Thermal Perturbation Nucleation and Controllable Growth of Silver

Article pubs.acs.org/crystal

Thermal Perturbation Nucleation and Controllable Growth of Silver Vanadate Crystals by Dynamic Template Route Xiao-Yu Yuan,† Feng-Rui Wang,† Jin-Ku Liu,*,† and Xiao-Hong Yang‡ †

Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ Department of Chemistry, Chizhou University, Chizhou 247000, P.R. China ABSTRACT: Silver vanadate (Ag4V2O7) nanotube clusters were successfully fabricated by the dynamic template route at room temperature. The sample presented a spherical cluster 8 μm in diameter at the macro level. While in the view of the microcosmic, it showed up with a cluster fabricated from mass nanotubes 1−2 μm in length and 18 nm in internal diameter. Detailed investigations were carried out designed to analyze the control procedure imposed on its reaction condition and heat characteristics. The growth mechanism of Ag4V2O7 nanotube clusters has also been elucidated, which indicated that the semipermeable structure of the eggshell membrane and the polymerization of acrylamide played an important role in the formation of Ag4V2O7 nanotube clusters. Furthermore, the nanotube clusters exhibited higher photocatalytic properties compared with the traditional Ag4V2O7 crystals. The new growth mechanism of the cluster has reference values for the preparation of nanotube structures.

1. INTRODUCTION

In this work, three-dimensional Ag4V2O7 nanotube clusters were synthesized through a dynamic template method mainly induced by the thermal perturbation nucleation effect and the template effect of polyacrylamide through the polymerization of acrylamide in the process of crystal growth. The properties of the Ag4V2O7 nanotube clusters were tested by the electrochemical characterization and the degradation of rhodamine B (Rh.B). This controllable growth will provide an efficacious and versatile way to fabricate optimized structure with outstanding properties.

Silver vanadate (AgVO3, Ag2V4O11, Ag3VO4, Ag4V2O7, etc.) has received great attention due to its specific photoelectronic and chemical properties.1,2 The distinctive properties of silver vanadate could be ascribed to the flexibility of the geometries, in which V, O, and Ag elements could adopt different stoichiometric and nonstoichiometric ratios.3 The hybridization of V 3d orbitals and Ag 5s orbitals constituted the conduction band of silver vanadate, and the valence band of silver vanadate associated with O 2p orbitals hybridized with Ag 3d orbitals, yielding a narrow band gap and highly dispersed conduction band and valence band.4,5 It is well-known that the properties of silver vanadate are highly dependent on its composition, morphology, crystal structure, and surface properties. Recently, many silver vanadate compounds with novel morphologies have been studied owning to their interesting photoelectronic and chemical properties. Nanotube materials have been extensively studied ever since the discovery of carbon nanotubes. Nanotubes have often shown outstanding thermal,6 optical,7 electrical,8 and magnetic9 properties, which would present good application performance in catalysis,10,11 sensors,12,13 batteries,14 supercapacitors,15−18 and so on. While the synthesis of nanotubes is still a puzzle to overcome in technology, which needs techniques like photoassisted deposition, metal organic chemical vapor deposition (MOCVD),19−21 electrodeposition,22,23 template-assisted multistep processes,24,25 electrospinning,26 sol−gel, and so on.27,28 So, developing new synthetic method was instructive to the fabrication of nanotube materials. © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Ag4V2O7 Nanotube Clusters by Dynamic Template Method. All chemicals including silver nitrate (AgNO3), ammonium meta-vanadate (NH4VO3), sodium hydroxide (NaOH), ethylenediamine, acrylamide (AM), polyacrylamide (PAM), pluronic F-68 (PF-68), and rhodamine B (Rh.B) were all analytical grade and used without further purification. Double-distilled water was used in the whole experimental process. The eggshell membrane was made from a fresh eggshell after removing the outer shell. In a typical synthesis, 0.12 g of NH4VO3 was dissolved in 20 mL of deionized water and adjusted with ethylenediamine to pH = 10. Twenty milliliters of AgNO3 (0.51 g) solution was added into another reactor, and then the reactor was sealed by the prepared eggshell membrane. Some cooperative reagents such as PAM and PF-68 were introduced into the system instead of AM to analyze the crystal growth. Received: April 25, 2017 Revised: June 11, 2017 Published: June 22, 2017 4254

