Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal

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Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal Method as Efficient Brønsted Acid Catalysts Wenqing Fan, Qinghong Zhang, Weiping Deng, and Ye Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400192q • Publication Date (Web): 28 Jul 2013 Downloaded from http://pubs.acs.org on August 1, 2013

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Chemistry of Materials

Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal Method as Efficient Brønsted Acid Catalysts Wenqing Fan, Qinghong Zhang,* Weiping Deng, Ye Wang* State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

ABSTRACT: This paper reports a novel bottom-up hydrothermal route for the synthesis of niobic acid nanosheets. This route is simpler and greener than the conventional top-down and multi-step route for the synthesis of hydrated metal oxide nanosheets via exfoliation of layered compounds, which typically requires the use of bulky organic cations. We have clarified that the pH of the suspension for hydrothermal treatment, the hydrothermal temperature and time, and the presence of NH4+ play roles in determining the morphology of the product. We propose that the nanosheet is formed from amorphous niobic acid nanoparticles through a dissolution-crystallization mechanism. The obtained niobic acid nanosheets are uniform with a thickness of ~2 nm and uniquely possess mainly Brønsted acid sites. As compared to the conventional amorphous niobic acid and several other typical solid acid catalysts, the niobic acid nanosheet synthesized by our bottom-up method exhibits significantly higher activity and selectivity for the Friedel-Crafts alkylation of anisole with benzyl alcohol. We have further demonstrated that our niobic acid nanosheet is a watertolerant and efficient catalyst for the hydrolysis of inulin, a polysaccharide-based biomass, into fructose.

KEYWORDS: nanosheet; niobic acid; hydrothermal synthesis; Brønsted acid; green chemistry

INTRODUCTION The development of new solid-acid catalytic materials to replace the liquid acids has attracted much attention in both green chemistry and materials chemistry.1-3 Moreover, solid acid catalysts have recently been expected to play crucial roles in the development of new green chemical processes such as the conversion of renewable biomass into chemicals or fuels.4 For this purpose, solid acid catalysts are usually required to be capable of working in water medium under hydrothermal conditions.4 Niobic acid, also known as hydrated niobium oxide (denoted as Nb2O5·nH2O), is a unique water-tolerant solid acid.1,5 While most of the solid acid metal oxides would lose acidity in the presence of water, Nb2O5·nH2O can function well in water medium at least under temperatures ≤ 373 K.1,5 In addition to functioning as an efficient acid catalyst for many reactions including hydration, dehydration, and hydrolysis in water medium,5-7 Nb2O5·nH2O could also work in the presence of various liquid media with different polarities and proticities such as decane, cyclohexane, toluene, methanol, and isopropanol.6 However, the nature of the acid sites on Nb2O5·nH2O is not fully understood yet. Both Brønsted and Lewis acid sites are known to co-exist on Nb2O5·nH2O, and these sites likely correspond to the OH groups associated with highly polarized NbO6 octahedra and the NbO4 tetrahedra.5,8 Nb2O5·nH2O generally functions as an acid catalyst in amorphous state, and the crystalline Nb2O5 after complete dehydration only possesses very weak Lewis acidity. The structure of amorphous

Nb2O5·nH2O is complicated, and this makes it difficult to further tune the acidity by structure manipulation. Nanosheets represent a new form of materials, and the high anisotropy of nanosheets with ultra-thin thickness can provide unique physicochemical properties.9 Recent studies demonstrated that some nanosheets of composite transition metal oxides or hydrated metal oxides with bridged OH groups such as HTiNbO5, HNbWO6, HNbMoO6, HTaWO6, and HNb3O8 functioned as efficient solid acid catalysts for Friedel-Crafts alkylation, esterification, hydrolysis, and dehydration.10-13 The two-dimensional structure of nanosheets can not only enhance the accessibility of bulky reactant molecules to almost all the acid sites, increasing the catalytic performance, but also may afford acid sites with unique properties owing to the anisotropic feature. The nanosheets of Nb-based composite metal oxides have been prepared mainly by a top-down route, which is a multi-step process via exfoliation of lamellar compounds and typically includes: (1) the synthesis of the parent lamellar compounds by a conventional solid-state reaction at high temperatures, (2) the protonation of the layered compounds in acidic solution, (3) the addition of bulky organic cations (e.g., tetrabutylammonium cations) to expand the interlayer spaces and to cause exfoliation, and (4) the addition of acid solution to the nanosheet solution to form aggregated nanosheets.9-13 This complicated route is not environmentally benign and not cost-effective because of the use of expensive organic compounds. It is highly desirable to develop a simpler, cheaper, and greener route for the synthesis of nanosheet materials.

