(16) 2010 Sol Gel, Influence of Zn Doping on the Photocatalytic Property of SrTiO3

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JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGYVolume 38, Issue 4, August 2010Online English edition of the Chinese language journal

Received: 04-Dec-2009; Revised: 20-Feb-2010* Corresponding author. E-mail: [email protected], Tel: +86 22 60202489, Fax: +86-22-26582427

Foundation item: supported by the National Natural Science Foundation of China (20241002) and Natural Science Foundation of Hebei (B2006000012).

Copyright © 2010, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2010, 38(4), 502−507

Influence of Zn doping on the photocatalytic property

of SrTiO

3

WANG Gui-yun, QIN Ya, CHENG Jie, WANG Yan-ji*

School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China

Abstract: The Zn doped SrTiO3 photocatalysts were synthesized and the photocatalytic activity of the doped samples for water

decomposition to hydrogen was measured under the irradiation of a high pressure Hg lamp (400 W). The influence of doping amount of

Zn and the calcination temperature on the photocatalytic activity were investigated. The physical properties of catalysts with and

without Zn doping were characterized using XRD, XPS, SEM and UV-visible diffuse reflectance spectra. The experimental resultsshow that Zn doping greatly improves the photocatalytic activity of SrTiO3. The optimum molar doping amount of Zn is about 1%, and

in this case, the suitable calcination temperature is about 950°C. Under this condition, the photocatalytic property of Zn doped SrTiO3

is 2.2 times higher than that of the undoped one. The characterization results indicate that 1% Zn doping does not change the crystal

structure and crystal perfection properties of SrTiO3. However, doping element Zn is enriched on the surface of SrTiO3 sample. In

addition, the crystal size increases within a certain range of Zn doping. It can be predicted that Zn2TiO4 is generated by the incorporated

Zn reacting with the Ti-rich phase existed on the SrTiO3 surface, which leads to the decrease of the surface defect concentration of

SrTiO3, and consequently increases the photocatalytic activity of Zn doped SrTiO3.

Keywords: Zn; strontium titanate; photocatalysis; water decomposition

Demand on energy is rapidly increasing in several-fold

along with the continuous development of social economy.

There is a risk for the limited fossil fuel to be used up in the

near future; therefore to develop replaceable energy is an

urgent task before us. Hydrogen is a kind of energy with the

characteristics of clean and efficient. Solar energy is the basic

of all kinds of the energy in the earth. To transfer solar energy

to hydrogen energy is one of the best ideal ways to develop

new energy, which is the main reason for hydrogen

preparation research to be highly focused by photocatalytic

decomposition of water with semiconductor catalysts. The

process for water decomposition to hydrogen and oxygen by

photocatalytic method is as follows: electron is excited toconduction band (CB) from valance band (VB) and at the

same time, hole is formed in VB when absorbing energy is

higher than that of band-gap E g 1240

(nm)(eV)

g E

λ ⎡ ⎤

≤⎢ ⎥⎢ ⎥⎣ ⎦

of

semiconductor catalyst. Electron has reducing capacity and

hole has oxidation capacity. Considering from view of

thermodynamics, when the electrical potential of the CB

bottom is lower than E (H+/H2), H

+ adhered on catalyst surface

can be reduced by excited electrons to release H2. If the hole

potential of the VB top is higher than E (O2/OH – ), OH

–

adhered on catalyst surface can be oxidized by holes to release

O2. In addition to the complicated photocatalytic process

mentioned above, photoexcited carrier may recombine, or

some possible reaction to corrode catalyst may occur. At the

present time, catalysts used for water decomposition that have

been reported can be divided into two parts: which can absorb

visible light and which can only absorb UV light. There is a

high absorbance rate on sun light for the former, however, due

to the narrow band-gap, the photoexcited electrons and holes

might recombine easily. Therefore in order to increase

utilization rate of photoexcited electrons, one kind of electron

donor such as CH3OH, HCOOH, Na2S, and Na2SO3 shouldusually be added for photocatalytic reaction to consume

holes[1–3]

. Adding of electron donor will limit the practical

application of corresponding catalyst system to a great extent.