DOI: 10.1021/acs.cgd.7b00594 Cryst. Growth Des. 2017, 17, 4254−4264

Crystal Growth & Design

Article

The direct precipitation method was also applied as the control. The samples prepared by the direct precipitation method, dynamic template method with AM, PAM, and PF-68 were denoted as AVOD, AVOA, AVOP, and AVOF, respectively. The effect of the AM concentration on the growth of Ag4V2O7 crystals was studied by adding different amounts of AM (0.01, 0.05, 0.1, and 0.2 g) into the solution. Temperature influences were explored by completing the reaction under 30, 50, and 70 °C. Ethanediamine and NaOH were used as the alkalinity regulators to adjust the pH of NH4VO3. The resulting products were washed with deionized water and absolute alcohol several times until no Ag+ was detected. Then the precipitations were dried in an oven at 55 °C for 12 h. 2.2. Characterizations. The structures of Ag4V2O7 nanotube clusters were characterized by X-ray powder diffraction (XRD) using a Shimadzu XD-3A diffractometer. X-ray photoelectron spectroscopy (XPS) (Shimadzu ESCA-3400, Mg Kα radiation) measurements were carried out so as to evaluate the surface electronic state and analyze the surface atom of sample. The microstructures and morphologies of Ag4V2O7 nanotube clusters were analyzed by transmission electron microscopy (TEM, Hitachi-800). The morphologies were tested by Philips S-4800 scan electron microscopy (SEM). The optical properties of the products were studied by the Fourier transition infrared (FT-IR) spectroscopy. UV−vis absorption spectrum was investigated at room temperature (25 °C) by a UV-2450 (Shimadzu) spectrometer. The specific surface area (BET) of the products was determined by isothermal nitrogen adsorption−desorption analysis (Micromeritics ASAP 2400). Calorimetry was applied to measure the reaction heat of the reaction. Thermal gravimetric (TG) and differential thermal analysis (DTA) measurements were performed using 2960 SDT TA Instruments. The reaction heat was detected by the Integrated Calorimeter (HADWC-RJ). 2.3. Photocatalytic Experiment. The photocatalytic performance of the Ag4V2O7 crystals was measured by the degradation of organic dyes. The photocatalytic experiment was carried out in a quartz photochemical reactor under the 300 W xenon arc lamp cutting off the ultraviolet light (λ > 400 nm). A total of 0.1 g of Ag4V2O7 crystals was added into 30 mL of Rh.B (2 × 10−5 mol/L) solution under magnetic stirring. The suspension was stirred for 30 min in the dark to reach adsorption−desorption equilibrium. Then the above suspension was exposed to the light at a temperature of 25 ± 1 °C. The absorbance of the solutions was test with a UV-2450 (Shimadzu) spectrometer. 2.4. Electrochemical Characterization. The photoelectrochemical experiment was carried out by a standard three-electrode cell on a CHI-660D electrochemical workstation with the catalyst samples as working electrode, platinum sheet as counter electrode, a saturated calomel electrode as reference electrode, respectively. A 0.1 M sodium sulfate solution was used as electrolyte. For working electrodes, a 5 mg Ag4V2O7 nanotube cluster and 10 μL Nafion solution (5 wt %) were mixed with 1 mL of absolute ethyl alcohol to get a homogeneous dispersed suspension liquid by 30 min of ultrasonic. The dispersed suspension was deposited on a piece of fluoride-doped tin oxidecoated (FTO) glass with 1 × 1 cm2 area by a spin-coating method on a spin coater. Then the electrode was dried in an oven at 60 °C for 3 h.

Figure 1. A representative optical image and the magnifying morphology of Ag4V2O7 nanotube clusters.

