A High Energy Density Azobenzene/Graphene Oxide Hybrid with Weak Nonbonding Interactions for Solar Thermal Storage

Incorporating photochromic chromophores into polymer composites provides the possibility of a reversible photoswitch of the intrinsic properties of these materials. In this paper we report a route to attach azobenzene (AZO) moiety covalently to graphene oxide (GO) to create chromophore/graphene oxide (AZO-GO) hybrid, in which GO is both part of the chromophore and the template. Due to the high grafting density of AZO moiety and the low mass of the novel structure, the hybrid is a potential solar thermal storage material with high energy density of about 240 Wh·kg−1. It is found that C-H···π interaction between the cis-AZO chromophores and the aromatic rings of the substrate induces collective electronic modifications of GO at critical percentage of cis-isomers and reduce the thermal barrier of π-π* transition of the chromophores directly, which results in two sections of first-order reactions during the photoisomerization of trans- to cis-hybrid and also thermally stabilizes the cis-hybrid. Our findings demonstrate that high-performance AZO–GO hybrid can be manipulated by optimizing intermolecular nonbonding interactions.

synthesis of AZo-Go hybrids. Firstly 10 mg GO, exfoliated by ultrasonication in 10 ml deionized water to attain an aqueous dispersion, was slowly added to ammonia and ammonium chloride buffer solution (90 mL, pH = 9) and was kept in an ice bath at 0-5 °C. Typically, 6 mL sodium nitrite (0.69 g, 10 mmol) solution was slowly added to the solution of aniline (0.93 g, 10 mmol) in hydrochloric acid (6 M, 10 mL) through a dropping funnel in 10 min. The solution was kept under strong magnetic stirring for 20 min. The solution of diazonium salt was slowly added to GO buffer solution at 0-5 °C with stirring for 0.5 h, then was kept at room temperature for 3 h. The resulting suspension was filtrated and washed with DI-water until the pH of the filtrate reached 6-7, followed by drying under vacuum at 40 °C overnight.
Characterization. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on NEXUS-670 spectrometer with a disc of KBr. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Kratos Axis Ultra DLD on model surface analysis system with a 450 W Mg Kα X-ray (1000-1500 eV) source at a base pressure in the 10 −8 to 10 −9 Torre range. Thermogravimetry-differential analyses (TGA) of the samples were conducted in air condition by using a STA PT1600 simultaneous thermal analyzer (Linseis Germany). The energy density of AZO-GO was evaluated by differential scanning calorimetry (DSC) on the same apparatus with the heating rate of 5 °C/min during the temperature range of 20 °C to 180 °C. The X-ray diffraction (XRD) patterns

Results
The interlayer effect and the crystallization of pristine GO and RGO were analyzed by XRD spectra (Fig. S1(a)) which showed that GO and RGO were well dispersed before functionalization with AZO 19 . According to the FT-IR spectra ( Fig. S1(b)), the oxygen functional groups attached to the GO and RGO surfaces mainly composed of hydroxyl (-OH), epoxy and alkoxy (C-O-C), and carboxylic (-COOH) groups. The IR band intensities corresponding to the oxygen-containing functional groups were much weaker for RGO due to chemical reduction.
The covalent linkage between AZO and GO (RGO) surface can be demonstrated by FT-IR spectra (Fig. 2). For both hybrids, the significant bands are those due to the aromatic ring, AZO chromophore (-N=N-), C-N stretching and other bands, e.g. C-H, etc. The C-H in plane vibration peaks are between 1200-1000 cm −1 , and out-of-plane vibration of C-H are between 910-665 cm −1 . The bands due to the aromatic region are in the range of 1400-1600 cm −1 . The absorption band of the AZO chromophore overlapped with that of the C=C stretching hence a broad band is observed around 1500 cm −1 (Fig. 2(a)). Compared with GO and AZO, the absorption of C-N is distinct from the C=C absorbing bands and its frequency ranges centered at 1312 cm −1 because resonance increases the bond order between the ring and the attached nitrogen atom (inset in Fig. 2(b,c)) 20 . The appearance of C-N bonds indicates the covalent bonding of AZO to GO surface. For the hybrids, another distinguish character happens in the region between 3200 and 3700 cm −1 , where the intense broad peak corresponding to the -OH stretching vibration in GO is replaced by several distinct peaks. The peaks at 3201 and 3025 cm −1 are C-H stretching vibration, which will be discussed at a later stage.
