Enhancement of photocatalytic activity of CaTiO3 through HNO3 acidification

Article abstract of DOI:10.1016/j.jphotochem.2016.02.012

CaTiO₃ synthesized by the hydrothermal method was treated to improve its photocatalytic activity using the HNO₃ acidification. Changes of morphology, structure, and composition of the acidified CaTiO₃ were investigated through various means such as XRD, SEM, BET, FTIR, Raman spectra, XPS, and UV–vis DRS. The results show that the enhancement of HNO₃ acidification led to the increase of substitutional N (O–Ti–N) compared to interstitial N (Ti–O–N and Ti–N–O) in the CaTiO₃ lattice. The apparent first-order reaction constant (k_obs) of methylene blue over the acidified CaTiO₃ at pH 2 was the largest and increased by 1.0 times than that over the not-acidified CaTiO₃, indicating that the acidified CaTiO₃ at pH 2 has the best photocatalytic performance among as-prepared samples. A negatively linear relationship between k_obs and |1 ? N_O–Ti–N/N_{Ti–O–N+Ti–N–O}| suggests that the improvement of photocatalytic activity of CaTiO₃ by HNO₃ acidification depends on synergistic effects of substitutional and interstitial N in CaTiO₃ lattice.

Full text of DOI:10.1016/j.jphotochem.2016.02.012

Accepted Manuscript
Title: Enhancement of photocatalytic activity of CaTiO3
through HNO3 acidification
Author: Chong Han Jingjing Liu Wangjin Yang Qianqian Wu
He Yang Xiangxin Xue
PII:
S1010-6030(15)30293-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.02.012
JPC 10141
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Journal of Photochemistry and Photobiology A: Chemistry
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Revised date:
Accepted date:
2-12-2015
11-2-2016
15-2-2016
Please cite this article as: Chong Han, Jingjing Liu, Wangjin Yang, Qianqian Wu,
He Yang, Xiangxin Xue, Enhancement of photocatalytic activity of CaTiO3 through
HNO3 acidification, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.02.012
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Enhancement of photocatalytic activity of CaTiO3 through HNO3 acidification
Chong Han , Jingjing Liu, Wangjin Yang, Qianqian Wu, He Yang, Xiangxin Xue
School of Metallurgy, Northeastern University, Shenyang, 110819, China
Abstract
CaTiO3 synthesized by the hydrothermal method was treated to improve its photocatalytic
activity using the HNO3 acidification. Changes of morphology, structure, and composition of
the acidified CaTiO3 were investigated through various means such as XRD, SEM, BET,
FTIR, Raman spectra, XPS, and UV-vis DRS. The results show that the enhancement of
HNO3 acidification led to the increase of substitutional N (O-Ti-N) compared to interstitial N
(Ti-O-N and Ti-N-O) in the CaTiO3 lattice. The apparent first-order reaction constant (kobs) of
methylene blue over the acidified CaTiO3 at pH=2 was the largest and increased by 1.0 times
than that over the not-acidified CaTiO3, indicating that the acidified CaTiO3 at pH=2 has the
best photocatalytic performance among as-prepared samples. A negatively linear relationship
between kobs and |1-NO-Ti-N/NTi-O-N+Ti-N-O| suggests that the improvement of photocatalytic
activity of CaTiO3 by HNO3 acidification depends on synergistic effects of substitutional and
interstitial N in CaTiO3 lattice.
Keywords: CaTiO3; HNO3; Substitutional N; Interstitial N
1. Introduction
The perovskite-type oxides with a structural formula of ABO3 (A is a rare or alkaline earth
metal and B is a first row transition metal) have excellent properties with wide applications in
electronics, ceramics, superconductor, nonlinear optics, and catalysis [1-4]. CaTiO3 is well
known as one of the most important perovskites and has attracted more and more focus in the
last decade. It can be prepared by many methods such as solid state [5-7], co-precipitation [8],
mechanochemical [9], sol-gel [10-12], hydrothermal [13-16], and solvothermal methods [17,
18]. Among these methods, hydrothermal or solvothermal routes for CaTiO3 have become the
popular methods because they can exactly control crystal growth, achieve novel crystal
Corresponding author. Tel.: +86-024-83684086; Fax: +86-024-83687719.
E-mail address: hanch@smm.neu.edu.cn (C. Han).

