Current Nanoscience - Volume 13, Issue 1, 2017

Background: During the last few decades, increasing interest has arisen in the field of nano-machining simulations. Due to the inability of other numerical techniques to properly describe the interactions in atomistic level, Molecular Dynamics method was shown to be capable of producing sufficiently accurate results and represent the mechanisms of nano-cutting. Significant progress has been noted since the first MD studies of nano-machining and it is worthwhile to present modeling techniques and details which have been employed in the literature. Methods: A significant amount of original work in the field on MD nano-machining simulations is reviewed and presented in order to underline the advances in this field during the past few decades. Then, based on the relevant literature, a brief and concise step-by-step methodology of nanomachining processes modeling using the MD method is presented with a view to provide the beginners with a practical guide for MD modeling. For that reason, the components of each modeling step are clearly introduced to the readers and the choice of parameters according to the characteristics of each process is justified. Results: Contemporary and older MD nano-machining studies are reviewed and valuable information about both common and state-of-the-art modeling techniques is gathered. Modeling of MD processes can be structured as step-by-step methodology which leads gradually to the definition of all parameters required for an MD simulation. Methods and modeling details can be useful for the set-up of a nano-machining simulation, design of new machining processes or variants of existing ones. Conclusion: Nano-machining processes are of increasing interest for state-of-the-art industrial and scientific applications. MD method is a reliable means of simulating these processes and obtaining valuable information about the characteristics of nano-machining that are impossible to be observed by experimental work. Modeling with MD method can be viewed as a multistep methodology with each step consisting of definitions of different parts of MD model. Successful use of modeling methodology can lead to efficient and accurate simulation of nano-machining processes, as well as help the design of new ones.

Background: The material removal in ultra precision machining is often at the nanometre scale with stringent form and surface finish accuracy. Currently, it is very difficult to observe nanometric phenomena through experiments, but the MD method has proved helpful in this respect. Many current MD simulation studies on nanometric cutting have been focused on single cutting pass or simple line-type groove. In practice, many machining processes involve the use of multiple passes to create a new surface. In this study, the effect of depth of cut and scribing feed on the simulation of multi-pass cutting, was investigated in the surface creation process, as in single point diamond turning. Methods: The MD method was employed in studying the effects of machining parameters in multipass nanometric machining of copper workpiece with a diamond tool. The copper-copper interactions were modelled by the EAM potential and the copper-diamond interactions were modelled by the Morse potential. The diamond tool was modelled as a deformable body and the Tersoff potential was applied for the carbon-carbon interactions. Results: The simulation results show the material removal mechanisms at the different depths of cut. For the first pass, the diamond tool approaches the workpiece at the onset of the simulation, and as the tool touches the workpiece, there is momentary adhesion of the tool atoms and the workpiece atoms. As this is overcome, the tool moves through the workpiece, by ploughing and cutting, depending on the depth of cut, At the cutting depth of 0.5nm, only ploughing occurs and at 1.5nm and 2.5nm, chip formation takes place. Conclusion: It has been observed that the average tangential and normal components of the cutting forces increase with increase in depth of cut and they reduced in consecutive cutting passes for each depth of cut. The ratios of the tangential to the normal force components decrease as the depth of cut increases, but remain constant after the depth of cut 1.5nm. The magnitudes of the cutting forces decrease from pass 1 to pass 2, but they are identical for both pass 2 and pass 3. The least resistance to cutting was observed at 2.0nm, which may indicate the existence of a critical depth of cut for tool wear reduction. With the variation of the cross feed, after the first pass, the average tangential and normal components of the cutting forces increase with increase in the feed. Also, there is always an increase in friction from pass 1 to pass 2. When carrying out multipass processes, the arrangement should be effected with minimum overlap in the runs, for efficient machining.

