Synthesis and characterization of N-doped TiO2 photocatalysts with tunable response to solar radiation

Applied Surface Science 305 (2014) 281–291

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Synthesis and characterization of N-doped TiO2 photocatalysts with tunable response to solar radiation Athanasia Petala a , Dimitris Tsikritzis a , Mary Kollia b , Spyridon Ladas a , Stella Kennou a , Dimitris I. Kondarides a,∗ a b

Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece Laboratory of Electron Microscopy and Microanalysis, School of Natural Sciences, University of Patras, GR-26504 Patras, Greece

a r t i c l e

i n f o

Article history: Received 13 November 2013 Received in revised form 29 January 2014 Accepted 9 March 2014 Available online 19 March 2014 Keywords: Titanium dioxide Doping Nitrogen Optical properties XPS

a b s t r a c t Modification of the electronic structure of wide band gap semiconductors by anion doping is an effective strategy for the development of photocatalytic materials operating under solar light irradiation. In the present work, nitrogen-doped TiO2 photocatalysts of variable dopant content were synthesized by annealing a sol–gel derived TiO2 powder under flowing ammonia at temperatures in the range of 450–800 ◦ C, and their physicochemical and optical properties were compared to those of undoped TiO2 samples calcined in air. Results show that materials synthesized at T = 450–600 ◦ C contain relatively small amounts of dopant atoms and their colour varies from pale yellow to dark green due to the creation of localized states above the valence band of TiO2 and the formation of oxygen vacancies. Treatment with NH3 at T > 600 ◦ C results in phase transformation of anatase to rutile, in a significant decrease of the specific surface area and in formation of TiN at the surface of the TiO2 particles. The resulting dark grey (T = 700 ◦ C) and black (T = 800 ◦ C) materials display strong absorption in both the visible and NIR regions, originating from partial reduction of TiO2 and formation of Ti3+ defect states. The present synthesis method enables tailoring of the electronic structure of the semiconductor and could be used for the development of solar light-responsive photocatalysts for photo(electro)chemical applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic and photoelectrochemical reactions that take place over irradiated semiconductor surfaces have attracted significant attention is recent years in view of their potential applications in purification of water and air [1–3], development of self-cleaning and self-sterilizing surfaces [4] as well as conversion and storage of solar energy [4–9]. The key parameters that determine the ability of a semiconductor to promote such reactions are its band gap energy (Ebg ) and the relative positions of the upper edge of the valence band (EVB ) and the lower edge of the conduction band (ECB ) with respect to the potential levels of redox couples present in the reaction mixture. The band gap energy determines the threshold wavelength, bg , of irradiation required for the excitation of the semiconductor and, therefore, its response to solar light, whereas the positions of EVB and ECB determine the ability of the semiconductor to promote a given reaction and its stability against photo-corrosion. In addition to thermodynamic criteria, photocat-

∗ Corresponding author. Tel.: +30 2610969527; fax: +30 2610991527. E-mail address: [email protected] (D.I. Kondarides). http://dx.doi.org/10.1016/j.apsusc.2014.03.062 0169-4332/© 2014 Elsevier B.V. All rights reserved.

alytic performance is determined by the quantum efficiency, which depends on the physicochemical properties of the material, and is generally higher for semiconductors with high crystalline quality and small particle size. Thermodynamic and kinetic considerations pose strong limitations and restrict significantly the number of efficient semiconductor photocatalysts. This is because, in most cases, the electronic properties of semiconductors do not satisfy all necessary requirements in terms of optical and catalytic properties as well as in terms of stability against photo-corrosion. In particular, wide band gap semiconductors, may be active and stable but are not efficient solar photocatalysts. On the other hand, materials with smaller band gap energies, which utilize more efficiently the solar light, are generally unstable during operation, especially in aqueous media [6]. In this respect, efforts in the field of photo(electro)catalysis are currently directed toward the development of efficient and stable semiconductor photocatalysts with enhanced response to solar radiation and indoor light [9–11]. Among the large number of semiconductors investigated to date [9–11], titanium dioxide (TiO2 ) is considered to be the best generalpurpose photocatalyst and is the material of choice in most research studies [12]. This is because TiO2 meets most of the requirements of an ideal semiconductor, including high chemical and