clusters presented when Ag4V2O7 was amplified by an electron microscope. Electron microscope images and XRD patterns were present to survey the morphology and structure of Ag4V2O7 nanotube clusters. Figure 2a is the low-magnified SEM image of the sample. It was obvious that Ag4V2O7 showed a spherical clusters structure with a diameter of 8 μm, which was composed of massive closely packed, ordered nanotubes. Figure 2b was the high-magnified SEM image which displayed more details of Ag4V2O7 nanotube clusters. Observations found that an individual nanotube had a mean outer diameter of 68 nm and inner diameter of 17 nm. Moreover, there was a mass of small nanoparticles anchored on the surface of Ag4V2O7 nanotube clusters, which was inferred as Ag nanoparticles photodissociated by Ag4V2O7 nanotube clusters under the natural light. TEM images further determined the tubular structure of Ag4V2O7 (Figure 2c,d). As shown in Figure 2c, Ag4V2O7 possessed an aggregate structure consisting of plentiful nanotubes, which was in accordance with the SEM image in Figure 2a. The tube structure was also found in higher magnified TEM images with a diameter about 17.43 nm (Figure 2d). Furthermore, the tube walls were supplied with small nanoparticles, agreeing with the presentation in SEM. The inset in Figure 2d (selected-area electron diffraction (SAED)) confirmed that Ag4V2O7 nanotube clusters presented as a polycrystal. The XRD pattern in Figure 2e illustrated the crystal structure of Ag4V2O7 nanotube clusters. It was found that all the peaks were in agreement with standard lattice planes of orthorhombic structure, corresponding to the Inorganic Crystal Structure Database (ICSD) card No. 38065.32,33 The preferred orientation was exposed to (040) referencing the standard XRD pattern. The space group of Ag4V2O7 crystals was Pbca, and each unit cell contained six formula units (Z = 16) according to the report reference. No other peaks were found in the sample, indicating that Ag4V2O7 nanotube clusters was a single phase. Conspicuous energy changes of the reaction system during the synthesis of Ag4V2O7 crystals could be detected by the determination of reaction heat (Figure 2f), elaborating the polymerization heat of AM would perturb the crystal growth of Ag4V2O7 crystals. The XPS measurement was further implemented to clarify the chemical compositions and the element states of Ag4V2O7 nanotube clusters. It was clear that only Ag, O, and V were

3. RESULTS AND DISCUSSION 3.1. Characterizations. Figure 1 showed the physical map and electron micrographs of Ag4V2O7 nanotube cluster synthesized by the dynamic template method. As a biology semipermeable membrane, eggshell membrane can control the transportation of ion on both sides of the membrane.29,30 The reaction system was composed of two glass containers separated by membrane.31 Because of the semipermeability of eggshell membrane, Ag+ transferred to the opposite. As a result, the precipitation and crystallization were carried out at the under layer of the reactor, generating silver vanadate crystals. As shown in Figure 1, the formed Ag4V2O7 on the eggshell membrane assumed a rodlike morphology, while nanotube 4255



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Figure 2. SEM images (a, b), TEM images (c, d), XRD pattern (e), and the thermal effect (f) of the Ag4V2O7 nanotube clusters.

Figure 3. XPS spectra of Ag4V2O7 nanotube clusters: survey (a); Ag 3d (b); O 1s (c); and V 2p (d).

visible region (400−700 nm). Figure 4b is the Kubelka−Munk plots40,41 of Ag4V2O7 nanotube clusters. According to eq 1:

detected in XPS survey (Figure 3a). Two peaks (Figure 3b) around 368.06 and 374.02 eV were attributed to Ag 3d5/2 and Ag 3d3/2, respectively. The two peaks were further separated as Ag+ and Ag0 by curve fitting. The binding energy at 368.06 and 374.07 eV were ascribed to Ag+, and others at 368.59 and 374.56 eV were assigned to Ag0.34−36 The O 1s XPS spectrum in Figure 3c showed a peak at 530.03 eV, indicating the bond of V−O.37 V 2p5/2 and V 2p3/2 of V5+ in Ag4V2O7 nanotube clusters were observed at 516.83 and 524.58 eV (Figure 3d).38,39 The XPS also showed the elementary composition of Ag4V2O7 nanotube clusters (Table 1). As can be seen from the UV−vis absorption spectrum (Figure 4a), Ag4V2O7 nanotube clusters had an obvious absorption covering the ultraviolet region (300−400 nm) and

αhυ = A(hυ − Eg)n/2

in which α, ν, A, and Eg are absorption coefficient, light frequency, proportionality constant, and band gap, respectively. The band gap energy of the Ag4V2O7 nanotube clusters was about 2.71 eV, which was consistent with the literature reports. The FT-IR spectrum in Figure 4c showed the characteristic absorption peaks of Ag4V2O7 nanotube clusters. The peak at 3429 cm−1 was due to the O−H stretching vibration of intermolecular hydrogen bonding.42 Peak at 1650 cm−1 was assigned to flexural vibrations of the OH in free water.43 The peak around 1383 cm−1 could be owning to the nitrate radical residue from the reactant. The peak at 853 cm−1 was assigned as the O−V−O stretching vibration. The symmetric and asymmetric stretching vibration peaks of the VO band were assigned to 798 and 520 cm−1.44,45 The TG and DTA curve of Ag4V2O7 nanotube clusters is shown in Figure 4d. The TG curve exhibited a major weight loss of 22 wt %, which could be divided into three sectors. The first sector weight loss below 180 °C was due to the volatilization of free water, which was