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In the present paper, we report a novel bottom-up route for the synthesis of niobic acid nanosheets (denoted as Nb2O5·xH2O) by a simple hydrothermal method from NbCl5 without using expensive bulky quarternary alkyl ammonium cations. The structure and acidity of the obtained Nb2O5·xH2O nanosheets have been characterized by various techniques. This paper will show that our Nb2O5·xH2O nanosheets with thickness of ~2 nm display a unique acid property, possessing mainly Brønsted acid sites. Furthermore, we will demonstrate that the Nb2O5·xH2O nanosheet is a superior catalyst for the Friedel-Crafts alkylation of anisole with benzyl alcohol, the hydrolysis of ethyl acetate, and the hydrolysis of inulin, which is a kind of inedible polysaccharide-based biomass, into fructose.

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quartz reactor was exposed to NH3 for adsorption at 373 K for 1 h after the pretreatment at 393 K for 1 h in He gas flow. The gas phase or the weakly adsorbed NH3 was then purged by high-purity He at 373 K. NH3-TPD was performed in the He flow by raising the temperature to 900 K at a rate of 10 K min−1, and the desorbed NH3 molecules were detected with a ThermoStar GSD 301 T2 mass spectrometer by monitoring the signal of m/e = 16. The amount of NH3 adsorbed on each catalyst was evaluated by quantifying the peaks of the desorbed NH3 molecules. Fourier-transform infrared (FT-IR) studies of adsorbed pyridine were performed with a Nicolet 6700 instrument equipped with an MCT detector. The sample was pressed into a self-supported wafer and was placed in an in situ IR cell. After pretreatment under vacuum at 423 K for 1 h, the sample was cooled down to 323 K. Then, pyridine was adsorbed onto the sample for a sufficient time at the same temperature. FT-IR spectra were recorded after the gaseous or weakly adsorbed pyridine molecules were removed by evacuation at 323 K. 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were measured at room temperature using a Bruker AVIII-400M spectrometer. Catalytic Reaction. The catalyst was evacuated at 353 K for 1 h prior to catalytic reactions. The Friedel-Crafts alkylation was performed in a batch-type reactor. Typically, 0.10 g of catalyst was added into a reaction vessel pre-charged with benzyl alcohol (10 mmol) and anisole (100 mmol). After the air in the reactor was replaced by N2, the reactor was placed in an oil bath and was heated to the reaction temperature (typically 373 K) in about 10 min. We confirmed that the conversion of benzyl alcohol was 99% at temperatures ≥363 K (Figure S4 in the Supporting Information). The yield of fructose was 87% at 363 K, and a further increase in reaction temperature rather decreased the yield of fructose due to the formation of humin, carbonaceous species, resulting from the side reactions of dehydration and oligomerization.8 Thus, the optimum reaction temperature for the hydrolysis of inulin to fructose was 363 K. We further performed repeated uses of the Nb2O5·xH2O nanosheet for the hydrolysis of inulin. Figure 14 shows that the inulin conversion and the fructose yield only decrease lightly from 90% to 86% and from 78% to 73%, respectively, after the initial three recycles. The inulin conversion of 86% and fructose yield of 73% could be sustained in the subsequent recycling uses. On the other hand, significant decreases in catalytic performances were observed during the repeated uses of Amberlyst-15 for the hydrolysis of inulin (Figure 15). The inulin conversion and the fructose yield decreased gradually from >99% and 88% to 62% and 53%, respectively, after five reaction cycles. This is possibly because of the leaching of SO3H groups from Amberlyst-15 to the aqueous solution during the reaction. These results demonstrate that the Nb2O5·xH2O nanosheet is stable under our reaction conditions and is an efficient water-tolerant Brønsted-acid catalyst.

