Fire Alarm Wallpaper Based on Fire-Resistant Hydroxyapatite

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Fire Alarm Wallpaper Based on Fire-Resistant Hydroxyapatite Nanowire Inorganic Paper and Graphene Oxide Thermosensitive Sensor Fei-Fei Chen,†,‡ Ying-Jie Zhu,*,†,‡ Feng Chen,† Li-Ying Dong,† Ri-Long Yang,†,‡ and Zhi-Chao Xiong*,† †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: Wallpaper with multiple functions, such as fire resistance and an automatic alarm in fire disasters, will be attractive for the interior decoration of houses. Herein, we report a smart fire alarm wallpaper prepared using fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires (HNs) and graphene oxide (GO) thermosensitive sensors. At room temperature, the GO thermosensitive sensor is in a state of electrical insulation; however, it becomes electrically conductive at high temperatures. In a fire disaster, high temperature will rapidly remove the oxygen-containing groups of GO, leading to the transformation process of GO from an electrically insulated state into an electrically conductive one. In this way, the alarm lamp and alarm buzzer connected with the GO thermosensitive sensor will send out the alerts to people immediately for taking emergency actions. After the surface modification with polydopamine of GO (PGO), the sensitivity and flame retardancy of the GO thermosensitive sensor are further improved, resulting in a low responsive temperature (126.9 °C), fast response (2 s), and sustained working time in the flame (at least 5 min). Compared with combustible commercial wallpaper, the smart fire alarm wallpaper based on HNs and GO (or PGO) is superior owing to excellent nonflammability and high-temperature resistance of HNs, which can protect the GO (or PGO) thermosensitive sensor from the flames. The smart fire alarm wallpaper can be processed into various shapes, dyed with different colors, and printed with the commercial printer and thus has promising applications in high-safety interior decoration of houses. KEYWORDS: hydroxyapatite, nanowires, graphene oxide, thermosensitive sensor, wallpaper smart fire-resistant and fire alarm wallpaper (FAW) will be very desirable if it can simultaneously prevent the fire from spreading and send out alerts in a fire disaster. This kind of FAW needs an ideal fire-resistant paper that can maintain its structural integrity in the flame and a thermosensitive sensor that can rapidly respond to the high temperature of fire. Commercial wallpaper made of plant cellulose fibers or synthetic polymer is lightweight, flexible, cheap, and increasingly popular for the interior decoration of houses. However, commercial wallpaper is highly flammable1 and will promote the spread of fire in a fire disaster. As a result, commercial wallpaper is not suitable for the substrate of FAW. To solve this problem, two solutions were proposed to improve the fire retardancy of the substrate. The first approach was to add the fire retardant into the flammable substrate or modify the surface chemistry.2−5 The fire retardancy of the flammable substrate could be efficiently improved after the treatment but was still limited, and the substrate was easily broken or shrunk in the flame.6−9 In addition, the chemicals used had potential

toxicity and were not environmentally friendly.1,10,11 The second approach was to use the inorganics with intrinsic nonflammability as alternatives.12−15 However, the inorganic material as the substrate of FAW should meet the requirements of wallpaper including environmental safety, white color, high flexibility, mechanical robustness, and excellent fire resistance. Hydroxyapatite (Ca10(OH)2(PO4)6), a well-known member of the calcium phosphate family with high biocompatibility, is the major inorganic component of bone and teeth in vertebrates, and thus, hydroxyapatite materials have been investigated for applications in biomedical fields.16−19 However, HAP materials usually exhibit high brittleness and poor flexibility because hydroxyapatite usually forms particles, short rods, or needles. Recently, we have found that ultralong hydroxyapatite nanowires (HNs) with ultrahigh aspect ratios

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© XXXX American Chemical Society

Received: January 3, 2018 Accepted: February 27, 2018

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Figure 1. Schematic illustration of fire alarm wallpaper (a), GO with abundant oxygen-containing groups (b), and HN/GF (glass fiber) paper with multilayered structure (c). (d) Transmission electron microscopy (TEM) image of GO. Inset: Digital image of the stable GO aqueous ink. (e,f) TEM images of HNs. (g) TEM image of GFs wrapped by HNs. (h) Scanning electron microscopy (SEM) image of the surface of the HN/GF paper. (i) SEM images of the cross section of HN/GF paper. (j) HN/GF paper can bear a weight of 500 g. (k) Twisting, bending, folding, or curling HN/GF paper. (l) Digital image of a large-sized HN/GF paper.

have high flexibility and can solve the problem of high brittleness and poor flexibility of hydroxyapatite materials.13 Ultralong hydroxyapatite nanowires are a promising kind of biomaterial with many advantages such as high biocompatibility, high flexibility, good mechanical properties, high thermal stability, and fire resistance.13 As a result, ultralong hydroxyapatite nanowires are excellent building materials for the construction of flexible fire-resistant HN-based inorganic paper.13,20,21 Considering the above-mentioned advantages, we determine that the flexible fire-resistant HN-based inorganic paper is suitable for the application as the substrate of FAW. There are some requirements for the thermosensitive sensor including flexibility, fire retardance, and rapid response and steady performance in the flame. Graphene and its derivatives show outstanding mechanical, optical, electrical, and thermal properties,22−25 which can be applied in transistors,26 triboelectric nanogenerators,27 energy conversion and storage,28−30 sensors,31−37 thermal conductive films,23 and so on. Among

them, graphene oxide (GO) has been intensively investigated owing to its water dispersibility and scaled-up production. GO is electrically insulated due to the presence of abundant oxygencontaining groups,38 which is unwanted for many applications. Some methods have been proposed to reduce GO.38−41 Among the reported methods, thermal treatment is the most direct and effective. High temperature can rapidly remove the oxygencontaining groups, which turns the electrically insulated GO into highly conductive reduced GO.42,43 This feature of GO is suitable for the application as the thermosensitive sensor of FAW. In a fire disaster, the temperature of fire is far higher than the temperature for the transformation of GO from the electrically insulated state into the highly conductive state. Therefore, the alarm lamp and buzzer connected with the GO thermosensitive sensor will send out the alerts to people as soon as GO is exposed to flame. Moreover, GO with functional groups can be easily modified to improve its fire retardancy.44−46 B