Table 1. Elements (Atomic %) of the Samples Determined by XPS element

Ag

O

V

contents (%)

39.02

44.72

16.26

(1)

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Figure 4. UV−vis spectrum (a), Kubelka−Munk plot (b), FT-IR spectrum (c), and TG-DTA curves (d) of Ag4V2O7 nanotube clusters.

about 9 wt %.46 The weight loss between 200 to 320 °C might be attributed to the removal of physically and chemically adsorbed water, which was about 7 wt %. The last main weight loss stage of 7 wt % was discovered from 320 to 420 °C, which was attributed to the removal of O2.47 Electrochemical impedance spectroscopy (EIS) was performed to scrutinize the interface resistance between the electrode and electrolyte on a frequency range from 0.1 Hz to 100 kHz.48,49 Figure 5 displays the EIS Nyquist plots of Figure 6. SEM images of Ag4V2O7 crystals prepared by the direct precipitation method (a) and template method (b).

of the eggshell membrane on the crystal growth. As a biologic semipermeable membrane, the eggshell membrane has a special channel structure,50,51 which could control the flow of Ag+ effectively. The prior transferred Ag+ ions through the eggshell membrane formed a nucleation center in the vanadate solution, which played an important role in the crystal growth. To further confirm the effect of the eggshell membrane on crystal growth, the Brunauer−Emmett−Teller (BET) measurement was investigated (Figure 7). The BET surface area of Ag4V2O7 crystals prepared by the direct precipitation method and template method was 0.10 m2/g and 11.95 m2/g, respectively. The adsorbed isotherm of Ag4V2O7 crystals prepared by direct precipitation was approximately linear. It was obvious that the adsorption property of Ag4V2O7 crystals prepared by the template method was typical type IV, which showed a flat curve in low relative pressure, and the isotherms rose rapidly in high relative pressure due to the capillary condensation. No apparent pore-size distribution data of Ag4V2O7 crystals prepared by direct precipitation method were detected (Figure 7b).52 However, Ag4V2O7 prepared by the template method showed a uniform pore structure. The results indicated that the eggshell membrane played an important role in the morphology of Ag4V2O7 crystals. PAM and PF-68 were added into the reaction system to analyze the effect of AM on the fabrication of Ag4V2O7 nanotube clusters. It was interesting that all the samples showed hierarchical globular structures but with different fine structures (Figure 8). By the amplified image, AVOA displayed

Figure 5. Nyquist plots of Ag4V2O7 nanotube clusters.

Ag4V2O7 nanotube clusters. The interface resistance of the working electrode can be informed by the radius of the arc of the EIS Nyquist plots. The smaller arc radius implies the lower charge transfer resistance and the higher charge transfer efficiency. The resistance Ag4V2O7 nanotube clusters was about 306.6 ohm deduced by the radius of the arc. 3.2. Formation Mechanism. Effect of Assisted Template. The Ag4V2O7 sample prepared by the direct precipitation method and template method are shown in Figure 6. All the products were aged for 24 h to exhibit the integrity of the crystal structure. Ag4V2O7 crystals prepared by the direct precipitation method were microsized bulk or schistose as shown in Figure 6a, while Ag4V2O7 crystals prepared by the template method presented a sphere with a fine structure (Figure 6b). The distinct morphologies showed the importance 4257



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Figure 7. Nitrogen sorption isotherm (a) and pore-size distribution of AVOD and AVOA.

