# Hydrothermal Preparation of Visible-Light-Driven N-Br-CodopedTiO2 Photocatalysts

Authors: Yonggang Sheng; Yao Xu; Dong Jiang; Liping Liang; Dong Wu; Yuhan Sun
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

Using a facile hydrothermal method, N-Br-codopedTiO2 photocatalyst that had intense absorption in visible region was prepared at low temperature (100oC), through a direct reaction between nanocrystalline anatase TiO2 solution and cetyltrimethylammonium bromide (CTAB). The results of X-ray photoelectron spectroscopy (XPS) showed the existence of N-Ti-N, O-Ti-N-R, Ti3+ (attribute to the doped Br atoms by charge compensation), and TiOxNy species, indicating the successful codoping of N and Br atoms, which were substituted for lattice oxygen without any influence on the crystalline phase of TiO2. In contrast to the N-doped sample, the N-Br-codoped TiO2 photocatalyst could more readily photodegrade methylene blue (MB) under visible-light irradiation. The visible-light catalytic activity of thus-prepared photocatalyst resulted from the synergetic effect of the doped nitrogen and bromine, which not only gave high absorbance in the visible-light range, but also reduced electron-hole recombination rate.


---

## Body

## 1. Introduction

The development of photocatalysis has attracted much attention in the recent
30 years [1–3]. Of the materials being developed for
photocatalytic applications, titanium dioxide (TiO2) remains the most promising due to its high
efficiency, low cost, chemical inertness, and photostability [4–6], and so forth. However, its wide-band gap (3.2 eV ≡ 380 nm for anatase) can only capture UV light, a small fraction (about 3–5%)
of the solar irradiance at the earth surface, compared to the 45% of visible
light [7]. Thus any red shift in the optical response of TiO2 from the UV band
to the visible band will have a profound positive effect on the photocatalytic
efficiency of the materials [8, 9].Doping TiO2 with transition-metal elements was used
as the initial approach to shift the optical response of TiO2 from the UV band
to the visible band [10–15]. However, these metal-doped TiO2 materials suffered from thermal instability and low-quantum efficiency because
of increased carrier trapping after doping, or require expensive facilities in
the ion implantation case [8, 16, 17]. In contrast, better visible optical response of TiO2 could be achieved by using nonmetal doping [8, 9, 18, 19]. Kinds of nonmetal
elements, including nitrogen [20–23], phosphor [24], carbon [25], sulfur [26, 27], boron [28],
fluorine [18, 29], iodine [30], chlorine, and bromine [31],
were found to be capable of making doped TiO2 respond to visible light.
In particular, the presence of substitutional N atoms in the TiO2 matrix enhanced the absorption in the visible band and then led to a
corresponding photocatalytic activity [32]. Moreover, doped halogen
atoms into the oxygen sites should be accompanied by the formation of Ti3+ to maintain the overall electroneutrality of the crystal lattice, and a certain
amount of Ti3+ reduced the electron-hole recombination rate and
further enhanced the photocatalytic activity [18, 33]. Thus, one may
want to know whether halogen atoms codoped in TiO2 together with
nitrogen have higher photocatalytic activity than that of N-doped TiO2.
To the best of our knowledge, there was no report on the visible-light-driven TiO2 photocatalyst codoped by nitrogen and bromine.Hereby, a simple process was utilized to prepare the N-Br-codoped TiO2 nanocrystals via the direct reaction between nanocrystalline TiO2 solution and cetyltrimethylammonium bromide (CTAB). Based upon the results of the
present work, we found that the N-Br-codoped TiO2 could be
considered as an effective visible-light-driven photocatalyst.


## 2. Experimental

### 2.1.  Materials Preparation

All chemicals, including Titanium tetraisopropoxide (TTIP, Tianchang Chemical Co., Ltd., Tianchang, China), nitric acid (65–68%, Shanghai chemicals, Shanghai, China),
CTAB (Tianjin Chemical
International Trading Co., Ltd., Tianjin, China), and
anhydrous ethanol (Tianjin Chemical International
Trading Co., Ltd., China), were of reagent grade and were
used without further purification. Water was doubly distilled and deionized.


#### 2.1.1. Preparation of Nanocrystalline TiO2 Solution

The preparation proceeded as follows: 15 mL of TTIP and 3 mL of ethanol were mixed and
then added dropwise into 100 mL of water in a 200 mL beaker under continuous
stirring. After 15 minutes ultrasonication, 2 mL of nitric acid were added to
the mixture, and then the hydrothermal crystallization was carried out at 100°C for 5 hours
to obtain a nanocrystalline TiO2 blurry solution (denoted as TOS for
conveniency, the original solution for the following preparation).