morphologies, and optimize physicochemical properties [14]. Hydrothermal or solvothermal
methods generally involved the facile reactions of titanium alkoxides or TiO2, calcium salt,
mineralizers, and water or organic solvent in a Teflon-lined stainless steel vessel at 100-220
oC [13-18].
Based on the hydrothermal method, rod-like and cubic-shaped CaTiO3 particles were
successfully synthesized by changing temperature and Ti-sources such as TiO2 and
titanium-triethanolamine [13]. Three-dimension aggregated prisms, butterfly-like dendrites,
and cross cubic shapes CaTiO3 particles were prepared by controlling NaOH concentration
during the hydrothermal process [14]. Single-crystalline hollow CaTiO3 cubes can be
produced through introducing 5% water during the solvothermal process [18]. Spherical,
cubic, and rectangular CaTiO3 particles can be obtained with varying content (1.25%-5%)
and addition order of water in the solvothermal process [17].
It has been widely reported that CaTiO3 exhibits a superior photocatalytic performance in
the removal of some organic and inorganic pollutants and the photolysis of water to hydrogen
and oxygen [19-23]. The photodegradation percentage of methyl orange can reach to 96%
using CaTiO3 under ultraviolet light irradiation [12]. 98.4% of As(Ⅲ) was oxidized to As(Ⅴ)
by photo-generated holes over fern-like CaTiO3 nanoparticles [19]. Hydrogen production rate
of 0.34 µmol/min was achieved with Pt-loaded CaTiO3 as photocatalyst to decompose water
[7]. CaTiO3 shapes can affect its photocatalytic activity. Compared to flower-like and
prism-like CaTiO3, butterfly-like dendrites CaTiO3 showed better photocatalytic activity to
degrade rhodamine B [14]. Rectangular CaTiO3 had higher activity than cubic and spherical
ones for photocatalytic degradation of methylene blue [17].
Because of its wide band gap energy (3.5 eV), CaTiO3 was photoactive only in ultraviolet
light region. Recently, the metal doping has been made to enhance the photocatalytic activity
of CaTiO3. For example, the photocatalytic activity of Ag-La codoped CaTiO3 for hydrogen
evolution increased dramatically than that of CaTiO3 when the doping amount was 3 mol%
[10]. Compared to CaTiO3, CaTi1-xCuxO3 (0.1≤x≤0.04) exhibited a higher activity for the
photocatalytic decomposition of water to oxygen and hydrogen [11]. Pt/CaTi0.9Fe0.1O3 had a
certain activity while Pt/CaTiO3 was inactive under visible light for photocatalytic generation
of hydrogen from methanol-water solution [21]. Our previous study found that the doping of

Fe in CaTiO3 can promote the photocatalytic degradation of methylene blue [5].
In this study, CaTiO3 synthesized using the hydrothermal method was modified to enhance
its photocatalytic activity by HNO3 acidification. The changes in the structures, compositions,
and properties of the modified CaTiO3 were detailedly characterized using various means
including X-ray diffractometer (XRD), scanning electron microscope (SEM), specific surface
area analysis, Fourier transform infrared spectrometer (FTIR), Raman spectrometer, X-ray
photoelectron spectroscopy (XPS), and UV-vis diffuse reflection spectra (UV-vis DRS). The
photocatalytic activity of CaTiO3 was evaluated through the degradation of methylene blue
(MB) under irradiation. Effects of HNO3 acidification on the photocatalytic activity of
CaTiO3 were also discussed.
2. Experimental
2.1 CaTiO3 synthesis
CaTiO3 particles were synthesized by the hydrothermal method. In a typical process, solid
Ca(NO3)2·4H2O (2.362 g) was dissolved by water in the Teflon-lined stainless steel vessel.
Tetrabutyl titanate (3.4 mL) was dropwise added into the solution under vigorous stirring.
The molar ratio of Ca to Ti was 1.0. Then, solid NaOH (0.8 g) was introduced as the
mineralization reagent. The mixture was continuously stirred at ambient temperature for 1.0 h.
o
Subsequently, the hydrothermal treatment was carried out for 24 h at 200 C. After cooling,
the precipitates were recovered by the filtration, washed several times with ethanol, diluted
acetic acid, and deionized water, and dried for 12 h at 80 oC.
2.2 HNO3 acidification of CaTiO3
The pH value of 20 mL deionized water was adjusted to 4 and 2, respectively, or the HNO3
concentration in 20 mL deionized water was adjusted to 2 mol/L using HNO3. These water
solutions containing HNO3 were used to modify CaTiO3. After the addition of CaTiO3 (0.2g),
o
the corresponding acidic solution was continuously stirred to evaporate water at 50 C. The
sample was further dried for 24 h at 80 oC and then was calcined for 3 h at 500 oC. Thus, the
acidified CaTiO3 by HNO3 at pH=4, pH=2, and HNO3 concentration of 2 mol/L was obtained.
As a comparison, the not-acidified CaTiO3 was also likewise treated without the addition of
HNO3.
2.3 CaTiO3 characterizations

