129 Original scientific paper  MIDEM Society Journal of Microelectronics, Electronic Components and Materials Vol. 47, No. 2(2017), 129 – 137 Highly Efficient Photocatalytic Activity in the Visible Region of Hydrothermally Synthesized N-doped TiO 2 Maja Lešnik1, Dejan Verhovšek1, Nika Veronovski1, Srđan Gatarić1, Mihael Drofenik2, Janez Kovač3 1Cinkarna Celje, d.d. Inc., Celje, Slovenia 2University of Maribor, Faculty of Chemistry and Chemical Engineering, Maribor, Slovenia 3Jozef Stefan Institute, Department of Surface Engineering and Optoelectronics, Ljubljana, Slovenia Abstract: Nanocrystalline rutile titanium dioxide (TiO2) samples doped with various amounts of nitrogen (N) atoms were prepared using a hydrothermal synthesis route and a polycrystalline TiO2 precursor. The doped rutile nanocrystallites were analysed with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–Vis spectroscopy. The Kubelka–Munk band-gap calculation were used to examine the UV–Vis reflectance spectra. The measurements of the photocatalytic activity were performed utilizing FT-IR. A remarkable increase in the photocatalytic activity of the doped rutile nanocrystallites was detected, when applying the isopropanol degradation method with UV–Vis light irradiation. Keywords: Nanoparticles, TiO2, Rutile, Visible photocatalyst Hidrotermalno sintentiziran TiO 2 dopiran z N z visoko fotokatalitsko aktivnostjo v vidnem delu svetlobnega spektra Izvleček: Z uporabo hidrotermalne sinteze in polikristalinične rutilne oblike nanodelcev TiO2 smo pripravili monokristalinične rutilne delce, dopirane z različnimi koncentracijami dopanta N. Za analizo dopiranih rutilnih TiO2 nanodelcev smo uporabili elektronsko mikroskopijo (TEM), fotoelektronsko spektroskopijo (XPS), rentgensko praškovno difrakcijo (XRD), UV-VIS spektroskopijo in kalkulacijo energijskih vrzeli s Kubelka Munk. Pri merjenju fotokatalitske aktivnosti z metodo degradacije izporopanola v alkohol smo ugotovili, da dopiranje rutilnega TiO2 z N povzroči izrazit premik delovanja fotokatalitske aktivnosti na vidni svetlobi svetlobnega spektra. Ključne besede: Nanodelci, TiO2, rutil, vidni fotokatalizator * Corresponding Author’s e-mail: maja.lesnik@cinkarna.si 1 Introduction Nanocrystalline titanium dioxide (TiO2) is the most widely investigated n-type semiconductor due to its high photocatalytic activity under UV light, which is important for numerous outdoor applications, such as wastewater treatment, air purification and self-clean- ing applications (walls, concretes). [1-8] Recently, new insights have been presented in the development of TiO2 photocatalysts that could efficiently utilize not only the UV light but also the visible light in the solar spectrum and could therefore be appropriate for inte- rior photochemical applications. [8-16] Chemical modi- fications of the TiO2 crystal lattice achieved by doping with cations or anions appear to be the most promis- ing approach to enhance the visible-light absorption power. [17] 130 The dopants can be interstitially or substitutionally incorporated into the TiO2 crystal lattice. Different concentration levels of these dopants might influ- ence the new electronic states localized in the gap or the electronic band edge narrowing, leading to an in- crease in the visible-light absorption efficiency. [14, 17] However, it is well known that the photocatalytic activ- ity under exposure to visible light, associated with the mobility of the excited electrons and holes and their recombination rate, differs, depending on the dopant type, its concentration and the lattice position that it occupies. [14, 17] On the other hand, anion doping has a tremendous effect on visible-light photocatalytically active TiO2. [18, 19] Among all the attempts at non- metal doping in TiO2, nitrogen doping has shown the greatest promise for achieving visible-light active pho- tocatalysts. The incorporation of nitrogen into the TiO2 crystal lattice is advantageous, due to it having a simi- lar atomic radius to oxygen and a lower electronega- tivity than oxygen. [20] The modification mechanism of N-doped