Meanwhile, maximum hydrogen production rate with

developed visible light catalyst system can only reach an

extent of several mmol/h. Water can be decomposed to

hydrogen and oxygen by UV light catalyst without adding

sacrificial agent. However hydrogen production rate in lab

with the irradiation of UV light can only reach several to

several hundred µmol/h[4,5]

. It can be seen that too low activity



WANG Gui-yun et al. / Journal of Fuel Chemistry and Technology, 2010, 38(4): 502− 507

for catalyst is the bottleneck to limit further development of

the technology. In order to improve the photocatalytic

hydrogen production rate, people have chosen many kinds of

ways to change the performance of catalyst. Doping of

semiconductor catalyst is the most commonly used modifyingmethod. Effective doping can improve catalyst activity by

expanding inductive range of catalyst by irradiation light[6,7]

,

promoting production and separation of photon-generated

carrier [8]

, increasing concentration of photon-generated carrier,

and improving surface structure and performance of

catalyst[10,11]

. As crystal particles with photocatalysis function,

adding of other components may at the same time affect

photocatalytic activity by improving crystallinity of catalyst. It

is found by the authors of this paper in the previous research

on SrTiO3 that crystalline perfection has a great effect on the

activity[12]

, which means crystal defect may offer

recombination center for photoexcited carrier and obviously

reduce the photocatalytic activity of catalyst. In order to

further improve the photocatalytic activity of SrTiO3, doping

modification has been carried out on SrTiO3, and this paper is

focused on corresponding research results.

1 Experimental

1.1 Preparation of catalysts

10 g of tetrabutyl titanate was diluted in 100 mL ethanol and

then 100 mL water was added dropwise slowly into thesolution with stirring and heating. After complete hydrolysis,

the filtered cake was mixed with pre-dissolved and

stoichiometric strontium nitrate, certain amount of zinc nitrate

and water solution of mineralizer potassium hydrate before

drying. Doping methods of other mixed elements were in the

same way and the raw materials were all nitrate of

corresponding elements. According to the literature[12]

, adding

amount of mineralizer potassium hydrate is 2.0% (w%, relative

to theoretical yield of SrTiO3 ). The precursor powder was

calcinated at a certain temperature for 6 h, then ground,

washed with deionized water, dried, and screened to obtain particles with particle size less than 38 µm for activity test.

Previous research indicated that CoO was an effective assistant

catalyst for SrTiO3, and 0.2% of CoO (w%) was loaded on the

catalyst surface to prepare CoO/SrTiO3 photocatalyst using the

adopting method as literature[12]

.

1.2 Characterization of catalysts

Crystallite sizes and shapes were observed using scanning

electron microscopy (SEM) (Philips XL30, Holland). UV-vis

absorption spectra of catalysts were obtained using a

UV-visible spectrophotometer (UV-2401PC, Shimadzu Co.,

Kyoto, Japan), with step length of 2 mm and slit-width of 5 nm

for rapid scanning. Element composition on catalyst surface

was tested with PHI-1600 type XPS produced by American PE

company. Mg K α was excitation source and the bonding

energy was calibrated by pollution C (1s) electron bonding

energy of 284.8 eV. The XRD patterns were recorded with aRigaku D/max 2500 V powder X-ray diffractometer using Cu

K α radiation.

1.3 Photocatalytic experiment

The photocatalytic reaction was carried out in an air free

closed gas circulation system with a reactor (1000 mL) made

of quartz. The catalyst powder (0.1 g) was dispersed in

deionized water (800 mL) containing Na2CO3 using a

magnetic stirring. The dispersed mixture was irradiated

externally under Ar atmosphere by a high pressure Hg lamp

(400 W). Argon was introduced into the system after the

reaction to make the gases well-mixed. The gaseous products

were measured using gas chromatography (MS-5A column, Ar

carrier, thermal conductivity detector) through a gas sampler

(ca.2 mL) which was directly connected to the reaction system.