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components of HNs, GFs, GO, PDMS, Ag paste, and Cu wire can be observed. Each GF is wrapped with a large number of HNs, forming a composite structure (Figure 1c). The surface of the GO thermosensitive sensor exhibits a typical wrinkled topography, as shown in Figure S2 in the Supporting Information. It is necessary for a wallpaper to be mechanically robust for practical applications. The following four factors are responsible for the mechanical robustness of the as-prepared fire-resistant HN/GF inorganic paper: (1) HNs have diameters of about 10 nm and ultralong lengths of >100 μm (up to several hundred microns), and these ultralong HNs prefer to interweave with each other, forming a hierarchical porous network (Figure 1e). The physical cross-linking and chemical interactions (van der Waals force and hydrogen bonds) among HNs are beneficial for enhanced mechanical properties. (2) High aspect ratios of ultralong HNs (up to >10000) contribute to the high flexibility of HNs,12 as indicated by the self-bending behavior (Figure 1f). (3) GFs are known as a biocompatible additive to improve the mechanical properties. By introducing the GFs, GFs are wrapped with HNs, resulting in a core−shell composite structure (Figure 1g,h), and this composite structure is also revealed by energy-dispersive X-ray (EDX) elemental mapping (Figure S3 in the Supporting Information). The large-area coverage of HNs on the surface of GFs and the homogeneous embedding of GFs in the HN framework (Figure 1i) mimic a classical architectural construction, that is, the “steel-reinforced concrete structure”. (4) After the paper-making process, HNs and GFs are assembled into a well-defined multilayered structure spontaneously (Figure 1i), which may be explained by the mechanical equilibrium between physical and chemical forces.47 The nacre-like multilayered structure is regarded as an effective strategy to balance the strength and toughness.48,49 When applying the external forces, the interlayer channel can serve as a buffer for energy dissipation. As shown in Figure 1i, under the through-plane shear force, a multiarch microstructure can be self-constructed in the HN/GF paper to discharge the stress, which is conducive to the fatigue resistance.50 In one word, the as-prepared fire-resistant HN/GF inorganic paper has good mechanical properties because of the above-discussed reasons. For example, the four pieces of the fire-resistant HN/ GF paper with 20 wt % GFs (15 × 25 mm, 67 mg) is able to withstand a weight of 500 g, which is ∼7462.7 times its own weight (Figure 1j). In addition, the as-prepared fire-resistant HN/GF inorganic paper is highly flexible, and it can endure physical deformation such as twisting, bending, folding, and curling and maintain its structural integrity without obvious damage (Figure 1k). Furthermore, a large-sized fire-resistant HN/GF paper with a diameter of 20 cm (Figure 1l) can be fabricated rapidly (within 15 min) using commercial papermaking equipment (Figure S4 in the Supporting Information). Figure 2a shows the X-ray diffraction (XRD) patterns of the fire-resistant HN paper, GFs, and the fire-resistant HN/GF paper with 10 wt % GFs. The diffraction peaks of the fireresistant HN paper and HN/GF paper with 10 wt % GFs can be indexed to the crystal phase of hydroxyapatite (JCPDF 740566). The XRD pattern of GFs indicates that GFs are amorphous in structure. The addition of GFs to the fire-resistant HN paper has a significant effect on the mechanical properties of the HN/GF paper. The typical stress−strain curves are shown in Figure 2b, and the measured results of the tensile strength, strain at failure, and Young’s modulus are summarized in Table S1 in the

Herein, we report a smart FAW that not only shows excellent resistance to both fire and high temperature but also sends out the alarms in a fire disaster. In this smart FAW, the highly flexible fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires serves as the substrate of FAW and GO is used as the thermosensitive sensor. At room temperature, the GO thermosensitive sensor is in a state of electrical insulation; however, it becomes electrically conductive at high temperatures. After being exposed to the flame, the high temperature induces instantaneous deoxygenation of the GO thermosensitive sensor, leading to the rapid transformation from the electrical insulation state to an electrically conductive one; thus, the alarm lamp and buzzer connected to the GO thermosensitive sensor are able to send out the alarms immediately. In addition, polydopamine is used as both a reductant and a capping agent to simultaneously improve the responsive sensitivity and flame retardancy of the GO thermosensitive sensor. The polydopamine-modified GO (PGO) thermosensitive sensor exhibits a low thermal responsive temperature (126.9 °C), rapid response time (2 s), and long alarm time in the flame (at least 5 min). Compared with the commercial wallpaper, the smart fire alarm wallpaper based on HNs and GO (or PGO) is superior owing to the excellent nonflammability and high-temperature resistance of HNs, which can protect the GO (or PGO) thermosensitive sensor from flames. The other desirable features of the HN inorganic paper include environmental friendliness, white color, mechanical robustness, and high flexibility. The smart fire alarm wallpaper reported herein can be processed into various shapes, dyed with different colors, and printed with the commercial printer, thus it is promising for applications such as high-safety interior decoration of houses.