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photochemical stability, biological inertness and low cost. The conduction band electrons of TiO2 are good reductants, able to reduce water to hydrogen and dioxygen to superoxide/hydroperoxide radicals, whereas valence band holes are excellent oxidants that can oxidize water to hydroxyl radicals. The major drawback of TiO2 is that, because of its relatively wide band gap, i.e., 3.23 eV (bg = 384 nm) for anatase and 3.02 eV (bg = 410 nm) for rutile [12,13], it can be excited only by photons in the near-UV region and, therefore, can make use of less than 5% of the solar energy that reaches the Earth’s surface. If the band gap energy of TiO2 could be decreased to levels corresponding to the energy of visible photons, then the fraction of solar energy that would be available for photocatalysis could be increased to about 50%. In this respect, numerous attempts have been made to improve the inherently low efficiency of TiO2 in harvesting sunlight by shifting its spectral response toward the visible spectrum [9–12]. Most common methods employed for this purpose include sensitization with dyes or organic semiconductors [5,14], coupling with narrow-band gap inorganic semiconductors [15,16], and doping with metal cations [17] or non-metallic elements like nitrogen [18], sulfur [19] and carbon [19,20]. In the latter cases, the aim is to decrease the bandgap of TiO2 and/or to introduce intra-band gap states, both of which may result in enhanced visible light absorption [12]. The same effect can be induced by reduction of TiO2 with H2 at moderate temperature and high pressure, which results in the formation of “black TiO2 ” that absorbs light at wavelengths as high as 900 nm [21]. It should be noted, however, that shifting the spectral response of a semiconductor toward the visible region does not necessarily result in materials with improved photocatalytic performance. One of the most promising strategies to enhance the response of TiO2 to visible light is doping with nitrogen. Results of several studies carried out over nanocrystalline powders [18,22–26], thin films [18,27–31], nanotubes [32] and single crystals [33] have demonstrated that doping with nitrogen can be used effectively to modify the optical and electronic properties of TiO2 . Incorporation of nitrogen in titanium dioxide has been achieved employing a variety of physical and chemical methods [4], including ion implantation [33], magnetron sputtering [28–30], chemical vapour deposition [62], sol–gel [23,24,26,34] and aerosol-assisted processes [35], and treatment of TiO2 with NH3 [18,22,25,27,36]. In all cases, nitrogen-doped TiO2 , hereafter N-TiO2 , was shown to exhibit enhanced absorption to visible light, in a manner that depends on the synthesis conditions employed and the level of nitrogen doping. Based on experimental results [22–36] and theoretical calculations [18,24,37], several attempts have been made to elucidate the origin of absorption of these materials to visible light and to identify the chemical nature and location of the dopant atoms in the solid. Asahi et al. [18] first proposed that visible light absorption of NTiO2 is due to band-gap narrowing induced by mixing of the N2p level with the valence band of TiO2 . Other researchers questioned this explanation [4] and suggested that nitrogen-doping results in the formation of localized mid-gap states above the valence-band edge [22,25,27,28,34,38]. It was also shown that N-doping facilitates the formation of oxygen vacancies [29,34], which result in the creation of mid-gap states below the conduction band of TiO2 [24,28,29,33,34]. Serpone and coworkers [38,39] argued that the red-shift of the adsorption edge of anion-doped TiO2 is mainly due to formation of colour centres associated with oxygen vacancies. Regarding the nature and location of the dopant in the solid, it has been proposed that nitrogen atoms can occupy substitutional [18,34,40] or interstitial [24,34,40] sites. In the first case, the N atom is bound to three Ti atoms by replacing a lattice oxygen atom in TiO2 , whereas in the latter case the N atom is bound to one or more O lattice ions and results in the formation of NOx -like lattice defects [23,24,39,41]. In spite of the extensive efforts made in this direction, no general consensus has been reached yet regarding

the chemical nature and location of the dopant in the solid as well as the electronic structure of N-TiO2 photocatalysts [4,34,39,40]. A detailed study is being carried out in the present laboratories in an attempt to develop efficient and stable solar light-responsive photocatalytic materials and to identify the key parameters that determine photocatalytic performance. In the present work, a set of N-doped TiO2 photocatalysts of variable nitrogen content have been synthesized by treating sol–gel derived TiO2 under flowing ammonia at temperatures in the range of 450–800 ◦ C, and were compared to undoped TiO2 samples calcined in air at the same temperatures. The synthesized materials were characterized with the use of several techniques, including BET, XRD, UV/vis DRS, TEM and XPS, in order to study their physicochemical and optical properties and to obtain information on the electronic structure of the Ndoped photocatalysts. The final aim is to utilize the present results in order to synthesize visible light-responsive materials with optimized physicochemical and optical properties that will be tested as photocatalysts and photoanodes for environmental and energyrelated applications. 2. Experimental 2.1. Catalyst preparation Titanium dioxide (TiO2 ) in powder form was synthesized employing a sol–gel method. For this, an aqueous ethanol solution was prepared and adjusted at pH 2 with the addition of HNO3 . The solution was then added under vigorous stirring into an ethanol solution of Ti[OCH(CH3 )2 ]4 and a gel was formed. The beaker containing the gel was put into a water bath for 7–8 h until nearly all the liquid was evaporated and then dried in an oven at 120 ◦ C for 12 h. In order to prepare the nitrogen-doped samples, portions of the above TiO2 powder were placed in a quartz tube and heated at the desired temperature (450, 500, 550, 600, 700 or 800 ◦ C) under flowing helium (He). The flow was then switched to 0.5% NH3 (in He) and the sample was maintained at that temperature for a period of 8 h. Finally, the powder was cooled down to room temperature under He flow and stored in sealed vials for further use. Materials thus prepared are denoted in the following as N-TiO2 (T), where T indicates the calcination temperature. For comparison purposes, undoped TiO2 catalysts, denoted as TiO2 (T), were prepared under the same conditions with the exception that they were heat-treated under a flow of synthetic air for 2 h (instead of NH3 flow for 8 h). The TiO2 (T) and N-TiO2 (T) materials thus prepared are listed in Table 1. 2.2. Measurements of specific surface areas The specific surface areas (SSA) of the synthesized materials were determined with the Brunauer–Emmett–Teller (BET) method with the use of a Micromeritics (Gemini III 2375) instrument, employing nitrogen physisorption at the temperature of liquid nitrogen (77 K). Prior to each measurement, the sample was outgased under dynamic vacuum at 250 ◦ C for 2 h. 2.3. X-ray diffraction (XRD) measurements Powder X-ray diffraction patterns were obtained using a Brucker D8 Advance instrument equipped with a Cu K␣ source operated at 40 kV and 40 mA. Data were collected in the 2 range of 2◦ to 85◦ at a scan rate of 0.05◦ s−1 and a step size of 0.015◦ . Phase identification was based on JCPDS cards. The anatase content of TiO2 (xA ) was estimated using the equation [42]:



xA = 1 + 1.26

 I −1 R

IA

(1)

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283

Table 1 Characteristics of the synthesized materials. Sample

Calcination temperature (◦ C) – gas atmosphere

TiO2 TiO2 (450) TiO2 (500) TiO2 (550) TiO2 (600) TiO2 (700) TiO2 (800) N-TiO2 (450) N-TiO2 (500) N-TiO2 (550) N-TiO2 (600) N-TiO2 (700) N-TiO2 (800) a b c