2 Results and discussion

Doping modification research had been carried out on

SrTiO3 by firstly selecting part of transition metals in the first

long period, with 1 mol% of adding amount of Ti for doping

elements. Calcination temperature for precursor was 1000°Cand activity test was carried out after loding 0.2% of CoO (w%)

on the prepared SrTiO3. The results indicate that photocatalytic

activity of SrTiO3 is obviously reduced by doping of Mn, Fe,

Co, and Ni, slightly reduced by adding Cr and Cu, and

obviously increased by doping Zn. Thus, further investigation

on the photocatalytic activities of Zn-doped samples were

carried out.

2.1 Photocatalytic performance of Zn doped SrTiO

3

Photocatalytic activities of CoO/SrTiO3 with differentdoping amount of Zn are shown in Table 1.

Table 1 Influence of Zn doping amount on the photocatalytic

activity of CoO/ SrTiO3

(loading amount of CoO, 0.2% (w%))

Rate of gas evolution / µmol·h –1

Amount of doping

wmol/% H2 O2

0

0.5

0.6

0.8

1

1.4

1.5

105

177

259

272

285

239

162

38

70

141

146

140

118

57




Table 2 Influence of calcination temperature on the photocatalytic

activity of Zn-doped SrTiO3

(loading amount of CoO, 0.2% (w%))

Rate of H2 evolution /µmol·h –1

Calcination temperature

t /°C1% Zn-doped

SrTiO3

undoped

SrTiO3

900

950

1000

1050

1100

254

315

285

269

239

85

119

105

139

132

The results indicate that there is an obvious improvement of

photocatalytic activity of SrTiO3 when the molar doping

amount of Zn is within 0.5% to 1.5%, and the optimum doping

amount is 1%.Calcination temperature is one of the main factors that affect

the performance of oxide catalyst. Photocatalytic activities of

prepared SrTiO3 with 1% Zn doping (mol%) and without Zn

doping at different precursor calcination temperatures are

shown in Table 2, with the CoO loading amount of 0.2%(w%).

The suitable calcination temperature for 1% Zn-doped (mol%)

catalyst is about 950°C, which is 100°C lower than that of

sample without Zn doped. The activities of catalysts with

Zn-doped are higher than those of without Zn doped in the

tested scope. When the samples are prepared at the optimum

calcination temperatures respectively, it can be seen that the

activity of sample with Zn doped is 2.2 times higher than that

of sample without Zn added, which means it increased by

120%.

2.2 Characterization results of Zn doped SrTiO

3

XRD and UV-vis diffuse reflection analysis are respectively

carried out on the SrTiO3 samples calcinated at 1000°C with

1% Zn doped (mol%) and without Zn added, which are shown

in Fig. 1 and Fig. 2. XRD analysis results in Fig. 1 show that

there is no impurity peak for the Zn doped SrTiO3 sample, and

there is no obvious difference in position of diffraction peak

and relative strength for with and without Zn doped samples,which indicates that Zn ion does not enter the crystalline

lattice of SrTiO3. Zn is scattered on the surface of SrTiO3

particles in a form of certain oxide. Fig. 2 indicates that the

intrinsic transition existing in the 1% Zn added (mol%) sample

is similar to that of without Zn added, and the band gap widths

calculated with absorption curve of both are about 3.2 eV. The

main difference between the two samples is that there is a few

impurity peaks for Zn-added sample in 300-350 nm, which

may be caused by UV absorption of Zn-containing substance.

SEM analysis results of SrTiO3 samples calcinated at

1000°C with 1% Zn doped (mol%) and without Zn added are

given in Fig. 3. It can be seen that the particle size of Zn doped

SrTiO3 particles is obviously bigger than that of without Zn

added, however there is no obvious change for morphology of

crystal particles.

Comparing SEM analytical results of samples with different

Zn doping amount of 0.0%, 0.5%, 1.0%, and 1.5%, the particle

sizes are 0.8 µm, 1.7 µm, 2.1 µm, and 2.0 µm, respectively. It

can be seen that the particle size of SrTiO3 increases with the

doping amount in a certain range.

