Highly Porous Double-shelled Hollow Hematite Nanoparticles for Gas

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Highly Porous Double-shelled Hollow Hematite Nanoparticles for Gas Sensing Tiantian Ma, Lingli Zheng, Yingqiang Zhao, Yongshan Xu, Jun Zhang, and Xianghong Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00228 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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ACS Applied Nano Materials

Highly

Porous

Double-shelled

Hollow

Hematite

Nanoparticles for Gas Sensing Tiantian Maa,‡ Lingli Zhenga,‡ Yingqiang Zhao,b Yongshan Xua, Jun Zhanga,c,*, Xianghong Liua,c,* aCollege

of Physics, Qingdao University, Qingdao 266071, China

College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal

b

University, Jinan 250014, China cKey

Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

‡ The authors contributed equally to this work. Email: [email protected], [email protected] 1

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ABSTRACT Multishelled hollow micro/nanostrutcures hold a great potential for a variety of important applications due to their fascinating properties. However, the synthesis of multi-shelled hollow micro/nanostrutcures mainly relies on template-involved methods. A template-free protocol for multishelled hollow structures remains quite challenging. In this work, a solution strategy based on inside-out and outside-in Ostwald ripening is developed to manipulate hematite hollow spheres with a single- or double shells without using any hard or soft templates. The structure evolution of the hematite hollow spheres is comprehensively studied in order to reveal the formation mechanism. As a potential application, the hematite hollow spheres have been utilized for chemical gas sensors to investigate the structure-property correlations. Results demonstrate that the double-shelled hematite spheres exhibit much higher response than the single-shelled due to structure sensitization. An improved sensor response is further obtained by catalytic sensitization from Au nanoparticles. Outstanding sensing performances in terms of response speed, sensitivity and detection limit towards acetone have been obtained by combining the dual sensitization effects. This work provides an alternative approach for the production of multi-shelled functional materials and a demonstration of the materials for application in high performance gas sensors.

KEYWORDS: hollow structure, double shells, iron oxide, Ostwald ripening, gas sensor

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1. Introduction Multi-shelled hollow micro/nanostructures have attracted great attention due to the intriguing properties derived from their peculiar structures, such as high specific surface area, low density, high accessibility to the inner surface and high loading capacity.[1-3] To date, multi-shelled hollow micro/nanostructures have been fabricated from metal oxides (Fe2O3[4], TiO2[5] MgCo2O4[6], NiCo2O4[7] and CoFe2O4[8]), metal sulfides (CuS[9], NiS[10]), and MOFs[11], which has delivered excellent performance in the fields of sensors[12,

13],

catalysis[18,

supercapacitors[14], lithium-ion batteries[15,

19].

The

traditional

synthesis

of

these

16],

solar cells[17] and

multi-shelled

hollow

micro/nanostructures generally requires the use of a template, either hard template such as carbon spheres[7, 12, 20, 21] or fibers[22], or soft template such as structure-directing surfactant[23]. While being effective in controlling the number of shells, the templateinvolved method usually suffers from low efficiency, low potential for scale production, high cost and time-consuming steps to remove the templates.[24, 25] Hematite Fe2O3 is an important semiconductor widely used in the area of gas sensor, catalysis, energy devices, ect., due to its structure stability, low toxicity, easy synthesis and large abundance.[26-29] The synthesis of hematite multi-shelled hollow micro/nanostructures have attracted significant interest. Xu et al.[26] prepared multishelled -Fe2O3 hollow microspheres with the sacrifice of carbonaceous template and proved the improvement of electrochemical performance, owing to large specific surface area and the porous on the shell. For example, Zhang et al.[28] synthesized multicomposition microboxes using metal-organic frameworks (MOFs) template. Recently, Sun et al.[29] reported the synthesis of multi-shelled Fe2O3 hollow fibers by using alginate fibers as the template. Importantly, the crucial role of the multi-shelled structure have been demonstrated by their advanced applications in gas sensing or energy storage devices. It is noted that the manipulation of hematite multi-shelled hollow micro/nanostructures using a straightforward non-template method has been unsuccessful. Herein, we report on the synthesis of single- or double-shelled Fe2O3 hollow nanospheres by a template-free solution strategy based on inside-out and outside-in 3