TiO2, its ability to absorb visible light and visible-light photocatalysis is still under investigation. There are three different hypotheses that could explain the phenomena. Firstly, in N-doped TiO2 the energies of the N 2p and O 2p states are similar. The consequence of this is band-gap narrowing and the ability to absorb visible light. [22] Secondly, oxygen sites are substituted by nitrogen atoms and the intermediate energy level is formed below the conductive band edge. [21] Thirdly, doping with nitrogen forms oxygen-vacancy defect sites, which are the major factor in visible-light photo- catalytic activity. [23, 24] Rutile, as an n-type TiO2 semi- conductor, exhibits oxygen vacancies on the surface. Nitrogen doping introduces additional oxygen vacan- cies, which leads to an even more efficient photocata- lytic activity. [25] It is well known that the absorption properties of N- doped anatase and raw rutile TiO2 are distinguishable. The structures of anatase and rutile differ in the posi- tion of the octahedron, resulting in a tetragonal struc- ture for both modifications. [3] The other reason for the different absorption is the electron density of the N-doped anatase or rutile. [26] Doping with nitrogen provides N 2p states located above the O 2p valence band. Since rutile has a smaller band gap than anatase, this furthermore enhances the valence band. [27] The same findings were confirmed by Yang and co-authors in their DFT calculations on nitrogen-doped structures of rutile crystals. [15] Liu, in his work, demonstrated that nitrogen-doped TiO2 with more rutile phase has more defects than the nitrogen-doped TiO2 with less rutile phase, which enhances the photocatalytic effi- ciency. [25] The photocatalytic efficiencies of rutile and anatase are related to the formation of hydroxyl radi- cals that prevent electron–hole recombination during exposure to sunlight. Some studies have demonstrated that the rutile crystal phase exhibits enough hydroxyl groups, which are believed to act as light photocata- lysts, i.e., to accept the holes generated by UV illumina- tion and form hydroxyl radicals and thus prevent elec- tron–hole recombination. [28, 29] The selection of nitrogen doping for the rutile crystal structure was based on theoretical studies published recently in the open literature, particularly on the nu- merous advantages of visible-light absorption and en- hanced photocatalytic activity. [26-29] In the present study we report a new synthesis proce- dure of N-TiO2 visible-light photocatalyst based on the hydrothermal synthesis using the polycrystalline rutile TiO2 nanocrystallites. 2 Experimental 2.1 Preparation method The hydrothermal synthesis of N-doped rutile TiO2 na- nocrystallites was performed in a Teflon-lined, stain- less-steel autoclave with a volume of 80 mL. To prepare the TiO2-doped sample the reactor was loaded with a 50-mL aqueous suspension of polycrystalline rutile TiO2 nanocrystallites provided by Cinkarna Celje, Inc., having a mass concentration between 60-150 g/L (cal- culated as TiO2) and 1 mass % (based on TiO2 content) of urea ((NH2)2CO, 99% w/w, Merck). The mixture of polycrystalline rutile TiO2 nanocrystallites and dopant was then stirred for at least 15 minutes. The autoclave was put into a preheated oven and was hydrothermal treated at 180 °C for 24 hours. At the end of the heat- ing process the autoclave was taken out of the oven and left to cool to room temperature. The as-prepared product was diluted with distilled water, washed on a laboratory centrifuge (MPW 350 – Med. Instruments, High brushless centrifuge, 4000 rpm, 20 minutes). The washing was continued until the conductivity of the ef- fluent was less than 900 µS/cm. The final product was an aqueous suspension of doped rutile having 10 mass % of TiO2 nanocrystallites. Samples with urea/TiO2 ra- tios of 0.01, 0.02, 0.03, 0.06, 0.08 w/w, labelled as sam- ples: B, C and D, E, F, were then prepared using the same process by varying the content of added urea. The sam- ple A was prepared using the same process, but with- out the addition of urea as a dopant. 