#### 2.1.2. Preparation of Photocatalysts

For N-Br-codoped TiO2, 18.6 g of CTAB was added into TOS and was
refluxed at 80°C for 3 hours under vigorous stirring, leading to a yellow milky solution.
The product was then evaporated under low pressure (20-mmHg) to a yellow
xerogel. Finally, the unreacted CTAB and other organic by-products were washed out with ethanol in a Soxhlet extractor and then the
N-Br-codoped TiO2 photocatalyst was obtained (denoted as TNB).For comparison, N-doped TiO2 photocatalyst (accordingly
designed as TDN) was prepared by a direct reaction between TOS and triethylamine as the above
process, and another undoped TiO2 photocatalyst
(denoted as TOX derived from TOS by a low-pressure evaporation) was also prepared.


### 2.2. Photocatalysts Characterization

Several techniques were used to characterize the structure and properties of
the above materials TDN and TNB. The X-ray diffraction (XRD) patterns were
obtained on a D8 Advance diffractometer (Bruker AXS, GmbH, Karlsruhe, Germany) using Cu Ka radiation. Line traces were collected over2θ values ranging
from 20° to 70°. The lattice spacing was observed by transmission electron microscopy
(TEM, JEOL JEM-2010). Diffuse reflection absorption spectra (DRAS) of the samples were
recorded using a UV-Vis-NIR spectrometer (UV-3150 spectrometer Shimadzu, Japan), and BaSO4 was used as the reference sample. The DRAS spectra were
used to evaluate the optical absorption properties of the samples [34].
The states of the charges were investigated by XPS (Mg kα as radiation source, PHI-5300X,
Perkin-Elmer physics electronics), and all the bonding energies were calibrated
to that of the adventitious C 1second peak fixed at 284.8 eV. In order to
investigate the depth profile of N and Br atoms in the lattice of TiO2 particles, XPS analysis of sample TNB using Ar etching was performed.For the undoped TiO2 sample, TOX, XRD, and DRAS spectra were
used to characterize the structure and properties.


### 2.3. Measurement of Photocatalytic Activity

#### 2.3.1. Photoreactor

For UV-activity determination, the photocatalytic experiments were carried
out on a photochemical reactor (SGY-1, Jiangsu Provincial Academy of Environmental
Sciences) with a 300-W high-pressure mercury lamp as the light source,
which provides a main irradiation wavelength about 365 nm.For vis-activity determination, the light source was a 250 W metal halide
lamp (Philips), which was positioned inside a cylindrical Pyrex vessel
surrounded by a circulating water jacket to cool the lamp. A cutoff filter was
also placed between lamp and the Pyrex jacket to remove the radiation below 420 nm, ensuring the reaction system to be irradiated only by visible light.


#### 2.3.2. MB, Photocatalytic Activity Determination

The photocatalytic experiments were carried out by adding 250 mg of samples
into 250 mL of methylene blue (MB) solution (50 ppm). The suspension was
stirred in dark for 30 minutes to obtain the saturated adsorption of MB before UV/visible
light illumination. The concentration of MB at this point was used as the
initial value for the further kinetic treatment of the photodecomposition
process. At given irradiation time intervals, the dispersion was sampled (4 mL), centrifuged, and subsequently filtered through a millipore filter (pore
size, 0.22μm) to separate the catalyst particles. The concentration of MB was
measured at 665 nm wavelength using UV-Visible spectrophotometer. Firstly, a
series of standard MB solutions (2.5, 5, 10, 15, and 20 ppm) were prepared and
measured using UV-Vis spectrophotometer. Then, a standard curve could be
obtained. The concentration of MB during the degradation process could be
obtained from the standard curve. Absorption spectra were recorded after 4-fold dilution.


## 2.1.  Materials Preparation

All chemicals, including Titanium tetraisopropoxide (TTIP, Tianchang Chemical Co., Ltd., Tianchang, China), nitric acid (65–68%, Shanghai chemicals, Shanghai, China),
CTAB (Tianjin Chemical
International Trading Co., Ltd., Tianjin, China), and
anhydrous ethanol (Tianjin Chemical International
Trading Co., Ltd., China), were of reagent grade and were
used without further purification. Water was doubly distilled and deionized.


### 2.1.1. Preparation of Nanocrystalline TiO2 Solution

The preparation proceeded as follows: 15 mL of TTIP and 3 mL of ethanol were mixed and
then added dropwise into 100 mL of water in a 200 mL beaker under continuous
stirring. After 15 minutes ultrasonication, 2 mL of nitric acid were added to
the mixture, and then the hydrothermal crystallization was carried out at 100°C for 5 hours
to obtain a nanocrystalline TiO2 blurry solution (denoted as TOS for
conveniency, the original solution for the following preparation).


