Efficient cycling utilization of solar-thermal energy for thermochromic displays with controllable heat output

Abstract

Use of solar energy as a source of heat is an important method of storing and providing clean energy for thermal management. However, the difficulties associated with combining the high-energy storage and high-rate heat release of solar thermal storage (STS) lower their ability to output heat controllably, and thus, prevent their application in temperature-sensitive or temperature-responsive systems. Herein, we report for the first time the closed-cycle utilization of photo-thermal energy for thermochromic displays by optimizing the solid-state high-rate heat output of STS films. By controlling the molecular interaction, a tri-azobenzene (Azo)-based templated assembly can be made to combine a maximum energy density of 150.3 W h kg⁻¹, a long half-life (1250 h), and a high power density of 3036.9 W kg⁻¹. The STS film can induce a reversible color change in a complicated thermochromic-patterned display by releasing heat to increase the temperature by 6–7 °C. We also realize variable heat release by controlling the heating rate and temperature to utilize photo-thermal energy efficiently. Efficient cycling utilization of photo-thermal energy using a tri-Azo assembly could be used to harness photo-thermal power for thermal management.

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Introduction

Solar thermal energy systems are one of the promising technologies for harnessing solar energy to generate heat by converting light into storable chemical energy followed by controllable release of heat.^1–11 Azobenzene (Azo) and its derivatives show great potential for solar thermal storage (STS) due to their trans (E)–cis (Z) reversible isomerization induced by light, heat and voltage.¹² However, Azo usually shows a low theoretical enthalpy per mole of chromophore (ΔH), a short half-life (τ_1/2) of Z-isomers and limited isomerization in the solid state.¹³ Many researchers spent much effort on increasing ΔH and τ_1/2 by introducing various substituents (chemical groups, bridging), designing multi-branched structures or devising templated assemblies (close-packing).^{5,13,14,16,17,24,26} However, it's difficult to achieve both a high ΔH and a long τ_1/2 in a single Azo molecule just by changing the substituents. Specifically, the ortho-substitution stabilizes the Z-isomer with a high barrier but the molecule has a lower ΔH. The push–pull groups increase the difference in dipole moments between the E and Z-forms, resulting in a high ΔH but a fast thermal reversion.¹⁴

One of the methods for improving ΔH and τ_1/2 is optimizing inter/intramolecular forces (the number and bond strength).¹⁵ This effect depends on the substituents (electronic effect, the number and the position), molecular orientation and distance (assembly) and steric configuration.¹⁴ Compared with the single molecule, multi-branched Azo molecules show a remarkable increase in ΔH and τ_1/2 due to intramolecular hydrogen bonds (H-bonds) and steric hindrance.^16–19 Previous studies demonstrated that the Z-isomer of azo units shows an increasing τ_1/2 (τ_{1/2 tris} > τ_{1/2 bis} > τ_{1/2 mono}) with a large chromophore size and an increasing number.^16–18 For bis-Azo molecules, two azo units with a bent structure show a higher ΔH than that of linear structures with a relatively long τ_1/2 due to the limited isomerization.^20,21 And the bent structure also enables a high-degree of isomerization compared with the linear structure.^20,21 Based on these results, a close-packed orderly assembly of multi-branched Azo is considered as one of the ideal molecularly templated candidates for high-energy and stable STS.²² Such a multi-branched Azo assembly favors the formation of intra/intermolecular interactions to increase the energy difference (ΔH) between E- and Z-isomers. Moreover, the assembly also enables controllability of E ↔ Z isomerization of different azo units (free or limited) by changing the molecular structures. This assembly shows great potential for absorbing light and releasing storable heat by increasing the degree and rate of isomerization.²³

Besides molecular engineering for high energy density, high-rate solid-state heat release and reversible temperature control are another two challenges for developing the application of STS. One of the core problems limiting fast solid-state heat release is a low rate and degree of isomerization due to steric hindrance.²⁴ The solid-state STS film usually releases a small amount of heat in a short time (induced by high temperature) or at a very low rate for a long time (induced by visible light).^16–19 Thus, a high heat-loss due to the transfer to the ambient atmosphere results in difficulty in collecting thermal energy for a high increase in temperature. The STS film releases heat for a temperature increase of only 2 °C induced by high-temperature (100 °C) due to its low power density.⁶ The low-rate heat release in a mild environment (25–60 °C) lowers the utilization efficiency of the STS film. Thus, exploring heat release at a relatively low temperature is practically useful for developing its application in thermal management.^{16–19,25–27}

An effective and feasible method to control the reversible temperature change by releasing heat is an important step for developing this application of STS. Although systematic investigation on the relationship between the performance (energy density & power density) and isomerization (degree and rate) gives an insightful understanding of charging and discharging processes, few related studies are reported. Furthermore, heat release of STS at different rates needs to be controlled for different temperature increases. Until now there have been no reports on controlling or inducing reversible thermochromic displays using heat-releasing STS based on multiple photo-thermal energy cycles.

