Author’s Accepted Manuscript N-doped TiO 2 nanoparticles obtained by a facile coprecipitation method at low temperature A. Sanchez-Martinez, O. Ceballos-Sanchez, C. Koop-Santa, Edgar R. López-Mena, E. OrozcoGuareño, M. García-Guaderrama www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)32851-1 https://doi.org/10.1016/j.ceramint.2017.12.140 CERI17020
To appear in: Ceramics International Received date: 2 September 2017 Revised date: 11 December 2017 Accepted date: 19 December 2017 Cite this article as: A. Sanchez-Martinez, O. Ceballos-Sanchez, C. Koop-Santa, Edgar R. López-Mena, E. Orozco-Guareño and M. García-Guaderrama, Ndoped TiO 2 nanoparticles obtained by a facile coprecipitation method at low t e m p e r a t u r e , Ceramics International, https://doi.org/10.1016/j.ceramint.2017.12.140 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
N-doped TiO2 nanoparticles obtained by a facile coprecipitation method at low temperature A. Sanchez-Martinez1,*, O. Ceballos-Sanchez2, C. Koop-Santa2, Edgar R. López-Mena3, E. OrozcoGuareño4, M. García-Guaderrama2 1
CONACYT-Departamento de Ingeniería de Proyectos, CUCEI, Universidad de Guadalajara, Av. José Guadalupe Zuno # 48, Industrial los Belenes, Zapopan, Jalisco 45157, México. 2
Departamento de Ingeniería de Proyectos, CUCEI, Universidad de Guadalajara, Av. José Guadalupe Zuno # 48, Industrial los Belenes, Zapopan, Jalisco 45157, México. 3
Escuela de Ingenieria y Ciencias, Tecnologico de Monterrey, Campus Monterrey, Monterrey, Nuevo León 64849, México. 4
Departamento de Química, CUCEI, Universidad de Guadalajara, Blvd. Marcelino García Barragán # 1451, Guadalajara, Jalisco 44430, México.
ABSTRACT
N-doped TiO2 nanoparticles (NPs) were synthesized using a facile synthesis route by coprecipitation method. The effect of the HNO3 volume and calcination temperature on the structural, morphological, optical and surface properties of the N-doped TiO2 NPs was studied. X-ray diffraction analysis showed particles of nanometric size (< 16 nm), which are consistent with HR-TEM micrographs. A slight shift of the absorption edge to higher wavelengths is observed as the HNO3 volume and calcination temperature increases. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) show the presence and stability of nitrogen in the N-doped TiO2 structure. The photocatalytic activity of the N-doped TiO2 NPs was assessed by testing the degradation of rhodamine B (RhB) under ultraviolet (UV) and visible light.
Keywords: N-doped TiO2 nanoparticles; synthesis at low temperature; photocatalysis; coprecipitation method.
Corresponding author: E-mail:
[email protected]
1
INTRODUCTION
Titanium dioxide (TiO2) or Titania is a well-understood material which is widely used due to its good stability, non-toxicity, and low cost [1–4]. Since Fujishima et al. reported on the ability of the TiO2 to split the water molecule different efforts have been made to improve its photocatalytic activity [5]. It is well known that TiO2 can only be activated by employing a photon energy greater than its band gap energy (3.0 eV for rutile phase and 3.2 eV for anatase phase) [6]. For this reason, several synthesis methods and routes for preparing TiO2 have been explored to narrow its band gap energy and to find suitable properties that allow a high photocatalytic performance [5,7–9]. Synthesis methods such as spray pyrolysis, solvothermal, hydrogel, microwave-assisted, hydrothermal and sol-gel have been explored for preparing doped and undoped TiO2 powder [10–14]. In order to expand the absorption ability of the TiO2 to visible light wavelengths, some metal ions (as Cu, Co, Ni, Cr, Mo, Fe) and non-metal ions (as N, S, B, I, F) have been incorporated into the TiO2 crystal structure [15–18]. The synthesis method and impurity source used for preparing N-doped TiO2 largely determines the physical and chemical properties of the semiconductor [19]. In particular, some methods for the N-doped TiO2 preparation involve the use of complex steps that are sometimes difficult to control during the synthesis process, they are relatively expensive, or possess large preparation times [20–22]. Nowadays, it is desirable to find simple and environmentally friendly methods that allow obtaining N-doped TiO2 at low temperatures. Chemical coprecipitation is a low-cost and straightforward method for preparing semiconductor materials [23–26]. C.-S. Kim et al. reported the synthesis of Cu/N-doped mesoporous TiO2 by using a template-free homogeneous coprecipitation method followed by a conventional impregnation method. The N-doped TiO2 synthesis was carried out at 80 °C and then the obtained powder was calcinated at 400 °C for 2 hours [27]. Z. Sheng et al. reported the
