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Research Article

Nano Adv., 2017, 2, 36−44.

2016, 1, X−X. Nano Advances

http://dx.doi.org/10.22180/na210 Volume 2, Issue 3, 2017

An Amorphous Mn, N-Codoped TiO2 Microspheres Photocatalyst Induced by High Defects with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Degradation

Mingming Zou, a Lu Feng, a Erum Pervaiz, ac Ayyakannu Sundaram Ganeshraja, a Tingting

Gao, a Heng Jiang, b* and Minghui Yang a,*

a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

b School of Chemistry and Materials Science, Liaoning Shihua University, Fushun 113000, China

c School of Chemical & Materials Engineering (SCME), National University of Sciences & Technology (NUST), Islamabad, 44000,

Pakistan

* Corresponding author Tel./Fax: +8641185168242; +862456860790; myang [at] dicp.ac.cn (Minghui Yang); hjiang78 [at]

hotmail.com (Heng Jiang).

Received March 4, 2017; Revised July 9, 2017

Citation: M. Zou, L. Feng, E. Pervaiz, A. S. Ganeshraja, T. Gao, H. Jiang and M. Yang, Nano Adv., 2017, 2, 36−44.

Titanium dioxide (TiO2) has attracted tremendous attention in photocatalysis due to its excellent properties. However,

the photocatalytic performance of TiO2 is still restricted by the wide band gap and high recombination rate of

electrons and holes. Herein, a simple approach to produce amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) by thermally

treating low crystallinity TiO2: (0.2 Mn) under ammonia gas is reported. These formed amorphous TiO2: (0.2 Mn, N -

400 °C – 2 h) with high suspension and adsorption ability, interstitial N doped form and many oxygen vacancies show

excellent photocatalytic activity. Compared to crystalline counterparts with high specific surface area of 112 m2 g–1,

amorphous TiO2: (0.2 Mn, N - 400 °C - 2h) with 43.1 m2g–1 has a much higher adsorption ability and photocatalytic

activity. The use of amorphous TiO2 (0.2 Mn) containing interstitial N as photocatalyst with enhanced visible light

absorption demonstrates that same could be extendable to other large-band gap semiconductors.

KEYWORDS: Amorphous photocatalyst; Suspending and adsorption ability; Interstitial N; Oxygen vacancies

1. Introduction

Solid-state materials basically can be divided into three forms:

crystalline, quasi-crystalline, and amorphous, which depends on

the degree of order in atomic arrangements. Compared to

crystals and quasi-crystals with long-range atomic orders,

amorphous materials have unique atomic arrangements:

short-range order but long-range disorder. So amorphous

materials revealed distinct mechanical, optical, electronic and

magnetic properties.1–3 In photocatalytic systems, well

crystalline materials with long-range atomic order have been

used for the efficient separation and diffusion of photo excited

charge carriers, which is crucial for obtaining high

photocatalytic efficiency.4–6 Amorphous semiconductors usually

have not considered to be employed in photocatalysis, mainly

due to their photocatalytic inactivity or less efficiency. Some

attempts at using amorphous semiconductors as photocatalysts

have limited success in obtaining high visible light activity.7, 8

Recent representative examples include the use of disordered

Co1.28Mn1.71O4 as a visible-light-responsive photocatalyst for

hydrogen evolution by Chen et. al.,9 black hydrogenated

titanium dioxide nanocrystals used as photocatalyst10 and an

amorphous carbon nitride photocatalyst for photocatalytic

hydrogen generation.11 A most remarkable advantage of

amorphous semiconductors is their much smaller bandgap than

their crystalline counterparts due to their band tails.9, 12 In

addition, contrary to highly ordered atom arrangements in

crystals, atomic arrangements in amorphous or semi-crystalline

materials are completely or partially disordered, which could be

energetically favourable for N doping.13

Generally, the efforts have been made in optical response of

TiO2 to shift from UV to the visible light region to harvest solar

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doi: 10.22180/na210

energy more efficiently and increasing the separation efficiency

of charge carrier pairs.14–21 An approach is to dope various

transition-metal cations (such as V, Cr, Mn, Fe and Ni) into

TiO2.22, 23 Another approach is to dope TiO2 with nonmetals such

as N, S, B, F and P. All of the nonmetal-doped TiO2 materials,

N-doped TiO2-based composites have been intensively

investigated. N-doping can result in intermediate energy levels

in the bandgap as a consequence of either mixing N 2p with O

2p states or by introduction of localized states.24 In addition,

doped N atoms can act as trapping centers for photogenerated

electrons and effectively separate the photogenerated

hole-electron pairs.25 Oxygen vacancies produced in the process

of nitriding under ammonia gas would promote the absorption of

visible light to a large extent.26–27 In photocatalytic degradation

systems, other than increasing the life time of photo induced

charges, adsorption ability of material for pollutant is crucial

factor that need further studies for understanding.