Herein, we design a closed-cycle utilization of photo-thermal energy using a tri-Azo-assembly PTF film for a thermochromic display based on light-harvesting (photo “charging”), storage, and heat release (“discharging”). The isomerization degree during the photo “charging” and “discharging” processes is controlled by the grafting density of the tri-Azo on a reduced graphene-oxide (rGO) template.²⁸ The STS film exhibits an excellent ability to store high levels of energy for >50 days and releases >80% of the stored heat at different rates. A controllable and continuous heat output from the tri-Azo/rGO film enables a reversible color change of thermochromic patterns for displays. The temperature change is also tracked to investigate the utilization efficiency and cycling performance of the film as a source of solar-thermal power.

Results and discussion

Grafting density for molecular engineering

The rGO supports the close-packed orderly tri-Azo via covalent bonds. The grafting degree and suitable steric configuration of tri-Azo are crucial for high energy density and good storage stability. Compared with other modes of covalent grafting on the boundary of GO via ester and amide bonds,²⁰ the diazo coupling reaction enables high-density grafting of tri-Azo throughout the nanosheet.⁴³ Thus, rGO with a partially restored conjugated structure and good dispersibility in water is one of the excellent assembled templates for high-density grafting to enable intermolecular H-bonds and steric hindrance.

The surface morphologies of rGO and tri-Azo/rGO were observed using scanning electron microscopy (SEM, Fig. S2†) and transmission electron microscopy (TEM, Fig. S3†). The single-layer rGO shows a smooth silk-like structure (with a size of 4–5 μm, Fig. S3†) with a hexagonal lattice. The partially reduced conjugated structure with good dispersibility provides a two-dimensional nano-template for the grafting of the tri-Azo. The close-packed tri-Azo that connects to its face increases the surface roughness of the nanosheets. As a result, tri-Azo/rGO exhibits a non-hexagonal pattern, indicating that the nano-template is covered by organic addends of tri-Azo (Fig. 1a and b).²⁹ This modification not only enables good dispersion in organic solvents such as N,N-dimethylformamide, ethanol, and H₂O but also favors bundling leading to molecular interaction in the solid state.

Fig. 1 The properties of the tri-Azo/rGO powder. (a) The high-resolution TEM image of tri-Azo/rGO. (b) The low-resolution TEM images and molecular structure of tri-Azo/rGO. (c) UV/vis absorption spectra of tri-Azo/rGO with different grafting densities (1/60, 1/68, 1/90, and 1/120), the degree of isomerization is calculated using eqn (S2),† giving results of 53.5%, 72.6%, 76.4%, and 83.0% respectively. (d) DSC first heating traces of tri-Azo/rGO with different grafting densities (1/60, 1/68, 1/90, and 1/120), with tri-Azo_1:68/rGO showing the maximum energy density of 150.3 W h kg⁻¹. (e) Energy density versus power density of tri-Azo/rGO with different grafting densities: , irradiated with blue light; , induced by heating at 1 °C min⁻¹, 10 °C min⁻¹, 20 °C min⁻¹, and 40 °C min⁻¹. The tri-Azo/rGO exhibited a tunable and high-power density of 3036.9 W kg⁻¹. (f) Energy density, power density, and half-life of different STS. : tri-Azo/rGO addressed in this study; : bis-Azo-based hybrid;⁷ mono-Azo-based material.^7,9,17,24

The E ↔ Z reversible isomerization of tri-Azo is essential to closed-cycle light harvesting, energy storage, and heat release. We found that the degree and rate of isomerization of the assembly are tuned by adjusting the density of tri-Azo on rGO based on a trade-off between the close-packing and free volume. Specifically, the high-density tri-Azo favors the formation of molecular H-bonds for increasing ΔH, but steric hindrance lowers the isomerization degree and results in poor cycling performance.^30–32 To the best of our knowledge, there have been few studies that have focused on the effects of grafting density on the storable and releasable energy (energy density) and heat release rate (power density) of the STS.

The covalent linkage of tri Azo to rGO is demonstrated by the –N=N– stretching in Fourier-transform infrared spectroscopy (FTIR, Fig. S4†) and three bands of –N=N– (401.7 eV), –N–CO– (399.7 eV) and C–N (284.8 eV) in X-ray photoelectron spectroscopy (XPS, Fig. S5†).^33–35 The value of I_D/I_G increases from 0.92 (rGO) to 1.14–1.27 (tri-Azo/rGO) in the Raman spectra (Fig. S6†).³⁶ Furthermore, the grafting density is measured comprehensively based on the element content and thermal stability (Fig. S7 and eqn (S1)†).³⁷ Although the three-branched steric configuration leads to problems with high-density covalent attachment, by optimizing diazotization, we can attain a series of tri-Azo/rGO assemblies with a grafting density from 1 : 150 (number of tri-Azo units to the number of carbon atoms) to 1 : 60 (Table S1†).⁴³ When the density is higher than 1 : 68, two adjacent Azo units can form intermolecular H-bonds with an appropriate molecular distance (4–5 Å) and steric configuration.^30,38 Molecular H-bonds are indicated by the red-shifted bands of S=O and O–H (Fig. S3†).³⁹ Due to the H-bonding and steric hindrance, the tri-Azo on rGO exhibits different abilities (degree and rate) to isomerize or reverse between the E- and Z-isomers under the stimuli.^{23,30–32,46}