X-ray diffractometer (XRD, X' Pert Pro, PANalytical) with Cu Kα radiation at 40 kV and
40 mA was used to analyze crystal structures of CaTiO3. A surface area & pore size analyzer
(NOVA 1200e, Quantachrome) was employed to measure specific surface areas of CaTiO3
by N2 adsorption/desorption at 77 K. Specific surface areas were determined by applying the
Brunauer-Emmett-Teller (BET) method to the adsorption isotherm in the partial pressure
range of 0.05-0.35. Scanning electron microscope (SEM, SSX-550, SHIMADZU) was used
to observe micro morphologies of CaTiO3. Fourier transform infrared spectrometer (FTIR,
Thermo Nicolet-380) was adopted to investigate functional groups of CaTiO3 with KBr as a
pelletizing medium. A UV resonance Raman spectrometer (UVR DLPC-DL-03) was used to
obtain Raman spectra of CaTiO3. The exciting radiation was a continuous diode pumped
solid state (DPSS) laser beam (325 nm), and the source power output was 60 mW. A surface
analysis system (ESCALAB 250, Thermo Fisher Scientific) with an Al Kα radiation was
employed to record X-ray photoelectron spectroscopy (XPS) of CaTiO3. The UV-vis diffuse
reflection spectra (UV-vis DRS) of CaTiO3 was recorded by a diffuse reflectance UV-vis
spectrophotometer (2550, SHIMADZU) with BaSO4 as the reflectance standard.
2.4 Photocatalytic activity test
The photocatalytic activity of CaTiO3 was measured by decomposing MB in an aqueous
solution under the irradiation of 500 W Xe lamp. During a typical MB removal experiment,
CaTiO3 (0.1 g) was dispersed into aqueous solution (200 mL) with initial MB concentration
of 10 mg/L. Before irradiation, the aqueous suspension was magnetically stirred for 1.0 h to
reach an adsorption/desorption equilibrium of MB on CaTiO3 in the dark. Subsequently, the
lamp was turned on and the photocatalytic activity test was performed at the temperature of
25 oC. Temporal changes of MB concentration were monitored by measuring the absorbance
of the solution at 664 nm (the maximum absorption wavelength for MB) using the UV-vis
spectrophotometer (UV-2550, SHIMADZU).
3. Results and discussion
3.1 Characterizations
Figure 1A shows XRD patterns of not-acidified and acidified samples. The peaks of
not-acidified sample can be indexed to the standard XRD data of orthorhombic CaTiO3
(JCPDS card No. 42-0423), determining that as-prepared sample through the hydrothermal

