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Scientific report

2011 October– December

(max 5 pagini)

Designing composite nanoarchitectures for hydrogen production and environmental depollution

(MO1) The design of TiO2/WO3 composites (aerogels and xerogels) by direct sol-gel process and the

controlled deposition of noble metal nanoparticles (Au or Pt) on the selected oxide’s surface – presents

the first main objective of the project and includes the preparation of the samples. The morpho-structural

analysis of the samples is foreseen in the next phase of the project/in next year. Thus, in order to obtain

xerogels/aerogels of TiO2-(1÷24%)WO3, their synthesis was performed in 5 stages: (a) preparation of the

TiO2-WO3 gels using sol-gel method; (b) aging of gels; (c) washing gels; (d) drying of gels at atmospheric

pressure at 600C (the obtained xerogels were denoted with X) or in supercritical conditions using liquid CO2

(the obtained aerogels were denoted with A); (e) thermal treating of the aerogels/xerogels.

(a) Obtaining TiO2 – (1-24%)WO3 gels using sol-gel method. Obtaining of TiO2-WO3 aerogels using sol-gel

method was based on the acid catalyzed hydrolysis and condensation (alkoxylation and oxalation) of the

organo-metalic Ti and W precursors. The catalyst we used was nitric acid (HNO3, 67%) and the

solvent/reaction medium was absolute ethanol (EtOH, > 99%; Chimopar SA) and distilled water. The TiO2

precursor was titanium tetraisopropoxide (TIPT, 98%; Merck), in the case of we used two type of

precursors, namely: tungsten hexaisopropoxide (HIPW, 5%; Alfa-Aesar) and tungsten tetraethoxide (TEOW,

ethanolic solution 8,7% WO(C2H5O)4) synthetised according to the method described in the literature1).

Table 1. Synthesis conditions of TiO2 - WO3 aerogels (A) and xerogels (X). Nr.

crt.

Sample

TiO2WO3 CWO3 [%]

Tungsten

precursor

TIPT/HIPW(TEOW)

/EtOH/H2O/HNO3

Timegelification

[min] Drying

Thermal

treatment

1 A1 7,43

HIPW

5/ 5/ 21/ 1.09/ 0.08 ~ 15

supercritical

5300C, 5h;

6000C, 1h

(40C/min)

2 A2 13,82 5/ 10/ 21/ 1.09/ 0.08 ~ 10

3 A3 24,29 5/ 20/ 10/ 1.09/ 0.08 <5

4 A4 1

TEOW

11.08/0.58/42/2.18/0.168 < 3

supercritical

5 X4 1 11.08/0.58/42/2.18/0.168 patm, 600C, 24h

6 A5 7 10.41/4.06/38/2.18/0.168 < 1

supercritical

7 X5 7 10.41/4.06/38/2.18/0.168 patm, 600C, 24h

8 A6 14 9.63/8.12/34/2.18/0.168 < 3

supercritical

9 X6 14 9.63/8.12/34/2.18/0.168 patm, 600C, 24h

10 A7 24 8.50/13.92/28/2.18/0.168 < 3

supercritical

11 X7 24 8.50/13.92/28/2.18/0.168 patm, 600C, 24h

1 Z. Novak, P. Kotnik, Z. Knez, J.Non-Crystal. Solids 350 (2004) 308-313

2

(b) Maturation of TiO2-WO3 gels. During the maturation of gels are taking place polymerization processes,

deriving from condensation, hydrolization, re-esterification reactions and phase-transformation routes

which can lead to compaction or shrinking of the gels’ structure. The obtained TiO2-WO3 were kept at

room temperature, in hermetically sealed recipients for 14 days.

(c) Washing of TiO2-WO gels. It was performed in order to eliminate the unreacted precursors, catalyst and

water. It was completed in 3-4 stages, using mixtures of ethanol/water in increasing reports. Finally, the gel

was washed just with EtOH in order to eliminate the water blocked in the pores of the gel-structure.

(d) Drying of TiO2-WO3 gels. Drying of the gels was performed in supercritical conditions, with liquid CO2 (T

> 35 °C and p > 1200 psi), using an SAMDRI-PVT 3D (Tousimis) for obtaining aerogels and at atmospheric

pressure, at 60 °C for 20h to obtain xerogels.

(e) Thermal treatment of TiO2-WO3 aerogels/xerogels. In order to determine the optimal temperature for

thermal treatment, TGA/DTA diagrams were recorded on selected TiO2-WO3 samples with different WO3

content (Fig. 1a). It can be observed that the mass losses are in agreement with major endo- and

exothermal events which are appearing and are associated with the elimination of water (~ 100 °C),

elimination of organic pollutants (~ 290 °C) and crystallization (between 450 and 490 °C, value dependent

on the WO3 content). Referring strictly to the formation of crystal structures it can be noted that the

crystallization temperatures are increasing in parallel with the concentration of tungsten-oxide. Since it

was intended to obtain TiO2-WO3 crystal structures to deposit in a controlled way Au/Pt nanoparticles on

the surface of the semiconductors, it was chosen the temperature of 530 °C for 5h, than 600 °C for 1h (for

stabilization of the crystalline structure), the heating rate being 4°C/min.