The N2 adsorption−desorption isotherms and Barrett− Joyner−Halenda (BJH) pore-size distributions of Ag4V2O7 crystals with AM, PAM and PF-68 are shown in Figure 9. It was obvious that the isotherms of all samples belong to typical type IV (Figure 9a), while there were some differences among adsorption isotherms and pore-size distribution when adding different assisted template agents. From Figure 9a, the isotherms of Ag4V2O7 crystals with AM showed a favorable adsorption−desorption capacity, while the adsorption capacity of Ag4V2O7 crystals prepared with PAM and PF-68 gradually declined, which showed a lower adsorbed volume. Figure 9b shows the pore-size distribution plots.55 A marked porous structure turned up in Ag4V2O7 crystals with AM. It was noteworthy that the mesoporosity was mainly thanks to the intraparticle pores instead of the interparticle pores according to the results of TEM and SEM. Ag4V2O7 with PAM and PF-68 presented no obvious pore structure, indicating the large particle size and the imporosity. At the same time, the BET surface area of Ag4V2O7 crystals with AM, PAM, and PF-68 was 11.95, 1.57, and 1.05 m2/g, respectively. The results showed that Ag4V 2O7 nanotube clusters presented outstanding adsorption capacity and pore structure on account of the loose assemble of nanotubes. The influences of AM content on the morphology of Ag4V2O7 crystals are presented in Figure 10. It was intuitive that the amount of AM had a big effect on the formation of Ag4V2O7 nanotube clusters. Figure 10a is the image of Ag4V2O7 crystals with 0.01 g of AM, in which the basic structure of Ag4V2O7 crystals presented chaotic and thin silks with an average diameter of 30 nm. When increasing the AM to 0.05 g (Figure 10b), the crystal grew to strips with a width of 70 nm and thickness of 5 nm on average. As 0.1 g of AM was added into the reaction system, an obvious tube structure appeared (Figure 10c). No substantially morphology changes occurred when AM was increased, which is verified in Figure 10d. This result illustrated that the amount of AM played a fatal role in

Figure 8. SEM images of AVOA (a); AVOP (b); and AVOF (c).

a fine structure of nanotube with an average diameter of 18 nm. Different from the tubular structure, Ag4V2O7 with PAM in Figure 8b was composed of abundant nets with a well macroporous structure. Ag4V2O7 with PF-68 was constitutive of a great deal of hexagonal nanopetals (Figure 8c). Besides, Ag4V2O7 crystals with AM and PAM were loose structures, but Ag4V2O7 crystals with PF-68 were dense in the structure. The differences illustrated the decisive role of AM in the formation of nanotube cluster structure. As a polymeric monomer, the AM polymerization reaction would be initiated under appropriate conditions. When the Ag4V2O7 nucleus formed, some thermal power was released because the nucleation is an exothermic reaction. Then PAM began to be generated with a great number of polymerization heat released, which promoted the nucleation of new Ag4V2O7 crystals.53 Meanwhile, the formed PAM provided a structure-directing effect to promote the formation of nanotubes.54

Figure 9. Nitrogen sorption isotherm (a) and pore-size distribution (b) of AVOA, AVOP, and AVOF. 4258



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Figure 12. SEM images of Ag4V2O7 crystals under 30 ◦C (a); 50 ◦C (b); 70 ◦C (c).

Figure 10. SEM images of Ag4V2O7 crystals with different amounts of AM 0.01 g (a); 0.05 g (b); 0.1 g (c); and 0.2 g (d).

presented as a big hollow sphere with some cracks on it (Figure 12b), and the sphere diameter was about 4 μm. As in Figure 12c, despite no obvious morphology change taking place in Ag4V2O7 crystals as the temperature rose to 70 ◦C, there was generous protuberance on the surface of the spherical shell. The protuberance might due to the dissolution of Ag4V2O7 crystals.56,57 It could be seen that the growth temperature played an important role in the morphology of Ag4V2O7. When temperature reached to a very high point, the energy provided in the system would be enough to overcome the potential barriers, and then the crystal nucleus was quickly generated, and the crystal growth rate was also accelerated gradually. In this case, a bulky crystal would be obtained rather than a nanotube. Effect of Ethylenediamine. Figure 13 is SEM images of Ag4V2O7 crystals with different alkalinity regulators. When the

