**Microwave Assisted Green Synthesis of Silver Nanoparticles Using Mulberry Leaves Extract and Silver Nitrate Solution**

#### **Le Ngoc Liem 1, \* , Nguyen Phuoc The <sup>2</sup> and Dieu Nguyen 2**


Received: 20 December 2018; Accepted: 31 December 2018; Published: 5 January 2019

**Abstract:** In this work, silver nanoparticles (AgNPs) were synthesized quickly and in an eco-friendly manner using the extract of Mulberry leaves and aqueous solution of silver nitrate without any toxic chemicals (Yuet et al. *Int. J. Nanomed.* **2012**, *7*, 4263–4267; Krishnakuma and Adavallan. *Adv. Nat. Sci. Nanosci. Nanotechnol.* **2014**, *5*, 025018). The Mulberry leaves extract functions as both a stabilizing and reducing agent. The UV-Vis spectroscopy shows a peak maximum at 430 nm. The transmission electron microscopy (TEM) image illustrated of synthesized AgNPs were nearly spherical-shaped particles whose sizes range from 15 to 20 nm. The TEM image of Nano Silver solution sample synthesized by the microwave assisted method shows nearly spherical particles, with an average particle size estimated at 10 nm. The absorption UV-vis spectrum of silver nanoparticles synthesized by the microwave assisted method (AgNPsmw) shows a sharp absorption band around 415 nm. The UV-Vis spectrum of AgNPsmw after two months of storage shows negligible peak changes of silver nanoparticles.

**Keywords:** silver nanoparticles; mulberry leaves extract

#### **1. Introduction**

Nanoparticles (NPs) are defined as small particles sized between 1 and 100 nm. Compared with the material bulk states, NPs have gained prominence in recent technological advancements due to their tunable physicochemical characteristics such as their melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption, better tunable optical properties, and higher reactivity. Various chemical and physical methods have been employed to prepare silver nanoparticles, including chemical reduction electrochemical techniques, and photochemical reduction. Among all the synthetic methods, chemical reduction is most commonly used. However, the chemical synthesis of nanoparticles may lead to the presence of some toxic chemicals. Several studies have focused on green synthesis approaches to avoid using hazardous materials. Synthesizing silver nanoparticles using mulberry leaves extract as a reducing agent not only offers many advantages but also uses less chemicals, thus reducing the pollution caused to the environment. In recent years, nanostructured materials have obtained many applications relevant to daily life. Silver nanoparticles (AgNPs) are used in a wide range of applications, including pharmaceuticals, cosmetics, medical devices, foodware, clothing and water purification, agriculture and in wastewater treatment, etc. due to their antimicrobial properties [1–7]. In this study, mulberry leaves extract had been used as a reduction agent and stabilizing agent. Synthesizing silver nanoparticles using mulberry leaves extract as a reducing agent not only offers many advantages but also uses less chemicals. The green synthesis combined with the microwave assistance make up a highly effective and eco-friendly method.

The mulberry (Figure 1) is a woody plant that grows quickly and within a short proliferation period. There are about 10 to 16 species of genus Morus that are found in the subtropical climates and the warm and temperate regions of Africa and North America [8]. Those species have been cultivated in many Asian countries such as China, India, Korea, Japan, Thailand and Vietnam, where the leaves have been used as food for silkworms [9]. Mulberry is known to be used in some traditional Chinese medicinal formulas. There are several studies have shown that it may provide health benefits. There are many biologically active compounds and many phytochemicals in mulberry leaves. There are also many important pharmacological properties such as antibacterial, antiviral [5–9], antitussive, hypoglycemic, antiatherogenic [10], hypotensive [11], diuretic, astringent, antioxidant [12,13] and α-amylase inhibitory effects [10]. α

