Visualization of the Formation and 3D Porous Structure of Ag Doped

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Visualization of the formation and 3D porous structure of Ag doped MnO2 aerogel monoliths with high photocatalytic activity Haojie Zhang, Chao Lin, Ting Han, Fuping Du, Yonghui Zhao, Xiaopeng Li, and Yuhan Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00578 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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ACS Sustainable Chemistry & Engineering

Visualization of the formation and 3D porous structure of Ag doped MnO2 aerogel monoliths with high photocatalytic activity Haojie Zhanga,b, Chao Linb, Ting Hana, Fuping Dub, Yonghui Zhaob, Xiaopeng Li b*, Yuhan Suna,c* a

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS), Shanghai 201210, China.

b

College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China.

c

School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

*

Corresponding author: [email protected], [email protected]

Supporting information

Keywords: Aerogels, supermacropore, photocatalytic degradation, manganese dioxide, inorganic nanowire

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Abstract Owing to their intriguing properties, aerogels with rich and hierarchical pore system are best appreciated in catalysis, sensing, and separation technologies. Herein, a comprehensive study is presented for inorganic monolithic aerogel of Ag doped MnO2 (Ag-MnO2), synthesized solely out of nanowires of diameter ~10 nm. We demonstrated a 3D image of the full bulk structure of the aerogel using X-ray computed tomography (µCT), providing a concise and statistical description of its porous structure for the first time. Interestingly, a flow-through supermacroporous system was observed as a result of freeze drying. Owing to the rich pore system, the Ag-MnO2 aerogel monolith exhibited photocatalytic degradation of organic water pollutants, which was superior in performance as compared to that of the MnO2 powder and compressed Ag-MnO2 pellet. A detailed study of photocatalytic mechanism was also carried out, indicating that Ag can simultaneously modulate the physical and chemical properties of MnO2. This work highlights the significance of porous system in monolith catalysts, and provides insight to design and prepare metal oxide aerogels for environmental and energy applications.

Introduction Porous materials are of fundamental interests for both, academia and industry, since they are ubiquitous in a broad variety of day to day life applications like energy absorption, filters, battery materials, as well as in industrial catalysis (e.g. as catalyst support, or 2 ACS Paragon Plus Environment

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zeolites).1-5 Developing micro- ( 10 µm) within the micro/mesoporous solids. The presence of super/macroporosity not only ensures an easy molecular transport and access throughout the bulk of the 3D architecture, but also prevents potential pore blockage while operating at high flow rates with low backpressure.13

Aerogels, also known as frozen smoke, are an extreme case of hierarchically porous materials, with more than 80% of porosity and an ultimate low mass densities ranging from 0.8-0.001 g/cm3.14-17 The low density mass also permits incorporation of higher amount of super/macropores. Owing to their advantageous morphological features like 3 ACS Paragon Plus Environment


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the continuous porous network and the rich open hierarchical pore system, aerogel has been considered to be of enormous potential in surpassing the traditional porous materials in terms of improvement in performance.18, 19 Aerogels can be prepared in the form of self-supporting monoliths, in which basic nanoscale building blocks are effectively connected and stabilized without any apparent agglomeration.10, 20 However, they are still far from being fully exploited, which is mainly due to two reasons. Firstly, the formation of aerogels strongly relies on self-assembly of nanoscale components; hence, poor controllability of the process mostly leads to mechanically brittle structures. This necessitates laborious drying techniques (e.g. supercritical drying), which limits practical applications.19,

21

Further, the complex pore systems of aerogels are so far poorly

characterized, especially in terms of their supermacroporous regions. The knowledge of the pore space morphology is essential to understand mass transport characteristics and catalytic performances. Common methods of obtaining information about pore structure includes mercury porosimetry (>50 nm) and gas physisoprtion (5-50 nm).19,