Background: The atomic surface roughness is very important in assessing the quality of high performance nano surfaces, in ultra-precision machining, and in silicon fabrication. The surface roughness of nano devices will invariably affect their quality and performance, so its understanding is very crucial. In this study, multi-pass nanometric atomistic simulations were employed in the characterization of this parameter to gain better insight. Methods: Multipass MD simulations were carried out to create nano surfaces by the nanomachining of copper workpiece with a diamond tool. The effects of machining parameters during the nanometric phenomena were investigated. The copper-copper interactions were modelled by the Embedded Atom Method (EAM) potential and the copper-diamond interactions were modelled by the Lennard- Jones (LJ) potential. The diamond tool was modelled as a deformable body and the Tersoff potential was applied for the carbon-carbon interactions. Results: The simulation results show two peaks and three valleys for all the depth of cut and the cutting velocity range used. These waviness displayed by the surface atoms contributing to the roughness, are due to the overlap caused by the consecutive passes, the tool geometry and the choice of the interatomic potentials used in the simulations. Conclusion: There is a noticeable variation of atomic surface roughness, Sa with machined thickness and machining velocity. The estimated Sa from the study, is in the range 0.189-0.345nm. The Sa increases initially up to 1.5nm depth of cut, then it decreases before showing another upturn. It seems there is no logical relationship between the depth of cut and the surface roughness. Furthermore, Sa increases and decreases for a certain range, as velocity increases. In conventional machining, the Sa should improve as the velocity increases. However, on the nanoscale, the parameters are very sensitive to small variations. This variation may either be due to size effects of the simulation model or some other factors. Also, the importance of adhesion is highlighted in the investigation of friction between the tool atoms and interacting workpiece atoms, as friction is shown to decrease with increase in the depth of cut. The importance of this study of atomic surface roughness and its clearer understanding can be useful in the assessment of atomically flat silicon.

Background: The so-called orthogonal cutting process constitutes a well-defined and transparent example of machining. Modern applications require an understanding of the defect formation processes occurring in nano-manufacturing. Methods: Molecular-dynamics simulation is employed to study the cutting of metals by a rigid tool made of C atoms. Fe, Al and Ti are selected as examples of bcc, fcc and hcp metals. We choose single- crystalline workpieces oriented such that multiple slip systems are activated. Atomistic analysis tools provide information on the crystal-plasticity effects induced by the cutting process. Results: The quantitative evolution of the forces needed to cut through the materials is similar for the three metals studied. However, the chip form and the roughness of the cut surface are quite distinct in the three metals, and reflect the different plastic processes occurring inside the material. Conclusion: The form of the cut depends on the details of the plastic processes proceeding in the interior. The form of the chip itself is strongly influenced by the constant arrival and absorption of dislocations at its surface; similarly the roughness of the cut surface stems from the emission of dislocations from the cutting edge.

Background: Unlike the traditional crystalline metals, metallic glasses are lack of longrange order and have short-range order. Metallic glasses as amorphous alloys have excellent physical, chemical and mechanical properties, and have a broad application prospects in military, aerospace and sports equipment due to their unique microstructure. Currently, machining has been considered as a promising method to obtain MGs components with low surface roughness and high dimensional accuracy. In addition, it was found that the normal cutting force is substantially equal to the tangential forces in nanometric cutting of MGs by molecular dynamics simulation, which is different from that of crystal alloys. Therefore, the objectives of this paper are to investigate the effect of machining parameters on cutting forces in nanometric cutting process and analyze the microstructure evolution of the metallic glass workpiece. Methods: The radial distribution functions were calculated to verify the amorphous state of workpiece. The microstructure of workpiece was used to analyze by the common neighbor analysis. In addition, the calculation of cutting forces was adapted to truncation radius method during all the simulations. Results: There are no obvious changes of lattice structure in nanometric cutting of Cu50Zr50 MGs. The cutting force increases with the increase of cutting speed and depth and then keeps on a steady value in stable cutting process. Furthermore, the normal cutting force is substantially equal to the main cutting force during all nanometric cutting simulations of MGs and larger elastic recovery was founded on the machined surface. Conclusion: Nanometric cutting at room temperature does not change the microstructures of Cu50Zr50MGs. There is no strain hardening in machining of MGs. Small elastic modulus will cause large elastic recovery on the machined surface. Therefore the normal cutting force is nearly equal to the main cutting force in nanometric cutting of MGs.