Herein, combined with the current research status of

photocatalyst, we show that amorphous Mn, N-codoped TiO2

microspheres used as an effective visible light photocatalyst.28–31

Amorphous TiO2 (0.2 Mn, N - 400 °C – 2 h) with a narrow

bandgap shows an obviously higher photocatalytic activity in

degradation of organic dye under visible light than the

crystalline counterparts. It is mainly due to the high suspending

and adsorption ability, interstitial N doped form and more

oxygen vacancies in present photocatalysts. These findings may

help to develop a class of amorphous photocatalysts for solar

energy conversion. A detailed characterization was carried out,

and the catalytic activities in the visible-light photodegradation

of RhB were explored and discussed below.

2. Experimental Section

2.1 Materials

All chemicals were analytical reagents and used without further

purification. The hexadecylamine (HDA; 90%), titanium(IV)

isopropoxide (TIP; 98%), manganous acetate tetrahydrate

(Mn(C2H3O2)2·4H2O), absolute ethanol, deionized water,

potassium chloride (AR) and anatase TiO2 were supplied by the

Sigma-Aldrich, China. Rhodamine B (AR grade, Sinopharm,

China) was used as model organic pollutants for evaluating the

activity of synthesized materials. In all experiments, doubly

distilled water was used.

2.2 Synthesis of Mn-doped mesoporous TiO2 microspheres

(Mn-TiO2)

Mn-TiO2 microspheres were synthesized by the solvothermal

reaction of TIP, HDA, KCl, and Mn(C2H3O2)2·4H2O, followed

by calcination. Briefly, calculated amount of

Mn(C2H3O2)2·4H2O (wherein the molar ratio of Mn : Ti is

0.2:100), HDA (1.98 g) and 1.6 mL KCl (5.5 × 10-3 M) aqueous

solution were dispersed in 200 mL ethanol under stirring and

stirred for 30 min. TIP (4.5 mL) as titanium source was dripped

into the mixture slowly under stirring. After 2 min, the light

brown precursor bead suspension was kept static for 18 h and

then washed with ethanol three times, then dried in air at room

temperature. After that, the samples were transferred into a

Teflon-lined autoclave with a capacity of 100 mL, and kept at

160 °C for 16 h. The low crystallinity TiO2: (0.2 Mn) sample

aged for another 2 days. The resulting precipitate was collected

by washing with ethanol and centrifugal separation, and dried in

air. Finally, the powders obtained were calcined at 500 °C for 2

h, leading to the formation of anatase and low crystallinity TiO2:

(0.2 Mn) microspheres, respectively. In this paper, TiO2:(0.2 Mn)

stands for 0.2 at % Mn doped TiO2 sample.

2.3 Synthesis of Mn, N-codoped mesoporous TiO2

microspheres (Mn-N-TiO2)

The resulting anatase and low crystallinity TiO2: (0.2 Mn)

microspheres were placed in a quartz boat, respectively. This

boat was then placed in a quartz tube with airtight, stainless steel

end-caps that have welded valves and connections to input and

output gas lines. The quartz tube was then placed in a tube

furnace, and the appropriate connections to the gas sources were

made. An argon gas flow through the tube was used for 15 min

to expel the air remaining in the tube before establishing the

flow of ammonia gas through the tube. The sample was then

heated in the tube to 400 °C and 500 °C at 4 °C/min speed,

respectively for 2 h. After 2 h calcination, the furnace power was

turned off and the product was cooled to room temperature in 4 h

under an ammonia gas flow. Before the quartz tube was taken

out of the tube furnace, an argon gas flow through the tube was

used to expel the ammonia gas remaining in the tube. In this

paper, amorphous TiO2: (0.2 Mn, N - 400 °C - 2 h) stands for

anatase low crystallinity TiO2: (0.2 Mn) samples and nitriding

treatment at 400 °C for 2 h.

