Supporting Information

Silver Vanadate Nanobelts: A highly Sensitive Material towards Organic Amines

Haitao Fu, Xiaohong Yang, Xuchuan Jiang,Aibing Yu

School of Materials Science and Engineering, The University of New South Wales,

Sydney, NSW 2052, Australia

1. Preparation of Ag0.35V2O5 nanobelts

The Ag0.35V2O5 nanobelts were synthesized by a hydrothermal method. Similarly, a

mixture of 1.5 mmol vanadium pentoxide powder and 0.6 mmol SDS was dissolved in 15

ml of pure water. The mixture was then placed into a Teflon-lined stainless steel

autoclave. After heating at 180 °C for 2 days, greenish gray precipitate was formed.

Finally, the precipitate was centrifuged and washed by pure water and ethanol several

times after being cooled down to room temperature, then dried at room temperature. To

investigate the effect of the ratios on the formation of the nanobelts, various molar ratios

of Ag to vanadium (0-100%) were added in the system. Also, to investigate the effect of

additives, SDS was replaced by CTAB and PVP, respectively.

Fig. S1a shows the XRD pattern corresponding to Ag0.35V2O5 prepared with the assistance

of SDS at the Ag/V molar ratio of 15%, while the TEM image shown in Fig. S1b

indicates the Ag0.35V2O5 product contained belt-like structure with nearly uniform size of

100 nm × 5 μm.

2. Preparation of VOx@Ag nanocomposites

To prepare Ag doped VOx via the method reported by our group under certain conditions,

[1] the results were not ideal, although the procedure of synthesis of VOx@Ag

nanocomposites was similar to that in our previous study.[1] Specifically, VOx@Ag

To whom correspondence should be addressed. Email: xcjiang@unsw.edu.au.

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nanocomposites were prepared by adding various amounts of Ag ions into the mixture

containing various ratios of V2O3 to V2O5 in aqueous solution at room temperature. The

molar ratio of V2O3 to V2O5 was fixed as 1:2, while the molar ratio of PVP to elemental V

was fixed at 3:1. The molar ratios Ag to V varied from 0.15, 0.1 to 0.05. The TEM

images and the corresponding XRD patterns are shown in Fig. S2. Here, in order to make

the Ag nanoparticles attach on the VOx particles evenly, PVP as a capping agent was

used to prevent the formation of Ag nanowires. It can be seen that the nanocomposites

were composed of V2O5 and a small amount of Ag. With the molar ratio of Ag to V

decreased, the amount of Ag nanoparticles decreased. It is noted from the XRD pattern

that only crystal V2O5 was prepared in the product.

XPS was employed to reveal the existence of other vanadium species, as shown in Fig.

S3a. The XPS pattern shows the coexistence of two vanadium species (V4+ and V5+) in

the nanocomposites, suggesting that V3+ could be oxidized to V4+. This is also the reason

why the nanocomposite was named as VOx@Ag, instead of V2O5@Ag. Fig. S3b shows

two strong peaks at the Ag region of 368.5 eV and 374.5 eV, assigned to Ag3d(5/2) and

Ag3d(3/2), respectively, in a good agreement with the previous study.[2] It is noted that

the multivalence of vanadium would not affect the sensing performance. According to the

testing procedure, the materials need to be aged at high temperatures (~300 °C) in the air

for a long time until the base line became stable. This means that all the V4+ could be

oxidized to V5+ so that the resistance of the materials will not change again.

3. Effect of experimental parameters on the synthesis of Ag2V4O11 and

Ag0.35V2O5 nanobelts

3.1 Effect of molar ratios of Ag to V

The effect of the molar ratios on the formation of the SVO was investigated with the

assistance of SDS at the different Ag/V molar ratios (0, 5%, 10%, 15%, 50%, and 100%).

The corresponding morphologies and sizes of the materials were observed by TEM, as

shown in Fig. . It was found that the structure without Ag (Ag/V molar ratio = 0) was soft

belts with wide distribution (Fig. a). With the ratios increase to 5%, the morphology of

the nanostructure changed little (Fig. b). With the further increase of the molar ratio to

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10%, some uniform nanobelts with 50 nm in width appeared. However, some wide soft

belts still existed (Fig. c). Continuously increasing the ratio (15%, Fig. d), the products

mainly contained the uniform nanobelts with 50 nm in width. When the ratio increased to

50% and 100% (Fig. e and f), the morphologies were similar to those obtained at the

molar ratio of 15%.