RESULTS AND DISCUSSION Figure 1a illustrates the design scheme of the smart fire alarm wallpaper prepared using fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires and the graphene oxide thermosensitive sensor. GO has abundant oxygen-containing groups (Figure 1b), thus the GO thermosensitive sensor is in a state of electrical insulation at room temperature. However, the GO thermosensitive sensor becomes electrically conductive at high temperatures because the oxygen-containing groups of GO will be rapidly removed at high temperatures, leading to the transformation process of GO from an electrically insulated state into a highly conductive one. In this way, the alarm lamp and buzzer connected with the GO thermosensitive sensor will send out the alerts to people immediately. In this work, the glass fiber (GF)-reinforced HN (HN/GF) fire-resistant inorganic paper with a multilayered structure and good mechanical properties (Figure 1c) is designed and prepared, and this will be discussed later. For the preparation of the GO thermosensitive sensor, GO is well dispersed in deionized water by vigorous stirring and ultrasonication (Figure 1d), and the stable GO aqueous ink is obtained (inset of Figure 1d). The GO thermosensitive sensor is installed on the HN/ GF fire-resistant inorganic paper by a facile drop-casting of the GO aqueous ink. The copper wires are connected to the two edges of the GO thermosensitive sensor as external electrodes with the help of silver paste and polydimethylsiloxane (PDMS). The microstructure of FAW based on the fire-resistant HN/GF paper and the GO thermosensitive sensor is shown in Figure S1 (Supporting Information). Figure S1 shows the GO thermosensitive sensor on the fire-resistant HN/GF paper, and all the C

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sensor thermally treated at 250 °C for 3 min is able to send out the strong alarm light and loud alarm sound from the alarm lamp and alarm buzzer (Figure 3a,b and Movie S1 in the Supporting Information), indicating that the thermal responsive temperature of the GO thermosensitive sensor is in the range of 200−250 °C. The as-prepared GO thermosensitive sensor after thermal treatment at different temperatures was investigated by X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG) analysis, elemental analysis, electrical conductivity, and XRD. Figure 3c shows the high-resolution XPS pattern of the C 1s peak of the GO thermosensitive sensor without thermal treatment. It exhibits the presence of C−C/CC groups (284.7 eV) and oxidized components including C−O (286.8 eV), CO (287.9 eV), and O−CO (288.9 eV). The strong peak of the C−O group is attributed to a large amount of oxidized carbon species (hydroxyl and epoxy).51 When the thermal treatment temperature increases, the intensity of the C−O peak decreases significantly (Figure 3d−f), and the peaks of oxidized components including C−O, CO, and O−CO are not obvious at 250 °C (Figure 3f). The TG curve of GO indicates that the oxygen-containing groups can be removed at high temperatures (Figure 4a), producing highly reduced GO. The removal of oxygen-containing groups is further verified by the EDX elemental analysis. As shown in Figure 4b, the atomic percentage of oxygen decreases with increasing thermal treatment temperature. The removal of the oxygen-containing groups of GO at high temperatures is accompanied by the abrupt change of the electrical performance (Figure 4c). The electrical conductivity of the GO thermosensitive sensor thermally treated at 250 °C is 289.6 S m−1, which is enhanced by about 3 orders of magnitude compared with that without thermal treatment (0.21 S m−1) and about 40 times that treated at 200 °C (7.26 S m−1). The high electrical conductivity of the GO thermosensitive sensor induced by deoxygenation at high temperatures can trigger the working state of the alarm lamp and buzzer. XRD is a useful tool to track the structural change of the materials. For the GO thermosensitive sensor without thermal treatment, there is a sharp diffraction peak located at 2θ = 10.902° (Figure 4d), corresponding to an interlayer spacing d = ∼8.12 Å. When the thermal treatment temperature increases to 150 °C, this diffraction peak shifts to 2θ = 11.443° (d = ∼7.73 Å). The decreased interlayer spacing can be explained by weakened repulsive forces between GO sheets due to the removal of oxygen-containing groups.52 This diffraction peak of the GO thermosensitive sensor thermally treated at 200 °C continues to shift to higher diffraction degree (2θ = 13.723°, d = ∼6.45 Å), and in addition, an additional broad peak at 2θ = ∼22.087° is observed (inset of Figure 4d), indicating the removal of the oxygen-containing groups to some extent. For the GO thermosensitive sensor thermally treated at 250 °C, the diffraction peak below 2θ = 15° totally disappears, and only a broad peak at 2θ = ∼23.388° is observed (inset of Figure 4d). This peak is close to the characteristic diffraction peak of graphite (2θ = ∼26.6°),41 indicative of a high degree of reduction.38 The detailed information on the electrical conductivity and elemental and XRD analysis results is shown in Table S2 in the Supporting Information. As discussed above, the experimental results indicate that the thermal responsive temperature of the GO thermosensitive sensor is located in the range of 200−250 °C. In order to accurately determine the thermal responsive temperature, the

Figure 2. (a) XRD patterns: (1) fire-resistant HN paper, (2) GFs, and (3) fire-resistant HN/GF paper with 10 wt % GFs; the diffraction peaks of HNs can be indexed to the crystal phase of hydroxyapatite (JCPDF 74-0566). (b) Typical stress−strain curves of the fire-resistant HN/GF inorganic paper sheets with different weight percentages of GFs.