the generation of nanotubes. Only when AM increased to a certain content, the chain length of PAM and energy would be enough to form nanotubes. Figure 11 were the adsorption isotherms and pore-size distribution of Ag4V2O7 crystals with 0.01 g, 0.05 g, 0.1 g, and 0.2 g AM, respectively. All the curves were typical type IV isotherm, while the adsorption capacity and pore-size distribution were disparate due to the generation of nanotube. The BET surface area of Ag4V2O7 crystals with 0.01 g, 0.05 g, 0.1 g, and 0.2 g AM were 4.69, 4.25, 11.95, and 19.96 m2/g, respectively. Correspondingly, the adsorbed volumes of Ag4V2O7 crystals with 0.01 g of AM and 0.05 g of AM were comparatively smaller than that of Ag4V2O7 crystals with 0.1 g of AM and 0.2 g of AM (Figure 11a). Some conspicuous disparities on the pore-size distribution of Ag4V2O7 crystals appeared in Figure 11b. The pore-size distributions of Ag4V2O7 crystals with 0.01 g of AM and 0.05 g of AM were broad with a low peak intensity, indicating that the pore was tiny and unsystematic. When AM was continuously increased, the poresize distributions curves became sharp and dense. The average pore sizes of Ag4V2O7 crystals with 0.1 and 0.2 g AM were about 18 and 20 nm. This meant that regular pore structure was formed along with the increase of AM. Effect of the Reaction Degree. It was familiar that the nucleation rate and the crystal growth rate were concerned with the reaction temperature. At room temperature, Ag4V2O7 crystals were nanotube clusters composed of hollow tubes (Figure 12a). By increasing the temperature, Ag4V2O7 crystals

Figure 13. SEM images of Ag4V2O7 crystals with alkalinity regulators of ethylenediamine (a) and sodium hydroxide (b).

Figure 11. Nitrogen sorption isotherm (a) and pore-size distribution (b) of Ag4V2O7 crystals with 0.01 g; 0.05 g; 0.1 g; and 0.2 g of AM. 4259



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Extrapolation has been adopted in the determination of Δt to reduce the system error and get accurate data. Figure 16 was

vanadate solution was adjusted with dilute ethylenediamine aqueous which belongs to organic base, Ag4V2O7 crystals showed nanotube clusters (Figure 13a), while sample in Figure 13b which was produced with inorganic sodium hydroxide as alkalinity regulators presented as flower-like clusters composed of nanoparticles with the size of 60 nm on average. The differences indicated that ethylenediamine was important for the formation of Ag4V2O7 nanotube clusters.58,59 The effect of alkalinity on the generation of Ag4V2O7 crystals is shown in Figure 14. Ag4V2O7 crystals presented as nanotube

Figure 16. Extrapolation method for the determination of Δt.

the t−τ curve in the electrical heating process. The slope of AB and CD demonstrated the temperature variation rate on account of the stirring heat exchange and radiating heat exchange. tB and tC are the initial and outlet temperatures when the current was switch on and switch off. However, the two temperatures could not be applied directly for the existence of system error, so Δt needed to be revised between tB and tC by the extrapolation. The average temperature (tE) of tB and tC was calculated and labeled on the curve as point E. Extrapolating the straight lines AB and CD, and drawing a vertical line through E intersected at G and H, corresponded to the temperature of tG and tH. As shown, the values of tB and tC ware 12.07 and 15.67 °C, respectively. The average temperature (tE) was 13.87 °C calculated by the formula

Figure 14. SEM images of Ag4V2O7 crystals under pH = 9 (a) and pH = 12 (b).

clusters when pH = 9 (Figure 14a). However, a higher alkalinity caused serious agglomeration of Ag4V2O7 crystals (Figure 14b). By this reaction we made a judgment that extreme alkalinity would destroy the structure of the eggshell membrane, which not only influenced but restricted the Ag+ transport. Therefore, the Ag4V2O7 crystals formed serious agglomerate blocks in extreme alkalinity. Effect of the Reaction Heat. The heat of chemical reaction in the system was calculated based on Hess’s Law.60,61 First, the reaction process was designed in two steps as shown in Figure 15. The total constant pressure enthalpy ΔrH was equal to the

t = (t1 + t 2)/2

The temperatures of tG and tH were inferred as 12.17 and 15.70 °C through extrapolation. The set heating voltage, current intensity, and electrifying time (U, I, and τ) were 0.59 A, 6.90 V, and 600 s. K was calculated as −691.95 J/K according to eq 5.62 In consideration of the requirements and the detection limit of the test system, the reaction solutions volumes and the acrylamide concentration were increased 2 times. A 60 mL vanadate solution was put in the test system first, and a certain amount of acrylamide was added, mixed, and dissolved. Then the test system was opened and run for 600 s to stabilize the system. Then a 60 mL silver nitrate solution was poured into the system rapidly after 600 s when the baseline leveled off. Temperature variation after 600 s was the energy change of the reaction system. The temperature in the whole was recorded in the computer. Energy change in the reaction process was present through the temperature variation. Figure 17 is the t−τ curves of pure Ag4V2O7 system and pure AM solution. The Δt of the pure Ag4V2O7 system was about 0.10 °C, which meant that there was 69.20 J heat released as Ag4V2O7 crystals generated.63,64 Pure AM solution showed no heat variation, indicating that pure AM system could not induce the polymerization of AM. The effect of AM on the generation of nanotube cluster was also investigated by the heat released in the reaction process. t−τ curves of Ag4V2O7 crystals with different amounts of AM were present in Figure 18. It was clear that Δt was tiny in the reaction system with 0.01 g of AM, while the temperature showed a marked rise as AM grew to 0.05 g. As the content of AM continued to increase to 0.1 g, the temperature increment increased more clearly compared with the former. Values of Δt