2.2 Characterization of samples The crystallinity of the particles was examined using X- ray diffraction (XRD) performed on a Cubi X PRO PW M. Lešnik et al; Informacije Midem, Vol. 47, No. 2(2017), 129 – 137 131 3800 instrument (PANanalytical) (Cu-Kα radiation (λ = 1.5418Å)). In order to acquire the TiO2 powders for the X-ray powder-diffraction (XRD) measurement, the suspensions were dried at 80 °C, ground and the pow- der pressed into pellets that were used to perform the measurements. The average crystallite size was determined using dif- fraction-peak (100) broadening and Scherrer’s formula based on the FWHM (Full Width at Half Maximum) of the XRD peak. The specific surface areas (SBET) of particles were deter- mined using Tristar 3000, the automatic gas analyser (Micromeritics Instrument Co.). The morphology and the size of the particles were ex- amined with a transmission electron microscope (TEM, Jeol JEM-2100, Jeol Ltd.,Tokyo, Japan). The samples for the TEM specimens were ultrasonically dispersed and the suspensions were collected using carbon-support- ed copper grids. The UV–Vis diffuse reflectance spectra were collected on an Agilent-Cary 300 UV–Vis spectrophotometer equipped with an integrating sphere (Varian Inc., USA). The measurements of the photocatalytic activity were performed in a sealed gas-solid reactor at room tem- perature and a relative humidity of 60%, utilizing FT-IR spectroscopy (Spectrum BX model Perkin Elmer spec- trometer). The model pollutant was isopropanol in the gas phase. During the photocatalytic reaction the iso- propanol oxidizes to acetone and subsequently to car- bon dioxide and water under UV irradiation (Xe lamp, 300 W). The light imitates the solar spectrum and emits both ultraviolet (UV) and visible (VIS) light. The reac- tor is at a distance of 4 cm from the lamp. The samples were dried under ambient conditions and prepared by milling 50 mg of the material. To perform the measure- ments, 20 µL of isopropanol was injected into the sys- tem. This volume represents around 2000 ppm of gas phase for the isopropanol in the system. The amount and ratio of isopropanol and the formed acetone were monitored in real time. The evaluation of the photocat- alytic activity is based on the acetone-formation kinet- ics and is given in ppm/h. [33] The chemical composition of the surfaces was deter- mined by X-ray photoelectron spectroscopy (XPS). XPS analyses were performed with a TFA XPS spectrometer, produced by Physical Electronics Inc., equipped with a monochromated Al–Kα X-ray source (1486.6 eV), un- der ultra-high vacuum (10-7 Pa). Samples in the form of powders were deposited on the adhesive carbon tape. The analyzed area was 0.4 mm in diameter and the analyzed depth was 3–5 nm. The high-energy reso- lution spectra were acquired with an energy analyzer operating at a resolution of about 0.6 eV and a pass energy of 29 eV. The XPS spectra were processed with the software MultiPak. Prior to the spectra process- ing, the same spectra were referenced to the C-C/C-H peak in the C 1s core level at a binding energy of 284.8 eV. The accuracy of the binding energies was about ± 0.2 eV. Quantification of the surface composition was based on the XPS peak intensities, taking into account the relative sensitivity factors provided by the instru- ment manufacturer. [38] Three different places were analyzed on each sample and the data were averaged. 3 Results and discussion 3.1 The crystallite phase and size of rutile particles Figure 1: XRD patterns of undoped and N-doped TiO2 samples with different ratios of urea to TiO2. Figure 1 presents the XRD patterns of the samples pre- pared using a modified hydrothermal process and vari- ous ratios of urea to TiO2. The presence of specific peaks (2θ = 27.38°, 36.06°, 41.19°) was taken as an attributive indicator of rutile titania. [20, 31] However, no N-de- rived peak is detected for N-TiO2, even when the ratio of urea to TiO2 was 0.08. It can also be seen from the XRD patterns that the nitrogen-doped samples show more intensive diffraction peaks, indicating a more pronounced