22

Unfortunately, mercury porosimetry is limited to a maximal pore size of 75 µm; moreover, the porous structure of aerogel is too open for an endemic intrusion of mercury.19, 23 On the other hand, the state-of-art focused ion beam – scanning electron microscopy (FIB-SEM) based tomography is capable of reconstructing 3D pore space morphology, but with a limited sample size of tens of micrometers.22, 24


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Herein, we report a unique Ag doped MnO2 aerogel monolith with hierarchical pore system, prepared by introducing a catalyzed hydrothermal nanowire growth. The obtained aerogel was constructed with nanowires as the building blocks. The monolith was used as a model system for X-ray µCT investigation of the aerogel pore structure. X-ray µCT is a non-destructive radiographic imaging technique, which can cover a wide range of scan sizes, ranging from micrometer to more than a few cm (size of the monolith).25-28 For the first time, we deliver a full-scale 3D image of the bulk structure of free-dried aerogel, followed by an accurate quantitative characterization of the supermacropore system. The MnO2 aerogel were tested for their photocatalytic performance in degradation of organic water pollutants; no loss of the nanowires was observed during the repeated circulation process. The change of physical and chemical properties of α-MnO2 upon Ag doping, and the photocatlytic mechanism were also investigated in detail.

Experimental details Materials: The MnO2 aerogel were prepared by a typical hydrothermal method. The corresponding procedure is briefly described as follow: a certain amounts of MnSO4·H2O (0.28 mol/L), (NH4)2S2O8 (0.28 mol/L), (NH4)2SO4 (1.14 mol/L), and deionized water were mixed together forming a clear solution in a Teflon vessel; next, 10 wt% of AgNO3 was added to the solution with vigorous agitation; followed by this, the Teflon vessel was 5 ACS Paragon Plus Environment


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sealed and heated to 140 ℃ for 12 hr in the oven. After cooling down to room temperature, the Ag-MnO2 hydrogel was obtained. The hydrogel was washed with deionized water until the pH value of the waste water reached around 7. Later, the aerogel was obtained after freeze drying for 48 hr. The non-doped α-MnO2 nanowire powder was prepared by the same method, except the addition of AgNO3.

Characterization: The phase purity of the aerogel was identified by powder X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.154178 nm) with a scanning speed of 4°/min. Morphologies of the as-synthesized samples were examined by scanning electron microscope (Carl Zeiss Microscopy GmbH Supra 55, Germany). Further, transmission electron microscope (TEM; FEI Tecnai G2, USA) was used to affirm the detailed nanostructures. The X-ray µCT experiments were carried out on the Phoenix v|tome|x s240 system (GE, USA). The N2 adsorption/desorption isotherms were measured using Tristar II set-up, while the surface area and porosity were measured using a Micromeritics analyzer. The samples were degassed at 120 ℃ for 6 hr under vacuum before the analysis. UV-visible spectra were measured in a UV-Visible spectrophotometer (Shimadzu UV-2700, Japan). Time-resolved photoluminescence (PL) was performed on a FLS980 Spectrometer (Edinburgh Instruments, UK) by applying laser excitation at 440 nm.

Visible light photocatalytic degradation of methylene blue (MB): MB was dissolved 6 ACS Paragon Plus Environment

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in deionized water (100 mL, 25 mg MB L-1), forming a transparent blue solution. 100 mg of the photocatalyst was then added to the solution. The MB solution was gently stirred for 2 h in dark, to achieve adsorption equilibrium between the MB and catalyst. Later, the mixture was irradiated with visible light (Xenon lamp; White light, 35 W). The light source was positioned approximately 7 cm from the top of the breaker. MB solutions, irradiated for different time periods, were collected and analyzed using UV-visible spectroscopy. Total organic carbon (TOC) was determined by a TOC analyzer (Shimazu 5050A, Japan). The intermediate compounds from photocatalytic degradation were identified by a liquid chromatography-mass spectrometry ((LC-)MS, Thermo Scientific™). Prior to UV-vis, TOC and (LC)-MS tests, photocatalysts were filtered off for avoiding any interference with spectroscopic measurements.