Ostwald ripening mechanism (Scheme 1). The structure evolution of the Fe2O3 hollow spheres has been systematically studied to elucidate the formation mechanism. Inspired by the intrigue properties afforded by the double-shell structure, the Fe2O3 hollow spheres have been tested as sensing materials for detection of gaseous chemicals, which delivers fast and high response. In an attempt to further optimize the sensing performances, the Fe2O3 spheres have been functionalized with catalytic Au nanoparticles, and superior performances including fast response speed, high sensitivity and detection limit have been registered. A synergistic mechanism combining the structure sensitization of double-shells and the catalytic sensitization effect of Au has been proposed for the significantly enhanced sensor performances.

Scheme 1. Schematic illustration of the formation mechanism of hollow Fe2O3 nanospheres.

2 Experimental Preparation of hollow Fe2O3 nanospheres. Hollow Fe2O3 nanospheres were synthesized by the hydrothermal method. Typically, 0.1976 g K3[Fe(CN)6] and 0.0261 g NH4H2PO4 were dissolved in 40 mL ultrapure water under stirring. Then, the mixture was poured into a 100 mL Teflon-lined stainless steel autoclave and heated at 200 oC for 0.5-18 h. After cooled to room temperature, the resultant product was centrifuged and washed several times with ethanol and ultrapure water alternately and dried at 80 oC

for 12 h. Solid Fe2O3 nanoparticles were prepared when the reaction time is 0.5 h.

Single- and double-shelled Fe2O3 hollow spheres are denoted as S-Fe2O3 and D-Fe2O3, respectively. Assembly of Au nanoparticles onto hollow Fe2O3 nanospheres. The decoration of Au nanoparticles was performed according to our previous publications.[30-32] 0.05 g of the obtained D-Fe2O3 was dispersed into 75 ml of ultrapure water under stirring, and 4


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then different amount of 0.01 M HAuCl4 (0.15, 0.3, 0.45 and 0.75 ml) and 0.01 M lysine (0.3, 0.6, 0.9 and 1.5 ml) were added. After stirring for 20 min, 2 ml of 0.01 M fresh NaBH4 solution was added to reduce HAuCl4 to metallic Au nanoparticles. After stirring for 1 h, the precipitate was centrifuged and washed with ethanol and ultrapure water alternately and dried at 80 oC for 12 h. To obtain Au/D-Fe2O3, the products were annealed in air at 300 oC for 0.5 h. And the final products were designated as Au/DFe2O3-x, x=1, 2, 3, respectively. 3 Results and discussion

Figure 1. (a, b) The TEM images and (c) STEM image with corresponding elemental mapping of Fe and O of D-Fe2O3. Inset shows the SAED of D-Fe2O3

Scheme 1 presents the formation mechanism of hollow Fe2O3 nanospheres. This process doesn’t require the use of any templates or structure-directing agents. The transformation of the single shell to a double-shell is a spontaneous process that can be realized by simply changing the hydrothermal reaction time. At short reaction time, the single shells are produced. At medium reaction time, the inner shells are formed due to the outside-in Ostwald ripening of the nanoparticles dissolved from the inner surface of 5



the pre-formed single shells. Further extension of reaction time will give rise to single shells due to the inside-out Ostwald ripening of the nanoparticles dissolved from the inner shells. The D-Fe2O3 hollow nanospheres are first examined by TEM. As shown in Figure 1a and b, a double-shell structure can be clearly observed. And the average diameters of the outer and inner hollow shells are ca. 260 and 120 nm. The SAED pattern in Figure 1a shows a low-crystallized or amorphous structure of FeOx.[33] To prove the amorphous structure, XRD text was performed. As shown in Figure S1 (Supporting Information), no obvious diffraction peaks can be observed in the XRD patterns of SFe2O3, D-Fe2O3 and S-Fe2O3 after calcination, indicating that the amorphous nature of Fe2O3. The STEM image and corresponding elemental mappings in Figure 1c also clearly reveal the double-shell structure of D-Fe2O3 and the hollow spheres are composed of Fe and O elements. This result is also consistent with the EDS spectrum of D-Fe2O3 (Figure S2), showing the presence of Fe and O elements. It is worth mentioning that this template-free synthesis of double-shelled Fe2O3 can be repeated with a success rate of 100%. The TEM images in Figure S3 shows that the spheres with a double-shell do not show much difference with those in Figure 1a.