Isomerization degree and stability

The amount of storable energy (“chargeability”), the storage period (“stability”), and amount of releasable heat (“dischargeability”) are vital for realizing the application of STS. The E-to-Z isomerization degree of tri-Azo determines its chargeability for light harvesting. The isomerization is characterized by the decrease in the intensity of the π–π* transition of the tri-Azo (at 380 nm) in the UV-vis absorption spectra (Fig. 1c and Table S2†) and the integration of the signals of the cis-isomer in ¹H-NMR (Fig. S8†).^40,41 According to the calculation (eqn (S2)†), four tri-Azo/rGO assemblies with densities of 1 : 60, 1 : 68, 1 : 90, and 1 : 120 (tri-Azo_1:60/rGO, tri-Azo_1:68/rGO, tri-Azo_1:90/rGO, and tri-Azo_1:120/rGO) exhibit an increasing E-to-Z isomerization degree of 53.5%, 72.6%, 76.4%, and then 83.0% (Table S2†) in the photo-stationary state after UV irradiation for 120, 80, 60, and 50 s, respectively. In particular, a drop in the grafting density (from 1 : 60 to 1 : 68) leads to a remarkable 19.1% increase in the isomerization degree. This indicates that the E-to-Z isomerization (“charging”) is seriously influenced by the molecular H-bonds and steric hindrance.^30–32,46 Thus, a slight increase in the free volume dramatically increases the degree of structural transformation. This analysis is also confirmed by the tri-Azo_1:68/rGO exhibiting a first-order rate constant of 1.74 × 10⁻⁴, which is much higher than that of tri-Azo_1:60/rGO (1.00 × 10⁻⁵), as shown in Fig. S9.†³⁸ Thus, based on optimizing the molecular interaction, the sacrifice of a certain degree of grafting density favors light harvesting and enhances the “chargeability” of tri-Azo/rGO STS. After light harvesting, the spontaneous Z-to-E isomerization in darkness implies the storage period (“stability”) of a STS. The reversion is tracked by time-evolved absorption (Fig. S10†). The close-packed assembly stabilizes the Z-tri-Azo isomers on rGO with long half-lifes (τ_1/2 > 1100 h) due to steric hindrance relative to randomly dispersed molecules (τ_1/2 = 87 h). As shown in Table S2,† the increase in the grafting density from 1 : 120 to 1 : 60 results in long half-lifes of 1100–1300 h. The resulting thermodynamically stable Z-tri-Azo enables a long-term photo-thermal storage with a very low rate of self-release of 0.08% h⁻¹, which is two orders of magnitude lower than that of tri-Azo (1.15% h⁻¹).

Energy density and power density

The “dischargeability” of STS is reflected by the releasable heat (the amount and the rate) in the solid state (powder) under different stimuli. Heat released by the tri-Azo/rGO assembly is tested by differential scanning calorimetry (DSC).⁴² The releasable heat depends on ΔH and the degree of isomerization (DI_E-to-Z × DI_Z-to-E), which can be tuned by adjusting the grafting density of the tri-Azo on the rGO.^23,38 As shown in Fig. 1d, tri-Azo/rGO assemblies with different grafting densities exhibit an obvious exothermic heat flow of >80% of storable heat at 80–125 °C at a heating rate of 10 °C min⁻¹. Those assemblies with a high grafting density release more heat than those with a low density. As a result, the tri-Azo_1:68/rGO assembly exhibits a maximum energy density of 150.3 W h kg⁻¹, which is 33.9% and 99.2% greater than the 1 : 90 and 1 : 120 assemblies, respectively. This high energy density arises from the combination of a high ΔH (stabilizing E-tri-Azo by H-bonds) and a relatively high isomerization degree (72.6% for DI_E-to-Z and 87.9% for DI_Z-to-E (Table S2†)). The red-shifted bands of S=O and O–H (Fig. S4†) and the weak endothermic peak during the first cooling (Fig. S11†) indicate the formation of H-bonds between two adjacent E-tri-Azo molecules.⁴⁵ The sufficient intermolecular space results in a relatively high degree of isomerization.^30–32 It is noticeable that, when the grafting density increases to 1 : 60, the energy density falls to 137.9 W h kg⁻¹, which is lower than that of tri-Azo_1:68/rGO. This can be attributed to the decrease in the isomerization (53.5% for DI_E-to-Z and 85.4% for DI_Z-to-E (Table S2†)), limited by the intermolecular steric hindrance.