Background: The widespread application of nanotechnology in the last decades, the increasing likelihood of human exposure to nano-sized materials, together with the still limited knowledge concerning their toxicological profile, require a careful risk assessment, particularly in occupational settings. However, a specific “risk assessment paradigm” for these peculiar xenobiotics has not yet been defined. Objective: The aim of this review was to address those critical aspects that currently prevent the achievement of a suitable risk evaluation in order to point out priorities of research helpful to develop and implement an effective guidance for nano-risk assessment. Method: Literature search concerning NM physico-chemical characterization, toxicological behavior and exposure assessment strategies was analyzed to extrapolate opportunities, challenges and criticisms in the application of the general chemical risk assessment steps to the nano-sized toxicological field. Results: Uncertainties on the role of the physico-chemical properties in nanomaterial toxicity, the complexity in extrapolating dose-response relationships, and practical difficulties in measuring nanomaterial exposure emerged as challenging issues for the application of a traditional risk assessment approach to nano-sized exposures. Conclusion: Future investigations on these topics appear necessary to define an effective, nanofocused risk evaluation strategy that should be dynamically improved and verified as more substantial information become available. Such a suitable risk assessment process should provide adequate estimates of nanomaterial risks to guide the adoption of appropriate risk communication and management strategies for the protection and the safety of the workers.

Background: The DNA tetraplex (TE) nanomotor is a new nanomotor family constructed through self-assembly of guanine-rich (G-rich) single strand DNA and produces controlled motion at the molecular scale. Objective and Method: In this study, the TE conformation stability of single strand 15-mer G-rich DNA, GGTTGGTGTGGTTGG, was studied in atomic scale using molecular dynamics simulation method for the aim of potential application in empowering nanosystems/nanoswitchers. Results and Conclusion: The study of dynamic behavior of TE conformation indicated that the 15- mer G-rich DNA is stabilized by eight Hoogsteen hydrogen bonds between NH…O and NH…N groups resulted in higher-order G-quadruplex conformation as an integral part of TE structure. Moreover, the higher-order G-quadruplex produces steric hindrance in the ion-free state, disorganizing rearrangement of Hoogsteen H-bonds leads to “wobble TE” conformation. Data showed that in the absence of the coordinated K+, the G bases initially tend to fluctuate and rotate while in the presence of coordinated ion, the system intends to form compact rigid TE. We propose the single strand 15-mer G-rich DNA as a nanomotor with maximum efficiency in attaching coordinated K+ ion by possessing compact TE formation.

Background: Until now various structures of photoanode are made by incorporating graphene in the photoanode structure and applying them in DSSCs. Literature review indicates that despite the considerable researches which are performed on the application of TiO2/Graphene nanocomposites in DSSCs, there is still no prevailing acceptable explanation for origin of the better performance by graphene. Methods: Here a TiO2 paste with 20-30 nm particle sizes is deposited by doctor blade method. Graphene sheets are synthesized by the chemical methods and their properties are investigated by the Raman spectroscopy and atomic force microscopy techniques. TiO2/Graphene nanocomposite photoanodes are made in four different types by chemical methods. Pt cathode layer is deposited on the FTO coated glass substrates by doctor blade method from a Pt paste. Photoanodes are sensitized by N719 dye. The active area of the cells was 0.25 cm2. Optical absorption of the photoanodes is investigated by Perkin-Elmer spectrophotometer. Morphology of the photoanodes is observed by scanning electron microscopy. Parameters of electron transport in the cells were measured by the electrochemical impedance spectroscopy (EIS). Results: We have shown that in spite of the micron size of graphene sheets, their scattering properties are not considerable in dye sensitized solar cells. It is shown that graphene sheets enhance the electron recombination resistance in the cells noticeably. We have shown that the electron diffusion length is enhanced from 20.32 microns to 35.41 microns after incorporating graphene sheets in the photoanode architectures. In the present study, efficiency of the cells with the TiO2 architecture (5.97 %) was improved to 6.46 % after incorporating graphene sheets in the photoanode architecture. Conclusion: Graphene sheets enhance the efficiency of the cells by improving electron diffusion length in the photoanode structure. As a general conclusion, our results show that in spite of theoretical prediction of considerable electron transfer rate of graphene sheets, this property is not noticeable in DSSCs.