Results and discussion Figure 1(a) shows the optical image of the obtained aerogel monolith. XRD confirmed that the product solely consists of α-MnO2, and that no traces of impurity phases were present (see in Figure S1). The SEM and the TEM images revealed the microscopic morphology and crystalline structure at the atomic scale respectively. The basic architectural elements of the aerogel were crystalline α-MnO2 nanowires, with diameter of ~ 10 nm and length of up to 10 µm. Apparently, the elongated nanowires tend to aggregate together, forming a secondary spherical cellular structure. These cellular 7 ACS Paragon Plus Environment


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structures are strongly interconnected/interlocked to each other via expanded nanowire bundles, forming a robust networked porous skeleton of the MnO2 aerogel with a mass density as low as ~49 mg/cm3. Figure 1(g-i) show the TEM images of intermediate products collected after 1 h of the reaction. Combing with the collected energy dispersive spectrum (EDS), and X-ray photoelectron spectroscopy (XPS), followed by cross checking with literature,29-33 we propose that the black nanoparticles located at the tip of MnO2 nanowire are of Ag2O (see in Figure S2 and S3), which strongly catalyzes the oxidation of Mn2+, while promoting anisotropic growth of nanowires via following reactions. S2O82- + Ag2O + H2O → 2AgO + 2H+ +2SO42-

(1)

H2O + 2AgO + Mn2+ → MnO2 + 2H+ + Ag2O

(2)

The total reaction is: Mn2+ + S2O82- + 2H2O → MnO2 + 4H++ 2SO42-

(3)

Such mechanism closely resembles the solution-liquid-solid (SLS) growth scenario.34 In the absence of silver sources, the nanowire hydrogel could not be formed, rather only short nanowires could be obtained after the hydrothermal reaction.35 As the reaction proceeded, the Ag2O nanoparticles eventually disappeared, since silver ions constantly and uniformly diffused/doped into the MnO2 lattice. Figure 2(a-d) shows the corresponding element mapping of MnO2 nanowires, illustrating the uniform distribution of Ag atoms in nanowires without the presence of any silver based nanoparticles after 8 ACS Paragon Plus Environment

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hydrothermal reaction. More details about the growth mechanism can be found in our recent publication.36

The bulk supermacropore system of the Ag-MnO2 aerogel was analyzed using X-ray µCT. A volume of 2.31×2.31×2.14 cm3 was imaged with 1601 projections and a spatial resolution of 20 µm. Figure 3(a, b) shows the optical micrographs, along with the corresponding 2D X-ray µCT images of the aerogel sample, revealing its foam structure. Figure 3(c) shows, an illustrative tomographic cross-section image taken at four different vertical planes in the aerogel (the solid body is represented in grayscale). Each section, as denoted by the colored squares at the top-left corner of the image, corresponds to the positions of colored line in Figure 3(b). A simple inspection of these images indicate that porosity distribution varies from top to bottom of the aerogel, with the bottom part exhibiting a more uniform distribution. The 3D tomographic image, as shown in Figure 2(d), gives a full-scale view of the aerogel body (in gray). A video demonstration is available in the supporting information. In order to visualize and assess in detail the pore connectivity in the supermacropore system, the entire pore system was extracted from its body, as shown in Figure 3(e-g). Interestingly, we found that there are two distinct kinds of supermacropore systems; one is interconnected with percolating pores (in pink), while the other is isolated with closed pores (in blue). Figure 3(e) gives a combined view of both the types of pore systems. It is evident that the interconnected supermacropore 9 ACS Paragon Plus Environment


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system runs throughout the aerogel body, while the closed supermacropores are scattered in the aerogel with higher density at the center and bottom parts. Figure 3(h-j) and (k-m) respectively shows the axial and orthogonal cross sections of the 3D tomography. Interconnected supermacropores marked in pink are clearly the majority with individual pore sizes lying in the range of 50-500 µm.

