Î³-MnOOH Nanowires Hydrothermally Reduced by Leaves for High

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#-MnOOH Nanowires Hydrothermally Reduced by Leaves for Highefficiency Electrocatalysis of the Glucose Oxidation Reaction Yu Qing Luo, Xiao Tian Guo, Mei Juan Yuan, Yan Yan, Changyun Chen, and Huan Pang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01106 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Sustainable Chemistry & Engineering

γ-MnOOH Nanowires Hydrothermally Reduced by Leaves for High-efficiency Electrocatalysis of the Glucose Oxidation Reaction Yuqing Luo, Xiaotian Guo, Meijuan Yuan,Yan Yan, Changyun Chen and Huan Pang* School of Chemistry and Chemical Engineering, Guangling College, Yangzhou University, Siwangting road, NO.180，Hanjiang district,Yangzhou, 225009, Jiangsu, P. R. China. College of Environmental Science, Nanjing Xiaozhuang University, Nanjing, 211171 Jiangsu, P. R. China. Email

address

of

the

corresponding

author:

[email protected];

[email protected] ABSTRACT: Among various manganese-based oxides or hydroxides, manganite (γ-MnOOH) has been regarded as an important participant for high-performance electrochemical energy storage and electrocatalysis. Efforts must be devoted to proposing more facile and environmentally benign synthetic routes. Herein, we have fabricated γ-MnOOH nanowires through a novel one-step hydrothermal approach with abundant biomass wastes (sapless leaves from the Magnolia grandiflora Linn tree) acting as the effective reductant. The influence of the temperature, reaction time and content of leaves on the morphologies of the products was further investigated. In addition, the optimized γ-MnOOH nanowires demonstrate excellent electrocatalytic activity and stability for the glucose oxidation reaction (GOR), which can be ascribed 1

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to the relatively high surface area, structural integrity and Mn (ⅠⅠⅠ) transitive valence state of the γ-MnOOH nanowires. KEYWORDS: γ-MnOOH, nanowire, biomass, glucose oxidation reaction INTRODUCTION In recent years, manganese-based oxides or hydroxides have been widely investigated and applied in the fields of electricity, magnetism and catalysis due to their natural abundance, low cost and environmental friendliness.1–3 Among these materials, nanosized manganite (γ-MnOOH), with controlled morphologies (nanowires, nanorods or wickers) and inherent redox reactions, manifests superior potential for electrochemical energy storage4,5 and electrocatalysis.6,7 Untill now, the methods of fabricating such one-dimensional (1D) γ-MnOOH nanomaterials mainly include the solvothermal

method,4

deposition,8,9

electrophoretic

template

method10

and

hydrothermal method.3 However, most synthetic processes require high reaction temperatures, specific molecular precursors or surfactants, making the synthesis more power-consuming and complicated.11 The exploration of more facile and easily controlled routes to fabricate γ-MnOOH is a major challenge. Since “green chemistry” has become a hot topic worldwide, a large number of biomass materials such as rice husks,12 shaddock peels13 and diatoms14 have been widely utilized as electrode materials for supercapacitors15,16 and rechargeable batteries.17,18 Commonly, biomass wastes function as templates,19 carbon sources or silicon sources.20,21 However, only a few reports are relevant to biomass wastes acting as the reducing reagents. In our previous work, we reported the facile synthesis of Cu 2


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superstructures via hydrothermal reduction with natural leaves, and the derived Cu-MnO222 and Cu-Co3O4 hybrids23 showed superiority for electrochemical energy storage. These findings indicate that sapless leaves can be renewable reducing substances, and this approach highly conforms to the purpose of “green chemistry”. Accordingly, with the identical manganese source, methylbenzene,6 β-cyclodextrin,24 ethanol25 and MnSO426 can be replaced by natural sapless leaves (from the Magnolia grandiflora Linn tree) to reduce KMnO4 for the manufacture of γ-MnOOH nanowires under hydrothermal conditions. This approach is a promising synthetic route since it is quite facile, economical and directly utilizes natural reductants. In this work, we successfully fabricated γ-MnOOH nanowires through a facile and controlled hydrothermal method. The possible effects of temperature, reaction time and the content of leaves on the nanowire morphologies were further explored. In addition, the prepared γ-MnOOH nanowires were applied as high-performance electrocatalysts for the glucose oxidation reaction (GOR) for the first time. The high surface area and the structural integrity endow γ-MnOOH nanowires with a low detection limit, excellent repeatability and anti-interference properties along with fabulous stability. The high electrocatalytic activity of γ-MnOOH makes it attractive for modern Mn-based electrocatalytic systems. STRUCTURAL CHARACTERIZATION The γ-MnOOH nanowires were directly fabricated from the hydrothermal reactions between sapless leaf pieces and KMnO4 in distilled water. (Figure 1) It was found that maintaining the reaction at 180 oC for 12 h was the optimum synthetic condition, 3