crystallinity for the N-doped crystallites. From Table 1 it is clear that the crystallite size increases with the amount of urea in the precursor suspension, i.e., the ratio of urea to polycrystalline TiO2 (w/w) pre- cursor changes from 0.01 to 0.08. The increase of the crystallite size, for identical hydrothermal conditions, due to a larger amount of urea, can be assigned to the vigorous thermally assisted decomposition reaction of the urea, which enhances the kinetics of mass transport during the dissolution, precipitation and growth of TiO2 nano-crystallites. Thus, a crystallite-size increase is straightforward and proportional to the amount of urea in the starting suspension. On the other hand, when the addition is a compound that is stable during the hydrothermal synthesis conditions it would, as ex- pected, hinder the crystallite growth and thus decrease M. Lešnik et al; Informacije Midem, Vol. 47, No. 2(2017), 129 – 137 132 the final crystallite size, which is the case when the sup- pression of crystallites size is planned. As a consequence, the specific surface areas (SBET) show a steady decrease in parallel with the crystallite size increase. Here, an exception proves sample B with a much smaller specific surface regarding the general trend in the sequence, which might be a consequence of the exaggerated crystallite agglomeration. However, in general the morphology of the crystallites follows the general expectation. Table 1: Average crystallite size, specific surface area and band-gap energies for various N-doped TiO2 samples. Sample Urea/TiO2 Specific surface area (m2/g) Crystallite size (100) (nm) Band gap (eV) A - 70.1 16.7 3.01 B 0.01 42.5 23.6 3.04 C 0.02 68.1 27.6 3.03 D 0.03 63.7 33.0 3.03 E 0.06 56.9 42.6 3.04 F 0.08 51.0 48.7 3.02 3.2 The morphology of the doped TiO 2 particles The morphologies of the N-doped TiO2 are shown in the TEM micrographs of Figure 2. The hydrothermally synthesized, doped, rutile TiO2 nanocrystallites have an oval/spherical morphology and are uniform in size. The crystallite sizes observed with the TEM match with those obtained from the Scherrer estimation using the peak broadening of the XRD spectra, which has shown a com- parable crystallite size up to a urea/TiO2 ratio of 0.03. On the other hand, the morphologies of samples E and F, prepared at ratios of urea to TiO2 of 0.06 and 0.08, respec- tively, exhibit a larger crystallite size (TEM images not shown). 3.3 UV–Vis diffuse reflectance spectra Figure 3: UV–Vis reflectance spectra of the undoped TiO2 and the TiO2 doped with different urea-to-TiO2 ra- tios, indicated in the legend. For the examination of the effects of doping on TiO2, an evaluation of the optical properties is the most appro- priate method. UV–Vis spectroscopy and diffuse reflec- tance spectroscopy were chosen as the techniques for the optical studies of N-doped TiO2. In our work diffuse reflectance spectroscopy was used to examine the vis- ible-light sensitivity. The influence of nitrogen doping on the UV–Vis spectra properties for the rutile TiO2 is demonstrated in Figure 3. The reflectivity dependence of the wavelength of the pure TiO2 has a typical sharp edge of reflection at around 420–400 nm. Compared with the spectrum of undoped TiO2, the N-doped sam- M. Lešnik et al; Informacije Midem, Vol. 47, No. 2(2017), 129 – 137 (a) (b) Figure 2: TEM micrographs of samples prepared with different ratios of urea to TiO2: (a) undoped TiO2 and (b) urea/TiO2 = 0.02. 133 ples exhibit a very similar curve progression; however, there is a small but distinguishable shift in the absorb- ance region of the visible range 400–550 nm. [30, 31, 34] The N-doped samples exhibit a slightly difference in the colour, which could provide a small absorbance in visible region. [41] An exception is observed for the 0.03-doped sample D, which shows a more notable red shift. So, based on the intensity of absorption for all the samples we can assume that the nitrogen entered the TiO2 crystal lattice under the reported hydrothermal condition. The same finding was reported by Huang. [31] It was reported