Figure 15. Decomposition view of heat of chemical reaction.

sum of ΔH1 and ΔH2 in the decomposed process. The equation could be expressed as ΔH = ΔH1 + ΔH2

(2)

Since the calorimeter is an adiabatic system, ΔrH1 = QP = 0, so the constant pressure enthalpy ΔrH in t1 Celsius degree is Δr H = Δr H2 = K (t1 − t 2) = −K (t 2 − t1)

(3)

K is the total thermal capacity of the system including calorimeter and solution. (t2 − t1) is the change of calorimeter temperature Δt. The value of K can be evaluated by the electric heating method: the temperature increases by Δt when the heating wire provides a certain amount of heat (Q) by electrical heating in the same system above. When heating voltage, current intensity, and electrifying time are U, I, and τ, respectively, the heat can be evaluated by

K Δt = IUτ

(5)

(4) 4260



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When the time was extended to 3 h (Figure 19c), the sphere did not grow further, while the prototype of nanotube appeared, which was attributed to the epitaxial growth of nanoparticles on the surface of Ag4V2O7 sphere. At the time of 4 h in Figure 19d, the Ag4V2O7 crystal became a nanotube cluster structure constituted with a nanotube in an intertwined style. The morphology hardly changed as the reaction time was prolonged, but the size increased further. Finally, the clusters remained stable with a diameter of around 8 μm. In conclusion, the generation of Ag4V2O7 nanotube clusters was contributed to the combined effect of the eggshell membrane, AM, ethylenediamine, and temperature. The release of Ag+ was controlled through the semipermeable property of the eggshell membrane, which controlled the nucleation and crystal growth of Ag4V2O7. PAM was formed by the polymerization of AM, which was initiated by the heat released during the nucleation of Ag4V2O7. Soon after, the thermal perturbation effect AM polymerization heat further perturbed the new nucleation of Ag4V2O7 and induced the epitaxial growth of Ag4 V 2O 7 crystals. Moreover, the generated polyacrylamide chains could act as a soft template to promote the formation of a tubular structure. 3.4. Formation Mechanism. The feasible growth schematic is illustrated in Figure 20. The growth of Ag4V2O7 nanotube clusters could be given combined with the synergistic influence of the eggshell membrane and acrylamide (AM) assisted templates; the eggshell and the AM both played a decisive role in the nanotube cluster structure. In addition, the temperature, alkalinity regulators, and the pH all had their influence on the nucleation. In the primary stage, Ag+ diffused across the eggshell membrane and rapidly reacted with V2O74−, and the Ag4V2O7 nucleated with some heat released. Then the AM was thermally initiated by the heat released during the nucleation of Ag4V2O7, forming polyacrylamide with low molecule weight. Subsequently, a regular sphere formed consisted of plenty of Ag4V2O7 nanoparticles due to the specific surface tension. Later, the heat liberated in the polymerization process perturbed the new nucleation of Ag4V2O7 due to the downgraded potential barriers of the critical nucleus, and the nanoparticles on the surface of Ag4V2O7 sphere epitaxial grew. As the polymerization degree of AM increased, long chains of PAM were generated and functioned as soft templates for the growth of nanotubes. With an increase of aging time, Ag4V2O7 crystals grew to long nanotubes and circularly formed the nanotube clusters. According to the growth stages, the nanotubes were closed at

Figure 17. t−τ curves of pure Ag4V2O7 system and pure AM solution.