**Figure 1.** Picture of Mulberry trees leaves.

#### **2. Materials and Methods**

## *2.1. Preparation of Mulberry Leaves Extract and Silver Nitrate Solution*

− Mulberry leaves were collected from a residential garden house in Hoi An, Quang Nam province, Vietnam. The leaves collected must be intact and at their prime (neither be too young or too old). Those fresh leaves were then cleaned with fresh water and let air dried by laying them out evenly. 10 g of fresh leaves was obtained, cut into thin strips, then placed into a 200 mL heat-resistant glass flask. Then, they were boiled with distilled water in 5 min, cooled and the mixture was filtered with Whatman filter paper using a vacuum filter [4–7]. The mulberry leaves extract had a light yellow color. The extracted solution was stored in a fridge for further use. Dissolved silver nitrate (AgNO3) from Sigma Aldrich was mixed with distilled water to get 4.10−<sup>3</sup> M aqueous AgNO<sup>3</sup> solution.

#### *2.2. Synthesis of Nano Silver Material*

#### 2.2.1. Non-Microwave Assisted Synthesis of Nano Silver

#### Visual Observation and UV-vis Spectral

− AgNO<sup>3</sup> 4.10−<sup>3</sup> M solution and mulberry leaves extract were mixed at different ratios as shown in Table 1. These mixtures were placed on a shaker and stirred for 30 min, 150 rpm at room temperature. After 30 min, the color of all the mixtures changed from light brown to dark red, except for mixtures M<sup>0</sup> and M1. The color of each mixture varies depending on the ratio of AgNO<sup>3</sup> concentration and

mulberry leaves extract that were used (Figure 2). This observation implied that Nano Silver particles were formed.


**Table 1.** Synthesized samples.

*Characterization Techniques*: In order to investigate the optical properties, we measured the UV-vis absorption spectra using GE Ultrospec 7000 UV-vis spectrophotometer (GE Lifesciences, Freiburg, Germany). Transmission Electron Microscope (TEM) analysis of silver nanoparticles was done using JEOL JEM 1010 (JEOL, Tokyo, Japan). X-ray diffraction spectra were measured by the diffractometer Bruker D8-Advance (Bruker, Karlsruhe, Germany), Fourier transform infrared (FTIR) Spectra. For mulberry leaves, extract was obtained in the range 400–4000 cm−<sup>1</sup> with IRAffinity-1S Shimadzu FTIR spectrophotometer (Shimadzu, Tokyo, Japan). −

**Figure 2.** Photograph color change of colloids.

The intensity of absorption within the wavelength of 200 to 250 nm is very strong. Therefore, before measuring UV-vis, samples were diluted 40 times. The UV-vis spectrum of the material shows a strong surface plasmonic resonance band centered at 430 nm. For the samples M2, M<sup>3</sup> and M4, the absorbance intensity increasesd as we increased the volume of mulberry leaf extract from 1 mL to 5 mL. For samples whose volume of mulberry leaves extract were more than 5 mL, the intensity decreased as the volume of mulberry leaves extract increased. Combining spectral information with qualitative observation, we could conclude that AgNPs has been formed when mixing AgNO<sup>3</sup> solution with mulberry leaves extract. Among our samples, the sample M<sup>4</sup> not only showed the highest peak intensity value but also had the darkest red color; the observations indicated that it had the most AgNPs particles. We can also infer that, at high concentration of mulberry leaves extract, the rate of AgNPs production is so rapid that it prevents the formation of protective layer between particles. For this to happen, the aggregation phenomenon between particles occurred and increased the particle size. The increased particle size reduced peak intensity and shifted the peak to a longer wavelength (as

in M5, M<sup>6</sup> and M<sup>7</sup> samples). The absorption spectrum of AgNO<sup>3</sup> aqueous solution, shown in the olive colored line in Figure 3, had no peak in the measuring range. The absorption spectrum of mulberry leaves extract, shown in orange line in Figure 3, had absorption peak in the short wavelength region due to an organic substance found in the extract solution and had no peak at wavelengths longer than 400 nm. The spectral information indicated that AgNPs particles were formed only when AgNO<sup>3</sup> was mixed with mulberry extract. When magnified from 380 to 700 nm of the absorption spectrum (Figure 3), discrete lines were shown.