method was assigned to well-crystallized CaTiO3. The peaks at 2θ of 23.27o, 33.13o, 47.54o,
59.37o, 69.49o, and 79.25o can be related to (101), (121), (202), (123), (242), and (161) planes
of orthorhombic CaTiO3, respectively. After HNO3 acidification at pH=4, pH=2, and 2 mol/L,
peak positions and intensities of CaTiO3 almost remained unchanged, implying that HNO3
acidification did not introduce impurity phases. However, as shown in Figure 1B, a careful
comparison in the range of 2θ=32-34o found that the peak positions of all acidified CaTiO3 by
HNO3 slightly shifted towards a lower 2θ value. The ionic radius (0.171 nm) of N3- is larger
than the one (0.140 nm) of O2-. According to the Bragg formula (nλ=2dsinθ, where n is
diffraction series, λ is the X-ray wavelength, d is the crystalline plane distance, and θ is the
diffraction angle), d value will increase when O2- is substituted by N3-, which can cause the
distortion of CaTiO3 crystal and result in the shift of peak positions in XRD patterns to lower
2θ values. This suggests that the N atom in HNO3 may partially substitute the O atom in
CaTiO3 lattice. The crystallite sizes of CaTiO3 were calculated using the Scherrer′ equation
(D=0.9λ/βcosθ, where D is the crystallite size and β is the full width at half maximum
(FWHM) of the most intense peak (121)). As summarized in Table 1, the crystallite sizes of
not-acidified and acidified CaTiO3 varied in the range of 43-44 nm, demonstrating the little
effects of HNO3 acidification on the crystallite sizes of CaTiO3.
Figure 1 XRD patterns of not-acidified and acidified CaTiO3
Figure 2 shows the morphologies of not-acidified and acidified CaTiO3 particles by SEM.

These images depicted that the morphologies and aggregate status of CaTiO3 particles greatly
varied with intensifying HNO3 acidification. Not-acidified CaTiO3 particles had irregular
rectangular shapes, the surfaces of which undulated and ruptured to form step-like structures.
Similar features were also observed for CaTiO3 particles prepared by the hydrothermal
progress of Ca(NO3)2·4H2O with TiO(OH)2 from the hydrolysis of tetrabutyl titanate [19].
For the acidified CaTiO3 particles at pH=4 and pH=2, only step-like structures with relative
large sizes still existed while the ones with small sizes disappeared. Some very small particles
adhered to the surface of step-like structures. The acidified CaTiO3 particles at 2 mol/L even
did not possess rectangular shapes and showed messy surfaces. These may be ascribed to the
erosion role of HNO3 in CaTiO3 surface. As shown in Table 1, the specific surface area of
not-acidified and acidified CaTiO3 at pH=4, pH=2, and 2 mol/L was 16.92 m2/g, 17.39 m2/g,
16.14 m2/g, and 14.90 m2/g, respectively. This suggests that HNO3 acidification has minor
negative effects on the specific surface area of CaTiO3.
Figure 2 SEM images of not-acidified and acidified CaTiO3
The functional groups of not-acidified and acidified CaTiO3 were examined by FTIR
spectra (Figure 3). The peaks at 440 and 554 cm-1 were related to Ti-O stretching and Ti-O-Ti
bridge stretching modes [1, 24-26]. The peak at 1635 cm-1 belonged to the bending vibration
of O-H band in hydroxyls and water adsorbed on CaTiO3 surface [27-29]. Two N-O groups
were observed in FTIR spectra. The band around 770 cm-1 and 1385 cm-1 can be assigned to

the in-plane bending vibration of N-O (v4) and the out-of-phase stretching vibration of N-O
(v3), respectively [30, 31]. HNO3 acidification insignificantly affected functional groups of
CaTiO3.
Figure 3 FTIR spectra of not-acidified and acidified CaTiO3
Raman spectra was applied to have better insight into the functional groups of CaTiO3. As
shown in Figure 4, functional groups including Ti-O and O-Ti-O (445 cm-1), N-O (721 cm-1
and 1348 cm-1), and O-H (1630 cm-1) that were detected in FTIR spectra were also observed
in Raman spectra [32-35]. Contrasted to the peaks in FTIR spectra, it was noticed that two
new bands at 848 cm-1 and 1042 cm-1 appeared in Raman spectra, which can be assigned to
the out-of-plane deformation N-O (v2) and the in-phase stretching vibration of N-O (v1),
respectively [34, 35]. Generally, v1 is Raman active, v2 is infrared active, and v3 and v4 are
both Raman and infrared active [30, 34]. Nevertheless, the v2 may become Raman active due
to the changes of its presence environment [30, 35]. Therefore, both of v3 and v4 were found
in FTIR and Raman spectra while v1 and v2 was observed only in Raman spectra.
Additionally, all peak intensities of acidified CaTiO3 at pH=2 and 2 mol/L were higher than
that for not-acidified CaTiO3 and acidified CaTiO3 at pH=4. The morphology and crystal size
of materials can strongly affect the strength of Raman signal because they can influence the
ratio of absorption and scattering events [36-40]. Here, the morphology enhancement effect
may be responsible for the increase of Raman peak intensities because the morphology of
CaTiO3 particles greatly varied but the crystal size almost remained unchanged with HNO3
acidification.

