Sea urchin-like microstructures pressure sensors with ultra-sensitivity and super working range

Sensitivity and pressure range are two significant parameters of pressure sensors. The existing pressure sensors are difficult to achieve both high sensitivity and a wide pressure range. In this regard, we proposed a new pressure sensor with a ternary nanocomposite Fe 2 O 3 /C@SnO 2 . Notably, the sea urchin-like Fe 2 O 3 structure promoted signal transduction and protected Fe 2 O 3 needles from mechanical breaking; while, acetylene carbon black improved the conductivity of Fe 2 O 3 . Moreover, one part of SnO 2 nanoparticles adhered to the surface of Fe 2 O 3 needles and formed Fe 2 O 3 /SnO 2 heterostructures whereas its other part of nanoparticles dispersed into the carbon layer and formed SnO 2 @C structures. Collectively, the synergy of the three structures (Fe 2 O 3 /C, Fe 2 O 3 /SnO 2 and SnO 2 @C) improved the limited pressure response range of a single structure. The experimental results demonstrated that the Fe 2 O 3 /C@SnO 2 pressure sensor exhibits high sensitivity (680 kPa -1 ), fast response (10 ms), broad range (up to 150 kPa), and good reproducibility (over 3500 cycles under a pressure of 110 kPa). This implies that the new pressure sensor has wide application prospects especially in wearable electronic devices and health monitoring.

Recently, different microstructure or nanostructure geometries such as (interlocked microstructures 17 , hollow-sphere microstructure 18 , micro-pyramid array 19 , and porous structure 20 ) has been explored to improve the sensitivity of piezoresistive pressure sensors. Among them, the tapering geometry or spine structure confers a clever design that not only promotes signal transduction for high sensitivity, but also protects the bristle from mechanical breaking 21,22 . Similar structural has been employed in mechanical sensors and yielded enhanced sensing performance. For example, Yin et al. noted that ZnO sea urchin-shaped microparticles with a low-temperature solution process exhibited a high sensitivity of 121 kPa -1 (pressure range 0 -10 Pa) 23 . Lee et al.
achieved a sensitivity of 2.46 kPa -1 (pressure range 0 -1 kPa) with a piezoresistive pressure sensor based on sea-urchin shaped metal nanoparticles 24 . In another work by Shi et al. studied the urchin-like hollow carbon spheres, and the sensitivity reached 260.3 kPa -1 at 1 Pa 25 . Based on the above background, the piezoresistive pressure sensors have high sensitivity only under a small pressure range. Additionally, without any additives, that is, relying only on its own structure and performance, it is difficult for the sensing material to achieve high sensitivity and wide pressure working range at the same time. Of note, the low conductivity of a single metal oxide semiconductor limits the pressure response range of the piezoresistive pressure sensors.
There are two widely researched metal oxide including Fe2O3 and SnO2, because of their low-cost, environmental friendliness, and natural abundance. Studies have reported that coupling metal oxide and carbon compounds to form metal oxide/C nanocomposite, may improve photocatalytic 26 and electrochemical performances 27 .
Despite metal oxide/C nanocomposite processing large specific surface area and strong conductivity, they have rarely been employed in fabricating flexible pressure sensors.
In this work, we proposed a nanostructure design of materials with ultra-sensitivity for an ultra-broad-range pressure sensor. Particularly, this strategy involves the use of acetylene black carbon as a carrier due to its strong conductivity and high specific surface. The acetylene black carbon encloses particles Fe2O3, thereby forming Fe2O3/C structure. Furthermore, one part of SnO2 nanoparticles were dispersed into the carbon layer and formed SnO2@C structures, whereas its other part of nanoparticle adhered to the surface of Fe2O3 needles and formed Fe2O3/SnO2 heterostructures. Carbon improves the conductivity of a single metal oxide. Collectively, the synergy of the three structures (Fe2O3/C, Fe2O3/SnO2 and SnO2@C) improved the limited pressure response range of a single structure. Notably, the Fe2O3/C@SnO2 (3:1:4) pressure sensor exhibited high sensitivity (680 kPa -1 ), fast response (10 ms), broad range (up to 150 kPa) and good reproducibility (over 3500 cycles under a pressure of 110 kPa).    6 The fabrication process used in this study is illustrated in Fig. 1. First, conductive materials were synthesized using hydrothermal method. Then, a clean melamine sponge was soaked in the sample solution. Finally, after the electrode connection, a pressure sensor with a melamine sponge substrate was obtained.
Besides, one part of SnO2 nanoparticles were well tightly attached to Fe2O3 needles, which indicates the formation of a heterojunction between the Fe2O3 and SnO2.

