Synthesis of Graphene Nanosheets
Ling Bing Kong,1,a,* Freddy Boey,1,b Yizhong Huang,1,c
Zhichuan Jason Xu, 1,d Kun Zhou, 2 Sean Li,3 Wenxiu Que,4
Hui Huang5 and Tianshu Zhang6
2.1. Introduction
One of the critical challenges in using graphene nanosheets for practical applications is the lack of an efficient way to reliably produce them in both large quantities and of high qualities. Also, it is still not possible to control the synthesis of graphene nanosheets with precisely defined size, shape and edge chemistry, which is crucial for most applications, especially when one or more of these parameters are important. For example, it is necessary to open up the energy band gap of graphene, so that it can be used as an active material in field-effect transistors (FETs) [1]. For electrode applications, electrical conductivity is more important than other parameters. In this case, the graphene nanosheets should have high purity and perfect structure. In this respect, the aim of this chapter is to summarize the progress in the synthesis of graphene nanosheets and their related materials. However, due to the huge number of references, it is impossible to provide a complete list; therefore representative examples will be presented for each synthetic method.
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore.
a E-mail: elbkong@ntu.edu.sg
b E-mail: mycboey@ntu.edu.sg
c E-mail: yzhuang@ntu.edu.sg
d E-mail: xuzc@ntu.edu.sg
2 School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore; E-mail: kzhou@ntu.edu.sg
3 School of Materials Science and Engineering, The University of New South Wales, Australia; E-mail: sean.li@unsw.edu.au
4 Electronic Materials Research Laboratory, School of Electronic and Information Engineering, Xi’an Jiaotong University, P. R. China; E-mail: wxque@xjtu.edu.cn
5 Singapore Institute of Manufacturing Technologies (SIMTech), Singapore; E-mail: hhuang@SIMTech.a-star.edu.sg
6 Anhui Target Advanced Ceramics Technology Co. Ltd., Hefei, Anhui, P. R. China; E-mail: 13335516617@163.com
* Corresponding author
2.2. Synthetic Methods
Two main strategies have been employed to synthesize graphene: top-down (exfoliation of graphite) and bottom-up (chemical synthesis) [2]. The top-down methods typically include mechanical exfoliation of HOPG [3, 4], chemical oxidation/exfoliation of graphite followed by reduction of graphene oxide (GO) [5–9] and solution-based exfoliation of graphite intercalation compounds (GICs) [10]. The bottom-up approaches for graphene synthesis comprise epitaxial growth on metallic substrates by means of CVD [11–13], thermal decomposition of SiC [14], and organic synthesis [15, 16] based on precursor molecules [17–20].
2.2.1. Liquid-Phase Exfoliation of Graphite
2.2.1.1.
Oxidation Exfoliation
Graphite oxide (GO), an oxygen-rich carbonaceous layered material, is produced by the controlled oxidation of graphite [5, 6]. Each layer of GO is essentially an oxidized graphene sheet commonly referred to as graphene oxide [21, 22]. It has been widely accepted that GO consists of intact graphitic regions interspersed with sp3 hybridized carbons containing hydroxyl and epoxide functional groups on the top and bottom surfaces of each sheet and sp2 hybridized carbons containing carboxyl and carbonyl groups mostly at the sheet edges [23–26]. Therefore, GO is hydrophilic and thus can be readily dispersed in water to form stable colloidal suspensions [27, 28].
Figure 2.1 summarizes the process in general, which has been modified in many alternative ways in the open literature [29, 30]. The process starts from the preparation of bulk quantities of chemically
Fig. 2.1. Schematic of the procedure followed for the production of chemically modified graphenes using graphite as the starting material. An oxidative treatment is initially performed to generate graphite oxide. This is followed by: (i) thermal reduction/exfoliation of graphite oxide to produce TR-GO, (ii) exfoliation by ultrasonication generating graphene oxide, (iii) chemical reduction of graphene oxide by using NaBH4 to produce CR-GO, (iv) electrochemical reduction of graphene oxide producing ER-GO. “OX” and “RED” stand for oxidation and reduction, respectively. Reproduced with permission from [30], Copyright © 2011, Wiley-VCH Verlag GmbH & Co.
modified graphite (CMG), which is derived from graphite microparticles that are oxidized by a mixture of HNO3, H2SO4 and KClO3 [31]. The oxidizing agents can be different. The resulting graphite oxide can be treated in several ways: (i) rapid heating of the sample in N2 atmosphere up to 1050°C resulting in the formation of TR-GO [32], (ii) ultrasonication of the graphite oxide dispersion in dimethylformamide (DMF) or ultrapure water leading to a dispersion of graphene oxide [33], (iii) chemical reduction of the water-dispersed graphene oxide with sodium borohydride (NaBH4, 50 mM) for 1 h resulting in CR-GO [8, 34, 35] and (iv) modification of the surface to form electrochemically modified graphene [36, 37].
Brodie’s method was developed more than 150 years ago, and is still widely used today [5]. In a typical process of this method, 10 g of natural graphite powder and 85 g of NaClO3 were mixed in a flask in an ice-bath. 60 ml of fuming HNO3 was added drop wisely [28]. Another portion of acid (40 ml) was added after 16 h and the slurry was heated to 60ºC and kept for 8 h. A slow heating rate of 1.5ºC/min was used to avoid strong deflagration. The reaction was terminated by transferring the mixture into 1 l of distilled water. The suspension was washed with 5 × 200 ml of 3 M HCl solution and with a copious amount of distilled water until the supernatant had a specific conductivity of 10 μS·cm‒1. The residual graphite oxide was decanted and dried. The first specimen of the graphite oxide series (GO-1) was oxidized forward (to GO-2), with the same procedure, except that a triple quantity (30 g) of GO-1 was used instead of graphite. Then, the whole oxidation procedure was repeated twice more in the same fashion to obtain the samples with the highest degree of oxidation. The four samples will be denoted as GO-1, GO-2, GO-3 and GO-4 where 1–4 correspond to the number of subsequent oxidation steps.
Dilute aqueous suspension of GO-1, 0.1 g·l‒1, pH = 4.5, was examined visually and by means of electron microscopy [28]. The partial disaggregation of GO-1 still formed a fine colloid, but coarse particles could be observed, which settled down in a short time. After 1 day aging, a clear supernatant and compact sediment were formed, as shown in the left image of Fig. 2.2. The platelet aggregates could not be cleaved. However, rapid and spontaneous exfoliation was observed, when NaOH was added, leading to the formation of a stable suspension, with more intensive light scattering, as shown in the right image of Fig. 2.2. The exfoliation effect of NaOH is schematically demonstrated in the figure as well. Figure 2.3 (a) shows TEM image of the GO at pH = 4.5, in which both coarse slabs and finer lamellae with micron-sized or smaller lateral dimensions could be observed. With the addition of NaOH (pH = 10), only thin carbon foils in random spatial arrangement and crumpled conformation could be found, which was indicative of the delaminated GO structure, as shown in Fig. 2.3 (b)
A typical process of Hummers’ method is described as follows [6]. 100 g of flake graphite powder and 50 g of sodium nitrate were mixed in 2.3 liters of sulfuric acid. The mixing was carried out in a container had been cooled to 0ºC in an ice-bath for the purpose of safety. While maintaining vigorous agitation, 300 g of potassium permanganate was added to the suspension. The rate of addition was controlled carefully to prevent the temperature of the suspension from exceeding 20ºC. The ice-bath was then removed and the temperature of the suspension was increased to 35 ± 3ºC, where it was maintained for 30 min. As the reaction progressed, the mixture gradually thickened with a diminishing in effervescence. At the end of 20 min, the mixture became pasty with the evolution of only a small amount of gas. The paste was
Fig. 2.2. Stability and laser light scattering of the GO suspensions at pH = 4 and pH = 10. Reproduced with permission from [28], Copyright © 2006, Elsevier.
Color image of this figure appears in the color plate section at the end of the book.