As prepared 450 – air 500 – air 550 – air 600 – air 700 – air 800 – air 450 – NH3 500 – NH3 550 – NH3 600 – NH3 700 – NH3 800 – NH3

SSAa (m2 g−1 )

TiO2 phase compositionb (% anatase)

172 111 91 60 17 0.2 <0.1 89 67 38 20 4 1

100 100 100 99 39 1.5 0 100 100 98 86 1.5 0

Primary crystallite sizec (nm)

Anatase

Rutile

TiN

5.7 7.7 9.6 13 24 31 – 8.7 11 17 27 33 –

– – – – 58 63 69 – – 32 45 67 68

– – – – – – – – – – – 22 23

Specific surface area, determined with the B.E.T. method. Percent anatase content, estimated with the use of Eq. (1). Primary crystallite size estimated from XRD line broadening with the use of Eq. (2).

where IA and IR are the intensities of the anatase (1 0 1) and rutile (1 1 0) reflections, respectively. The primary crystallite size of nanocrystals was estimated by means of the Debye–Scherrer’s formula d=

0.9 B cos 

(2)

where  is the X-ray wavelength corresponding to Cu K␣ radiation (0.15406 nm),  is the diffraction angle and B is the line broadening (in radians) at half maximum. Diffraction peaks used for the estimation of the d values were located at angles (2) of 25.3◦ for anatase TiO2 (1 0 1), 27.5◦ for rutile TiO2 (1 1 0) and 43.2◦ for TiN. 2.4. Transmission electron microscopy (TEM) The morphological characteristics of the synthesized samples were examined using transmission electron microscopy on a JEOL JEM-2100 system, operated at 200 kV (point resolution 0.23 nm). TEM Images were recorded by means of an Erlangshen CCD Camera (Gatan Model 782 ES500 W), while films (Kodak SO-163) were used for HRTEM images. The specimens were prepared by dispersion in water and spread onto a carbon-coated copper grid (200 mesh). 2.5. Diffuse reflectance spectroscopy (DRS) The diffuse reflectance spectra were recorded on a UV–vis spectrophotometer (Varian Cary 3) equipped with an integration sphere, using BaSO4 as a reference. The catalyst powder was loaded into a quartz cell and the spectrum was obtained at room temperature in the wavelength range of 200–800 nm. The DR measurements were converted into the equivalent absorption coefficient by applying the transformation based on the Kubelka–Munk function: F(R∞ ) =

(1 − R∞ )2 K() = 2R∞ S()

(3)

where K and S are the absorption and scattering coefficients, respectively, and R∞ = R/Rref is the reflectance. The optical band gaps of the semiconductors were evaluated based on the following expression [43] (˛h)

1/n

= B(h − Ebg )

(4)

where a is the absorption coefficient, h is the incident photon energy, Ebg is the band gap energy, B is a constant related to the effective masses of charge carriers associated with valance and conduction bands, and n is a factor that depends on the kind of

optical transition induced by photon absorption. Band gap energies (absorption thresholds) were estimated assuming that F(R) values are proportional to the optical absorption coefficients and that the synthesized materials are indirect semiconductors, as is TiO2 , for which n = 2. Thus, the values of Ebg were obtained from the plot of [F(R)h]1/2 versus h (Tauc plot) in the region of high absorption and the extrapolation of the linear region to the horizontal axis, at zero F(R) [43–45]. 2.6. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) measurements were carried out in a MAX200 (LEYBOLD/SPECS) Electron Spectrometer using non-monochromatized Mg K␣ radiation (1253.6 eV) and an EA200-MCD analyser operated at a constant pass energy of 100 eV. The analysis was performed on the as-received samples along the surface normal and the analysed specimen area was a ∼4 × 7 mm2 rectangle near the specimen centre. The binding energy (BE) scale in all reported spectra was corrected for electrostatic charging using the main C1s peak at 284.8 eV due to atmospheric surface contamination. The uncertainty in the corrected BE values is of the order of ±0.15 eV. Further details on spectrometer calibration and data treatment can be found elsewhere [46]. 3. Results and discussion 3.1. Textural characteristics and phase composition The specific surface areas of the TiO2 (T) and N-TiO2 (T) catalysts, estimated with the BET method, are listed in Table 1. It is observed that the as prepared TiO2 sample obtained after drying at 120 ◦ C for 12 h has a relatively high SSA of 172 m2 g−1 . Calcination of this sample in air for 2 h at temperatures in the range of 450–600 ◦ C results in a progressive decrease of SSA from 111 to 17 m2 g−1 . Further increase of the calcination temperature at 700 and 800 ◦ C has a detrimental effect on SSA, which drops to values lower than 0.2 m2 g−1 . Qualitatively similar results were obtained for the nitrogendoped materials prepared following treatment with ammonia for 8 h in the same temperature range (Table 1). The X-ray diffractograms of the TiO2 (T) samples are shown in Fig. 1A. It is observed that samples treated under flowing air at temperatures up to 550 ◦ C are characterized by reflections attributed to the anatase phase of TiO2 . XRD peaks due to the rutile phase appear for samples calcined at 600 ◦ C, and their relative intensity increases

A. Petala et al. / Applied Surface Science 305 (2014) 281–291

R

TiO2

R

R

R

2

R

A

R

R

RR

R

Signal (a.u.)

R

800 C 700 C 600 C

A

A

AA

A

AA

120

(A)

-1

R

(A)

Specific surface area (m g )

284

550 C 500 C

100

TiO2

80 60 40

N-TiO2

20 0

450 C

450

As prepared TiO

500

550

600

650

700

750

800

o

20

30

40

50

60

70

Calcination temperature ( C)

80

Angle (2θ)

R

R

R

R

N-TiO2 R

R

A

Signal (a.u.)