so the following characterization analysis and the electrocatalytic tests for GOR were all based on the sample prepared in that manner. Based on a series of explorations, the possible reduction mechanism of the sapless leaves can be proposed: during the hydrothermal processes, the cellulose or other carbohydrate molecules of the leaves might be slowly released and promoted the controlled hydrothermal reduction of KMnO4 into MnOOH.22 Sapless leaves from the Magnolia grandiflora Linn tree play a guiding role in the synthetic route. With such biomass wastes acting as novel renewable reductants, the reactions can occur at relatively low temperature without surfactants or templates. The cost of synthesis will be greatly reduced, and the route will become more eco-friendly and easily-controlled, thus making the practical applications possible.

Figure 1. Schematic illustration of the synthesis of γ-MnOOH nanowires. Temperature and reaction time are two crucial factors for hydrothermal synthesis, so the effects of these two factors on the morphologies were investigated. First, the products were fabricated under hydrothermal conditions at different temperatures (100, 120, 140, 160 and 200 oC) for identical times (12 h). No precipitates formed at 100 and 120 oC; instead, only brown solutions were obtained. The morphologies of the products synthesized at 140, 160 and 200 oC for 12 h are shown in Figure S1 for comparison. Under these conditions, the γ-MnOOH nanowires all successfully formed. 4


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At a relatively low temperature of 140 oC, the distribution and morphology of the nanowires were not very uniform, with many intersections between different nanowires. Some clustered impurities covered the surface of the nanowires, making the surface slightly rough. (Figure S1a, b) In contrast, when the temperature increased, the surface of the nanowires got more became smoother without impurities. At 160 oC, nanowire intersections still existed, and the average width of the as-obtained nanowires was approximately 180 nm. (Figure S1c, d) At 200 oC, each nanowire was separated individually without intersections, and the average width decreased to around 50 nm. (Figure S1e, f) The average length of the nanowires did not obviously change as the temperature increased. Second, another series of synthesis was conducted for different periods of time (6, 8, 10 and 24 h) at 180 oC, which could help explore the possible formation mechanism of the γ-MnOOH nanowires. After only 6 h of hydrothermal reaction, some precipitates formed; however, they were clustered small lumps instead of nanowires. (Figure S2a, b) When the hydrothermal reaction time increased to 8 h, some nanowires could be observed, (Figure S2c, d) still with clustered lumps covering the surface. At 10 h, the morphology of the nanowires became clearer and more homogeneous. (Figure S2e, f) Nevertheless, the surface of this sample was still rougher if compared with that of the most ordered γ-MnOOH nanowires synthesized for 12 h. If extending the time up to 24 h, the nanowires became inhomogeneous and were partly cracked. (Figure S2g, h) Furthermore, Figure 2 displays the photographs and the corresponding SEM images of the samples synthesized under the same hydrothermal condition (at 180 oC 5



for 12 h) with the addition of different content of leaves. The sample without leaves manifests violet solution with no precipitates, resembling the initial KMnO 4 solution before the hydrothermal process. This result further proves the effective reducing function of sapless leaves. When adding only 0.5 mg of leaves, the structure of the sample was not satisfactory, which could be attributed to the insufficiency of reductant. (Figure 2b, c) Adding excessive amounts of leaves (over 2 mg) would also affect the state of the samples. The samples were no longer seemingly viscous precipitants, and their inner structures were changed. The nanowires got more inhomogeneous and were clustered together with the content of leaves increased. (Figure 2d-g) It has been found that adding 1.0 mg of leaves is suitable.

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Figure 2. a) Photographs of the samples synthesized at 180 oC for 12 h with the addition of different content of sapless leaves; b-g) SEM images of the obtained samples, b, c) 0.5 mg; d, e) 2.0 mg; f, g) 4.0 mg (leaf content).

