Limitations of CVD graphene when utilised towards the sensing of heavy metals

Abstract

We explore the utilisation of a CVD grown graphene electrode towards the electrochemical detection of lead(II) via anodic stripping voltammetry. Owing to specific limitations that arise during the cathodic pre-treatment step, namely surface instability issues, we find that the respective analytical sensitivity of the CVD-graphene electrode is poor and unfavourable in comparison to the current electrochemical literature concerning the detection of heavy metals. We offer insight into the possible limitations facing the application of CVD-graphene electrodes within the sensing of heavy metals, assisting those concerned with its generation/fabrication.

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The experimental discovery of graphene in 2004¹ sparked a ‘gold rush’ among scientists in terms of global efforts to investigate and exploit its unusual physical, chemical and electrical properties.^2,3 A vast degree of interest has focused upon the electrochemical properties and applications of graphene,^4,5 where numerous literature reports have emerged indicating that graphene has enormous potential within all aspects of this discipline, ranging from energy storage and generation through to the electrochemical sensing of various gaseous and aqueous analytes.^4–10

In terms of the electrochemical monitoring of heavy metal ions, which is of substantial interest and importance owing to the severe toxic nature of such analytes,^11,12 graphene has been reported to be a highly beneficial electrode material and has been utilised in the urgently required exploration of fast, sensitive and reliable detection methods.^12–14 For instance, Li et al.¹² have reported the trace level detection limit of 0.02 μg L⁻¹ for lead(II) via the utilisation of a graphene–Nafion modified in situ plated bismuth film glassy carbon electrode, moreover, recent work by Gong et al.¹³ has established a highly sensitive and selective mercury(II) sensor which is based upon a monodispersed gold nanoparticle decorated graphene electrode and is reported to achieve a detection limit as low as 6 ng L⁻¹, significantly below the guideline value of the World Health Organisation.¹³ Considering such reports, future electrochemical prospects for graphene appear promising; note however, contrasting reports have emerged indicating that graphene may not be such a ‘wonder material’ in all aspects of electrochemistry.^15–17

The fabrication of graphene via Chemical Vapour Deposition (CVD) is one of numerous methodologies;^3,4 however this technique appears most promising for the application of graphene within electrochemistry given that it is reported to yield high-quality volume-produced uniform graphene sheets that exhibit large surface areas, are readily transferable and possess ‘outstanding’ electrical conductivities.^18,19 There has been an overabundance of recently emerging reports in which CVD-graphene has been beneficially utilised within electrochemistry,^20–25 most notably for example, Li et al.²⁰ have reported a 10-fold increase in electron transfer rates (towards ferrocenemethanol) at CVD-graphene when contrasted to the basal plane of bulk graphite, similarly Dubuisson et al.²¹ report a beneficial response at anodised epitaxial graphene towards the sensing of DNA and a highly sensitive CVD-graphene hydrogen sensor has also been reported,²⁴ moreover, Qu and co-workers²² have reported improved electro-catalytic activity and long-term operational stability towards the reduction of oxygen at a nitrogen doped CVD-graphene electrode when compared to commercially available platinum loaded carbon, Zhou et al.²³ have explored the advantageous use of CVD-graphene films as transparent conductive electrodes in organic photovoltaic cells and CVD-graphene has been utilised within the fabrication of a supercapacitor which achieved a high specific capacitance value of 330 F g⁻¹.²⁵

Recently we have shown proof-of-concept that CVD grown graphene is not a continuous layer of graphene, but rather is heterogeneous in nature and has few- and multi-layered graphitic domains (viz graphitic islands) comprising its surface, which can dominate the electrochemical response;²⁶ an insight which is only evident when differing redox probes, viz inner- and outer-sphere redox probes are employed. This was further explored towards biologically prevalent molecules where again these graphitic islands were shown to dominate the observed electrochemical response,²⁷ where in comparison CVD-graphene exhibits voltammetry akin to a highly ordered pyrolytic graphite electrode.²⁸

Note that the above literature reports^26–28 have exclusively explored the anodic potential region (typically from 0 up to +1.5 V (vs. SCE)), predominantly due to the analytes studied. A key application, never explored to date with CVD-graphene, is the sensing of heavy metals—which exclusively involves the cathodic potential region.

Consequently, in this study we explore for the first time the utilisation of a CVD grown graphene electrode towards the electrochemical detection of lead(II) via anodic stripping voltammetry, incorporating a cathodic pre-treatment step and utilisation of the cathodic potential region which to date have not been previously explored at a CVD-graphene electrode.