In order to further confirm the covalent linkage between AZO and GO (RGO), X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical structure and composition of GO, RGO, AZO-GO and AZO-RGO powder samples. The deconvolution of each C 1s spectrum has been summarized in Table 1. According to XPS spectrum, it is found that the C/O ratio is 2.96 for GO, indicating a high oxygen atomic percentage in the as-prepared GO though it is a high-bound estimate for the real value 19,21 . The C 1s core level XPS of GO ( Fig. 3(a)) shows a peak at 284.6 eV, corresponding to sp 2 carbon framework in a conjugated honey-comb. Three components centered at 286.4 eV, 287.1 eV and 288.7 eV are related to C-OH, C-O-C (epoxy and alkoxy), and C=O groups, respectively, which are in good agreement with the previous reports 21,22 . For RGO (Fig. 3(b)), as listed in Table 1, the intensities of C 1s peaks, especially the peaks assigned to C-O (epoxy and alkoxy) and C=O, decrease dramatically due to the reduction of oxygen-containing functional groups, which confirms the XRD and FT-IR examinations. But the relative content of C-OH hardly changes. As shown in Table 1, the composition percentage of C-OH bond is 19.8% in GO and 18.7% in RGO. The slight difference in binding energy of each bond compared with that in GO can be attributed to the different chemical environment which it is in.
The N atoms in AZO-GO and AZO-RGO hybrids may form three kinds of bonds, the N=N bond and C-N bond in the AZO moiety, and the C-N bond connecting AZO to GO (RGO) substrate. The key difference between the two C-N bonds is that the C in the aromatic ring of AZO is at sp 2 state, and the one in GO is at sp 3 state, which has been well proved by the N 1s XPS spectra of the two hybrids ( Fig. 3(e,f)). The N 1s spectra of AZO-GO can be decomposed into three components, the one located at 399.0 eV is attributed to N=N bonding, the other two peaks with higher binding energies locate at 402.3 eV and 403.8 eV, respectively, being attributed to sp 3 and sp 2 C-N bonding. According to the experimental data (Table 1), obviously the percentages of the three components are nearly the same, which proves the covalent linkage between AZO and the substrate. As to AZO-RGO, the situation is quite similar except the slight changes of the peak positions. Figure 3(c,d) display the C 1s XPS of AZO-GO and AZO-RGO hybrids. From the deconvoluted spectra, it is found that in addition to the four aforementioned components in GO and RGO, there is a new one centered around 285.4 eV for the two hybrids, which can be associated with sp 3 type carbon due to the formation of C-N bond. The appearance of C-N bond provides evidence for the covalent linkage between -N=N-group and GO (RGO) substrate. Note that the percentage of C-N is 15.9% in AZO-GO, comparable to that of 12.8% in AZO-RGO. This observation supports our suggestion that the -N=N-group covalently attaches to the aromatic ring with -OH group in GO (RGO) via a diazo coupling reaction (Fig. 1). According to the elemental composition and the atomic percentage of the emerging nitrogen from XPS, as shown in Fig. 3(e), the N/C ratio is 0.089 for AZO-GO, indicating a functionalization density of one AZO moiety for every sixteen GO carbon atoms (noted as 1/16). As to AZO-RGO, the N/C ratio is 0.067 and the functionalization density is 1/24. That is, the average nearest distance between two AZO moieties is ~3.76 Å (~4.26 Å) in AZO-GO (AZO-RGO), which is comparable to previous studies 9 .
