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Photocatalytic hydrogen production with visiblelight over Mo and Cr-doped BiNb(Ta)O4

Cristiane G. Almeida a, Rafael B. Araujo c, Rafael G. Yoshimura a,Artur J.S. Mascarenhas a,b, Antonio Ferreira da Silva c,Carlos Moyses Araujo d, Luciana A. Silva a,b,*a Instituto de Quımica, Universidade Federal da Bahia, Campus de Ondina, CEP 40170-290 Salvador, BA, Brazilb Instituto Nacional de Ciencia e Tecnologia de Energia e Ambiente INCT-E&A, Universidade Federal da Bahia,

Campus de Ondina, CEP 40170-290 Salvador, BA, Brazilc Instituto de Fısica, Universidade Federal da Bahia, Campus de Ondina, CEP 40170-290 Salvador, BA, BrazildMaterials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala,

Sweden

a r t i c l e i n f o

Article history:

Received 31 July 2013

Received in revised form

28 October 2013

Accepted 7 November 2013

Available online 8 December 2013

Keywords:

Bismuth niobate

Bismuth tantalate

Chromium

Molybdenum

Hydrogen

Photocatalysis

* Corresponding author. Instituto Nacional dCampus de Ondina, CEP 40170-290 Salvador

E-mail addresses: las@ufba.br, luciana@c0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.11.0

a b s t r a c t

In the present work, we prepared pure and Cr(III) and Mo(V)-doped BiNbO4 and BiTaO4

by the citrate method. Pure BiNbO4 and BiTaO4 were obtained in triclinic phase at 600 �C

and 800 �C, respectively. The metal doping influenced strongly the crystal structure as

well as the photocatalytic activity of the oxides. The XRD data could prove that the

Mo(V) doping induces the orthorhombic phase, while the Cr(III) doping favors the

triclinic phase for both oxides. Metal doping also modified the photosensitivity of the

oxides, extending the absorption toward the visible light region. The photocatalytic

activity in water splitting under visible light irradiation was evaluated by monitoring the

H2, CO2 and CO evolution. The results showed that Cr(III)-doped BiTaO4 and BiNbO4, in

general, are more selective for hydrogen production, while Mo(V)-doped materials are

more selective for CO2 generation. Comparing the photocatalytic activity of BiTaO4 and

BiNbO4, the former shows higher activity for hydrogen production as well as for CO2

generation, specially when the concentration was 2% in Cr(III) and Mo(V), respectively.

Those results are in agreement with the computational study to access the effect of

doping on the electronic structure. For Mo(V)-doped materials a negligible change of

conduction band minimum potential was found, indicating that there might be no

improvement on the reduction power of the material following the substitutional

doping. In Cr(III)-doped BiNbO4 there is a slight shift of the CBM potential increasing a

little bit the reduction power. However, the effect is much stronger in the Cr(III)-doped

BiTaO4.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

e Ciencia e Tecnologia de Energia e Ambiente INCT-E&A, Universidade Federal da Bahia,, BA, Brazil. Tel.: þ55 71 3283 6881.altech.edu (L.A. Silva).2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.19

Fig. 1 e XRD patterns of the powders calcined at 600 �C,700 �C and 800 �C and ICSD patterns of a and b-BiNbO4.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 7 1221

1. Introduction

Semiconductor-mediated photocatalytic processes have been

widely investigated in recent years as potentially efficient,

economic and environmentally friendly methods for

hydrogen production. Since 1972, after the pioneering work of

Honda and Fujishima [1], who discovered that water can be

photo-electrochemically decomposed into hydrogen and ox-

ygen using a semiconductor (TiO2) electrode under UV irra-

diation, a large number of metal oxides has been reported to

have photocatalytic activity for water splitting. Nowadays,

research efforts have attempted mainly to extend the photo-

sensitivity of semiconductor photocatalysts toward the visible

light region in order to fully harvest solar energy, since it ac-

counts for approximately 43% of the incoming solar energy

spectrum against only about 3% of UV light.

Several strategies for the development of visible-light-

driven photocatalysts have been tested, such as dye sensiti-

zation [2], band gap modification by nonmetals doping [3e5]

and transition metal doping and co-doping [3,6,7]. In gen-

eral, doping of a foreign element into UV-active photo-

catalysts withwide band gaps, in order to introduce a donor or

an acceptor level in the forbidden band, is an effective way to

design the visible light-driven photocatalyst and has been

intensively practiced [7].