sion or formation of bulky reactants or products. As expected, H-beta, which possessed larger sizes of micropores than HZSM-5, showed higher activity than H-ZSM-5. The exploitation of mesoporous H-ZSM-5, which was prepared by treating H-ZSM-5 with an aqueous solution of NaOH followed by ion exchange using NH4+ and subsequent calcination,25 increased the conversion of benzyl alcohol and the yield of benzyl anisole. Amberlyst-15 exhibited higher selectivity and yield of benzyl anisole than the zeolites but was inferior to the Nb2O5·xH2O nanosheet. Particularly, the Nb2O5·xH2O nanosheet exhibits significantly higher selectivity of benzyl anisole. This clearly demonstrates the favorable catalytic function of the Nb2O5·xH2O nanosheet in the reaction requiring Brønsted acid sites. It should be noted that the formations of two isomers, i.e., o-benzyl anisole and p-benzyl anisole, were observed over our catalysts. The ratios of o-benzyl anisole to p-benzyl anisole over different catalysts were similar, being in a range of 37/63 to 45/55 (Table 3). To make a better comparison of the activity among these solid acids, we have further evaluated the turnover frequency (TOF) for the formation of benzyl anisole by calculating the moles of benzyl anisole produced per mole of surface acid site per hour. The acid density for each catalyst is expressed by using the adsorption amount of NH3, which has been measured by NH3-TPD. The result summarized in Table 3 clearly demonstrates that the Nb2O5·xH2O nanosheet possesses the highest TOF among the solid acid catalysts examined in this work. The TOF over the Nb2O5·xH2O nanosheet was about one order of magnitude higher than that over Amberlyst-15, which was also an excellent Brønsted-acid catalyst. The TOF obtained at a lower conversion by shortening the reaction time to 20 min over the Nb2O5·xH2O nanosheet was also significantly higher than that over Amberlyst-15. This further suggests that, besides the stronger acidity, the easier accessibility of the acid sites located on the two-dimensional Nb2O5·xH2O nanosheet by the bulky reactant molecules should contribute to its higher catalytic efficiency. We also performed the hydrolysis of ethyl acetate at 343 K, a catalytic reaction in water medium with smaller reactant molecules, over the Nb2O5·xH2O nanosheet and the amorphous Nb2O5·nH2O catalysts together with several typical solid acids. In this case, the TOF for ethyl acetate conversion over H-ZSM-5 was higher than that over H-beta (Table 4), although the latter zeolite possessed a larger pore size. The use of mesoporous H-ZSM-5 only slightly increased the TOF for ethyl acetate conversion. This is likely because the smaller reactant molecules can also access the acid sites located in the micropores of zeolites easily. Table 4 reveals that the TOFs over the amorphous Nb2O5·nH2O and the Nb2O5·xH2O nanosheet are higher than those over other solid acid catalysts examined. This suggests that niobic acids are more efficient catalysts in water medium. Moreover, the Nb2O5·xH2O nanosheet demonstrates higher catalytic performance than the amorphous Nb2O5·nH2O. Inulin is an inedible polysaccharide consisting of fructose units linked by β-2,1-glycosidic bonds with the polymer chains terminating in a glucose unit (Scheme 2). Inulin exists as the main component in many plants such as helianthus tuberosus and chicory, which are cold- and heat-resistant and are easy to cultivate even in desert areas. The hydrolysis of inulin can produce mainly fructose, which can be used in food and pharmaceutical industries26 or can be exploited for the production of 5-hydroxymethylfurfural (HMF), a versatile intermedi-

CONCLUSIONS We succeeded in synthesizing niobic acid nanosheets via a novel bottom-up route by simply treating the precursor obtained from the hydrolysis of NbCl5 dissolved in ethanol with NH3·H2O under hydrothermal conditions. The nanosheet was suggested to be formed from amorphous niobic acid nanoparticles via a dissolution-crystallization mechanism. The pH of the suspension, the hydrothermal temperature and time were key factors for the formation of nanosheets. We further clarified that the NH4+ ions played a crucial role in determining the morphology of the product. Organic ammonium cations could also be used to direct the assembly of niobic acid nanosheets instead of the NH4+ ions. Our characterizations revealed that the Nb2O5·xH2O nanosheet possessed a thickness of ~2.0 nm with an average lateral size of ~130 nm. The crystalline structure of the Nb2O5·xH2O nanosheet was similar to that of the 6