NaOH, exfoliation
Graphite oxide suspension at pH4
Graphite oxide suspension at pH10
brownish-gray in color. At the end of 30 min, 4.6 liters of water were slowly stirred into the paste, causing violent effervescence and an increase in temperature to 98ºC. The diluted suspension, brown in color, was maintained at this temperature for 15 min. The suspension was then further diluted to about 14 liters with warm water and treated with 3% hydrogen peroxide to reduce the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. Upon treatment with the peroxide, the suspension turned bright yellow. The suspension was then filtered, resulting in a yellow-brown filter cake. The filtering was conducted while the suspension was still warm in order to avoid precipitation of the slightly soluble salt of mellitic acid formed as a side reaction. After washing the yellowish-brown filter cake three times with a total of 14 liters of warm water, the graphitic oxide residue was dispersed in 32 liters of water to 0.5% solids. The remaining salt impurities were removed by treating with resinous anion and cation exchangers. The dry form of graphitic oxide was obtained by using centrifugation, followed by dehydration at 40ºC over phosphorus pentoxide. This method has been widely used in the open literature.
A modified Hummers’ method has been developed to synthesize graphite oxide nanosheets in high yield of 122 wt% based on the raw graphite and 68% based on the recovery of carbon [21]. The GO nanosheets had an average thickness of several nanometers and an average size of about 20 μm. Furthermore, excellent flexibility of the nanosheets was observed. When embedded in polymer matrix polymer, they were present in two types of secondary conformations, i.e., (i) lamination-layer-aggregate and (ii) randomshape-aggregate. Figure 2.4 shows the various categories of the GO nanosheets: oxidized form, partially oxidized form and reduced form.
Figure 2.5 shows SEM image of the product reduced with nascent hydrogen from aluminium powder and hydrochloric acid. They were random shaped aggregates. There were both singular and plural aggregates of the GO nanosheets, which were irregularly bent and deformed like crumpled paper [38]. This shows that, when the affinity between the nanosheets and the dispersion medium was very low, the nanosheets tended to aggregate like a linear flexible polymer. It was also found that the XRD peak corresponding to
Fig. 2.3. TEM images of GO-1 suspension at pH = 4.5 (a) and at pH = 10 (b). Reproduced with permission from [28], Copyright © 2006, Elsevier.
I Reduced-form I (graphene)
partiallyreduced
I Oxidized-fonn J (graphene oxide)
(graphit1
I Other-forms I (graphite o
Fig. 2.4. Classification of the graphene and graphene oxide nanosheets. Reproduced with permission from [21], Copyright © 2004, Elsevier.
Fig. 2.5. SEM images of a random shaped aggregate: (b) is a detail view of (a). Reproduced with permission from [21], Copyright © 2004, Elsevier. One SeveralSeveral tens m ) (single-layer) (multi-layer)
the spacing of 0.83 nm disappeared, but it was present in the original nanosheets in the oxidized form. This implied that the degree of both the interparticular orientation and interlayer orientation inside each nanosheet was reduced due to the deformation and aggregation.
Figure 2.6 (a) shows SEM image of a part of the random shaped aggregate, while Fig. 2.6 (b) demonstrates TEM image of a center of bend in the random shaped aggregate. Although GO nanosheets possessed a dense carbonaceous skeleton, they were very flexible, due to their extremely small thinness and very high aspect ratios. In this case, each of the fundamental layers of the particle is also flexible. The single-layered nanosheets were similar to a two-dimensional flexible macromolecule.
Dispersion behaviors of GO nanosheets in various solvents have been well-studied in the open literature [33, 39, 40]. For example, as-prepared graphite oxide can be dispersed in N,N-dimethylformamide, N-methyl-2-pyrrolidone, tetrahydrofuran and ethylene glycol. In all these solvents, full exfoliation of the graphene oxide into individual single-layer graphene oxide sheets was achieved with the aid of sonication [33]. The graphene oxide dispersions exhibited long-term stability, consisting of sheets with sizes ranging from hundreds of nm to a few μm, similar to the dispersion of graphene oxide in water. These results provide guidance while selecting solvents to disperse graphene-based materials.
with permission from [21], Copyright © 2004, Elsevier.
The as-prepared graphite oxide material was dispersed in water and 13 organic solvents with a nominal concentration of 0.5 mg·ml‒1 with the aid of ultrasonication. The dispersions were then allowed to settle for several weeks. Figure 2.7 shows photographs of all the dispersions immediately after the sonication (top) and 3 weeks after the sonication (bottom). For the just dispersed samples, graphene oxide could be well-dispersed in most of the solvents, while it was not stable in dichloromethane and n-hexane, and was dispersed in methanol and o-xylene to a lesser extent. However, some dispersions were stable only for a short while, from hours to days, such as those in acetone, ethanol, 1-propanol, DMSO and pyridine. Four solvents, including ethylene glycol, DMF, NMP and THF, offered long-term stability, which is comparable to that of dispersion in water. In the five most stable dispersions, only a small amount of precipitate was observed during the first few days after the sonication and they were kept stable without further precipitation.
Fig. 2.6. Fine structure of the random shaped aggregate: (a) SEM image of the bends and (b) TEM image of the center of one bend. Reproduced
Fig. 2.7. Photographs of the as-prepared graphene oxide dispersed in water and 13 organic solvents prepared with bath ultrasonication for 1 h. Top: dispersions immediately after the sonication. Bottom: dispersions 3 weeks after the sonication. Reproduced with permission from [33], Copyright © 2008, American Chemical Society.
Color image of this figure appears in the color plate section at the end of the book.
Those precipitates were attributed to graphite oxide particles that were not sufficiently exfoliated. In other words, if these graphite oxide particles are properly exfoliated, they can be dispersed.
To disperse graphene oxide, the organic solvents should be polar molecules, because the graphene oxide sheets contain a large quantity of polar oxygen-containing groups, such as hydroxyl, carbonyl, carboxyl, etc., which would trigger a strong graphene oxide sheet solvent interaction. Water and the four good organic solvents have high electrical dipole moment values, while the relatively poor solvents possess lower dipole moment values.
The GO exfoliated by using the above methods has strong interlayer hydrogen bonds between the oxygen functional groups of adjacent graphene oxide layers. Therefore, GO is usually hydrophilic, so the direct exfoliation of GO in non-aqueous solvents is not effective. This is because organic solvents are unable to penetrate the interlayer spaces of GO and disrupt these hydrogen bonds, leading to poor exfoliation. One idea is to reduce the density of the hydrogen bond groups, such as hydroxyls, through chemical functionalization, so that the graphene oxide layers become less hydrophilic and the strength of interlayer hydrogen bonding is decreased. In this case, it is possible to exfoliate GO in organic solvents. It has been shown that graphite oxide can be functionalized to form chemically modified graphite oxide derivatives [41]. Examples include the silylation of butyl amine-intercalated graphite oxide [42, 43].
Functionalized graphite oxides were prepared by treating them with organic isocyanates [44]. The isocyanate-treated GOs (iGOs) could be exfoliated into functionalized graphene oxide nanoplatelets, so that stable dispersions in polar aprotic solvents could be readily achieved. According to FT-IR spectroscopy and elemental analysis, the isocyanate treatment led to the functionalization of the carboxyl and hydroxyl groups in the GO through the formation of amides and carbamate esters, respectively, as shown in Fig. 2.8. The degree of the functionalization could be controlled through either the reactivity of the isocyanate or the reaction time.
Figure 2.9 shows the dispersions of the parent GO in DMF (left), phenyl isocyanate-treated GO in water (middle) and phenyl isocyanate-treated GO in DMF (right), all at a concentration of 1 mg·ml‒1. The vials with parent GO in DMF and phenyl isocyanate-treated GO in water contained visible precipitates, indicating their poor dispersing stability. In contrast, the dark brown dispersion of the phenyl isocyanatetreated GO in DMF had no visible precipitate, and was stable for weeks.
