Development of Gold Phosphorus Supported Carbon Nanocomposites

Abstract

Metal-containing carbon nanocomposites have shown significance promise in the area of energy storage, heterogeneous catalysis and material science because of their morphology and combined properties. Phosphorus-doped carbon nanocomposites with gold nanoparticles were developed by applying a simple impregnation method and metal deposition technique. Gold-phosphorus supported carbon nanocomposites with two sized (25 and 170 nm) were prepared from economical petroleum pitch residue as the carbon source using an advanced silica template method. These nanocomposites will lead to the novel applications in the field of material science with the combined properties of gold, phosphorus and carbon. The newly prepared gold phosphorus supported carbon nanocomposites were fully characterized using a range of different physico-chemical techniques.

Introduction

Porous carbonaceous materials,1 metal oxides (TiO2, Al2O3, ZrO2, MgO),2 zeolites and mesoporous siliceous materials (SBA–15, MCF)3 have been recommended as solid support materials by scientists all over the world. Carbon and silica based materials are the most widely used supports for hybrid organic–inorganic composite materials. These hybrid composites have driven significant interest for range of applications including drug delivery, biosensors, energy conservation, methane oxidation, heterogeneous catalysis and chemical synthesis.2b,4 Over the last two decades, porous carbon materials have been studied widely because of their outstanding mechanical properties, electrical conductivity and thermal stability. These characteristic properties of porous carbon materials have prompted fundamental studies into areas, such as band structure and chirality.5 These materials are considered to be advanced contenders for a wide range of scientific applications, such as field emission display sources, energy storage, semiconductor probes and interconnects, and conversion devices.6 In addition, carbonaceous materials have attracted significant attention as a potential adsorbent and heterogeneous catalyst. The advantages of porous carbon composites for gas storage are the rapid adsorption–desorption mechanism and extremely lightweight material. Hybrid carbon composites have recently been developed by focusing on nano–technology with modern doping tool. The doping effect imparts not only the mechanical properties of carbon composites but also enhance their applications in the semiconductor, optical, magnetic and biomedical industries.7 These hybrid metal nanocomposites of porous carbon have revealed strong research interest due to the individual properties of metal and carbon materials and the materialization with new added properties capable of endeavoring innovative applications.8

Recently, improvisation and controlling the properties of the porous carbon matrix, through a new functionalization process, have been studied successfully. Doping the carbon framework with a foreign element has shown great promises for the preparation of hybrid carbon composites because the addition of organic/inorganic elements can transform the characteristic (electrical, physical and chemical) properties of porous carbon.9 Boron (B) and nitrogen (N), which are the neighbors of carbon in the periodic table, have been used in the porous carbon matrix using a doping technique. N-doped carbon composite, such as carbon nanotube cups,10a carbon nanotube carpets,10b vertically aligned carbon nanotubes, 10c ordered mesoporous graphitic arrays,10d nanoshell carbons10e and graphene sheets10f exhibited high electrocatalytic activity for the oxygen reduction reaction in acid or alkaline fuel cells because of the unique electronic conjugation between the nitrogen lone–pair of electrons and graphene π system.11a To design a novel foreign atom–loaded carbon composites material, phosphorous (P), a N–group element, was recently used as a dopant atom to alter the structural properties of porous carbon with similar chemical properties to nitrogen.11b

Uniformly, gold nanoparticles with sizes in the range of nanometers have shown considerable growth in diverse fields of sciences (physics, chemistry, material science, medicine, and biology) because of their unique magnetic, optical, mechanical, electronic, physical and chemical properties. Gold nanoparticles have novel properties in areas of science and are one of the most studied nanoparticles in industry and academia. A gold inserted carbon matrix exhibits not only the original properties of gold and carbon but also the combined properties of both materials as well as the emergence of novel properties for new applications. These nanocomposites provide high thermal and mechanical potency, sufficient surface area, carbon resistance to acid/base and the optical, electrical and catalytic properties of gold.12

In continuous interest for the preparation of hybrid composite materials, we have demonstrated range of metal (copper, nickel, potassium, manganese, gold, molybdenum and vanadium) supported carbon nanocomposites for energy storage and heterogeneous catalysis.1b,8b,13 In this mode of operation, ongoing research has focused on the development of advanced hybrid gold–phosphorus coupled carbon nanocomposites. Among the several methods available, the engineering and construction of hybrid metal–carbon nanoreactors with two sized (25 and 170 nm) was achieved using a simple silica template method and metal deposition technique with low-cost pyrolysis oil (PFO) based pitch with advanced aromatic content from petroleum residue. Petroleum pitch is considered as a cheap and suitable flexible material for designing a variety of porous carbon matrices owning to its individual structural property and extremely rich aromatic content.14 These hybrid nanocomposites can provide an interesting new class of support materials for scientific advancement.