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nanosheet synthesized by the exfoliation of the layered HNb3O8·H2O. The characterization further suggested that the Nb2O5·xH2O nanosheet was composed of two layers of single sheets. NbO6 octahedra were found to be the predominant building units without NbO4 tetrahedra present in our Nb2O5·xH2O nanosheet. As compared to the conventional amorphous Nb2O5·nH2O particles, the Nb2O5·xH2O nanosheet possessed enhanced acid strength and acid density. Furthermore, the Nb2O5·xH2O nanosheet contained mainly Brønsted acid sites and almost no Lewis acid sites. The Nb2O5·xH2O nanosheet showed unique catalytic behaviors in the FriedelCrafts alkylation of anisole with benzyl alcohol, the hydrolysis of ethyl acetate, and the hydrolysis of inulin to fructose. As compared to several typical solid acids, the Nb2O5·xH2O nanosheet showed higher yields and TOFs for the target products for these reactions. It is proposed that the stronger acidity and the two-dimensional structure of the nanosheet, which makes the acid sites easier to be accessed by the bulky reactant molecules, contribute to its superior catalytic performances. The Nb2O5·xH2O nanosheet also demonstrated a particularly higher selectivity for the Friedel-Crafts alkylation because of the lack of Lewis acid sites. Furthermore, the Nb2O5·xH2O nanosheet displayed higher stability during the repeated uses than Amberlyst-15, which also showed excellent activity for the conversion of inulin to fructose.

ASSOCIATED CONTENT Supporting Information TG curves for several niobic acid samples, distribution of lateral sizes of the Nb2O5·xH2O nanosheets, time courses for the Friedel-Crafts alkylation of anisole with benzyl alcohol, and dependence of catalytic performances of the Nb2O5·xH2O nanosheets for the hydrolysis of inulin on reaction temperature. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected], Tel: +86592-2186156, Fax: +86-592-2183047

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2013CB933100 and 2010CB732303), the Natural Science Foundation of China (Nos. 21173172, 21103143, 21161130522, 21033006), and the Program for Changjiang Scholars and Innovative Research Team in Chinese University (No. IRT1036).

REFERENCES (1) (a) Tanabe, K.; Hölderich, W. F. Appl. Catal. A: Gen. 1999, 181, 399. (b) Okuhara, T. Chem. Rev. 2002, 102, 3641. (c) Busca, G. Chem. Rev. 2007, 107, 5366. (2) Clark, J. H. Acc. Chem. Res. 2002, 35, 791. (3) (a) Harmer, M. A.; Farneth, W. E.; Sun, Q. Adv. Mater. 1998, 10, 1255. (b) Xing, R.; Liu, N.; Wu, H.; Jiang, Y.; Chen, L.; He, M.; Wu, P. Adv. Funct. Mater. 2007, 17, 2455.

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Table 1. The content of H2O in the Nb2O5·xH2O samples prepared by the hydrothermal method under different conditions. Hydrothermal temperature [K]

Hydrothermal time [h]

x in Nb2O5·xH2O

x1 in Nb2O5·xH2Oa

x2 in Nb2O5·xH2Ob

513

0

2.5

n.d.c

n.d.c

513

1

1.6

0.54

513

3

1.5

0.59

0.93

513

12

1.2

0.51

0.67

513

24

1.1

0.34

0.75

513

72

1.1

0.31

0.76

423

24

2.1

n.d.

n.d.

453

24

1.6

0.75

0.83

483

24

1.2

0.49

0.73

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Table 3. Catalytic behaviors of some solid acid catalysts for the Friedel-Crafts alkylation of anisole by benzyl alcohol.a

Crystalline Nb2O5 Amorphous Nb2O5·nH2O

1.05

x1 denotes the number of H2O in Nb2O5·xH2O desorbed in the lower temperature region. bx2 denotes the number of water in Nb2O5·xH2O desorbed in the higher temperature region. cNot detected.