R

RR

R

*

*

*

*

R

800 C 700 C 600 C

A A A

A

AA

A

AA

Anatase content (%)

(B)

100

(B)

80 60

N-TiO2

TiO2

40 20 0

550 C

450

500

550

600

650

700

500 C

750

800

o

Calcination temperature ( C)

450 C

20

30

40

50

60

70

80

Angle (2θ) Fig. 1. X-ray diffraction patterns of (A) TiO2 and (B) N-TiO2 catalysts calcined at the indicated temperatures. Diffraction peaks denoted as “A”, “R” and “*” are due to anatase TiO2 , rutile TiO2 and titanium nitride (TiN) phases, respectively. All curves are displaced vertically by arbitrary amounts.

significantly with further increase of temperature. The sample treated at 800 ◦ C is completely converted to rutile. This behaviour is rather expected since temperatures typically higher than 600 ◦ C are required for the transition of anatase to rutile. The XRD patterns obtained for the N-TiO2 (T) catalysts are shown in Fig. 1B. Similar to what was observed for TiO2 (T) samples, transformation of anatase to rutile takes place at temperatures around 600 ◦ C. In addition to the reflections attributed to the two allotropic phases of TiO2 , two additional peaks, located at angles (2) of 37.2 and 43.2◦ , can be clearly discerned in the XRD spectra of N-TiO2 (700) catalyst, together with less clearly distinguishable peaks located at ca. 62.6◦ , 75.1◦ and 79.1◦ . These peaks, which are attributed to the presence of titanium nitride (TiN) [47,48], increase in intensity with increase of calcination temperature at 800 ◦ C. It is of interest to note that the most intense TiN peak at 43.2◦ observed for the N-TiO2 (800) sample is about 20 times weaker, compared to that of the rutile (1 1 0) reflection at 27.5◦ (Fig. 1B). This indicates that a relatively small portion of TiO2 is reduced to crystalline TiN under the present conditions. It has been reported that complete conversion of TiO2 into TiN can be achieved upon heating in ammonia atmosphere at temperatures of 850 ◦ C for 6 h [36]. The XRD patterns of Fig. 1 were used to estimate the anatase content of TiO2 and the mean crystallite size of the three phases

Mean crystallite size (nm)

As prepared TiO

Anatase Rutile TiN

80

(C)

TiO2 N-TiO2

60

40

20

0 450

500

550

600

650

700

750

800

o

Calcination temperature ( C) Fig. 2. Effects of calcination temperature on (A) specific surface area, (B) anatase content, and (C) mean crystallite size of the synthesized TiO2 (T) and N-TiO2 (T) catalysts.

detected, according to Equations (1) and (2), respectively, and results obtained are listed in Table 1. For comparison purposes, results of BET and XRD experiments are shown graphically in Fig. 2, where the SSA (Fig. 2A), anatase content (Fig. 2B) and mean crystallite size (Fig. 2 C) of the TiO2 (T) and N-TiO2 (T) samples are plotted as functions of calcination temperature. It is observed that treatment of the as prepared TiO2 sample at a given temperature under air flow for 2 h or under ammonia flow for 8 h results in materials with similar characteristics. A notable exception is the considerably higher anatase content of N-TiO2 (600) compared to that of TiO2 (600) (Fig. 2B), which indicates that incorporation of nitrogen atoms into the crystal matrix of TiO2 prevents, to some extent, the

A. Petala et al. / Applied Surface Science 305 (2014) 281–291

285

Fig. 3. TEM (left side) and HRTEM (right side) images of (A) undoped TiO2 (500) sample and N-TiO2 (T) photocatalysts synthesized at temperatures (B) 500 ◦ C, (C) 600 ◦ C and (D) 800 ◦ C.

phase transition of anatase to rutile. It is also worth mentioning that doping with nitrogen has a small but measurable beneficial effect on the SSA of samples calcined at temperatures higher than 550 ◦ C (Fig. 2A), despite the higher calcination period used for N-TiO2 (T) (8 h) compared to TiO2 (T) (2 h) catalysts. These observations are in general agreement with results of previous studies, which showed

that doping of TiO2 with cations or anions partly retains the specific surface area and stabilizes the anatase structure of TiO2 in a manner that depends on the dopant concentration and the synthesis conditions employed [49]. For both sets of catalysts investigated, increase of calcination temperature results in a progressive growth of the grain size (Fig. 2C), caused by the high surface energy of

286

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1/2

(A)

[F(R)*hv ]

N-TiO2(500) TiO2(500)

2.30 eV

3.06 eV 3.17 eV

2.0

2.5

3.0

3.5

4.0

4.5

Energy (eV)

(B)

the nanocrystalline particles, which leads to a densification of the material and, concomitantly, to decreased surface areas (Fig. 2A). TEM and HRTEM images of selected samples are presented in Fig. 3. It is observed that the undoped TiO2 (500) sample comprises irregular spherical nanoparticles of approximately 10 nm diameter with the structure of anatase, as indicated by HRTEM (Fig. 3A). In consistency with XRD results, no notable difference is observed in the particle size and the structure of TiO2 after the N doping at 500 ◦ C (Fig. 3B). Increase of synthesis temperature at 600 ◦ C is accompanied by a change in nanoparticle morphology, characterized by crystal growth and appearance of rutile TiO2 (Fig. 3C), which is the only allotropic phase of TiO2 that is detected for the sample treated at 800 ◦ C (Fig. 3D). Interestingly, crystalline TiN was not detected with HRTEM. This is in general agreement with results of XRD experiments, which indicate the presence of a very small quantity of crystalline TiN in the N-doped samples synthesized at 700 and 800 ◦ C (Fig. 1B). Based the above, and taking into account the results of XPS experiments (see below), it may be argued that TiN is mainly present as an amorphous layer of the surface of photocatalyst particles (Fig. 3C). 3.2. Optical properties 3.2.1. Absorption thresholds of TiO2 and N-TiO2 catalysts In Fig. 4 are shown the absorption spectra of the undoped TiO2 (500) catalyst (trace a) and the N-TiO2 (T) catalysts synthesized at temperatures in the range of 450–800 ◦ C (traces b–g). It is observed that the undoped sample absorbs photons of wavelengths lower than ca. 400 nm, as expected for anatase TiO2 . The spectra of the N-TiO2 photocatalysts synthesized at temperatures of 450 ◦ C and 550 ◦ C are characterized by the presence of absorption “tails” which extend up to ca. 550 nm (traces b–d), whereas materials prepared at higher temperatures exhibit strong absorption in both the visible and near infrared (NIR) spectral regions (traces e–f). As a general trend, absorption at wavelengths higher than 400 nm increases monotonically with increase of calcination temperature, which is evident by visual inspection of the N-TiO2 samples (inset of Fig. 4). In particular, the initially white TiO2 powder turns to pale yellow (T = 450 ◦ C), yellow (T = 500 ◦ C), yellow-green (T = 550 ◦ C), green (T = 600 ◦ C), grey (T = 700 ◦ C) and, eventually, black (T = 800 ◦ C), upon increasing calcination temperature under flowing ammonia. In contrast, the undoped TiO2 (T)