The heat-release rate under different stimuli (“discharging” mode) determines the power density of solid-state STS.⁴² The variability of heat release as a function of temperature obeys Arrhenius kinetics. However, the isomerization also depends on molecular interaction and steric hindrance (free volume), for example in the solid state or in solution. The tri-Azo/rGO film with different microstructures (ordered packing or random dispersion) and densities (close-packing) exhibits different rates of isomerization (heat release) at the same temperature. Thus, it's meaningful to investigate the variable heat output and cycling performance of the tri-Azo/rGO film induced under different stimuli (thermal-irradiation and light). Fig. S12† shows the exothermic heat flow for different ranges (55–160 °C) induced by heating at a rate of 1–40 °C min⁻¹. A high heating rate leads to a fast heat release (“discharge”) over a narrow temperature range (1–2 min at 40 °C min⁻¹ and 20–23 min at 1 °C min⁻¹, Table S3†). When induced by heating at 40 °C min⁻¹, the tri-Azo_1:68/rGO assembly exhibits a high power density of 3036.9 W kg⁻¹ (Fig. 1e and Table S4†). Importantly, when induced at 40 °C min⁻¹, the releasable heat (energy density) of all the tri-Azo/rGO assemblies falls by 25–35% compared with that at 10 °C min⁻¹ (Fig. 1e and Table S4†). Moreover, the loss of heat release is clearly higher in the case of the assembly with high grafting density. Specifically, the tri-Azo_1:68/rGO exhibits a 34% decrease in the energy density, which is higher than that of tri-Azo_1:90/rGO (30.2%) and tri-Azo_1:120/rGO (27%). This effect is attributed to a decrease in the isomerization degree (Table S6†) over a relatively short release time (1–3 min). This transformation, affected by the steric hindrance, is more serious in a solid-state film due to the bundling effect.⁴⁶

Any improvement in the photo-thermal performance, including the energy density, storage period (half-life), and power density, mainly relies on molecular engineering (ΔH, orderly assembly) and E–Z isomerization (free volume and interaction) for light-harvesting and heat release.^30–32 By optimizing H-bonds and steric hindrance, the tri-Azo_1:68/rGO template assembly exhibits an excellent photo-thermal utilization ability with a high density (150.2 W h kg⁻¹), excellent thermal stability (1250 h) and high-rate heat release (3036.9 W kg⁻¹). The comparison with the related STS with different molecules or assemblies is shown in Table 1.^{5–10,16,20,24,26,27,38,46} The tri-Azo_1:68/rGO assembly outperforms all other STS (for example: anthracene 111.1 W h kg⁻¹; NBD 20.4 W h kg⁻¹ and 6.3 h; Azo-polymer 67.7 W h kg⁻¹ 27.8 h). Thus, the tri-Azo/rGO exhibits great promise as a solar-thermal power source, being able to supply heat when subjected to a range of stimuli.

Table 1 Energy density of STS

		ΔH (kJ mol^−1)	Energy density (W h kg^−1)	Power density (W kg^−1)	τ 1/2 (h)	Ref.
Molecule
1	Anthracene	72.3	111.1			8
2	9-Styrylacridine	5.0	4.97	22.95		9
3	2,3,5,6-Tetramethylstyrylacridine	55.0	45.33	209.1		10
4	NBD-H2	85.3	20.4		6.3	6
5	Diaryl-substituted norbornadienes	81.4	110	942.86	5	7
6	Azo	71.7	41.14	411.4		38
7	Liquid azobenzene derivative	51.8	47	352.5		5
8	Azo-compound	87.0	30	211.76		27
9	Bis Azo	80.9	32	768	0.83	20
10	Tri-Azo	135.1	40.7	813	87	Our paper
Hybrids
1	Diacetylene–azobenzene polymer	176.2	67.7	1354	27.8	24
2	Azo/SWCNT	92.0	56	480	0.5	46
3	Azo-phase-change hybrids	79.3	55.6			26
4	Azo-alkyl polymer	89.0	26	156	55	16
5	Azo-rGO	341.8	138.08	828.48	1248	38
6	rGO-bis Azo	582.9	131	2517	900	20
7	Tri-azo/rGO	943.0	150.8	3036.9	1250	Our paper

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Solid-state STS films for thermochromic displays

A high-rate Z-to-E isomerization (“discharging”) in a solid-state film is a great challenge affecting the heat output from STS. To date, no studies of thermochromic displays controlled by STS films have been reported due to the difficulty in releasing a large amount of heat at a high rate. We prepared several uniform self-supported tri-Azo/rGO films via a solution-processing method.⁴⁴ The thickness can be tuned within a range of 20–30 μm to ensure good light harvesting (3–5% transmittance at 365 nm) and sufficient volume for the isomerization.