We believe that the formation of such interconnected flow-through supermacropore system is due to the freeze drying process. As in the case of commonly used supercritical drying technique, nearly the entire pore structure can be retained identical to that in the hydrogel form.37 In the freeze drying method, however, MnO2 hydrogel was placed in a refrigerator at a temperature far below the freeze point of water. Nucleation of ice crystals quickly followed and the relatively soft nanowire sponges were pushed into the interstitial spaces between the growing ice fingers. Due to the low density of the MnO2 nanowires, most of the ice fingers merged together. Subsequently, the ice was sublimed in the freeze drying process, forming the interconnected supermacropore network as shown in Figure 3(g).

Orthogonal cross sections of the 3D tomography, as shown in Figure 3(k-m), offers a hint for the formation mechanism of the monolithic aerogel body. The observed morphological texture show high resemblance with the convection flow lines in the 10 ACS Paragon Plus Environment

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heated aqueous system. It is important to note that the hydrothermal reaction was carried out under static condition. The bottom of the sealed autoclave has better heat transfer in the oven as compared to the other parts. Therefore, it is more likely that the convection occurred during the initial stage of the reaction. The convective fluid flow and the created temperature gradient, strongly influences the transport of reactants and reaction rates, leading to different packing density of nanowires in different regions of the aerogel. A higher packing density of nanowires could block the ice-crystal interconnection, giving rise to a higher concentration of closed pores at the bottom and central part of the aerogel.

The X-ray µCT results (see Figure 4(a, b)) provided the diameter and volume distribution of supermacropores; the interconnected supermacropore system is treated as a single pore and highlighted in green. The diameter of this single pore was found to be up to 31.2 mm (equivalent to the diagonal of the cylindrical sample), with a pore volume of 1.55 ×103 mm3, occupying 86.5% of the total pores detected within the resolution of X-ray µCT and 17.59% of the total volume. In contrast, the volume of those supermacropores which are closed, is 0.39×103 mm3, occupying only 2.74% of the total sample volume. The pore diameter of 93.3% of the closed pores is distributed within the range of 50-500 µm. At this point it is important to emphasize that whole monolith, which is constituted of nanowires as basic units, has effectively an open pore structure, as observable in the SEM and TEM images. Due to its limited resolution, X-ray µCT could not detect mesopores 11 ACS Paragon Plus Environment


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(5-50 nm), and part of macropore (50 nm-20 µm), which are well connected to the closed and interconnected supermacropore system

In order to gain insights of the meso-range, N2 physisorption was carried out for obtaining the adsorption and desorption isotherm of the Ag-MnO2 aerogel. The isotherms exhibit a type-IV characteristic, typical of mesoporous materials.18 The Brunauer–Emmett–Teller (BET) specific surface area of Ag-MnO2 aerogel, calculated from the nitrogen adsorption isotherm, was about 124.7 m2/g. The pore size distribution, calculated using the Barrett-Joynes-Halenda (BJH) method, assumed a narrow unimodal envelope centered at 25 nm. The occurrence of mesopores might be originated at the interstitial spaces within the α-MnO2 spherical cellular structures (see Figure 1). In contrast, the non-doped α-MnO2 nanowires, exhibited a much smaller surface area of 94.3 m2/g; the corresponding TEM images are shown in Figure S4. In addition, they showed a broader distribution of pore diameter, possibly caused by the random stacking of nanowires in the freely available space. Presently, observation of the mesopores in the already carried out 3D tomographic imaging is not possible due to the limited resolution, however, our ongoing efforts in that direction are expected to extend the resolution to nanoscale.

In order to exploit the hierarchical pore structure along with the continuous supermacropore network, we conducted experiments of photocatalytic degradation of 12 ACS Paragon Plus Environment

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methylene blue (MB).38 Three different samples were compared, which includes the non-doped α-MnO2 nanowire powder, the Ag-MnO2 aerogel, and an Ag-MnO2 pellet (Figure 5(a)). Prior to the experiment, α-MnO2 nanowire powder was sonicated for 30 min to achieve a homogenously dispersed solution, which, based on literature reports, would be stabilized for hours without precipitation, thereby maximizing the contact with the reactant species.38, 39 On the other hand, the Ag-MnO2 aerogel and the compressed pellet were both directly placed in the MB solution (Figure 4(b)). The concentration of MB was 25 mg/L, 2-5 times higher than that used in most of reported works.38, 39 Gentle stirring was maintained throughout the photocatalytic degradation procedure for simulating the mild water flow. The mass of the Ag-MnO2 aerogel was recorded before and after the reaction (Figure 4(c-d)). There was little mass loss after photocatalytic degradation.

