Study on adsorption and desorption of ammonia on graphene
© Zhang et al. 2015
Received: 8 July 2015
Accepted: 27 August 2015
Published: 16 September 2015
The gas sensor based on pristine graphene with conductance type was studied theoretically and experimentally. The time response of conductance measurements showed a quickly and largely increased conductivity when the sensor was exposed to ammonia gas produced by a bubble system of ammonia water. However, the desorption process in vacuum took more than 1 h which indicated that there was a larger number of transferred carriers and a strong adsorption force between ammonia and graphene. The desorption time could be greatly shortened down to about 2 min by adding the flow of water-vapor-enriched air at the beginning of the recovery stage which had been confirmed as a rapid and high-efficiency desorption process. Moreover, the optimum geometries, adsorption energies, and the charge transfer number of the composite systems were studied with first-principle calculations. However, the theoretical results showed that the adsorption energy between NH3 and graphene was too small to fit for the experimental phenomenon, and there were few charges transferred between graphene and NH3 molecules, which was completely different from the experiment measurement. The adsorption energy between NH4 and graphene increased stage by stage which showed NH4 was a strong donor. The calculation suggested that H2O molecule could help a quick desorption of NH4 from graphene by converting NH4 to NH3 or (NH3)n(H2O)m groups, which was consistent with the experimental results. This study demonstrates that the ammonia gas produced by a bubble system of ammonia water is mainly ammonium groups of NH3 and NH4, and the NH4 moleculars are ideal candidates for the molecular doping of graphene while the interaction between graphene and the NH3 moleculars is weak.
Ammonia detection has a great significance in many areas, such as environmental protection and industrial inspection. Furthermore, ammonia detection has good prospects in medicine diagnose. For instance, measurements of exhaled ammonia may differentiate between viral and bacterial infections in lung diseases to justify the use of antibiotics , and ammonia detection may be used to indirectly measure urea levels for renal disease monitoring . Identifying these signaling metabolites (disease markers) and measuring them in trace concentrations is not a trivial problem, and the low concentrations of analyte molecules presents a major challenge, along with the specificity to a given analyte. Recently, low-dimensional materials used for gas detection has become a trend [3, 4], it has been reported that it is possible to use graphene as a gas sensor with high sensitivity and high accuracy for detecting ammonia groups [5, 6]. Graphene is considered to be an excellent kind of sensor material due to its following properties: (i) graphene is a single atomic layer of graphite with a larger specific area than any other materials, which maximizes the interaction between the surface dopants and the adsorbates; (ii) as a kind of special material with zero bandgap, graphene has a extremely low Johnson noise , for which a slight change of carrier concentration can cause a notable variation of electrical conductivity; (iii) graphene has limited crystal defects, which ensures a low level of excess noise caused by thermal switching . F. Schedin et al.’s study showed that micrometer-sized sensors made from graphene were capable of detecting individual gases, and the study found out that the changes in graphene conductivity during chemical exposure were quantized, with each event signaling adsorption or desorption of a single NH3 molecule, and proved that NH3 was acting as a strong donor for graphene and the desorption was relatively difficult . Hugo E Romero et al. considered that the slow NH3 desorption rate from supported graphene FETs was consistent with a Fickian diffusion process of molecules in the SiO2/graphene interface . Lakshman K. Randeniya et al. has developed a new method by adding the flow of water-vapor-enriched air at the beginning of the recovery stage to realize a rapid desorption, and researchers believe that shifts in the substrate defect states of graphene caused by the water adsorption can weaken the interaction between NH3 and defective graphene . However, O. Leenaerts et al. investigated the adsorption of NH3 on graphene substrate with first-principle calculations, and the results illustrated a very small adsorption energy (31 meV) and a small charge transfer (0.027 ± 0.001 electrons per molecule at pristine sites) from NH3 to graphene .
These explanations revealed the truth from some degree, but some of these explanations are still not theoretically grounded or lack experimental verifications. Therefore, it is hard to treat them as a fully complete conclusion. In this work, we studied the adsorption and desorption of ammonia groups on graphene theoretically and experimentally. Our study demonstrates that the interaction between ammonia groups NH4 and graphene is much stronger than that of ammonia groups NH3. We take the view that hydrogen atom plays an important role in the adsorption between NH3 and graphene, and we find that H2O helps NH4 to take off one hydrogen atom and achieves a rapid desorption from graphene sheets.