The releasable heat of a solid-state STS film relies on the microstructure and heating rate. The amount and rate (“discharging”) of the heat released by a tri-Azo/rGO film were measured by DSC.⁴² The tri-Azo/rGO film releases 27–35% less heat than the powder (Table S7,†Fig. 2a and b). This decrease in the amount of heat release arises from a low degree of Z-to-E isomerization (Table S7†) due to steric hindrance (bundling effect).^30–32,46 This effect is more serious in the case of high-rate “discharging.” The film exhibits a high-speed exothermic flow over a narrow temperature range (95–155 °C) at 10 °C min⁻¹, relative to that (55–80 °C) at 1 °C min⁻¹ (Fig. 2a). The change in the degree of isomerization is demonstrated by time-evolved absorption (Fig. 2d and Table S7†). For the film, at 10 °C min⁻¹, only 56.5% isomerization can be used for the “discharging,” which is 7.5% lower than 1 °C min⁻¹. In particular, at a high heating rate, the degree of isomerization further decreases to 64% and 56.5%. (Table S7†).⁴² As a result, the tri-Azo/rGO film exhibits an energy density of 108.3 W h kg⁻¹ and 98.6 W h kg⁻¹, induced by heating at 1 °C min⁻¹ and 10 °C min⁻¹, respectively.⁴² Despite this, the tri-Azo/rGO STS film enables a high-rate heat release (14.3% min⁻¹) at 95–155 °C. This high-rate heat release provides important information on how to control the output of the STS film.

Fig. 2 The properties of the tri-Azo/rGO film. (a) DSC traces of the tri-Azo_1:68/rGO powder and film at a heating rate of 1 °C min⁻¹ and 10 °C min⁻¹. (b) Energy density of the tri-Azo_1:68/rGO powder and film at different heating rates of 1 °C min⁻¹, 10 °C min⁻¹, 20 °C min⁻¹, and 40 °C min⁻¹. (c) Time-evolved absorption spectra of tri-Azo/rGO for different conditions (before/after UV-irradiated. Heating rates: 1 °C min⁻¹, 10 °C min⁻¹). (d) Time-evolved absorption spectra of tri-Azo/rGO films under different conditions (before/after UV-irradiated. Heating rates: 1 °C min⁻¹, 10 °C min⁻¹). (e) Time-evolved absorption spectra of the tri-Azo film during the heating process after UV irradiation. The temperature and time correspond to those shown in (e). (f) UV/vis absorption spectra show the “discharge” of the tri-Azo/rGO film with low temperature (5 °C) at different time stages induced by blue light.

Heat release at a relatively low temperature of the STS film is also important for this application. According to Arrhenius kinetics, tri-Azo/rGO shows a slow E–Z isomerization at a low temperature. This effect is favorable for long-term energy storage, but the heat could be released only by blue-light irradiation. The photo-charging time, τ_1/2 and power density of the tri-Azo/rGO film at 0 °C, 5 °C and 25 °C are given in Table S8.† The tri-Azo/rGO film shows a lower power density (10 W kg⁻¹) and a longer half-life (1630 h) at 0 °C than that at 5 °C and 25 °C. And the time-evolved isomerization induced by blue light at 5 °C and by a hot plate is shown in Fig. 2e and f, and blue light is able to induce heat release of the STS film even at 5 °C. But a long-term heat release lowers the temperature change because of heat transfer to the environment. Thus, a high-rate “discharging” is essential to improve the power density of the STS for heat output.

A closed cycle of light harvesting (“charging”), storage, and then heat release (“discharging”) by a STS film was investigated to demonstrate its ability to utilize photo-thermal energy for a temperature rise. A uniform STS film (30 mm × 30 mm × 25 μm, transmittance at 365 nm: 3.17%) was prepared. The right half (30 mm × 15 mm) was irradiated with UV light to “charge” it (12 h), while the left half was covered by a mask. After being stored in darkness for 24 h, film-I was heated on a hot plate, from 40–63 °C, at a rate of 1 °C min⁻¹ and then held at 63 °C for 23 min (Condition I). Film-II was heated from 40–77 °C and then held at 77 °C for 37 min (Condition II). The temperature difference (ΔT) between the left and right half was tracked using a high-resolution (±0.03 °C) IR thermal-imaging camera.

The time-evolved ΔT under two conditions is summarized in Fig. 3. The tri-Azo/rGO STS film starts to release heat at 50–55 °C, which is consistent with the DSC results. When heating to 63 °C and 77 °C (Stage one), the film exhibits (Fig. 3a and b) a maximum ΔT of 2–3 °C under Condition I and 6–7 °C under Condition II. This indicates that the heat output of the STS film increases the temperature. STS of previous studies reported a fast heat release for a ΔT of 2 °C based on high temperature (>100 °C).¹⁶ Based on high-energy storage and controllable isomerization, we realize a high-rate heat output with a ΔT of 2–3 °C at a relatively low temperature (55–63 °C) with only 24.0% isomerization degree below 63 °C. Subsequently, the film maintains a ΔT of 2–3 °C and 6–7 °C over the following 23 and 37 min when the hot plate is held at 63 and 77 °C (Stage two), respectively. The heat release in Stage two is confirmed by the time-evolved absorption spectra (Fig. 3c and d). A low-temperature-induced solid-state heat output of the hybrid film (rather than powders) is important for realizing this application.