Supporting Information. With increasing weight fraction of GFs (0−20 wt %), the ultimate tensile strength of the fire-resistant HN/GF paper increases significantly at the expense of maximal strain at failure (Figure S5, Supporting Information). However, further increasing of GFs to more than 20 wt % will lead to the decrease in the mechanical properties of the HN/GF paper (Figure 2b and Figure S5 and Table S1 in the Supporting Information). Figure S6 in the Supporting Information shows SEM images of the surface and cross section of the fire-resistant HN paper and the HN/GF paper sheets with different weight percentages of GFs. On the basis of experimental results, the fire-resistant HN/GF paper with 20 wt % GFs is chosen for the fabrication of FAW because it exhibits the optimum mechanical properties including high tensile strength (2489.31 ± 136.03 kPa) and Young’s modulus (420.73 ± 31.99 MPa). The thermal responsive behavior of the as-prepared GO thermosensitive sensor was investigated. The as-prepared fireresistant FAW was thermally treated at different temperatures for 3 min, and then the GO thermosensitive sensor was connected with an alarm lamp and an alarm buzzer and was supplied with a low applied voltage (2.5 V), as illustrated in Figure 3a. Figure 3b shows that GO is electrically insulated at relatively low temperatures, and as a result, the alarm lamp and alarm buzzer are not able to be activated and send out the alerts. However, the increased temperature can effectively remove the oxygen-containing groups of GO.40 Thus, high temperatures will induce the transformation from the electrical insulation state to the electrical conduction state. The asprepared fire-resistant FAW with the GO thermosensitive D

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Figure 3. (a) Schematic illustration shows the changes of the alarm lamp, alarm buzzer, and electrons in the electrical circuit after thermal treatment of the FAW with the GO thermosensitive sensor at different temperatures for 3 min (RT: room temperature). (b) Corresponding digital images. (c−f) High-resolution X-ray photoelectron spectra of the C 1s peak of the GO thermosensitive sensor after thermal treatment at different temperatures for 3 min.

as-prepared FAW was heated in the electric oven, and the changes of the alarm lamp, alarm buzzer, and electrical current with increasing temperature were monitored (Figure 5a). The experiments indicate that the electrical current through the GO thermosensitive sensor is zero below 223.5 °C (Figure 5b), and thus the alarm lamp and alarm buzzer do not work (inset of Figure 5b). After that, the electrical current increases with increasing temperature up to 236.7 °C and then levels off. The real-time monitoring process between 228.9 and 232.0 °C is shown in Movie S2 in the Supporting Information. It is noteworthy that the alarm light and sound are obvious when the electrical current is higher than 0.10 mA (231.3 °C, inset of Figure 5b), and we define the temperature at this point as the thermal responsive temperature (Table 1). The thermal responsive temperature of the as-prepared FAW is much lower than the high temperature of the flame, implying that the FAW is able to send out timely alarms in a fire disaster. In a practical fire disaster, the temperature of fire will go up very fast. To simulate the real situation, the FAW is directly exposed to the flame of the alcohol lamp, as shown in Figure 5c. As expected, the GO thermosensitive sensor rapidly responds to the high temperature of fire within 5 s (Figure 5d). The electrical current at 5 s is 0.13 mA, which is high enough to activate the alarm lamp and alarm buzzer, and then they send

out the alerts (Figure 5e and Movie S3 in the Supporting Information). The time needed to send out the alarms is defined as the response time (Table 1). A major problem of the GO thermosensitive sensor is its inability to survive for a long time in the flame.44,53 As shown by the TG curve (Figure 4a), the residual weight of GO at 690 °C is only 2.81%. Consequently, the GO thermosensitive sensor is easily burnt out (inset of Figure 5d). The electrical current after 7 s is lower than 0.10 mA, and its changing trend is to decrease with time (Figure 5d,e and Movie S3 in the Supporting Information). The time period for maintaining electrical current higher than 0.10 mA is defined as alarm time. In the above experiment, the alarm time of the GO sensor is very short (only 3 s, Table 1). The thermal responsive temperature and response time relate to the sensitivity of the GO thermosensitive sensor, and the alarm time is associated with the thermal stability of the GO thermosensitive sensor in the flame. An ideal FAW should have low thermal responsive temperature, fast response time, and long alarm time. The fire retardancy of the GO thermosensitive sensor can be improved by using polydopamine (PDA). Dopamine provides an effective solution owing to the following advantages: (1) dopamine is known as a hormone and neurotransmitter and is biocompatible and thus is an excellent biomaterial.54−56 (2) E

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Figure 4. (a) TG curve of GO: weight loss before 100 °C is ascribed to the evaporation of adsorbed water, and the major weight loss between 100 and 300 °C results from the decomposition of oxygen-containing groups. The atomic percentages of carbon and oxygen (b), electrical conductivities (σ) (c), and XRD patterns (d) of the GO thermosensitive sensor after thermal treatment at different temperatures. Inset of (d) is the XRD patterns of the GO sensor after thermal treatment at 200 and 250 °C.