Figure 5(e) shows an illustrative series of UV-vis spectra of Ag-MnO2 aerogel. The amount of the photocatalytically degraded product was calculated using the BeerLambert law, according to which the change of MB concentration (C/Co, Co = 25 mg/L) is proportional to the normalized absorption value (A/Ao). It is interesting that prior to the light irradiation, a 50% decrease in the MB concentration was observed for MnO2 nanowires, indicating that MnO2 inherently possess strong affinity for MB. Consequently, due to relatively larger surface area of the Ag-MnO2 aerogel, 55% of MB is adsorbed, as 13 ACS Paragon Plus Environment


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shown in Figure 5(f). In contrast, for the case of Ag-MnO2 pellet, which is made by simply compressing aerogel under high pressure, hardly exhibits any supermacroporous structures. As a result, only 15% of MB is adsorbed by the Ag-MnO2, attributed to an insufficient mass transport. For excluding the adsorption behavior during photocatalytic degradation experiments, the MB adsorption time in the dark was extended up to 12 hr (Figure S5). However, the extra 10 hr of adsorption time also gave 3% more of MB removal for non-doped MnO2 nanowires, and 7% more for Ag-MnO2 aerogel. Therefore, 120 min period for MB adsorption would be enough prior to photocatalytic degradation experiments.

Upon visible light irradiation, MB was fully degraded within 300 min in the presence of well dispersed MnO2 nanowires. In contrast, the degradation rate of MB for the Ag-MnO2 aerogel was much faster; total degradation only required 180 min of irradiation. However, only 25% of MB was degraded within 300 min for the Ag-MnO2 pellet. The reaction rates of the three samples were further evaluated by fitting the photocatalytic degradation data to the first-order reaction kinetics.38 The photocatalytic degradation leads to the conversion of MB into other organic derivatives or complete mineralization into CO2 and inorganic ions (e.g. ammonium and sulfate ions). The TOC plot (Figure 5(g)) shows that non-doped MnO2 nanowire powder only reached 20% of mineralization; while Ag-MnO2 aerogel can achieve 38% of mineralization. The rate constant (kobs) is derived from the 14 ACS Paragon Plus Environment

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slope of the linear fit, as shown in Figure 5(h). The kobs for Ag-MnO2 aerogel is 0.0181 min-1, nearly 3 times higher than that for MnO2 nanowire powder, and 10 times higher than that for the compressed Ag-MnO2 pellet. All the results further highlighted the importance of the 3D continuous supermacroporous structure of the Ag-MnO2 aerogel, which ensures facilitated mass transport and sufficient contact between catalyst and reactants. The cyclic performance of a photocatalyst is also crucial for its commercialization. Nano-photocatalysts in the powder form has to be cycled via tedious steps like centrifugation and filtering, where the quality loss is unavoidable. Monolithic photocatalysts with sufficient mechanical strength, on the contrary are extremely simple to use, since their macroscopic structural integrity remains preserved under the mild cyclic fluid flow. The cycling performance of Ag-MnO2 aerogel was measured for the MB concentration of 10 mg/L, as shown in Figure 5(i). Remarkably, there was less than 3% of quality loss for Ag-MnO2 aerogel after 5 cycles, maintaining an excellent photocatalytic activity. It has to be also noted that it is the sample handling and the freeze-drying process which accounts for majority of the (permanent) quality loss, rather than the photodegradation step itself. XRD and XPS measurements were also performed (Figure S6). There was no change of crystalline structure and chemical valence of Ag-MnO2 aerogel, again verifying the structural stability of Ag-MnO2 aerogel.