preparation of Mn-Ce/TiO2 composites calcinated at 400 °C for 2 hours by using the coprecipitation method for the low-temperature selective catalytic reduction of NOx with ammonia [28]. Y.C. Zhang et al. prepared nanocrystalline of N-doped TiO2 using HNO3, tetrabutyl titanate and ethanol by one-step solvothermal route. They found that the nanocrystals prepared with HNO3 as the nitrogen source presented a higher photocatalytic activity in comparison to nanocrystals prepared with NH3-H2O [29]. C. Leyva-Porras et al. reported the preparation of TiO2 nanoparticles by an acid-assisted sol-gel method at low temperatures. They obtained a well-defined TiO2 crystal structure at 80 °C, which was mainly related to the formation of the anatase phase [30]. In this work, N-doped TiO2 was prepared using a facile coprecipitation method. The synthesis of the N-doped TiO2 NPs was carried out at room temperature employing HNO3 as the doping source. The effect of the variation of the HNO3 volume and the calcination temperature on the morphological, optical, and structural properties was investigated. The photocatalytic activity of the N-doped TiO2 NPs was assessed by testing the degradation of RhB under UV and visible light. 2
EXPERIMENTAL SECTION
2.1
Preparation of N-doped TiO2 nanoparticles
Titanium (IV) isopropoxide (TTIP) (Aldrich, 97%), nitric acid (HNO3) (70%), and ammonium hydroxide (NH4OH) (29.3%) were used for the preparation of the N-doped TiO2 NPs employing a modified synthesis to that reported in the reference [31]. TTIP (3.6 g) was added in 100 mL of deionized water (DIW), and then different volumes of HNO3 (10, 20, 30, and 40 mL) were incorporated into the mixture. Subsequently, NH4OH was slowly added to the transparent solution as a precipitant agent until a pH = 11 was reached. The final mixture was vigorously stirred throughout the process at room temperature. The precipitate (yellow color) was vacuum
filtered and dried at 100 °C for 5 h. Afterwards, the N-doped TiO2 powder was collected and calcinated at 200, 300, 400, and 500 °C for 2 h in air atmosphere. Four sets of N-doped TiO2 NPs were obtained varying in HNO3 volume, and for each set a temperature sweep from 200 to 500 °C was done. The samples were labeled as C-T, where C and T refer to the HNO3 volume and the calcination temperature, respectively. 2.2
Characterization
The crystal structure of the N-doped TiO2 NPs was investigated using an Empyream (PANalytical) diffractometer with a Cu Kα radiation source (λ = 1.5405Å). XRD patterns were taken from a 20° to 80° (2theta) range, and a 0.02° step size. Absorption spectra were obtained using
a
Cary-300
UV-Vis
(Agilent
Technologies)
spectrometer
equipped
with
a
polytetrafluoroethylene (PTFE) integration sphere. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was employed to investigate the presence of nitrogen in the TiO2 structure. ATR-FTIR spectra were recorded in the 4000 to 700 cm-1 range using a Nicolet iS50 FT-IR (Thermo Scientific) spectrometer. The elemental composition and chemical states were investigated using X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out employing a monochromatic Al Kα (
eV) X-ray source. The C 1s, Ti 2p,
O 1s, and N 1s core levels were collected at 90° (the electron take-off angle is defined from the sample surface and the analyzer input lens axis) and 15 eV pass energy. The peak-fitting software employed was AANALYZER [32]. High resolution transmission electron microscopy (HRTEM) micrographs were acquired with a JEM-ARM200F Transmission Electron Microscope operated at 200 KV, STEM mode.
2.3
Photocatalytic activity
The photocatalytic activity of the N-doped TiO2 NPs was assessed by testing the degradation of rhodamine B (RhB) under irradiation of UV (365 nm) and visible light at room temperature. The tests under UV light irradiation were carried out using a Pyrex beaker with a plug cap and a quartz tube in the center. The UV lamp used for the photocatalytic test was a pen-ray mercury lamp (UVP, 365 nm). To keep the temperature of the reaction (25 ± 1 °C), the photo-reactor was placed inside a recirculating water bath system. For the tests under visible light irradiation, a borosilicate reactor (300 mL) equipped with a borosilicate tube at the center and a recirculating water system was employed. A LED (6000 K) was used as visible light source, which was placed at the center of the photo-reactor. The photocatalytic tests under UV and visible light were done by dispersing 0.1 g and 0.2 g of catalyst in 100 mL and 200 mL of a RhB aqueous solution (5 mg/L), respectively. The mixture was magnetically stirred for 30 min in the dark to reach the adsorption-desorption equilibrium. After that, UV/visible light was turned on, keeping the mixture under continuous stirring during the degradation process. Aliquots were taken every 10 min and 60 min for the UV and visible tests, respectively. These aliquots were centrifuged at 4000 rpm for 10 min. The residual RhB concentration was analyzed using a Cary-300 UV-Vis (Agilent Technologies) spectrometer. The RhB concentration was calculated from a calibration curve of the RhB solution. The mineralization degree of the RhB solution was monitored by using a total organic carbon analyzer (TOC-VCSH, Shimadzu, Japan). Prior to TOC analysis, aliquots exposed under UV light irradiation were collected each 20 min for 1 h and centrifuged at least twice. The BET specific superficial area was determined by multipoint BET method at the relative pressure range of 0.05-0.3 P/P0. Nitrogen desorption volume was used to determine the average pore volume and the average pore size at a relative pressure (P/P0) of 0.99 by using the Barrett-Joyner-Halenda (BJH) method.