0.14

0.24

0.3

39 (45:55)

3.3

0.24

7.1

79

85 (37:63)

67

0.51

65

95

96 (40:60)

91

0.51

88

16

81 (37:63)

13

0.51

83

21

52 (40:60)

11

1.3

4.2

38

56 (39:61)

21

1.1

9.5

Yield [%]

0 0.61

23 (43:57)

8.5

Amorphous Nb2O5·nH2Oe Nb2O5·xH2O nanosheet Nb2O5·xH2O nanosheete Nb2O5·xH2O nanosheet f H-ZSM-5 (16.5)g Mesoporous H-ZSM-5

a

TOFd [h-1]

0

Acid densityc [mmol g-1] [c] 0

Benzyl anisole selectivityb [%] −

Conversion [%]

Catalyst

g

0

43

56 (43:57)

24

0.96

12

H-beta (25)e,g

87

66 (45:55)

76

0.96

38

Amberlyst-15 Amberlyst-15f

85 16

68 (44:56) 62 (45:55)

58 9.9

4.7 4.7

6.2 7.0

H-beta (25)

a

Reaction conditions: anisole, 100 mmol; benzyl alcohol, 10 mmol; catalyst, 0.10 g; reaction temperature, 373 K; reaction time, 2 h. bThe number in the parentheses denotes the ratio of o-benzyl anisole/p-benzyl anisole. The byproduct was dibenzyl ether. cThe acid density was evaluated by NH3-TPD. dTOF was evaluated by the rate of benzyl anisole formation per acid site. e Reaction temperature, 403 K. fReaction time, 20 min. gThe number in the parenthesis denotes the Si/Al ratio.

Table 2. The strength and density of acid sites determined by Hammett indicators and titration using n-butyl amine. Solid acid (acid density [mmol g-1])a Hammett indicator

pKa

Nb2O5

Amorphous Nb2O5·nH2O

Nb2O5·xH2O nanosheet

Neutral red

+6.8

+ (0.11)

+ (0.95)

+ (1.30)

Methyl red

+4.8

+ (0.01)

+ (0.36)

+ (0.71)

Methyl yellow

+3.3

−

+ (0.16)

+ (0.40)

Benzeneazodiphenylamine

+1.5

−

+ (0.10)

+ (0.33)

Dicinnamalacetone

−3.0

−

−

+ (0.06)

Anthraquinone

−8.2

−

−

−

Table 4. Catalytic behaviors of some solid acid catalysts for the hydrolysis of ethyl acetate.a

a

“+” and “−” represent occurrence and no occurrence of color change, respectively. The number in the parenthesis is the acid density measured by titration using n-butyl amine.

Catalyst

Rate [µmol g-1 min-1]

Acid densityb [mmol g-1]

TOFc [h-1]


5.2

0.24

1.3


18

0.51

2.1

d

15

1.3

0.68

Mesoporous H-ZSM-5

16

1.1

0.89

H-beta (25)d

6.0

0.96

0.37

Amberlyst-15

63

4.7

0.80

H-ZSM-5 (16.5)

a

Reaction conditions: ethyl acetate, 15 mmol; H2O, 10 mL; catalyst, 0.20 g; reaction temperature, 343 K. bThe acid density was evaluated by NH3-TPD. cTOF was evaluated by the rate of ethyl acetate conversion per acid site. dThe number in the parenthesis denotes the Si/Al ratio.

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Table 5. Catalytic behaviors of some solid acid catalysts for the hydrolysis of inulin.a Conv.