The commercially available CVD-graphene utilised in this study has been electrochemically characterised previously in the literature.^26–28 CVD processes generally utilise hydrocarbon gases as precursors and transition metal surfaces for the growth of graphene.²⁹ Nickel is commonly the preferred metal of choice owing to its high carbide solubility, which can be utilised beneficially to control the layer thickness of graphene sheets; additionally, nickel reportedly has the advantage of producing continuous large-area graphene films.^29,30

We first consider the anodic stripping voltammetry of the target heavy metal lead(II) which involves the nucleation of lead metal onto the electrode surface (deposition step) and the subsequent transition of in situ formed lead metal back to lead(II) ions (stripping step) which produces the resultant voltammetric response. As a starting point we utilised a deposition potential of −0.8 V (vs. SCE) and a time of 40 s and explored the electroanalytical response of the CVD-graphene electrode towards the sensing of lead(II) ions in aqueous solution (pH 1.5, HCl).

Fig. 1 depicts typical voltammetric responses arising from the successive additions of lead(II) ions where a characteristic stripping peak is evident at ∼−0.52 V. We find that the lowest ‘detectable’ addition to be observed using these experimental parameters corresponds to 400 μg L⁻¹, which is significantly deviated from that currently reported in the literature using carbonaceous electrode surfaces.^31,32 Analysis of the peak height (I_P) as a function of concentration, as depicted in Fig. 1, reveals a linear response over the range of 400 to 2000 μg L⁻¹ (I_P (μA) = 1.61 × 10⁻² μA/μg L⁻¹ – 4.14 μA; R² = 0.993, N = 9), however, the accessible linear range is largely deviated from that reported in the literature and is thus unfavourable.^31,32 Note however, the % Relative Standard Deviation of the observed stripping peak was calculated to be 4.8% (N = 3, see Fig. S1, ESI†) which is analytically acceptable.¹⁶

Fig. 1 Typical square-wave voltammetric responses for the detection of lead(II) ions in pH 1.5 aqueous HCl solution at a CVD-graphene electrode. Blank solution is represented by the dotted line and additions of lead(II) made are; 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2000 μg L⁻¹ (solid lines). A deposition potential of −0.8 V (vs. SCE) for 40 s was utilised. Inset: calibration plot indicating the relationship between the concentration of lead(II) ions and the observed peak height.

It is evident that the analytical response at the CVD-graphene electrode is somewhat limited. We consequently explore the origin of why this large deviation from that reported in the literature at carbonaceous surfaces arises, especially compared to when graphene is utilised towards the sensing of the heavy metal lead (see introduction).¹²

In order to improve the analytical response, as observed in Fig. 1, there are two parameters that can be varied. The first is the accumulation time, where increasing this will allow for a greater quantity of material (nucleated lead) to accumulate onto the electrode surface, which consequently on the stripping step results in a larger amount of material that is voltammetrically stripped and hence a larger analytical signal will be observed. However, this significantly increases the overall time to conduct the analytical measurement. The other parameter that may be varied in order to optimise the response is the deposition potential. Typically, one would vary the magnitude of the negative potential and monitor the size of the stripping signal. Fig. 2 shows the effect of deposition potential towards the resultant peak height, where in our case, the maximum stripping signal is observed when utilising a deposition potential of −0.8 V. It is clear that altering the deposition potential towards higher electronegative potentials (up to −1.8 V) induces a detrimental effect on the analytical peak height.

Fig. 2 The effect of varying the deposition potential (vs. SCE) upon the resultant peak height for the detection of 2000 μg L⁻¹ lead(II) ions in pH 1.5 aqueous HCl solution at a CVD-graphene electrode via anodic stripping voltammetry (square-wave voltammetry). A deposition time of 40 s was utilised.

Given the poor analytical performance of the CVD-graphene electrode and the unfavourable outcome when attempting to optimise the deposition potential, it is interesting to revisit and explore the voltammetric response shown in Fig. 1. Closer inspection reveals that on the anodic scan (stripping step), at potentials more positive than −0.4 V, an uprising in the current is observed; it is this we investigate further.

