Harvesting the Gas Molecules by Bioinspired Design of 1D/2D Hybrids Toward Sensitive Acetone Detecting

Single‐atom catalysts (SACs) are highly attractive in many surface‐dependent reactions due to their extraordinary reactivity and maximum atomic efficiency. However, in a practical catalytic system, guiding and transferring the reactant molecules to target single‐atom sites are crucial processes for achieving high‐efficient reactions. Herein, gas‐microchannels on TiO2 nanorod‐supported Pt single‐atom catalyst by in situ growing derivative TiO2 nanosheets are constructed. Those perpendicular‐grown TiO2 nanosheets can guide gas molecules to the reactive single Pt sites and thus realize an intact system of guidance‐transfer‐reaction. Experimentally, the local coordination and electronic states of Pt species decrease with the growth of nanosheets, which could significantly strengthen the adsorption and activation of oxygen molecules toward the acetone sensing. The final Pt1/TiO2 catalysts exhibit a high sensing response (13.6–50 ppm) to acetone gas, which are approximately three times higher than that of catalysts without scaly shell (4.8–50 ppm).


Introduction
The creatures in nature can selectively develop specific surface structures to capture required substances (such as water, air) from the environment to meet the needs of survival. [1][2][3][4][5][6] For example, the cactuses are able to live in highly arid deserts where they cannot get enough water only from roots. Thus, they evolve an special spine structure with microscale and nanoscale textures to effectively harvest fog from the atmosphere as an additional water supply. [7,8] Meanwhile, the hierarchical grooves structure on the spine surface also can create a Laplace pressure gradient to directionally transport water molecules from the spine tip to the base. The process is beneficial to the sufficient contact and quick absorption of water molecules for the cactuses stem. Thus, such a biologic textured configuration can not only help to gather molecules passing through the surface but rapidly transport them to the target region. This phenomenon may exhibit significant advantages for the design of catalysts in surface-dependent reactions, which enlightens us to achieve highefficient reactions by regulating the transportation of reactant molecules. [9,10] effectively catalyze the accessible reactants and show excellent activity and selectivity. However, the surfaces of most singleatom catalysts are smooth and most of the active reactive single metal sites are confined within the top surface. [20,21] This makes it difficult for reactant molecules to accumulate and make full contact with the reactive single metal sites. Guiding the diffusion direction and maximizing the accessibility of reaction molecules are of great significance to further improve the performance of single-atom catalysts. [22,23] Inspired by the cactus spine structure, introducing multilevel textures on smooth support surface as molecule assembling and transport microchannels and then realizing an intact system of guidance-transfer-reaction may be an effective strategy for practical catalytic design, but it's still challenging.
Herein, we conceive multilevel textures with the capacity to transfer gas on the TiO 2 nanorod surface for confining reactants close to the single Pt atom. First, the atomically dispersed Pt species are deposited on the support to increase reactivity (Pt 1 /TiO 2 NRs, Scheme 1B) compared with unloaded TiO 2 nanorods (TiO 2 NRs, Scheme 1A). And then, numerous channels are constructed through in situ growing the scaly-like nanosheets (NSs) to the original Pt 1 /TiO 2 (Pt 1 /TiO 2 NRs@NSs, Scheme 1C). Due to the 2D nanosheets grow perpendicularly on the 1D building block, they can harvest and guide the reactants rather than obscure the active sites, especially for gas molecules. Moreover, abundant distortions and defects along the generated 1D/2D interfaces are induced and thus result in the lowercoordinated structure conversion of Pt species. Near ambient pressure X-Ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations exhibit that the special Pt sites significantly strengthen the adsorption and activation of oxygen molecules. The combined advantages of single Pt atom and local spatial structure enable the Pt 1 /TiO 2 NRs@NSs catalysts to exhibit improved acetone sensing capability with a high sensitivity and excellent stability during the measurements.