It is known that the relative positions of the valence band

and conduction band are critical variable in determining the

feasibility of semiconductor for hydrogen production. In the

last decades a lot of attention was devoted to study bismuth

niobates and tantalates. These materials are envisaged as

interesting photocatalysts for eliminating organic pollutants

from domestic or industrial effluents [4e6,8e10], as well as for

producing H2 by the photocatalytic decomposition of water

under UV light irradiation [7,11]. Recently, we prepared

nanosized triclinic BiTaO4 (known as type b polymorph, stable

at high temperatures) by polymeric precursor method with

BET surface area of 3.31 m2 g�1 [12,13]. It is seven times higher

than the surface area of BiTaO4 obtained from solid-state re-

action. The band gap have also been estimated to be 2.65 and

2.45 eV using photoacoustic spectroscopic (PAS) method and

density functional theory (DFT) calculations, respectively, as

well as the reduction and oxidation levels positions with

respect to the vacuum level were identified. Results showed

that the energy level for water reduction (Hþ/H2) exists 0.5 eV

lower than the conduction band (CB), thus leading to inject

electrons into the solution phase for hydrogen production.

Oxidation level (O2/H2O) exists about 0.7 eV higher the valence

band (VB), so the holes can spontaneously transfer to the

oxidizing potential of the water splitting with oxidation power

0.7 eV, which confirms that BiTaO4 can be a good photo-

catalyst for hydrogen production. However, despite the nar-

row band gap energy (Eg < 3 eV) and the suitable VB and CB

positions, no photocatalytic activity is observed above 400 nm,

probably due to the too low absorption intensity of BiTaO4 in

the visible region.

Zou et al. [11] observed hydrogen evolution from a meth-

anol aqueous solution and pure water with BiTa1�xNbxO4

under ultraviolet irradiation and, in general, the orthorhombic

(a phase) samples exhibited much higher activity than that of

triclinic (b phase) compounds. Zang et al. [7] showed that b-

BiTaO4 doped with copper, prepared by solid-state reaction

process, is an effective photocatalyst for overall water split-

ting and DFT calculations revealed that the Cu 3d states act as

donor level above valence band, which contributes to the

small band gap of BiTaO4 doped with copper. In the present

study, we evaluated the photocatalytic activity in water

splitting of Mo and Cr-doped BiNbO4 and BiTaO4 under visible

light irradiation.

2. Experimental section

2.1. Reagents and materials

Pure b-BiTaO4 was prepared by the citrate method according

to themethod described elsewhere [12]. In a typical synthesis,

bismuth citrate and tantalum pentachloride (TaCl5) are used

as starting materials and citric acid and ethylene glycol as

chelating agent and reaction medium, respectively. Pure b-

BiNbO4 was also prepared by citrate method following the

same procedure for b-BiTaO4. In this case ammonium nioba-

te(V) oxalate hydrate was used as niobium precursor and the

polymeric precursor was fractionated and subjected to calci-

nation at different temperatures (600, 700 and 800 �C) for 3 h in

order to investigate the ideal calcination temperature. Metal

dopingwas carried out by introducing appropriate amounts of

ammonium molybdate tetrahydrate or chromium(III) nitrate

nonahydrate in the start suspension, resulting in concentra-

tions from 1% to 3% (mol/mol) Mo(V) and Cr(III).

2.2. Characterizations

The powders were characterized by X-ray diffraction (Shi-

madzu XRD6000), using CuKa, Ni-filtered radiation, and

scanning rate of 2� 2q min�1, in a 2q range of 5e80�, at 35 kV

and 15 mA. Diffuse reflectance spectra were recorded on a

spectrometer Thermo Scientific Evolution 600 UVevis by

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 71222

using a praying mantis accessory. SEM images were taken in

JSM-6610LV scanning electron microscopy (JEOL) operated at

20 kV after gold metalization.

2.3. Photocatalytic experiments

A high-pressure HgeXe arc lamp, with power fixed at 500 W,

was used as the light source for the photocatalytic reactions.

The collimated light beam was passed through an IR filter

and a 418 nm cutoff filter before reaching the photocatalytic

cell, whichwas air cooled tomaintain a constant temperature.

Before each experiment, the photocatalytic cell was purged

with argon for 30 min to eliminate O2. H2, CO2 and CO gases

evolution was measured by gas chromatography (Shimadzu

GC2014) operating with thermal conductivity detection

(TCD) for hydrogen detection and flame ionization detector

(FID) with methanator for CO2 and CO detection. Because He

and H2 have similar conductivity values, argon was used as a

carrier gas.