Figure 4 Raman spectra of not-acidified and acidified CaTiO3
The chemical compositions and states of surface elements in not-acidified and acidified
CaTiO3 were investigated by XPS spectra. As seen in Figure 5, Ti 2p XPS spectra showed
two peaks at the binding energy of 456.8-457.6 eV and 462.6-463.4 eV, which are assigned
to Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively [41-44]. There exhibited no shoulder at lower binding
energies that are classically attributed to Ti3+ due to oxygen vacancies [45], indicating the
existence of only Ti4+ in all CaTiO3 samples. Additionally, the binding energies of Ti4+ 2p3/2
and Ti4+ 2p1/2 in acidified CaTiO3 shifted to higher values compared with that in not-acidified
CaTiO3. This indicates different electronic interactions of Ti4+ and other ions and may be
related to a decrease in the electron density around Ti atom because of HNO3 acidification
[27, 40, 46, 47]. It was speculated that the substitute element may be permeated into the
acidified CaTiO3 lattice.
Figure 5 Ti 2p XPS spectra of not-acidified and acidified CaTiO3

Figure 6 shows O 1s XPS spectra of not-acidified and acidified CaTiO3. O 1s XPS spectra
can be fitted into a main peak at 527.9-528.6 eV and a shoulder peak around 529.4-530.3 eV.
The fitting results coincided well with experimental spectra and were credible. The former
belonged to oxygen in the CaTiO3 lattice and the latter corresponded to oxygen in surface OH
groups [29, 41, 42]. Compared to that for not-acidified and acidified CaTiO3 at pH=4, the two
peaks of O 1s for the acidified CaTiO3 at pH=2 and 2 mol/L positively shifted by 0.3-0.7 eV
and 0.1-0.9 eV. This may be attributed to a little adsorbed nitrate at high HNO3 concentration
since O 1s peaks in nitrate usually appear at 532.6-533.6 eV [42].
Figure 6 O 1s XPS spectra of not-acidified and acidified CaTiO3. (A) not-acidified CaTiO3;
(B) acidified CaTiO3 at pH=4; (C) acidified CaTiO3 at pH=2; (D) acidified CaTiO3 at 2
mol/L.
Figure 7 shows N 1s XPS spectra of not-acidified and acidified CaTiO3. There was a broad
N 1s peak in the range of 386-404 eV. After curve fitting, two peaks can be obtained around
392.1-394.1 eV and 396.8-398.9 eV, respectively. The fitting results were well coincident
with experimental spectra and were believable. The peak at the lower binding energy was
assigned to the substitutional N in the O-Ti-N linkages by the replacement of oxygen atom in
CaTiO3 lattice, and the other peak at the higher binding energy was ascribed to the interstitial
N in the forms of Ti-O-N and Ti-N-O [41, 44, 47, 48]. Thus, the substitutional and interstitial

