Solar-thermochromism of a hybrid film of VO₂ nanoparticles and Co^II–Br–TMP complexes

Abstract

A hybrid film consisting of VO₂ nanoparticles and Co^II–Br–TMP complexes was prepared. The Co^II–Br–TMP complexes containing cobalt(II), trimethylolpropane (CH₃CH₂C(CH₂OH)₃, TMP) and bromine were selected to be combined with VO₂ nanoparticles for the first time. Evident colour-change behaviour in response to temperature change and enhanced optical performance of this composite were reported.

---

Introduction

Thermochromic materials can contribute to the enhancement of energy utilization efficiency of building environments, houses and vehicles for instance, and hence to the reduction of consumption and CO₂ emission.^1–3 In this regard, vanadium dioxide(VO₂), as the most promising thermochromic material, has received particular interest. It has a reversible phase transition from an infrared-transparent semiconducting state to an infrared-translucent metallic state and thus can partly block near infrared (NIR) light while remaining transparent when temperature increases.^4–8 However, relevant products have not been commercialized yet primarily because of problems related to performance including insufficient solar modulation efficiency (ΔT_sol), lower luminous transmittance (T_lum) and constant unsightly colour.^9–11 Numerous methods were employed to enhance the optical performance, especially (i) doping with F or Mg to widen the band gap of VO₂,^12,13 (ii) depositing a single/multi-layered antireflection structure,^14,15 (iii) preparing nanoporous structured film¹⁶ and (iv) forming biomimetic nanostructures like moth-eye¹⁷ as well. However, it is still insufficient to match the requirements for practical usage. Fortunately, the above problems can be effectively ameliorated by the introduction of Co^II–Br–TMP complexes known as a Co^II-based ligand exchange thermochromic system (CLETS) which can partly block visible light and change colour as a response to temperature change.^18–21 This letter describes, for the first time, the successful combination of such CLETS and VO₂ nanoparticles (NPs), and the preparation of the hybrid film, which presents improved optical performance and evident temperature-dependent colour change.

The CLETS used in this work consists of cobalt(II), trimethylolpropane (CH₃CH₂C(CH₂OH)₃, TMP) and bromine, and synthesis and thermochromic characterization of which have been reported elsewhere.^18,22 Briefly, the CLETS presents reversible macroscopic chromic behaviour from nearly transparent to blue during heating. The hydroxyl group in the TMP is the donor group responsible for the coordination reaction given below,^23–25

where 0 < x ≤ 2.

Hybrid film preparation

Hybrid films that contain VO₂ NPs and Co^II–Br–TMP complexes were simply prepared by two prime steps: (i) VO₂ NPs were synthesized according to our resent paper.²⁶ Typically, 1.63 g VOSO₄ was dissolved in 40 mL deionized (DI) water with continuous stirring, then 0.5 mL N₂H₄·H₂O was dropped into the solution. The pH value was then adjusted to 7 by adding NaOH solution (0.1 mol L⁻¹) resulting in a precipitate forming, followed by centrifuging and washing with DI water. Afterward, the precipitate was dispersed in 40 mL DI water with a hydrothermal treatment at 240 °C for 36 h. The obtained black slurry and 30 mL DI water, 5 mL NH₃·H₂O (28 wt%) were added to 280 mL ethanol, followed by dropping 0.5 mL TEOS into it and reacting for 4 h. Finally, the product was collected under centrifugation, washed with DI water and ethanol, and dried in an oven at 80 °C for 2 h, subsequently annealed at 600 °C for 20 min at nitrogen atmosphere and milled with alcohol and dispersant by bead mill to form VO₂ NPs dispersion. (The crystal phase and particle morphology of the VO₂ NPs were characterized by XRD pattern and TEM image respectively shown in Fig. S1.†)

(ii) The mixture of CoBr₂, TMP and tetrabutyl ammonium bromide were dissolved at a prescribed molar ratio (1 : 16 : 5) in the VO₂ NPs dispersion. In order to keep VO₂ NPs dispersed, an appropriate amount of ANTI-TERRA-U (dispersant, BYK-Chemie GmbH) was needed to form a homogeneous dispersion. Afterward, a small quantity of film-forming resin and curing agent were added into the dispersion to obtain VO₂/Co^II–Br–TMP complexes hybrid paint. Ultimately, the paint was cast onto a float glass substrate and dried at 80 °C for 1 h to finally form the hybrid film.

