This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 6293–6295 6293

Cite this: Chem. Commun., 2011, 47, 6293–6295

Nitrogen doping in ion-exchangeable layered tantalate towards

visible-light induced water oxidationw

Xu Zong,aChenghua Sun,

abZhigang Chen,

cAniruddh Mukherji,

aHao Wu,

aJin Zou,

cd

Sean C. Smith,bGao Qing Lu*

aand Lianzhou Wang*

a

Received 8th December 2010, Accepted 16th February 2011

DOI: 10.1039/c0cc05440b

An ion-exchangeable layered photocatalyst CsCa2Ta3O10 was

doped with nitrogen. The as-prepared photocatalyst (N-doped

CsCa2Ta3O10) is stable and demonstrates high performance for

catalyzing water oxidation under visible light irradiation.

The solar-driven splitting of water to produce H2 and O2 with

semiconductor photocatalysts represents an attractive pathway

towards solving important energy and environmental problems.1

As half reactions of water splitting, water oxidation is particularly

demanding compared with water reduction because it requires the

removal of four protons and four electrons and the formation of

an oxygen–oxygen double bond.2 Thus, the development of

photocatalysts capable of oxidizing water to O2 is of critical

importance for realizing overall water splitting.3

Ion-exchangeable oxide semiconductors containing Ti4+,

Nb5+ or Ta5+ cations, typically in layered structures, are of

great interest in the scheme of solar energy utilization.4 These

materials are supposed to provide spatially separated reduction

and oxidation reaction sites, which thereby renders photo-

catalytic reactions more favorable.5 However, as is commonly

the case, these oxide semiconductors with wide band gaps only

demonstrate photocatalytic activity under UV irradiation, which

impairs the utilization of visible light that accounts for 43% of

the solar spectrum. To overcome this limitation, a range of

strategies have been developed to enhance the visible light

response of pristine metal oxide photocatalysts. Amongst these

strategies, partial or total nitrogen doping has been demonstrated

to be effective through mixing of the dopant states with the upper

valence band states of the bulk material.6 Recently, homo-

geneous nitrogen doping in layered titanates was realized, which

leads to high photocatalytic activity for the degradation of

pollutants under visible light.4d Because the electronic structures

of these oxide semiconductors are predominantly determined by

their constituent cations, it is expected that the extension

of this strategy from Ti4+ to Ta5+ and Nb5+-based layered

oxide semiconductors will lead to interesting photocatalytic

properties.1b Especially, the water oxidation properties may be

drastically modified because Ta5+ and Nb5+-based (oxy)nitrides

have previously demonstrated high activity for water oxidation

reactions.6b

With this concept in mind, various Ta5+ and Nb5+-based ion-

exchangeable layered oxides such as RbTaO3, CsLa2Ti2TaO10,

and CsCa2Ta3O10, were first synthesized and then calcined from

973 to 1123 K under an ammonia gas flow.7 The as-obtained

nitrogen-doped layered materials can be briefly classified into

two types. The first type consists of materials (such as nitrogen-

doped CsLa2Ti2TaO10) with white, grey or black color,

indicating an undesirable doping level in the materials. This

cannot be avoided when changing the preparation conditions.

The second type includes materials with intriguing colours

ranging from bright yellow to red, indicating their possible

functionality in the visible light region. However, it was found

that after contacting with air or water, the colour of most of these

materials (such as nitrogen-doped RbTaO3) faded and finally

disappeared. This may be ascribed to the susceptibility of these

materials to unwanted reactions with air or water.8 Therefore,

the synthesis of a stable nitrogen-doped layered oxide capable of

oxidizing water under visible light remains a challenge.

Herein we present a new type of stable Ta5+-based

ion-exchangeable layered photocatalyst doped with nitrogen

(N-doped CsCa2Ta3O10), which demonstrated high performance

for catalyzing water oxidation under visible light. To our

knowledge, no prior study focused on the utilization of nitrogen-

doped ion-exchangeable layered photocatalysts for water oxida-

tion under visible light.

N-doped CsCa2Ta3O10 catalysts with different nitrogen

dopants were prepared by heating a CsCa2Ta3O10 precursor at

temperatures from 973 to 1073 K in an ammonia flow (see

detailed preparation procedure in ESIw). The N dopants in

CsCa2Ta3O10 samples prepared at 973, 1023, and 1073 K were

estimated to be 0.43, 0.90, and 1.80 at%, respectively. The color

of the resultant catalysts ranges from pale yellow to red with

increasing heating temperatures, which corresponds well with the

a ARC Centre of Excellence for Functional Nanomaterials,School of Chemical Engineering and AIBN,The University of Queensland, QLD 4072, Australia.E-mail: l.wang@uq.edu.au, maxlu@uq.edu.au;Fax: +61 7 33654199; Tel: +61 7 33654218

bCentre for Computational Molecular Science, AIBN,The University of Queensland, QLD 4072, Australia

cMaterials Engineering, The University of Queensland, QLD 4072,Australia

dCentre for Microscopy and Microanalysis,The University of Queensland, QLD 4072, Australia

w Electronic supplementary information (ESI) available: Details ofexperimental procedures, characterization (including theoreticalcalculations, XRD and XPS), photocatalytic activity tests, andsupporting images. See DOI: 10.1039/c0cc05440b