2.4 Characterization

The crystalline structures of the samples were characterized by

X-ray Diffraction (XRD) using a Miniflex 600 X-ray

diffractometer with monochromatic Cu Kα radiation (λ = 0.1542

nm, accelerating voltage 40 kV, applied current 15 mA) at

scanning rate of 1°/min. Fourier Transform Infrared (FTIR)

spectra were recorded on a Bruker Tensor 27. Raman spectra

(532 nm excitation) were recorded on Bruker Optics Senterra.

The morphology and compositions were performed using a field

emission scanning electron microscope (FE-SEM) instrument

(JSM-7800F, Japan). Transmission electron microscope (TEM)

characterization was performed on a JEM-2100 electron

microscope operating at 200 kV. UV-Vis diffuse reflectance

spectra (UV-Vis DRS) were recorded with a Hitachi U-3900

spectrometer in the range of 200-850 nm, using a BaSO4

standard used as the reference. X-ray photoelectron spectroscopy

(XPS) measurements were carried out on an X-ray Photoelectron

Spectrometer (ESCALAB250Xi) using Al Kα (1486.6 eV)

X-rays as the excitation source. C 1s (284.6 eV) was chosen as

the reference. Surface area measurements were performed by

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doi: 10.22180/na210

nitrogen adsorption using Brunauer-Emmet-Teller (BET) area

method on accelerated surface area and porosimetry system

(ASAP 2420) to obtain the value ofspecific surface area, pore

volume and mean pore size. Electron paramagnetic resonance

(EPR) was recorded at 104 K (Brucker EPR A200 spectrometer,

center field, 3450 G; microwave).

2.5 Measurement of photocatalytic properties: degradation

of Rhodamine B (RhB) and methylene blue (MB)

The photocatalytic measurements were carried out in an open

double jacket glass thermostatic photoreactor. Before light

irradiation, a suspension containing 15 mg L–1 RhB (MB)

solution (200 mL) and the solid catalyst (120 mg) was sonicated

for 10 min, and then stirred for 1 h in the dark to ensure an

adsorption-desorption equilibrium. The suspension was

irradiated under continuous stirring using a visible light (300 W

Microsolar 300 UV-Xe lamp with a UV-cutoff filter (420 nm))

and was positioned 10 cm away from the reactor (removing the

effect of light on photocatalysis). All the experiments were

performed at 25 °C under constant stirring. At a given time

interval of irradiation, 5 mL of the solution was withdrawn and

centrifuged for measurement with a UV-Vis absorption (Hitachi

U-3900 spectrometer) at the maximal absorption wavelength for

RhB and MB, which have characteristic absorption peaks of 554

nm (λRhB) and 664 nm (λMB), respectively.

Polycaprolactone (PCL, Mw: 70000-90000), trimethylene

carbonate (TMC), rhodamine B (RhB), and phosphate buffered

saline (PBS) were purchased from Sigma-Aldrich. Ferric

chloride (FeCl3·6H2O), ferrous chloride (FeCl2·4H2O),

hydrochloride acid (HCl, 36–38%), and sodium hydroxide

(NaOH) were obtained from Sinopharm Chemical Reagent Co.

Ltd. Ultrapure water was obtained from a Millipore pure water

system. All chemicals were of analytical-reagent grade and used

without further purification.

3. Results and discussion

Anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) sample has two typical

nitrogen species models (Figure 1(A)) for instance substitutional

and interstitial nitrogen impurities. In substitutional model, the

nitrogen atom is bound to three Ti atoms and replaces lattice

oxygen in TiO2. In the interstitial model, the nitrogen atoms are

bound to one or more oxygen atoms. And for amorphous TiO2:

(0.2 Mn, N - 400 °C – 2 h) (Figure 1(B)) could be destroyed in

the long-range order by the more defects, which were actually

induced by nitriding treatment, while it gives disorder status.

Figure 2 shows the Powder X-ray Diffraction (PXRD)

patterns of anatase TiO2: (0.2 Mn), anatase TiO2: (0.2 Mn, N -

400 °C – 2 h), low crystallinity TiO2: (0.2 Mn) and amorphous

TiO2: (0.2 Mn, N - 400 °C – 2 h) microspheres prepared in this

work. The PXRD peak positions for all the samples are matched

well with anatase TiO2, in the space group I41/amd with refined

lattice parameters of a = 3.7842(2) Å and c = 9.5146 (2) Å using

GSAS package as shown in Figure S1 (Supplementary

Information). The mesoporous anatase TiO2: (0.2 Mn) sample

exhibited well crystallinity. After nitriding treatment at 400 °C

for 2h, intensity of diffraction peaks observed to decrease and

sample can be termed as low crystalline owed to defects

introduced by nitriding treatment as induced defects could

Figure 1. Crystal structures of (A) Crystal anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) (B) amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h).