Figure 2. TEM images of product obtained with different reaction time: (a) 0.5 h, (b) 1 h, (c) 3 h, (d) 6 h, (e) 7 h, (f) 8 h, (g) 10 h, (h) 12 h and (i) 18 h 6


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To further investigate the formation process of the double-shell structure of Fe2O3 nanospheres, control experiments are conducted and the products collected at different reaction time are characterized by TEM. Interestingly, the structure of the products evolves from solid nanoparticles (Figure 2a), single-shell (Figure 2b-d) to double-shell (Figure 2e and f), and then single-shell (Figure 2g-h) with the extension of reaction time from 0.5 to 18 h. Apart from the number of shells, the reaction time also has an important influence on the size of the hollow spheres. As shown in Figure 2a, the solid Fe2O3 nanospheres are formed at 0.5 h and have a large size distribution. When the reaction time extends to 1 h and 3 h (Figure 2b and c), all the particles are observed to have a clear inner cavity. The conversion from solid to hollow particles is probably related to the Ostwald ripening process driven by the high surface energy.[34-36] At 3 h (Figure 2c), The Fe2O3 hollow spheres exhibit a single-shell structure with a wide range of diameter from 100 to 200 nm. Many small hollow spheres are seen to attach on the big ones. When the reaction time extends to 6 h (Figure 2d), a thin, and stable singleshell hollow sphere completely formed. The small hollow spheres that has been observed in Figure 2b are absent under TEM and the diameter of these spheres is also increased. It is deduced that with extension of reaction time, the small hollow sphere are consumed by the big ones to give a larger size. Due to the porous structure of the shells, the nanoparticles can transport through the pores on the shell.[37-39] With further extension to 7 h (Figure 2e), thin and discontinuous shells start to appear at the near region of the inner surface of the hollow structures. The higher surface energy drives the nanoparticles inside the hollow spheres to aggregate and start ripen, and this leads to the formation of the inner shells (Figure 2e). For a reaction time of 8 h, it is seen that inner-shells are clearly observed within the hollow spheres, as shown in Figure 2f. When the reaction time is extended to 9 h, the nanospheres with a defined double-shell structure are successfully derived, which is demonstrated in Figure 1. The formation of the inner shells are resulted from the outside-in Ostwald ripening of the nanoparticles dissolved from the inner surface of the pre-formed hollow shells, as shown in Scheme 1. Based on the inside-out and outside-in Ostwald ripening process, the single-shell and 7



double-shell structures can be controlled simply by varying the reaction time. However, on increasing the reaction time to 10 h (Figure 2g), the inner shells of the hollow spheres are observed to partially dissolve. The dissolved nanoparticles again re-aggregate to the outer shells, which give rise to a loose and thick shell. Compared with the stable outer shells, the inner shells possibly have a metastable phase, which completely disappear when the reaction time is 12 h (Figure 2h). At this time, the hollow spheres appear to have a condensed shell. Further extending reaction time to18 h (Figure 2i), the hollow spheres are seen to have a broken shell.

Figure 3. Structure evolution of Fe2O3 spheres along with reaction time.