The inherent structure features including 3D continuous porous structure, large surface 15 ACS Paragon Plus Environment


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area and high interconnection of each nano-unit make Ag-MnO2 aerogel quite unique and advantageous as photocatalytst. However, in order to further understand the greatly improved photocatalytic activity of Ag-MnO2 aerogel, the physical and chemical properties of MnO2 nanowire before and after Ag doping require to be fully characterized and understood. Figure 6(a) shows a classical Tauc plot, used to derive the optical bandgap (Eg).40 The Eg of Ag-MnO2 aerogel is estimated to be 1.46 eV, which is 0.32 eV smaller than that for the non-doped MnO2 nanowire. This implies that Ag doping can effectively modulate the optical properties of MnO2 and enhance the light absorption towards broader light spectrum. Time resolved PL was conducted to probe the dynamics of photon generated carriers in Ag-MnO2 aerogel. The typical decay profiles are shown in Figure 6(b) and were well fitted by a biexponential model.41 I(t) = A1exp(-t/τ1) + A1exp(-t/τ2) where I(t) is intensity, A1 and A2 are relative magnitudes, and τ1 and τ2 are decay times. For non-doped α-MnO2 nanowire powder, τ1 is 0.21 ns and τ2 is 2.57 ns. The fast component τ1 can be attributed to decay from free exciton states, while the slow component τ2 is ascribed to the bound exciton states. Ag-MnO2 aerogel shows slightly improved lifetime of emission with τ1=0.27 ns and τ2 =2.72 ns. This indicates that Ag doping does not quench the photoactivity of MnO2; instead, it enhances the lifetime of photon generated carriers. Both measurements confirm the beneficial roles of Ag in promoting the physical properties of MnO2. However, to rule out the photosensitization 16 ACS Paragon Plus Environment

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effect under light irradiation, we carried out the photocatalytic degradation towards the colorless phenol. Figure 6(b, c) depicts that the intrinsic photocatalytic activity of Ag-MnO2 aerogel is higher than that of α-MnO2 nanowire powder.

Besides the positive change of physical properties, we also investigated the change of chemical properties of MnO2 by XPS. Figure 7(a-d) shows high resolution XPS spectrum of Mn2p, Mn3s and O1s of non-doped α-MnO2 nanowire powder and Ag-MnO2 aerogel. The Mn2p3/2 peaks of both samples exhibit asymmetry, suggesting the presence of multiple Mn oxidation state (i.e. Mn3+ and Mn4+). The Mn3s spectrum delivers more accurate information of the Mn valence.35 From Figure 7(b), we found that the extent of Mn3s doublet splitting is nearly same for both samples, which indicates that the valence of Mn remains unchanged after Ag doping. Figure 7(c, d) shows that the O1s spectrum of Ag-MnO2 is wider and more asymmetric than that of α-MnO2 nanowire powder. Through deconvoluting the O1s spectrum into three different peaks A, B and C, oxygen species at the surface of MnO2 can be identified. The major peak A at the binding energy (BE) of 529.88 eV is usually ascribed to the lattice oxygen. The middle peak B at BE of 531.58 eV is assigned to the oxygen adsorption species at the surface oxygen defect, and peak C at BE of 533.08 eV is attributed to the minor attached water. Peak B related with the surface oxygen vacancy in Ag-MnO2 aerogel is 25.5% much higher than 16.1% in α-MnO2 nanowire powder. Surface oxygen vacancies have been constantly found owning 17 ACS Paragon Plus Environment


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promotional effects on photocatalytic activity of metal oxides such as CeO2,42 ZnO,43 and SrTiO3,44 because oxygen vacancies can create new sites for reactant adsorption and act as electron sink facilitating the charge separation within metal oxide nanostructures. Therefore, we believe that surface oxygen vacancies induced Ag doping also contributes to the enhancement of the photocatalytic activity of Ag-MnO2 aerogel.