### 2.1.2. Preparation of Photocatalysts

For N-Br-codoped TiO2, 18.6 g of CTAB was added into TOS and was
refluxed at 80°C for 3 hours under vigorous stirring, leading to a yellow milky solution.
The product was then evaporated under low pressure (20-mmHg) to a yellow
xerogel. Finally, the unreacted CTAB and other organic by-products were washed out with ethanol in a Soxhlet extractor and then the
N-Br-codoped TiO2 photocatalyst was obtained (denoted as TNB).For comparison, N-doped TiO2 photocatalyst (accordingly
designed as TDN) was prepared by a direct reaction between TOS and triethylamine as the above
process, and another undoped TiO2 photocatalyst
(denoted as TOX derived from TOS by a low-pressure evaporation) was also prepared.


## 2.1.1. Preparation of Nanocrystalline TiO2 Solution

The preparation proceeded as follows: 15 mL of TTIP and 3 mL of ethanol were mixed and
then added dropwise into 100 mL of water in a 200 mL beaker under continuous
stirring. After 15 minutes ultrasonication, 2 mL of nitric acid were added to
the mixture, and then the hydrothermal crystallization was carried out at 100°C for 5 hours
to obtain a nanocrystalline TiO2 blurry solution (denoted as TOS for
conveniency, the original solution for the following preparation).


## 2.1.2. Preparation of Photocatalysts

For N-Br-codoped TiO2, 18.6 g of CTAB was added into TOS and was
refluxed at 80°C for 3 hours under vigorous stirring, leading to a yellow milky solution.
The product was then evaporated under low pressure (20-mmHg) to a yellow
xerogel. Finally, the unreacted CTAB and other organic by-products were washed out with ethanol in a Soxhlet extractor and then the
N-Br-codoped TiO2 photocatalyst was obtained (denoted as TNB).For comparison, N-doped TiO2 photocatalyst (accordingly
designed as TDN) was prepared by a direct reaction between TOS and triethylamine as the above
process, and another undoped TiO2 photocatalyst
(denoted as TOX derived from TOS by a low-pressure evaporation) was also prepared.


## 2.2. Photocatalysts Characterization

Several techniques were used to characterize the structure and properties of
the above materials TDN and TNB. The X-ray diffraction (XRD) patterns were
obtained on a D8 Advance diffractometer (Bruker AXS, GmbH, Karlsruhe, Germany) using Cu Ka radiation. Line traces were collected over2θ values ranging
from 20° to 70°. The lattice spacing was observed by transmission electron microscopy
(TEM, JEOL JEM-2010). Diffuse reflection absorption spectra (DRAS) of the samples were
recorded using a UV-Vis-NIR spectrometer (UV-3150 spectrometer Shimadzu, Japan), and BaSO4 was used as the reference sample. The DRAS spectra were
used to evaluate the optical absorption properties of the samples [34].
The states of the charges were investigated by XPS (Mg kα as radiation source, PHI-5300X,
Perkin-Elmer physics electronics), and all the bonding energies were calibrated
to that of the adventitious C 1second peak fixed at 284.8 eV. In order to
investigate the depth profile of N and Br atoms in the lattice of TiO2 particles, XPS analysis of sample TNB using Ar etching was performed.For the undoped TiO2 sample, TOX, XRD, and DRAS spectra were
used to characterize the structure and properties.


## 2.3. Measurement of Photocatalytic Activity

### 2.3.1. Photoreactor

For UV-activity determination, the photocatalytic experiments were carried
out on a photochemical reactor (SGY-1, Jiangsu Provincial Academy of Environmental
Sciences) with a 300-W high-pressure mercury lamp as the light source,
which provides a main irradiation wavelength about 365 nm.For vis-activity determination, the light source was a 250 W metal halide
lamp (Philips), which was positioned inside a cylindrical Pyrex vessel
surrounded by a circulating water jacket to cool the lamp. A cutoff filter was
also placed between lamp and the Pyrex jacket to remove the radiation below 420 nm, ensuring the reaction system to be irradiated only by visible light.


### 2.3.2. MB, Photocatalytic Activity Determination

The photocatalytic experiments were carried out by adding 250 mg of samples
into 250 mL of methylene blue (MB) solution (50 ppm). The suspension was
stirred in dark for 30 minutes to obtain the saturated adsorption of MB before UV/visible
light illumination. The concentration of MB at this point was used as the
initial value for the further kinetic treatment of the photodecomposition
process. At given irradiation time intervals, the dispersion was sampled (4 mL), centrifuged, and subsequently filtered through a millipore filter (pore
size, 0.22μm) to separate the catalyst particles. The concentration of MB was
measured at 665 nm wavelength using UV-Visible spectrophotometer. Firstly, a
series of standard MB solutions (2.5, 5, 10, 15, and 20 ppm) were prepared and
measured using UV-Vis spectrophotometer. Then, a standard curve could be
obtained. The concentration of MB during the degradation process could be
obtained from the standard curve. Absorption spectra were recorded after 4-fold dilution.