**Figure 3.** UV-Vis spectra of AgNO<sup>3</sup> solution (M1), Mulberry extract (M0) and AgNps prepared at different Mulberry leaves extract.

#### 2.2.2. Microwave Asissted Systhesis of Nano Silver

#### Visual Observation and UV-vis Spectral

We mixed 50 mL of AgNO<sup>3</sup> solution with 6 mL of mulberry leaves extract and divided it into 2 equal portions. Then, we put the first portion (M8) on a shaker machine and stirred for 30 min, 150 rpm at room temperature. The other portion (M9) was heated in a microwave for one minute. The changed colors of M<sup>9</sup> from light yellow to dark red within one minute in the microwave indicated that the efficiency of (AgNPsmw) synthesis using the microwave assisted method was higher than the non-microwave assisted method. The sample colors are shown in Figure 4.

**Figure 4.** Photograph of silver nanoparticles (AgNPs) (M<sup>8</sup> ) and of AgNPsmw (M<sup>9</sup> ).

The UV-vis spectra (Figure 5) shows that absorption intensity at around 415 nm of M<sup>9</sup> is higher than that of M<sup>8</sup> and the maximum wavelength of M<sup>9</sup> is smaller than the M8. After two months of storage, the M<sup>9</sup> sample was renamed M10. Compared to the spectrum of M9, the UV-vis spectrum taken for M<sup>10</sup> (the blue line of Figure 5) shows a slight decrease in the peak intensity and the peak shift of 5 nm towards a longer wavelength. The above observations indicated that the synthesized colloid solution is highly stable. Compared with the work of other authors, our study resulted in a smaller particle size, short-time synthesis, and a higher absorption peak intensity.

**Figure 5.** UV-Vis spectra of AgNPs (M<sup>8</sup> ), of AgNPsmw (M<sup>9</sup> ) and of AgNPsmw after two months of storage (M10).

#### *2.3. X-ray Diffraction (XRD) Studies*

θ Cloth was cut into 25 cm<sup>2</sup> pieces and dipped into a colloids solution of AgNPs for 10 min, and another piece of cloth was not dipped into a colloids solution of AgNPs. The XRD pattern of the cloth dipped into a colloids solution AgNPs and the cloth not dipped into a colloids solution AgNPs are shown in Figure 6. The XRD pattern of the undipped cloth showed no characteristics of peak silver. On the other hand, the XRD pattern of the cloth that was dipped in the colloids AgNPs showed the characteristic of peak silver. Four main characteristics of the diffraction peak for Ag were observed at 2θ values of 38.2◦ , 44.1◦ , 64.5◦ , and 77.6◦ , which correspond to the (111), (200), (220) and (311) crystallographic planes of face-centered cubic (fcc) Ag crystals, respectively (JCPDS 00-004-0783) [14,15]. Because the AgNps concentration was low and the cloth's internal structure had lots of empty spaces, not many AgNPs particles were deposited onto the cloth surface, thus the peaks intensity was low.

**Figure 6.** X-ray diffraction of a pieces of cloth dipped into a colloids solution AgNPs (blue line), and a pieces of cloth not dipped into a colloids solution AgNPs (black line).

#### *2.4. Transmission Electron Microscope (TEM) Analysis*

The size and shape of the AgNPs was further confirmed by TEM analysis, is shown in Figure 7. The TEM image of M<sup>8</sup> showed a relatively uniform spherical particle size, ranging from 15–20 nm. The TEM image of M<sup>9</sup> showed particle size about 10 nm. The particle sizes were more uniform and no sign of nanoparticle clustering was observed.

**Figure 7.** Transmission Electron Microscope (TEM) images of silver nanoparticles (M<sup>8</sup> and M<sup>9</sup> ).

Figure 8 shows the histogram of size distribution of silver nano particles. The average particle size measured from the TEM image is 10 nm. This large variation in particle size was due to the presence of a few irregular shaped particles.