In summary, the nanostructures of the sapless leaves might have different rates/efficiency of releasing reducing substances when the reaction conditions change, thus affect the growth of γ-MnOOH nanowires in terms of their specific surface-interface properties and physical-chemical absorption. Hence, the reduction 7



works effectively when the temperature is above 140 oC and meanwhile the reaction time is at least 8 h. Meanwhile, the amounts of sapless leaves also make a difference since the concentration of the released reducing substances will change. For more direct information, the morphology and nanostructure of the as-obtained sample was characterized via scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and energy-dispersive X-ray spectrum (EDX)-elemental mapping. A panoramic view of the sample (Figure 3a) displays representative nanowires, which are orderly distributed. The width of the nanowires is in the range of 50-100 nm, while the length is measured to be 2-4 μm. (Figure 3b) To further elucidate the architecture of the γ-MnOOH nanowires, TEM images (Figure 3c-e) were obtained. The nanowires are all solid without mesopores inside, and their surface is relatively smooth. Magnified from the chosen area in Figure 3c, the average lattice spacing of γ-MnOOH is approximately 0.28 nm, which corresponds to the (111) lattice plane. (Figure 3f) The crystallinity of the γ-MnOOH nanowires is quite good and such material belongs to the typical monoclinic system, which is suggested by the SAED pattern in Figure 3g. Moreover, EDX-elemental mapping images (Figure 3h-j) exhibit the presence of Mn and O elements with a uniform distribution throughout the sample surface. The BET surface area is approximately 44 m2 g-1.26

8


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Figure 3. a) Low-magnification and b) high-magnification SEM images; c) TEM image of the γ-MnOOH nanowires; d) TEM image of a single nanowire; e) Magnified TEM image of the selected area in d); f) HRTEM image of the selected area in c); g) SAED pattern; h-i) EDX-elemental mapping images of Mn and O for γ-MnOOH nanowires. The crystallographic structure and phase purity of the sample were tested by X-ray powder diffraction (XRD). A typical XRD pattern of the product is shown in Figure 4a. The diffraction peaks can be readily assigned to the standard γ-phase MnOOH (JCPDS No. 41-1379). The strong and sharp peaks in the diffraction pattern also show the excellent crystallinity of the sample and the unit cell parameters (a = 0.888 nm, b = 0.525 nm, c = 0.571 nm) can be easily calculated.

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The fourier transform infrared (FTIR) pattern of the product is demonstrated in Figure S3. The broad characteristic peak at 3426.55 cm-1 can be attributed to the stretching mode of O-H, while three peaks at 1084.56, 1110.0 and 1151.91 cm-1 can be ascribed to γ-OH, δ-2-OH and δ-1-OH, respectively. More importantly, strong and sharp peaks at 594.75, 491.19, and 448.15 cm-1 result from the characteristic vibrations of Mn-O. A special combination band at 2088.95 cm-1 belongs to the stretching mode of O-H in hydrogen bonds (2704.12 cm-1) coupled with the excited Mn-O lattice vibration (594.75 cm-1), which agrees with the reported literature.27 This FTIR pattern further confirms the formation of γ-MnOOH. Furthermore, Figure S4 reveals the Raman spectra of the product, which can also be indexed to γ-MnOOH since the standard characteristic peaks of γ-MnOOH are located at 358, 388, 528, 555 and 620 cm-1. X-ray photoelectron spectroscopy (XPS) can help gain more insight into the chemical composition and surface electronic state in the products. Figure 4b (full-spectral analysis) displays the existence of Mn and O elements in the sample. Figure 4c and d demonstrate detailed-scanned Mn2p and O1s spectra, respectively. As shown in Figure 4c, the product presents two peaks at 642.9 and 641.4 eV,28 which can be ascribed to the surface Mn4+ and Mn2+, respectively, while their corresponding Mn 2p1/2 peaks can also be observed. The average oxidation state of Mn is Mn(III) which is consistent with γ-MnOOH. In addition, two peaks centered at 529.8 and 531.2 eV are assigned to the lattice oxygen (Mn-O bond) and the Mn-OH bonds brought out by γ-MnOOH itself or absorbed -OH, H2O.29 (Figure 4d) 10


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Figure 4. a) XRD patterns of the γ-MnOOH nanowires; XPS patterns of the γ-MnOOH nanowires b) wide survey, c) Mn 2p and d) O 1s. In addition, thermogravimetric (TG) analysis was conducted to investigate the thermal behavior of the γ-MnOOH nanowires in an air atmosphere. (Figure S5) According to weigh loss calculations, the peak below 300 oC, with a weight loss of approximately 3%, can be attributed to the removal of the absorbed water and the dehydroxylation of the sample. Subsequently, peaks in the range of 300 to 500 oC result from the oxidation of Mn3+ to Mn4+ with the formation of MnO2, accompanied by phase transformation (γ-MnO2 to β-MnO2 at around 400 oC). The last weight loss occurred at approximately 560 oC corresponds to the conversion of MnO2 to the final Mn2O3. This curve is typical of γ-MnOOH decomposition, as reported in other 11



studies.30 TG curves show the thermostability of Mn-based oxides, which will provide a deeper insight into their conversion and applications. The valence of Mn is rich, thus Mn-based materials show great potential for the electrocatalysis.