The amounts of AZO moiety functionalized on GO and RGO were also estimated by TGA ( Fig. 4(a)). The TGA weight loss curve of GO shows two steps of weight losses in the temperature region of 25-500 °C. A mass loss of about 25% can be seen at 175 °C, which is resulted by the evaporation of physically adsorbed water on the GO surface. A sharper mass loss happens hereafter and the mass loss of about 45% can be seen at 200 °C, which is assigned to the burning of labile oxygen containing functional groups, yielding CO and CO 2 as by-products 23,24 . While the AZO-GO hybrid exhibits a steady weight loss from room temperature to 100 °C where the mass loss is about 7%, then the curve drops sharply till 175 °C, where the mass loss is about 65%. Thermal decomposition temperatures of AZO-RGO is slightly increased incomparison with AZO-GO. AZO-RGOis thermally stable from 25 °C to 111 °C, and subsequently shows a sharp weight loss of 44.7% between 111 °C and 175 °C followed by slow loss from 175 °C to 500 °C 25 . Obviously, both hybrids show higher functionalization degrees than GO, confirming the covalent attachment of AZO molecules on GO and RGO surfaces. The grafting degree was calculated to be 38% (20.6%) from the weight losses of the AZO-GO (AZO-RGO) assemblies at different heating stages using Equation 1 in ref. 8 , from which the functionalization density is approximately 1/15 (1/25) (Equation S1 in ref. 9 ), agrees well with the XPS measurements.
The room temperature photoisomerization properties of AZO-GO and AZO-RGO hybrids are investigated by UV-vis spectra 7,26 . Typical time evolutions of the absorption spectra of the AZO-GO and AZO-RGO hybrids in ethyl alcohol upon UV light irradiation at 365 nm are shown in Fig. 5(a,b). It is well known that for -N=Ngroup, the two lowest electronic transitions of isolated trans-AZO are the ππ* band due to the S 0 → S 2 transition centered at 320 nm and the nπ* band due to S 0 → S 1 transition centered at 440 nm 26 . Accordingly, the strong band peaked at 354 nm (λ max ) in the UV-vis spectra of AZO-GO and AZO-RGO hybrids characterize the π-π* transition, while the red shifts compared with that in isolated AZO originate from electronic interactions between www.nature.com/scientificreports www.nature.com/scientificreports/ AZO and GO (RGO). The weak absorption band centered at 280 nm is the n-π* transition and the strong band at 234 nm is related to the π-π* transition of the GO substrate 15,18 .
Photoisomerization of AZO-GO ( Fig. 5(a)) and AZO-RGO ( Fig. 5(b)) hybrids can be well indicated by continuous decrease in the band intensity at 354 nm owing to the transformation from trans-to cis-isomers with the increasing irradiation time. A photostationary state between the trans-and cis-isomers is established after 800 minutes irradiation, much shorter than that of AZO/graphene hybrid 7 . Hereinafter the trans-hybrid after irradiating under UV light for 800 minutes at room temperature is called cis-hybrid for convenience. The www.nature.com/scientificreports www.nature.com/scientificreports/ isomerization degree for cis-hybrid, R, can be evaluated from the relation: R = (A 0 − A ∞ )/A 0 × 100, where A 0 is the initial absorbance and A ∞ is the absorbance at the photostationary state, which yields R = 68.9 wt%. This result is comparable to previous reports about dense packing AZO hybrids 8,27 . An interesting observation is that the main UV absorbing band red shifted from 354 nm to 358 nm after radiation longer than 360 minutes, while the center of n-π* transition of GO substrate blue shifted for about 6 nm at the same time. The reasons will be discussed latter.