In a typical photolysis experiment, 120 mg of the target

material was dispersed in an aqueous solution (total

volume ¼ 60 mL) containing 30% isopropanol and 40 mL of 8%

H2PtCl6 (w0.3 wt. % Pt), with pH adjusted to pH 9.5 by the

Fig. 2 e XRD patterns of pure and doped BiNbO4 and BiTaO

addition of KOH solution. Metallic platinum was deposited in

situ on the photocatalyst surface by the photodecomposition

of [PtCl6]2�. The photocatalytic cell was equipped with argon

gas inlet/outlet tubes, which serve to collect and transfer

gaseous products to the analytical system. An aliquot of gas

phase was injected after 3 h of reaction. In order to ensure the

accuracy of determinations, a standardmixture containing 5%

H2, 2%CO2 and 500 ppmCOdiluted in argonwas injected in GC

system before each experiment; besides, the experiment for

each photocatalyst was performed at least twice to observe

the repeatability.

2.4. Computational details

The calculations were carried out within the framework of

generalized gradient approximation (GGA) to density func-

tional theory (DFT) [16,17] and using projector augmented

plane wave (PAW) method [18], as implemented in VASP code

[19]. We used a cutoff energy of 500 eV for the plane-wave

basis. In all calculations, self-consistency was achieved with

a tolerance in the total energy of 0.1 meV. The DOS were

calculated by means of the modified tetrahedron method of

Blochl et al. [20].

4: (a) and (b) Cr(III) doping and (c) and (d) Mo(V) doping.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 7 1223

In order to model the substitutional cationic doping we

have built up a 3 � 3 � 2 supercell consisting of 288 atoms for

both triclinic and orthorhombic phases. According to our

experimental findings the doping with Cr always favors the

triclinic phase while Mo favors the orthorhombic phase. Since

our aimwith this computational study is to access the effect of

doping on the electronic structure, and correlate such effect

with the photocatalytic activity, we restricted ourselves to

model the doping on those corresponding phases, i.e., Cr in the

triclinic phase and Mo in the orthorhombic phase. In both

cases the impurity atoms were substituted on the cation

sublattice. We have worked then with the following super-

cells: Bi48Ta47MoO192 (orthorhombic), Bi48Ta47CrO192 (triclinic),

Bi48Nb47MoO192 (ortohombic), Bi48Nb47CrO192 (triclinic). As can

be seen we are modeling here 2% of doping. In order to

compare the density of state of the doped and pristine com-

poundswe have aligned the semi-core states of a oxygen atom

place far from the impurity site. This approach takes care of

Fig. 3 e SEM micrographs of pure and Mo

the arbitrariness of the zero energy in the solid-state calcu-

lation. It should also be pointed out that we have performed

spin-polarized calculations.

3. Results and discussion

Pure BiTaO4 was synthesized according to the procedure

described previously by our group [12]. BiNbO4 was also pre-

pared following the same procedure; however, in this case, the

calcination temperature was investigated. The calcined ma-

terials were submitted to XRD analysis and compared with

PDF patterns of a-BiNbO4 and b-BiNbO4 in crystallographic

databases.

From XRD data showed in Fig. 1 it is possible to conclude

that pure b-BiNbO4 (triclinic phase) was formed at 600 �Cinstead of 800 �C as observed for b-BiTaO4 prepared by the

same method [12]. Increasing the calcination temperature up

- and Cr-doped BiNbO4 and BiTaO4.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 71224

to 700 �C, the material keeps the same phase, however, when

the temperature is increased up to 800 �C, it is possible to note

some a phase contamination. Therefore, in this work, for pure

and doped BiTaO4 preparation the polymeric precursors were

calcined at 800 �C for 3 h, while for pure and doped BiNbO4 the

polymeric precursors were calcined at 600 �C. Fig. 2 shows the

XRD patterns of Cr(III) (Fig. 2(a) and (b)) andMo(V) (Fig. 2(b) and

(c)) doped materials compared with a and b phase patterns.