structure of PGO may be responsible for its corrugated microstructure (Figure S9 in the Supporting Information). In addition, a sharp peak at 2θ = 10.780° in the XRD pattern of the PGO thermosensitive sensor corresponds to the d-spacing of ∼8.21 Å (Figure 6c). Compared with the GO thermosensitive sensor (d = ∼8.12 Å), the larger interlayer spacing in the PGO thermosensitive sensor is derived from the intercalation of PDA into interlayers of GO. Moreover, an additional broad peak appears at 2θ = ∼25.427° in the XRD pattern, suggesting the reduction of GO to some extent. The sensitivity of the as-prepared PGO thermosensitive sensor was investigated (Figure S7 in the Supporting Information). The microstructure of the as-prepared FAW based on the fire-resistant HN/GF paper and the PGO thermosensitive sensor is shown in Figure S10 in the Supporting Information. It should be mentioned that the PGO thermosensitive sensor is weakly electrically conductive (σ = 11.19 S m−1, Figure 6d and Table S2 in the Supporting Information) owing to a certain degree of reduction of GO by PDA. However, the alarm lamp connected with the PGO thermosensitive sensor at a low applied voltage (2.5 V) exhibits only weak visible light, and the alarm buzzer does not send out any audible sound. As shown in Figure S7 and Movie S4 (Supporting Information), the bright light and loud sound are sent out from the PGO thermosensitive sensor after thermal treatment at 150 °C for 3 min, and the electrical conductivity is 188.83 S m−1 (Figure 6d and Table S2 in the Supporting Information). Consequently, the thermal responsive temperature is determined in the range of 100−150 °C. The properties of the PGO thermosensitive sensor with increasing temperature were also investigated. As oxygencontaining groups of the PGO thermosensitive sensor are removed to some extent by PDA, the peak intensities of oxygen-containing groups in the XPS spectrum of the PGO thermosensitive sensor (Figure S7c) are lower than those of the

Dopamine can self-polymerize to PDA. In this process, GO can be not only reduced but also attached with the PDA coating.51,57 (3) PDA plays dual roles as a reductant and a capping agent. As a reductant, reducing GO will remove some oxygen-containing groups and improve the sensitivity of the sensor, whereas as a capping agent, PDA can improve the fire retardancy of the GO thermosensitive sensor. PDA has multiple flame-retardant actions: (1) the nitrogen element in PDA is considered as a flame-retardant element, which produces noncombustible gases such as N2 and NH3 in the flame and further hinders the oxygen transfer.1,44,58 (2) During the burning process, PDA transforms into the protective char as a physical barrier to the flame. (3) Catechol groups of PDA may scavenge free radicals to suppress the fuel supply.46,59,60 The successful fabrication of polydopamine-modified GO thermosensitive sensor is verified by XPS (Figure 6a) and Fourier transform infrared (FTIR) spectra. An additional N 1s peak at 400.1 eV emerges in the XPS pattern of the PGO thermosensitive sensor (Figure 6a), and the high-resolution C 1s peak in the XPS pattern of the PGO thermosensitive sensor shows the presence of the C−N group (Figure S7 in the Supporting Information), suggesting the successful attachment of PDA on the GO surface. FTIR spectra of GO, PDA, and PGO (Figure S8 in the Supporting Information) further confirm the successful coating of PDA on the GO. In addition, the GO and PGO thermosensitive sensors were characterized by Raman spectroscopy and XRD analysis. Raman spectra of the GO and PGO thermosensitive sensors (Figure 6b) exhibit two typical bands at 1340 cm−1 (D-band) and 1587 cm−1 (Gband). However, the intensity ratio of ID/IG increases from 0.97 for the GO thermosensitive sensor to 1.14 for the PGO thermosensitive sensor. The higher intensity ratio is attributed to more defects and disorder in the structure of the PGO thermosensitive sensor,38,44 which is a common phenomenon for the reduced GO.33 More defects and disorder in the F

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Figure 5. (a) Schematic illustration of an electrical circuit for determining the thermal responsive temperature of the GO thermosensitive sensor. (b) Electrical current (I) through the GO thermosensitive sensor with increasing temperature. Inset: Digital images of the alarm lamp and alarm buzzer connected with the GO thermosensitive sensor at different temperatures. (c) Schematic illustration of an electrical circuit for monitoring the working process of the GO thermosensitive sensor in the flame. (d) Electrical current through the GO thermosensitive sensor in the flame with time. Inset: Schematic illustration of the FAW in the flame, and the GO thermosensitive sensor is burnt out. (e) Realtime monitoring of the changes of the alarm lamp, alarm buzzer, and GO thermosensitive sensor in the flame.

deoxygenation. The EDX analysis also shows the increase of atomic percentage of carbon and the fall of atomic percentage of oxygen with increasing thermal treatment temperature (Figure 7a). As discussed above, the experimental results indicate that the thermal responsive temperature of the PGO thermosensitive sensor is within the range of 100−150 °C. The accurate thermal responsive temperature of the PGO thermosensitive sensor was further determined by real-time monitoring of the electrical current change with increasing temperature in an electric oven (Figure 7b). The uptrend of electrical current through the PGO thermosensitive sensor is similar to the GO thermosensitive sensor but in a different fashion. Compared with the faster change of electrical current through the GO thermosensitive sensor with increasing temperature (Figure 5b), the gentle increase of electrical current through the PGO thermosensitive sensor is a result of its enhanced thermal stability. The thermal responsive temperature (electrical current = 0.10 mA) of the PGO thermosensitive sensor is determined to be 126.9 °C, which is much lower than that of the GO thermosensitive sensor (231.3 °C, Table 1). Movie S5 in the Supporting Information shows the real-time monitoring of the thermal responsive behavior of the PGO thermosensitive