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To measure the piezoresistive characteristics of is pressure sensor, we set up a custom-made system composed of a universal testing machine and a digital source meter. The formula for calculating sensitivity was based on S = (ΔI/I unloading)/Δp, where ΔI = I loading-I unloading, which refers to relative current change, while Δp refers to the change of pressure. The measurement results are depicted in Fig. 4a, Fig. S3, and Table. S1. The sensitivity of Fe2O3/C@SnO2 pressure sensor was higher than that of Fe2O3, Fe2O3/C, and SnO2@C pressure sensors. The sensitivity of Fe2O3/C@SnO2 (3:1:4) sensors is S1 ~ 680 kPa -1 when the pressure is below 10 kPa, is S2 ~ 98 kPa -1 within the pressure range of 10 -50 kPa, and is S3 ~ 35 kPa -1 within the pressure range of 50 -150 kPa. It is apparently higher than that of ZnO sea urchin-like, carbon sea urchin-like, Ag/Au sea urchin-like, and other pressure sensors, as shown in Table S2. Although the sea urchin-like Fe2O3 structure promoted signal transduction and protected Fe2O3 needles against mechanical breakage, the sensitivity of the sensor (3 kPa -1 ) was still lower, because Fe2O3 has poor conductivity. Also, the change in current was still smaller even under the effect of larger pressure. Fig. S3a and Table. S1 shows the current response of the pressure sensors under the different mass ratio of Fe2O3 and carbon, which indicates that the mass ratio of (3:1) has the highest sensitivity (203 kPa -1 ) compared to the other lower ratios (2:1, 1:1 and 1:2) and also the large one (4:1), let alone the pure carbon. A major reason for the high sensitivity of the Fe2O3/C pressure sensor can be explained as follows. On one hand, the microfibers of sponge are composed of nanocomposites so that the contact area is increased and thus leading to an increase of current, as viewed in Fig. S6. On the other hand, when adding carbon into the Fe2O3 system, the Fe2O3/C nanocomposite exhibits a larger current variation compared to that of pure carbon. Under the constant mass of Fe2O3, followed with increased amounts of carbon, the sensitivity of the Fe2O3/C pressure sensor increases due to the increase of the conductive path. However, the excess addition of carbon to the sensor may significantly increase conductivity (when the mass ratio of carbon and  Table S1). Furthermore, when the two semiconductors were brought in contact and subjected to high-temperature calcination, the hand alignment occurred driven by the equilibration of the Fermi level as shown in Fig. S4  The key feature of the Fe2O3/C@SnO2 (3:1:4) pressure sensor is high sensitivity in a wide pressure range. In this respect, to evaluate the sensitivity of this sensor under high pressure, it was tested as follows. Specifically, the pressure sensor was subject to different pressure values at 1.5, 10 and 50 kPa, as illustrated in Fig. 5a-c. First, the sensor was compressed to the set pressure value, followed by consecutive addition of three coins, each weighing about 3.19 g, which is equivalent to a pressure of 86 Pa.
Each pressure increment caused a step increase of current, and the current signal is stable. In another experiment, a pressure sensor with a volume of V = 19×19×4 mm 3 was placed under the front wheel of a car (the weight of the car is 1670 kg) as shown in Fig. 5d. Thereafter, a carton of milk weighing 4 kg put on the driving seat of the car and then taken away as indicated in Fig. 5e. Consequently, the changes in current were successfully detected. Likewise, when the male passenger with a weight of 73 kg gets into or out of the car, the current changed significantly (Fig. 5f). The circled vibrations in Fig. 5f demonstrated that the sensor can accurately capture the movement of the male passenger getting on or off the car. It shows that the sensor still has high sensitivity under the high pressure.