(LC)-MS was conducted for revealing the byproducts formed during degradation of MB. For non-doped α-MnO2 nanowire powder, reduction of peak intensity at m/z = 284 with increase of dark equilibrium time (Figure 8(a-c)) was solely caused by simple adsorption behavior, and no chemical degradation of MB was observed prior to the light irradiation. Other peaks in the MS spectrum (from -720 min to 0 min) were from the flow and calibrating organic agent. The byproducts from MB degradation upon light irradiation (Figure 8(d)) were identified as thionine, azure A, B and C with their chemical formulas and molecular weight (MW) listed in Table 1. The photocatalytic degradation of MB by α-MnO2 nanowire proceeded mainly through demethylation by cross checking literature.45 From Figure 8(e-h), we found that the photocatalytic degradation of MB by Ag-MnO2 aerogel also followed the demethylation mechanism. However, a striking feature was discovered in Figure 8(f, g). MB degraded into thinione, azure A, B and C even in the darkness. This indicates that Ag dopants embedded in MnO2 crystals are highly active in catalyzing the demethylation of MB even without the assistance of 18 ACS Paragon Plus Environment

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photon energy. Upon light irradiation, Ag dopants continued to catalyze the MB degradation process. The MS spectrum (Figure 8(h)) shows that MB was fully removed by degradation and mineralization within 300 min, in consistence with the observation from UV-vis spectrum (Figure 5(e, f)).

In summary, beneficial effects on the photocatalytic activity of MnO2 from Ag are multiple. Addition of Ag precursor in hydrothermal synthesis generates unique structure merits. 3D continuous supermacropore system effectively eliminates the mass transport issue; meanwhile highly nanostructure Ag-MnO2 aerogel provides large surface area for photocatalytic reactions. On the other hand, the intrinsic properties of α-MnO2 are effectively modulated. The optical bandgap of MnO2 is narrowed, allowing light absorption in broader spectrum. Ag doping also improves the photon generated carrier lifetime. Moreover, Ag doping also alter the surface chemistry of MnO2 creating more beneficial oxygen vacancies and Ag dopant itself act as highly active center catalyzing the demethylation of MB even without the presence of photon energy.

Conclusion A novel aerogel was synthesized, solely constituted of Ag doped MnO2 nanowires. Introduction of silver catalyst into the hydrothermal reaction, promoted the nanowire growth and simultaneously induced a unique self-assembly behavior, which consequently 19 ACS Paragon Plus Environment


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led to a robust aerogel structure. With the assistance of time resolved PL, high resolution XPS, (LC)-MS etc., we found that Ag doping effectively modulate the physical and chemical properties of MnO2 nanowires such as narrowed bandgap, creation of surface oxygen vacancies and improved carrier lifetime., which make Ag-MnO2 aerogel ideal photocatalysts. With the help of X-ray µCT, a full-scale vivid 3D image of the supermacroporous structure in the freeze-dried aerogel was illustrated for the first time. The 3-D tomographic data allowed us to deliver an accurate statistical data pertaining to the pore volume and diameter distribution; moreover, it also provided a good idea about the formation of the whole monolithic structure, within the sealed hydrothermal reaction. Application of Ag-MnO2 aerogel in an illustrative photocatalytic degradation experiment demonstrated excellent performance, which was found to be superior to that of the MnO2 nanowire powder and the compressed pellet. This highlighted the importance and functionality of open interconnected supermacropore network in catalytic reactions. We believe that the aerogel developed in the present work will find a wide range of applications, mainly on account of its competitive fabrication cost, robustness, scalability, and the unique structural features.

Acknowledgements Financial supports from the National Natural Science Foundation of China (No. 21403280 and No. 21403277), Natural Science Foundation of Shanghai (14ZR1444600) 20 ACS Paragon Plus Environment

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and SARI-CAS Interdisciplinary Innovation Funding (Y426475231) are acknowledged.