Based on the discussion above, the structure evolution of Fe2O3 spheres on varied reaction time is summarized in Figure 3. In the present reaction system, when reaction time is 0.5 h, the product is solid nanospheres aggregated by nanoparticles. Singleshelled hollow spheres are produced at 3-6 h or 10-18 h, while double-shells can be obtained within 7-10 h. The porous structure and the specific surface area of the hollow spheres have been characterized by nitrogen adsorption-desorption tests. Figure 4a-d show the nitrogen adsorption-desorption isotherm and the corresponding pore size distribution of S-Fe2O3 and D-Fe2O3. As shown in Figure 4a and c, it is worth noting that there are obvious hysteresis loops, corresponding to the mesoporous structures for S-Fe2O3 and D-Fe2O3. The Brunauer-Emmett-Teller (BET) specific surface area of D-Fe2O3 (117.7 m2/g) is larger than that of S-Fe2O3 (108.2 m2/g), indicating that the double-shell structure can provide more active sites. The pore size distribution of the samples is calculated by the Barret-Joyner-Halenda (BJH) method, as shown in Figure 4b and d. The measurement reveals that both samples have a mesoporous shell with a pore diameter in the range of 20-30 nm. The porous shell is highly advantageous to improve the functional properties, 8


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as it can facilitate the mass diffusion of gas molecules within the sensing layers.

Figure 4. N2 adsorption-desorption isotherm and BJH pore-size distribution plots of (a, b) SFe2O3 and (c, d) D-Fe2O3.

Figure 5. (a) TEM, (b) HRTEM and (c) STEM image with corresponding elemental mappings of Fe, O and Au of Au/Fe2O3. 9



Surface functionalization of porous metal oxides proves to be a very effective strategy to improve their functional properties. Herein, the D-Fe2O3 hollow spheres have been further employed as the host material to anchor Au nanoparticles. The presence of Au in the hybrids is confirmed by XRD (Figure S1). In the XRD pattern of Au/D-Fe2O3-3, the two diffraction peaks at 38.22o and 44.45o correspond to (111) and (200) planes of Au (JCPDS 89-3697). Compared with the pristine D-Fe2O3 (Figure 1b), no obvious change is observed for Au/D-Fe2O3-3 (Figure 5a), indicating the doubleshell structure of D-Fe2O3 is well preserved after functionalization with Au nanoparticles. The high-resolution TEM (HRTEM) image in Figure 5b shows the lattice fringe spacing of 0.239 and 0.245 nm, which are attributed to the (110) plane of Fe2O3 and (111) plane of Au. The appearance of (110) plane is attributed to partially crystallized Fe2O3 domains in the hollow shells.[40] The elemental mapping results shown in Figure 5c clearly identify the distributions of Fe, O and Au in the Au/D-Fe2O3 nanocomposite. The Au nanoparticles are observed to homogenously distribute on both the inner and outer shells of D-Fe2O3.

Figure 6. XPS spectra of Au/Fe2O3 and D- Fe2O3 after calcination at 300 oC. (a) survey, (b) Au, (c) Fe and (d) O. 10


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To further analyze the surface chemical states of D-Fe2O3 and Au/D-Fe2O3-3, XPS test is carried out. The survey spectra are shown in Figure 6a. Compared with D-Fe2O3, a new peak of Au 4f can be observed, indicating that the presence of Au nanoparticles in the nanocomposites. The high resolution XPS spectrum of Au 4f (Figure 6b) shows signals of Au 4f7/2 and Au 4f5/2 at the binding energies of 83.6 and 87.2 eV.[41, 42] The actual amount of Au in the Au/D-Fe2O3 nanocomposites is 0.56 at.% according to XPS analysis. Figure 6c exhibits the XPS spectrum of Fe 2p, the two peaks at around 711.5 and 725.3 eV is attributed to Fe 2p3/2 and Fe 2p1/2 of Fe3+, while the other two peaks at 719.0 and 733.3 eV correspond to the satellite peaks.[43, 44] The XPS spectrum for O 1s in Figure 6d has been fitted into three components at 529.8, 531.1 and 533.5 eV, which can be attributed to lattice oxygen (O2-) ions, surface-adsorbed oxygen (O-/OH-) and surface chemisorbed oxygen ions(O2-/O22-).[45, 46] In order to check the valence state of Fe in the as-synthesized D-Fe2O3, Figure S4a presents the XPS spectra of Fe 2p and O 1s for D-Fe2O3 before and after calcination. It is seen that thermal calcination doesn’t cause much difference for Fe 2p, since the binding energies of the Fe 2p signals are over-lapped. While for O 1s, thermal annealing obviously increases the intensity of lattice oxygen fitted at 529.8 eV. Gas sensing performances Fe2O3 nanostructures have been extensively studies for gas sensors.[47, 48] To the best knowledge of us, there are few reports on the gas sensing properties of amorphous Fe2O3 hollow structures functionalized with Au nanoparticles. Therefore, we have fabricated a series of devices from the Fe2O3 and Au/Fe2O3 hollow nanospheres to test their gas sensing properties. The scheme of the sensor device based on D-Fe2O3 is shown in Figure 7a. Operating temperature is known to largely affect the sensing performance of metal oxide semiconductors.[49-52] We studied the response to 10 ppm acetone at various operating temperatures from 140 to 320 oC in order to find the optimal temperature of the sensor, as shown in Figure 7b. Obviously, the D-Fe2O3 sensor generally shows a higher response than that of S-Fe2O3 under all operating temperatures. This is due to the 11