Figure 2. XRD patterns of (a) anatase TiO2: (0.2 Mn), (b) anatase TiO2: (0.2

Mn, N - 400 °C – 2 h), (c) low crystallinity TiO2: (0.2 Mn) and (d) amorphous

TiO2: (0.2 Mn, N - 400 °C – 2 h).

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Nano Adv., 2017, 2, 36−44.016, 1, X−X.

doi: 10.22180/na210

reduce the crystallinity of the samples. Also the long aging time

after solvothermal reaction have caused the good attachment of

acetate ion and surfactant. Therefore after calcination, the

surfactant has not decomposed completely. Similarly, the more.

Coincidentally, the amorphous TiO2: (0.2 Mn, N- 400 °C – 2 h)

sample was synthesized by nitriding low crystallinity TiO2: (0.2

Mn) sample. The diffractograms of all the samples do not show

any diffraction peaks of manganese or manganese compounds,

which can be attributed to very low dopant amount and the

similar ionic radius of metals in these samples which make Mn

ions to, replace the Ti ions more easily. Also XRD patterns do

not show any peaks related to TiN phase after nitriding treatment,

as the temperature and time of nitriding was not sufficient to

produce TiN.

Figure 3(a) illustrates the simulated diagrams of as prepared

samples. Morphology and surface features of the prepared

samples are presented in Figure 3(b-e) that clearly portrays the

uniform microspherical structure exist for all samples. Field

emission scanning electron microscopy (FESEM) image of

anatase TiO2: (0.2 Mn) and low crystallinity TiO2: (0.2 Mn)

clearly show particle diameters around 600-800 nm. Anatase

TiO2: (0.2 Mn) microspheres contain nanocrystals with pores on

the surface of the beads produced during calcinations as a result

of template removal. In comparison to crystalline counterparts,

low crystalline TiO2 and amorphous TiO2 possess smoother

surfaces with no pores and grains as can be observed in Figure

3(d-e). This can be attributed to strong bonding between acetate

ion and surfactant, during aging. After nitriding treatment at low

temperature, the morphology of two different types of samples is

basically retained. Figure 3 (f-i) shows the TEM images for all

samples. Anatase TiO2: (0.2 Mn), and anatase TiO2: (0.2 Mn, N -

400 °C - 2h) (Figure 3(f and g)) reveal a high density of

channels/pores in a single microsphere. As different contrast was

observed from the surface and inside lattice, whereas a solid core

Figure 3. (a) Schematic diagram, (b-e) FESEM images with the insets showing enlargements of single microspheres, (f−i) TEM images, (j−m) lattice

resolution HRTEM images, and (n−q) SAED patterns of anatase TiO2: (0.2 Mn), anatase TiO2: (0.2 Mn, N - 400 °C – 2 h), low crystallinity TiO2: (0.2 Mn) and

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)).

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Research Article Nano Advances

Nano Adv., 2017, 2, 36−44.016, 1, X−X.

doi: 10.22180/na210

and smooth surface (Figure 3(h and i)) can be observed for low

crystallinity TiO2: (0.2 Mn) and amorphous TiO2: (0.2 Mn, N -

400 °C – 2 h) respectively. In the selected area electron

diffraction (SAED) patterns (Figure 3(p and q)), one can see that

the low crystallinity TiO2: (0.2 Mn) becomes amorphous phase

due to a mass of defects induced by nitriding treatment.

Furthermore, there is still weak and less crystalline lattice in the

HRTEM image of amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)),

that was examined as shown in Figure 3(m). This implies that

the long-range atomic order has been destroyed but short-range

atomic order remains unchanged. The lattice-resolved HRTEM

images (Figure 3(j, k and l)) clearly demonstrated the crystal

lattice fringes of the anatase TiO2 (101) crystal planes (d = 3.562

Å) within the primary nanocrystals. The SAED pattern (Figure

3(n-p)) of the TiO2 samples presents only the diffraction of pure

anatase TiO2 phase as (101), (103), (200), (202), and (204)

planes.