Fig. 3 Solid-state controllable heat release of the tri-Azo_1:68/rGO film under (a and c) Condition I and (b and d) Condition II, inducing temperature changes tracked by IR. The IR image of the tri-Azo_1:68/rGO film after Stage one. The color bar indicates the relative magnitude of the temperature. The difference in the average temperature between the left and right sides is 2.5 °C (Condition I) and 6.4 °C (Condition II), respectively. Temperature changes in the tri-Azo_1:68/rGO film are induced by the solid-state heat release. The film exhibits a maximum ΔT of 2–3 °C and 6–7 °C in Stage two (in which the temperature is held constant at 23 and 37 min, respectively). Time-evolved absorption spectra of tri-Azo_1:68/rGO films after different stages. And the average temperature of (e) the film with a heating rate of 0.5 °C min⁻¹ and (f) the film with a heating rate of 5 °C min⁻¹.

We also investigated the temperature change at different heating rates (0.5 °C min⁻¹, 5 °C min⁻¹, Fig. 3e and f). The corresponding temperature is traced by a high-resolution IR image. A high heating rate results in fast heat release of the tri-Azo/rGO film. Specifically, Stages one and two (Fig. 3e and f) take 80 and 8 min at 0.5 °C min⁻¹, 5 °C min⁻¹, respectively. Moreover, ΔT remarkably increases with increasing heating rate due to low heat exchange. As indicated by Table S3,† the tri-Azo/rGO film realizes a ΔT of 1.6 and 8.2 °C at 0.5 °C min⁻¹ and 5 °C min⁻¹, respectively. ΔT controlled by different-rate heat release points to the possibility of operating thermochromic displays and thus realizing thermal management. In addition, we examined the temperature change of the tri-Azo/rGO STS film without UV irradiation, in which case no ΔT was detected (Fig. S13†).

This result indicates that ΔT is caused by the heat release from the charged right half, rather than by any other reaction. The tri-Azo/rGO film with the ability to increase the temperature by releasing heat points to its great potential for use as a photo-thermal power source. The heat output from the tri-Azo_1:68/rGO film as a photo-thermal power source for a thermochromic display was characterized. A schematic illustration of this closed cycle is shown in Fig. 4a. Two kinds of reversible thermosensitive ink (rare earths) with thermochromic temperatures (CT) of 65 °C (Ink I: red → colorless) and 80 °C (Ink II: blue → colorless) were used to display the temperature change and thus evaluate the utilization of photo-thermal energy. The color change of the two thermochromic inks at the CT (65 °C and 80 °C) can be selectively induced and tuned by releasing heat from the STS film. Then, a two-dimensional code and the Tianjin University logo were printed using the two inks on a thin polypropylene film (transmittance at 365 nm of 85%) for display, which was pasted onto the tri-Azo_1:68/rGO films to form a multilayer structure (Fig. 4a). As shown in Fig. 4b, after UV-irradiation, when the temperature of the hot-plate reaches 63 °C in Stage one, the film displays the blue two-dimensional code and the Tianjin University logo without the red background, indicating that >98% of Ink I (area ratio) in the right-hand pattern changes from red to colorless. Meanwhile, the red background on the left is also observed, indicating that the temperature of the film is lower than the CT. Ink II shows no color change until the temperature reaches 76 °C. At this temperature, the blue two-dimensional code and Tianjin University logo disappear, indicating that >99% (area ratio) of Ink II in the right-hand pattern becomes colorless, while there is no change in the blue pattern on the left-hand side. The tri-Azo_1:68/rGO film is able to release heat resulting in a ΔT of 2–3 °C (Condition I) and 6–7 °C (Condition II) for the thermochromic display. Thus, in Stage one, when the hot-plate temperature was lower than CT, both Inks I and II exhibited obvious color changes because the temperature was increased by the heat output from the STS film. In Stage two (in which the temperature of the hot plate was maintained at 77 °C), the right-hand side also remained colorless for 40 min due to the continuous heat release. In Stage three, after the hot-plate was turned off, >50% of Ink II remained colorless for 7 min, while the temperature difference between the film and the ambient atmosphere was >3 °C. Subsequently, the film gradually became blue again, and then became red in the next 15 min. The thermochromic display of many complicated patterns at temperatures under CT can also be attained by controlling the rate of the heat output of the tri-Azo/rGO STS film (Fig. 4c). This indicates that the thermochromic display exhibits universal applicability. The utilization of photo-thermal energy based on the tri-Azo/rGO STS film had an excellent cycling performance over 100 closed cycles without degradation (because of alternate irradiation with UV light (365 nm) and blue light (430 nm) or heating at 80 °C as shown in the ESI data (Fig. S14†)).

