peaks position. Quite surprisingly, the intensity of N1s peak of Ns is sharply raised and reached ca. 90% for
Comparing the bandgap energy values of
The catalytic response under UV light exposure was observed for
2.2. Nitrogen-doped ZrO2/TiO2 films
2.2.1. Low content of ZrO2 in the target
Incorporation of transition metal oxides into titania improved the photocatalytic activity due to the mutual action of metal ions and anatase resulting in increased specific surface area, surface acidity, and changed photo-electrochemical properties. Coupling of two semiconductors is useful to achieve a more efficient separation of photogenerated electron–hole pair that led to the improvement of the photoactivity. It is found that the photocatalytic activity depends on the phase composition and crystalline size that modify the TiO2 bandgap . Zirconium incorporation into TiO2 lattice by sol–gel synthesis was achieved with the formation of Ti1−xZrxO2 solid solution for the film with a Zr content up to 10 mol% as evidenced by XRD and XPS. It was demonstrated that the formation of Zr–O–Ti bonds has an influence on the thermal stability during sintering of the mesoporous structure of the films, surface texture, and optical properties as well as in the changes of number of surface active sites for nanocomposite films [10, 58].
Pulsed laser-deposited (2.5, 5, and 10%) ZrO2/TiO2 thin films were synthesized under various gas atmospheres (see Table 1). We inferred the characteristic Ebg values for pure titania and zirconia of 3.3 and 4.2 eV. All nonmetal-doped films showed more intensive absorption extending into the visible-light region comparing
For films synthesized at 3 Pa, the calculation of Ebg was impossible because of the strong absorption in the visible region (Figure 6 inset) . Similar broad absorption was reported for N-doped TiO2 after the nitridation by ammonia at 650°C or above and explained by the metallic property of samples as indicated by the XRD and transmission electron microscopy (TEM) result .
The N1s line at 395.8 eV attributed to Ns is present for all nonmetal-doped samples. The XPS spectra of
Such effective nitrogen incorporation in oxide matrix can be the effect of an interaction between carbon (and/or hydrogen) and laser ablated oxygen atoms forming CO2 during PLD that leads to the oxygen deficiency and the formation of nonstoichiometric TiO2−xNx structure. This can be confirmed by the lower ratio (1.6) of OTi1s/Ti2p for samples synthesized at 3 Pa in comparison with pristine TiO2 where this ratio equals 2 .
A significant intensity of peak at 457.5 eV assigned to the Ti–N bonds was observed for
Additionally, the ratio of N1s line at 395.8 eV to Ti2p3/2 line at 457.5 eV corresponds as 1:2 and 2:1 for
Certainly, the non-doped ZrO2/TiO2 films (columns 7, 8, and 9 in Figure 9) exhibit much lower activity under both UV and visible light compared to nitrogen-doped synthesized at 100 Pa. The highest yield under both UV (51% reduced ions) and visible light (14%) was demonstrated by
It must be emphasized that all films obtained at lowest pressure are completely inactive under both UV and visible-light irradiation (not shown here). In order to understand in depth this phenomenon, the electrical conductivity (Figure 10) measurements were performed. The samples obtained in the range of (3–10) Pa are black colored (Figure 11) and exhibit the electrical conductivity pointing on the appearance of oxygen vacancies, which results in metallic character of these materials. The formation of nonstoichiometric TiO2−xNx containing in excess of Ti–N bonds exhibiting a low semiconductive behavior is concluded.
Moreover, no electrical conductivity (Figure 10, curves 4 and 5) was observed for the samples obtained at 50 and 100 Pa providing their semiconductive behavior. This indicates that anatase in conjunction with a certain numbers of Ti–N bonds is beneficial on the effective photocatalytic transformations.
As seen from Figure 11, the level of nitrogen incorporation strongly affects the optical properties of the films which turned from black to dark yellow color.
2.2.2. High weight content of ZrO2 in the sample
The next series of the films used in our investigation was based on the mixed titania–zirconia films with the component ratio 50:50 wt%. The synthesis conditions are detailed in Table 1. TiO2/ZrO2 films prepared in ambient N2 and/or CH4 atmosphere had different absorption spectra. The shift of the absorption band edge to the red region of the spectra with the increase of N2 content in the gas mixtures was observed (Figure 12). The influence of the doping agent nature and its content was observed in the variation of bandgap energy values (Table 1).