Fig. 3 depicts the anodic stripping voltammetry of lead(II) where, after the initial cathodic pre-treatment step, the potential is swept from −0.8 up to +0.6 V (vs. SCE). Upon inspection of Fig. 3 it is evident that a peak occurs at ∼−0.52 V, corresponding to the stripping of lead,^31,32 while a second peak is also evident at ∼+0.12 V which is likely due to the stripping of the underlying nickel upon which the graphene surface is grown/supported. Furthermore, after acquiring the two stripping peaks observed in Fig. 3 a subsequent scan was attempted (again following the cathodic pre-treatment step), in which case the voltammetric response exhibited no peaks and instead a capacitative current was observed, as would be expected if utilising a relatively non-conductive electrode material. Thus it is evident that within the first analysis, where a second stripping peak is observed the underlying nickel is consequently stripped and as a result the graphene ‘working’ electrode surface is removed (thus both components become detached from the surface), such that the oxidised silicon wafer, on which the nickel and then graphene were deposited, is the sole component remaining (within the defined working area—see experimental section) giving rise to the subsequent non-conductive/capacitative response. Our inference is supported via visible observations of the CVD-graphene chip after utilising it in this manner (see Fig. 4), where within the centre of the chip (the defined working area) there is a clear circular section/indentation of the electrode surface, which is confirmed via Energy Dispersive X-ray spectroscopy (EDX) coupled with a Scanning Electron Microscope (SEM) to be exclusively composed of the underlying oxidised silicon wafer (atomic compositions of 44 and 54% for Si and O respectively, the remaining 2% was atomic C; note, 0% corresponded to Ni), which is non-conductive, confirming the absence of the previously present graphene surface upon a nickel film. Furthermore, areas outside the defined working region can be observed to remain ‘intact’ and consist of the graphene modified nickel film.

Fig. 3 Typical square-wave voltammetric response for the detection of 800 μg L⁻¹ lead(II) ions in pH 1.5 aqueous HCl solution at a CVD-graphene electrode. Cathodic pre-treatment; a deposition potential of −0.8 V (vs. SCE) for 40 s was utilised.

Fig. 4 SEM (A) and optical (B) images of the commercially obtained CVD-graphene chip following its utilisation as a working electrode within anodic stripping voltammetry (square-wave voltammetry) towards the detection of 800 μg L⁻¹ lead(II) ions in pH 1.5 aqueous HCl solution. Cathodic pre-treatment involved a deposition potential of −0.8 V (vs. SCE) for 40 s, after which the potential was swept anodically from −0.8 up to +0.6 V.

We next explore the stripping/detachment of the nickel layer further, where in the absence of lead(II) ions we hold the potential for 40 s at −0.2, −0.4 and −0.8 V before sweeping the potential from the corresponding pre-conditioning potential up to +0.6 V (vs. SCE). Experimental observations (not shown) indicate that the magnitude of the stripping peak ‘for nickel’ increases upon increasing the potential to more negative values, again this would also coincide with increasing the pre-conditioning time, where with increased hold time an increment in the quantity of nickel stripped would be expected.

Based upon our previous work^26–28 we have always explored the electrochemical oxidation of analytes which occur at potentials ∼0.0 V (vs. SCE) and upwards to more positive potentials. The only case where we have explored the cathodic region is that of the electrochemical reduction of hexaammine-ruthenium(III) chloride, which, at best, occurs at a negative potential of ∼−0.25 V (vs. SCE) and within this ‘cyclic voltammetric’ analysis there is no degradation of the underlying nickel layer.²⁶ However, when utilised for the detection of the heavy metal ion lead(II), we have found that when potentials more negative than this are applied, a new voltammetric peak is observed at ∼+0.12 V (see Fig. 3). It is evident that cycling of the potential window within the negative region does not detrimentally affect the CVD-graphene chip, however, when cathodic pre-conditioning (i.e. holding the electrode surface at a negative potential for a definite time) is employed it is clear that the graphene–nickel surface is disrupted somewhat and as a result under said conditions the graphene–nickel surface is unstable upon the stripping step and is consequently lost/stripped/detached from the surface of the electrode when an anodic current is applied, as a consequence the working surface breaks-down to leave the underlying oxidised silicon wafer base exposed (as evidenced within Fig. 4). Given insights obtained from varying the deposition potential (see Fig. 2) it is clear that at larger electronegative potentials this effect is increased, resulting in a higher degree of instability of the electrode surface where stripping of the graphene/nickel surface is then more likely upon anodic scans.