The energy disperse spectroscopy (EDS) (Figure S2 (a) in

Supplementary Information) indicates that microspheres

consisted of Mn, Ti and O. Moreover, the elemental mapping of

anatase TiO2: (1 Mn) is also performed by EDS area scans. The

maps (Figure S2(b-d) in Supplementary Information) of O, Ti

and Mn are well defined with sharp contrast. The profile of Mn

is close to that of O and Ti, which indicates that Mn and Ti are

distributed uniformly and densely throughout the whole

composite. The Mn, N peaks of anatase TiO2: (0.2 Mn, N -

400 °C – 2 h) is not obvious in EDS due to the low content of

Mn, N and also due to an overlap between N peak and a high O

peak (In addition, due to the similar synthetic process of anatase

TiO2: (0.2 Mn, N - 400 °C – 2 h) and amorphous TiO2: (0.2 Mn,

N - 400 °C – 2 h), EDS results of these two samples are almost

the same.). EDS result of the sample only provides the chemical

composition of sample. Other information can be given by XPS.

The nature of the functional groups and bond vibration

positions of all the samples was further revealed by FTIR and

Raman spectrum, which are sensitive to the local (or short-range)

structure of materials. The FTIR spectra of all the samples

(Figure 4(A)) show broad peaks assigned to hydroxyl group or

water around 3400 cm–1 and 1650 cm–1, and a typical mode of

Ti-O-Ti network at 500 - 800 cm–1. 32 After nitriding treatment,

deformation vibration modes of C-H bonds of acetate ion was

observed at around 1375 cm–1, revealing that the similar surface

groups of anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) and

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) were exposed.

After treatment by acetic acid, the amount of the exposed C–H

of anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) has an obvious

increase. It can be observed in SEM images that highly smooth

surface of low crystallinity TiO2: (0.2 Mn) was retained after

calcinations which can be attributed to strong bonding between

acetate ion and surfactant, during aging. After nitriding treatment,

the large amount of the exposed acetate ion C–H of amorphous

TiO2: (0.2 Mn, N - 400 °C – 2 h) is mostly present. In addition,

the stretching of alkane C–H33 (at around 2850 cm−1) is clearly

observed only on amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)

Figure 4. IR (A), Raman (B), N2 adsorption-desorption isotherms and pore size distribution curve (inset) (C) and high resolution N 1s XPS

spectra ((D), the right shows the area ratio of N species after fitting) of ((a) anatase TiO2: (0.2 Mn), (b) anatase TiO2: (0.2 Mn, N - 400 °C – 2

h), (c) anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) treatment by acetic acid, (d) low crystallinity TiO2: (0.2 Mn) and (e) amorphous TiO2: (0.2 Mn,

N - 400 °C – 2 h)).

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sample that can be ascribed to the presence of some surfactant

after calcination. In Figure 4(B), Raman spectroscopy was used

to detect the possible influence of nitrogen on the geometric

structure of TiO2. Anatase TiO2 gives six active modes, namely,

Eg(1), Eg(2), Eg(3), B1g(1), B1g(2) and A1g at corresponding

frequencies of 144, 197, 639, 399, 519, and 513 cm–1,

respectively.34 Besides these six typical modes (the B1g(2) and

A1g modes usually appear as an overlapped peak), an additional

active mode ranging from 250 cm–1 to 400 cm–1 is observed in

the low crystallinity TiO2: (0.2 Mn) (Figure 4(B)-d). This mode

cannot be ascribed to other phases of TiO2. Furthermore, no such

mode was detected in the common nitrogen-doped anatase TiO2

(Figure 4(B)-b). A possible explanation for this new peak is that

low crystallinity of TiO2: (0.2 Mn) sample with more defects

breaks down the Raman selection rules to generate a new active

mode by lowering the geometric symmetries of TiO2.10 The

broad Raman peak of amorphous TiO2: (0.2 Mn, N - 400 °C – 2

h) can be attributed to amorphous character; it was well in

agreement with the results in XRD patterns. The textural

properties of mesoporous anatase and amorphous TiO2: (0.2 Mn,

N - 400 °C – 2 h) are analysed by N2 adsorption desorption.

After the nitriding treatment, type IV isotherms with a sharp

capillary condensation step at high relative pressures (P/Po ≈ 0.7

- 0.9), and H1 type hysteresis loops are observed for typical

mesoporous anatase TiO2: (0.2 Mn, N - 400 °C – 2 h)

microspheres. This specifies the relatively large pore sizes and

uniform pore size distribution of this sample.35 Mesoporous

anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) sample had a specific

surface area of 112 m2 g–1 and a narrow pore size distribution

centred at 5.0 nm. For the amorphous TiO2: (0.2 Mn, N - 400 °C

– 2 h) microspheres, the surface area is 43.1 m2 g-1 and almost

two - three times lower than that of mesoporous anatase sample.