Thermochromism

The colour variations during heating and cooling in pure Co^II–Br–TMP complexes (CLETS for short) film and VO₂/CLETS hybrid film were visually inspected and recorded with a digital camera and a CM2600d spectrophotometer as is presented in Fig. 1. When heating to 80 °C, the pure CLETS film instantly changed its colour from colourless to blue while the colour of hybrid film shifted from yellow to green. This colour-temperature relation was perfectly reversible for both films. Fig. 1a–d reports these observations, which are more prominent in chromaticity diagram of Fig. 1e–f. The distinct colour change of the new hybrid film is extremely conductive for function exhibiting, publicity and promotion of the smart windows.

Fig. 1 Pure CLETS film (a) at 20 °C and (b) at 80 °C; VO₂/CLETS hybrid film (c) at 20 °C and (d) at 80 °C. (e and f) The chrominance of the above four samples in (a–d); (e) the chromaticity diagram and chromaticity coordinates (the positive a* value indicates reddish, negative a* value indicates greenish; the positive b* value indicates yellow, negative b* value indicates bluish); (f) the enlarged chromaticity diagram of the selected circular area in (e).

UV-vis-NIR spectra were recorded on pure CLETS film, pure VO₂ film and their hybrid films at different thicknesses (50 μm, 30 μm and 20 μm, decided by SEM pictures of cross-sections in Fig. S2†), as shown in Fig. 2. The calculated optical performance (ΔT_sol and T_lum) was summarized in Table 1. To ensure the reliability of comparison, the same amount of solid contents in the hybrid was added in the pure VO₂ film and the same thickness of 50 μm of the hybrid was adopted for the pure CLETS film in characterization (Fig. 2a).

Fig. 2 (a) UV-vis-NIR spectra at 20 °C and 80 °C of pure CLETS film, pure VO₂ film and VO₂/CLETS hybrid film. The yellow-orange area indicates the normalized values of the visible (yellow) and NIR (orange) spectra spectral irradiance, and the cyan area indicates the values of eye sensitivity function. (b) Optical transmittance spectra of the three VO₂/CLETS hybrid samples with different thicknesses at 20 °C and 80 °C.

Table 1 Summary of optical properties for pure CLETS film, pure VO₂ film and VO₂/CLETS hybrid films of different thickness (50 μm, 30 μm and 20 μm)

Sample	Luminous transmittance Tlum (%)		Solar transmittance Tsol (%)		Solar regulation efficiency ΔTsol (%)
	20 °C	80 °C	20 °C	80 °C
Pure CLETS	92.09	90.12	91.40	84.63	6.77
Pure VO2	62.12	57.83	65.10	49.04	16.06
VO2/CLETS hybrid 50 μm	62.73	55.77	66.13	45.32	20.82
30 μm	66.63	61.42	69.28	51.51	17.77
20 μm	72.56	67.49	74.48	58.77	15.71

---

The optical transmittance spectra for pure CLETS film, pure VO₂ film and VO₂/NLETS hybrid film are presented in Fig. 2a. The spectra features of the pure CLETS film and VO₂/CLETS hybrid film are in agreement with the visual observations. They exhibit notable contrasts of the transmittance around 710 nm at different temperatures, absorbing red light and presenting blue. However, it could be seen from Table 1 that the pure CLETS and the hybrid film maintain a relative high T_lum of 90.12% and 57.83% respectively at 80 °C, which is attributed to the absorption peaks staggered with the extremum of the eye sensitivity function (shown by cyan area in Fig. 2a). From Fig. 2a, we can also see that the hybrid film exhibits the characteristic peaks of both pure VO₂ film and pure CLETS film, which means that there is no chemical action between VO₂ NPs and Co(II)–Br–TMP complexes and they could friendly coexist in matrix. Additionally, the ΔT_sol of the pure CLETS (6.77%) is mainly originated from a giant transmittance contrast in the visible region (yellow area in Fig. 2a), but that of the pure VO₂ (16.06%) is predominantly based on a large absorbance in the near IR (NIR) range (orange area in Fig. 2a). As a result, the VO₂/CLETS hybrid film we prepared is capable to modulate both visible region with CLETS component and NIR range with VO₂ particles, enhancing ΔT_sol to 20.82%. The hybridizing influence is in accordance with that of the organic thermochromic material to VO₂ which has been by researched by Zhou et al.²⁷