## 2.3.1. Photoreactor

For UV-activity determination, the photocatalytic experiments were carried
out on a photochemical reactor (SGY-1, Jiangsu Provincial Academy of Environmental
Sciences) with a 300-W high-pressure mercury lamp as the light source,
which provides a main irradiation wavelength about 365 nm.For vis-activity determination, the light source was a 250 W metal halide
lamp (Philips), which was positioned inside a cylindrical Pyrex vessel
surrounded by a circulating water jacket to cool the lamp. A cutoff filter was
also placed between lamp and the Pyrex jacket to remove the radiation below 420 nm, ensuring the reaction system to be irradiated only by visible light.


## 2.3.2. MB, Photocatalytic Activity Determination

The photocatalytic experiments were carried out by adding 250 mg of samples
into 250 mL of methylene blue (MB) solution (50 ppm). The suspension was
stirred in dark for 30 minutes to obtain the saturated adsorption of MB before UV/visible
light illumination. The concentration of MB at this point was used as the
initial value for the further kinetic treatment of the photodecomposition
process. At given irradiation time intervals, the dispersion was sampled (4 mL), centrifuged, and subsequently filtered through a millipore filter (pore
size, 0.22μm) to separate the catalyst particles. The concentration of MB was
measured at 665 nm wavelength using UV-Visible spectrophotometer. Firstly, a
series of standard MB solutions (2.5, 5, 10, 15, and 20 ppm) were prepared and
measured using UV-Vis spectrophotometer. Then, a standard curve could be
obtained. The concentration of MB during the degradation process could be
obtained from the standard curve. Absorption spectra were recorded after 4-fold dilution.


## 3. Results and Discussion

### 3.1. Crystal Phase Identification

Figure1 shows the XRD patterns of samples, TOX, TDN, and TNB. The
diffraction lines at 2θ = 25.3°,
37.8°, 48.1°, 53.9°, 55.2°, and 62.7° demonstrate that all the three
samples are of the anatase phase. The average crystallite size, DA,
determined from the diffraction peak of the anatase (101) planes, is about 6.5 nm estimated from the Debye-Scherrer equation [35]. TEM (see Figure 2) of sample TDN and TNB clearly
revealed the fringes of (101) planes of anatase with a lattice spacing of about
0.352 nm [36], which corresponds to the XRD
patterns. Among the three main polymorphs, anatase TiO2 is
believed to exhibit the highest photocatalytic activity [31]. The
anatase phase of the photocatalysts will contribute to a good photocatalytic
activity.Figure 1
XRD patterns of
samples, TOX, TDN, and TNB. Sample TOX was undoped TiO2, sample TDN
was N-doped TiO2, and sample TNB was N, Br-codoped TiO2.TEM images of samples, TDN (a) and TNB (b).
(a)(b)