The composition of the products synthesized with various Ag/V molar ratios was

confirmed by XRD technique, as shown in Fig. . Obviously, the different ratios resulted

in the change of the composition of the products. It was found that the crystallinity of the

nanoparticles increased with the molar ratios. When the ratio was lower than 1%, the

products were close to amorphous, and it was difficult to identify its composition. When

the ratios were between 5% and 15%, the typical patterns corresponding to Ag0.35V2O5

appeared. With the ratio further increased, the composition of the products changed to

Ag2V4O11. That means two materials can be prepared by adjusting the Ag/V molar ratios.

And the Ag/V molar ratio affected not only the morphology of the products but also the

composition and crystallinity. Moreover, SDS has little effect on the formation of

Ag2V4O11 nanobelts.

3.2 Effect of additives on the formation of Ag0.35V2O5 nanobelts

The effect of vanadium precursors, the additives and the existence of Ag ions on the

formation of the Ag0.35V2O5 nanobelts were studied in this section. Sodium vanadates

(Na3VO4 or NaVO3) as precursors in the system can lead to impurity by the formation of

other sodium vanadates (like NaV6O15 in Fig. a) which can co-precipitate with silver

vanadates. To avoid the impurity, NH4VO3 and V2O5 were selected as the vanadium

precursor. For the additives, besides SDS, PVP and CTAB were also considered.

Although CTAB can cause the formation of AgBr (precipitates) in the presence of Ag+,

previous studies suggested that Ag nanoparticles will be eventually formed and the AgBr

only affected the route of the formation of the final product, instead of introducing

impurity.[3, 4] Therefore, in this work, CTAB was also considered as a surfactant to

investigate the effect on the formation of the nanocomposites.

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The belt-like structured products prepared with different reactants at 180 °C for 2 days

are shown in Fig. , in which the figures on the left-hand side are the TEM images and

those located on right-hand side are the XRD patterns corresponding to left-hand figures.

Fig. a shows the TEM image and XRD pattern of the product synthesized by NH4VO3

and SDS, while Fig. b shows the TEM image and XRD pattern of the product obtained by

NH4VO3, SDS and 10% (molar ratio) of Ag. In comparison, it is found that the structure

of the products synthesized with Ag was “harder” than that synthesized without Ag as

some twisted nanobelts appeared in the TEM image (Fig. a), as marked by red arrows.

The discrepancy of the structures is probably attributed to the change of the

compositions. From XRD patterns, the product synthesized without Ag mainly contained

VO2 (B) phase. However, due to the existence of sodium ions (from SDS), an impurity

corresponding to NaV6O15 can also be observed. According to the XRD pattern in Fig. b,

the product was mainly composed of Ag0.35V2O5 and metallic Ag. Compared with the

products in Fig. a (Curve b and c), it was found that NH4VO3 could increase the yield of

both Ag0.35V2O5 and Ag, based on the intensity of the corresponding peaks. However, this

vanadium precursor can cause the impurity of the product (Ag particles). This requires

further investigation. Fig. c & d, and e show that the products obtained with various sets

of reactants. The TEM images show that the belt-like structures of the products were

retained. On the other hand, the XRD reveals the difference of the compositions of the

products. Without any additives like SDS or PVP (Fig. c), only V2O5 was found.

However, the morphology significantly changed from the platelike structure (Fig. S2c) to

the beltlike layered structure. Fig. d shows the morphology and composition of the

product produced by V2O5 and PVP. It can be found that the nanobelts were VO2 (B)

phase. Fig. e reveals that the product comprised VO2 (B) nanobelts and small amount of

Ag nanoparticles. Fig. f corresponds to the material prepared by V2O5, CTAB, and 10%

(Ag/V molar ratios) Ag+ suggesting that the nanobelts were amorphous, and no Ag and

Ag0.35V2O5 appeared. This may be attributed to the formation of poorly crystalline CTAV

which were formed by the cation [CTA]+ (from CTAB) and vanadium, as reported by

Luca et al.[5, 6]

On the basis of the above discussion, it can be concluded that: (i) Ag0.35V2O5 cannot be

prepared without SDS; (ii) both SDS and PVP can reduce V5+ in the absence of Ag+,

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however, SDS will introduce small amount of sodium; (iii) the reducing property of PVP

is higher than that of SDS, because in the present of Ag+, PVP can totally reduce V2O5

and Ag+ to VO2 and Ag while SDS can partially reduce V2O5 and Ag+ to Ag0.35V2O5; (iv)

compared to NH4VO3, V2O5 as the vanadium precursor can lead to purer products; (v)

CTAB in this system can lead to an amorphous beltlike nanostructure, instead of

Ag0.35V2O5; (vi) the appropriate reactants for synthesis of Ag0.35V2O5 are V2O5, SDS, and

Ag+.