double-shell structure of D-Fe2O3 with a porous structure. The porous shells can offer good permeability for gas adsorption and gas diffusion, hence sufficiently more active sites for gas sensing reactions.

Figure 7. (a) Schematic diagram of the D-Fe2O3 samples-based sensor and TEM image of DFe2O3 samples (inset, a); (b) Response of five sensors to 10 ppm acetone versus operating temperature and (c) Dynamic sensing curves of four sensors to 10 ppm acetone at 200 oC

In addition, the content of Au nanoparticles is observed to have an important influence on the response of D-Fe2O3. Among them, the sensor of Au/D-Fe2O3-1 shows the highest response of about 5.3 at an optimum temperature of 240 oC, while the highest response of Au/D-Fe2O3-2 and Au/D-Fe2O3-3 is about 5.6, and 6.1 obtained at an optimum temperature of 200 oC. With an increasing of Au content, the optimal temperature has been lowered from 240 to 200 oC. Due to the catalytic sensitization of Au, the adsorption and dissociation of gases on the materials surface are promoted, which significantly enhances the response to acetone and lower the optimum operating temperature.[47] Given the lower power consumption and the highest response of Au/DFe2O3-3 sensor at 200 oC, in the following gas sensing tests, we mainly studied the sensing performance of Au/D-Fe2O3-3. 12


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Figure 8. (a, c, e, g) Sensing transients of S-Fe2O3, D-Fe2O3, Au/D-Fe2O3-2, Au/D-Fe2O3-3 to different concentrations of acetone at 200 oC; (b, d, f, h) Fitted sensor response as a function of acetone concentration

To further evaluate the effect of Au functionalization on sensing characteristics, the dynamic response-recovery curves for all sensors versus 10 ppm acetone are shown in Figure 7c. It can be seen that D-Fe2O3 shows relatively fast response and recovery compared with S-Fe2O3. The faster response time may be caused by the unique double 13



shell structure of D-Fe2O3 and the relatively large specific surface area. The hollow interior and the porous shell can promote the diffusion of the test gas, and the larger specific surface area can adsorb more test gas and oxygen. Obviously, the response speed of the Au/D-Fe2O3 sensor is faster than that of D-Fe2O3, the response and recovery time of Au/D-Fe2O3-2 is 6 and 30 s, while the response and recovery time of Au/D-Fe2O3-3 is reduced to 5 and 20 s. These results indicate that the loading of Au nanoparticles conduce to improve the sensor response and dynamics in response and recovery time. Figure 8a, c, e and g display the dynamic sensing transients of four samples. The DFe2O3 hollow nanospheres demonstrate relative faster response and recovery than SFe2O3 to acetone concentration in the range of 1-200 ppm and the sensor response is observed to increase as the acetone concentration increases. Obviously, after functionalization with Au nanoparticles, the sensing performances are significantly enhanced due to the catalytic sensitization of Au nanoparticles.[53, 54] The fitting results demonstrate four sensors possess a good linear relationship between the sensor response and acetone concentration, as shown in Figure 8b, d, f and h. It can be seen that within the range of 1–200 ppm, the response of all sensors displays a good linearity along with increasing concentration. In Figure 8h, it also shows that the Au/D-Fe2O3-3 has a much higher sensitivity (0.465 ppm-1) than other sensors. The limit of detection (LOD) of the Au/D-Fe2O3-3 sensor is calculated to be ca. 0.132 ppm (3σ/s).[55] To further explore the detection ability, the Au/D-Fe2O3-3 sensor has been exposed to even lower acetone concentration. And the response-recovery transient to 0.2 and 0.5 ppm acetone is shown in Figure 9a. It can be seen that the sensor still has a relatively good response to sub-ppm concentration.