XPS analysis on 1% Zn doped (mol%) SrTiO3 sample

shows that the element contents are C (61.8%), O (28.7%), Sr

(4.2%), Ti (4.4%), Zn (0.7%), K (0.2%). The Zn/Ti mol ratiois 1:6.3, which is obviously higher than the batch charging

ratio of 1:100. XPS is a surface analysis technology and it can

only detect atoms within 4 nm of surface[13]

. Thus, after SrTiO3

catalyst has been doped with Zn and calcinated, the Zn content

on the surface or the adjacent surface of SrTiO3 is much higher

than that in the bulk phase.

Fig. 1 XRD patterns of Zn-doped SrTiO3

a: without doping; b: doped with 1.0% Zn

Fig. 2 UV-vis diffuse reflection absorptive spectra of

Zn-doped SrTiO3

a: without doping; b: doped with 1.0% Zn




Fig. 3 SEM photos of Zn-doped SrTiO3

(a): without doping; (b): doped with 1.0 % Zn

2.3 Result analysis

Radius of Zn2+

is about 0.74 nm, bigger than that of Ti4+

,

0.68 nm and smaller than that of Sr 2+

, 1.13 nm. If the

substitutional doping occurred in the bulk phase of SrTiO3, its

lattice constant would change and diffraction peak position of

XRD would shift, or there would be a crystalline lattice

deformation of the doped sample with increased deficiency.

Therefore there will be an obvious affect on morphology of

SrTiO3 particles with 1% (mol%) doping amount. However,

the analytical results indicate that there is no obvious change

for crystalline structure, morphology characteristics,

absorption performance on UV light of doped sample, from

which it can be seen that there is low possibility for the

substitutional doping. It is also found in doping modification

research on TiO2 photocatalyst by Wang et al that it is not easy

for substitution of Zn2+

for Ti4+[14]

. Combining the analysis

result of XPS, it can be inferred that added Zn can not enter

the inside of crystal lattice and Zn is segregated on SrTiO3

surface in a form of ZnO or titanium zinc composite oxide.

The raw materials for preparation of SrTiO3 are Sr(NO3)2

and tetrabutyl titanate which have certain absorbency for both

of them. Therefore it is a little difficult to accurate weight the batch charging strictly according to stoichiometry. Even

though the suitable batch charging ratio was determined for

each batch of raw materials, there was a little difference

between element atom ratio and stoichiometric proportion of

Sr and Ti. For the formation energy of ''''

TiV is very high, it is

difficult to obtain SrTiO3 with Ti ion vacancy[15]

. When there

is an excessive amount of Sr, it will precipitate in form of SrO

or SrCO3 (formed during the roasting of raw catalyst materials

in air). When SrTiO3 is covered by SrO and SrCO3, the

photocatalytic activity is reduced because SrO and SrCO3 are

both insulation ionic crystal. When Ti is a little excessive,Ti-rich phase can be formed under circumstances to keep

crystal structure of SrTiO3." ..

2 2 Ti O Sr 1/ 2O TiO Ti 3O 2V h× ×+ + + + (1)

The following reaction may be occurred in Ti-rich phase:'' . '

Sr Sr V h V + (2)

' .

Sr Sr V h V ×+ (3)

That is to say, defect sites containing in the Ti-rich phase

includeSr

V ×

,'

Sr V , and

''

Sr V , in which

Sr V

× and

'

Sr V are

electron traps, which can capture photoemission electron to

reduce the photocatalytic activity of catalyst. At the same time

Ti-rich phase has type P conduction characteristics, which willmake electron in SrTiO3 bulk phase shift to it to gain

grain-boundary barrier. When the photocatalytic reaction

happens, SrTiO3 will absorb photon and stimulate electrons

and holes. Some holes will move towards Ti-rich phase under

function of potential barrier and then pass through the Ti-rich

phase to jump to surface to oxidize water and release oxygen.