 

Experimental

Reagents, Standards and Samples. Reagents such as ethanol, toluene, ammonium hydroxide, phosphoric acid (85%), sodium citrate dehydrate, (Dae-Jung Chemicals & Metals Co. Ltd., Korea), tetraethylorthosilicate (TEOS) (Aldrich, USA), hydrofluoric acid (J. T. Baker, USA) and hydrogen tetrachloroaurate (III) hydrate (HAuCl4) (Kojima Chemicals Co. Ltd, Japan) were used as received. Pyrolysis fuel oil (PFO) was obtained from Yeocheon Naphtha Cracking Center (YNCC) Korea. All the solvents were purified using known method.15

Characterization and Measurements. Hybrid metal nanocomposites of phosphorus and gold with two sized (25 and 170 nm) were characterized by powder X–ray diffraction (PXRD, Phillips X’pert MPD diffractometer, Almelo, The Netherlands) over the 2θ range (1–10 and 10–80) at scan step 0.02°. Fourier transform infrared (FT-IR, Perkin– Elmer Spectrometer, Massachusetts, USA) spectroscopy was carried out using a KBr self–supported pellet technique. Microanalysis of the products was determined out using a CHN analyzer (CE instruments, UK) and the metals entering into the carbon cage were determined by inductively coupled plasma–optical emission spectroscopy (ICP–OES, JY Ultima 2CHR). The BET surface area was completed by N2–sorption data measured at 77 K using a volumetric adsorption set up (Micromeritics ASAP-2010, USA). The pore diameter of the samples was determined from the desorption branch of the nitrogen adsorption isotherm employing the Barret–Joyner–Halenda (BJH) model. The thermal measurements and microstructural evaluation of these samples were examined by thermo–gravimetric analysis (TGA, SDT600, TA instrument, USA) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM–EDS, LEO–1430, VP, UK) and transmission electron microscopy (TEM, JEM 2011, Jeol Corporation, Japan).

Figure 1.Schematic illustration for the synthesis of NSB, CC, CGN, CPN and CGPN.

Preparation of NSB-25/170, CC-25/170 and CGN-25/170. Metal incorporated carbon nanocomposites and its precursors were synthesized as shown in Figure 1. The synthesis procedure for the two–sized nano silica ball (NSB-25/ 170), carbon cage (CC-25/170) and carbon gold nanocomposite (CGN-25/170) are explained in the supporting information.13b

Synthesis of Carbon Phosphorus Nanocomposite (CPN-25/170). Typically, 3 g of CC-25/170 was mixed with 10 mL of distilled water in a beaker followed by the addition of 0.4 mL of the H3PO4 solution (85%). The resulting suspension ultrasonicated for 10 min. This process was followed by the evaporation of excess water and drying in oven at 140 °C for 12 h. The material was activated at 800 °C for 30 min in a tubular furnace in the presence of argon gas. After activation, the phosphorus doped carbon nano composite was cooled to room temperature in flowing argon. To remove the excess H3PO4, the samples were washed extensively with hot water in a Soxhlet extractor until the pH of the washing became neutral. The sample was then dried in an oven at 110 °C to give a black solid carbon phosphorus nanocomposite (CPN-25/170) with a high carbon content. (CPN-25); Yield: 3.3 g, BET surface area of CPN-25: 70 m2/g and FTIR (KBr) 1084, 1206, 1580, 3434 cm-1. Elemental analysis and ICP showed that CPN-25 contained > 72 wt % carbon and > 0.9 wt % phosphorus (P) after activation at 800 °C. (CPN-170); Yield: 3.4 g, BET surface area of CPN-170: 178 m2/g and FTIR (KBr) 1229, 1586, 3498 cm−1. Elemental analysis and ICP results showed that CPN-170 contained > 82 wt % carbon and > 1.5 wt % phosphorus (P) after activation at 800 °C.