Figura 1. TGA/DTA diagrams (A) and diffractograms (B) of TiO2-WO3 samples with different composition:

TiO2-7,4 WO3 (a), TiO2-13,8 WO3 (b) and TiO2-24,3WO3 (c).

After obtaining the samples, the crystal structure of the samples was verified by performing X-ray

diffraction measurements (XRD) on the same set of samples (Fig. 1b). It can be clearly observed the

features that indicate the presence of TiO2 crystal structures, mainly anatase.

(B)

(a) (b)

(c)

DTA

TGA

Temperatura [0C]

4500

C

4600C

4900C

(A)

10 20 30 40 50 60 70 80 90

(c)

(b)

Inte

nsita

te

2

(a)

3

2. Synthesis of TiO2-WO3 composites with 1%Pt or Au) content

In order to obtain Pt,Au/TiO2-WO3 composites, it were followed two methods which are described in the

following sections.

2.1. Impregnation of thermally treated TiO2-WO3 aerogels/xerogels with Pt or Au colloid

This method consist of two steps discussed below.

2.1.1. Preparation of Au colloid. Preparation of the Au colloid was performed following the method

described by Turkevich2, which is based on reduction of Au ions with sodium citrate. As Au precursor we

have used tetrachloroauric-acic (HAuCl4·3H2O, 99,5%, Merck), and sodium citrate as reductive agent

(Na3C6H5O7·2H2O, > 99%, Merck). Practically, 100 ml of aqueous solution of HAuCl4·3H2O, 10-3 M was

heated with stirring until boiling temperature. It was added 6,5 ml of aqueous solution of Na3C6H5O7·2H2O,

25,2 mM and the boiling for 10 minutes under continuous stirring. Then, the heating was stopped and the

stirring was continued for 5 minutes. The thus obtained colloidal Au particles had an average size of (see

inset of Fig. 2a) and showed absorption spectra with a maximum at 531 nm (see Fig. 2a).

2.1.2. Preparation of Pt colloid3. To prepare the Pt colloid, similar to preparation of Au colloid, acidic Pt

precursor was used, namely hexacloroplatinic-acid (H2PtCl6·6H2O, > 98%, Merck). As reducing agent for Pt

ions, anhydrous trisodium citrate was used (Na3C6H5O7, Alfa Aesar). Basically, 1 ml of aqueous solution of

Pt precursor (H2PtCl6·6H2O, 1%) was added to 100 ml of distilled water than heated until boiling

temperature. It was added 3 ml from reducing agent (Na3C6H5O7, 3%). After 30 minutes of boiling, the

initially yellow solution became darker (plain brown), while the overall volume decreased about 40 ml. The

colloidal Pt particles had a dimension between 5 - 7 nm (see inset of Fig. 2b). In order to verify the colloidal

suspension with those described in the literature, it were recorder both UV-vis spectra and TEM

micrographs, confirming the size of Pt nanoparticles (Fig. 2).

Figure 2. UV absorption spectra of colloidal Au suspension (a) and Pt (b) and TEM micrographs of the

nanoparticles (insets have a scale of 100 nm).

2 J. Turkevich, P. C. Stevenson, J. Hillier, "A study of the nucleation and growth processes in the synthesis of colloidal gold", Discuss. Faraday.

Soc. 1951, 11, 55-75. 3 M. Huang, Y. Shao, X. Sun, H. Chen, B. Liu, S. Dong, Langmuir 21 (2005) 323-329; H. D. Jang, H. Chang, K.Cho, F. Kim, K. Sohn, J.Huang, Aerosol

Science and Technology 44 (2010) 1140-1145; K. Hikosaka, J. Kim, M. Kajita, A. Kanayama, Y. Miyamoto, Colloids and surfaces B: Biointerfaces

66 (2008) 195-200

300 400 500 600 700

Ab

sorb

anta

Lungimea de unda [nm]

531

200 300 400 500 600

Ab

sorb

anta

Lungimea de unda [nm]

(a) (b)

4

2.1.3. Impregnation of thermally treated TiO2-WO3 aerogels/xerogels with Pt or Au colloids.

Aerogels/xerogels were immersed in different volumes of colloidal solution of noble-metals in order to

obtain compozites with Au/Pt solution of 1 % (Table 2). Immersion time was 65 h at room temperature.