Scheme 1. Schematic diagram of the possible electrocatalytic process of the γ-MnOOH nanowires for the GOR. ELECTROCATALYTIC PERFORMANCE To investigate the electrocatalytic property of the γ-MnOOH nanowires for the determination of glucose (Glu), a series of electrochemical tests were carried out by employing a typical three-electrode configuration.

Figure 5 exhibits the

electrocatalytic performance of the sample with a mass loading of 5 mg mL -1. First, the cyclic voltammetry (CV) curves were tested to study the oxidation and reduction of Glu. As shown in Figure 5a, two obvious redox peaks (ranging from -0.1-0.5 V) were well maintained, and the current changed upon the addition of different concentrations of Glu into 0.1 M NaOH, demonstrating that Glu can be easily oxidized to gluconolactone on the γ-MnOOH/glassy carbon electrode (GCE) over a 12


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wide concentration window.31 The possible electrocatalytic mechanism is shown in Scheme 1 with the possible electrocatalytic equations as follows: ………… (1)

MnOOH + OH- ↔ MnO2 + H2O + e-

2MnO2 + C6H12O6 (glucose) → 2MnOOH + C6H10O6 (gluconolactone)………… (2) The current-time response curves when adding 20 μM Glu at different potentials (0.30, 0.40, 0.50 and 0.60 V) in 0.1 M NaOH were measured for the γ-MnOOH/GCE before the detection of Glu. (Figure 5b) It is acknowledged that a higher working potential can increase the current and improve the sensitivity to some extent, but at the cost of stability, which is suggested by the unstable curve at 0.60 V. In terms of both the large sensitivity and stability of the detection, the results indicate that 0.50 V is the optimal potential for all the following tests. Figure 4c exhibits the amperometric i-t responses at 0.50 V with continuous addition of different amounts of Glu stirred in 0.1 M NaOH. (the whole concentration range: 0.5-6065.5 µM) Furthermore, the plot of the electrocatalytic current of Glu versus its concentrations in the intercepted range of 0.5 to 265.5 µM is shown in Figure 5d, and the corresponding linear equation is I(μA)= 0.49C(μM) + 0.555, R=0.99859 with the calculated sensitivity of 6.936 μA mM-1 cm-2. Meanwhile, the limit of detection is estimated to be 0.25 µM for the γ-MnOOH/GCE (signal-to-noise ratio: S/N =3). Except

for

the

abovementioned

parameters,

the

repeatability

and

anti-interference properties are also significant criteria for judging the electrocatalytic performance of γ-MnOOH nanowires towards the GOR. As demonstrated in Figure 5e, the repeatability is decent in a relatively low concentration range. This result is 13



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because when the concentration of Glu increases, the amperometric response will gradually reach saturation. Thus, the surface of the γ-MnOOH/GCE is partly covered with the adsorbed reactive intermediates and can no longer offer enough active sites to catalyze Glu oxidation. Moreover, Figure 5f displays the current-time response for the successive addition of 100 µM Glu, ascorbic acid (AA, 5 μM), uric acid (UA, 5 μM), dopamine (DA, 5 μM), NaCl (5 μM) and 100 µM Glu. The results reveal that the abovementioned four types of interference all produce negligible current responses

for

the

electrode,

demonstrating

the

excellent

selectivity

and

anti-interference property of the γ-MnOOH nanomaterial. Moreover, SEM images in Figure S6 demonstrate that even after 4000 s cycling for the GOR, the γ-MnOOH samples modified on the GCE can well maintain their former morphologies, indicating good stability. It can be observed that the nanowires become shorter and thinner after the long-term GOR, and the substance dispersed on the wire surfaces may be adhesive or reaction intermediates. Furthermore, electrochemical impedance spectroscopy (EIS) of the γ-MnOOH/GCE electrode displays that the electron transfer resistance increases after the Glu cycling tests. (Figure S7) In order to further evaluate the electrocatalytic performance of the γ-MnOOH/GCE for GOR, the correlative reference of the properties of other Mn-based materials is demonstrated in Table S1 for comparison. In regard of detection limit, testing range and anti-interference, the properties of the as-prepared γ-MnOOH nanowires were found to be superior to most of the recently reported 14


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Mn-based materials. This can be possibly ascribed to the following facts: 1) The nanowire structures of the γ-MnOOH exhibit a higher specific surface area, which offers more space for electrolyte diffusion and makes OH- intercalation and deintercalation easier; 2) The intrinsic rich valence states of Mn endows γ-MnOOH with enhanced electrocatalytic activity.