Fig. 2b exhibits the transmittance spectra of hybrid films with different thicknesses (20 μm, 30 μm and 50 μm). It could be noticed from the spectra and Table 1 that T_lum improves from 62.73% (20 °C) and 55.77% (80 °C) to 72.56% (20 °C) and 67.49% (80 °C) while ΔT_sol drops from 20.82% to 15.71% as the VO₂/CLETS hybrid film thickness decreases from 50 μm to 20 μm. The decrease of both VO₂ and CLETS amount is responsible for the increase of T_lum and the slight weakening of ΔT_sol. The optical performance of the hybrid films with three different thicknesses, ΔT_sol = 20.82% and T_l,lum = 62.73% at 50 μm, ΔT_sol = 17.77% and T_l,lum = 66.63% at 30 μm, ΔT_sol = 15.71% and T_l,lum = 72.56% at 20 μm, is all very promising for the practical applications. Thus, with further optimization, these new hybrid films can be applied to the internal surface of a double-glazed window (for instance) for intelligent modulation of solar heat into buildings.²⁸

Morphology and composition characterization

A further thought is that what is the spatial relationship between the VO₂ NPs and the Co(II)–Br–TMP complexes. The cross-section back scattered SEM (BS-SEM) picture of VO₂/CLETS hybrid film in Fig. 3a indicates that both VO₂ NPs and Co(II)–Br–TMP complexes distribute in resin uniformly, VO₂ NPs are encased in the Co(II)–Br–TMP complexes in peculiar cases. Through the distribution of VO₂ NPs and Co(II)–Br–TMP complexes, it seems that there is no apparent interaction between them. This suggests that VO₂ NPs and Co(II)–Br–TMP complexes evenly coexist in the hybrid film almost without interference and interaction, exploiting respective advantage and covering each other's shortage. The EDS element mapping of Co, Br, C, O and V confirmed the component distribution (Fig. 3b–f). As expected, Co and Br signals are intensely strong on the light grey spots, and V signal is mainly from the white dots. Besides, the Co and Br signals are roughly overlapped but opposite to C signal, which demonstrates that most of Co(II)–Br complexes have probably formed instead of Co(II)–Br–TMP complexes turning colour to blue during measurement. This phenomenon, which is also verified by EDS line scanning patterns in Fig. S3,† is owing to increasing temperature of the film exposed to high energy electron beam bombarding during the test. Similar observation has been previously reported by others.²⁹

Fig. 3 (a) Back scattered SEM picture of the cross-section of VO₂/CLETS hybrid film, and (b–f) EDS element mapping images of Co, Br, C, O and V respectively.

Conclusions

In conclusion, this work reported a novel VO₂/Co^II–Br–TMP hybrid film that exhibits evident temperature responsive colour change from yellow to green, which facilitates the application, function exhibiting, publicity and promotion of smart windows. Meantime, the hybrid film demonstrates splendid optical properties (ΔT_sol = 20.82% and T_l,lum = 62.73%), which is extremely promising for utilitarian application on architectures. Ultimately, this study provided a new solution for optimizing and enriching the properties of VO₂-based thermochromic films.

Acknowledgements

This study was financially supported by the high-tech project of MOST (No.: 2014AA032802), the Key Research Program of the Chinese Academy of Sciences (No.: KFZD-SW-403), and the Science and Technology Commission of Shanghai Municipality (STCSM, No.: 13NM1402200).