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Figures

Figure 1. (a) Optical image of MnO2 nanowire powder and MnO2 aerogel, (b-c) SEM and (d-f) TEM of MnO2 aerogel at different magnifications. The inset picture in panel (f) is the EDS analysis showing the presence of Ag in MnO2. (g-i) TEM view of reaction intermediates collected with hydrothermal reaction time of 1 h.


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Figure 2. Electron microscopy image of Ag-MnO2 aerogel and corresponding element mapping of Mn, O and Ag.



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Figure 3. (a) Optical image of MnO2 aerogel, (b) 2-D X-ray image of MnO2 aerogel. (c) Four selected axial cross-section images taken from 3-D tomography. 3-D rendering of (d) solid aerogel body (gray), (e) total extracted pores (yellow), (f) interconnected pores (pink), and (g) closed pores (blue). (h-j) axial and (k-m) orthogonal cross sections of the


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3D tomogram. Yellow colored images represent total extracted pore combining closed pores (blue) and interconnected pores (pink) together.

Figure 4. (a) Pore diameter and (b) pore volume distribution of Ag-MnO2 aerogel obtained based on the X-ray tomography. (c) Nitrogen adsorption-desorption isotherm of non-doped MnO2 nanowire powder (black line) and Ag-MnO2 aerogel (red line). (d) Pore size distribution calculated by the BJH method.



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Figure 5. (a) Optical image of non-doped α-MnO2 nanowire powder, Ag-MnO2 aerogel and Ag-MnO2 pellet. (b) Optical image of one Ag-MnO2 aerogel immersed in the blue MB solution under light irradiation. (c, d) The weight of Ag-MnO2 aerogel recorded by an electronic balance before and after 5 times photodegradation 30 ACS Paragon Plus Environment

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experiments. (e) Variation in the UV-vis absorption spectrum of MB aqueous solution before and after irradiating with light. (f) Photocatalytic degradation curves (MB) of α-MnO2 nanowire powder, Ag-MnO2 aerogel and pellet. (g) Kinetic of disappearance of total organic carbon (TOC) by α-MnO2 nanowire powder and Ag-MnO2 aerogel. (h) First-order reaction kinetics derived from data shown in (f). (i) Repeated photocatalytic degradation experiments with Ag-MnO2 aerogel.

Figure 6. (e) Tauc plot of non-doped α-MnO2 nanowire powder and Ag-MnO2 aerogel for deriving the optical bandgap. (b) Decay profiles of the exciton emission of α-MnO2 31 ACS Paragon Plus Environment


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nanowire powder and Ag-MnO2 aerogel. (c) Variation in the UV-vis absorption spectrum of phenol aqueous solution before and after irradiating with light. (d) Photocatalytic degradation curves (phenol) of α-MnO2 nanowire powder and Ag-MnO2 aerogel

Figure 7. High resolution XPS spectrum of (a) Mn2p and (b) Mn3s. Black line is non-doped α-MnO2 nanowire powder, and red line is Ag-MnO2 aerogel. O1s spectrum of (c) α-MnO2 nanowire powder and (d) Ag-MnO2 aerogel. The O1s spectrum is resolved into three peaks A, B and C.


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Figure 8. (LC)-MS spectrum of MB and MB degradation products using non-doped α-MnO2 nanowire powder and Ag-MnO2 aerogel before and after irradiating with light. 33 ACS Paragon Plus Environment


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The X axis denotes α-MnO2 nanowire powder (left) and Ag-MnO2 aerogel (right). The Y axis is the timeline of photocatalytic irradiation. -720 min and -420 min mean the sample equilibrated with MB in the darkness.

Table 1. Methylene blue (MB) and the major products detected by (LC-)MS after adsorption and photocatalytic degradation.


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Graphic abstract

Synopsis: Hierarchically structured Ag-MnO2 aerogel owns a unique open interconnected supermacropore network, which facilitate the mass transport and enhance the photocatalytic reaction kinetics.


Visualization of the Formation and 3D Porous Structure of Ag Doped

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