Results and Discussion
The TiO 2 NRs substrate with a diameter of 40-110 nm and a length of several microns are synthesized and shown in Figure 1A. Then, the single Pt atoms were uniformly dispersed on the surface of TiO 2 NRs by deposition-precipitation method ( Figure 1B and S1, Supporting Information). When further modifying the Pt 1 /TiO 2 NRs with lots of scale-like nanosheets to form hierarchical structures, the attached TiO 2 NSs exhibit a perpendicular growth nature on the NRs and possess a width of about 30 nm ( Figure 1C). The morphology of as-prepared catalysts is further confirmed by the scanning electron microscope (SEM) images ( Figure S2, Supporting Information). The TiO 2 shell will induce abundant 1D/2D interfaces on Pt 1 /TiO 2 NRs, which is shown in the typical high-resolution TEM (HRTEM) image ( Figure 1D, highlighted by a yellow dashed line). Of note is that the fast Fourier transformation (FFT) analyses ( Figure S3, Supporting Information) in different regions indicate that the Pt 1 /TiO 2 NRs@NSs is composed of anatase NR core and TiO 2 (B) NS shell. Further X-Ray diffraction (XRD) patterns of Pt 1 /TiO 2 NRs@NSs ( Figure S4, Supporting Information) display the additional diffraction peaks except for that of the NRs substrate (anatase), which are ascribed to the monoclinic TiO 2 (B) phase (JCPDS No. 74-1940). [24] The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM, Figure 1E) reveals that no Pt nanoparticles were found on the Pt 1 /TiO 2 NRs@NSs and energy dispersive X-Ray spectroscopy (EDS) mapping images in Figure 1F show that the Ti, O, and Pt are homogeneously dispersed, suggesting no aggregation of supported Pt species during further modification processes. It is worth noting that the Pt content decreased during this process (Table S1, Supporting Information), which may be due to the increase in the mass of the supports.
A surface evolution from smooth to scaly can be observed between Pt 1 /TiO 2 NRs and Pt 1 /TiO 2 NRs@NSs, during which  several gas microchannels were produced (Figure 2A,C). This process is accompanied by more structure change, especially in the heterointerface region. The lattice structure of corresponding local regions was carefully analyzed by AC HAADF-STEM ( Figure 2D, the corresponding schematic diagram is shown in Figure S5B). It is observed that there are numerous vacancies (circled in yellow dotted line) and dislocations (marked by yellow ⊥) around the interface, which are in contrast to the smooth surface and perfect lattice of Pt 1 /TiO 2 NRs ( Figure 2B and S5A, Supporting Information). For most heterostructures, the misfit strain that arises from mismatched lattices of adjacent phases is an important factor affecting the interface structure. A large number of crystalline imperfections such as dislocations are induced to relax the strain. During the growth of the scaly shell, the estimated lattice mismatch between the anatase NR core (2.33 Å) and TiO 2 (B) NS shell (3.05 Å) was up to 31.2% ( Figure S6, Supporting Information), which promotes the formation of crystalline defects on the substrate surface. [25,26] More detailed structural information is acquired from the near-edge X-Ray absorption fine structure (NEXAFS). Figure 2E shows O K-edge NEXAFS spectra of the Pt 1 /TiO 2 NRs and Pt 1 /TiO 2 NRs@NSs. There are two bands (t 2g and e g ) situated at the range of 528-535 eV, which ascribed to the O 2p states hybridized with the empty split Ti 3d bands. [27,28] The high-energy part of the spectra is dominated by three peaks (C, D, and E), which are attributed to the delocalized antibonding O 2p states coupled with Ti 4sp band. [27] For the Pt 1 /TiO 2 NRs@NSs, the intensity of all peaks decreases, suggesting the generation of defects on the surface of original supports. In addition, the Ti L-edge NEXAFS spectra are presented in Figure 2F. On account of the 2p spin-orbit coupling, both samples show two sets of bands (L 3 and L 2 ), which can be assigned to excitation of Ti 2p3/2 and Ti 2p1/2 core levels into empty Ti 3d states, respectively. [29] A slight red-shift of the L-edge absorption onset is observed for Pt 1 /TiO 2 NRs@NSs, further proving the existence of oxygen vacancies. [30] In consideration of these surface reconstruction phenomena that came up around the origin of Pt species, it can be speculated that the coordination environment and spatial configuration of Pt species may alter accordingly. To analyze the electronic states and local coordination of Pt from Pt 1 /TiO 2 NRs to Pt 1 /TiO 2 NRs@NSs, X-Ray photoelectron spectroscopy (XPS) and X-Ray absorption fine structure (XAFS) were performed. The binding energy of Pt4f peak for two samples is higher than Pt 0 (71.2 eV) but lower than that of Pt 4þ (74.0 eV), according to the ionic Pt δþ (0 < δ < 4) nature of the Pt SAs ( Figure 2G). [31] Moreover, Pt 4f 7/2 binding energy of Pt 1 /TiO 2 NRs@NSs is 72.3 eV, which shifts to the lower energy compared with Pt 1 /TiO 2 NRs (72.9 eV), indicating the lower oxidation state of Pt atom. Through the extended X-Ray absorption fine structure (EXAFS) observation, only a dominant peak around 1.81 Å is observed in the R-space spectrum ( Figure 2H) and no PtÀPt characteristic peak is detected, which confirms the isolated atomic dispersion of Pt. [32] The corresponding quantitative analysis was carried out by least-square EXAFS fitting and the structure parameters are listed in Table S2, Supporting Information. For the Pt 1 /TiO 2 NRs@NSs, the first coordination number of the central Pt is about 4, whereas the coordination number of Pt 1 /TiO 2 NRs is about 5. The fitting results are displayed in Figure S7 and S8, Supporting  Information. The X-Ray absorption near-edge structure (XANES) curves in Figure 2I show the white-line intensity of Pt 1 /TiO 2 NRs@NSs not only locates between Pt foil and PtO 2 but also is slightly lower than that of Pt 1 /TiO 2 NRs, demonstrating the lower oxidation state of Pt in the catalyst after refactoring. This is in accordance with the XPS results.