It is expected that the Brodie, Staudenmaier and Hummers’ methods and their modified versions will be continuously used to produce GO nanosheets [45–49]. However, due to the use of strong acids and oxidants during the processing, the structural defects caused by the chemical reactions disrupted the electronic structure of graphene, so that GO usually has a sheet resistance that is several orders of magnitude higher than that of pristine graphene or graphite. Therefore, chemical or thermal reduction
Wavenumber (cm–1)
Fig. 2.8. (a) Proposed reactions during the isocyanate treatment of GO, where the organic isocyanates reacted with the hydroxyl (left oval) and carboxyl groups (right oval) of the GO sheets to form carbamate and amide functionalities, respectively. (b) FT-IR spectra of GO and phenyl isocyanate-treated GO. Reproduced with permission from [44], Copyright © 2006, Elsevier.
of GO is necessary in order to recover electrical conductivity for some applications, such as electrodes. However, the complete reduction of GO to defect-free high quality graphene is still a challenge [50–54]. Reduction of GO has been mainly carried out by using chemical methods, by using various reducing agents. Among them, hydrazine is the most widely used one, including hydrazine monohydrate [55–57], dimethylhydrazine [58, 59] and anhydrous hydrazine [8]. Efficient chemical reduction of GO has mostly been achieved in solutions, because both sides of the sheets can be in contact with the reducing agents. Hydrazine is able to effectively remove the in-plane functional groups, like epoxy and hydroxyls, but is unable to eliminate the edge moieties, including carboxyl and carbonyl [7].
Due to their hydrophobic characteristics, graphite or graphene sheets cannot be stably dispersed in water without using any dispersing agents. Therefore, when GO is reduced to graphene, precipitation occurs. A strategy, known as electrostatic stabilization, has been proposed to disperse chemically reduced graphene nanosheets from graphite derived GO in water [7]. This discovery makes it possible to develop largescale production of aqueous graphene dispersions without the need for polymeric or surfactant stabilizers. In this process, the as-synthesized graphite oxide was suspended in Ultrapure Milli-Q® water to form a brown dispersion, with the residual salts and acids being completely removed through dialysis. The as-purified graphite oxide suspensions were then dispersed in water at a dispersion concentration of 0.05 wt%. In the chemical reduction of graphite oxide to graphene, the homogeneous dispersion (5.0 ml) was mixed with 5.0 ml of water, 5.0 ml of hydrazine solution (35 wt% in water) and 35.0 ml of ammonia solution (28 wt% in water). The weight ratio of hydrazine to GO was about 7:10. After being vigorously
Fig. 2.9. Vials containing the dispersions (1 mg·ml‒1) of GO in DMF (left), phenyl isocyanate-treated GO in water (middle) and phenyl isocyanate-treated GO in DMF (right). The top image shows the dispersions 24 h after preparation. The bottom image shows the inverted dispersions with the precipitate clearly shown on the bottom of the left and middle vials. Reproduced with permission from [44], Copyright © 2006, Elsevier.
shaken or stirred, the vial was put in a water bath at 95ºC for 1 h. The concentration of hydrazine in the reduction mixture can be varied from 0.0175 wt% to 1.75 wt%. In order to obtain stable dispersions with concentrations of > 0.0175 wt%, the excessive hydrazine should be removed by dialysis against a 0.5% ammonia solution after the reduction is finished. A RN2H4/GO ratio of 7:10 was the optimal ratio to obtain stable dispersions of highly conducting graphene nanosheets.
Figure 2.10 shows a schematic diagram demonstrating the solution-based route to obtain hydrophilic graphite oxide, which is exfoliated as individual graphene oxide (GO) sheets by ultrasonication in water. However, chemically reduced graphene nanosheets obtained through this method precipitate as irreversible agglomerates due to their hydrophobic nature. The resulting graphene agglomerates are insoluble in water and organic solvents, which cannot be used for further processing.
Surface charge or zeta potential studies have indicated that the as-prepared GO nanosheets are negatively charged when they are dispersed in water, as shown in Fig. 2.11 (a). This is attributed to the ionization of the carboxylic acid and phenolic hydroxyl groups on surfaces of the GO nanosheets [23, 60]. Therefore, the formation of stable GO colloids in water is attributed to the electrostatic repulsion, rather than just the hydrophilicity of GO. Because it is difficult to reduce the carboxylic acid groups with hydrazine, these groups still remain after the reduction, which has been confirmed by FT-IR analysis, as demonstrated in Fig. 2.11 (b). This result implied that the surfaces of the graphene nanosheets in aqueous solution are still charged after the chemical reduction. As a result, the electrostatic repulsion mechanism could be used to form well-dispersed graphene colloids.
It has been shown that, in most colloids, the colloidal stability of an electrostatically stabilized dispersion is strongly dependent on pH, the concentration of electrolytes and the electrolytes of dispersed particles [61]. Based on these facts, chemically reduced graphene nanosheet stable colloids through electrostatic stabilization have been achieved, as shown schematically in Fig. 2.10. Graphene oxide
Fig. 2.10. Scheme showing the chemical route to the synthesis of aqueous graphene dispersions. (1) Oxidation of graphite (black blocks) to graphite oxide (lighter colored blocks) with greater interlayer distance. (2) Exfoliation of graphite oxide in water by sonication to obtain GO colloids that are stabilized by electrostatic repulsion. (3) Controlled conversion of GO colloids to conducting graphene colloids through deoxygenation by hydrazine reduction. Reproduced with permission from [7], Copyright © 2008, Nature Publishing Group.
Wavenumber (cm–1)
Fig. 2.11. Surface properties of the GO and chemically converted graphene (CCG). (a) Zeta potential of GO and CCG as a function of pH, in aqueous dispersions at a concentration of 0.05 mg·ml‒1. (b) FT-IR spectra of the GO and CCG. The absorption band at 1,700 cm‒1 is attributed to carboxyl groups. The absorption of the CCG sheets at this range is observable, but not as pronounced as that observed for the GO, due to the overlapping of the strong absorption of graphene sheets in this region. Reproduced with permission from [7], Copyright © 2008, Nature Publishing Group.
nanosheets in the dispersions can be directly converted to graphene nanosheets that can be dispersed in water, without precipitation. In this case, the metal salts and acids that generally remained in the starting graphite oxide should be completely removed, because they can neutralize the charges on the graphene nanosheets and thus destabilize the dispersions. Ammonia was used to control the pH of the suspensions to be about 10. Volatile ammonia can be easily removed after the processing of the graphene nanosheets.
As the GO dispersions had concentrations of < 0.5 mg·ml‒1, reduction with hydrazine under optimized conditions caused no increase in the particle size of the resulting graphene nanosheets, as shown in Fig. 2.12 (a) Figure 2.12 (b) shows an atomic force microscopy (AFM) image of the graphene nanosheet on silicon substrates. The flat graphene nanosheets had a thickness of 1 nm. In the graphene nanosheet suspensions, ammonia and hydrazine dissociated into ionic items with behaviors like electrolytes. If the amount of hydrazine exceeded the optimal level, i.e., hydrazine:GO = 7:10 by weight, the stability of the dispersion decreased with the increasing concentration of hydrazine. Therefore, excessive hydrazine must be immediately removed from the dispersions after the reduction. Also, the concentrations of the GO dispersions should be < 0.5 mg/ml.
Besides hydrazine, other reducing agents, such as hydrides, hydroquinone [62, 63], and p-phynylene diamine [64], vitamin C and amino acid [65, 66], wild carrot root [67], potassium iodide [68], iron (Fe) [69], zinc (Zn) [70], and so on, have also been used for GO reduction. Hydrides include sodium borohydride [34, 62, 71, 72] and sodium hydride [73]. It has been reported that these residual edge groups can be removed by using concentrated H2SO4 after the initial reduction step [74]. As an alternative to chemical methods, hydrogen plasma treatment has also been shown to result in efficient reduction [56]. Some examples are discussed in the following part.
Reduced with N2H4 but without NH3
Reduced with N2H4 and NH3
Fig. 2.12. Colloidal and morphological characterization of the CCG dispersions. (a) Effect of the addition of ammonia on the dispersion state of the CCG sheets, characterized by measuring average particle sizes over a long period of time. The photographs shown in the inset were taken two days after the reduction reaction was complete with (left) and without (right) the addition of ammonia. The concentration of the starting GO solution is 0.5 mg·ml‒1. (b) Tapping mode AFM image of the CCG sheets with a height profile (blue curve with scale bar of 1 nm) taken along the red line. The sample was prepared by drop-casting a dilute CCG dispersion onto a silicon wafer. Reproduced with permission from [7], Copyright © 2008, Nature Publishing Group.