1/2

N-TiO2(600)

[F(R)*hv ]

Fig. 4. UV–vis diffuse reflectance spectra of (a) the undoped TiO2 (500) sample, and the N-TiO2 (T) photocatalysts synthesized at temperatures of (b) 450, (c) 500, (d) 550, (e) 600, (f) 700 and (g) 800 ◦ C. Inset: Picture of selected catalysts.

TiO2(600)

2.07 eV

2.61 eV

2.0

2.5

2.98 eV

3.0

3.5

4.0

4.5

Energy (eV) Fig. 5. Tauc plots obtained for the TiO2 and N-TiO2 catalysts synthesized at temperatures of (A) 500 ◦ C and (B) 600 ◦ C.

powders annealed under air flow at the same temperatures are all white in colours and do not absorb in the visible range (spectra not shown for brevity). This implies that the improved absorption of NTiO2 (T) samples at wavelengths longer than ca. 400 nm is due to the incorporation of nitrogen into the crystal matrix of TiO2 , in agreement with results of previous studies [23,25,26,30,32,34,41,50]. The optical band gaps of the synthesized materials were determined according to the Tauc method (Eq. (4)), and representative results obtained for TiO2 (T) and N-TiO2 (T) samples calcined at 500 ◦ C and 600 ◦ C are shown in Fig. 5. Regarding the TiO2 (500) sample (Fig. 5A), it is observed that extrapolation of the linear part of the Tauc plot at zero F(R) yields an absorption threshold of 3.17 eV, which corresponds to the band gap energy of anatase TiO2 (∼3.2 eV) [12,13]. The optical band gap of N-TiO2 (500) is red-shifted by ca. 0.1 eV, compared to that of TiO2 (500), and a second absorption threshold can be distinguished in the visible region, located at ca. 2.3 eV (Fig. 5A). Increase of the calcination temperature at 600 ◦ C results in partial transformation of anatase to rutile (see Fig. 2B) and, as a result, the band gap of TiO2 (600) decreases to ca. 3.0 eV (Fig. 5B), which is characteristic of the rutile phase of TiO2 [12,13]. Regarding the N-TiO2 (600) sample, two absorption thresholds can be distinguished located at ca. 2.6 eV and 2.1 eV (Fig. 5B). An additional absorption threshold should be present in the NIR region for N-TiO2 samples synthesized at temperatures higher than 600 ◦ C (Fig. 4, traces e–g), but its position cannot be determined by the present UV–vis DR spectra, which are restricted to wavelengths shorter than 800 nm. Qualitatively similar results, showing the presence of two or three absorption thresholds, have been reported for a number of anion-doped TiO2 materials [20,50–53].

A. Petala et al. / Applied Surface Science 305 (2014) 281–291

3.6

Ebg (eV)

3.2 2.8

(I)

(II)

0.1 eV

a

0.7 eV

b

(III) 0.2 eV 0.4 eV

2.4

c 2.0 1.6

400

500

600

700

800

900

o

Calcination tempeature ( C) Fig. 6. Absorption thresholds of the synthesized catalysts as functions of calcination temperature, estimated with the Tauc method: (a) primary band gap of TiO2 (T) catalysts; (b) primary band gap of N-TiO2 (T) catalysts; (c) intra band gap of N-TiO2 (T) catalysts. Regions (I), (II) and (III) are discussed in the text.