Reversion of cis-to trans-isomerization of both hybrids was studied using UV-vis absorption spectra. The cis-hybrids were irradiated under violet (400-430 nm) at room temperature. The results are shown in Fig. 6(a,b). As can be seen, the intensity of the main band centered at 354 nm increases with time, which proves the reversibility of the trans-to cis-photoisomerization of the hybrids. Thermal reversion of cis-hybrid was also studied by UV absorption spectra. The cis-hybrid samples were kept in darkness (in a transparent glass) for 60 days before the measurements. As shown in Fig. 6(c,d), the band in the region 320-450 nm in the UV spectrum is rather weak for each hybrid, indicating good thermal stability of the cis-hybrid.  www.nature.com/scientificreports www.nature.com/scientificreports/ The energy density of the AZO-GO and AZO-RGO hybrids were determined by DSC analyses. As shown in Fig. 4(b), the exothermic heat flow of cis-hybrid due to cis → trans isomerization is over 20-65 °C, which corresponds to a bulk gravimetric energy density of 240 Wh·kg −1 (199 Wh·kg −1 for AZO-RGO). Obviously, higher grafting density results in higher energy density. The energy density is much higher than previous reports about similar hybrids 7-10 , it is even larger than soft-packing Li-ion batteries (90-150 Wh·kg −1 ) 28 . Accounting for the cis-and trans-compositions of the photostationary state from XPS, the enthalpy difference ∆H between the cisand trans-isomers was calculated to be 173.7 kJ·mol −1 (114.2 kJ·mol −1 for AZO-RGO), which is also outstanding compared with reported materials 7,9 . The high grafting density of AZO moiety in the hybrid is one reason for the improved energy density. The capacity of thermal storage is another key factor to decide the energy density. First principles calculations of ΔH for per AZO moiety is performed based on density functional theory (DFT). The model is that one AZO moiety is supported by 24 carbon atoms of RGO. (Details of the calculations are presented in the supporting information.) Using the relaxed steric structures of trans-and cis-isomers (Fig. S4), the calculated ΔH is about 0.86 eV/f.u., much larger than the value of ~0.56 eV of an unsubstituted AZO molecule in gas phase 3,7 . More importantly, due to the novel structure, where GO is not only the template but also acts as a component of the chromophore, the hybrid has much smaller molar mass than the other materials, which increase the energy density per unit weight greatly.
To investigate the kinetics of the trans-cis photoisomerization of the hybrids, the rate constant and the activation barrier ΔE a (energy required for isomerization of per trans-AZO moiety) were studied. Previous studies demonstrate that the trans-to-cis isomerization of AZO in the presence of UV light follows first-order kinetics 29,30 and the time dependence of the rate constant κ can be written as follows: where A 0 , A t and A ∞ are the absorbance before irradiation, at irradiation time t and after irradiation for a prolonged time. Three times of independent UV-vis absorption measurements have been performed using hybrids synthesized in different batches, the average value of for each hybrid is plotted as the function of t in Fig. 7.
curve has two discrete segments and for each segment the curve behaves linearly. The discontinuity happens at about t c = 360 min (340 min) for AZO-GO (AZO-RGO), after that the curve drops much steeper with time. Note that for both AZO-GO and AZO-RGO hybrids, the proportion of cis-hybrid is about 60% at t c . The rate constant κ can be deduced from the slope, which yields κ 1 = (1.93 ± 0.19) × 10 −5 s −1 when 0 < t < t c , κ 2 = (9.51 ± 1.93) × 10 −5 s −1 when t c < t < t ∞ for AZO-GO. As to AZO-RGO, (9.51 ± 1.93) × 10 −5 s −1 and κ 2 = (9.61 ± 0.58) × 10 −5 s −1 . Interestingly, the kinetics of cis-to-trans isomerization also shows deviation from the first order kinetics (Fig. S3 for details). where τ 1/2 is the time required for half of the trans-hybrids to transform to cis version and τ 1/2 = ln2/κ, T is the temperature, k B , R and h are the Boltzmann constant, gas constant and Plank constants, respectively. As shown in Table 2, τ 1 1/2 = 10 h for AZO-GO samples stimulated less than t c , while τ 2 1/2 = 2 h for those under irradiation for times longer than t c . Applying equation (2) and the calculated τ 1/2 , we obtained the ΔE a = 1.02 eV (0.96 eV) for 0 < t < t c (t c < t < t ∞ ). Clearly, after t c the isomerization energy barrier is lower and the process proceeds faster. The calculations match with the observed red shift of the π-π* transition band in UV-vis absorption spectral after t c .