Synthesis of the Carbon Gold Phosphorus Nanocomposites (CGPN-25/170). In a mixture of distilled water (10 mL) and 3 g of CGN-25/170, 0.4 mL of H3PO4 solution (85%) was added and the resulting mixture was sonicated for 10 min. The process was followed by the evaporation of excess water and drying in an oven at 140 °C for 12 h. The material was activated at 800 °C for 30 min in a tubular furnace through an argon gas purge. The resulting nanocomposite was cooled to room temperature in flowing argon. The excess reactant H3PO4 was removed by extensively water washing through Soxhlet extraction until a neutral pH was reached. The material was dried in oven at 110 °C to obtain a carbon gold phosphorus nanocomposite (CGPN-25/ 170) material. (CGPN-25); Yield: 3.4 g, BET surface area of CGPN-25: 40 m2/g and FTIR (KBr) 1084, 1216, 1579, 3427 cm−1. Elemental analysis and ICP showed that CGPN-25 contained > 71 wt % carbon with > 10.9 wt % gold (Au) and > 0.9 wt % phosphorus (P) after activation at 800 °C. (CGPN-170); Yield: 3.4 g, BET surface area of CGPN-170: 172 m2/g and FTIR (KBr) 1183, 1580, 3486, 3678 cm−1. Elemental analysis and ICP results showed that CGPN-170 contained > 75 wt % carbon with > 4.8 wt % gold (Au) and > 3.3 wt % phosphorus (P) after activation at 800 °C.

 

Results and Discussion

Characterization. The synthesized gold and phosphorus incorporated carbon nanocomposites were fully characterized by microanalysis, N2 adsorption–desorption isotherm, SEM–EDS, TEM, ICP, TGA, PXRD and FTIR (See figures 2 to 17 in supporting information).

Powder X–ray Diffractions (PXRD) Analysis. Wide–angle and low–angle and powder X–ray diffractions (PXRD) was carried out to characterize the all nanocomposites. The low–angle PXRD (1–10° ) measurements did not show any characteristic peaks for the nanocomposites suggesting short range order or disordered phases present in all the nanocomposites prepared (see supporting information).

The wide–angle PXRD profile of NSB-25 showed (Figure 2) a single broad peak assigned at 2θ = 23.3° , corresponding to the diffraction peak of amorphous silica.16a Two additional peaks, with a lower intensity corresponding to the graphite–type reflection from the (002) and (100) planes, were observed at 2ϴ values of 24.9° and 42.9°, respectively, upon switching to CC-25 from NSB-25.16a,16b PXRD of the CGN-25 material showed four additional peaks at 2θ = 38.2°, 44.4°, 64.6° and 77.6°, which corresponded to reflections of the Au planes (111), (200), (220) and (311) respectively, denoting the formation of noble gold particles with a face centered cubic structure.12a,16c The PXRD measurement of CPN-25 did not show any additional characteristic peak except for the graphite–type reflection from the (002) and (100) planes of CC-25. Similarly, CGPN-25 showed a marginally shifted PXRD pattern at 2θ = 25.3°, 39.2°, 45.6°, 66.3° and 79.9°, which corresponded to reflections of the Au plans. All hybrid nanocomposites exhibited broad peaks centered at 2θ value of around 20–30° which is consistent with the typical amorphous nature of the carbon/silica foundation. PXRD of the CGN-25 and CGPN-25 nanocomposites showed strong signals for gold in the carbon foundation.

Figure 2.Wide-angle PXRD patterns of NSB-25 (a), CC-25 (b), CGN-25 (c), CPN-25 (d) and CGPN-25 (e).