Table 1. Sensitivity and Thermal Stability of the GO and Polydopamine-Modified GO (PGO) Thermosensitive Sensor substrate HN/GF paper HN/GF paper commercial wallpaper

sensor

responsive temperature (°C)

response time

alarm time

GO PGO PGO

231.3 126.9 126.9

5s 2s 2s

3s >5 min 7s

GO thermosensitive sensor (Figure 3c). The peak intensities of oxygen-containing groups of the PGO thermosensitive sensor are at low levels after thermal treatment (Figure S7d−f in the Supporting Information). The deoxygenation of the PGO thermosensitive sensor after thermal treatment is confirmed by the XRD (Figure 6c) and EDX analysis (Figure 7a). A sharp XRD peak of the PGO thermosensitive sensor at 2θ = 10.780° shifts to a higher degree (2θ = 10.863°, d = ∼8.14 Å) after thermal treatment at 100 °C (Figure 6c). The decreased interlayer spacing can be explained by the removal of some oxygen-containing groups. This diffraction peak totally disappears for the PGO thermosensitive sensor after thermal treatment at greater than 150 °C, indicating the high degree of G

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Figure 6. XPS patterns (a) and Raman spectra (b) of the GO and PGO thermosensitive sensor. XRD patterns (c) and electrical conductivities (σ) (d) of the PGO thermosensitive sensor after thermal treatment at different temperatures.

Figure 7. (a) Atomic percentages of carbon and oxygen of the PGO thermosensitive sensor after thermally treating at different temperatures. (b) Electrical current (I) through the PGO thermosensitive sensor with increasing temperature; the inset shows digital images of an alarm lamp and alarm buzzer connected with the PGO thermosensitive sensor at different temperatures. (c) TG curves of the GO and PGO thermosensitive sensors. (d) Electrical current through the PGO thermosensitive sensor on the fire-resistant HN/GF paper in the alcohol flame with time; the inset shows a schematic illustration of the intact PGO thermosensitive sensor on the HN/GF paper after the test in the alcohol flame. (e) Real-time monitoring of the alarm lamp, alarm buzzer, and PGO thermosensitive sensor on the HN/GF paper during the testing process in the alcohol flame.

H

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Figure 8. (a) TG curves of GFs, HNs, and commercial wallpaper (CW). (b) Electrical current (I) through the PGO thermosensitive sensor on commercial wallpaper with time in the alcohol flame; the inset is a schematic illustration of the broken PGO thermosensitive sensor on the commercial wallpaper after the burning process. (c,d) Fire resistance tests of the HN/GF inorganic paper (c) and commercial wallpaper (d). (e) Real-time monitoring of the changes of the alarm lamp, alarm buzzer, and PGO thermosensitive sensor on commercial wallpaper during the burning process.

sensor on the fire-resistant HN/GF paper between 117.5 and 129.8 °C. The enhanced thermal stability of the PGO thermosensitive sensor was investigated. As shown in Figure 7c, the TG curve shows that the residue weight of the PGO thermosensitive sensor is 58.69% of the initial weight at 1000 °C in air; in contrast, the residue weight of the GO thermosensitive sensor is only 1.96% of the initial weight, indicating the enhanced thermal stability of the PGO thermosensitive sensor compared with the GO thermosensitive sensor. In addition, as discussed above, the PGO thermosensitive sensor has multiple flameretardant actions including the production of noncombustible gas and formation of protective char and free radical scavenging. The performance of the PGO thermosensitive sensor in the flame was investigated. After the FAW was directly exposed to the alcohol flame, the PGO thermosensitive sensor rapidly responds to the high temperature of fire. The electrical current in a very short time period (2 s) reaches as high as 0.20 mA, and consequently, the thermal response time is determined to be 2 s (Table 1). Compared with that of the GO thermosensitive sensor, the faster response of the PGO thermosensitive sensor to the fire results from lower thermal responsive temperature. More importantly, the PGO thermosensitive sensor is able to steadily work and send out the alarms in the flame for a relatively long period of time (at least 5 min, Figure 7d,e). The electrical current through the PGO thermosensitive sensor reaches a maximal value of 0.32 mA at 8 s, and then it is relatively stable for several minutes in the flame. The alarm time of the PGO thermosensitive sensor is able to last for more than 5 min in the flame (Table 1). The

real-time monitoring of changes of the alarm lamp, alarm buzzer, and PGO thermosensitive sensor on the fire-resistant HN/GF paper for the first 60 s in the alcohol flame is shown in Movie S6 in the Supporting Information. The experiments indicate that the sensitivity and thermal stability of the thermosensitive sensor can be improved by modifying GO with PDA, and the as-prepared PGO thermosensitive sensor shows lower thermal responsive temperature, faster response time, and longer alarm time, which can send out timely and sustained alarms in a fire disaster. The excellent thermal stability and fire resistance of the substrate is also indispensable for high-performance FAW. The synergistic effect between the substrate and thermosensitive sensor contributes to the long-lasting service of FAW in a fire disaster. To understand such synergistic effects, commercial wallpaper was used as a control sample (Figures S11 and S12 in the Supporting Information). Commercial wallpaper made of plant cellulose fibers shows high combustibility and poor thermal stability. The TG curve of the commercial wallpaper shows that the weight loss is as high as 93.41% at 1000 °C in air (Figure 8a). In contrast, both HNs and GFs have high inherent thermal stability, and the TG curves show that the weight loss of HNs and GFs is only 2.90 and 0.33%, respectively (Figure 8a). The vertical combustibility method was adopted to test the fire resistance of the HN/GF inorganic paper and commercial wallpaper.2,61 The experiment indicates that HN/GF inorganic paper has excellent fire resistance, and the HN/GF paper can be well preserved in the flame for a long period of time (Figure 8c and Movie S7 in the Supporting Information). In contrast, commercial wallpaper is easily burnt out in a very short time I