The lower activation barrier of AZO-RGO (ΔE a = 0.98 eV) than that of AZO-GO before t c is related to the lower density of AZO moiety in AZO-RGO. The trans-to-cis isomerization is influenced by steric hindrance and strong intermolecular interaction between AZO units. In the AZO-RGO hybrids, the lower AZO density results in larger separation distance of neighboring AZO molecules and weaker intermolecular interaction, which reduces the steric hindrance, hence lowers the trans to cis activation barrier. After t c , the content of cis-isomers has exceeded the trans-isomers in both hybrids. Consequently, the distance between the neighboring trans-isomers increases and the intermolecular interaction decreases, which attenuates the difference in steric effect to the trans-to-cis isomerization in the two hybrids. Then the reaction barriers are mainly dominated by the electronic structures of the -N=N-group and the two hybrids have the same ΔE a after t c .
To investigate the mechanism of the two-stage first-order reaction kinetics of trans-to-cis photoisomerization of the hybrid, we recorded a series of Raman spectra, which can be used to elucidate the in situ properties of molecules containing the AZO chromophore 31 . Figure 8(a) shows the results of AZO-GO samples after exposed under UV (365 nm) light for different times, the assignments are given in Table 3.
A strong Raman signal was obtained for the as prepared trans-hybrid. The Raman active mode located at about 999 cm −1 , indexed as Mode A, is ascribed to the aromatic rings breathing. Mode N1 and N2 centered at 1165 cm −1 and 1180 cm −1 , respectively, are predominately C-N symmetrical stretches coupled with C-H in-plane bending contributions of the AZO phenyl ring 31,32 . The three bands in the -N=N-stretching region, 1400-1465 cm −1 , are named N3, N4 and N5. The band at about 1600 cm −1 , named as Mode C, involves the stretching of the aromatic ring. The bands in the region of 1200-1260 cm −1 can be attributed to the combination of the C-C stretching and the C-H in-plane bending, the band in the region 1280-1340 cm −1 can be ascribed to the conjugation between the oxygen containing groups and the phenyl ring 31 . These two bands are less important in probing the dynamics Figure 7. The kinetics constants for trans→cis photoisomerization. www.nature.com/scientificreports www.nature.com/scientificreports/ in photoisomerization process hence were elided in the following. Contributions from other molecular constituents are much less pronounced. It is found that, compared with the pristine GO and RGO (Fig. S2), the G and D band of graphene are absent in Fig. 8(a). This can be understood as the result of proton transfer from AZO to GO during chemisorbing, which inducing blue shift of G band and significant decrease of I D /I G 33 . The blue shift of G band makes it overlap with the C band of AZO, while the D band is masked by the N3 band of AZO, which is broad and strong.
Obviously, Raman scattering of the trans-hybrid is the strongest for modes which elongate the molecule along its length causing a change in polarization of the molecule 31,34 . After exposing under UV light, the samples have composition of cis-isomers, which results in weaker spectrum due to the reduced symmetry. In the meanwhile, a new C-N mode appears at about 1136 cm −1 , named N0. Band N0 requires population of excited-state S 1 35 . In trans-hybrid the intense contributions of ground state S 0 to the Raman Spectrum may hinder the isolation signals from S 1 hence N0 is absent. For irradiated samples, the appearance of N0 can be considered as an important signature of isomerization.