### 3.2. Optical Properties and XPS Analysis

The samples, TOX, TDN, and TNB show different color. TOX looks white, TDN
yellow while TNB tan. From Figure3 (DRAS of the samples), TOX have no
absorption in the visible band; while the other two samples both have distinctly
great absorption in the visible band. Compared to sample TDN, sample TNB showed obvious enhanced absorption
in the visible range of 400–600 nm. The broad absorption below 500 nm is because the N 2p orbits are localized above the top of the O 2p valence
bands, whereas the absorption above 500 nm is mainly caused by oxygen vacancies [37]. The band gap energy of the synthesized samples was estimated to
be about 3.12 eV for TOX, 2.03 eV for TDN, and 1.88 eV for TNB by the
absorption edge. The band gap narrowing will result in the obvious red-shift in
the optical response, which is helpful for visible photocatalytic activity.Figure 3
UV-Vis diffuse reflection absorption spectra of samples, TOX, TDN, and TNB.The chemical composition of surface was determined by XPS (see Figure4).
The C 1s, O 1s, Ti 2p, and N 1s peaks components are discernible in the wide
scan spectrum of the two samples, but obvious peaks of Br 3d can be found only
for TNB. The N 1s core-level spectra of the two samples
are showed in Figure 5. For TDN sample (Figure 5(a)), the feature at 396.7 eV is
generally known as the “nitride” peak and can be assigned to the N2- anions that replace oxygen atoms in the TiO2 lattice to form N-Ti-N [38, 39].
N 1s peak of 399.3 eV can be attributed to the binding energy of the N atom in special
environment of O-Ti-N-R (R: alkyl) [38–40]. The weak N 1s peak at
401.7 eV indicates the trace amount of nitrogen in the form of Ti-N-O [39].
For TNB sample, No N 1s peaks from N+ of CTAB (at 402.3 eV) can be
found in Figure 5(b) [38, 41], indicating that residual CTAB has been
completely removed by the Soxhlet extracting method or at least the residual
CTAB was too little to be detected. The two main N 1s peaks at 396.5 eV and
399.3 eV can be attributed to the binding energy of the N atom in special environment
of N-Ti-N and O-Ti-N-R (R: alkyl) [38–40].Figure 4
XPS wide scan spectra of samples, TDN and TNB.(a) N 1s core-level spectrum (deconvolution of N 1s bands) of TDN. (b) N 1s core-level spectrum (deconvolution of N 1s bands) of TNB.
(a)(b)Figure6 showed Ti 2p core-level spectra of TDN and TNB. For TDN sample (Figure
6(a)), the deconvolution of Ti 2p bands yields two major doublets (2p3/2 and 2p1/2) encompassing the set of two 2p3/2 peaks. The
main Ti 2p3/2 peak at 458.6 eV is typical for pure TiO2 [38, 42] and can be attribute to those unreacted TiO2, the other Ti
2p3/2 peak at 457.2 eV for TiOxNy
[38, 42].
For TNB sample (Figure 6(b)), the deconvolution of the complex Ti 2p bands
yields three major doublets (2p3/2 and 2p1/2)
encompassing the set of three Ti 2p3/2 peaks. The strongest Ti 2p3/2 peak at 458.7 eV is typical for pure TiO2 [38] and can be attribute
to those unreacted TiO2, other peaks, at 455.7 eV typical for Ti3+ [38], and at 457.2 eV for TiOxNy [38, 42].
Doped nitrogen atoms result in the formation of TiOxNy [42],
meanwhile, the formation of Ti3+ might attribute to the substitution
of bromine atoms for oxygen just like the case of fluorine doping [33, 43].(a) Ti 2p core-level spectrum (deconvolution of Ti 2p bands) of TDN. (b) Ti 2p core-level spectrum (deconvolution of Ti 2p bands) of TNB.
(a)(b)From the results of the depth profile of N and Br atoms of sample TNB (see
Table1), we found that N and Br atoms were mainly at the surface of TiO2 particles. Besides, the results of XPS indicate that N and Br atoms were actually
existed in the lattice of the titania crystal.Table 1
Results of
XPS analysis using Ar etching, sample TNB.
NameContent/%BrNBefore etching0.313.68First etching0.111.81Second etching01.79Third etching01.17


### 3.3. Photocatalytic Activity

#### 3.3.1. UV-Activity

The photodecomposition of the MB in aqueous solution was carried out under
UV light. The time-dependent concentration of MB is shown in Figure7(a). With
high crystallization, P25 can more readily photodegrade MB. The
photocatalysts, TOX, TDN, and TNB, exhibit similar and good photocatalytic
activity, which due to their similar crystallization and microstructure. Because
the doping process was performed under low temperature, then there is little
change of crystallization and microstructure.(a) Comparison of the UV photocatalytic activities
of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under UV light. (b) Comparison of the visible photocatalytic
activities of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under visible light.
(a)(b)


#### 3.3.2. Vis-Activity

The photodecomposition of the MB in aqueous solution was carried out under
visible light. The time-dependent concentration of MB in solution is shown in Figure7(b). Compared with the visible-inactive P25 and TOX, both of TDN and TNB
exhibit excellent photocatalytic activity, and obviously TNB can more readily photodegrade MB
than TDN. One may raise the doubt of whether it is
the photocatalyst that plays the key role in decomposing MB because MB can
absorb visible light itself. This doubt, however, is not necessary considering
the case using P25, TOX, TDN, or TNB as photocatalyst. If the decomposition of MB
is due to the light absorbance itself, then the efficiency of the decomposition
using a different photocatalyst may not vary so much as is shown in Figure 7(b).
Also, one may consider such dyes degradation on photocatalysts surface under
visible light to be a sensitization mechanism. If the decomposition of MB is
due to a sensitization mechanism, then the efficiency of the decomposition
using TOX may be similar to that of TDN and TNB due to their similar
crystallization and microstructure.The stability of the catalysts has been tested by using the photocatalysts
repeatedly three times. No visible change of the photoactivity has been
observed throughout these three runs. To ascertain the role of light radiation
in the decomposition process, we have tested the effect of TNB without light
radiation and the result is not unexpected (see Figures7(a) and 7(b)).
Without light, the decomposition process could hardly proceed.Altogether, the excellent visible-light photoactivity of TDN and TNB can
be explained by the doped microstructure. Because the N 2p orbits are localized
above the top of the O 2p valence bands, the doped N atoms will result in band
gap narrowing and improve the visible light absorption of anatase TiO2.
The improved visible light absorption can increase the number of photoinduced
electron. Undoubtedly, this can enhance the photocatalytic activity. Moreover,
the N-doping is considered to have a contribution in the formation of oxygen vacancies [44–47]. Incorporation of two N atoms into the oxygen sites should be
accompanied by the formation of one oxygen vacancy to maintain the overall
electroneutrality of the crystal lattice. Many studies have confirmed that the formation
of the superoxide (O2⋅−) and hydroxyl radicals (OH⋅), two
important active species for initiating a photocatalytic reaction [48–52],
required oxygen vacancy sites [35, 53, 54]. For sample TNB, The contribution
of the doped Br atoms to the visible photocatalytic activity is achieved mainly
by the improvement of the quantum efficiency. The doped Br atoms can convert Ti4+ to Ti3+ by charge compensation, and the existence of a certain amount of Ti3+ reduces the electron hole recombination rate and further enhanced the quantum efficiency and photocatalytic activity [43].
Moreover, halogen element, such as Br, Cl, F, doping for oxygen in TiO2 can also cause the absorption edge of TiO2 to shift to the lower
energy region [31], though the red-shift is weak, which can enhance
the photocatalytic activity too. Thus, the synergetic effect of doped N and Br
atoms contributes to a higher visible photocatalytic activity of TNB.