The reduction of GO to reduced GO (rGO) by using sodium hydride (NaH) is described [73]. When GO dispersion in methanol was treated with fresh sodium hydride powder at a dosage of 100 mg/ml, instantaneous reduction of GO to rGO occurred in less than 1 min. Accordingly, the yellow colored GO suspension became dark black. After being kept stationary for 10 min, the reduced suspension centrifuged at a high speed of 11000 g for 60 min at 20ºC, this was followed by washing with DI water to remove the sodium methoxide (NaOCH3) deposits on the rGO nanosheet surface. Stable rGO dispersion was formed by redispersing the rGO powder with adsorbed NaOCH3 in pure methanol.
For the as-prepared GO, the addition of the electronegative oxygen and the removal of unsaturated π electrons caused the GO sheets to be distorted, due to the introduction of nonplanar sp3 bonds, as shown in Fig. 2.13 (top). When NaH powder was dropped into the GO suspension, the immediate reduction was accompanied by the generation of hydrogen gas bubbles. At the same time, methanol was deprotonated into methoxy ions by sodium hydride [75], which in turn stabilized the rGO nanosheets in methanol, as shown schematically in Fig. 2.13. With respect to the initial mass of the graphite flakes, the process offered a high yield of about 68%, with most rGO nanosheets having 1–4 layers. Nearly 100% GO could be reduced to rGO in this process. Due to the electrostatic repulsion between individual rGO nanosheets in methanol, which was induced by the adsorbed methoxide ion (Na+CH3O–), the rGO suspension was highly stable with no visible settling.
Instantaneous Reduction
Ox ide (GO)
Fig. 2.13. Schematic diagram of the hydride reduction process. Photographs of glass vials containing the dispersion of GO in methanol (left) and the stable dispersion of rGO in methanol (right). Center right, bottom: picture of a vial with NaH in methanol. The dark black color of the stabilized rGO in contrast to the yellow color of the GO indicates the partial restoration of the interlayer π network of the rGO nanosheets. The cartoon and the three-dimensional (3D) chemical structures, with the gray, red and blue balls representing carbon, oxygen and sodium atoms, respectively, in the ball-and-stick model, show the reduction of GO to rGO with hydride, together with the release of hydrogen gas and the stabilization of the rGO suspension by sodium methoxide ions. Reproduced with permission from [73], Copyright © 2010, John Wiley & Sons.
Microstructural analysis results have indicated that sodium methoxide with dendritic structures was deposited on the surface of the resulting rGO nanosheets. The rGO suspension was destabilized when deionized (DI) water was added, due to the removal of the stabilizing methoxide ions from surface of the rGO nanosheets. It has been confirmed that sodium was completely removed, so that the rGO nanosheets had a smooth surface, as demonstrated by SEM image in the inset of Fig. 2.14 (b). When a 300-nm thick silicon dioxide substrate was contacted with an rGO solution for 10 min, Fig. 2.14 (c) shows that the rGO had a large areal coverage on SiO2/Si substrates. AFM observation indicated that the rGO had a thickness of about 0.6 nm, which was thinner than the original GO (about 1.2 nm), as illustrated in Fig. 2.14 (a, b). This observation could be attributed to the presence of (i) partially unreduced nonplanar oxy-functional groups, (ii) remnant sp3 C–C bonds or (iii) gas/solvent molecules trapped between the substrate and the rGO nanosheets [76]. In addition, the rGO deposits exhibited a higher degree of folding, due to the π–π interaction energy within the nanosheets. A possible folding mechanism is shown in Fig. 2.14 (c, left). Also due to the negatively charged methoxide adsorbed on the rGO nanosheet surfaces, the rGO deposits had no agglomeration during deposition.
Figure 2.15 shows Raman spectra of GO and rGO. GO had an ID/IG ratio of 1.88 ± 0.25, while the ID/IG ratio of the rGO was 1.08 ± 0.15, which indicated that the hydride reduction decreased the relative content of the sp3 carbon atoms and other related defects. In addition, the reduction of GO to rGO also led to a decrease in ID’/IG ratio, which is usually used to quantify the content of weak defects, induced intravalley scattering (D’ peak) and the graphenic region (G peak).
Fig. 2.14. (a, b) Atomic force microscopy (AFM) images of the GO nanosheets (a) and rGO nanosheets (b) spin-coated on 300-nm-thick silica wafers. The height profiles shown as the insets indicate that the thickness of the GO monolayer is about 1.2 nm, while that of the rGO monolayer is 0.6 nm. The top left inset in (b) shows an SEM image of the washed rGO nanosheets deposited on a 300-nm-thick silica substrate. The nanosheets had wrinkles (W) and folds (F). (c) Optical image of the rGO nanosheets deposited on a 300-nm-thick silica substrate, which shows large-area coverage of sporadic folding (F). The stars indicate the possible residual sodium methoxide deposits on the rGO nanosheets and on the substrate. The schematic diagram of the carbon structure (right) depicts a possible mechanism of folding of the rGO nanosheets. Reproduced with permission from [73], Copyright © 2010, John Wiley & Sons.
Fig. 2.15. Raman spectra of the GO and rGO nanosheets. The ID/IG and ID’/IG ratios for rGO were decreased after the reduction process, which suggested the presence of the long-range crystallographic order of the sp2 carbon atoms in the rGO nanosheets. The peak marked (*) in the rGO spectrum was attributed to residual surface-adsorbed sodium methoxide molecules. 3D chemical structures of the GO and rGO nanosheets are shown on the right. Reproduced with permission from [73], Copyright © 2010, John Wiley & Sons.
Reduced Graphene Oxide (RGO)
Graphene Oxide (GO)
Raman Shift (cm–1)
Fe is a cheap and widely available element. Fe powder has been used to reduce exfoliated graphite oxide at room temperature, with the principle being shown schematically in Fig. 2.16 [69]. Due to the presence of residual Fe, the resulting graphene nanosheets exhibited a high adsorption capacity of 111.62 mg·g‒1 for methylene blue at room temperature. More importantly, the items could be recycled from the suspensions by using magnetic separation after the absorption. In a representative experiment, 1 g of Fe powder with an average particle size of 10 m and 20 ml of HCl (35 wt%) were directly added into 100 ml of GO suspension at room temperature. The mixture was stirred for 30 min and then kept stationary for a period of time. After reduction, 15 ml of HCl (35 wt%) was added into the suspension in order to fully remove the excessive Fe powder. The resulting graphene nanosheet powder was collected with filtration.
Figure 2.17 (A) shows that the brown colored GO suspension was rapidly darkened in 60 min upon the addition of Fe and H+. Due to the hydrophilic characteristics of the oxygenated graphene layers, GO is easily exfoliated in aqueous media [27, 28]. As a result, GO sheets with a thickness of 1 nm can be readily dispersed in water to form a stable colloidal suspension, as shown in Fig. 2.17 (A). With the presence of H+, Fe reacted with H+ to form Fe2+ ions at the surface of the Fe particles. In this case, the GO nanosheets with negative charges (‒37 mV) could be attracted onto the surface of the positively charged Fe particles to form spherical structures, as shown in Fig. 2.17 (D, E). Therefore, the Fe particles were covered by the GO nanosheets, in close contact, so that reduction of the GO to rGO became possible, through the fast electron transport from Fe/Fe2+ to GO [37].
After reduction for 360 min, rGO powder was obtained, which comprised of randomly aggregated crumpled thin sheets, as illustrated in Fig. 2.17 (F, H). Figure 2.17 (I) shows cross-sectional TEM image of the rGO nanosheets. The thickness of the rGO nanosheets was between 1‒5 nm, consisting of approximately 2‒10 stacked monatomic graphene layers. It was found that, without HCl, the reduction of GO was very slow. The introduction of HCl dissolved the passive film of the Fe particles and promoted the reduction potential of Fe/Fe2+.