**Figure 8.** Histogram showing the particle sizes of AgNPs corresponding to TEM images M<sup>9</sup> .

#### *2.5. FT-IR Spectrum*

− − − − 0.5 mL of the AgNO<sup>3</sup> solution was dropped into 50 mL of mulberry leaves extract to prevent the extracted solution from rotting before taking the spectrum for mulberry leaf extract (From the sample synthesis to the FT-IR spectrum takes about 5 days). An FT-IR spectrum of silver nanoparticles synthesized by this green method is shown in Figure 9. A number of absorption peaks at 3261 cm−<sup>1</sup> and 1637 cm−<sup>1</sup> . The peaks at 3261 cm−<sup>1</sup> corresponds to O-H and N-H bonds, the peak at 1637 cm−<sup>1</sup> corresponds to the C=O bond, indicating the biomaterial bind to the silver nanoparticles through amine and C=O of amide I and amid II of the protein [1,6,7,16]. These results indicate that mulberry leaves extract acts as a reducing and stabilizing agent for silver particles. According to studies by A.K. Mittal et al., the extract of Dhatura metel contained alkaloids, proteins, enzymes, amino acids, alcoholic compounds, and polysaccharides, which were said to be responsible for the reduction of the silver ions to nanoparticles. Quinol and chlorophyll pigments present in the extract also contributed to the reduction and stabilization of the nanoparticles [17]. Leaf extracts of Mulberry also contain similar leaf extracts of Dhatura metel [18]. Therefore, mulberry leaf extract also played a role in the reduction and stabilization of the nanoparticles. − − − −

**Figure 9.** FTIR spectrum of mulberry leaves extract.

## **3. Conclusions**

The silver nanoparticles (AgNPs) colloid solution has been successfully synthesized, using a green and eco-friendly method. This is a quick, highly effective and less chemical-consuming method. The Microwave assisted green synthesis method is more effective than the non-microwave assisted method. The UV-Vis spectrum of AgNPs has an absorbance peak ranging from 425 nm–435 nm. Nano-silver particles are spherical shaped with size ranging from 15 nm to 20 nm. UV-vis spectrum of AgNPsmw shows peak at 415 nm, with an average particle size of 10 nm. The results observed after two months of storage of AgNPsmw are quite stable.

**Author Contributions:** Conceptualization, L.N.L.; Methodology, L.N.L. and N.P.T.; Validation, L.N.L. and N.P.T.; Formal analysis, L.N.L.; Investigation, L.N.L., D.N. and N.P.T.; Writing—Original draft preparation, L.N.L.; Writing—Review and Editing, L.N.L., D.N. and N.P.T; supervision, L.N.L.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Letter* **Facile Fabrication of Macroscopic Self-Standing Ni or Co-doped MnO2 Architectures as Catalysts for Propane Oxidation**

#### **Long Chen 1,2 and Xiping Song 1, \***


Received: 17 October 2019; Accepted: 8 November 2019; Published: 11 November 2019

**Abstract:** The fabrication of macroscopic self-standing architectures plays a key role in the practical applications of nanomaterials. A facile strategy to assemble MnO<sup>2</sup> nanowires into macroscopic self-standing architectures via hydrothermal reaction followed by ambient pressure drying was developed. The obtained sample was robust and showed excellent mechanical strength with a Young's modulus of 127 MPa, which had the possibility for practical applications. In order to promote the catalytic activity for propane oxidation, Ni or Co doping into MnO<sup>2</sup> was studied. The results showed that the obtained macroscopic self-standing Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> architectures exhibited enhanced catalytic activities for propane oxidation. Specifically, the conversions of propane over Co-MnO<sup>2</sup> and Ni-MnO<sup>2</sup> samples at 400 ◦C were 27.3% and 25.7% higher than that over pristine MnO<sup>2</sup> sample.