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Figure 9. (a) The transient response-recovery curves of the Au/D-Fe2O3-3 sensors to 0.2 and 0.5 ppm; (b) The selectivity of the sensors based on as-prepared samples to 10 ppm various gases at 200 oC; (c) The stability of the sensor based on as-prepared samples to 10 ppm acetone at 200 oC

and (d) Sensors response of as-prepared samples on different humidity environment

Selectivity is another important parameter that determines the actual application of the sensor. These sensors have been tested for various analyte. Figure 9b manifest the responses of four sensors to 10 ppm acetone as well as other gases such as ethanol, methanol and formaldehyde at 200 oC. The Au/D-Fe2O3-3 sensor clearly exhibits the highest response towards acetone, indicating an impressive selectivity toward acetone. One possible reason for this improved selectivity is that the catalytic sensitization of Au nanoparticles promotes the reaction of oxygen species with acetone gas molecules and increases the response to acetone. From a practical point of view, gas sensors are expected to have good repeatability and stability. Figure 9c shows a dynamic transient of six cycles at 200 °C for a sensor based on Au/D-Fe2O3-3 to 20 ppm acetone vapor. The response of the sensor has not decreased significantly, which means high stability and good reproducibility of Au/D15



Fe2O3-3.[56] Humidity is another factor that must be considered. For practical applications, because it usually affects the gas sensing properties of metal oxide semiconductors. The response of Au/D-Fe2O3-3 to 10 ppm acetone at various relative humidity was tested. As shown in Figure 9d, the relative humidity (RH) has a large effect on the sensor response. The sensor response amplitude decreases as the RH increases. The adverse effects of high humidity on the performance of the gas sensor can be attributed to the adsorption of water molecules on the sensing layer[57], which reduces the active site, i.e. the number of adsorbed oxygen species, and reduces the adsorption of target molecules.[58] Gas sensing mechanism For metal oxide semiconductor gas sensors, the most widely accepted sensing mechanism is the electron depletion layer model.[59] In air, oxygen molecules will adsorb to the oxide surface and form chemisorbed oxygen species (O2-, O2-, and O-) by trapping electrons from the conduction band of the semiconductor. In this process, oxygen molecules act as electron acceptors, resulting in a decrease in electron concentration. Therefore, the sensor shows a high resistance.[56, 60] When the sensor is exposed to ethanol, acetone or other reducing gases, the surface-adsorbed oxygen species will react with these gas molecules. This process releases the trapped electrons back into the conduction band, which reduces the resistance of the sensor. The reaction can be expressed as[61]: 𝑂2(𝑔𝑎𝑠)→𝑂2(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) ― 𝑂2(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) + 𝑒 ― →𝑂2(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) ― ― 𝑂2(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) + 𝑒 ― →2𝑂(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) ― ― 𝑂(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑) + 𝑒 ― →𝑂2(𝑎𝑑𝑠𝑜𝑟𝑏𝑒𝑑)

When the sensing material is exposed to acetone, the acetone molecules will react with the surface-adsorbed oxygen ions by the following equation[62]: 𝐶𝐻3𝐶𝑂𝐶𝐻3 + 8𝑂 ― = 3𝐶𝑂2 + 3𝐻2𝑂 + 8𝑒 ―

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Figure 10. The mechanism diagram of the gas sensing principle of different hollow spheres: (a) S-Fe2O3, (b) D-Fe2O3 and (c) Au/D-Fe2O3