3
RESULTS AND DISCUSSION
3.1
X-ray diffraction
Fig. 1 shows the XRD patterns of the N-doped TiO2 powders for different HNO3 volumes: a) 10 mL, b) 20 mL, c) 30 mL, and c) 40 mL as a function of calcination temperature. It is possible to observe that the N-doped TiO2 NPs present well-defined diffraction peaks at 100 °C. The diffraction peaks located at 25.2°, 37.0°, 37.8°, 38.6°, 48.0°, 53.9°, 55.1°, 62.1°, 62.7°, 68.8°, 70.3°, 74.0°, 75.0°, and 76.1° are mainly related to the anatase phase (JCPDS card No. 21-1272) with Miller indices labeled as (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (107), (215), and (301), respectively. The N-doped TiO2 NPs prepared with 10, 20, and 30 mL HNO3 volumes are observed to show the presence of a mixture of anatase and brookite phases. A small diffraction peak centered at 30.8° is related to the (121) brookite family of planes (JCPDS card No. 29-1360), see Fig. 1a-c.
Anatase Brookite
(b)
20-500°C
(116) (220)
(204)
10-100°C 60
70
80
20
30
40
Anatase Brookite
60
70
80
Anatase
(d)
♦ Rutile
(121)
♦ (301)
♦ (002)
(215)
(116) (220)
40-300°C (204)
♦ (111)
(004)
(101)
Intensity (a. u.)
30-300°C
♦ (211) ♦ (220)
40-400°C
♦ (101)
30-400°C
♦ (110)
40-500°C
(215)
(204)
30-500°C
(116) (220)
(105) (211)
(200)
(004)
(c)
50
2 (degree)
(200)
50
(105) (211)
40
2 (degree)
Intensity (a. u.)
(200)
20-100°C
(121)
30
20-200°C
(121)
10-200°C
20
(105) (211)
(004)
20-300°C
(101)
10-300°C
20-400°C
(101)
Intensity (a. u.)
(215)
(204)
(116) (220)
10-400°C (105) (211)
Intensity (a.u.)
(004)
(200)
10-500°C
(215)
Anatase Brookite
(a)
40-200°C
(101)
30-200°C
40-100°C
30-100°C
20
30
40
50
60
2 (degree)
Fig. 1
70
80
20
30
40
50
60
70
80
2 (degree)
XRD patterns of N-doped TiO2 powder prepared at a) 10 mL, b) 20 mL, c) 30 mL, and c) 40 mL of HNO3. For each set of samples, the calcination temperature was also varied.
The N-doped TiO2 NPs prepared with a 40 mL HNO3 volume presented a mixture of the anatase and rutile phases. The diffraction peaks associated with the rutile phase are consistent with JCPDS card No. 75-1748, see Fig. 1d. The phase percentage was calculated using the following equation:
Here
represents the percentage of anatase phase,
(101) peak and
the integrated intensity of the anatase
the integrated intensity rutile (110) peak [33]. The percentages of anatase-rutile
phase obtained for 40-100, 40-200, 40-300, 40-400, and 40-500 NPs were around 93-7, 93-7, 919, 85-15, and 81-19, respectively. It should be noted that for a high HNO3 volume (40 mL), the N-doped-TiO2 NPs tend to be less crystalline (see Fig. S1, supplementary information). Fig. 2 shows the dependence of crystallite size as a function of the calcination temperature. These values were calculated on the basis of the anatase (101) diffraction peak using the Scherrer equation.