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Figure 4. Electrochemical performances of the γ-MnOOH nanowires. a) CV curves in 0.1 M NaOH when adding different concentrations of Glu; b) Current-time response with the addition of 20 μM Glu at different potentials (0.30, 0.40, 0.50 and 0.60 V) in 0.1 M NaOH; c) Current-time response at 0.50 V with successive addition of different volumes of Glu in 0.1 M NaOH; d) A plot of the electrocatalytic current of Glu vs its concentrations in the range of 0.5 μM to 265.5 μM; e) Current-time response with successive addition of 100 μM Glu 10 times into 0.1 M NaOH at 0.50 V; f) Current-time response with successive addition of 100 μM Glu, 5 μM AA, 5 μM UA, 5 μM DA, 5 μM NaCl, and 100 μM Glu into 0.1 M NaOH at 0.50 V. The influence of the mass loadings on the electrocatalytic property of the GOR was also explored by dispersing different amounts of γ-MnOOH (3, 4 and 6 mg) into Nafion solution. The corresponding mass loadings were 3, 4 and 6 mg mL-1. In the experiments, it was found that when the mass loading increased to 6 mg mL-1, the sample film could not be completely modified onto the GCE surface, thus showing inferior performance. A series of analogous tests were conducted for comparison. The CV curves are of a similar shape, with couples of redox peaks in the -0.1-0.5 V range, and the order of the oxidation current is 5 mg mL-1 > 4 mg mL-1 > 3 mg mL-1. (Figure S8a, b) As shown in Figure S8c and d, the selected suitable potentials for the 3 and 4 mg samples are both 0.6 V. Based on necessary measurements and calculations, the results demonstrated that with the mass loadings decrease (5, 4, 3 mg mL-1) within limits, the sensitivity (4 mg mL-1: 0.326 μA mM-1 cm-2 ; 3 mg mL-1: 0.156 μA mM-1 cm-2) and the resolution of the electrode will significantly decline. Meanwhile, the detection limit (4 mg mL-1: 0.5 μM; 3 mg mL-1: 1.0 μM S/N =3) will increase. It can be inferred that as the mass loadings increase, the quantity of active substances on the GCEs will simultaneously increase, thus resulting in improved electrocatalytic performance. Meanwhile, the repeatability will be slightly enhanced, and the 16


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anti-interference properties are still satisfactory as the mass loadings decrease. (Figure S9) Moreover, the curves (Figure S10) represent the stability of the response current for the γ-MnOOH/GCE with different loadings in the presence of 100 μM Glu in 0.1 M NaOH over 4000 s. Accordingly, the response of Glu is very stable for a long time without loss. CONCLUSIONS In summary, γ-MnOOH nanowires were successfully prepared via a facile hydrothermal approach. With biomass wastes (sapless leaves from the Magnolia grandiflora Linn tree) replacing other common experimental reductants, the synthetic route became more envieromentally friendly. Subsequently, the obtained γ-MnOOH nanowires were applied as novel electrocatalysts for the GOR. We anticipate that our present work will not only facilitate the exploration of more efficient and green biomass reductants but also open up more opportunities for Mn-based nanomaterials for a series of electrocatalytic applications.

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ASSOCIATED CONTENT Supporting Information Additional text describing synthesis, material characterization, and electrocatalytic measurements; figures showing SEM, FTIR, raman spectra, TG, CV, current-time response curves and electrocatalytic data; one table listing the GOR data. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC-21671170, 21673203, and 21201010), the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), Program for New Century Excellent Talents of the University in China (NCET-13-0645), Postgraduate Research & Practice Innovation Program of Jiangsu Province (XKYCX17-038), the Six Talent Plan (2015-XCL-030), and Qinglan Project. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University. REFERENCES

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Diatoms

for

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γ-MnOOH nanowires have been prepared by a novel one-step hydrothermal approach with biomass materials acting as the reductant, and the good electrocatalytic performance for the glucose oxidation reaction can be attributed to the high surface area, structural integrity and Mn transitive oxidation state.

ToC figure

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Î³-MnOOH Nanowires Hydrothermally Reduced by Leaves for High

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