## 3.1. Crystal Phase Identification

Figure1 shows the XRD patterns of samples, TOX, TDN, and TNB. The
diffraction lines at 2θ = 25.3°,
37.8°, 48.1°, 53.9°, 55.2°, and 62.7° demonstrate that all the three
samples are of the anatase phase. The average crystallite size, DA,
determined from the diffraction peak of the anatase (101) planes, is about 6.5 nm estimated from the Debye-Scherrer equation [35]. TEM (see Figure 2) of sample TDN and TNB clearly
revealed the fringes of (101) planes of anatase with a lattice spacing of about
0.352 nm [36], which corresponds to the XRD
patterns. Among the three main polymorphs, anatase TiO2 is
believed to exhibit the highest photocatalytic activity [31]. The
anatase phase of the photocatalysts will contribute to a good photocatalytic
activity.Figure 1
XRD patterns of
samples, TOX, TDN, and TNB. Sample TOX was undoped TiO2, sample TDN
was N-doped TiO2, and sample TNB was N, Br-codoped TiO2.TEM images of samples, TDN (a) and TNB (b).
(a)(b)


## 3.2. Optical Properties and XPS Analysis

The samples, TOX, TDN, and TNB show different color. TOX looks white, TDN
yellow while TNB tan. From Figure3 (DRAS of the samples), TOX have no
absorption in the visible band; while the other two samples both have distinctly
great absorption in the visible band. Compared to sample TDN, sample TNB showed obvious enhanced absorption
in the visible range of 400–600 nm. The broad absorption below 500 nm is because the N 2p orbits are localized above the top of the O 2p valence
bands, whereas the absorption above 500 nm is mainly caused by oxygen vacancies [37]. The band gap energy of the synthesized samples was estimated to
be about 3.12 eV for TOX, 2.03 eV for TDN, and 1.88 eV for TNB by the
absorption edge. The band gap narrowing will result in the obvious red-shift in
the optical response, which is helpful for visible photocatalytic activity.Figure 3
UV-Vis diffuse reflection absorption spectra of samples, TOX, TDN, and TNB.The chemical composition of surface was determined by XPS (see Figure4).
The C 1s, O 1s, Ti 2p, and N 1s peaks components are discernible in the wide
scan spectrum of the two samples, but obvious peaks of Br 3d can be found only
for TNB. The N 1s core-level spectra of the two samples
are showed in Figure 5. For TDN sample (Figure 5(a)), the feature at 396.7 eV is
generally known as the “nitride” peak and can be assigned to the N2- anions that replace oxygen atoms in the TiO2 lattice to form N-Ti-N [38, 39].
N 1s peak of 399.3 eV can be attributed to the binding energy of the N atom in special
environment of O-Ti-N-R (R: alkyl) [38–40]. The weak N 1s peak at
401.7 eV indicates the trace amount of nitrogen in the form of Ti-N-O [39].
For TNB sample, No N 1s peaks from N+ of CTAB (at 402.3 eV) can be
found in Figure 5(b) [38, 41], indicating that residual CTAB has been
completely removed by the Soxhlet extracting method or at least the residual
CTAB was too little to be detected. The two main N 1s peaks at 396.5 eV and
399.3 eV can be attributed to the binding energy of the N atom in special environment
of N-Ti-N and O-Ti-N-R (R: alkyl) [38–40].Figure 4
XPS wide scan spectra of samples, TDN and TNB.(a) N 1s core-level spectrum (deconvolution of N 1s bands) of TDN. (b) N 1s core-level spectrum (deconvolution of N 1s bands) of TNB.
(a)(b)Figure6 showed Ti 2p core-level spectra of TDN and TNB. For TDN sample (Figure
6(a)), the deconvolution of Ti 2p bands yields two major doublets (2p3/2 and 2p1/2) encompassing the set of two 2p3/2 peaks. The
main Ti 2p3/2 peak at 458.6 eV is typical for pure TiO2 [38, 42] and can be attribute to those unreacted TiO2, the other Ti
2p3/2 peak at 457.2 eV for TiOxNy
[38, 42].
For TNB sample (Figure 6(b)), the deconvolution of the complex Ti 2p bands
yields three major doublets (2p3/2 and 2p1/2)
encompassing the set of three Ti 2p3/2 peaks. The strongest Ti 2p3/2 peak at 458.7 eV is typical for pure TiO2 [38] and can be attribute
to those unreacted TiO2, other peaks, at 455.7 eV typical for Ti3+ [38], and at 457.2 eV for TiOxNy [38, 42].
Doped nitrogen atoms result in the formation of TiOxNy [42],
meanwhile, the formation of Ti3+ might attribute to the substitution
of bromine atoms for oxygen just like the case of fluorine doping [33, 43].(a) Ti 2p core-level spectrum (deconvolution of Ti 2p bands) of TDN. (b) Ti 2p core-level spectrum (deconvolution of Ti 2p bands) of TNB.
(a)(b)From the results of the depth profile of N and Br atoms of sample TNB (see
Table1), we found that N and Br atoms were mainly at the surface of TiO2 particles. Besides, the results of XPS indicate that N and Br atoms were actually
existed in the lattice of the titania crystal.Table 1
Results of
XPS analysis using Ar etching, sample TNB.
NameContent/%BrNBefore etching0.313.68First etching0.111.81Second etching01.79Third etching01.17