The photocatalytic activity of the catalyst with Ti-rich phase is

lower than that of without Ti-rich phase. This means that the

stoichiometric deviation of Sr to Ti ratio can reduce the

photocatalytic activity of sample. However, compared with the

condition of Sr excess, slight excess of Ti can cause a little

activity decrease of SrTiO3. Previous Sr/Ti test results indicate

that the activity decrease tendency of SrTiO3 with excess of Ti

is slow and the tendency with excess of Sr is rapid. Actually,

there is more or less Ti-rich phase on the SrTiO3 surface

during the optimization of raw materials with an aim at

improving catalyst activity. Therefore, the activity of prepared

catalyst is lower than that of ideal crystal which is strictly

composed of stoichiometry ratio.

Zn may react with Ti-rich phase of SrTiO3 to form

Zn2TiO4[16,17]

at a corresponding calcination temperature. The

formed Zn2TiO4 has an inverse spinel structure, which is

greatly different from that of SrTiO3. So it is not easy to

continuously grow and cover on SrTiO3 surface and it may




segregate to two or more crystal boundaries formed by SrTiO3

crystal particles. That means the added Zn has the functions to

reduce the amount of Ti-rich phase and the defection

concentration on the surface of SrTiO3, which may be the main

reason for the activity of Zn-added SrTiO3 improved. Surfacedefect is the main factor to prevent particles from further

growing. Therefore defection concentration on catalyst surface

is reduced and the catalyst particles become bigger after

doping with certain amount of Zn.

When doping amount of Zn is too low, the Ti-rich phase on

the SrTiO3 surface can not be eliminated. When adding

amount of Zn is relatively high, the excessive Ti will combine

with Zn and Sr will precipitate in a form of SrO, which will

cause the reduction of photocatalytic activity. In addition, ZnO

will be formed as well as Zn2TiO4 when doping amount is too

high, then enough amount of Zn2TiO4 and ZnO impurity phase

will cover on the SrTiO3 surface to prevent further growing of

SrTiO3, to reduce effective surface exposure of SrTiO3, to

reduce absorption rate on illumination light by SrTiO3, and

therefore rapidly reduce the photocatalytic activity of SrTiO3.

Thus, a suitable amount of Zn doping can get a maximum

photocatalytic activity and particle size of SrTiO3 is firstly

increased and then decreased along with the doping amount of

Zn.

If the calcination temperature is too low, when preparing Zn

added SrTiO3, it is not easy for guarantee the purity and

integrity of crystallization of SrTiO3 because of the low

reaction speed and the incomplete precursor reaction. However,if the calcination temperature is too high, because of the low

sintering temperature[18]

Zn2TiO4 in crystal boundary may

soften and frit to partially cover the effective surface of SrTiO3

and therefore reduce its activity. So the optimum roasting

temperature exists for the doping samples. Compared with the

sample without doping, its suitable roasting temperature is

lower. That is because the determination factor of suitable

calcination temperature for both is different. Suitable

calcination temperature for sample without doping is caused

by crystalline perfection of SrTiO3, particle growing and

sintering. Roasting temperature for SrTiO3 is a little high,therefore its suitable calcination temperature is a little higher

than that of Zn-doped SrTiO3.

3 Conclusions

The photocatalytic activity of SrTiO3 for water

decomposition to hydrogen can be obviously improved by Zn

doping. Hydrogen production rate for 1% Zn (mol%) doping

SrTiO3 sample is 120% higher than that of the undoped sample.

The suitable calcination temperature to form 1% Zn doping

SrTiO3 catalyst is 950°C, which is 100°C lower than that of

the catalyst sample without Zn added.

There is not obvious difference between the Zn doped and

undoped SrTiO3 in crystalline morphology characteristics,

XRD diffraction peak position and relative strength, and the

absorption performance on Vis-UV light. However, particle

size of Zn doped catalyst is a little increased. XPS analysis

result indicates that the added Zn is enriched on the surface ofSrTiO3.

The added Zn may react with Ti-rich phase existing on the

surface of SrTiO3 to form Zn2TiO4, which reduces the possibly

existing impurities phase on SrTiO3 surface. Therefore, the

defection concentration on Zn doped SrTiO3 surface is reduced

and the photocatalytic activity is increased.

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Documents

(16) 2010 Sol Gel, Influence of Zn Doping on the Photocatalytic Property of SrTiO3