Here λ is the wavelength of the X-ray source (Cu Kα =1.5405 Å), maximum (FWHM) of the diffraction peak and
is the full width half-
is the Bragg diffraction angle. As the
calcination temperature increases from 100 to 500 °C, the crystallite size of the N-doped TiO2 NPs also increases. A summary of crystallite size obtained from XRD is listed in Table 1, where it is also compared to the values obtained by HR-TEM. 18 10 mL HNO3
Crystallite size (nm)
16
20 mL HNO3 30 mL HNO3
14
40 mL HNO3
12 10 8 6 4 2
100
200
300
400
Temperature (°C)
500
Fig. 2
Crystallite size as a function of the calcination temperature for N-doped TiO2 powder using different HNO3 volumes.
The effect of HNO3 volume over particle size can be understood using the activation energy required to form a critical nucleus of the solid radius r* [34]:
Here
is defined as the interfacial tension between the precipitate and the solution,
precipitate density,
is the molecular weight of the precipitate, and
is the
the supersaturation. The
last one is the fraction between concentrations of the hydrolyzed species divided by the concentration of the solution in equilibrium. According to this equation, when the HNO3 volume varies,
varies and therefore
can increase or decrease, allowing for different values of r*,
which is related to the particle size. Regarding the rutile presence in N-doped TiO2 powder, Zhang et al. explained that the anatase, brookite, and rutile phases are crystallite size dependent due to that the formation energies of these three polymorphs are almost equal and that they can be reversed by small differences in surface energy [35].
Table 1. Summary of crystallite size, band gap energy ( some N-doped TiO2 NPs. Sample
a
Crystallite size (nm)
a
Particle size (nm)
b
Eg
), superficial area and porosity for
SBET (m2/g)
(eV)
Pore volume 3
Pore size
(cm /g)
(nm)
10-100
6.2 nm
5.9
3.09
255.70
0.27
4.89
10-500
14.1 nm
17.7
3.07
102.70
0.25
7.80
40-100
4.5
4.8
2.94
324.5
0.36
4.32
40-500
10.2
10.0
3.03
144.90
0.36
7.79
b
Average crystallite size was obtained from XRD analysis. Average particle size was obtained from HR-TEM micrographs.
3.2
BET superficial area and pore size
The effect of the HNO3 volume and calcination temperature on the specific superficial area, pore size and average pore volume was investigated. Fig. 3a-b display the nitrogen adsorptiondesorption isotherms and pore size distribution of the a) 10-100, 10-500, 40-100, and 40-500 Ndoped TiO2 NPs. It can be seen that the NPs present isotherms of type IV with a H2 hysteresis loop, that suggest a uniform mesoporous structure [36]. The shift of the hysteresis loop as increases the calcination temperature shows a larger average pore size, see Fig. 3a. For the case where the HNO3 volume increases, it can be seen that the N-doped TiO2 NPs presents a better capability of nitrogen absorption, which is directly related to a higher specific superficial area (see Table 1). On the other hand, it is reported that there is a relationship between average pore size and crystallite size [37,38]. The latter is consistent with the results presented in the Fig. 2 and Fig. 3b, where it can be observed that as the calcination temperature increases the pore size distribution and the crystal size increases. The BET specific superficial area results showed that the calcination temperature has a strong impact on the N-doped TiO2 NPs properties in comparison with the variation of the HNO3 volume. Lin Yu-Hao et al. observed a similar behavior when N-doped TiO2 powder underwent different calcination temperatures [39]. Note that even at 500 °C, the specific superficial area is higher than that of the value reported for Degussa-P25 (45.7 m2/g) [37].
(a) 200 10-100 10-500 40-100 40-500
3
Absorbed volume (cm /g, STP)
250
150 100 50 0 0.0
3
dV(logd) (cm /g)
2.0
0.2
(b)
0.4 0.6 0.8 Relative pressure (P/P0)
1.0
10-100 10-500 40-100 40-500
1.5
1.0
0.5
0.0
Fig. 3
3.3
0
2
4
6 8 10 12 14 16 18 20 Pore diameter (nm) a) Nitrogen adsorption-desorption isotherms, and b) pore size distribution curves for some N-doped TiO2 NPs.
UV-Vis spectroscopy
The optical properties of N-doped TiO2 NPs were studied through the UV-Vis diffuse spectra. Fig. 4a-b show the absorbance spectra of N-doped TiO2 NPs varying the calcination temperature, where it is possible to observe the absorption edge around 320 to 420 nm. A slight shift of this absorption edge towards visible light wavelengths is observed as the calcination temperature increases. This behavior was more evident when the N-doped TiO2 NPs were prepared using 40 mL of HNO3. The absorbance spectra were converted to the Kubelka–Munk function, described as [40].