**Keywords:** macroscopic self-standing architectures; Ni-doped MnO2; Co-doped MnO2; propane oxidation; mechanical properties

#### **1. Introduction**

Amongst various transition metal oxides, manganese dioxide has been considered as one of the most potential low-temperature catalysts due to its environmental friendliness as well as its low cost [1]. In order to further improve the catalytic activity of MnO2, one of the most common strategies is doping with a different cation, such as Ni [2,3] and Co [4,5]. On the other hand, self-assembly of nanomaterials into macroscopic architectures provides the possibility for exploring the practical applications. However, the method to assemble nanomaterials into macroscopic architectures remains a challenge [6,7].

Long et al. [8] obtained a macro-assembly with MnO<sup>2</sup> nanowires via a hydrothermal method followed by a freeze-drying process, which showed selective adsorption of cationic dyes. Jung et al. [9] constructed MnO<sup>2</sup> nanowire hydrogel/aerogels via hydrothermal synthesis (over four days) and supercritical drying, which could be used to remove heavy metal ions and toxic organic contents in water. Suib et al. [10] constructed macroscopic free-standing OMS-2 sponges through hydrothermal reaction (250 ◦C for four days) and freeze-drying process, which could be used to separate oil and water. Rong et al. [11] fabricated a three-dimensional manganese dioxide framework combining δ-MnO<sup>2</sup> nanosheets and α-MnO<sup>2</sup> nanowires, which had interconnected network structures and showed excellent oxidation activity for ppm-level HCHO to CO<sup>2</sup> at low temperatures (≤120 ◦C). However, the reported process was time-consuming and high cost, and the catalytic application at higher temperature was rarely concerned. Therefore, it is necessary to explore simpler preparation methods and study the catalytic performance at higher reaction temperatures.

This research aims to fabricate macroscopic self-standing architectures with metal doped MnO<sup>2</sup> nanomaterials via facile hydrothermal reaction followed by ambient pressure drying and study the catalytic activity for propane oxidation. To the best of our knowledge, the study of macroscopic architectures with metal doped MnO<sup>2</sup> nanowires as catalysts for propane oxidation has been scarcely reported.

#### **2. Materials and Methods**

The macroscopic self-standing MnO<sup>2</sup> architectures were prepared by a modified hydrothermal process followed by ambient pressure drying. Typically, the A aqueous solution consisting of manganese acetate (Mn(CH3COO)2·4H2O, 1.18 g) and ammonium sulfate ((NH4)2SO4, 3 g) was added slowly into the B aqueous solution consisting of potassium permanganate (KMnO4, 0.506 g) and cetyl trimethylammonium bromide (C19H42BrN, 0.09 g) with continuous stirring. Afterwards, the resultant slurry was treated under hydrothermal conditions at 140 ◦C and kept for 6 h. Then the produced wet gel was washed in distilled water at 50 ◦C repeatedly. After ambient pressure drying, the macroscopic free-standing MnO<sup>2</sup> with the specific shape of the drying vessel was obtained. The schematic diagram of the preparation process for macroscopic self-standing architectures was shown in Figure 1. Mn(CH3COO)2·4H2O, (NH4)2SO4, KMnO4, C19H42BrN, wa

**Figure 1.** The schematic diagram of the preparation process for macroscopic self-standing architectures.

The macroscopic self-standing Ni- or Co-doped MnO<sup>2</sup> architectures were prepared by a similar procedure including the addition of nickel nitrate or cobalt nitrate precursors. The doping amounts of Ni (Ni/Mn molar ratio of 3/5) or Co (Co/Mn molar ratio of 1/5) were optimized in previous research [12,13] and the obtained samples were designated as Ni-MnO<sup>2</sup> and Co-MnO2, respectively.