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6294 Chem. Commun., 2011, 47, 6293–6295 This journal is c The Royal Society of Chemistry 2011

diffuse reflectance UV-Vis spectra of these catalysts shown in

Fig. 1. Based upon the absorption edge in Fig. 1, the band gap of

pristine CsCa2Ta3O10 was calculated to be 3.8 eV. After nitrogen

doping, the absorption of the as-obtained catalysts gradually red

shifted with increasing heating temperature. The band gap

of N-doped CsCa2Ta3O10 catalyst prepared at 1073 K was

estimated to be ca. 2 eV, indicating the drastic influence of

nitrogen doping on the electronic structures of the pristine oxide.

Noting that the origin of long wavelength shift is still unclear,

while the typical layered structure having lamellar galleries may

be one possible reason for facilitating this drastic change upon

nitrogen doping.

This marked influence of nitrogen doping on the electronic

structure of CsCa2Ta3O10 can be clearly demonstrated by

density functional theory calculations (see computational

details in ESIw).9 As shown in Fig. 2, the valance band (VB)

and conduction band (CB) of an undoped tantalate

are dominated by O2p and Ta5d (Fig. S1 and S2, ESIw),respectively. After the substitutional nitrogen doping, the VB

was spanned by O2p and N2p, which leads to a substantial

narrowing of the band gap from 2.07 to 1.55 eV for undoped

and nitrogen-doped samples, respectively. Therefore, the

contribution of nitrogen to the top of the VB is expected to

play an important role in extending the absorption of the

nitrogen doped samples into the visible region.

X-Ray diffraction characterizations were then used to

investigate the influence of nitrogen-doping on the crystal

structures of the as-prepared photocatalysts. It can be seen

clearly that the diffraction peak intensities of the as-obtained

catalysts decrease slightly with increasing nitridation temperature

(Fig. S3, ESIw). However, the catalysts preserve the original

crystal structure of CsCa2Ta3O10, indicating the minimal

influence of low nitrogen doping on the crystal structure of

CsCa2Ta3O10.1g Therefore, nitrogen doping can significantly

modify the electronic structure of CsCa2Ta3O10 without

drastically changing its crystal structure. Moreover, the

as-prepared N-doped CsCa2Ta3O10 photocatalysts are very stable

in the presence of air and water, which will facilitate further

intercalation and exfoliation processes. In our initial work, we

found that the Cs ions in the N-doped CsCa2Ta3O10 can be

successfully exchanged by protons (not shown here). Further

experiments on the exfoliation of N-doped CsCa2Ta3O10 into

visible light responsive nanosheets are under way.

The presence of nitrogen in the N-doped CsCa2Ta3O10

samples was also ascertained by XPS and TEM character-

izations. As shown in Fig. 3, no signal for nitrogen species was

observed for CsCa2Ta3O10. After calcination in NH3 gas flow

at high temperatures, a peak centered at 395.8 eV appeared

and its intensity increased with the calcination temperature, a

clear evidence of enhanced nitrogen doping in the CsCa2Ta3O10

catalysts at elevated temperatures. The binding energies of Ta 4f

and O 1s were shifted to lower regions while those of Cs 3d and

Ca 2p remain intact (Fig. S4, ESIw). Moreover, the Ta 4f peaks

after nitrogen doping are broadened probably due to the

formation of Ta–N bonds, which is very similar to the case of

Cs0.68Ti1.83O4 doped with nitrogen.4d Fig. 4 presents the TEM

and elemental mapping images of CsCa2Ta3O10 and N-doped

CsCa2Ta3O10. It is very clear that N was presented in the

N-doped CsCa2Ta3O10 particles with very strong contrast

(Fig. 4c), whereas only weak N signal was detected in pristine

CsCa2Ta3O10 particles (Fig. 4f).

Photocatalytic reactions on the as-prepared photocatalysts

were then carried out under visible light irradiation (l>400 nm).

Fig. 5 describes the rate of O2 evolution on the N-doped

CsCa2Ta3O10 photocatalysts prepared at different temperatures

together with that of CsCa2Ta3O10 for a comparison. No O2

evolution was observed for CsCa2Ta3O10 under visible light.