Fig. 2(a) and (b) show the XRD patterns of pure and Cr(III)-

doped BiNbO4 and BiTaO4, respectively. The XRD analysis

reveals that at lower concentrations of chromium the triclinic

phase (b phase) is predominant in both cases. However, as the

concentration of chromium increases the contamination

with orthorhombic phase (a phase) also increases. For Cr(III)-

doped BiNbO4, it can be evidenced through the increase in

intensity of the peaks 30.5� and 31.48� (2q), that are charac-

teristic of a phase. The increase in Cr(III) concentration

also increases the a phase contamination for Cr(III)-doped

BiTaO4. Such contamination is also evident in XRD patterns

in Fig. 2(b), where one can observe the increase of the relative

intensity of the peak 25.1�, typical of the a phase, compared to

the peaks 23.44� and 23.9� (2q), characteristic of the b phase. In

any of the caseswe observe chromium segregated phase, such

as Cr2O3, CrO2 or CrO3.

Fig. 4 e Diffuse reflectance spectra (DRS) of pure

Fig. 2(c) and (d) show the XRD patterns of pure and

Mo(V)-doped BiNbO4 and BiTaO4, respectively. Unlike Cr(III)-

doped BiNbO4 and BiTaO4, the XRD patterns for Mo(V)-

doped BiNbO4 and BiTaO4 present the orthorhombic phase

as predominant phase in both cases. At lower concentra-

tions of Mo(V), 1% and 2%, it is possible to detect the

presence of b phase, which disappears when the concen-

tration is increased to 3%. In both cases the pure ortho-

rhombic phase is obtained, without any molybdenum

segregated phase. Therefore, the Mo(V) doping induces the

orthorhombic phase while the Cr(III) doping favors the

triclinic phase.

The absence of chromium or molybdenum species in

segregated phases, associated with the large similarity in

the XRD patterns among pure and doped oxides, suggest

an isomorphic replacement of metal doping without

structure distortion. Since the ionic radii of Cr(III) and

Mo(V), 75.5 and 75 pm, respectively, are quite similar to

Nb(V) and Ta(V), both 78 pm, in an octahedral environ-

mental, and are too different of Bi(III), 117 pm, the

isomorphic substitution takes place in the octahedral sites

of Nb(V) and Ta(V).

Themorphologies of the pure andmetal doped oxideswere

depicted by SEM images. As shown in Fig. 3 themorphology of

and Mo and Cr-doped BiNbO4 and BiTaO4.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 7 1225

the powders was not uniform, nevertheless, the materials

present a spongeous appearance, suggesting that the ag-

glomerations could be formed by nanosized particles. It can be

seenmore clearly in the SEM images of 2%MoeBiTaO4 and 2%

CreBiTaO4.

The diffuse reflectance spectra of the samples indicate the

extending photosensitivity of doped semiconductor photo-

catalysts toward the visible light region (Fig. 4). In general, as

the doping concentration increases the absorbance toward

the visible light region also increases.

The hydrogen production was measured from an aqueous

solution containing isopropanol (30%) as sacrificial reagent

under visible light irradiation (l � 418 nm) for each target

photocatalyst. The results demonstrate that pure b-BiNbO4

and b-BiTaO4 are inactive under visible light irradiation,

however, metal doped oxide photocatalysts with different

concentrations of Cr(III) and Mo(V) show some activity with

respect to hydrogen production, especially those doped with

chromium (Fig. 5).

Besides hydrogen, CO2 and CO concentrations were also

evaluated in all experiments. Fig. 5 shows that Cr(III)-doped

BiTaO4 and BiNbO4, in general, are more selective for

hydrogen production, while Mo(V)-doped BiTaO4 and BiNbO4

photocatalysts are more selective for CO2 generation, indi-

cating that the former aremore efficient forwater reduction to

hydrogen and the last ones are more efficient for minerali-

zation of organic compounds. For the photocatalysts more

active with respect to hydrogen production, the XRD patterns

revealed that there is a predominance of triclinic phase

induced by chromium doping.

From the comparison of the photocatalytic activity of

BiTaO4 and BiNbO4, the former shows higher activity as for

hydrogen production as well for CO2 generation, when the

concentration was 2% in Cr(III) and Mo(V), respectively.

In Fig. 6(a) and (b), we present the density of states of the

Mo-doped BiTaO4 and BiNbO4, respectively. As can be

observed, the substitution of Mo on the cation-sublattice

leads to the formation of impurity states near the conduc-

tion band minimum (CBM) of the host material. These states

are not populated and result from the charge transfer be-

tween Mo and the coordinating oxygen atoms, i.e., the

oxidation of Mo. In fact, this impurity bands are mainly

composed of Mo d-states. An important effect for the

Fig. 5 e Hydrogen, CO and CO2 evolution over Cr(III) and

Mo(V)-doped BiNbO4 and BiTaO4 photocatalysts

(l ‡ 418 nm).

application on photocatalysis is the doping induced modi-

fication of the conduction band minimum potential, which

is connected to the materials reduction power. We have

found a negligible change of such potential indicating that

there might be no improvement on the reduction power of

the material following the substitutional doping. Actually,

the localized state formed near the CBM may act as trapping

states worsening the charge transfer mechanism during the

chemical reaction.