An X-ray diffractometer (XRD, D8 Advance A25, Bruker, Germany) was employed to identify the phase structure. The morphologies were investigated with Scanning Electron Microscopy (SEM, SUPRATM 55, Carl Zeiss, Germany). An Instron 5940 universal testing machine (Shanghai, China) was employed for compression testing of pristine MnO2, Ni- or Co-doped MnO<sup>2</sup> samples.

ed n b g was The catalytic activities of propane oxidation over pristine MnO2, Ni- or Co-doped MnO<sup>2</sup> samples were evaluated in a fixed-bed reactor with continuous flow. The feed gas consisted of 1000 ppm propane, 5% oxygen in a nitrogen balance gas, and total flow rate was 300 mL/min. A mixture of 0.4 g sample and quartz sands was used, and the heating rate of the oxidation reaction was 5 ◦C/min. The maximum pressure reached inside the reactor was 103 kPa. The propane concentration in the outlet gas was on-line monitored with a MultiGas analyzer (MKS MultiGas ™ 2030, USA). The propane conversion was defined according to the following equation:

$$\text{Propane conversion (\%)} = (1 - [\text{C}\_3\text{H}\_8]\_{\text{outlet}} / [\text{C}\_3\text{H}\_8]\_{\text{inlet}}) \times 100\text{\%} \tag{1}$$

#### **3. Results and Discussion**

The appearance of prepared macroscopic self-standing MnO<sup>2</sup> architectures with cylindrical shapes was shown in the inset of Figure 2. The appearance of Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> were similar to that of MnO2. It demonstrates that the Ni or Co doping did not interrupt the assembling process of macroscopic self-standing MnO<sup>2</sup> architectures. The XRD patterns (Figure 1) suggested that the crystal phases of pristine MnO2, Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> samples corresponded well to the tetragonal MnO<sup>2</sup> (JCPDS no. 44-0141). No impurity phase was observed, which validated the high dispersion state of Ni or Co within the MnO<sup>2</sup> framework. The statistical average bulk densities of MnO2, Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> samples were 0.72, 0.69 and 0.70 g cm−<sup>3</sup> , respectively. The morphologies of pristine MnO2, Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> samples were shown in Figure 3. The basic component unit of pristine MnO<sup>2</sup> was nanowire, with a length of several hundred micrometers. The bundles of ultra-long nanowires intertwined and assembled into a network structure of macroscopic self-standing MnO<sup>2</sup> architecture. After Ni or Co doping, the morphology of nanowires did not change distinctly. Thus, the Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> also showed the disordered network structure assembled by nanowires.

**Figure 2.** The X-ray diffractometer (XRD) patterns of pristine MnO<sup>2</sup> , Ni-MnO<sup>2</sup> , and Co-MnO<sup>2</sup> samples. The inset is the appearance of the macroscopic self-standing MnO<sup>2</sup> architecture.

On the basis of the aforesaid results, the fabrication process of macroscopic self-standing Mn-based architectures included the formation of wet gel with nanowires via hydrothermal reaction and the removal of water via ambient pressure drying maintaining the self-standing architecture simultaneously. In previous research, the MnO<sup>2</sup> short nanofibers were prepared and the wet gel was not formed without the addition of ammonium sulfate during the hydrothermal reaction [12,13]. The role of ammonium sulfate included: (1) modification of the nanostructure unit; the nanowires with large length-to-diameter ratio were obtained with the addition of ammonium sulfate and this morphology was favourable for the assembly, and (2) assistance to assembly; the ammonium sulfate functioned as bridging ligands and promoted the assembling of MnO<sup>2</sup> nanowires. It has been proposed that sulfate ions have two coordination sites which can bond water molecules via hydrogen bonds forming a three-dimensional network, and simultaneously coordinate metal nanoparticles, thus promoting nanoparticles self-assembly [14–16]. Therefore, the solvent of water also participated in the building process of the macroscopic assembly.