states of N coexisted in all CaTiO3 samples. The molar ratio of O-Ti-N to Ti-O-N and Ti-N-O
(NO-Ti-N/NTi-O-N+Ti-N-O) was 0.62, 0.77, 0.91, and 1.18, respectively, for not-acidified CaTiO3
and acidified CaTiO3 at pH=4, pH=2, and 2 mol/L. It indicates that the enhancement of
HNO3 acidification mainly results in the increase of amount of substitutional N in CaTiO3
lattice. Besides, the third weak N 1s peak was also observed at 405.9 and 406.4 eV for the
-
acidified CaTiO3 at pH=2 and 2 mol/L, which was related to the N in the adsorbed NO3 on
CaTiO3 surface [42, 49, 50]. The adsorbed N, which only accounted for 3.0-5.0% of total N,
should have a slight contribution to the signals of N-O bands in FTIR and Raman spectra
(Figure 3 and 4). The results of N 1s XPS spectra can well explain the shift of peaks in XRD
patterns. Figure 7A demonstrates that the hydrothermal process can cause the permeation of
N from Ca source (Ca(NO3)2·4H2O) into CaTiO3 lattice and then generate N-doped CaTiO3,
which has been never reported in previous papers using the hydrothermal method with
Ca(NO3)2·4H2O as Ca source to prepare CaTiO3 [17, 19]. For acidified CaTiO3 (Figure
7B-D), substitutional and interstitial N may originate from Ca(NO3)2·4H2O and HNO3 while
the adsorbed N only emerged from HNO3.
Figure 7 N 1s XPS spectra of not-acidified and acidified CaTiO3. (A) not-acidified CaTiO3;
(B) acidified CaTiO3 at pH=4; (C) acidified CaTiO3 at pH=2; (D) acidified CaTiO3 at 2
mol/L.
Figure 8A displays UV-vis DRS of not-acidified and acidified CaTiO3. Compared with

not-acidified CaTiO3, acidified CaTiO3 at pH=4 and pH=2 showed a stronger adsorption in
the region of 350-410 nm and their adsorption edges exhibited a slight red-shift. According to
XPS results (Figure 7B and C), the red-shift of adsorption edge may be related to the increase
of substitutional N in CaTiO3 lattice with increasing acidity. Substitutional N can lead to a
narrower bandgap by mixing with O2p states and generate an isolated state above the valence
band [47, 51-53]. Nevertheless, acidified CaTiO3 at 2 mol/L displayed a blue-shift of the
adsorption edge compared to acidified CaTiO3 at pH=2, and it had similar light adsorption
properties to not-acidified CaTiO3. Although the acidified CaTiO3 at 2 mol/L contained more
substitutional N (Figure 7D), there may be the optimum amount of substitutional N to
accomplish the narrowest bandgap and the best photoresponsive ability. CaTiO3 is an indirect
inter-band transition material [2]. As shown in Figure 8B, the bandgap of CaTiO3 can be
calculated by plotting (F(R∞)hν)1/2 (F(R∞)=(1-R∞)2/2R∞, R∞ is diffuse reflectance, and hν is
the photon energy) versus the energy of absorbed light. The bandgap energy of not-acidified
CaTiO3 and acidified CaTiO3 pH=4, pH=2, and 2 mol/L was 3.50 eV, 3.46 eV, 3.42 eV, and
3.51 eV (Table 1), respectively.
Figure 8 UV-vis spectra (A) and plots of transformed Kubelka-Munk function versus the
energy of absorbed light (B) for not-acidified and acidified CaTiO3
3.2 Photocatalytic activity
The Langmuir-Hinshelwood kinetic model was usually used to describe the photocatalytic
degradation process of MB by catalysts [2, 43, 47]. The first-order reaction kinetic function
was expressed as ln(Ct/C0)=kobst, where C0 is initial MB concentration, Ct was the MB