,
Here,
is the absolute reflectance, which is calculated by a %Rsample/%Rspectralon ratio. The
spectralon disk used in the diffuse reflectance measurements has a diffuse reflectance of around 50 %. The band gap energy ( , considering
) was calculated by using the Tauc’s relation and an indirect allowed transition (
4a-b show the extrapolating of the linear portion of
with the
). The inserts in Fig. axis for N-doped
TiO2 NPs as a function of the calcination temperature. 6
1.5 1/2
(F(R)*h)
Absorbance (a. u.)
10-100°C 10-200°C 10-300°C 10-400°C 10-500°C
5
1.0
4 3 2
Eg
1 0 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 hv (eV)
0.5
(a) 0.0 250 300 350 400 450 500 550 600 650 Wavelength (nm) 6
(F(R)*h)
Absorbance (a. u.)
1/2
5
1.5
1.0
4 3 2
40-100°C 40-200°C 40-300°C 40-400°C 40-500°C Eg
1 0 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 h (eV)
0.5
(b)
Fig. 4
0.0 250 300 350 400 450 500 550 600 650 Wavelength (nm) UV-Vis spectra of N-doped TiO2 NPs varying the calcination temperature for samples prepared by using a) 10 mL of HNO3, and b) 40 mL of HNO3.
Fig. 5 shows the dependence of
as a function of calcination temperature for N-doped TiO2
NPs prepared at different HNO3 volumes. It is possible to observe that for the N-doped TiO2 NPs prepared at 10 and 40 mL of HNO3 there is a clear decrease of the
as the calcination
temperature increases. This behavior is more evident for the N-doped TiO2 NPs prepared with 40 mL of HNO3. It is reported that the nitrogen incorporation modifies the TiO2 band structure, shifting the absorption edge to visible light wavelengths that lead to narrowing the band gap energy [41]. Noted that for the 10-T and 40-T samples as the crystallite size increases the value decreases (see Fig. S2, supplementary information). 3.20 3.15
Eg(eV)
3.10 3.05
10-T 20-T 30-T 40-T
3.00 2.95 2.90
100
200
300
400
500
Temperature (°C) Fig. 5
3.4
Band gap energy ( ) as a function of the calcination temperature for different types of N-doped TiO2 NPs prepared with 10, 20, 30 and 40 mL of HNO3.
FT-IR spectroscopy
The presence of nitrogen in the N-doped TiO2 NPs was investigated using the FT-IR spectroscopy. Fig. 6 shows the FT-IR spectra of the N-doped TiO2 NPs varying the HNO3 volume and calcination temperature. The 2500 to 3600 cm-1 region displays three vibration modes located at ~3380 cm-1, ~3182 cm-1 and ~3025 cm-1 which correspond to O–H, N–H, and
C–H stretching vibration modes [42,43]. The band located at 1640 cm-1 is associated to the O–H bending vibration [44]. The band located at 1421 cm-1 corresponds to N–H bending vibration modes, and this was associated with nitrogen incorporated in the TiO2 crystal lattice [31]. The band located at 1313 cm-1 can be related to a N–O stretching vibration of
and
species,
which are characteristics of interstitial nitrogen [41,45,46]. The effect of the variation of the HNO3 volume on molecular structure of the N-doped TiO2 NPs is clearly observed in Fig. 6a-b. At low calcination temperature (100 °C), the presence of the N–H and N–O vibration modes depend on the HNO3 volume, while for a high calcination temperature (500 °C) these vibration modes are below the detection limits of the equipment. A similar behavior is observed for when the N-doped TiO2 NPs were prepared varying the calcination temperature (10-T and 40-T samples), see Fig. 6c-d. Then, both the HNO3 volume and the calcination temperature have a strong influence on the stability of nitrogen in N-doped TiO2 NPs.
100 (b)
Transmittance (%)
Transmittance (%)
100 (a) 90 -1
80
1640 cm
-1 70 3380 cm
-1
1421 cm -1
3182 cm
60
-1
3025 cm
70
-1
3380 cm
-1
3240 cm
10-500 20-500 30-500 40-500
1313 cm
60
50 4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm ) 100 (d)
90 1640 cm-1
80 70 3380 cm-1 3182 cm-1 3025 cm-1
-1
10-100 1421 cm 10-200 1313 cm-1 10-300 10-400 10-500
Transmittance (%)
100 (c)
Transmittance (%)
80
-1
1640 cm
-1
10-100 20-100 30-100 40-100
50 4000 3500 3000 2500 2000 1500 1000 -1 Wavenumber (cm )
60
90
90 -1
80
1640 cm
-1 70 3380 cm
-1
-1
3182 cm
60
-1
3025 cm
40-100 1421 cm -1 40-200 1313 cm 40-300 40-400 40-500
50 50 4000 3500 3000 2500 2000 1500 1000 4000 3500 3000 2500 2000 1500 1000 -1 -1 Wavenumber (cm ) Wavenumber (cm ) Fig. 6 FT-IR spectra of N-doped TiO2 powder varying (a)-(b) the HNO3 volume, and (c)- (d) the calcination temperature.