To explore the influence of microchannel design and single Pt sites modification on catalytic activity, the gas-sensing performances of as-prepared catalysts were performed. For the sensing process, the oxygen molecules could first adsorb onto TiO 2 surface and form O x À species by extracting the electrons from the conduction band of n-type TiO 2 , leading to the formation of the electron depletion layer. [33] Once exposed to reducing gases like acetone, these oxygen species can react with the gas molecules and release the trapped electrons, resulting in the decreased resistance. Moreover, noble metal additives are commonly used as chemical sensitizers to activate the dissociation of molecular oxygen. [34] These chemisorbed oxygen species on the surface increase the electron withdrawal to thicken the electron depletion layers. On the other hand, constructing an open structure to enhance gas diffusion is also an ideal way to improve sensing efficiency.
In general, the working temperatures show a strong influence on the activity of the sensing materials. As described in Figure 3A, the response (defined as S ¼ R a /R g , where R a is the sensor resistance under air ambient and R g is the resistance measured under analytes) of Pt 1 /TiO 2 NRs@NSs reaches its maximum at 280°C. The gas sensors present a fast response toward acetone from 5 to 500 ppm ( Figure 3B and S9A, Supporting Information), and the sensitivity continuously increases in response to varying concentrations ( Figure 3C). It's worth noting that the Pt 1 /TiO 2 NRs@NSs (13.6-50 ppm) exhibit a response approximately three times and four times higher than that of Pt 1 /TiO 2 NRs (4.8-50 ppm) and TiO 2 NRs (3.5-50 ppm), respectively. Such a response exceeds that of most reported acetone sensors ( Figure S9B and Table S3, Supporting Information). The Brunauer-Emmett-Teller (BET) surface areas are calculated by recording N 2 adsorption and desorption isotherms ( Figure S10, Supporting Information), where the surface area of Pt 1 /TiO 2 NRs@NSs is larger than that of TiO 2 NRs and Pt 1 /TiO 2 NRs. The selectivity of sensors toward six kinds of typical vaporous molecules (ethanol, methanol, formaldehyde, toluene, ammonia, and nitric oxide) with a concentration of 50 ppm was also  investigated. As shown in Figure 3D, the negligible response to the interfering gases is observed. Furthermore, the catalyst exhibits a good stability during the cyclic exposures to acetone and in the long-term test ( Figure S11, Supporting Information) and there is no obvious change in morphology and phase structure of the catalysts after reaction ( Figure S12, Supporting Information).