The magnetization hysteresis loop of the rGO powder is shown in Fig. 2.18, which is an S-like curve with a saturation magnetization value of 1.66 emu·g‒1. Moreover, the rGO exhibited typical superparamagnetic behavior, without coercivity and remanence magnetization. The rGO powder exhibited a strong adsorption to MB in aqueous solution. More importantly, the magnetic rGO could be readily separated from the solution by using a magnetic field. In summary, this is a cost-effective, environmentally friendly and large-scale method to produce graphene nanosheets.
Another interesting example is the reduction of GO in strong alkaline solutions [77]. It was incidentally found that a stable graphene suspension could be quickly prepared by simply heating an exfoliated GO suspension under strongly alkaline conditions at moderate temperatures of 50–90ºC, as shown in Fig. 2.19 (a). The finding originated from an experiment using free-radical addition to exfoliate GO through the introduction of functional groups, which was demonstrated in CNTs [78]. NaOH was added to the GO suspension in order to increase the solubility of the alkyl free-radical initiator, with carboxyltermination. However, the suspension experienced a rapid color change from yellow-brown to dark black,
Fig. 2.16. Illustration of preparation of the graphene nanosheets with Fe powder as the reducing agent. Reproduced with permission from [69], Copyright © 2011, American Chemical Society.
GO + aH+ + be– → Reduced –GO + cH2O
Fig. 2.17. (A) Photographs of aqueous dispersions (0.5 mg·ml‒1) of the GO before and after reduction by using Fe powder for different reaction times. (B) AFM image of the GO dispersed on mica and (C) the corresponding line profile. (D) Photograph and (E) SEM image of the rGO after reduction for 30 min without the presence of HCl. (F) SEM and (H, I) TEM images of the rGO after reduction for 360 min. The inset of panel F shows a piece of pressed graphene paper. Reproduced with permission from [69], Copyright © 2011, American Chemical Society.
Fig. 2.18. Magnetic hysteresis loop of the graphene nanosheets obtained by using the Fe reduction for 360 min without acid treatment at room temperature. Reproduced with permission from [69], Copyright © 2011, American Chemical Society.
Fig. 2.19. (a) Illustration of the deoxygenation (reduction) of exfoliated GO under alkaline conditions and (b) photographs of the exfoliated-GO suspensions (0.5 mg·ml‒1) before and after the reaction. The control sample in (b) was prepared by heating the pristine exfoliated-GO suspension without the presence of NaOH or KOH at 90ºC for 5 h with the aid of sonication. No obvious color change was observed in the control suspension, even when it was heated for a prolonged time period at relatively higher temperatures. Reproduced with permission from [77], Copyright © 2008, John Wiley & Sons.
which implied that the exfoliated GO was deoxygenated (reduced), leading to stable aqueous graphene suspensions, as demonstrated in Fig. 2.19 (b). At a suitable temperature, e.g., 80ºC, the yellow-brown exfoliated GO suspension became black in a few minutes.
The graphene suspensions obtained in this way exhibited very high stability, which made it possible for further processing, such as spin-coating, casting or spraying to take place. Because the negatively charged oxide functional groups were not completely removed, the stability of the graphene suspension was attributed to a strengthened electrostatic stabilization caused by the strong alkaline conditions. This was because the repulsion between negatively charged graphene nanosheets was increased at higher pH values. The deoxygenation (reduction) of the exfoliated GO under the strong alkaline conditions could be understood to be the reverse process of the oxidation reaction of graphite in strong acids [6]. This hypothesis was supported by the dependency of the deoxygenation reaction on pH value, i.e., the higher the pH value of the exfoliated-GO suspension, the faster the reaction would be. More importantly, if the pH value of the suspension was sufficiently high, the reaction could take place at room temperature or even lower. On the other hand, it is not surprising that all the reducing agents in the chemical reductions of exfoliated GO are intrinsically strong alkalis, such as hydrazine and dimethylhydrazine, as discussed previously [7, 56, 58, 79].
Other routes, including electrochemical reduction [37, 80, 81], photocatalytic reduction [82] and flash conversion [83], have been explored to reduce exfoliated GO to rGO nanosheets. However, these methods are more suitable for the reduction of thin films than for bulk or solutions.
A simple, efficient, low-cost and environmentally friendly electrochemical method has been reported to fabricate electrochemically reduced graphene oxide (ER-GO) films with an O/C ratio of less than 6.25% [37]. When coupled with a spray coating technique, the reduction method can be used to prepare large-area patterned ER-GO films with thicknesses ranging from monolayer to several microns on various conductive and insulating substrates. GO slurry was prepared by using the modified Hummers’ method [84], which was used to fabricate GO films on substrates by using spray coating, as demonstrated schematically in Fig. 2.20. Each spraying cycle consisted of spraying for 1 s and drying for 20 s. By controlling the concentration of the dispersion and the number of the spray coating cycles, the thickness of GO films could be readily controlled. By applying templates, patterned GO films could be obtained, as shown in Fig. 2.20.
The GO film could be directly reduced to rGO by using the potentiostatic method. Digital camera photographs showed that as the electrochemical reaction proceeded, a yellow–brown/black and near circular
Carbon Oxygen Hydrogen OH–
(a)
(b)
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By this time the man had brought his truck to a stop, a little distance from the place where Russ had fallen and where the doll had been lying.
“That was a narrow escape for you, youngster!” exclaimed the man rather sternly. “You ought not to do things like that!”
“I didn’t want Vi’s doll run over,” explained Russ, as his mother and sisters hurried toward him.
And while Russ is brushing the dust from his clothing and while Vi is looking over her doll, to make sure it is all right, I shall take a moment to let you know who the Bunkers are. And I shall also speak of the other books in this series telling about them. I think it is much better to read about people after you know who they are and what they have done.
The first book introducing the children is called “Six Little Bunkers at Grandma Bell’s.” At the opening of that story you find the Bunkers living in Pineville, a Pennsylvania town.
Bunker was the family name, and as there were six children, none of them very large, it was the most natural thing in the world to speak of them as the “six little Bunkers.” Of course there was a father and mother Bunker. Mr. Bunker’s name was Charles, and he was in the real estate business. His wife was named Amy, and there were a number of relatives, all of whom loved the six little Bunkers and all of whom the six little Bunkers loved.
As for the children the eldest was Russ—the one who was just in such danger. Russ seemed destined to become an inventor, for he was always making new things—make-believe houses, engines, automobiles, steamboats and the like. And as he worked he whistled merrily.
Rose might be called a “little mother,” for she was very helpful about the house, and Mrs. Bunker often said:
“I don’t know what I’d do without Rose to help look after the younger children.”
Violet and Laddie, who were twins, needed much looking after They were both rather peculiar. That is, Violet was given to asking questions. Her father said she could ask more in an hour than could be rightly answered in a week. As for Laddie, he was fond of asking riddles such as:
“You can have a house full and a hole full but you can’t keep a bowl full. What is it?” The answer, of course, is “smoke,” but nothing gave Laddie more pleasure than to find some one who couldn’t answer that or some other riddle he asked. Sometimes he made up riddles himself, or he might ask one that came out of a book. A queer little chap was Laddie.
Then there was Margy, who was seldom called by her real name of Margaret, and Mun Bun, otherwise known as Munroe Ford, as I have mentioned.
Now you have met all the six little Bunkers and I hope you will like them. As for their aunts, their uncles, their cousins and their other relatives—well, there are books telling about these different characters. The children often went to visit their cousins and aunts and had many adventures.
For instance there is the time they stayed for a while at Aunt Jo’s, or the occasion of their visit to Cousin Tom’s. They had fun at both these places, but no more than at Grandpa Ford’s or Uncle Fred’s. When they spent several weeks at Captain Ben’s the six little Bunkers had delightful times, and Russ thought there never was such a chap as Cowboy Jack, at whose ranch they spent some time. The other children liked Cowboy Jack, too.
Just before the events I am going to tell you about in this book took place, the children had been down South. You may find out all that happened by reading the volume, “Six Little Bunkers at Mammy June’s.” The family was now at home again in Pineville, ready for more adventures.
“You certainly gave me a fright, boy,” said the truck driver, as he got down off his high seat and looked at Russ. “Why did you run out into the road like that?”
“I wanted to get my sister’s doll,” answered Russ, still brushing the dust from his clothes.