4. Formation of Ag0.35V2O5 nanobelts

The precise formula of Ag0.35V2O5 can be represented as Ag(I)0.35V(IV)0.35V(V)1.65O5,

which is a non-stoichiometric solid solution of silver in V2O5. The silver atoms in the

solid solution are singly ionised.[7] Therefore, the formation mechanism of the 1D

nanostructure is similar to that of V2O5, as stated in the reference.[8]

In addition, a reducing agent with suitable reducing properties is significant for the

formation of Ag0.35V2O5 by hydrothermal methods. This kind of reducing agents must be

weak enough to reduce a small part of V5+ in the V2O5 to V4+, instead of totally reducing

the V5+ so as to form VO2, especially under hydrothermal conditions (high temperature

and long reaction time). As compared in Fig. , only V2O5 and Ag+ cannot lead to the

formation of Ag0.35V2O5 (Fig. c), and PVP can totally reduce V2O5 and Ag+ to VO2 and Ag

(Fig. d and e), while Ag0.35V2O5 cannot be prepared without SDS although the different

reactants can cause the production of impurity (Fig. a and b). That is, SDS owns a very

weak reducing property at high temperatures in aqueous solution, even weaker than that

of PVP.

The reducing property of SDS is likely to generate from the decomposition. Prolonged

heating at 40 °C or higher in aqueous solution can cause hydrolysis of SDS into fatty

alcohols and sodium sulphate.[9] As known, fatty alcohols own weak reducing property.

Therefore, the true reducing agents in this case might be the fatty alcohols decomposed

from SDS.

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Fig. S1 (a) The XRD patterns corresponding to Ag0.35V2O5; (b) An overview TEM image of Ag0.35V2O5 nanobelts.

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Fig. S2 TEM images of the nanocomposites prepared at different Ag/(V2O3+V2O5) ratios: (a) 15%, (b) 10%, and (c) 5%. (d) XRD patterns of the nanocomposites shown in (a), (b), and (c), while the XRD patterns a, b, and c correspond to the products shown in Figure (a), (b) and (c), respectively.

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Fig. S3 XPS patterns of (a) vanadium species and (b) Ag species in the VOx@Ag nanocomposites. Black line is original curve obtained by measurement. Green line corresponds to the background, while blue line and red line are the simulated curves corresponding to V4+ and V5+, respectively.

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Fig. S4 The TEM images of the Ag2V4O11 nanobelts obtained at different pH: (a) 13, (b) 10, (c) 6 (original reaction system), and (d) 1.

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Fig. S5 (a) TEM image and (b) HRTEM image of the corresponding area of the Ag2V4O11

nanobelt with Ag nanoparticles.

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Fig. S6 TEM images of the products obtained with different molar ratios of Ag to V: (a) 0, (b) 5%, (c) 10%, (d) 15%, (e) 50%, and (f) 100%.

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Fig. S7 TEM images of the products obtained with different molar ratios of Ag to V: (a) 0, (b) 5%, (c) 10%, (d) 15%, (e) 50%, and (f) 100%.

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Fig. S8 TEM images (left) and XRD patterns (right) of the products obtained by hydrothermal methods under various reactants. The reactants are: (a) NH4VO3 + SDS; (b) NH4VO3 + SDS + Ag (10%); (c) V2O5 + Ag (10%); (d) V2O5 + PVP + Ag (10%); (e) V2O5 + PVP; (f) V2O5 + CTAB + Ag (10%).

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References

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[2] C. Mao, X. Wu, H. Pan, J. Zhu, H. Chen, Single-crystalline Ag2V4O11 nanobelts: hydrothermal synthesis, field emission, and magnetic properties, Nanotechnology, 16 (2005) 2892.

[3] S. Liu, J. Yue, A. Gedanken, Synthesis of Long Silver Nanowires from AgBr Nanocrystals, Advanced Materials, 13 (2001) 656-8.

[4] D. Yu, V. W. W. Yam, Controlled Synthesis of Monodisperse Silver Nanocubes in Water, Journal of the American Chemical Society, 126 (2004) 13200-1.

[5] V. Luca, D. J. MacLachlan, J. M. Hook, R. Withers, Synthesis and Characterization of Mesostructured Vanadium Oxide, Chemistry of Materials, 7 (1995) 2220-3.

[6] V. Luca, J. M. Hook, Study of the Structure and Mechanism of Formation through Self-Assembly of Mesostructured Vanadium Oxide, Chemistry of Materials, 9 (1997) 2731-44.

[7] J. Van Den Berg, A. Broersma, A. J. Van Dillen, J. W. Geus, A thermal analysis study of V2O5 and Ag0.35V2O5, Thermochimica Acta, 63 (1983) 123-8.

[8] J. Livage, Hydrothermal Synthesis of Nanostructured Vanadium Oxides, Materials, 3 (2010) 4175-95.

[9] Chemical Abstract1970.

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