After immersion the samples were centrifuged (10 minutes at 104 rot/min), filtered, washed in distilled

water and dried at 22 h la 105 °C.

Tabelul 2. Sample preparation conditions of Au,Pt/TiO2-WO3 compozites using impregnation with noble-

metal colloid.

Sample name

Au,Pt/TiO2-WO3

Metal used in

composite

CWO3 in

composite

[%]

Ratio

(g TiO2-WO3: ml col. sol. Au/Pt)

A4-Au

1%Au

1 0,35 : 15

A5-Au 7 0,35 : 15

A6-Au 14 0,35 : 15

A7-Au 24 0,20 : 8,70

X4-Au 1 0,50 : 22

X5-Au 7 0,50 : 22

X6-Au 14 0,50 : 22

X7-Au 24 0,19 : 8,30

A4-Pt

1%Pt

1 0,35 : 0,72

A5- Pt 7 0,35 : 0,72

A6- Pt 14 0,35 : 0,72

A7- Pt 24 0,19 : 0,41

X4- Pt 1 0,50 : 1

X5- Pt 7 0,50 : 1

X6- Pt 14 0,50 : 1

X7-Pt 24 0,50 : 1

2.2. Impregnation of thermally treated TiO2-WO3 aerogels/xerogels with Pt or Au precursors followed by

their photoreduction.

It were used the same precursors of noble metal like at 2.1., namely: HAuCl4 (1,6 ml; 2,53 mM) and H2PtCl6

(3,45 ml; 5,93 mM). Noble metals, Pt or Au (1% m/m, ±0,02), were deposited in same conditions on the

surface of thermally treated TiO2-WO3 aerogels/xerogels (Table 3). In order to perform the deposition of

the noble metal only on the surface of WO3 suspension of TiO2-WO3 (5g/l; V=80ml) containing the noble-

metal precursor was irradiated with visible light. For deposition of noble metal on the surface of both

oxides, the continuously stirred suspension was irradiated with UV light. Basically, the TiO2-WO3

suspensions containing the noble metal precursor was irradiated for 6 hours, using 3 UV lamps (Lightech,

40 W) with the λmax ≈ 365 nm, or 3 lamps with visible light (cold light indoor lamp Lightech, 40 W). In order

to eliminate the Cl anions occurring during reactions, the samples were washed 3-4 with oxalic acid (1mM)

and with distilled Milli-Q water, the sample being centrifuged at 5000rot/min, for 5 minutes. The resulted

samples were dried at 80 °C, for 24 h.

It should be noted that in this phase of the project, although this activity was not included in this

phase of the work plan, we sent an application for evaluation to Scientific Committee of the MAX IV

laboratory in Lund, Sweden with the title “The EXAFS-assisted structural investigation of the contact zones

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in WO3/TiO2/noble metal nanoarchitectures- Proposal ID: 20110343” in order to investigate using XAFS

the structures prepared in the present project, the proposal being recently accepted.

Tabelul 3. Sample preparation conditions of Au,Pt/TiO2-WO3 composites using impregnation with noble-

metal precursor and reducing by UV/Vis irradiation of nanoparticles.

Sample name

Au,Pt/TiO2-WO3

Metal used in

composite

CWO3 in composite

[%]

Ratio

(g TiO2-WO3: ml col.

sol. Au/Pt)

Irradiation used

X4-Au-UV

~ 1%Au

1

0,4/1,6 ml sol. col.

Au 2,53 mM

UV X5-Au-UV 7

X6-Au-UV 14

X7-Au-UV 24

X4-Au-Vis 1

Visible X5-Au-Vis 7

X6-Au-Vis 14

X7-Au-Vis 24

X4- Pt-UV

~ 1%Pt

1

0,4/3,45 ml sol. col.

Pt 5,93 mM

UV X5- Pt-UV 7

X6- Pt-UV 14

X7-Pt-UV 24

X4- Pt-Vis 1

Visible X5- Pt-Vis 7

X6- Pt-Vis 14

X7-Pt-Vis 24

Conclusions

In the present phase of the project the TiO2-(1÷24%)WO3 aerogel/xerogel structures were obtained, using

sol-gel method, followed by evaluation of the crystallization temperature (TGA/DTA) and the effect of the

thermal treatment together with the investigation of the crystallite structures by XRD. In final, Au and Pt

nanoparticles were deposited on the surface of the thermally treated TiO2-WO3 by impregnation of the

Pt/Au colloids and by impregnation of semiconductor composited with noble-metal precursors. In the last

case, the deposition of the noble-metal nanoparticles was performed by photoreduction of the precursors.

In conclusion, we can say that the objectives were fulfilled properly regarding this stage of the present project.

Project manager,

Assoc. Prof. dr. Lucian BAIA