“Um! Well, don’t do it again—that’s all I ask!” begged the man. “I was afraid I was going to run right over you!”
“Yes, it was a very dangerous thing for him to do,” said Mrs. Bunker. “He shouldn’t have tried it. I’m sorry he caused you trouble.”
“Oh, it wasn’t exactly trouble,” said the man, and he smiled a little. “I was going to stop around here, anyhow. I’m looking for a family named Bunker. Do you know if they live around here?”
“We’re the Bunkers!” quickly answered Russ. “Anyhow, we’re the most of ’em,” he added, laughing. “All but daddy and——”
“Oh!” murmured the driver of the truck. “Are there more of you?”
“It is rather a large family,” said Mrs. Bunker. “I have two more boys.”
“My daddy’s in his office,” volunteered Violet, who was now satisfied that her doll, Esmeralda, was all right except for a little dirt.
“And Laddie and Mun Bun are digging a hole to China,” added Margy.
“Oh,” and again the man smiled.
“Are you looking for a Mr. Charles Bunker?” asked Mrs. Bunker.
“That’s the name, yes, ma’am,” the truck driver replied, glancing at a slip of paper in his hand. “I have a load of flowers for him.”
“Oh, flowers! Is that what’s on your auto?” cried Rose, for the sides of the truck were covered with canvas and it could not be seen what it was laden with. Without waiting for an answer, Rose hurried around to the rear There she saw a number of pots of flowers and plants, and, being very fond of them, she reached up to pull nearer to her the pot closest to the end of the truck.
Perhaps the sudden stopping of the vehicle had made the pot unsteady, for, as Rose touched it, the pot was upset and rolled out of the truck toward the little girl.
“Oh! Oh!” cried Rose.
“What is the matter now?” asked Mrs. Bunker, going around to the rear of the truck. She was just in time to see a shower of brown earth from the pot splattering around Rose. The pot fell to the ground and was broken, the flower in it being knocked out.
“Not much damage done as long as the little girl isn’t harmed,” said the driver. “I’ve got some extra pots on the truck and I can easily plant this flower again,” and he picked up the geranium, which was a pink one in full blossom.
“Let me ’mell!” begged Mun Bun who, with Laddie, had now come out in the street to see why his mother and the other little Bunkers were gathered there.
“There isn’t much smell to that geranium,” laughed the driver. “But I have other flowers that do smell.”
“Are all these for us?” asked Mrs. Bunker, as she saw the mass of blossoms inside. “Rose, dear, are you sure you aren’t hurt?”
“Yes, Mother, I’m all right,” was the answer. “But, oh, where did all the pretty flowers come from?”
“They’re from Mr. Joel Todd,” answered the driver.
“Farmer Joel?” asked Mrs. Bunker.
“Yes, some folks call him that,” was the reply, and Mrs. Bunker remembered a rather odd character whom her husband knew. Mr. Bunker had often spoken of “Farmer Joel,” but had said nothing about a load of flowers coming from him.
“Did my husband order these?” asked Mrs. Bunker.
“No, I don’t know that he did, exactly,” the driver answered. “Farmer Joel had more plants than he could use, so he told me to bring these in to you, as I had to come this way anyhow with a load of produce.”
“Mother, who is Farmer Joel?” asked Rose, in a whisper.
“He has a farm about forty miles from here,” answered Mrs. Bunker. “Your father and I were there some years ago. Farmer Joel has orchards, bees, flowers, chickens, cows, and horses.”
“Oh, what a lovely place that would be to go to for the rest of the summer!” exclaimed Rose.
“Could we go there, Mother?” begged Vi.
“I—now—I know a riddle about a horse,” spoke up Laddie. “When is a boy a little horse?”
“We haven’t time for riddles now, dear,” said his mother. “I must tell this man where to leave the flowers that Farmer Joel was so kind as to send us.”
“Well, then I’ll tell you when a boy is a little horse,” went on Laddie. “It’s when he has a cold.”
“Pooh! Being hoarse when you have a cold isn’t being a horse on a farm,” declared Rose.
“It’s good enough for a riddle,” replied Laddie. “Oh, I want a ride!” he cried, as he saw the driver climbing up on his seat after Mrs. Bunker had pointed out her house.
“No, Laddie! Keep off the truck,” his mother warned him.
“Farmer Joel!” said Russ, in a musing tone as they all turned to go back home. “I wonder if we could go there?”
“Maybe you’ll have the chance,” his mother said, smiling.
“Oh! Oh! Oh!” cried the six little Bunkers in delight.
“But I can’t tell you any more now,” Mrs. Bunker went on. “It’s a secret!”
CHAPTER III
THE SECRET
Mrs. Bunker could not have said anything more exciting than the word “secret” if she had tried for a week. Hearing it, the six little Bunkers fairly jumped for joy.
“Oh, ho! A secret!” cried Russ.
“Let me guess what it is!” begged Laddie, acting as though he thought it a riddle.
“Oh, tell me!” cried Rose. “I won’t tell the others, Mother.”
“No, no!” laughed Mrs. Bunker. “When it is time to tell the secret you shall all know it at once.”
“Is it about us?” asked Violet, with what she thought a cunning air, hoping she might surprise something of the secret from her mother
“Yes, it’s about all of you,” was the answer.
“Is it good to eat?” was what Mun Bun wanted to know.
“Yes, the secret is good to eat,” answered Mrs. Bunker, with laughing eyes, as she looked at Farmer Joel’s truck driver.
“Is it good to play with?” was the question Margy asked.
“Yes, it’s good to play with, too,” said her mother
This set all the six little Bunkers to guessing, and they named first one thing and then another, but Mrs. Bunker only shook her head, laughed, and told them they would have to wait to find out about the secret.
“You’ve got your hands full with those youngsters, I can see that,” chuckled the truck driver, who had said his name was Adam North. “They must keep you busy.”
“They do. But they are good children,” Mrs. Bunker said, while Rose was murmuring:
“I can’t think what kind of a secret it can be that you can eat and play with. Can you, Russ?”
“Not unless it’s a candy cane—the kind we used to get for Christmas,” he answered.
“Oh, it couldn’t be that!” quickly declared Rose. “Mother wouldn’t make a secret about a candy cane. I think it must have something to do with this Farmer Joel.”
“Maybe,” agreed Russ. “But I have to go into the house and brush my clothes. I didn’t think they were so dusty. It’s like sliding for first base when you’re playing ball.”
By this time the six little Bunkers in charge of their mother were ready to walk back toward their house. They made a pretty picture as they stood in the street, Mun Bun and Margy were first, side by side, and holding hands as the two youngest generally did. Then came the twins, Violet and Laddie, next largest in size, and back of them were Rose and Russ, while Mrs. Bunker came behind the two oldest, smiling at her “brood,” as she sometimes called them, pretending they were hungry chickens.
“Well, we’re generally hungry all right,” Russ would say with a laugh when his mother spoke thus.
“I suppose we look like a procession, don’t we?” asked Mrs. Bunker of Adam North, as he prepared to start his truckload of flowers.
“Well, a little, yes,” he agreed, with a laugh. “But it’s a mighty nice procession. I guess Farmer Joel wishes he had one like it.”
“That’s so, he has no children, has he?” remarked Mrs. Bunker. “It’s been some time since I have seen him, and I thought perhaps he might have married.”
“No,” went on Mr North, while the six little Bunkers listened to the talk, wondering, the while, what the wonderful secret might be.
“Farmer Joel is still a bachelor He lives with his sister Miss Lavina. She keeps house for him, you know.”
“Oh, yes, I know Lavina Todd very well,” said Mrs. Bunker “She and I were old chums. We went to school together when we lived in the same country town as girls. But that was quite a number of years ago, and I thought Farmer Joel might have married in all that time.”
“No—old bachelor,” replied Adam North. “But he’s the kindest, jolliest soul you’d want to meet and he loves children. That’s why I say he’d like a procession like yours. Now then, where do you want these flowers? I’ve got quite a load of ’em.”
“Indeed you have a wonderful load of blossoms,” said Mrs. Bunker. “It was very kind of Farmer Joel to send them. But I’m afraid I can’t set them out all alone.”