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Figure 9. (a,b) As-prepared fire-resistant HN/GF inorganic paper can be folded into complex shapes such as a paper airplane (a) or paper crane (b). (c) Fire-resistant HN/GF paper sheets dyed with different colors. (d−f) Various colorful patterns and images can be printed on the fire-resistant HN/GF paper using a commercial printer.

colored patterns and shapes. The HN/GF paper with a white color, mechanical robustness, high flexibility, and excellent fire resistance can exactly satisfy these requirements. As shown in Figure 9a,b, the fire-resistant HN/GF paper can be folded into complex shapes such as the paper airplane and paper crane. By introducing a small amount of dyes into the initial HN/GF aqueous suspension, the HN/GF paper sheets with different colors like purple, pink, red, blue, yellow, and orange can be fabricated (Figure 9c). More attractively, the fire-resistant HN/ GF inorganic paper can be well printed with a commercial printer, and the colorful patterns and images can be clearly printed on the fire-resistant HN/GF paper (Figure 9d−f).

(within 10 s) (Figure 8d and Movie S8 in the Supporting Information). The fire resistance tests of the HN/GF inorganic paper (in first 60 s) and commercial wallpaper are shown in Movies S7 and S8 in the Supporting Information, respectively. The slight color change of the HN/GF inorganic paper in the fire results from a small amount of adsorbed oleic acid molecules on the surface of HNs. Although the PGO thermosensitive sensor on the commercial wallpaper has the same thermal responsive temperature and response time as those on the HN/GF paper (Table 1), the PGO thermosensitive sensor is unable to work steadily and continuously because the commercial wallpaper is burnt out rapidly in the fire. The electrical current drops to 0.03 mA very rapidly (at 9 s) (Figure 8b), and the alarm lamp and alarm buzzer cannot work after that time (Figure 8e and Movie S9 in the Supporting Information). As shown in the inset of Figure 8b, the devastation of the commercial wallpaper from flames is accompanied by the damage of the PGO thermosensitive sensor. The alarm time of the PGO thermosensitive sensor on commercial wallpaper is very short (only 7 s), which is much shorter than that on the HN/GF inorganic paper (>5 min, Table 1). When the HN/GF paper is used as the substrate of FAW, it can maintain the structural integrity and well protect the PGO thermosensitive sensor during its exposure to the flame (inset of Figure 7d). For the interior decoration of houses, it is highly desirable that the fire-resistant FAW can be processed into various

CONCLUSION In summary, we report a smart fire alarm wallpaper by using fire-resistant HN/GF inorganic paper as a substrate and GO (or PGO) as a thermosensitive sensor, which can prevent the fire from spreading and automatically send out the alarms to people when a fire disaster occurs. When being exposed to high temperature, the electrically insulated GO thermosensitive sensor transforms into an electrically conductive state, and as a result, the alarm lamp and alarm buzzer connected with the GO thermosensitive sensor are activated and send out timely alarms to people for taking emergency actions. The sensitivity and flame retardancy of the GO thermosensitive sensor can be improved by modifying GO with polydopamine to form the PGO thermosensitive sensor. The PGO thermosensitive sensor J

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ACS Nano on the fire-resistant HN/GF paper has low thermal responsive temperature (126.9 °C), rapid response time (within 2 s), and long alarm time (>5 min) in the flame. In addition, the fireresistant HN/GF inorganic paper with a white color, mechanical robustness, and high flexibility can be easily processed into various shapes and can be easily printed with a commercial printer. The materials used in the as-prepared smart fire alarm wallpaper including GO, PDA, HNs, and GFs are environmentally friendly, which is promising for applications in interior decoration of houses. The as-prepared smart fire alarm wallpaper provides insights into the design and preparation of flame-retardant materials with smart responsive behaviors and broadens the applications of the ultralong hydroxyapatite nanowire-based fire-resistant inorganic paper and graphene-related materials. In addition, the smart fire alarm wallpaper reported herein will be helpful for public safety by saving human lives and reducing the loss of property in a fire disaster if it can be widely used.

Dyeing of the HN/GF Fire-Resistant Inorganic Paper. The HN/GF fire-resistant inorganic paper was dyed with the help of a color fixing agent (poly(vinyl alcohol), PVA). Fifty milligrams of the dye (methyl blue, methyl orange, Congo red, crystal violet, brilliant yellow, or rose bengal) and PVA aqueous solution (5 mL, 2.5 wt %) were added into a 40 g aqueous suspension containing 40 mg of HNs and 10 mg of GFs, and the resulting mixture was stirred for 10 min and aged for 30 min before making the paper. After filtration, the HN/ GF paper sheets with different colors were obtained. Characterization of Samples and Instruments. The samples were characterized by transmission electron microscopy (H-800, Hitachi, Japan), scanning electron microscopy (SU-8220 and TM3000, Hitachi, Japan), X-ray photoelectron spectroscopy (Al Kα radiation, ESCAlab 250XI, Thermo Scientific), X-ray diffraction (Cu Kα radiation, λ = 1.54178 Å, D/max 2550 V, Rigaku, Japan), and Fourier transform infrared spectroscopy (FTIR-7600, Lambda Scientific, Australia). The elemental mapping and elemental analysis were carried out on an energy-dispersive X-ray spectrometer (XMAXN, Oxford Instruments). Thermogravimetric analysis was performed on a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) with a heating rate of 10 °C min−1 in flowing air. Electrical conductivities were measured on a physical property measurement system (Quantum Design, USA). The thickness of the fire-resistant HN/GF paper was measured on a high-precision paper thickness gauge (GDH-3, Qingtongboke Automation Technology Co., Ltd., China). The tensile tests of the fire-resistant HN/GF paper (40 × 10 mm2) were performed on a universal testing machine (DRK-101B, Drick, China) at a loading rate of 0.2 mm min−1. The voltage was applied by a direct current power supply (KXN-305D, Zhaoxin Electronics, China). The electrical current was measured by a digital multimeter (MT-1280, Pro’skit). The fire-resistant HN/GF paper was printed with a commercial printer (Epson Stylus Photo R330).