The absorption thresholds of all TiO2 (T) and N-TiO2 (T) samples synthesized in the present study were estimated in a way similar to that shown in Fig. 5 and results obtained are summarized in Fig. 6. Regarding the undoped TiO2 (T) catalysts, it is observed that increase of calcination temperature above 550 ◦ C results in a decrease of the band gap energy by about 0.2 eV (trace a). As discussed above, this is due to the temperature-induced transformation of anatase to rutile TiO2 , which is in agreement with results of XRD experiments (Fig. 2B). The N-TiO2 (T) catalysts display two absorption edges in the UV–vis region (Fig. 5), which correspond to the two absorption thresholds reported in Fig. 6: the primary absorption threshold (trace b) is related to the onset of the absorption at short wavelengths near the UV region, whereas the secondary absorption threshold (trace c) corresponds to absorption in the visible spectral region. For convenience, the data presented in Fig. 6 can be discussed by dividing them into three regions, based on the effects of the synthesis temperature on their optical properties. In region (I) (T = 450–500 ◦ C), the primary absorption threshold of N-TiO2 catalysts is about 0.1 eV lower, compared to that of the undoped TiO2 samples calcined at the same temperatures. The NTiO2 catalysts treated in this temperature range have a yellowish colour (Fig. 4), which is related to doping of N in the TiO2 lattice. Treatment with ammonia at higher temperatures (region (II)) results in a further decrease of the band gap by ca. 0.4 eV (trace b). The corresponding samples are characterized by a yellowish green (550 ◦ C) or dark green (600 ◦ C) colour (Fig. 4), which may be attributed to a mixture of yellow (N-doped TiO2 ) and blue (partially reduced TiO2 ). It is known that the blue colour of reduced TiO2 arises from the visible tail of an IR absorption band peaking at about 0.75–1.18 eV, and its intensity depends on the degree of reduction of the material [54]. Regarding the secondary absorption threshold, it decreases progressively from ca. 2.4 to 2.1 eV with increasing synthesis temperature from 450 to 600 ◦ C (trace c). Absorption thresholds in this region have been previously reported for N-doped TiO2 films and nanotubes [30,50,55]. Further increase of calcination temperature under flowing NH3 (region (III)) results in materials which absorb strongly in the visible and NIR regions so that their colour becomes dark grey (700 ◦ C) or black (800 ◦ C). As a result, the DR spectra of the N-TiO2 (700) and N-TiO2 (800) samples do not allow accurate determination of their absorption thresholds. 3.2.2. On the origin of the enhanced visible light absorption of N-TiO2 catalysts Results of DRS measurements (Figs. 4–6) show clearly that heat treatment of the sol–gel derived TiO2 with ammonia results in materials that absorb in the visible and NIR regions, and that the

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optical properties of these materials can be tuned by proper selection of the calcination temperature. The enhanced absorbance of N-doped TiO2 at wavelengths longer than 400 nm can be attributed to several reasons, including narrowing of the band gap of the semiconductor induced by a delocalized mixing of N2p states and O2p states [18], introduction of localized states above the valence band of TiO2 [27,34,56–58], creation of colour centres associated with oxygen vacancies [38,39,50,59,60], and formation of Ti3+ states due to chemical reduction of TiO2 [61]. Experimental results [22,27,28,33] and theoretical studies [37,56] have indicated that the enhanced response of N-TiO2 catalysts to visible light should not be attributed to a “rigid” upward shift of the valence band, as proposed in earlier studies [18], but to the creation of localized states above the valence band of TiO2 [27,34,57,58]. True narrowing of the intrinsic band gap of TiO2 would necessitate heavy doping of the semiconductor and, therefore, would result in materials with a completely different chemical composition and electronic structure than that of TiO2 [39,60,62]. Based on the above, an attempt will be made to discuss the observed variation of the primary absorption threshold of the present N-TiO2 (T) catalysts by taking into account the electronic transitions that may take place from localized states in the band gap of TiO2 , generated by the doping process, to the conduction band of the semiconductor. In particular, results of Fig. 6 will be discussed based on the simplified energy diagrams of Fig. 7, which show the possible electronic band structures and energy levels of the synthesized materials. Starting with the undoped TiO2 (T) samples, the optical band gaps plotted in Fig. 6 (trace a) correspond to the energy separation between the extended states of the semiconductor, which is ca. 3.2 eV for anatase (region (I)) and 3.0 eV for rutile (region (III)). The valence-band (VB) and conduction-band (CB) states of anatase TiO2 as well as the band-to-band electronic transition that can be induced by absorption of a UV photon (fundamental absorption) are shown schematically in Fig. 7(a). It is reminded that the VB and CB states of TiO2 are derived mainly from the O2p orbitals and the Ti3d orbitals, respectively. Regarding the N-TiO2 samples, results of Fig. 6 (trace b) show that the primary absorption threshold of materials synthesized at temperatures of 450 ◦ C and 500 ◦ C is about 3.1 eV (region (I)), and decreases to ca. 2.6 eV for samples treated in the temperature range of 550 ◦ C to 700 ◦ C (region (II)). According to the literature, these observations can be explained by considering that substitutional and interstitial nitrogen-doping of TiO2 results in the creation of new, localized states into the band gap of the semiconductor. In particular, Di Valentin et al. [37,40] provided theoretical evidence that substitutional N-doping of anatase TiO2 results in the creation of N2p states located slightly above (0.14 eV) the valence band edge of the semiconductor. This is in very good agreement with the observed decrease of the optical band gap of the present N-TiO2 catalysts in region (I) by ca. 0.1 eV, compared to the undoped samples (Fig. 6). Evidence for the presence of such states has been provided by UPS experiments, which showed that, for rutile TiO2 , the N2p states extend up to 0.4 eV above the VB maximum [33]. Regarding interstitial nitrogen-doping, it has been found that the N O bond of interstitial N species generates localized states with  character, the two antibonding states of which lie higher in the gap, compared to the N2p states [24]. Interestingly, the highest localized state for the interstitial species was found to be located 0.73 eV above the valence band of TiO2 [24,40]. The computed band gap for these systems was estimated to be around 2.6 eV [24], which is in very good agreement with the absorption threshold values determined in region (II) (Fig. 6). Based on the above, it may be argued that treatment of the as prepared TiO2 under ammonia flow at temperatures lower than 550 ◦ C results, mainly, in substitutional N-doping of TiO2 , whereas treatment at higher temperatures results in interstitial N-doping of the semiconductor. Both dopant configurations

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Fig. 7. Simplified schemes showing the possible changes that might occur to the electronic structure of TiO2 upon doping with nitrogen atoms, and the corresponding transitions that may lead to absorption of visible photons: (a) pristine TiO2 ; (b–d) N-doped catalysts prepared at temperatures corresponding to the three regions shown in Fig. 6 (energies not to scale).