Catalyst

[%]

Selectivityb [%] Fructose yield [%] Fructose

Glucose

Acid densityc -1

[mmol g ]

TOFd [h-1]


23

67

14

16

0.24

2.1


>99

87

2.3

87

0.51

5.3

Nb2O5·xH2O nanosheete

68

81

5.7

55

0.51

6.7

H-ZSM-5 (16.5)f

54

69

7.4

37

1.3

0.88

89

79

3.5

70

1.1

2.0

14

63

8.8

8.6

0.96

0.28

Amberlyst-15

>99

90

3.3

90

4.7

0.59

Amberlyst15g

48

80

5.5

38

4.7

2.5

Mesoporous H-ZSM-5 H-beta (25)

f

a

Reaction conditions: inulin, 0.10 g; H2O, 10 mL; catalyst, 0.10 g; Ar, 0.5 MPa; reaction temperature, 363 K; reaction time, 2 h. bThe residual byproducts were sucrose and humin. c The acid density was evaluated by NH3-TPD. dTOF was evaluated by the rate of fructose formation per acid site. eCatalyst, 0.050 g. fThe number in the parenthesis denotes the Si/Al ratio. g Catalyst, 0.010 g.

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Scheme 1. Friedel-Crafts alkylation of anisole by benzyl alcohol to benzyl anisole.

Figure 1. Representative SEM images. (a) Nb2O5·xH2O nanosheets, (b) amorphous Nb2O5·nH2O.

Scheme 2. Hydrolysis of inulin to fructose and glucose.

Figure 2. SEM images of niobic acids synthesized by the hydrothermal treatment under different conditions. At 513 K for different times: (a) 0, (b) 1 h, (c) 3 h, (d) 12 h, (e) 48 h, (f) 72 h. At different temperatures for 24 h: (g) 423 K, (h) 453 K, (i) 483 K.

Figure 3. XRD patterns of niobic acids synthesized by the hydrothermal method under different conditions together with the amorphous Nb2O5·nH2O. 10


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Figure 6. SEM images of niobic acids synthesized by the hydrothermal method with the suspension controlled at a pH of 8 by adding different bases. (a) NaOH, (b) NH3·H2O, (c) n-butyl amine, (d) TBAOH.

Figure 4. SEM images of niobic acids synthesized by the hydrothermal method with the suspension controlled at different pH values. (a) pH = 6, (b) pH = 7, (c) pH = 10, pH = 12.

Figure 7. XRD patterns of niobic acids synthesized by the hydrothermal method with the suspension controlled at a pH of 8 by adding different bases. (a) NaOH, (b) NH3·H2O, (c) n-butyl amine, (d) TBAOH.

Figure 5. XRD patterns of niobic acids synthesized by the hydrothermal method with the suspension controlled at different pH values.

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Figure 10. Raman spectra. (a) Nb2O5 crystallites, (b) amorphous Nb2O5·nH2O, (c) amorphous Nb2O5·nH2O after dehydration by evacuation at 473 K for 2 h, (d) Nb2O5·xH2O nanosheet, (e) Nb2O5·xH2O nanosheet after dehydration by evacuation at 473 K for 2 h.

Figure 8. TEM images of the Nb2O5·xH2O nanosheets synthesized by the hydrothermal method at 513 K for 24 h. (a) Top view, (b) side view, (c) enlarged image of (b), (d) HRTEM of the center part, (e) enlarged image of (d), (f) HRTEM of the edge part.

Figure 11. NH3-TPD profiles for crystalline Nb2O5, amorphous Nb2O5·nH2O, and Nb2O5·xH2O nanosheet together with some typical solid acids.

Figure 9. AFM image and the corresponding roughness profile for the Nb2O5·xH2O nanosheets synthesized by the hydrothermal method at 513 K for 24 h.

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Figure 12. Pyridine-adsorbed FT-IR spectra for the Nb2O5 crystallites, amorphous Nb2O5·nH2O, Nb2O5·xH2O nanosheet, and HZSM-5.

Figure 14. Repeated uses of the Nb2O5·xH2O nanosheet for hydrolysis of inulin to fructose. Reaction conditions: inulin, 0.10 g; H2O, 10 mL; Ar, 0.5 MPa; catalyst, 0.075 g; temperature, 363 K; time, 2 h.

Figure 13. 1H MAS NMR spectra for the amorphous Nb2O5·nH2O and the Nb2O5·xH2O nanosheet.

Figure 15. Repeated uses of Amberlyst-15 for hydrolysis of inulin to fructose. Reaction conditions: inulin, 0.25 g; H2O, 10 mL; Ar, 0.5 MPa; catalyst, 0.010 g; temperature, 363 K; time, 2 h.

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Table of Contents Graphic

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Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal

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