Fig. 1 Diffuse reflectance spectra of (a) CsCa2Ta3O10 and (b)–(d)

N-doped CsCa2Ta3O10 samples prepared by calcining CsCa2Ta3O10 in

an ammonia gas flow at 973, 1023, and 1073 K, respectively.

Fig. 2 Total density of states of (a) undoped CsCa2Ta3O10, and

(b) N-doped CsCa2Ta3O10 photocatalysts. The structure models of

CsCa2Ta3O10 and N-doped CsCa2Ta3O10 for theoretical calculations

are shown in the ESI.w

Fig. 3 High-resolution XPS spectra of N 1s and Ta 4f measured on

(a) CsCa2Ta3O10 and (b)–(d) N-doped CsCa2Ta3O10 samples prepared

by calcining CsCa2Ta3O10 in an ammonia gas flow at 973, 1023, and

1073 K, respectively.

Fig. 4 TEM images of (a, d) general morphologies, (b, e) O maps,

and (c, f) N maps of (a–c) N-doped CsCa2Ta3O10 and (d–f) pristine

CsCa2Ta3O10, respectively.

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 6293–6295 6295

It is reasonable considering its wide band gap requiring an

excitation energy of 3.8 eV. After nitrogen doping, the

as-obtained photocatalysts demonstrated high efficiency for

O2 evolution under visible light. The amount of O2 evolved

was about 5.7, 33 and 72 mmol for the samples prepared at 973,

1023, and 1073 K in the first hour, respectively. This trend

corresponds well with the absorption of the photocatalysts in

the visible light region. However, the O2 evolution on N-doped

CsCa2Ta3O10 catalysts prepared by successive calcinations at

1023 and 1073 K was about 44 mmol even though more

nitrogen is anticipated to be doped into CsCa2Ta3O10. Because

different factors such as calcination temperature and time, and

flow rate of ammonia can influence the performance of

as-prepared photocatalysts,6e it is expected that the efficiency

of N-doped CsCa2Ta3O10 photocatalysts can be further

enhanced by modifying the preparation parameters. In our

work, we also tested the N-doped layered Cs0.68Ti1.83O4�xNx

photocatalyst for O2 evolution. Although Cs0.68Ti1.83O4�xNx

demonstrated high efficiency for the degradation of dyes under

visible light, no O2 evolution was observed during the reaction.

This suggests more favorable electronic or surface structures

of N-doped CsCa2Ta3O10 for the oxidation of water compared

with those of Cs0.68Ti1.83O4�xNx photocatalyst.

It is worth noting that the N-doped CsCa2Ta3O10 catalysts

demonstrated high efficiency for the reduction of Ag+ to Ag

and the subsequent oxidation of water to O2 because the

yellow–red colour of the photocatalysts in the reaction

solution turned black in the initial five minutes and most of

the O2 was evolved in the first half of hour (Fig. S5, ESIw).Further reaction did not produce substantial O2 because the

continuous deposition of Ag on the N-doped CsCa2Ta3O10

photocatalyst will block the absorption of light and the active

sites (Fig. S6, ESIw). This has been pointed out to be the major

reason for the decreased activity for O2 evolution in other

heterogeneous systems.1b As far as we know the present work

is the first report of using nitrogen-doped ion-exchangeable

tantalate for photocatalytic O2 evolution under visible light

and the efficiency is much higher than that of transition metal

doped ion-exchangeable oxide photocatalysts.10 N-doped

CsCa2Ta3O10 was also tested for the photocatalytic H2 evolution,

which shows no activity in the presence of methanol as the

sacrificial reagent. However, considering the unique crystal

structure and the high efficiency for water oxidation, N-doped

CsCa2Ta3O10 will be a promising O2-evolution photocatalyst for

the establishment of an efficient Z-scheme system for overall

water splitting.

In summary, we present a new type of stable layered

photocatalyst (N-doped CsCa2Ta3O10) capable of producing

O2 from water splitting under visible light. It is anticipated

that the galleries of ion-exchangeable layered photocatalysts

with intriguing visible light photocatalytic properties may be

further enriched by combining different doping anions and the

constituent cations (Ti4+, Nb5+ or Ta5+). Further studies on

the exploration of anion-doped ion-exchangeable layered

photocatalysts with the capability of producing H2 from water

splitting are under way.

This project was supported by Australian Research Council

(through its Centres of Excellence grant and DP programs)

and Queensland State Government (NIRAP).

Notes and references

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Fig. 5 Rates of photocatalytic O2 evolution on (A) CsCa2Ta3O10 and

N-doped CsCa2Ta3O10 photocatalysts prepared by calcining

CsCa2Ta3O10 in an ammonia gas flow at (B) 973 K (15 h), (C) 1023 K

(15 h), (D) 1073 K (15 h), and (E) 1023 K (15 h) + 1073 K (15 h).

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