induce the formation of localized states in the gap [24], which are responsible for the absorption thresholds measured in regions (I) and (II), respectively. The corresponding energy levels and associated optical transitions are shown schematically in Fig. 7(b and c), respectively. Inspection of the DRS spectra of Fig. 4 shows that N-TiO2 samples treated at temperatures higher than 550 ◦ C display strong optical absorption above 500 nm (traces e–g), which is not the case for the undoped TiO2 catalysts (e.g., trace a) or for the N-TiO2 samples synthesized at lower temperatures (traces b–d). In addition, it is observed that absorption in the visible and near IR regions increases significantly with increasing calcination temperature from 600 to 800 ◦ C (traces e–g). This behaviour can be understood by considering that NH3 decomposes into N2 and H2 at temperatures around 550 ◦ C. Therefore, samples treated under NH3 flow at higher temperatures are exposed to a highly reducing environment, which results in partial reduction of TiO2 and formation of intrinsic defects such as oxygen vacancies and Ti3+ states [22,63]. Formation of such defects is physically favoured in order to compensate the excess negative charge induced by the replacement of O2− with N3− anions [24,59]. Experimental results obtained over rutile TiO2 have indicated that the Ti3+ defect states are located in the band gap, about 0.7–1.0 eV below the bottom of the conduction band [61,64,65]. Qualitatively similar results were obtained from theoretical calculations, which showed that a variety of Ti3+ states, located from 0.3 to 0.8 eV below the CB of TiO2 , may exist in chemically reduced or doped anatase TiO2 samples [63,66]. It may then be argued that such states, that extend deep into the band gap, are related to the absorption of the lower energy photons (<1.5 eV) in region (III) and are responsible for the bluish and grey colour of samples annealed in NH3 flow at temperatures higher than 600 ◦ C (Fig. 4). The corresponding optical transitions are shown schematically in Fig. 7(d). Regarding the absorption tails observed for N-TiO2 samples in the visible region (400–550 nm) (Fig. 4) and the corresponding absorption thresholds in the visible range, which decrease from 2.35 eV to 2.1 eV with increase of synthesis temperature from 450 to 600 ◦ C (Fig. 6, trace c), they can be attributed to the presence

of oxygen vacancies [67]. Experimental results and first-principle calculations revealed that N-doping of TiO2 leads to the creation of such species and that the combination of N impurity and oxygen vacancies accounts for the observed absorbance of N-TiO2 catalysts to visible light [68]. In particular, it has been proposed that the absorption tail that appears at around 500 nm is due to the acceptor level (oxygen vacancy) located about 1 eV above the valence-band maximum [68]. The position and amplitude of this defect state were found to depend on the amount of the oxygen vacancies or surface reduction. An alternative way to explain the optical properties of the present N-TiO2 samples is to attribute them to the formation of colour centres, as proposed by Serpone and coworkers [38,39,60,62]. Based on the similarities observed in the absorption spectra of reduced TiO2 as well as of several anion- and cationdoped TiO2 samples, it has been argued that the optical absorption of all these materials in the visible region should have a common origin [38,39,60,62]. In particular, it has been proposed that the three absorption bands commonly observed in the visible spectral region, denoted by the authors as AB1 (2.75–2.95 eV), AB2 (2.50–2.55 eV), and AB3 (2.00–2.30 eV) are due to of F-type colour centres associated with oxygen vacancies [60]. Interestingly, the above three absorption bands are very close to the values of the absorption thresholds estimated for the present N-TiO2 samples (Fig. 6). 3.3. Surface composition and electronic structure of N-TiO2 catalysts In Fig. 8 are shown the Ti2p, N1s and O1s regions of the XP spectra obtained from the undoped TiO2 (500) sample and the NTiO2 (T) photocatalysts synthesized at T = 500, 600, 700 and 800 ◦ C. Spectra obtained for N-doped materials synthesized at 450 and 550 ◦ C are not shown, for clarity. It is observed that for temperatures up to 550 ◦ C nitrogen appears in very small quantities in the form of NHx species, as evidenced by the N1s photopeak at ∼400 eV BE (Fig. 8B), whereas titanium remains in the TiO2 form with the

Table 2 Atomic ratios N/Ti(tot) estimated from XPS measurements of the N1s/Ti2p area ratio (experimental uncertainty is of the order of ±8%).

(a) (b) (c) (d) (e) (f) (g)

Sample

Total N

NHx (∼400 eV BE)

NOx (∼399 eV BE)

N-nitride (∼396 eV BE)

TiO2 (500) N-TiO2 (450) N-TiO2 (500) N-TiO2 (550) N-TiO2 (600) N-TiO2 (700) N-TiO2 (800)

0.013 0.026 0.034 0.022 0.027 0.144 0.500

0.013 0.026 0.034 0.022 ∼0.012 ∼0.012 ∼0.04

– – – – – ∼0.025 ∼0.08

– – – – ∼0.015 0.11 0.38

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Ti2p3/2 peak located at 458.5 eV (Fig. 8A). Detectable TiO2 reduction with characteristics attributed mainly to TiN (Ti2p3/2 BE ∼456 eV) is observed at synthesis temperatures above 600 ◦ C with a concomitant decrease of the O1s peak (Fig. 8C). Nitridic N (N1s BE ∼396 eV) also appears, accompanied by a smaller contribution from oxynitrides (N1s BE ∼399 eV). The N/Ti atomic ratios calculated from the measured N1s/Ti2p peak area ratios are presented in Table 2. The ratios shown in Table 2 are based on the total Ti2p area, Ti(tot), and include both the overall N content and a rough estimate of the contributions from the various N species detected at synthesis temperatures above 600 ◦ C, where reduction of TiO2 takes place. It is noteworthy that the O1s spectra obtained from the strongly nitrided specimens (synthesized at 700 and 800 ◦ C), besides the oxidic O attenuation (O1s BE at ∼529.7 eV), exhibit a distinct contribution near 534.5 eV BE, which is indicative of molecular-type oxygen. According to reference [69], nanocrystalline mesoporous Ndoped partly reduced TiO2 films prepared under similar conditions as those in the present study, can be described by the general formula Ti1−y 4+ Tiy 3+ O2−(3x/2)−(y/2) Nx V(x+y)/2