EXPERIMENTAL SECTION Materials and Chemicals. CaCl2, NaOH, NaH2PO4·2H2O, methanol, and tris(hydroxymethyl)aminomethane were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleic acid and dopamine hydrochloride were purchased from Aladdin Industrial Corporation. Graphene oxide, conductive silver paste, polydimethylsiloxane (Sylgard 184, Dow Corning Corporation), glass fibers, and copper wires were commercially available. The materials and chemicals were used as received without further purification. Synthesis of Ultralong Hydroxyapatite Nanowires. HNs were synthesized via the calcium oleate precursor solvothermal method.12,13 In brief, NaOH (10.500 g) aqueous solution (150 mL), CaCl2 (3.330 g) aqueous solution (120 mL), and NaH2PO4·2H2O (9.360 g) aqueous solution (180 mL) were separately added into a mixture of deionized water (135 mL), methanol (60 mL), and oleic acid (105 mL) under mechanical stirring. The resulting mixture was transferred into a Teflon-lined stainless steel autoclave (1 L), sealed, and thermally treated at 180 °C for 24 h. After cooling to room temperature, the obtained HNs were separated, washed with ethanol and deionized water three times, respectively, and dispersed in deionized water for further use. Fabrication of Ultralong Hydroxyapatite Nanowire/Glass Fiber Fire-Resistant Inorganic Paper. GFs were dispersed in deionized water under ultrasound treatment for 3 min. During ultrasound treatment, an aqueous suspension containing HNs was slowly added into the aqueous suspension of GFs, and the resulting mixture was stirred magnetically for 3 min to form a uniform aqueous suspension to make paper (Figure S4 in the Supporting Information). After filtration, the HN/GF paper was pressed under an applied pressure of 4 MPa for 3 min, followed by drying at 90 °C for 3 min (Figure S4 in the Supporting Information). Fabrication of Graphene Oxide and Polydopamine-Modified GO Aqueous Ink. For the preparation of the GO aqueous ink, GO (60 mg) was dispersed in deionized water (30 mL); the resulting GO aqueous suspension was stirred magnetically for 1 h and then subjected to ultrasound treatment for 30 min, and the GO aqueous ink was obtained at a concentration of 2 mg mL−1. For the preparation of the PGO aqueous ink, the dopamine hydrochloride (60 mg) and tris(hydroxymethyl)aminomethane (36 mg) were added in the GO aqueous ink at a concentration of 2 mg mL−1, and the resulting suspension was stirred magnetically at 60 °C in air for 12 h to obtain the PGO aqueous ink. Preparation of Fire Alarm Wallpaper. The thermosensitive sensor was installed on the fire-resistant HN/GF inorganic paper by drop-casting the GO or PGO aqueous ink. After being dried at 60 °C, two edges of the thermosensitive sensor were connected with Cu wires (0.2 mm) as external electrodes using conductive silver paste and PDMS (base agent/curing agent = 10:1), and the as-prepared FAW was thermally treated at 80 °C for 30 min.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00047. Figures S1−S10 and Tables S1 and S2 provide additional experimental and analysis results for the GO thermosensitive sensor, PGO thermosensitive sensor, HN/GF fire-resistant inorganic paper and other samples including SEM images, TEM images, digital images, EDX elemental mapping, XPS spectra, FTIR spectra, mechanical properties, and other analysis results (PDF) Movie S1: alarm lamp and alarm buzzer connected with the GO thermosensitive sensor on the fire-resistant HN/ GF paper after being thermally treated at 250 °C for 3 min (AVI) Movie S2: real-time monitoring of the thermal responsive behavior between 228.9 and 232.0 °C of the GO thermosensitive sensor on the fire-resistant HN/GF paper (AVI) Movie S3: real-time monitoring of the thermal responsive behavior of the GO thermosensitive sensor on the fire-resistant HN/GF paper in the alcohol flame (AVI) Movie S4: alarm lamp and alarm buzzer connected to the PGO thermosensitive sensor on the fire-resistant HN/ GF paper after being thermally treated at 150 °C for 3 min (AVI) Movie S5: real-time monitoring of the thermal responsive behavior of the PGO thermosensitive sensor on the fire-resistant HN/GF paper between 117.5 and 129.8 °C (AVI) Movie S6: real-time monitoring of changes of the alarm lamp, alarm buzzer, and PGO thermosensitive sensor on K

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ACS Nano the fire-resistant HN/GF paper for the first 60 s in the alcohol flame (AVI) Movie S7: fire resistance test of the HN/GF paper for the first 60 s (AVI) Movie S8: fire resistance test of the commercial wallpaper (AVI) Movie S9: real-time monitoring of the thermal responsive behavior of the PGO thermosensitive sensor on the commercial wallpaper (AVI)

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Feng Chen: 0000-0002-1162-1684 Zhi-Chao Xiong: 0000-0002-0216-5699 Notes

The authors declare no competing financial interest.

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