The mechanical properties of samples were investigated and the stress-strain curves were shown in Figure 4. The curves of all samples had a similar shape and contained elastic and plastic regions. The calculated Young's moduli in the linear region of pristine MnO2, Ni-MnO2, and Co-MnO<sup>2</sup> samples were 127, 73.8, and 172 MPa, suggesting they had good resistance to elastic deformation under load. The yield strengths of pristine MnO2, Ni-MnO2, and Co-MnO<sup>2</sup> samples reached 4.5, 3.2, and 4.2 MPa, respectively. In the plastic regions, when the samples were compressed to strain of 50%, the corresponding stresses of pristine MnO2, Ni-MnO2, and Co-MnO<sup>2</sup> samples were 6.2, 4.1, and 5.7

MPa. It suggested that all samples were robust, compared with manganese oxide sponges in previous research [10].

**Figure 3.** The Scanning Electron Microscopy (SEM) images of pristine MnO<sup>2</sup> (**A–C**), Ni-MnO<sup>2</sup> (**D–F**), and Co-MnO<sup>2</sup> (**G–I**) samples.

**Figure 4.** The mechanical properties of pristine MnO<sup>2</sup> , Ni-MnO<sup>2</sup> , and Co-MnO<sup>2</sup> samples.

The catalytic activities of propane oxidation over pristine MnO2, Ni-MnO2, and Co-MnO<sup>2</sup> samples were investigated and the results were shown in Figure 5. The *T*<sup>50</sup> (the temperature at which 50% C3H<sup>8</sup> was converted) was usually used to compare the performance of different samples. It was found that after incorporation of Ni or Co, the *T*<sup>50</sup> shifted towards lower temperatures. Moreover, the *T*<sup>50</sup> of Co-MnO<sup>2</sup> (278 ◦C) was lower than that of Ni-MnO<sup>2</sup> (289 ◦C), suggesting that Co-MnO<sup>2</sup> showed higher activity for propane oxidation than Ni-MnO2. When the temperature reached 400 ◦C, the conversion of propane over Co-MnO2, Ni-MnO2, and MnO<sup>2</sup> samples was 85.9, 84.3, and 58.6%, respectively. The results verified the promotional effect of Ni or Co doping on the catalytic activity for propane oxidation over MnO2. After the reactions, the self-standing architectures did not deform or collapse, suggesting they had good resistance to thermal shock.

**Figure 5.** The catalytic activities for propane oxidation of pristine MnO<sup>2</sup> , Ni-MnO<sup>2</sup> and Co-MnO<sup>2</sup> samples.

− − − − − − − − − Furthermore, the turnover frequency (TOF) values were calculated to compare the catalytic activities of the three different catalysts. The detailed calculation method was described in previous research [17]. Based on the light-off curves and surface areas, the calculated results were shown in Table 1. It was found that the TOFs of samples increased in the order of MnO<sup>2</sup> (4.20 × 10−<sup>10</sup> mol m−<sup>2</sup> s −1 ) < Ni-MnO<sup>2</sup> (4.53 × 10−<sup>10</sup> mol m−<sup>2</sup> s −1 ) < Co-MnO<sup>2</sup> (5.10 × 10−<sup>10</sup> mol m−<sup>2</sup> s −1 ), which suggested that the incorporation of Ni or Co remarkably enhanced the catalytic activity for propane oxidation.



TOF = turnover frequency.

#### **4. Conclusions**

Macroscopic self-standing MnO<sup>2</sup> architecture was fabricated via hydrothermal reaction followed by ambient pressure drying. The addition of ammonium sulfate played an important role in the assembling of MnO<sup>2</sup> nanowires. After the Ni or Co doping, the macroscopic self-standing architectures could be maintained with excellent mechanical properties, which showed enhanced catalytic activities for propane oxidation. This study demonstrated a facile strategy to develop macroscopic self-standing Mn-based architectures, having great possibilities for practical applications.

**Author Contributions:** L.C. performed the experiments, analyzed the data and editted the paper; X.S. contributed review and supervision.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank the ecomaterials laboratory of the School of Materials Science and Engineering, Tsinghua University, for performing the catalytic activity for propane oxidation tests.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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