Fig. 4 A thermochromic display controlled by heat release from the STS film. (a) Schematic illustration of the thermochromic display device with a thermochromic ink pattern on a polypropylene film on a hotplate. The “R” area is photo-induced (“charged”) by UV light, while the “L” area is covered by a mask. The patterns printed using two thermochromic inks with a CT of 65 °C and 80 °C, respectively, are used to indicate the change with temperature from red (Ink I) or blue (Ink II) to colorless. (b) The color of the two-dimensional code and Tianjin University logo changes at 63 °C and 76 °C, which is 2–4 °C lower than the CT, respectively. The thermochromic display is induced by solid-state heat output of the tri-Azo_1:68/rGO STS film. The thermochromic pattern returns to its original color upon cooling. The reversible thermochromic display controlled by the STS film points to the possibility of utilizing photo-thermal energy. (c) Complicated patterns printed using two inks for thermochromic displays controlled by heat output from the STS film.

The utilization of photo-thermal energy for heat output is calculated using eqn (1), where Q is the heat output for “discharge”; E is the light energy for “charge”; C is the specific heat (C_tri-Azo/rGO is 0.8 J g⁻¹ °C⁻¹ and C_PP is 1.9 J g⁻¹ °C⁻¹); m is the weight (m_tri-Azo/rGO is 2 g and m_pp is 1 g); ΔT_d is the temperature difference; P_I is the power density of UV light for irradiation (10 mw cm⁻²); A is the area (30 mm × 15 mm) and t is the irradiation time (0.5–12 h).

The heat output from the STS film increases the temperature of a thermochromic display. By optimizing light-harvesting and heat release, the tri-Azo_1:68/rGO STS film exhibits a photo-thermal conversion efficiency of 10.2% (Table 2).

Table 2 The PE and cycling performance of STS controlled by different conditions^a

STS	t (h)	DIE-to-Z (%)	ΔT (°C)	PE (%)	Cycling	Loss	Ref.
a t is the UV irradiation time; DIE-to-Z (%) is the degree of trans-to-cis isomerization.
Tri-azo/rGO	0.5	23.6	2.3	10.2	100	0.24%	Our paper
	1	33.5	4.2	9.27
	3	47.1	6.2	4.31
	6	50.8	6.7	2.1
	12	53.0	7.1	0.9
DHA/VHF	n DHA = 5 mL h^−1, αVHF = 81.3%			0.13	70	0.18%	11
NBD/QC	α QC = 60%, ΔT = 17 °C			0.9	100	0.20%	3

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The efficiency can be further tuned by changing the irradiation time and the heating rate. It's worth noting that a relatively short-time “charging” inevitably lowers the storable heat, which can also be controlled by heating rates for “discharging”. In this paper, the tri-Azo/rGO film (25 μm, 3–5% transmittance) is charged by low-intensity UV light (10 mW cm⁻²). Increasing the intensity of incident light reduces the irradiation time for charging and vice versa. The film UV-charged for 6 h releases heat to achieve a ΔT of 6.7 °C for thermochromic displays. When photo-charging the film for 0.5–6 h, the tri-Azo_1:68/rGO STS film exhibits a PE of 10.2–2.1%, which is higher than those of previous studies even though PE is calculated by different methods.^3,11 The PE only exhibits a 1.12% decrease after storage for 30 d regardless of how low the temperature drops (Fig. S15b†). This performance basically outperforms solar thermal technology (solar collectors). The collector needs to accumulate (at high temperature) and transfer heat into an engine or other equipment (thermoelectric conversion systems) to generate usable energy. However, this technology exhibits a high-rate of heat self-release of about 49% per day, meaning that half of the light energy is lost during storage for 1 day, especially at very low ambient temperatures.⁴⁷ Photo-thermal utilization indicates that the tri-Azo_1:68/rGO STS film combining high energy and long-term storage with a high-rate heat output is an excellent photo-thermal source of power for supplying heat continuously. The thermochromic display induced by the heat output by a STS at different rates points to the possibility of controlling the heat release and thus realizing thermal management.

Conclusion

A tri-Azo/rGO assembly film was demonstrated as a source of solar-thermal energy for application in thermochromic displays by outputting solid-state heat based on a closed cycle of light harvesting, energy storage, and heat release. The heat released from a STS film can be improved by increasing the degree of E–Z isomerization of tri-Azo on rGO with different grafting densities during photo “charging” and “discharging.” By optimizing the molecular interaction and steric hindrance, the tri-Azo_1:68/rGO assembly exhibits a high energy density of 150.3 W h kg⁻¹ and a high power density (3036.9 W kg⁻¹). The film can release 23.6–69.7% of the stored heat which results in a temperature increase of 2–7 °C. The heat output is applied to a thermochromic display by effectively inducing color changes in patterns at temperatures below the CT. The high-resolution and broad applicability of the thermochromic display indicate a uniform heat release from the STS film at different rates. As a result, the tri-Azo_1:68/rGO film reaches a high utilization efficiency of 10.2%, even after storage at temperatures as low as 0 °C for 5 d. This closed-cycle efficient utilization provides useful insights into the development of STS films as an excellent source of photo-thermal power for thermal management.