The enhanced sensing properties of the Au-modified porous hematite hollow spheres can be interpreted by the scheme in Figure 10. Due to the highly porous structure, the sphere-shells have a high permeability to enable a fast diffusion of molecules within the hollow spheres. As shown in Figure 10a, oxygen molecules are absorbed on the surface of S-Fe2O3 to form oxygen ions (O- or O2- )[63] Compared with S-Fe2O3, the inner shells of D-Fe2O3 can greatly add up the surface area to adsorb more oxygen species serving as the active sites for sensing reactions. When acetone is introduced and reacted with oxygen ions, the electrons rapidly return to the material, resulting in a change in the electrical conductivity of the hollow structures. The change in electronic conductivity of D-Fe2O3 will be much larger than of that S-Fe2O3. Therefore, the improved sensing response of D-Fe2O3 over S-Fe2O3 can be attributed to the structure sensitization, as shown in Figure 10a and b. For Au/D-Fe2O3 hollow spheres, the enhanced sensing performance indicates that there must be other important contributions in addition to the effect of structure sensitization. This contribution is probably related to the catalytic spillover effect of Au nanoparticles (catalysis sensitization, in Figure 10). In fact, Au nanoparticles has been intensely employed as a chemical sensitizer to increase the sensitivity of semiconductor gas sensors because of its eminent catalytic activity.[53, 64] As displayed in Figure 10c, the spillover effect of Au nanoparticles strongly contribute to the adsorption and dissociation of molecular O2 into ionized oxygen species (O- or O2-).[53] The increased concentration of oxygen ions could trigger more sensing reactions taking place on the 17



hematite surface, thereby inducing an even larger change in the electrical conductivity of the sensor. Basically, the enhanced response to acetone is attributed to the resistance changes due to the formation of Schottky junction at the Au/D-Fe2O3 interface. In addition, the Au nanoparticles on the D-Fe2O3 hollow spheres can serve as the catalyst to facilitate the adsorption and dissociation of oxygen. Consequently, more sensing reactions between the oxygen ions and the reducing gas take place on the hollow shells, giving rise to higher response. To further evaluate the sensing capability of the Au/D-Fe2O3 hollow spheres, the performances of various Fe2O3-based hybrid materials toward acetone detection are compiled in Table S1. It is seen that the operation temperature of Fe2O3-based sensor materials is generally higher than 250 oC, the Au/D-Fe2O3 in this work shows a relatively low temperature (200 oC). Although some sensor can work at temperature even lower than 150 oC), they suffer from a low sensor response. Given the good sensing stability and the low detection limit discussed above, the Au/D-Fe2O3 nanospheres are potentially useful for acetone detection.

4. Conclusion In summary, a template-free protocol has been developed for creation of single- or double-shelled hematite hollow structures simply by changing the reaction time. The formation mechanism has been successfully clarified by recording the structure evolution of the hematite hollow spheres along with time. As a potential application, these hematite hollow spheres have been investigated for gas sensors to address the structure-property correlations. The double-shelled hematite spheres display much higher response than the single-shelled due to structure sensitization. An enhanced sensor response is further obtained by virtue of catalytic sensitization from Au nanoparticles. This work provides an optional method to prepare the multi-shelled hollow micro/nanostructures and the outstanding sensing performances also give some hints to the design of high performance gas sensors.

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Supporting Information Materials and characterization; Figures showing (1) XRD of S-Fe2O3, D-Fe2O3, SFe2O3 after calcination at 300 oC and Au/D-Fe2O3 after calcination at 300 oC; (2) EDS spectrum of D-Fe2O3 after calcination at 300 oC; (3) TEM images of D-Fe2O3 obtained from two repeated synthesis at 9 h and (4) XPS spectra of D-Fe2O3 before and after calcination at 300 oC; Table S1 summarizing the sensing performance of various Fe2O3based materials for detecting acetone.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No.

21601098

and

51602167),

Shandong

Provincial

Science

Foundation

(ZR2016EMB07 and ZR2017JL021) and Key Research and Development Program (2018GGX102033), and Qingdao Applied Fundamental Research Project (17-1-1-81jch and 16-5-1-92-jch).

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Highly Porous Double-shelled Hollow Hematite Nanoparticles for Gas

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