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Due to the high sensitivity, faster response, and broad pressure regime of the sensor (Fe2O3/C@SnO2 (3:1:4)), it can thus be applied in many fields. For instance, it can be used to detect voice, wrist pulse, and human motion activities. Fig. 6a shows the realtime wrist pulse detecting using the Fe2O3/C@SnO2 (3:1:4) pressure sensor. The testing curves revealed strong characteristic peaks of the human sphygmic waveforms, and the pulse rate at about 73 times/min, which is the normal level. Notably, based on the excellent performance of the Fe2O3/C@SnO2 (3:1:4) pressure sensor, it can be used for monitoring human health. Subsequently, this pressure sensor was attached to the human throat to monitor and distinguish subtle differences during the muscle motions near the throat, when the words "one", "two" and "three" were spoken as shown in Fig. 6b.
Interestingly, this technique can be applied to deaf and mute people who are unable to speak. It is well-known that their vocal cords can vibrate, and thus vibration produced can be transformed into the required sound 7 . Moreover, the pressure sensor was mounted on the cheek to monitor the occlusion movement of humans as displayed in Fig. 6c. Upon occlusion, the current changed significantly, which once again proves the excellent performance of the sensor. Also, the pressure sensor was attached to the arm to detect the radial muscle contraction, which occurs when making a fist as viewed in Fig. 6d. When the tester made a fist, the current signal increased as well as the compression of the sensor. Therefore, this result illustrates its valuable potential application in physical training and curing muscle damage. The response of the sensor for continuously bending six different motions of the finger is elucidated in Fig. 6e.
Consequently, it was noted that the sensor showed different responsive current signals to different motions of the finger. In particular, the current signal exhibited a slight increase when the finger bent in small-scale (motion-I, motion-II, and motion-III), whereas larger-scale bending led to the sharp increase of current value (motion-IV, motion-V, and motion-VI). The large-scale movements resulted in a strong compression of the sensor and thus forming more conductive pathways. These findings demonstrated that the pressure sensor can precisely distinguish different-scale motions of the finger.
In addition, Fig. 6f shows the pressure sensor mounted on foot using tape to monitor walking states. The response signals of walking motion were stable and repetitive. This suggests that it can be applied for gait recognition and motion monitoring. Conclusively, these outcomes enumerate that the Fe2O3/C@SnO2 pressure sensor holds broad application prospects in the fields of medical health and wearable electronic devices.

Conclusion
In summary, we present a sea urchin-shaped microstructure Fe2O3/C@SnO2, which was synthesized using a simple and environmentally friendly hydrothermal method. We also provide a pressure sensor with high sensitivity and a large working range based on a simple dip-coating process method. Notably, the Fe2O3/C@SnO2 pressure sensor exhibits high sensitivity (680 kPa -1 ), fast response (10 ms), broad range (up to 150 kPa), and good reproducibility (over 3500 cycles under a pressure of 110 kPa). Interestingly, multiple human physiological activities (such as pulse, pronouncing, joint bending, and walking, among others) could be monitored using the Fe2O3/C@SnO2 pressure sensor. Lastly, based on the above excellent performance of the device, it has significant implications especially in wearable electronics, health monitoring, and measuring pressure distribution.

Synthesis of Fe2O3
The detailed processing method for the synthesis of Fe2O3 used in this study can be found in one of the authors' previous works 32 . In particular, the Fe2O3 was synthesized through a hydrothermal process, where 0.405 g FeCl3.6H2O and 0.205 g Na2SO4 were first dissolved in distilled water (30 mL) and stirred for 10 min. Afterward, the mixture was heated at 120 ºC for 6 h in a Teflon-lined stainless-steel autoclave.
Lastly, after cooling, filtering, drying, and thermal annealing at 400 ºC for 3 h under air, Fe2O3 was obtained.

Preparation of Fe2O3/C composites
Besides adding different masses of carbon, the same process was used for the synthesis of Fe2O3/C composites.

Preparation of conductive sponges and electrode
Melamine sponge was cut into a cuboid with a length of 19 mm, width (19 mm), and height (4 mm). Then it was the melamine sponge was washed several times with ethanol and dried at 45℃. Thereafter, Fe2O3/C@SnO2 (SnO2@C, Fe2O3/C, Fe2O3, SnO2, and C) and Polyvinylidene fluoride (PVDF) binder were dissolved in N-methyl pyrrolidone (NMP) in a weight ratio of 10:1 and mixed to form a slurry. Next, the melamine sponge strip was immersed in slurry until full and dried at 45℃ in a vacuum.
The copper wire was soldered on the copper tape with a size of 19 mm×19 mm while the sponge was sandwiched between two copper tape.

Characterization of structures and performance of the sensors
The crystal structures of the samples were explored using X-ray diffraction (XRD, PANalytical X'Pert Powder), whereas, the morphology was characterized using scanning electron microscopy (SEM; Quattro S), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping and transmission electron microscopy (TEM; Talos F200S). The binding energy of products was investigated using X-ray photoelectron spectroscopy (XPS; ESCALAB250Xi).
Moreover, the loading of pressure was examined using a universal testing machine (ETM-5038, Shenzhen Wance Testing Machine Co., Ltd.), while the electrical signals of the pressure sensors were recorded at the same time using a Keithley 2450 digital meter at a constant voltage of 0.1 V. Finally, to assess the response time of the pressure sensor, a Keithley DAQ6510 multimeter was used.