Fig. 8. XP spectra in (A) the Ti2p region, (B) the N1s region, and (C) the O1s region obtained for (a) the undoped TiO2 (500) sample and the N-TiO2 (T) photocatalysts synthesized at temperatures of (c) 500, (e) 600, (f) 700 and (g) 800 ◦ C.

whereby V designates anionic vacancies and x corresponds to nitridic N, which is detected in the present study at synthesis temperatures above 600 ◦ C (Fig. 8B). The above formula can be used to estimate the amount of anionic vacancies and the O/Ti(tot) ratios of the present N-TiO2 catalysts, by taking into account the spectra shown in Fig. 8A and the values listed in Table 2. In particular, for specimen N-TiO2 (600), y ∼ 0 and x ∼ 0.015, hence O/Ti(tot) ∼2 and vacancies are less than 1%; for specimen N-TiO2 (700), y∼0.06 and x = 0.11, hence O/Ti(tot) ∼1.80 and vacancies amount to ∼9%; for specimen N-TiO2 (800), y∼0.37 and x = 0.38, hence O/Ti(tot) ∼1.25 and vacancies amount to ∼38%. These values are in good agreement with XPS measurements for the atomic ratios O/Ti(tot)–2.0, 1.85 and 1.5, respectively, excluding disordered oxygen – as obtained from data in Fig. 8C. The small oxygen excess could be tentatively attributed to vacancies adsorbing some molecular oxygen, thus explaining the high BE O1s contribution in Fig. 8C. As a final comment on the chemical composition of the strongly nitrided specimens, it should be noted that the TiN content estimated by XPS is much higher, compared to that obtained from the XRD measurements (Fig. 1B). Since XPS is a surface-sensitive technique, it may be argued that TiN is located mainly at the surface of the photocatalyst particles, possibly in an amorphous phase indicated by HRTEM (Fig. 3C). Fig. 9 shows the valence band region of the XP spectra obtained from the undoped TiO2 (500) sample (trace a) and the N-TiO2 (T) photocatalysts synthesized at temperatures of 450–800 ◦ C (traces b–g). It is observed that the valence band of undoped TiO2 is characterized by a broad peak at around 6 eV and a narrow peak at ca. 8 eV, attributed to bonding and antibonding O2p orbitals (trace a). The basic features of the VB of TiO2 are maintained for N-doped materials synthesized at temperatures up to 600 ◦ C (traces b–e), with a small contribution from N2p states associated with adsorbed NHx species on top of the valence band. At higher synthesis temperatures, where the TiO2 phase transformation is completed and considerable amount of N is incorporated into the TiO2 lattice, the shape of the valence band features changes (traces f and g). The BE position of N2p sates from nitridic N in specimens N-TiO2 (700) and N-TiO2 (800), where nitrided TiO2 has the rutile structure, as well as the O2p-dominated valence band onset are in agreement with theoretical and experimental studies [37,40]. This picture is consistent with the lower part of the simplified energy diagrams in Fig. 7, which is accessible via the XPS measurements. The transitory appearance of Ti3d states at ∼1.7 eV above the valence band

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Fig. 9. XP spectra in the valence band region obtained for (a) the undoped TiO2 (500) sample, and the N-TiO2 (T) photocatalysts synthesized at temperatures of (b) 450, (c) 500, (d) 550, (e) 600, (f) 700 and (g) 800 ◦ C. The left panel shows the full spectra and the right panel is an expanded view of the spectra around the valence band onset. Zero binding energy corresponds to the analyser Fermi level.

onset in specimen N-TiO2 (600) (trace e), is attributed to Ti3+ defect sates, which are known to be created upon rutile reduction but are attenuated upon nitridation [70]. The fact that the valence band onset is not significantly affected by extensive nitridation (traces f and g) is consistent with the energy diagrams in Fig. 7, whereby the observed decrease of the effective band gap upon nitridation is attributed mainly to the creation of Ti3+ defect states near the conduction band. 4. Conclusions N-doped TiO2 catalysts of variable nitrogen content and tuneable light absorption properties have been synthesized by heat treatment of sol–gel derived TiO2 under flowing NH3 (0.5% in He) in the temperature range of 450–800 ◦ C. Increase of synthesis temperature results in a monotonic increase of absorption at wavelengths longer than 400 nm, which is accompanied by a progressive increase of the mean crystallite size and a decrease of the specific surface area of the materials. The N-TiO2 (T) catalysts synthesized at temperatures of 450–600 ◦ C contain relatively small amounts of dopant atoms (N/Ti ∼ 0.025), probably in the form of NHx species, and their colour varies from pale yellow to dark green. The DR spectra of these samples are characterized by the presence of two absorption thresholds in the UV/vis region. The primary absorption threshold is red-shifted by ca. 0.1 eV for T = 450–500 ◦ C and ca. 0.4 eV for T = 550–600 ◦ C, compared to that of undoped TiO2 (T) samples (3.2 eV for anatase and 3.0 eV for rutile). This behaviour has been attributed to the creation of localized states above the valence band of the semiconductor possibly due to substitutional and interstitial nitrogen-doping of TiO2 , respectively. The secondary absorption threshold, which decreases progressively from 2.35 eV to 2.1 eV with increase of synthesis temperature from 450 to 600 ◦ C, may be associated to the dopant-induced formation of oxygen vacancies in the crystal matrix of the semiconductor. Treatment of the sol–gel derived TiO2 under NH3 flow at temperatures higher than 600 ◦ C results in a significant increase of the dopant content (N/Ti = 0.144 at T = 700 ◦ C and 0.500 at 800 ◦ C), which is

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