Theoretically, the density functional theory (DFT) calculations were performed with CASTEP ; the generalized gradient approximation (GGA) was used to deal with the exchange and correlation term. It is well known that the local density approximation (LDA) is normally inaccurate in describing the van der Waals-like interaction, and the advantage of GGA over LDA in this work is that the GGA will not lead to a strong bonding of the molecules as LDA does . A plane wave basis set with a cutoff energy of 800 eV and pseudopotentials of the Troullier–Martins type and non-spin-polarized calculations were used in this study. The total system consisted of a 3 × 3 graphene supercell (18 C atoms) with a single molecule adsorbed and a distance of 16 Å between adjacent graphene layers. The Brillouin zone integration was performed within the Monkhorst–Pack scheme using 5 × 5 × 1 k points . For the calculation of the density of states (DOS), a 20 × 20 × 1 Monkhorst–Pack grid and a Gaussian smearing of 0.05 eV were used .
Results and discussion
where C is a constant related to electrode geometry structure and dielectric material of the device, K is the slope of linear region (K h for hole, K e for electron), and μ is the mobility (μ h for hole and μ e for electron). From the above expression, the mobilities of the hole and electron were almost same and which value was about 2000 cm2(V•S)−1.
where E system is the total energy of the optimized equilibrium configuration of the graphene and the adatom; E graphene is the total energy of the isolated graphene; E molecule is the total energy of the corresponding adsorption molecules in its ground state; n is the number of gas molecules in the system.
The adsorption energy E a and the charge transfer ∆Q from H2O to graphene with three different geometries
E a (H2O)/meV
The adsorption energy E a and the charge transfer ∆Q from NH3 to graphene with six different geometries
E a (NH3)/meV
These results were in agreement with those already reported in previous studies on the binding energies of physisorbed NH3 molecules on graphene [5, 6]. All of these results showed that ammonia molecules NH3 are not good candidates for effective molecular doping of graphene. Obviously, there are other chemical groups that physisorb more strongly on graphene sheets and shift the Fermi energy inside the valence or conduction bands. Bradley et al. found that vacuum-degassed SWNT-FETs were insensitive to NH3; they suggested that to detect NH3 gas in the FET response, one should dissolve the NH3 in a H2O monolayer which forms on the nanotube FETs under ambient lab conditions. They proposed that the NH3 molecule in this environment could become a cation with the charge compensated from the SWNTs, i.e., acting as an n-dopant . In fact, hydrogen is an ordinary impurity in electronic devices [21, 22], playing an important role in many reactions between physisorbed NH3 molecules and hydrogen adatoms to generate chemical species that have stronger interactions with the graphene layer. Specifically, an isolated H impurity on graphene can be captured by an NH3 molecule in an exothermic reaction that releases 1.00 eV of energy .
The adsorption energy E a and the charge transfer ∆Q from NH4 to graphene with three different geometries
E a (NH4)/meV
and the experimental equilibrium constant K b for this reaction at room temperature is 1.77 × 10−5; therefore, the concentration of NH4 + and OH− in water solutions, although extremely small, is about 4.21 × 10−3 mol/dm3.
The adsorption energy E a and the charge transfer ∆Q from NH3•H2O and NH3•2H2O to graphene
E a (NH3•H2O)/meV
E a (NH3•2H2O)/meV
In conclusion, the adsorptions of ammonia on graphene were investigated with experiment and first-principle calculation. The study demonstrates that the remarkable variation of the electrical conductivity is induced by the ammonia adsorption, and that the graphene gas sensor possesses an excellent characteristic of high sensitivity for ammonia gas detection. It was found experimentally that ammonia molecules produced by a bubble system of ammonia water were strongly adsorbed onto the graphene sheets; however, ammonia molecules were only weakly adsorbed onto the intrinsic graphene with small binding energy value and large distance between the NH3 molecules and graphene from first-principle calculations. The electronic structure and electrical conductivity of the intrinsic graphene have limited changes due to the adsorption of NH3 molecules. Moreover, this study found that the ammonium (NH4) had strong interactions with graphene, forming a strong bond that introduces a large amount of shallow donor states into the system. When adding water molecules into the desorption process, the whole desorption process was greatly accelerated. We considered that ammonium molecules and water molecules generated the ammonia-water cluster and found that the cluster has weak adsorption with graphene sheets by calculation. The results are in accordance with the experiment. In a word, this study demonstrated that the ammonia gas produced by a bubble system of ammonia water were mainly molecular groups of NH3 and NH4, and the NH4 moleculars are ideal candidates for the molecular doping of graphene. However, the NH3 moleculars have weak interaction with graphene.
The authors are grateful to Natural Science Foundation of China (No. 11104348), School Pre-research of National University of Defense Technology (JC11-02-08) for the financial support to this work. The calculations were performed on computers in the High Performance Computing Center of Central South University, China.
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