Similar to the bandgap values of low zirconia content films (Section 2.2.1), Ebg values correlate with dinitrogen content in the gas mixture and applied pressure. For non-doped ZrO2/TiO2 (
Unexpectedly, no crystallization of titania but incipient crystallization corresponding to the (101) line of ZrO2 were confirmed by XRD investigation (not shown here) . It can be explained by the existence of undetectable very small TiO2 particles or by the excess of ZrO2 in the film matrix inhibiting the titania crystallization.
The sample synthesized at the high content of N2 in the gas mixture (
In summary, an appropriate selection of gas mixture, pressure, and temperature during PLD synthesis is identified for the optimum photocatalytic activity of semiconductive materials. The highest photocatalytic conversion yields in the dichromate ions reduction are obtained for nitrogen-doped 10% ZrO2/TiO2 synthesized in N2:CH4 = 5:1 at 100 Pa and at 450°C under both UV and visible light. This is the result of effective nitrogen atom substitution into titania lattice as confirmed by N1s XPS line at 395.8 eV. When the films were synthesized at low pressure of 3 Pa, the only Ns peak with the binding energy 395.8 eV was observed and photocatalytically inactive nonstoichiometric TiO2−xNx structures were deposited. The absence of semiconductive properties of the films obtained at the low pressures is a direct evidence of no anatase formation and, respectively, their photocatalytic performance remained at the level of blank experiments. It can be suggested that carbon and hydrogen radicals/atoms forming as a result of laser action react with oxygen species causing the subsequent oxygen substitution by nitrogen atoms. This mechanism is supported by the XPS evidence, according to which the intensity of N–Ti peak at 395.9 eV is lower for the samples synthesized in a pure N2 atmosphere. Thus, the effective incorporation of substitutional nitrogen in oxide matrix can be caused by a direct interaction of carbon or hydrogen atom with oxygen species during PLD resulting in an oxygen deficient lattice, taking into account that pure methane used in PLD prevents anatase formation providing oxygen atoms deficiency. At high pressure, a number of collisions between C or H species and ablated oxygen atoms are less probable owing to the screening effect of other atoms/species (Figure 14a). When pressure is decreased, the deposition rate is enhanced in addition to much higher probability for carbon/oxygen and hydrogen/oxygen interactions (Figure 14b). This can result in the higher efficiency of Ti–N as compared with Ti–O fragments formation.
As seen from Figure 15, the percentage of reduced dichromate ions under UV light is correlated with the contributions of N1s at 395.8 eV which is in turn dependent on the zirconia content. The ratio of line intensities of N1s at 395.8 eV to Ti2p3/2 at 457.5 eV is pointing to the large number of O–Ti3+–N bonds with the increase of ZrO2 contents. It is obvious that photocatalytic activity under UV light strongly depends on the efficiency of substitutional nitrogen incorporation inside titania matrix. The high ZrO2 content (50%) induces the deceleration of anatase crystallization rate leading to the activity decrease, while a much lower zirconia amount (2.5 and 5%) is still not enough to stimulate an appropriate quantity of substituted nitrogen embedded in titania matrix. The correlation between the zirconia content and the efficiency of substitutional N incorporation is established. The distortion of Ті4+О2 lattice with the advent of Ti3+ states occurred due to the larger radius of zirconium ions (RZr4+ = 0.720 Å vs. RTi4+ = 0.650 Å). The relative high stability of Ti3+ states is assigned to the presence of Zr4+ ions.
In general, the low photocatalytic efficiency is explained by the limited photo-excitation of electrons in the intragap localized states, the very low mobility of photogenerated holes , and the high recombination rate due to the creation of oxygen vacancies by doping . Hence, the photoreactivity of doped TiO2 appeared to be a complex function of the doping agent concentration, the energy level of doping agent within the TiO2 lattice, their d electronic configurations, the distribution of doping agent, the electron donor concentrations, and light intensity.
The debates concerning the preferred N substitutional or interstitial sites which induce the highest photocatalytic action is still alive in the literature [62–64]. Our investigation suggests that the substitutional nitrogen (Ti–N) that belongs to XPS lines at 395.2–395.8 eV is basically responsible for the observed photoactivity. Additionally, it can be proved by the next two observations. (i) The peak at 397.8 eV has the negligible relative intensity (2.8%) for most active
All authors acknowledge the collaboration between Romanian Academy and National Academy of Sciences of Ukraine. Romanian authors acknowledge with thanks the support of CNCS-UEFISCDI under the contract ID304/2011 and Core Programme 2016–2017 LAPLACE-IV.