Experimental section

Materials

Unless otherwise noted, the reagents were obtained from commercial sources and used without further purification. Graphite with a particle size of 20 mm was obtained from Qingdao Huarun graphite Co. Ltd., China. All other chemicals were purchased from Sigma-Aldrich, USA.

Synthesis of tri-Azo/rGO PTF

Details of the experimental and computational methods can be found in the Supplementary methods within the ESI.†

Characterization of the morphology and chemical structure of tri-Azo/rGO PTF films

The chemical structure of the films was characterized by using FTIR spectra recorded on a Tensor 27 spectrometer (Bruker, USA) with a disc of KBr. ¹H NMR spectra were collected on a 500 MHz spectrometer (INOVA, Varian, USA) with trimethylsilyl as an internal standard. High-resolution mass spectrometry (HRMS) was performed on an APEXIV 4.7 T Fourier-transform ion cyclotron resonance mass spectrometer (Bruker Daltonics, USA). X-ray photoelectron spectroscopy (XPS) analysis was performed on a surface analysis system (PHI 1600, ULVAC-PHI, Japan) with a 450 W MgKα source. Thermal analysis was performed with a thermogravimetric analyzer (TGA; STA 449C, NETZSCH, Germany) under the protection of 50 mL min⁻¹ nitrogen purging at a heating rate of 5 °C min⁻¹ from 30 to 800 °C. The morphology analysis was performed by using a transmission electron microscope (TEM; Tecnai G2 F20, FEI, USA) as well as a scanning electron microscope (SEM; S-4800, Hitachi, Japan). Time-evolved UV/Vis absorption spectra of tri-Azo and tri-Azo/rGO hybrids (1 cm path-length quartz cuvettes, DMF) were measured using a UV/Vis spectrophotometer (330, Hitachi, Japan). The tri-Azo/rGO hybrids were irradiated using a mercury lamp (Beijing Changtuo Technology Company, China) at 500 W, with a filter set at 365 nm or 450 nm as the light source positioned at a distance of 20 cm from the samples at 30 °C. The cis-to-trans thermal reversion process was investigated using a UV/Vis spectrophotometer after the samples were stored in darkness, covered with foil. Alternatively, the samples were irradiated with visible light at 450 nm at room temperature, while the rate of thermal reversion was calculated based on the change in intensity of the transition bands of Azo on rGO. STS films caused by heat release were measured using an IR thermal imaging camera (Fluke TiX640 Expert HD, USA).

Preparation of the tri-Azo/rGO film

100–300 mL tri-Azo/rGO DMF (0.3 mg mL⁻¹) was filtered in a vacuum with a polyvinylidene fluoride filter (Millipore, USA) and dried to obtain a solid-state PTF film with a thickness of 20–30 μm.

UV irradiation for energy storage

(1) Solution: tri-Azo/rGO in an acetonitrile solution was irradiated with UV light at 365 nm (20 mW cm⁻², 25 °C) for 4 h to induce trans-to-cis isomerization until the photostationary state was reached. The energy was stored in the metastable cis-isomer of Azo on rGO. (2) Film: the tri-Azo/rGO uniform film (20–30 μm) was irradiated with the same UV light for 12 h to obtain a photostationary state. Long-duration irradiation was used to induce trans-to-cis isomerization of Azo in the dense film because of steric hindrance.

Heat release

(1) Powder: the UV-absorbed solution was dried in a vacuum to obtain a powder followed by heating (DSC) to induce cis-to-trans isomerization accompanied by heat release. (2) Film: the UV absorbed film was induced by heat (DSC) at the same rate, reversing to the ground state, accompanied by simultaneous heat release. The difference in energy density of the PTF powder and PTF film was an indication of the different isomerization degrees of azobenzene molecules on the graphene.

Heat release induced by blue light

The UV-absorbed solution was irradiated with green light at 450 nm (20 mW cm⁻², 25 °C) for 1–2 h. The irradiation accelerated the cis-to-trans isomerization, accompanied by rapid heat release.

Heat release induced by heat

The heat flow of the tri-Azo/rGO powder or film sample is evaluated by differential scanning calorimetry (DSC; TA Q20, TA Instruments, USA) (page 11). The powder or film was transferred to a hermetically sealed DSC pan. Heat release was analyzed using the following method: equilibration at 25 °C; heating to 180 °C at 0.5–40 °C min⁻¹; and cooling to 25 °C at 5 °C min⁻¹; re-heating to 180 °C at 5 °C min⁻¹. No heat flow in the second heat cycle indicates that heat flow in the first cycle is released by the solar-energy-stored tri-Azo/rGO assembly.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (No. 2016YFA0202302), the National Natural Science Funds for Distinguished Young Scholars (No. 51425306), the State Key Program of National Natural Science Foundation of China (No. 51633007), and the National Natural Science Foundation of China (No. 51573125 and 51573147).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta05333b

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