3.5
XPS analysis
The elemental composition and superficial chemistry of the N-doped TiO2 NPs was investigated by XPS analysis, see Fig. 7. High resolution XPS spectra were acquired for the 10-100, 40-100, and 40-500 N-doped TiO2 NPs. All spectra were aligned to C 1s signal located at 284.8 eV, which is associated mainly to adventitious carbon. Fig. 7a shows the wide-scan XPS spectra of the N-doped TiO2 NPs, where it is possible to observe only the presence of titanium, oxygen, carbon, and nitrogen in the samples. On the other hand, high resolution XPS spectra show important changes in chemical environments of the elements present in the sample. Fig. 7b shows the N 1s spectra where it is possible to observe the presence of one chemical component centered
at 400.2 eV for all samples. A weak nitrogen peak centered at 396.2 eV was observed for the 10100 and 40-100 samples, while the 40-100 sample showed an additional peak centered at 407.38 eV. Previous studies have shown the presence of these peaks in around 396 to 410 eV, and their assignation strongly depends on the synthesis method and the doping source [47]. Some reports suggest that the chemical components centered between ~ 396 to 398 eV correspond to nitrogen in TiN, mostly as substitutional nitrogen in the TiO2 crystal lattice [41,48–50]. The chemical component centered on ~ 400.2 eV was assigned to nitrogen forming Ti-O-N or Ti-N-O bonds [48,51,52]. Furthermore, the chemical component centered at 407.38 eV was associated to and
species adsorbed on the catalyst surface. It should be mentioned that the chemical
components located at higher binding energies (~ 400.2 and 407.38 eV) are characteristics of interstitial nitrogen. Then, according to chemical components observed in the N 1s spectra, interstitial nitrogen is present in the three N-doped TiO2 NPs, while only the samples prepared at low temperatures (10-100 and 40-100) contain a small amount of nitrogen atoms replacing the oxygen atoms in the TiO2 crystal lattice. When the HNO3 volume increases there is a higher contribution of interstitial nitrogen and absorbed
and
species on the surface. This is
consistent with those observed from the FT-IR spectra, where is possible to observe the effect of the HNO3 volume and calcination temperature on the intensity of the vibration modes related to substitutional and interstitial nitrogen.
O 1s
1000
800
600
400
200
0
Intensity (a. u.)
C 1s
Ti 3s Ti 3p
Ti 2p
40-500 40-100 10-100
N 1s
1200
(b) 40-500
Survey
Ti 2s
O Auger
Intensity (a. u.)
(a)
40-100
10-100
Ti 2p3/2 Ti 2p1/2
E=0.16 eV
459.0 eV 457.14 eV
468 466 464 462 460 458 456 454 Binding energy (eV) Fig. 7
O 1s
(d)
Ti 2p
40-100
10-100
396.20 eV
400.20 eV
Binding energy (eV)
40-500
Intensity (a. u.)
Intensity (a. u.)
40-500
407.38 eV
410 408 406 404 402 400 398 396 394
Binding energy (eV) (c)
N 1s
40-100
10-100
534
O1
O2
532
530
528
526
Binding energy (eV)
a) XPS spectra (survey), and high resolution XPS spectra for b) N 1s, c) Ti 2p and d) O 1s core levels of some N-doped TiO2 samples.
The Ti 2p region was fitted using two doublets with a split orbit separation of around 5.7 eV, Fig. 7c. The first doublet located at 459.0 eV was associated to Ti4+ (TiO2), while the second one centered at low binding energies (457.14 eV) was related to oxygen vacancies induced by the presence of Ti3+ species [53–55]. Note that for a calcination temperature of 500 °C, the Ti 2p spectrum presented a shift of 0.16 eV to lower binding energy, which can be associated with changes in the electronic density of the TiO2 structure probably by the nitrogen and the Ti4+ reduction to Ti3+ species [49]. It is reported that the presence of Ti3+ improves the photocatalytic activity, since it creates located states below the conduction band [41,56]. Also, it is believed that the presence of nitrogen and Ti3+ species can improve the visible light absorption [57,58]. The
O 1s spectra were fitted using two singlets labeled O1 and O2, in Fig. 7d. These peaks were associated with oxygen signals from TiO2 and hydroxyl groups or adsorbed water molecules on the catalyst surface, respectively. For the O 1s spectrum of the 40-500 sample, a slight shift on its binding energy was observed which can be related to the creation of oxygen vacancies by increasing the calcination temperature [59].
Table 2. Atomic percentage of elements presents in the N-doped TiO2 NPs obtained by XPS.