that the visible-light absorption could be brought about by band-gap narrowing. However, it was also reported that the localized N states within the band gap and the Ti3+ defects could also provide the absorption red shift. [34, 35] In addition, Hu showed that the band gaps of the doped samples were the same, indicating that N doping did not change the band gap of the TiO2. [32] The doping of TiO2 with N atoms improved its visible-light absorption, increased the numbers of photons in the photocatalytic reaction and thus enhanced the photocatalytic activity in the visible region. The band-gap energies of the rutile nanocrystallites, estimated using Kubelka–Munk model are summarised in Table 1. [35, 36] The values of the band-gap energies of the doped samples were compared with a control sample (undoped rutile nanocrystallites), which was calculated to have a band gap of 3.01 eV. The calcu- lated value for the undoped rutile nanocrystallites is in agreement with the theoretical value of 3.0 eV for the rutile modification. [37] The results show that the band- gap energies of all the N-doped samples are practically the same as the control sample. A possible explanation is that the visible-light absorption occurs due to the colour centres formed by the N-doping process rather than by a narrowing of the band gap. The research was conducted on various N-doped metal-oxide nanoparti- cles. The band-gap narrowing does not occur, even for significantly high doping levels, such as 25 % doping. [32, 34]. It can be concluded that the main effect of N doping is a slightly improved absorption at long wave- lengths, which enhances the visible photocatalytic ac- tivity of these material. It could be concluded that the main effect of N doping is the improved absorption at long wavelengths due to the shallow trap states inside the TiO2 crystal lattice, which enhances the visible pho- tocatalytic activity of these materials. [22, 24, 25] 3.4 Investigation of chemical states of TiO 2 samples The surface chemical composition and the chemical states of the TiO2 samples were analyzed by means of XPS. The survey spectra (not shown) are similar and indi- cate the presence of Ti, O and C in all the samples, while N is visible only in the spectra from the urea-modified TiO2 samples and confirms a successful treatment. The surface chemical compositions are presented in Table 2. The carbon on the surface of the undoped sample can be related to the surface contamination and the synthesis conditions. For the TiO2 samples treated with urea a nitrogen signal appeared. The highest nitrogen concentration (0.8 at.%) was observed on the surface of the TiO2 sample D (urea/TiO2 ratio 0.03). On the sam- ple treated with a higher urea concentration, sample F (urea/TiO2 ratio 0.08), we observed less nitrogen (~ 0.3 at.%). The amount of nitrogen on the surfaces of the analyzed samples correlates with the photocatalytic activity. High-energy-resolution C 1s, O 1s, N 1s and Ti 2p XPS spectra were acquired to further understand the chemical bonding. In the high-energy-resolution O 1s spectra (not shown here) we were able to observe the presence of two different components by using a fit- ting procedure. The main contribution is attributed to the Ti-O in the TiO2 (529.9 eV) and the other minor peak can be ascribed to the surface hydroxyl Ti-OH (531.4 eV). [39] A comparison of the O 1s spectra from the undoped sample with the treated samples shows no major differences. Nitrogen was only detected in the urea-treated TiO2 samples. High-energy-resolution N 1s XPS spectra from the undoped TiO2 and the TiO2/urea ratio of 0.03 are shown on Figure 4. The maxima of the N 1s spectra, for all the treated samples, are located at 400 eV, which indicates interstitial nitrogen integrated into the TiO2 lattice. It is known that the peak at around 400 eV is related to the N-O, N-C or N-N type of bonds. [40] A comparison of the high-energy-resolution Ti 2p spec- tra from all the analysed samples is shown in Figure 5. In the acquired Ti 2p spectra a doublet peak is visible, containing both Ti 2p3/2 and Ti 