Note that in terms of surface morphology prior to the loss of the graphene modified nickel film, the CVD-graphene surface is highly disordered and heterogeneous in nature with the prevalence of single and multiple layered graphene crystals (viz graphitic islands) orientated both parallel and perpendicular to the surface; as is evident in Fig. S2 and S3 of the ESI†via Atomic Force Microscopy (AFM) images and Raman spectroscopy respectively, which indicate both single- and multiple-layered graphene domains persist across the CVD-graphene surface prior to use. With respect to the mechanism of loss/breakdown of the CVD-graphene surface, once perturbed the underlying nickel layer (nickel metal film) is lost through electrochemical oxidation via the following stripping process:

Ni_(s) → Ni²⁺_(aq) + 2e⁻

It is likely that surface morphology plays a key role in this process where some of the heterogeneous domains would be more susceptible to pitting which must lead to the ultimate demise of the structure. We found that no intermediate image was obtained, that is, we could only observe Fig. S2 and S3† and Fig. 4. However, to gain further insights an in depth study utilising in situ AFM and Raman microspectroscopy is suggested as a function of time/applied potential to detail surface changes that occur during the course of analysis,³³ further work on this is currently underway.

We have thus shown that the sensitive detection of lead(II) ions is not possible when utilising a CVD grown graphene electrode given that, owing to its prior exposure to negative potentials, the underlying nickel interferes with the subsequent analysis. Therefore the application of a CVD-graphene electrode that has been synthesised upon a nickel substrate has limitations when employed after cathodic pre-treatment. Manufacturers concerned with the CVD synthesis of graphene should thus focus efforts towards the utilisation of various other metal catalysts/substrates or provide the CVD grown graphene only after transferring it onto an appropriate insulator/dielectric material, given that the CVD-graphene topography will be strongly affected by the substrate used in CVD.

In conclusion we have provided new insights when utilising CVD-graphene for the potential application of heavy metal sensing which invokes applying a highly negative electrode potential; here we have shown for the first time that the nickel layer (which is the underlying support) is detrimentally perturbed as a result of specific cathodic pre-treatments, which thus interferes with subsequent electrochemical analysis. This gives researchers working in the fabrication of CVD-graphene further insights on how to produce task specific CVD-graphene surfaces.

Experimental section

All chemicals used were of analytical grade and were used as received from Sigma-Aldrich without any further purification. All solutions were prepared with deionised water of resistivity not less than 18.2 MΩ cm and were vigorously degassed prior to electrochemical measurements with high purity, oxygen free nitrogen. Voltammetric measurements were carried out using an ‘Autolab PGSTAT 101’ (Metrohm Autolab, The Netherlands) potentiostat. All measurements were conducted using a three electrode system; a commercially obtained CVD synthesised graphene film grown directly onto a nickel film deposited on an oxidised silicon wafer was utilised as the working electrode,³⁴ with a platinum wire counter and a saturated calomel electrode (SCE) reference (Radiometer, Copenhagen, Denmark) completing the circuit. Note, for the employment of the CVD chip working electrode an electrochemical cell was utilised as described previously by our group,^27,28 where essentially the CVD chip was secured into a PTFE housing unit with a steel contact making connection to the back of the chip (which via the use of silver conductive paint (applied to cover the back and sides of the chip in their entirety—that are otherwise not electrically conductive) ensures electrical conductivity from the front graphene ‘working surface’ of the electrode) and a silicone O-ring defining the working surface (diameter, 3.9 mm). Fig. S4 of the ESI† details the experimental set-up. Note that typically within electrochemistry, graphene is studied upon graphite/carbon surfaces and thus it is sometimes hard to de-convolute the response of graphene, hence studying graphene on a nickel surface is a more informed approach since the electrochemistry of nickel is surprisingly limited.²⁶ Furthermore, note that lifting/transferring the graphene from the CVD surface onto an insulating substrate is a hard and complicated process¹⁹ where in this case an electrical connection is still needed and required to make contact with the new material—given the complexity of this task and the fact that we have previously reported nickel control experiments in which we have shown that the underlying nickel layer does not affect/influence the observed electrochemistry of the graphene layer, transfer of the graphene is not required as it appears that the graphene is the only electrochemically active material partaking in the reaction^26,27—until the employment of the cathodic pre-treatment step of course as we observe and report this limitation for the first time in this work.

Commercially available CVD-graphene was obtained from ‘Graphene Supermarket’ (Reading, MA, USA)³⁴ and is known as ‘CVD-Graphene™: Graphene Film on Nickel’ where single to few layer continuous graphene films with low defect density are grown directly onto a nickel film deposited on an oxidised silicon wafer via a CVD process, as previously reported and characterised.^34,35

Note, square-wave voltammetry was conducted utilising an amplitude of 20 mV and a frequency of 25 Hz (equivalent scan rate: 62.5 mV s⁻¹).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra01279k/

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