“Oh, I’ll stay and help you plant the flowers,” offered Adam North, who was something of a farmer and gardener himself. “Mr. Todd said I was to do that. I’ve got to stay, anyhow, to see Mr. Bunker. He’ll be home soon, I expect.”
“Yes, he’ll come home to supper,” replied Mrs. Bunker. “I hope you can stay and have a meal with us,” she added.
“Well, I might—yes,” was the slow answer. “In fact, I was going to stay over at the hotel all night, as it’s a long ride back to Cedarhurst, and I don’t like to drive the truck after dark if I can help it.”
“Oh, then you can stay at our house,” quickly said Mrs. Bunker. “We’d be delighted to have you. There is plenty of room.”
“And you can tell us about the farm,” added Rose.
“And about the bees,” added Mun Bun. “Does they sting?”
“Sometimes,” laughed Mr. North.
THE CHILDREN HELPED AS MUCH AS THEY COULD.
Six Little Bunkers at Farmer Joel’s (Page 31)
“And tell us about the cows and chickens,” begged Laddie. “I know a riddle about—now—about a cow, only I can’t think of it.”
“Maybe it’s the cow that jumped over the moon,” joked Mr. North.
“No, it isn’t that,” Laddie answered. “Maybe I’ll think of it after a while.”
“I’d like to hear about the horses,” suggested Violet. “How many horses does Farmer Joel have and do they ever run away and did they ever run away with you and did you get hurt and are there any little horses? I don’t believe they’d run away, would they? And if a horse runs away does he run back again and——”
“Violet! Violet!” cried her mother. But the little girl had stopped herself, for she was out of breath.
“Does she often get spells like that?” asked Adam North, with a laughing look at Mrs. Bunker.
“Sometimes,” was the smiling answer. “But generally she asks her questions one at a time. I don’t know what made her take such a streak. But come, children, I want to get these flowers set out before daddy comes home. Come along.”
“We can plant some in the hole we dug,” said Laddie.
“No! No!” cried Mun Bun. “That’s a hole to China and we don’t want any flowers in it!”
“Easy, Mun Bun! Don’t get so excited,” soothed Russ. “Maybe the people in China would like some of these flowers.”
“Oh, all right. I give some flowers to Chiweeze,” agreed Mun Bun.
By this time the truck had rolled into the driveway of the Bunker home, and the family of children and their mother soon followed. The doll, which had been the cause of so much excitement, and not a little trouble, was put in the house where no wandering dog could carry her off again. Then Adam North began unloading the pots of
flowers, some of which needed to be set out in the ground to make them grow better.
It was toward the end of spring, with summer in prospect and just the time to start making a flower garden, Mr. North said. Farmer Joel raised many kinds of plants and blossoms, his sister Miss Lavina Todd helping him. They had so many that it had been decided to send some to Mr. Bunker.
“But I never thought he could spare all these,” remarked Mrs. Bunker, when she saw the geraniums, the begonias, the fouro’clocks, the petunias, the zinnias, the marigolds and many other kinds of “posy-trees,” as Mun Bun called them.
“Oh, yes, we have more flowers at Cedarhurst than we know what to do with,” said Adam North, as he began setting out the blossoms.
The children and Mrs. Bunker helped as much as they could, but except for what Russ, Rose and Mrs. Bunker did there was really not much help. For Violet, Margy, Mun Bun and Laddie would start to dig a hole in which to set out a plant, then they would forget all about it in running to see a new kind of blossom that was taken from the truck.
So it was that there were a number of half-dug holes about the garden, with nothing planted in them. But Adam North knew his business well, and soon he had turned the formerly dull Bunker yard into a veritable flower-show, with bright blossoms here and there.
“Now if you’ll just give ’em a little wetting down with the hose so they won’t wilt, they’ll come up fresh and strong by morning,” he said, when the last plant was set out.
“I’ll use the hose!” offered Russ.
“I’ll help!” said Rose.
“So will I!” cried the other four little Bunkers. Using the hose was something they all delighted to do.
“No, my dears,” said Mrs. Bunker firmly “Russ will do the sprinkling and all the others must come in and get washed ready for supper. Daddy will soon be home and then——”
“Will you tell us the secret?” asked Rose.
“I think so—yes,” was the reply, and this gave the smaller children something to think about so they did not mind not being allowed to use the hose.
“I wouldn’t dare let them take turns wetting the new plants,” said Mrs. Bunker to Adam. “Russ is all right, but the others would shower every one passing in the street.”
“I reckon so, and wash out all the new plants besides,” chuckled Farmer Joel’s hired man. “And now,” he went on, “since you have been so kind as to ask me to stay to supper and remain all night, I’d like to wash up myself. I’m pretty dirty,” he added, with a laugh, as he looked at his grimy hands, for he had been delving in the dirt to set out the flowers.
“Come with me,” said Mrs. Bunker “And, Russ,” she added, “be careful about the hose. Don’t spray on any people who may be passing.”
“I’ll be careful,” he promised.
Ordinarily when Russ used the hose all the other little Bunkers stood around anxiously waiting for their turn. But now, with the prospect of hearing a secret, they went willingly to the bathroom and soon were as shining as soap and water could make them.
Adam, as the children soon began to call him, for he was very friendly, ran the big truck up alongside the garage, as there was not room for it inside. Then, after he had washed and prepared for supper, he went out to see that Russ did not spray too much water on the newly set out plants.
Norah, the cook, had supper almost ready and Adam had told Russ enough water had been used when the boy, looking down the street, saw his father approaching.
“Here comes daddy!” he cried.
Mr Bunker waved his newspaper and as he reached the gate and saw the visitor a pleasant smile came over his face and he cried:
“Well, Adam North! Glad to see you! How’s Farmer Joel?”
“Right hearty! I brought you those flowers.”
“That’s good! Hello, Russ! How’s everything here?”
“All right, Daddy!”
“Daddy! Daddy!” came in a chorus from the other little Bunkers, and their father was overwhelmed in a joyous rush.
“What’s the secret?”
“Tell us the secret!”
“Can Mother tell us the secret now?”
These were only a few of the words Mr. Bunker heard as he was hugged and kissed.
“Secret?” he exclaimed, looking at Adam. “What secret?”
“Oh, you know!” laughed Rose. “It must be about Farmer Joel!”
“Oh, that!” chuckled Mr. Bunker. “Yes, the secret is about him,” he admitted. “But how did you all know it?”
“There’s been a lot of excitement in the last hour,” said Adam. “I nearly ran over a doll, just missed smashing Russ, and there’s a secret in the air. Oh, nobody’s hurt,” he quickly added, for he saw that Mr. Bunker looked a little alarmed at the mention of what had so nearly been an accident.
“That’s good,” said Daddy Bunker.
“The secret! The secret!” begged the children.
“All right. Come into the house and I’ll tell you the secret,” he promised.
With whoops of delight, in trooped the six little Bunkers.
CHAPTER IV
WHERE IS LADDIE?
“Supper is all ready, Daddy! We’ll sit right down,” called Mother Bunker, as the happy crowd entered. “I see you have already met Farmer Joel’s man,” she added, nodding and smiling.
“Oh, yes, Adam and I are old friends,” Mr. Bunker said. “And I’m glad supper is ready, for I’m hungry. Let me see now——”
“The secret! The secret!”
“You promised to tell us the secret!”
“Tell us now!”
“Don’t wait until after supper!”
Thus cried the six little Bunkers.
“Quiet, children! Please be quiet!” begged their mother. “What will Adam North think of you?”
“Oh, let ’em go on! I like it!” chuckled the truck driver.
“I think perhaps I had better tell the secret,” said Mr. Bunker. “It is the only way we shall have any peace and quiet. Now all of you sit down to the table,” he ordered, “and when you can compose yourselves I will tell you what I have to say.”
It took some little time for all of the six little Bunkers to get quiet, but finally each one was sitting nicely in his or her chair, with their father at one end of the table and their mother at the other, Adam having a place next to Mr. Bunker.
“Now,” said Mr. Bunker, when all was quiet, “in order that you will not eat too fast, to get through supper quickly to hear the secret, I am going to tell it to you now.”
“Oh, I can hardly wait!” murmured Rose.
“What is it?” asked Violet.