To further clear the structural superiority of Pt 1 /TiO 2 NRs@NSs in surface-dependent reaction, the O1s peaks were carefully characterized by NAP-XPS, where measurements were carried out under air atmosphere ( Figure 4A). To better accord with the actual condition, a temperature of 550 K was used. Compared to TiO 2 NRs material, the ratio between chemisorbed oxygen species and lattice oxygen species for Pt 1 /TiO 2 NRs increased slightly, while the value for Pt 1 /TiO 2 NRs@NSs reached the maximum among the three catalysts. [35,36] It indicates the superior ability of Pt 1 /TiO 2 NRs@NSs to chemisorb oxygen, [16] which attributes to its improved guidance-transfer capacity and lower-coordinated Pt sites. To confirm this point, the DFT calculations are further implemented. As seen in Figure S13, Supporting Information, two models for O-coordinated Pt species are first constructed on the anatase TiO 2 (101) surface. The Pt-O 5 configuration, where a single Pt atom replaces the topmost fivefold-coordinated Ti atom, will convert into Pt-O 4 when an oxygen vacancy is created. The band gap of TiO 2 NRs, Pt 1 /TiO 2 NRs, and Pt 1 /TiO 2 NRs@NSs decreases sequentially ( Figure S14, Supporting Information). As shown in Figure 4B, the projected density of states (PDOS) analysis suggests that the d-band center of Pt-O 4 in Pt 1 /TiO 2 NRs@NSs (À2.69 eV) is upshifted compared to that of Pt-O 5 in Pt 1 /TiO 2 NRs (À3.30 eV), indicating the stronger adsorption capacity of the Pt-O 4 sites. [37] When exposed to O 2 and acetone atmosphere, it can be found that the reactant molecules are more readily adsorbed on the surface of Pt 1 /TiO 2 NRs@NSs than that of Pt 1 /TiO 2 NRs, which is verified by the lowest adsorption energy of the Pt-O 4 sites ( Figure 4C). Moreover, the O─O bond length of O 2 adsorbed on the Pt-O 4 is longer than that of O 2 adsorbed on Pt-O 5 (Table S4, Supporting Information), which promotes O 2 dissociation and increases the surface chemisorbed oxygen species ( Figure 4D). [16] The charge density difference of O 2 and acetone adsorption ( Figure 4E) clearly exhibit that the adsorbed O 2 gains electrons from the surface of catalysts while the acetone molecules act as an electron donor, conforming to the resistance variation trend of catalyst in different atmospheres.

Conclusion
In summary, we constructed an intact system of guidancetransfer-reaction by bioinspired design for surface-dependent reaction. Plenty of gas-microchannels room wrapped on the surface of Pt 1 /TiO 2 by growing the derivative nanosheets, which endows the Pt 1 /TiO 2 NRs@NSs with the ability to effectively  guide and transfer the passing gas molecules to the accessible single Pt sites. Due to the optimized lower-coordinated structure induced by the generated 1D/2D interfaces, these active sites significantly strengthen the adsorption and activation of oxygen molecules, and thus enhance catalytic performance. The Pt 1 /TiO 2 NRs@NSs catalyst shows acetone-sensing performances with a high sensitivity, high selectivity, and good stability. This work provides a strategy for building a reaction system that takes into account the accessibility and reactivity of single metal sites and brings more opportunities for high-efficiency catalytic process.
Synthesis of TiO 2 Nanorods (TiO 2 NRs): In a typical procedure, [24] 1.5 g commercial P25 was added to 30 mL NaOH aqueous solution (10 M) and thoroughly stirred for 10 min. Then, the mixture was sealed in a 50 mL autoclave and kept at 180°C for 24 h. The product was filtered and washed with deionized water until it was neutral. Afterward, the product was dispersed into 500 mL HCl aqueous solution (0.1 M) and stirred for 24 h. Finally, the acidified product was washed to be neutral and dried at 80°C before it was calcined at 700°C for 2 h under air to obtain TiO 2 nanorods.
Synthesis of Pt 1 /TiO 2 Nanorods (Pt 1 /TiO 2 NRs): The single Pt atoms were anchored on the surface of the as-prepared TiO 2 nanorods by deposition-precipitation method. Briefly, 100 mg TiO 2 nanorods powders were dispersed into 20 mL deionized water. After stirring for 2 h, 265 μL H 2 PtCl 6 ·6H 2 O aqueous solution (Pt: 3.77 mg mL À1 ) was added to the mixture and stirred for another 12 h. Then, the precipitates were centrifugated and washed several times with deionized water. Finally, the product was dried at 60°C and calcined at 200°C for 1 h under flowing H 2 /Ar gas (5 vol% H 2 and 95 vol% Ar) to produce Pt 1 /TiO 2 nanorods.
Synthesis of Pt 1 /TiO 2 Nanorods@TiO 2 (B) Nanosheets (Pt 1 /TiO 2 NRs@NSs): First, 8 mg of the as-prepared Pt 1 /TiO 2 nanorods were dispersed into 7.5 mL ethylene glycol to obtain a uniform suspension by ultrasound. Then, 10 μL TiCl 4 was added to the suspension under stirring conditions. After adding 1.5 mL 1-dodecanethiol drop by drop, the mixture was stirred for 5 min and transferred into a 25 mL autoclave kept at 180°C for 14 h. The precipitates were centrifugated and washed separately several times with ethanol and deionized water. Finally, the prepared Pt 1 /TiO 2 NRs@NSs was dried at 70°C.