2p1/2 components, which appear at 458.6 eV and 464.3 eV, respectively, with 5.7 eV spin-orbital splitting. This corresponds to a Ti4+ va- Table 2: Surface composition in at. % of the undoped TiO2 and TiO2 modified with urea using different con- centrations. Sample Urea/TiO2 C O Ti N A - 26.1 52.2 21.7 B 0.01 21.5 55.8 22.3 0.3 C 0.02 23.4 54.6 21.4 0.7 D 0.03 24.8 53.4 21.1 0.8 E 0.06 20.0 56.7 22.7 0.6 F 0.08 25.2 53.1 21.4 0.3 M. Lešnik et al; Informacije Midem, Vol. 47, No. 2(2017), 129 – 137 134 lence state. The peaks are narrow and no significant dif- ferences, like shifting in the binding energy, between the undoped and treated samples were observed (Fig- ure 5). Figure 4: N 1s XPS spectrum of undoped TiO2 (a) and TiO2 modified with a TiO2/urea ratio = 0.03 (b). Figure 5: XPS spectra of Ti 2p from all the samples. 3.5 Photocatalytic activity measurements To evaluate the photocatalytic activity of the undoped and N-doped TiO2 in the visible range, the degradation of isopropanol under UV+VIS and Vis irradiation was in- vestigated. The results of the photocatalytic activities are presented in Figure 6, based on the acetone-for- mation kinetics, and are given in ppm/h. As illustrated in Figure 6, different N-doped TiO2 catalysts differ in the degradation of isopropanol under the same ex- perimental conditions. One can see that i) in general, N-doped TiO2 samples achieve a higher photocatalytic activity than the undoped TiO2 samples and ii) the pho- tocatalytic activity increases with the surface-nitrogen concentration. Among all of the investigated N-doped TiO2 samples, sample E, with a urea/TiO2 ratio of 0.06 and a corre- sponding surface-nitrogen concentration of 0.6 at %, displays the highest photocatalytic efficiency for iso- propanol degradation. Nearly the same photocatalytic efficiency was also detected with the sample C, hav- ing otherwise a lower urea/TiO2 ratio of 0.02; however, it exhibits a similar surface-nitrogen concentration of 0.7 at %. In addition, sample E exhibits a lower specific surface area than the sample C with a urea/TiO2 ratio of Figure 6: Photocatalytic activity of undoped TiO2 and TiO2 doped with different ratios of urea under a) UV+Vis irradiation and b) Vis irradiation. (a) (b) M. Lešnik et al; Informacije Midem, Vol. 47, No. 2(2017), 129 – 137 135 0.02, Table 1. Therefore, the high photocatalytic activ- ity of sample C with a surface nitrogen concentration of 0.7 at % is not a consequence of a higher specific surface area, but the result of a high surface-nitrogen concentration. This is in accordance with the general trend that the N∙ centres enhanced the photocatalytic activity in the visible range. As a result, an increase in the surface area does not automatically produce an increase in the photocata- lytic activity, demonstrating that the higher activity is a consequence of a high surface-nitrogen concentration and not of the surface-regulated process. On the other hand, a greater surface area provides more active sites on the TiO2 surface for the degradation of the organic pollutant. [20, 32] 4 Conclusions N-doped rutile TiO2 nanocrystallites that exhibit a strong increase in their photocatalytic activity were successfully prepared using the hydrothermal method. The absorbance of N-TiO2 in the visible-light region is the most important when concerning the material’s application since it can be activated with solar light and thus exhibits an enhanced photocatalytic visible-light activity. The narrowing of the band gap does not occur, indicating that the major effects of N doping are an en- hanced absorption at long wavelengths and the hole- trapping sites, which retards the hole–electron recom- bination and might be useful in enhancing the visible photocatalytic activity of these materials. The maxima of the N 1s spectra, for all the treated samples, indicate that the interstitial nitrogen is integrated into the TiO2 lattice. The N-doped TiO2 samples achieved a higher photocatalytic activity in the UV and visible-light re- gions than the undoped sample. 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