In addition, there is no obvious porous character as shown in

the SEM image (Figure 3(e)) and the corresponding pore size

distribution curve (Figure 4(C)-e). As a whole, the specific

surface area is not the critical factor for photocatalytic activity.

In order to compare the different existence forms and the states

of doped N in samples by treated in ammonia gas, XPS

technique was conducted. As shown in Figure 4(D), three peaks

were centred at binding energies of 396.07 eV, 399.49 eV and

402.66 eV for anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) sample,

while the binding energies were obtained at 396.74 eV, 399.48

eV and 401.1 eV for amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)

sample, and these are respectively endorsed to N species in

N-Ti-O bonds (substitution of N ion for O ion), interstitial N in

TiO2 and N-H.34–35 According to the XPS results, the total

amount of N doping is 0.32 at.% for anatase TiO2: (0.2 Mn, N -

400 °C – 2 h) sample. The peak-area ratio of substitutial N is

37.74%, which is more than that of interstitial N. And for

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) sample, the amount

of total N doping is 0.54 at.%, and interstitial N with the

peak-area ratio 41% has taken a leading role. We have also

examined the chemical states of Ti, O and Mn in all the samples

before and after nitrogen doping, as shown in Figure S3 in

Supplementary Information. Generally, the peak at around 530

eV is assigned to the Ti-O bonds and the peak at around 458 eV,

which corresponds to the binding energies of Ti 2P3/2 levels for

Ti4+ 36, 38. The binding energies of Ti 2P3/2 and O 1s are shifted

from 458.6 and 529.8 eV to 458.37 and 529.71 eV for anatase

TiO2: (0.2 Mn, N - 400 °C – 2 h) sample, and are shifted from

458.5 and 529.9 eV to 458.4 and 529.7 eV for amorphous TiO2:

(0.2 Mn, N - 400 °C – 2 h) sample, respectively. In addition, the

XPS spectrum shows binding energy of Mn 2p for anatase TiO2:

(0.2 Mn) and anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) samples

all have weak signals. It was mainly due to the low content of

Mn on the surface. The broad peaks were observed between

640.56 eV and 639.89 eV for low crystallinity TiO2: (0.2 Mn)

and amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)) samples,

respectively, and this can be ascribed to Mn2+ of Mn 2p. MnO

has a satellite feature (around 647 eV), which is not present for

either Mn2O3 or MnO2.39 These energy shifts can be explained as

the increased outer electron densities of Ti, O and Mn by N

atoms of a smaller electronegativity (3.04) versus O atoms (3.44).

According to the weak order degree of low crystallinity TiO2:

(0.2 Mn) sample, substitutial N formation is difficult. But it is

easier for nitrogen doping.

In Figure 5(A), compared with the absorption spectrum of

anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) sample, an add-on

shoulder was clearly imposed onto the edge of the absorption

Figure 5. (A)UV-Vis DRS of ((a) anatase TiO2: (0.2 Mn, N - 400 °C – 2 h), (b)

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h), (c) anatase TiO2: (0.2 Mn, N -

500 °C – 2 h) and (d) anatase TiO2: (0.2 Mn, N - 600 °C – 2 h)) and (B) EPR

spectra of (a) anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) and (b) amorphous

TiO2: (0.2 Mn, N - 400 °C – 2 h).

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spectrum for amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h)

sample. The wavelength increased to 510 nm and 520 nm as the

NH3 treatment temperature and duration increased to 500 °C (2

h), and 600 °C (2h) for anatase TiO2: (0.2 Mn, N) samples. The

form of the absorption spectrum of amorphous TiO2: (0.2 Mn, N

- 400 °C – 2 h) and anatase TiO2: (0.2 Mn, N - 500 °C – 2 h)