To get further insight of the temporal structural information of the hybrids under UV light before and after t c , we calculate the intensity of each mode compared with that of Mode N3, the strongest peak for all cases, at specific exposing time and show the results in Table 3. Due to the weak intensity and strong overlapping with N3 after irradiation, it is hard to differentiate the variation tendency of Mode N4 hence not shown. As shown in Table 3, the intensity ratio of Mode N5 is nearly constant till the exposing time limit, while the relative intensity of Mode A, N0, N1, N2 decreases suddenly after exposing for t c , meanwhile, the intensity of Mode C increases abruptly. Clearly, distinct intensity anomaly appears for modes related to the vibrations of the aromatic rings after t c . Simultaneously, the peak position of the characteristic -N=N-stretching, Mode N4, shifts toward lower frequency side for 4 cm −1 immediately after t c . While in cis-hybrid, all modes undergo red shift compared with trans-hybrid, for example, Mode N1 and N2 are shifted by about 3.0 cm −1 , mode A and mode C are shifted by about 2 cm −1 .
The anomalies in intensities and positions of these modes before and after t c represent the abrupt variations of the vibration modes after exposing under UV light for t c . Comparative analysis of the infrared spectra of trans-and cis-hybrid provide further evidences. As shown in Fig. 8(b), for both hybrids, the significant bands corresponding to the aromatic ring, AZO chromophore (-N=N-), C-N stretching are similar in positions and profiles, the increased intensity of C-N and N=N modes in cis-hybrid is reasonably ascribed to the reduced symmetry of cis-hybrid. However great changes take place for C-H modes. The C-H in-plane bending mode and C-H out-of-plane bending mode are in the region 900~1100 cm −1 and 650-900 cm −1 , respectively (inset of Fig. 8(b)), while the C-H stretching vibration modes are observed in region 3000-3100 cm −1 . As can be seen, for cis-hybrid, the phenyl ring's C-H stretching vibration is enhanced while the C-H bending bands are strongly prohibited compared with trans-hybrid. This observation indicates a vertical or a tilted orientation of the AZO phenyl ring to the substrate for trans-isomer whereas the AZO moiety is more parallel to the GO substrate for cis-isomer.

Discussion
With use of the Cambridge Structure Database, Yoji Umezawa and coworkers put forward a method for exploring X-H···π interaction 36 . According to this method, whether there is a C-H···π interaction in AZO-GO hybrid depends mainly on two factors, the first is the distance d between H atom and the aromatic ring underneath, the second is the angle of θ (=∠HC AZO C GO1 , C AZO is the carbon atom on the AZO moiety which connected directly with the H atom, C GO1 is the nearest carbon neighbor of H atom on the aromatic ring of GO). It is found that C-H···π interaction happens given appropriate parameters, i.e., d max < 3.05 Å and θ < 60°. Based on the calculated bond lengths and bond angles of AZO moiety in ref. 37 , and set the dihedral angle between AZO aromatic ring and GO surface as 30° (90°) for cis-isomer (trans-isomer), it is found that the d cis = 3.01 Å for cis-isomer, while d trans = 3.82 Å for trans-isomer. Obviously, d trans is much larger than d max , which means the C-H···π interaction cannot occur in trans-isomer while it is possible in cis-isomer. Moreover, the energy difference in ΔE a of AZO-GO before and after t c is about 1.38 Kcal/mol (0.06 eV), matching with the typical energy of C-H···π bonds 38 .
According to this assumption, the intensity and Raman shift mentioned above can be well explained. As shown in the inset of Fig. 8(b), the intensities of C-H stretching mode are enhanced in cis-hybrid, while the intensity of C-H bending mode reduces. It is believed that the coupling of C-N vibration with C-H in plane bending strengthens the band intensity of C-N in Raman spectra 32 . The formation of C-H···π stacking hinders the C-H in-plane bending and reduces the coupling with C-N related modes hence weaken the band intensity. Similarly, coupling of C-H stretching with the aromatic ring increase the polarizability of C=C stretching mode and increase the intensity of Raman Mode C, at the same time the ring breathing mode is limited hence reducing the band intensity of Mode A.