3.6
Sample
N (at. %)
O (at. %)
C (at. %)
Ti (at. %)
Ti3+/Ti4+
10-100
1.27
51.75
18.98
24.62
0.10
40-100
1.95
46.10
26.90
21.97
0.03
40-500
0.36
47.83
25.58
22.78
0.12
TEM Analysis
Fig. 8 shows the TEM (left side) and HR-TEM (right side) micrographs for the 10-100, 10-500, 40-100, and 40-500 N-doped TiO2 NPs. From the TEM micrographs it is possible to observe that the N-doped TiO2 NPs have a semi-spherical shape with an average particle size less than 16 nm, Fig. 8 (left side). These values are consistent with those obtained by XRD analysis, see Table 1. The HR-TEM micrographs show that the N-doped TiO2 present a polycrystalline structure due to the presence of nanoparticles with different crystal orientations, in Fig. 8 (right side). The spacing between two adjacent parallel fringes is around 0.34 nm, which is consistent with the (101) plane of anatase-phase TiO2 NPs [48]. The selected area diffraction pattern (SAED) shows typical concentric rings that clearly support the presence of anatase-phase for all the N-doped TiO2 NPs [30]. It is possible to observe that the 40-500 sample presents a well-defined diffraction pattern, which is associated with a high crystallinity degree. The morphology was not affected by changing the HNO3 volume.
Fig. 8
TEM (left side) and HR-TEM (right side) micrographs of (a)-(b) 10-100, (c)-(d) 10-500, (e)-(f) 40-100, and (g)-(h) 40-500 N-doped TiO2 NPs.
3.7
Photocatalytic activity
The photocatalytic activity of the N-doped TiO2 NPs was assessed at room temperature by measuring the degradation efficiency of the RhB under irradiation of ultraviolet (UV-365 nm) and visible light. Prior the photocatalytic test, the RhB solution along with the catalyst was left in dark and stirred for 30 min to reach the adsorption-desorption equilibrium between the organic compound and catalyst surface.
0.8 0.6 UV light irradiation
0.4 0.2
0.0 200
300
1.0
Ansorbance (u. a.)
Absorbance (a. u.)
(a)
Sample: 10-500
1.0
RhB 5ppm 30 min Dark 10 min 20 min 30 min 40 min 50 min 60 min 70 min 80 min
400
(b)
Sample: 40-500 Visible light irradiation
0.8
0h 1h 2h 3h 4h 5h 6h 7h 8h 9h
0.6 54.4 nm
0.4 0.2
500
600
450
(c)
Wavelenght (nm) 1.0
0.0 400
700
500
550
600
650
Wavelengh (nm)
(c)
(d)
1.0
C/C0
C/C0
0.8 0.8 Photolysis 10-100 10-500 40-100 40-500
0.6
Photolysis 10-100 10-500 40-100 40-500
0.6 0.4 0.2 0.0
0.4
0
20
40
Time (min)
Fig. 9
60
80
0
1
2
3
4
5
6
7
8
Time (h) Absorption spectra of RhB (5 mg/L) under a) UV light (365 nm) and b) visible light as a function of exposition time in presence of the 10-500 and 40-500 catalysts, respectively. c)-d) Degradation curves of RhB for different types of Ndoped TiO2 catalysts.
9
10
The intensity variation of the absorbance spectra at different times under irradiation of UV and visible light is presented in Fig. 9a-b. For the case where UV light irradiation was employed, it is possible to observe a slight shift to the left of the maximum absorption peak (~2.3 nm) using the 10-500 catalyst. When visible light irradiation was employed, this behavior was significantly more evident and the maximum absorption peak was shifted around 54.4 nm when 40-500 catalyst was employed. This phenomenon is related to the formation of intermediates during the photocatalytic degradation process of the RhB solution in the presence of N-doped TiO2 catalyst and visible light irradiation [60,61]. A similar behavior during the RhB degradation was observed when the other N-doped TiO2 catalysts were employed. The maximum absorption peak of the RhB solution was shifted around 49 nm, 49.5 nm 52.7 nm when the 10-100, 10-500 and 40-100 catalysts were used, respectively. Fig. 9c-d show the relative RhB concentration ( ⁄ ) as a function of irradiation time for various types of N-doped TiO2 catalysts under UV and visible light. The photodegradation rate of the RhB solution was studied using a pseudo-first-order kinetic: ⁄ Here
is the residual concentration of RhB at time ,
apparent reaction rate constant and
is the initial RhB concentration,
is the
is the reaction time. The photolysis test shows that around
9.5 % of RhB was decomposed within 80 min under UV light and 15 % of RhB when visible light was used during 9 h; both processed without any type of catalyst. Results show that when the N-doped TiO2 catalyst was incorporated to the photocatalytic test the RhB degradation was efficiently accelerated both under UV and visible light. Table 3 summarizes the kinetic parameters as: kinetic constant ( ), half-life of the RhB (
⁄
) and percentage of RhB photo
degradation using UV and visible irradiation. It is worth noting that the higher degradation percentage
⁄
was obtained when the N-doped TiO2 catalysts were calcinated at 500
°C. To corroborate the degradation percentage, TOC analysis was done to the RhB solution when the 10-500 powder was used as catalyst. The test showed that around 33 % of total organic carbon was removed within the first 60 min under UV light irradiation. This is consistent with what is observed from the degradation curves showed in Fig. 9c.