Notes and references

1. X. J. Wei, L. P. Yu, X. B. Jin, D. H. Wang and G. Z. Chen, Adv. Mater., 2009, 21, 776–780.
2. Y. Zhou, Y. F. Cai, X. Hu and Y. Long, J. Mater. Chem. A, 2014, 2, 13550–13555.
3. A. Seeboth, J. Kriwanek and R. Vetter, Adv. Mater., 2000, 12, 1424–1426.
4. Y. F. Gao, H. J. Luo, Z. T. Zhang, L. T. Kang, Z. Chen, J. Du, M. Kanehira and C. X. Cao, Nano Energy, 2012, 1, 221–246.
5. Y. F. Gao, S. B. Wang, H. J. Luo, L. Dai, C. X. Cao, Y. L. Liu, Z. Chen and M. Kanehira, Energy Environ. Sci., 2012, 5, 6104–6110.
6. J. Wei, Z. Wang, W. Chen and D. H. Cobden, Nat. Nanotechnol., 2009, 4, 420–424.
7. J. Jeong, N. Aetukuri, T. Graf, T. D. Schladt, M. G. Samant and S. S. Parkin, Science, 2013, 339, 1402–1405.
8. M. Zhou, J. Bao, M. S. Tao, R. Zhu, Y. T. Lin, X. D. Zhang and Y. Xie, Chem. Commun., 2013, 49, 6021–6023.
9. Z. Chen, Y. Gao, L. Kang, C. Cao, S. Chen and H. Luo, J. Mater. Chem. A, 2014, 2, 2718–2727.
10. M. Li, X. Wu, L. Li, Y. Wang, D. Li, J. Pan, S. Li, L. Sun and G. Li, J. Mater. Chem. A, 2014, 2, 4520–4523.
11. M. E. A. Warwick and R. Binions, J. Mater. Chem. A, 2014, 2, 3275–3292.
12. L. Dai, S. Chen, J. Liu, Y. Gao, J. Zhou, Z. Chen, C. Cao, H. Luo and M. Kanehira, Phys. Chem. Chem. Phys., 2013, 15, 11723–11729.
13. J. D. Zhou, Y. F. Gao, X. L. Liu, Z. Chen, L. Dai, C. X. Cao, H. J. Luo, M. Kanahira, C. Sun and L. M. Yan, Phys. Chem. Chem. Phys., 2013, 15, 7505–7511.
14. G. Xu, P. Jin, M. Tazawa and K. Yoshimura, Sol. Energy Mater. Sol. Cells, 2004, 83, 29–37.
15. N. R. Mlyuka, G. A. Niklasson and C. G. Granqvist, Phys. Status Solidi A, 2009, 206, 2155–2160.
16. L. Kang, Y. Gao, H. Luo, Z. Chen, J. Du and Z. Zhang, ACS Appl. Mater. Interfaces, 2011, 3, 135–138.
17. X. Qian, N. Wang, Y. Li, J. Zhang, Z. Xu and Y. Long, Langmuir, 2014, 30, 10766–10771.
18. H. J. Byker, P. H. Ogburn, D. A. V. Griend, B. S. Veldkamp and D. D. Winkle, US Pat., US 8018639 B2, 2011.
19. T. S. Markova and O. V. Yanush, Russ. J. Appl. Chem., 2008, 81, 779–785.
20. S. Gadzuric, M. Vranes and S. Dozic, Sol. Energy Mater. Sol. Cells, 2012, 105, 309–316.
21. V. D. Nguyen and K. R. Birdwhistell, J. Chem. Educ., 2014, 91, 880–882.
22. H. J. Byker, P. H. Ogburn, D. A. V. Griend, B. S. Veldkamp and D. D. Winkle, US Pat., US 8431045 B2, 2013.
23. A. Seeboth, R. Ruhmann and O. Muehling, Materials, 2010, 3, 5143–5168.
24. D. E. Scaife and K. P. Wood, Inorg. Chem., 1967, 6, 358–365.
25. X. Wei, L. Yu, D. Wang, X. Jin and G. Z. Chen, Green Chem., 2008, 10, 296–305.
26. J. Zhu, Y. Zhou, B. Wang, J. Zheng, S. Ji, H. Yao, H. Luo and P. Jin, ACS Appl. Mater. Interfaces, 2015, 7, 27796–27803.
27. Y. Zhou, Y. Cai, X. Hu and Y. Long, J. Mater. Chem. A, 2015, 3, 1121–1126.
28. C. A. Balaras, A. G. Gaglia, E. Georgopoulou, S. Mirasgedis, Y. Sarafidis and D. P. Lalas, Build. Environ., 2007, 42, 1298–1314.
29. C. D. Gu and J. P. Tu, RSC Adv., 2011, 1, 1220–1227.

---

Footnote

† Electronic supplementary information (ESI) available: The XRD pattern and TEM image of the VO₂ nanoparticles we prepared, cross-section SEM pictures of VO₂/CLETS hybrid films with different thicknesses and EDS line scanning patterns. See DOI: 10.1039/c6ra14232j

---