We would ascribe the origin of the two segments of linear dynamics during the trans-to-cis isomerization as a direct consequence of the existence of C-H···π interaction. When a cis-isomer comes into being, the C-H···π hydrogen bond makes the H atom of the AZO benzene moiety act like an electron donor, while the aromatic ring of the GO substrate beneath acts like an acceptor, which results in local electron-rich region in the GO surface 38 . The perturbations to the electronic structures of GO will enhance the polarizability of GO substrate. The area of electron-rich region grows with the extension of irradiation time. When there are enough percentage amount of cis-isomers, about 60% at t c , the area of electron-rich region links up and the perturbation is strong enough to modify the electronic distributions of the GO substrate collectively, which influence the electronic structure of trans-hybrid and cis-hybrid directly. For simplicity, the effect of increasing the electron-rich region is analogy to increasing the polarity of the solvent, which decreases (increases) the energy gap between π-π * (n-π * ) states. As depicted in Fig. 5, the π-π * transition of -N=N-group in trans-hybrid red shifted while the n-π * transition of GO blue shifted after t c . The energy gap reduction of π-π * also leads to red shift of Mode N4 in Raman spectra and decreased activation energy ΔE a during the trans-to-cis isomerization, which also contributes to the thermal-stabilization of cis-hybrid. The red shifts of other Raman active modes in cis-hybrid indicate coupling of the -N=N-stretching to the C=C and C-H modes of the aromatic rings.
Our observations prove that, though C-H···π interaction is a weak nonbonding interaction, it is capable to tune the band structures of GO. A recent theoretic calculation reports that a band gap of ~90 meV is opened by C-H···π interaction for graphene 14 . The modifications of the band structures originate from the broken of inversion symmetry of the two carbon sub-lattices of graphene due to heterogeneous distributions of C-H groups. So it is inferred that the key factors to decide t c are the grafting density of AZO and the spatial uniformity of C-H groups above GO, as well as the intensity of the C-H···π interaction. Therefore, besides changing the functionalization degree of the hybrid by modifying preparation method, such as multiple iterating reaction 8 , substituting partial H atoms of the AZO aromatic ring by -XH groups (such as -NH 2 , -OH, -CH 3 , etc.) can influence t c directly 39 . Substituting induces asymmetric distribution of nonbonding interactions, what's more, the substituent can have a dramatic impact on XH···π interactions, which depends strongly on the nature of the group 14 .
Now there are two key problems towards commercial applications of AZO-based solar thermal fuels. One is to increase the energy storage density and the second is to reach a suitable thermal barrierΔE for the cis → trans back reaction. Dense molecular packing of the covalently attached AZO molecules on carbon substrates can effectively increase the bulk energy density, as have been proved here and in refs 3,4,7,29 . More importantly, it provides possibility to systematically control the inter-or intra-molecular interactions in the hybrids 7-10 . However single mechanism usually improves in ΔH at the cost of decreasing ΔE 7 . This work provides a thought to adjust the stability of cis and trans states independently hence to decouple and increase both the energy density and thermal barrier of the hybrids since C-H···π interaction happens only between the cis-AZO moiety and the GO substrate due to the peculiar structure of AZO-GO hybrid. To improve the stability of the trans-hybrid, the intermolecular hydrogen bond can be introduced by proper substituent of the AZO phenyl ring. Other adjustable parameters include molecular assembly density, chemical composition and position of the joining groups, etc.

Conclusion
AZO-GO (RGO) hybrid is prepared by diazotization method. XPS and FT-IR results prove that the AZO monomer is covalently bonded to the GO (RGO) substrate and the functionalization density is approximately 1/16 (1/24) for AZO-GO (AZO-RGO). The UV-vis spectra of AZO-GO hybrid prove reversible photoisomerization in region of 300-400 nm. The novel hybrid exhibits a high energy density up to 240 Wh·kg −1 and good thermal stability of cis-hybrid. Vibrational spectra of the hybrids highly suggest the existence of C-H···π interaction between the aromatic ring of the AZO to that of the GO matrix in cis-isomer, which induces collective electronic modifications of the hybrid and influences the π-π* transition of the chromophores directly. The kinetics of the trans to cis photoisomerization includes two discrete linear segments. The discontinuity happens at t c when the percentage of the cis-hybrid is about 60%, after that the activation energy drops and the photo isomerization process proceeds faster.