Table 3. Kinetic parameters of the degradation of RhB using different catalysts. UV light irradiation Sample
(min-1)
% degradation after 80 min
⁄
(min)
10-100
42.5
6.9
100.4
10-500
50.2
8.2
84.5
40-100
45.4
4.5
154
40-500
40.3
6.8
102
Visible light irradiation (min-1)
% degradation after 540 min
⁄
(min)
10-100
94.3
4.3
161.1
10-500
93.7
4.3
161.1
40-100
96.6
4.8
144.4
40-500
99.2
7.7
90.0
The photocatalytic tests show that the N-doped TiO2 NPs can be activated by using both UV and visible light. It is expected that N-doped TiO2 NPs will be excited by UV light due to photon energy being greater than the energy band gap. For the case where visible light is employed, the photocatalytic activity of N-doped TiO2 NPs can be mainly related to the presence of impurities located within the energy band gap. From XPS results, it is possible to prove two important aspects that can determinate the photocatalytic activity of N-doped TiO2 NPs under visible light. The nitrogen presence in the mesoporous TiO2 structure slightly decreased the band gap energy of the N-doped TiO2. This can be a consequence of the creation of localized impurities levels close to the valence band [62]. Furthermore, some reports showed that oxygen sites are partially
replaced by nitrogen atoms in the TiO2 crystal structure, which increases the oxygen vacancies leading to the formation of Ti3+ species that improve the photocatalytic efficiency [56,62]. The incorporation of nitrogen and the presence of Ti3+ species in the TiO2 structure produce the formation of midgap energy levels on the N doped TiO2 band gap and more electrons can be transferred to the conduction band [57,58,63]. The photo-induced electrons into the conduction band of N-doped TiO2 reduce O2 leading to the formation of superoxide ions (
), while the
holes tend to form •OH radicals that decompose the RhB molecules. Fig. 10 shows the mechanism for the generation of charge carriers and the photodegradation process of RhB in presence of light (UV or visible).
Fig. 10 Scheme of the photocatalytic mechanism for the RhB degradation employing N-
doped TiO2 powder.
Reports suggest that the photocatalytic activity occurs when the catalyst presents a small crystallite size and a high superficial area [64]. However, the presence of impurities plays fundamental role in the photocatalytic activity of the N-doped TiO2. Then, we consider that the synergic effect between the crystal structure and the impurities level play a key role in the photocatalytic activity of TiO2 NPs.
4
CONCLUSIONS
N-doped TiO2 NPs was synthesized using a facile synthesis route by coprecipitation method. Both the calcination temperature and the HNO3 volume had a strong influence on the structural properties of the N-doped TiO2 NPs. As the calcination temperature was increased, the crystallite size increases. N-doped TiO2 NPs less than 16 nm were obtained, which are consistent with the HR-TEM micrographs. By FT-IR and XPS measurements the presence of nitrogen into the Ndoped TiO2 structure was confirmed. Also, XPS analysis showed the presence of the impurities related to Ti3+ states. These impurities play a fundamental role in the photocatalytic performance of the N- doped TiO2 by introducing localized states within the band gap energy of the N-doped TiO2. The photocatalytic activity of the N-doped TiO2 was assessed under irradiation of UV and visible light. The photocatalytic results showed that NPs can be activated by using both UV and visible light. The N-doped TiO2 NPs calcinated at 500 °C presented higher photocatalytic efficiency, which can be associated to the synergic effect between the high specific superficial area, crystal structure and band-gap narrowing. 5
ACKNOWLEDGMENTS
This research was supported by CONACYT (CB-2011-166366, Cátedras-CONACYT 144), Universidad de Guadalajara (PRO-SNI 237642-2017), and Tecnológico de Monterrey-Campus Monterrey through the Research Chair in Nanotechnology and Devices. We are expressing our sincere thanks to CIMAV-Chihuahua and Departamento de Ecomateriales y Energía of the FICUANL from Mexico for their valuable support given for this work. 6
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