Background: The electrochemical determination of nicotine was investigated at the poly (alizarin red S) modified graphene/screen printed carbon electrode (poly (ARS)-GR/SPCE). The peak value of nicotine at poly (ARS)-GR/SPCE was increased comparing with the unmodified GR/SPCE, suggesting that the disposable GR/SPCE was efficiently modified by poly (ARS) to fabricate the good working area. Characterization of the modified electrode was realized with electrochemical impedance spectroscopy and scanning electron microscopy. Under ideal experimental conditions, differential pulse voltammetry of nicotine showed oxidation at +600 mV (vs. Ag/AgCl) in phosphate buffer solution pH 7.0. Methods: The standard calibration curve was achieved in the nicotine (NIC) concentration range of 30–1000 μM and the detection limit was found to be 4.6 μM at a signal-to-noise ratio of 3. The poly (ARS) modified disposable GR/SPCE can be employed for the direct evaluation of NIC in real electronic cigarette juice and real-world water samples with satisfactory results. Finally, the method was performed to the electrochemical assay of NIC in food samples with unsatisfactory results. Results: A rapid and simple procedure of constructing an effective alternative modified electrode for NIC detection was established by electrochemical polymerization of ARS in PBS. The developed method has a higher electrochemical activity toward the oxidation of nicotine as compared with that for either SPCE or GR modified SPCE. This electrochemical approach uses miniaturized detection system, and portable devices to allow simple, rapid and onsite measurements. The poly(ARS) modified GR/SPCE may serve as a voltammetric sensor for NIC detection and can also be used as an electroanalytical method in the quality control process for other tobacco products and water samples. This approach could be an alternative procedure for the assay of nicotine in the future with its sufficient long term stability, sensitivity and good reproducibility. Conclusion: In recent years, there has been increased interest in the detection of nicotine in several often consumed vegetables and fruits, as well as in some of their processed products. In this study, the detection of NIC in food matrices was examined without solid phase extraction. According to the obtained recoveries (27-34%), the voltammetric detection of nicotine in spiked mixtures are not quantitive. The detection of NIC in vegetables and fruit samples represents a laborious a problem for the analytical researcher because of nicotine’s basic attributes. As almost all vegetables and fruits demonstrate acidic pH. Nicotine is bound to the matrix as a salt in this environment, hence is very difficult to identify directly and under this circumstance, hard extraction is needed.

Background: To realize the high performance of direct methanol fuel cells (DMFCs), controlled construction of well-dispersed Pt-based bimetallic nanoparticles either of free standing or supported on carbon-based materials is highly sought. This work demonstrates the feasibility of reduced graphene oxide (RGO)-supported Pt-Pd bimetallic nanodendrites as electrocatalysts for enhanced methanol oxidation. Methods: RGO-supported Pt-Pd bimetallic nanodendrites were conveniently synthesized by simultaneously reducing chloroplatinic acid and potassium tetrachloro palladate with ascorbic acid on RGO supports. X-ray diffraction and transmission electron microscopy were utilized for the physical characterization of Pt-Pd/RGO nanodendrites. Electrocatalytic activity of synthesized catalysts was evaluated in 0.5 M HClO4 + 1 M CH3OH solution. Results: Experimental results on the electrocatalytic activity of methanol oxidation demonstrate that Pt-Pd/RGO bimetallic nanodendrites synthesized using poly(diallydimethylammonium chloride) (PDDA) surfactant exhibits higher methanol electro-oxidation activity compared to Pt-Pd bimetallic nanodendrites synthesized using polyvinylpyrrolidone (PVP) as a surfactant. Conclusions: This work offers a convenient synthesis strategy to fabricate Pt-Pd nanodendrites (NDs) with improved dispersivity on RGO support with the aid of PDDA and PVP surfactants. Further, these findings suggest that utilization of PDDA as a surfactant yields Pt-Pd NDs with high dispersion on RGO.