Characterization: TEM images were recorded on a Hitachi H-7700 operated at 100 kV. The HRTEM and the corresponding EDS mapping were conducted on a Talos F200X. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (aberrationcorrected HAADF-STEM) images of samples were recorded on a JEM-ARM 200 F at 200 kV. SEM was carried out by a GeminiSEM 500. The XRD patterns were recorded on a Rigaku Miniflex-600 operating at the voltage of 40 kV and the current of 15 mA with Cu Kα radiation (λ ¼ 1.5406 Å). XPS was measured on Thermo ESCALAB 250 using Al Kα (hv ¼ 1486.6 eV) radiation source. NAP-XPS was conducted on www.advancedsciencenews.com www.small-structures.com equipment manufactured by SPECS Surface Nano Analysis GmbH, Germany. The analysis chamber included a microfocus monochromatized Al Kα X-Ray source, a PHOIBOS NAP hemispherical electron energy analyzer, a SPECS IQE-11 A ion gun, and an infrared laser heater. The powdered samples dissolved in ethanol were dropped onto a copper foil and dried at room temperature. The measurements were performed under 1 mbar air and the temperature was maintained at 550 K. Soft X-Ray absorption spectroscopy (Soft-XAS, O K-edge, and Ti L-edge) tests were measured at photoemission end-station (BL10B) beamline of National Synchrotron Radiation Laboratory (NSRL, Hefei in China). The hard XAFS (Pt L 3 -edge) data were collected at BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) operated at 3.5 GeV under "top-up" mode with a constant current of 240 mA. Pt foil and PtO 2 were used as references. The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages.
Gas Sensing Measurement: The as-prepared catalysts were dispersed in ethanol to form a uniform suspension. Then, the suspension of each catalyst was drop-coated on a sensing substrate (area, 1.5 mm Â 1.5 mm; thickness, 0.25 mm). The sensing tests were conducted using a commercial CGS-8 Gas Sensing Analysis System (Beijing Elite Tech Co., Ltd.). In detail, the sensing measurements were carried out in a sealed chamber (volume, 20 L) and the operating temperature of the sensors was modulated by using a Ru microheater that was applied by a certain voltage using a DC power supply. After stabilizing in air, the sensors were exposed to acetone (concentration, 1-500 ppm) and then recovered in the fresh air. Resistances of the sensors were monitored using an acquisition system in real time. The sensors were also exposed to 50 ppm of various analytes, including ethanol (C 2 H 5 OH), methanol (CH 3 OH), formaldehyde (HCHO), toluene (C 7 H 8 ), ammonia (NH 3 ), nitric oxide (NO). The response was defined as S ¼ R a /R g , where R a was the baseline resistance in air and R g was the resistance measured under analytes.
Density Functional Theory Calculations: All density functional theory (DFT) computations were performed in the Vienna ab initio simulation package (VASP) [38] under projector augmented wave (PAW) potentials to describe the interactions. The generalized gradient approximation (GGA) [39] in the Perdew-Burke-Ernzerhof (PBE) functional was used to treat the exchange correlation between electrons. The way of DFT-D3 [40] was adopted to correct van der Waals force. A cutoff energy of 500 eV for plane wave expansions was used and all geometric structures were set to a 15 Å vacuum layer in the z-direction. The 10 À5 eV for conventional energy and 0.02 eV Å À1 for force under a 3 Â 3 Â 1 sheet k-point mesh were taken as the convergence criteria to optimize the structures. The electronic structures of the density of state (DOS) were calculated with 7 Â 7 Â 1 k-points. In all of the structure optimization, the bottom half of the slab in the vertical z-direction was constrained, while the top half of the slab and the adsorbates were fully relaxed. In this calculation, the anatase TiO 2 (101) support was modeled by a 3 Â 3 supercell and used to anchor the single Pt atoms.
The adsorption energy (ΔE) of oxygen and acetone on as prepared catalysts can be given by where E total , E adsorbates , and E substrate are the energy of oxygen or acetone adsorption on the substrate, energy of oxygen or acetone, and energy of substrate, respectively.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.