Figure 18. t−τ curves of Ag4V2O7 with different amount of AM 0.01, 0.05, and 0.1 g.

in the system with 0.01, 0.05, and 0.1 g AM in 30 min were about 0.28, 0.99, and 1.21 °C, respectively, and the homologous heat of chemical reaction was calculated as 193.75, 685.03, and 837.26 J through eq 3, which would also represent the polymerization of AM. We could infer from the consequences that the heat of chemical reaction and AM content positively correlated with Ag4V2O7 crystal growth. 3.3. Growth Stages. The morphology evolution of Ag4V2O7 nanotube clusters was studied by SEM tests under different aging times. During the initial stage, Ag+ infiltrated into the ammonium vanadate solution through the eggshell membrane. Ag+ and V2O74− interacted, forming amorphous crystal nuclei. As in Figure 19a, the morphology of Ag4V2O7 crystals presented an anomalous bulk with plenty of nanoparticles with an average diameter of 21 nm. When increasing the aging time to 2 h, Ag4V2O7 crystals grew to a regular sphere (Figure 19b) composed of nanoparticles, and the average diameter of the sphere was about 2 μm. The regular spheric structure could be attributed to the specific surface tension.

Figure 19. SEM images of Ag4V2O7 under different aging times: (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6, (g) 9, and (h) 12 h. 4261



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Figure 20. Growth schematic of the Ag4V2O7 nanotube clusters.

Figure 21. Degradation curves (a) and −ln(Ct/C0)−t (h) plots (b) for Rh.B degraded by AVOD; AVOA; AVOP; and AVOF.

one end, which could also be testified through the N2 adsorption−desorption measurements. 3.5. Performance. Figure 21 shows the degradation curves and −ln(Ct/C0)−t (h) plots for Rh.B degraded by Ag4V2O7 crystals. Before the degradation, the suspension was stirred for 30 min in the dark to reach adsorption−desorption equilibrium, and about 16.39% of Rh.B was decomposed after 8 h by AVOD. Especially, Ag4V2O7 nanotube clusters showed the best photocatalytic activity among the samples, which has degraded Rh.B for 24.48%. The degradation rates of Rh.B by AVOP and AVOF were 14.92% and 7.83%, respectively. Compared with AVOD, the photocatalytic activity of AVOA was enhanced by 60.4%. The higher photocatalytic activity was due to the larger specific surface area, which could increase the contact area. It was affirmed that the formation of nanotubes promoted the degradation of Rh.B on account of the capillarity of nanotubes. As reported before, the photocatalytic degradation reaction was in accord with first order reaction kinetics, which follow the formula: ln(Ct /C0) = −kt

Table 2. Equation of Linear Regression and Linear Parameter of −ln(Ct/C0) versus time (h) equation y = a + bx sample

AVOD

AVOA

AVOP

AVOP

Pearson’s r adj R-square intercept slope

0.9881 0.96846 −0.01168 0.02164

0.98917 0.97127 −0.01395 0.03182

0.95535 0.88359 −0.02022 0.01811

0.96476 0.90768 −0.00352 0.00948

The −ln(Ct/C0)−t plots verified that the degradation of Rh.B conformed with first order reaction kinetics and AVOA gave the maximal slope, implying the highest photocatalytic efficiency.

4. CONCLUSIONS In summary, the Ag4V2O7 nanotube clusters with high photocatalytic activity and electric property were successfully prepared by the dynamic template method. A large number of embryos of Ag4V2O7 were created with the diffusive release of Ag+ through the eggshell membrane. With the polymerization of AM, the released polymerization heat perturbed the new nucleation of Ag4V2O7 and urged the epitaxial growth of Ag4V2O7 crystal. Furthermore, the PAM chains which functioned as soft templates promoted the generation of nanotubes. The investigation of the formation mechanism of Ag4V2O7 nanotube clusters indicated that the controllable growth provided an effective and advisable procedure to prepare nanotube materials.

(6)

where C0 is the initial concentration of Rh.B, Ct is the real concentration of Rh.B on the response time t, and k is the observed rate constant for the degradation reaction. The ln(Ct/ C0)−t (h) plots of Rh.B solutions degraded by AVOD, AVOA, AVOP, and AVOF are constructed in Figure 21b to verify the reaction kinetics. Linear-fittings of ln(Ct/C0)−t (h) were carried out to elucidate the reaction kinetics. The coefficients of linear regression equation were calculated and expressed in Table 2. The adjusted R square (R2) of AVOD, AVOA, AVOP, and AVOF were 0.97, 0.97, 0.88, 0.91, respectively, illustrating the significant linear correlation between −ln(Ct/C0) and t (h). 4262



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jin-Ku Liu: 0000-0001-9580-8309 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 21341007), Fundamental Research Funds for the Central Universities (Grant 222201717003), and Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (Grant SKL201605SIC).

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