## 3.3. Photocatalytic Activity

### 3.3.1. UV-Activity

The photodecomposition of the MB in aqueous solution was carried out under
UV light. The time-dependent concentration of MB is shown in Figure7(a). With
high crystallization, P25 can more readily photodegrade MB. The
photocatalysts, TOX, TDN, and TNB, exhibit similar and good photocatalytic
activity, which due to their similar crystallization and microstructure. Because
the doping process was performed under low temperature, then there is little
change of crystallization and microstructure.(a) Comparison of the UV photocatalytic activities
of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under UV light. (b) Comparison of the visible photocatalytic
activities of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under visible light.
(a)(b)


### 3.3.2. Vis-Activity

The photodecomposition of the MB in aqueous solution was carried out under
visible light. The time-dependent concentration of MB in solution is shown in Figure7(b). Compared with the visible-inactive P25 and TOX, both of TDN and TNB
exhibit excellent photocatalytic activity, and obviously TNB can more readily photodegrade MB
than TDN. One may raise the doubt of whether it is
the photocatalyst that plays the key role in decomposing MB because MB can
absorb visible light itself. This doubt, however, is not necessary considering
the case using P25, TOX, TDN, or TNB as photocatalyst. If the decomposition of MB
is due to the light absorbance itself, then the efficiency of the decomposition
using a different photocatalyst may not vary so much as is shown in Figure 7(b).
Also, one may consider such dyes degradation on photocatalysts surface under
visible light to be a sensitization mechanism. If the decomposition of MB is
due to a sensitization mechanism, then the efficiency of the decomposition
using TOX may be similar to that of TDN and TNB due to their similar
crystallization and microstructure.The stability of the catalysts has been tested by using the photocatalysts
repeatedly three times. No visible change of the photoactivity has been
observed throughout these three runs. To ascertain the role of light radiation
in the decomposition process, we have tested the effect of TNB without light
radiation and the result is not unexpected (see Figures7(a) and 7(b)).
Without light, the decomposition process could hardly proceed.Altogether, the excellent visible-light photoactivity of TDN and TNB can
be explained by the doped microstructure. Because the N 2p orbits are localized
above the top of the O 2p valence bands, the doped N atoms will result in band
gap narrowing and improve the visible light absorption of anatase TiO2.
The improved visible light absorption can increase the number of photoinduced
electron. Undoubtedly, this can enhance the photocatalytic activity. Moreover,
the N-doping is considered to have a contribution in the formation of oxygen vacancies [44–47]. Incorporation of two N atoms into the oxygen sites should be
accompanied by the formation of one oxygen vacancy to maintain the overall
electroneutrality of the crystal lattice. Many studies have confirmed that the formation
of the superoxide (O2⋅−) and hydroxyl radicals (OH⋅), two
important active species for initiating a photocatalytic reaction [48–52],
required oxygen vacancy sites [35, 53, 54]. For sample TNB, The contribution
of the doped Br atoms to the visible photocatalytic activity is achieved mainly
by the improvement of the quantum efficiency. The doped Br atoms can convert Ti4+ to Ti3+ by charge compensation, and the existence of a certain amount of Ti3+ reduces the electron hole recombination rate and further enhanced the quantum efficiency and photocatalytic activity [43].
Moreover, halogen element, such as Br, Cl, F, doping for oxygen in TiO2 can also cause the absorption edge of TiO2 to shift to the lower
energy region [31], though the red-shift is weak, which can enhance
the photocatalytic activity too. Thus, the synergetic effect of doped N and Br
atoms contributes to a higher visible photocatalytic activity of TNB.