Background: TiO2 photocatalysts are well-known for variety of applications in a range of important technological areas. The main drawback associated with TiO2 photocatalysts is the use of these particles in presence of UV light for practical applications and hence these particles utilizes very less part of sunlight. The purpose of this work is to introduce new energy levels between the conduction band and valence band, which results in reduction of the band gap. This permits TiO2 to be active under visible light irradiation and increases the degradation activity in sunlight. Methods: This research contain the preparation of Ce-TiO2 NPs through wet dispersion followed by impregnation method and activity study of these catalysts for degradation of methylene blue under direct sunlight. Results: Physicochemical characterization of Ce-TiO2 NPs has been studied by using different techniques such as XRD, IR, UV, SEM, and EDAX. XRD reveals the nanocrystalline nature of all the samples with anatase and rutile phase. The crystallite size of all samples was found in the range of 21-24 nm. UV-Visible absorption measurement revealed that the optical band gap of the doped samples decrease with increase in dopant concentration from 1.0 - 5.0 mol%. The photocatalytic activities of bare/doped TiO2 samples were demonstrated for the degradation of methylene blue (MB) dye under direct sunlight irradiation. Enhanced activity for photocatalytic degradation of methylene blue is shown by doped TiO2 samples as compare to bared TiO2. As dopant content increases, the photocatalytic activity of doped TiO2 NPs also increases due to the lower optical band gap energy than its bare form. Conclusion: In summary, prepared Ce-TiO2 found to be nanocrystalline and visible light active. These prepared catalysts has shown better activity for photocatalytic degradation of methylene blue under sunlight than bared TiO2.

Background: The recent progress in photocatalytic oxidation technique is concentrated on developing new photocatalytic materials. Photocatalytic activity of strontium titanates depends on the phase composition and structure of these materials. Recently, the authors also reported the enhanced photocatalytic activity of SrTiO3 after loading on a HZSM-5 zeolite. Methods: The raw NaZSM-5 powders were treated in hydrochloric acid solution to prepare HZSM-5 through Na+-H+ ion exchange. The ηHZSM-5 supported strontium titanate was synthesized through a sol-gel route followed by thermal treatment. The composites were measured by X-ray diffraction, scanning electron microscopy, infrared spectrum and N2 desorption analyses. Results: SrTiO3 is mainly supported on the surface of microporous HZSM-5 without apparently blocking the inner pores of the zeolite. HZSM-5 is responsible for the porous structure in the composites. Surface areas of the composites are much larger than that of pure SrTiO3. Photocatalytic activity of SrTiO3 is apparently improved after supporting on HZSM-5. The activity of 30%SrTiO3/ηHZSM-5 can be affected by the variation of hydrochloric acid concentration with the optimal value of 0.3 mol/L. The absorption of RBR-X3B solution in visible region and most ultraviolet region are not found after 90 min of degradation on 30%SrTiO3/0.3HZSM-5. Conclusion: SrTiO3 disperses on the surface of HZSM-5. Photocatalytic activity of 30%SrTiO3/ ηHZSM-5 can be greatly affected by hydrochloric acid concentration with the optimal value of 0.3 mol/L. Not only RBR-X3B molecules can be decolorized during photocatalytic process, but also the dye can be almost fully degraded on the supported SrTiO3 after a certain time.