concentration at the irradiation time t, and kobs is the apparent first-order reaction constant.
Figure 9 shows photocatalytic degradation kinetics of MB. Plots of ln(Ct/C0) with t exhibited
a well linear relationship, determining that the photocatalytic degradation of MB by CaTiO3
followed pseudo first-order kinetics. The kobs of MB self-degradation was (1.09±0.03)×10-3
min-1. The kobs of MB degradation over not-acidified CaTiO3 was (1.80±0.04)×10-3 min-1
while it was (2.46±0.08)×10-3 min-1, (3.59±0.05)×10-3 min-1, and (3.10±0.10)×10-3 min-1,
respectively, for MB degradation over acidified CaTiO3 at pH=4, pH=2 and 2 mol/L. All
acidified CaTiO3 had higher photocatalytic activity than not-acidified CaTiO3. The kobs of MB
over acidified CaTiO3 at pH=2 was the largest and increased by 1.0 times compared to that
over not-acidified CaTiO3, confirming that acidified CaTiO3 at pH=2 exhibits the best
photocatalytic activity among as-prepared samples.
Figure 9 Photocatalytic degradation kinetics of MB over not-acidified and acidified CaTiO3
As shown in Figure 7, the enhancement of HNO3 acidification caused the increase of the
ratio of O-Ti-N to Ti-O-N and Ti-N-O (NO-Ti-N/NTi-O-N+Ti-N-O). Furthermore, the substitutional
N in the O-Ti-N linkages and the interstitial N in the forms of Ti-O-N and Ti-N-O can play
important roles in the light adsorption ability and the photocatalytic activity of materials [41,
44, 47, 48]. Therefore, the quantitative relationship of NO-Ti-N/NTi-O-N+Ti-N-O and photocatalytic
activity may be attempted to be built. Figure 10 displays the plots of kobs versus the absolute
value of subtraction of 1 and NO-Ti-N/NTi-O-N+Ti-N-O (|1-NO-Ti-N/NTi-O-N+Ti-N-O|). Interestingly, the
kobs and |1-NO-Ti-N/NTi-O-N+Ti-N-O| exhibited a negatively linear relationship with a math
equation of kobs=-(6.11±0.60)×10-3×|1-NO-Ti-N/NTi-O-N+Ti-N-O|+(4.10±0.17)×10-3, meaning that

CaTiO3 would have the highest photocatalytic activity when NO-Ti-N/NTi-O-N+Ti-N-O is equal to 1.
These confirm that the improvement of photocatalytic activity of CaTiO3 though HNO3
acidification significantly depends on synergistic effects of substitutional and interstitial N in
CaTiO3 lattice.
Figure 10 Plots of kobs versus |1-NO-Ti-N/NTi-O-N+Ti-N-O
|
4. Conclusions
HNO3 acidification had little effects on the crystallite sizes and specific surface areas of
CaTiO3 while it can destroy step-like structures on CaTiO3 surface. FTIR and Raman spectra
detected functional groups Ti-O, Ti-O-Ti and N-O. Ti 2p and O 1s XPS spectra confirmed
the existence of Ti4+ and O in the CaTiO3 lattice and surface OH groups in not-acidified and
acidified CaTiO3. N 1s XPS spectra suggested that substitutional N in the O-Ti-N linkages
and interstitial N in the forms of Ti-O-N and Ti-N-O coexisted in not-acidified and acidified
CaTiO3. The enhancement of HNO3 acidification increased the amount of substitutional N in
CaTiO3 lattice. The acidified CaTiO3 at pH=4 and pH=2 showed a stronger adsorption in the
region of 350-410 nm and a slight red-shift of their adsorption edges. The kobs of MB over
acidified CaTiO3 at pH=2 increased by 1.0 times compared to that over not-acidified CaTiO3,
suggesting that acidified CaTiO3 at pH=2 has the best photocatalytic activity. The kobs and
|1-NO-Ti-N/NTi-O-N+Ti-N-O| exhibited a negatively linear relationship, which confirms that the
improvement of photocatalytic activity of CaTiO3 by HNO3 acidification is dependent on
synergistic effects of substitutional and interstitial N in CaTiO3 lattice.

Acknowledgements
This research was financially supported by the National Natural Science Foundation of
China (21407020) and the Fundamental Research Fund for the Central Universities
(N130302004).
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Table 1 Crystallite sizes, specific surface areas, and bandgaps of not-acidified and acidified
CaTiO3 particles
Samples
CaTiO3
Crystalline grain sizes Specific surface areas
Band gaps
3.50 eV
3.46 eV
3.42 eV
3.51 eV
43.5 nm
43.2 nm
43.9 nm
43.4 nm
16.92 m2/g
17.39 m2/g
16.14 m2/g
14.90 m2/g
CaTiO3(pH=4)
CaTiO3(pH=2)
CaTiO3(2 mol/L)