Then came a moment of eager, anxious waiting.
“We are all going to spend the summer at Farmer Joel’s,” said Mr. Bunker suddenly.
“Oh! Oh! Oh!” came the murmurs of delight. Mrs. Bunker, with laughter shining in her eyes, looked at the happy faces around her.
“They sure will have fun out there!” said Adam.
“Do you really mean it?” asked Russ. “Are we going?”
“Surely,” said his father “Farmer Joel’s sister, who has been keeping house for him, is going away on a visit. When he told me this he said he didn’t know what he was going to do, as he didn’t want a strange woman coming in to look after the place. Then I said I would bring my six little Bunkers up there and they would keep house for him.”
“Did you really say that, Daddy?” Rose asked eagerly. “I surely did.”
“Well, I can keep house a little bit,” Rose went on. “But to cook for a farmer——”
Rose began to look worried, so her mother said:
“You won’t have to do it all alone. I am going with you, and so is Norah, and we’ll see that Farmer Joel doesn’t get hungry.”
“Oh, if mother is coming it will be all right,” said Violet.
“Fine! Yes!” cried the other little Bunkers. You can see they thought a great deal of their mother.
“So that is how it came about,” went on Mr. Bunker. “Farmer Joel’s sister is going away on a long visit—to remain all summer. We are going up there to live on his farm.”
“And can I help get in the crops?” asked Russ, who liked to be busy.
“Yes, we’ll all help,” his father promised. “I think you need a lot of help on a farm in summer, don’t you, Adam?” he asked.
“That’s right,” answered Farmer Joel’s hired man. “The more help we have the better. I’m pretty well rushed myself in the summer.”
“And can we see the horses?” asked Violet.
“And the cows?” came from Laddie.
“And the sheep?” Mun Bun wanted to know.
“And the apple trees?” asked Margy.
“I’d like to see the bees make honey,” remarked Rose, who, herself, was often as busy as any bee.
“You shall see everything there is to see,” promised Daddy Bunker “There! Now you know the secret. We are going off to Farmer Joel’s for the summer, and I think we shall have a fine time. Now eat your suppers!”
And the six little Bunkers did.
After supper there was more talk about going to the farm, and Mr. Bunker said:
“I have been talking with Adam, and this seems the best way to go. Cedarhurst, where Farmer Joel lives, is about forty miles from here. It is not on any railroad, so we shall need to go in the automobile. As our car is hardly large enough to take us all and the trunks we shall need this is what we can do.
“Adam and I will ride to Cedarhurst in the big auto truck that brought the flowers. In that we can also take the baggage—the trunks of clothes and the like. The children can also ride in the truck with me. We’ll fill it full of straw.”
“Oh, that will be fun!” cried Russ.
“A regular straw ride!” added Rose.
“But what about mother?” asked Violet. “Is she going in the truck with us?”
“Your mother and Norah will drive up in our own touring car,” said Mr. Bunker.
“When can we go?” asked Russ.
“In a few days,” his father answered.
“Then I won’t bother to make the seesaw here,” went on Russ. “I’ll save the nails and take them to Farmer Joel’s.”
“That’s a good idea,” agreed Rose. “We can make a lovely teetertotter up there, and have lots of fun.”
In the early evening, after supper, not much was talked of by the six little Bunkers but the coming visit to Farmer Joel’s. Mrs. Bunker, who had been to the farm some years before with her husband, told the children about it. There were many places where they could have fun, she said.
The evening was passing. Mun Bun and Margy, in spite of their hard work to keep awake, were fast falling asleep, their little heads nodding from side to side and their eyes closing.
“It’s time they were in bed!” cried Mrs. Bunker, when she finally noticed them. “It’s long past their hour. And Laddie and Vi, too! They must go to bed!”
“I’ll carry up Mun Bun,” offered Mr. Bunker.
“And I’ll take Margy,” said Adam, for both the smallest children were now asleep.
“Come, Vi,” suggested her mother. “You and Laddie can go up by yourselves.”
“Laddie isn’t here,” said Violet.
“He isn’t? Where is he?” asked her mother “Perhaps he has fallen asleep in a corner of the porch,” for they were sitting out on the piazza talking over the coming visit to Farmer Joel’s.
“No, he isn’t here,” went on Violet. “He got up and walked off a little while ago.”
“Then I guess he went up to bed by himself,” said Mr Bunker, as he went into the house carrying Mun Bun, while Adam followed with Margy. “I’ll see if he’s in his room,” he added to his wife.
But a little later, when Mr. Bunker called down: “Laddie isn’t up here!” there was some excitement.
“Where can he be?” asked Mrs. Bunker.
“Maybe he’s out in the yard trying to catch lightning bugs,” suggested Rose, for she and Russ were to be allowed to remain up a little later than the smaller children.
“It’s too early for lightning bugs,” replied Mrs. Bunker. “Where can the child have gone? Laddie! Laddie!” she called, raising her voice. “Where are you?”
But the only sound was the singing of the frogs down in the pond —that is, if you call the noise the frogs make “singing.” There was no answer from Laddie.
“He may have wandered down into the garden, to look at some of the flowers you set out,” suggested Mr. Bunker.
“He couldn’t see flowers in the dark,” objected Mrs. Bunker.
“He might if he took a flashlight,” said Russ. “Maybe that’s what he did. I’ll go and look for him.”
“I’ll come and help you,” offered Adam.
But a search through the garden and more calling of Laddie’s name brought no answer from the little fellow.
“Where can he have gone?” exclaimed Mrs. Bunker “I’m afraid he’s lost.”
CHAPTER V
OFF TO THE FARM
Mr. Bunker saw that his wife was growing a little alarmed over Laddie’s absence, so he said:
“Now don’t worry, we’ll find Laddie.”
“I’ll help you look for him,” said Adam. “He can’t have gone very far.”
“Maybe he fell asleep in the summer-house,” suggested Russ, for at the end of the garden was a rustic summer-house, or pavilion, in which the children sometimes played. But Laddie was not there.
“Could he have fallen into the brook?” asked Rose.
“If he did, all that could happen would be that he got wet,” her father answered, with a laugh.
“And if Laddie fell into the brook I guess he’d yell and we would hear him,” Rose said, nodding her head.
“’Tisn’t very deep, anyhow,” added Russ.
They looked farther in the garden for Laddie and called his name, but there was no answer. Mr. Bunker was just beginning to get worried when the telephone in the house suddenly rang.
“Maybe that’s some news of him!” exclaimed the mother of the missing little fellow. She started toward the telephone, but Laddie’s father reached it first.
“Hello! Hello!” called Mr. Bunker into the telephone.
The others listened to what he had to say.
“Yes! Yes,” he went on. “Oh, then he’s all right. I’m glad of that. Thank you! Yes, I’ll be right down after him.”
“Evidently it’s about Laddie?” said Mrs. Bunker in a questioning voice.
“Yes,” answered her husband, as he hung up the receiver “Laddie is in the police station.”
“The police station!” cried Russ.
“Is he arrested? What for?” Rose queried wonderingly.
Daddy Bunker laughed, which let them all know it could not be very serious.
“What is it?” asked his wife.
“As nearly as I can make out,” said Mr. Bunker, “Laddie wandered away from here and went to the police station about some riddle.”
“A riddle!” cried Adam North. “Good gasoline! That boy must dream of riddles!”
“I sometimes think he does,” sighed his mother. “But what sort of riddle is it this time?” she asked her husband.
“The officer at the police station didn’t just know,” was Mr. Bunker’s answer. “He said they had Laddie there and asked me to come and get him, as they didn’t want to send him home with a policeman for fear the neighbors would think something had happened. As nearly as I can make out, Laddie must have thought of a riddle and have gone to the police station to see if any one could guess it.”
“Why didn’t he ask one of us?” his mother wanted to know “He generally does ask us first.”
“We’ll find out all about it when I bring him home,” replied Mr Bunker. “I’ll go right after him.”
“Will you take the car?” asked Mrs. Bunker
“Yes, I think I’d better. Laddie may have fallen asleep, and he’s pretty heavy to carry.”
“I’ll go with you,” offered Adam, and soon they were at the police station.