Transmission Electron Microscopy (TEM) analysis. A comparison of the high magnification TEM images of the NSB-170 template (Figure 3(a)) and replicated composite carbon materials (CC-170) (Figure 3(b)) showed that the porous structure of the silica template had been replicated well. The consistent hollow cores of the hierarchically porous CC-170 were identical and strongly connected to each other.17 TEM image of CGN-170 and CPN-170 confirmed that the Au and P nanoparticles were dispersed homogeneously and adhered to the carbon cage cores (Figures 3(c) and 3(d)). TEM of CGPN-170 revealed combined Au and P nanoparticles on the interconnected hierarchically wider hollow cores (~170 nm) of the carbon structure (Figures 3(e) and 3(f)).

Figure 3.TEM images of NSB-170 (a), CC-170 (b), CGN-170 (c), CPN-170 (d) and CGPN-170 (e and f).

SEM was coupled with energy dispersive spectroscopy (EDS) was used to assess the purity and elemental composi- tion of the composite materials (Figure 4). EDS of CGN-25/170, CPN-25/170 and CGPN-25/170 revealed strong signals for gold, phosphorus and combined (Au-P) along with a carbon foundation, respectively (Figure 4).

Figure 4.SEM-EDX spectra of CGN-25 (a), CGN-170 (b), CPN-25 (c), CPN-170 (d), CGPN-25 (e) and CGPN-170 (f).

Nitrogen Adsorption-desorption Isotherm Study. Typical isotherms for CPN-25/170 and CGPN-25/170 are shown in Figures 5 and 6, which represent a type IV pore structure and confirms the well arranged mesopores. The BET surface area, BJH pore diameter, total pore volumes found are summarized in Table 1. NSB-25 showed reasonable BET surface area (30 m2/g), total pore volume (0.086 cm3/g) and BJH pore diameter (116 Å). A large increase in the BET surface area was observed (30-82 m2/g) upon the preparation of the carbon cage (CC-25). Consequently, a decrease in the BJH pore diameter from 116 to 58 Å and tiny rise in total pore volume from 0.086 to 0.120 cm3/g were observed. On the other hand, a small decrease in BET surface area of 82 to 75 m2/g, pore volume from 0.120 to 0.108 cm3/g, and pore diameter from 58 to 57 Å was observed upon gold incorporation in the carbon cage. This suggests that the internal pores of the CC-25 are occupied by gold particles (CGN-25). Similarly, a small decrease in BET surface area (from 82 to 70 m2/g), pore volume (from 0.120 to 0.105 cm3/g) and a slight increase in pore diameter (from 58 to 60 Å) were noticed upon phosphorus insertion in the carbon matrix. This indicates that the pores of the carbon matrix (CC-25) in CPN-25 were filled by phosphorus. Moreover, a decrease in BET surface area (from 75 to 40 m2/g), pore volume (from 0.108 to 0.075 cm3/g) and a small increase in pore diameter (from 57 to 76 Å in CGN-25 as a result of phosphorus incorporation were obsereved. This supports the suggestion phenomenon of filling the internal pores of CGN-25 with phosphorus in CGPN-25. The typical isotherms for all nanocomposites NSB, CC, CGN, CPN and CGPN (25 and 170 nm) are presented in the supporting information.

Figure 5.Nitrogen adsorption-desorption isotherms of CPN-25 and CPN-170.

Figure 6.Nitrogen adsorption-desorption isotherms of CGPN-25 and CGPN-170.

Table 1.Physico-chemical data of NSB, CC, CGN, CPN and CGPN (25 and 170 nm)

 

Conclusions

Gold and phosphorus and jointly–loaded, two sized (25 and 170 nm) carbon nanocomposites were prepared by an easy template method and metal impregnation using an inexpensive petroleum pitch residue as the carbon resource. The nanocomposites were examined by PXRD, TEM, SEM-EDX and ICP measurements. The physico-chemical analysis results confirmed the presence of P and Au elements in the porous carbon matrix and the carbon network was preserved intact upon the functionalization immobilization of the foreign atoms. These solid hybrid nanocomposites showed the best stability. Therefore, they can be used and recycled as a heterogeneous material for further catalysis and adsorption studies. Controlling the size of the carbon matrix and insertion of Au and P in CC will improve significantly the chemical and physical properties of the hybrid carbon nanocomposites and will provide new research areas. In particular, Au/P carbon nanocomposites are a new field of research and more research needs to will be needed to entirely understand the features of hybrid nanocomposites and their applications.