In Fig. 7(a) and (b), we show the density of states of the

Cr-doped BiTaO4 and BiNbO4, respectively. The character-

istics are quite different from the one obtained for Mo-

doped systems. In BiNbO4 the impurity bands merge with

the valence and conduction bands of the host material and

there is a slight shift of the CBM potential increasing a little

bit the reduction power. However, the effect is much

stronger in the BiTaO4. In this system, the CBM potential is

shifted by about 143 meV, which can be quite effective for

improving the reduction power of the material. Besides that,

the impurity bands are placed deeper in the conduction

band what may diminish the trapping mechanism of the

excited electrons. These results are consistent with our

experimental observations.

Fig. 6 e Density of states of the Mo-doped BiTaO4 (a) and

BiNbO4 (b).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 2 2 0e1 2 2 71226

Considering both reactions, water reduction to produce

hydrogen and isopropanol oxidation to yield CO2, the overall

reaction can be described as follows:

C3H7OH þ 5H2O / 9H2 þ 3CO2 (1)

The theoretical stoichiometry ratio H2/CO2 for Equation (1)

is three. Among photocatalysts, only 1% CreBiNbO4 and 1%

CreBiTaO4 have presented ratio near the theoretical value.

However, for the best photocatalyst in relation to hydrogen

production, 2% CreBiTaO4, this ratio is 35, which means iso-

propanol is not being totallymineralized in the evaluated time

of reaction. Although, the theoretical calculation results

confirm an improvement in the reduction power of the 2%

CreBiTaO4, as a consequence of CBM potential shift, the

experimental results suggest that VBM potential is inefficient

to oxidize isopropanol completely under our reaction condi-

tions, suggesting that the reaction can take place via hydroxyl

radical. The photocatalytic dehydrogenation of aliphatic al-

cohols, such as methanol and ethanol, is mediated by hy-

droxyl radical via hydrogen abstraction of alpha carbon

[14,15]. In the case of the present work, the hydroxyl radical,

Fig. 7 e Density of states of the Cr-doped BiTaO4 (a) and

BiNbO4 (b).

formed by reaction represented in Equation (3), reacts with

isopropanol giving rise the hydroxyisopropyl radical (4). Then,

the hydroxyisopropyl radical reacts with photogenerated

holes in the valence band of the photoexcited semiconductor,

probably, yielding propanone (5) as the mean product. A pro-

posed mechanism compatible with this discussion is pre-

sented below:

H2O þ e� / 1/2H2 þ OH� (at high pH) (2)

OH� þ hþbv/

�OH (3)

CH3CHOHCH3 þ �OH/CH3C�OHCH3 þH2O (4)

CH3C�OHCH3 þ hþ

bv/CH3COCH3 þHþ (5)

Hþ þ e�bc/1=2H2 (6)

4. Conclusions

The citrate polymeric precursor method has proven efficient

in the preparation of pure b-BiNbO4 at mild temperatures as

we could also verify in the preparation of b-BiTaO4 in a

previous work. However, the calcination temperature for

the former was lower than for the last, 600 �C and 800 �C,respectively. The metal doping strongly influences the

crystal structures of the oxides, while Cr(III) doping favors

the triclinic structure, Mo(V) doping induces the ortho-

rhombic structure. In both cases we observe an iso-

morphical replacement of Nb(V) and Ta(V) by metal doping

without structure distortion. The photocatalytic activity

evaluation showed the Cr(III)-doped oxides are more effi-

cient for water reduction to hydrogen, while Mo(V)-doped

oxides are more efficient for mineralization of organic

compounds. The improvement in the reduction power of

the Cr(III)-doped BiTaO4 is justified by the CBM potential

shift with the substitution of Cr on the cation-sublattice,

confirmed by theoretical calculations.

Acknowledgments

The authors acknowledge the Brazilian research funding

agencies Conselho Nacional de Desenvolvimento Cientıfico e

Tecnologico (CNPq) and Fundacao de Amparo a Pesquisa do

Estado da Bahia (FAPESB) for financial support. The authors

are also thankful to Laboratorio Multi-Usuario de Microscopia

Eletronica da UFBA (LAMUME) for the SEM analyses.

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