samples is almost same. The extension of the absorption from

the UV to the visible range and increased absorption intensity

are mainly due to the contributions of both nitrogen atoms and

the oxygen vacancies in the lattice. For the interstitial doping, it

induced the local states near the valence band edge. And the

oxygen vacancies (or F centers) gave rise to the states below the

conduction edge. Excitation from such local states is consistent

with the one on the absorption edge.40, 41 Compared with anatase

TiO2: (0.2 Mn, N - 400 °C – 2 h) sample, the add-on shoulder

onto the edge of the absorption spectrum and increased

absorption intensity of amorphous TiO2: (0.2 Mn, N - 400 °C – 2

h) sample are mainly due to the more interstitial N and oxygen

vacancies due to induced defects. This demonstrates the ease of

N doping for low crystallinity sample. Furthermore, the electron

paramagnetic resonance (EPR) spectrum (Figure 5(B)) recorded

at 104 K, has confirmed the significant presence of oxygen

vacancies (VO) in the amorphous TiO2: (0.2 Mn, N - 400 °C – 2

h). A strong signal at g = 2.003, can be ascribed to the electrons

trapped on oxygen vacancies (VO).42 But the oxygen vacancy

signal is not obvious for anatase TiO2: (0.2 Mn, N - 400 °C – 2 h)

sample, which agrees well with the UV-Vis DRS results in

Figure 5(A).

The photocatalytic activity of the TiO2 Degussa P-25 (P25)

powders, Mn-doped anatase TiO2, N-doped anatase TiO2,

anatase and amorphous TiO2: (0.2 Mn, N - 400 °C- 2 h) samples

and the anatase TiO2: (0.2 Mn, N - 400 °C- 2 h) sample after

surface acetic treatment (Figure 7(A)) was evaluated by

monitoring the decomposition of RhB under visible light

irradiation. The order of photocatalytic degradation rate for TiO2

and anatase series samples is Mn-doped anatase TiO2 ≈ N-doped

anatase TiO2 > anatase TiO2: (0.2 Mn, N - 400 °C- 2 h) > TiO2

Degussa P-25 (P25) powders. P25 TiO2 with a wide band gap

under visible light has no contribution in photocatalytic

performance. Also at low nitriding treatment duration and

temperature, the N doped TiO2 mainly contains interstitial

nitrogen. A bit local states near the valence edge induce by the

interstitial N dopant can capture the electron motivated from

valence band, but can easily return. And the codoped level of

Mn and N can be the recombination centers for electron and hole.

Interestingly, amorphous TiO2: (0.2 Mn, N - 400 °C- 2 h) sample

with specific surface area of 43.1 m2 g–1 shows better adsorption

activity (almost 8-10 times) than both anatase TiO2: (0.2 Mn, N -

400 °C- 2 h) sample and the anatase TiO2: (0.2 Mn, N - 400 °C-

2 h) sample by surface acetic treatment. In addition, the

absorbance of methylene blue (MB) (Figure S4 in

Supplementary Information) decreases 79 % after 1 h

(irradiation time). As shown in Figure 4(A), acetate ion and

alkane C–H bands exposed on the surface of amorphous TiO2:

(0.2 Mn, N - 400 °C-2 h) sample. It is mainly due to nitriding

effect of ammonia gas and strong bonding energy of acetate ion

and surfactant. Equated with this, anatase TiO2: (0.2 Mn, N -

400 °C- 2 h) sample shows less peak intensities about these

vibration positions. After treatment by acetic acid, the peak of

acetate ion enhanced obviously, but the significant adsorption

Figure 6. Water contact angles for (a) anatase TiO2: (0.2 Mn), (b) anatase

TiO2: (0.2 Mn, N- 400 °C – 2 h), (c) anatase TiO2: (0.2 Mn, N- 400 °C – 2 h)

treated by acetic acid and (d) amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h).

Figure 7. (A)Time-dependent photocatalytic degradation of RhB solution

upon exposure to visible light using the obtained the samples ((a) TiO2

Degussa P-25 (P25) powders, (b) Mn-doped TiO2, (c) N-doped TiO2, (d)

anatase TiO2: (0.2 Mn, N - 400 °C – 2 h), (e) anatase TiO2: (0.2 Mn, N -

400 °C – 2 h) treatment by acetic acid and (f) amorphous TiO2: (0.2 Mn, N -

400 °C – 2 h)). (B) Representative variations of the characteristic

absorption of RhB under visible light irradiation by using amorphous TiO2:

(0.2 Mn, N - 400 °C – 2 h) sample (-60 to 0 as absorb time, 0-240 as

photocatalysis time).