## 3.3.1. UV-Activity

The photodecomposition of the MB in aqueous solution was carried out under
UV light. The time-dependent concentration of MB is shown in Figure7(a). With
high crystallization, P25 can more readily photodegrade MB. The
photocatalysts, TOX, TDN, and TNB, exhibit similar and good photocatalytic
activity, which due to their similar crystallization and microstructure. Because
the doping process was performed under low temperature, then there is little
change of crystallization and microstructure.(a) Comparison of the UV photocatalytic activities
of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under UV light. (b) Comparison of the visible photocatalytic
activities of P25, TOX, TDN, and TNB on MB photodecomposition. Time-dependent concentration
of MB in solution under visible light.
(a)(b)


## 3.3.2. Vis-Activity

The photodecomposition of the MB in aqueous solution was carried out under
visible light. The time-dependent concentration of MB in solution is shown in Figure7(b). Compared with the visible-inactive P25 and TOX, both of TDN and TNB
exhibit excellent photocatalytic activity, and obviously TNB can more readily photodegrade MB
than TDN. One may raise the doubt of whether it is
the photocatalyst that plays the key role in decomposing MB because MB can
absorb visible light itself. This doubt, however, is not necessary considering
the case using P25, TOX, TDN, or TNB as photocatalyst. If the decomposition of MB
is due to the light absorbance itself, then the efficiency of the decomposition
using a different photocatalyst may not vary so much as is shown in Figure 7(b).
Also, one may consider such dyes degradation on photocatalysts surface under
visible light to be a sensitization mechanism. If the decomposition of MB is
due to a sensitization mechanism, then the efficiency of the decomposition
using TOX may be similar to that of TDN and TNB due to their similar
crystallization and microstructure.The stability of the catalysts has been tested by using the photocatalysts
repeatedly three times. No visible change of the photoactivity has been
observed throughout these three runs. To ascertain the role of light radiation
in the decomposition process, we have tested the effect of TNB without light
radiation and the result is not unexpected (see Figures7(a) and 7(b)).
Without light, the decomposition process could hardly proceed.Altogether, the excellent visible-light photoactivity of TDN and TNB can
be explained by the doped microstructure. Because the N 2p orbits are localized
above the top of the O 2p valence bands, the doped N atoms will result in band
gap narrowing and improve the visible light absorption of anatase TiO2.
The improved visible light absorption can increase the number of photoinduced
electron. Undoubtedly, this can enhance the photocatalytic activity. Moreover,
the N-doping is considered to have a contribution in the formation of oxygen vacancies [44–47]. Incorporation of two N atoms into the oxygen sites should be
accompanied by the formation of one oxygen vacancy to maintain the overall
electroneutrality of the crystal lattice. Many studies have confirmed that the formation
of the superoxide (O2⋅−) and hydroxyl radicals (OH⋅), two
important active species for initiating a photocatalytic reaction [48–52],
required oxygen vacancy sites [35, 53, 54]. For sample TNB, The contribution
of the doped Br atoms to the visible photocatalytic activity is achieved mainly
by the improvement of the quantum efficiency. The doped Br atoms can convert Ti4+ to Ti3+ by charge compensation, and the existence of a certain amount of Ti3+ reduces the electron hole recombination rate and further enhanced the quantum efficiency and photocatalytic activity [43].
Moreover, halogen element, such as Br, Cl, F, doping for oxygen in TiO2 can also cause the absorption edge of TiO2 to shift to the lower
energy region [31], though the red-shift is weak, which can enhance
the photocatalytic activity too. Thus, the synergetic effect of doped N and Br
atoms contributes to a higher visible photocatalytic activity of TNB.


## 4. Conclusions

In this paper, N-Br-codoped TiO2 photocatalyst was prepared via
a facile two-step homogeneous hydrothermal route at low temperature. As
expected, the prepared photocatalyst showed a high-photocatalytic activity
under visible irradiation, which was attributed to its anatase crystalline
framework, low-electron hole recombination rate and high absorbance in the
visible light range. A synergetic effect of both doped N atoms and Br atoms should be responsible for the
enhanced visible photoactivity.



---

*Source: 563949-2008-03-18.xml*