42

Research Article Nano Advances

Nano Adv., 2017, 2, 36−44.016, 1, X−X.

doi: 10.22180/na210

activity of anatase TiO2: (0.2 Mn, N - 400 °C- 2 h) sample was

not observed. It is suggested that the alkane C-H may play a key

role in improving the adsorption activity of amorphous TiO2:

(0.2 Mn, N - 400 °C- 2h). In order to test the affinity of samples

to dye water solution, the contact angles were taken. The contact

angles (Figure 6) for the as obtained anatase TiO2: (0.2 Mn) by

calcination treatment were about 23.1°. It increased to 41.5°

under nitriding treatment in ammonia gas at 400 °C for 2 h and it

decreased slightly to 39.9° after being treated by acetic acid, due

to the hydrophilic group - acetic ion. Compared with this, the

existence of the hydrophobic group - alkane (C-H) have induced

hydrophobic surface, hence the contact angle of amorphous TiO2:

(0.2 Mn, N - 400 °C- 2 h) sample reached 55.9°. Moderate

hydrophobicity of the samples would improve suspension and

dispersion of the samples in solutions.43 This resulted in the

interfacial contact and thereby adsorption ability of the samples

enhanced accordingly. In addition, the photocatalytic

degradation efficiency of amorphous TiO2: (0.2 Mn, N - 400 °C-

2 h) sample is much higher than that of the other two samples.

The significant activity is mainly due to (i) the increase in

interfacial area and adsorption ability of the material in solution;

(ii) the extension of the absorption from the UV to the visible

range and increased absorption intensity mainly due to the

contributions of large amounts of interstitial N and oxygen

vacancies, and (iii) easier nitriding degree of low crystallinity

sample to high crystallinity at the same temperature. In addition,

the representative variations in the absorption of RhB (λmax =

554 nm) under visible light irradiation for amorphous TiO2: (0.2

Mn, N - 400 °C- 2 h) can be got in Figure 7(B). The

characteristic absorption of RhB decreases obviously as the

irradiation time increases, the hypsochromic shift of the

absorption maximum was obvious, indicating that

N-deethylation and a dominant cleavage of the whole conjugated

chromophore structure produced at the same time. Prolonging

the irradiation time further, RhB decomposed completely.44

4. Conclusions

Amorphous Mn, N-codoped TiO2 has been synthesized and

analyzed for photocatalytic performance in comparison to the

crystalline Mn, N-codoped TiO2. The precursor of amorphous

TiO2: (0.2 Mn, N - 400 °C – 2 h) was synthesized by aging for 2

days after solvothermal synthesis route. Long aging time has

relatively increased the binding ability of surfactant and metal

salts. Compared with large BET surface area (112m2 g–1)

mesoporous nanocrystalline anatase TiO2: (0.2 Mn, N - 400 °C –

2 h), smooth surface and solid structure of amorphous TiO2: (0.2

Mn, N - 400 °C – 2 h) was maintained after calcination although

a decrease in BET surface area (43.1 m2 g–1) has been observed.

After nitriding treatment, hydrophobic group (alkane C–H) was

exposed, and its suspending and adsorption ability in dye

solution were found to be increased. The N doped content of

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) is found almost two

times higher than that of anatase TiO2: (0.2 Mn, N - 400 °C – 2

h), and interstitial N with the peak-area ratio 41% has taken a

leading role. Further strong EPR signal at g = 2.003 for

amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) induced by trapped

electrons confirmed the presence of oxygen vacancies (VO)

intuitively. Similarly, the more interstitial N and oxygen

vacancies of amorphous TiO2: (0.2 Mn, N - 400 °C – 2 h) than

those of anatase TiO2: (0.2 Mn, N - 400 °C – 2 h) result in shift

of the absorption edge to visible light region, which could

advance the extension of the photocatalyst for visible light

absorption. Totally, compared with their crystalline counterparts,

amorphous TiO2: (0.2 Mn, N - 400 °C - 2h) microspheres with

small BET surface area, high adsorption ability (taken by alkane

C-H), easily N doped property have been observed as the

superior materials for decomposing organic pollution in the

future.

Acknowledgments

This work is supported by the National Natural Science

Foundation of China through Grant No. 21471147 and the

Liaoning Provincial Natural Science Foundation through Grant

No. 2014020087. M. Yang would like to thank for the National

"Thousand Youth Talents" program of China.

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How to cite this article: M. Zou, L. Feng, E. Pervaiz, A. S.

Ganeshraja, T. Gao, H. Jiang and M. Yang, Nano Adv., 2017, 2,

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