Graphene Based Sensors for Air Quality Monitoring – Preliminary Development Evaluation

: Indoor air pollution can induce adverse health effects on building occupants and pose a significant role in health worldwide. To avoid such effects, it is extremely important to monitor and control common indoor pollutants such as CO 2 , VOCs and relative humidity. Therefore, this work focuses on recent advances in the field of graphene-based gas sensors, emphasizing the use of modified graphene that broadly expands the range of nanomaterials sensors. Graphene films were grown on copper by chemical vapor deposition (CVD) and transferred to arbitrary substrates. After synthesis, the samples were functionalized with Al 2 O 3 by ALD and characterized by a large set of experimental techniques such as XPS, Raman and SEM. The results demonstrated that graphene was successfully synthesized and transferred to SiO 2 , glass and polymer. As a proof-of-concept, ALD of Al 2 O 3 was performed on the graphene surface to produce a graphene/metal oxide nanostructure towards the development of nanocomposites for gas sensing. From this perspective, a laboratory prototype device based in measuring the electrical properties of the graphene sample as a function of the gas absorption is under development.


INTRODUCTION *
Volatile organic compounds (VOCs), particulates (PM) of diverse sizes and microbial contaminants deteriorate indoor air quality (IAQ) and have subsequent effects on human health [1].According to World Health Organization (WHO), it is estimated that, in 2016, household air pollution is estimated to have caused 3.8 million deaths from non-communicable diseases (including heart disease, stroke and cancer) and acute lower respiratory infections.In the same year, in Portugal, the mortality rate attributed to household and ambient air pollution was 9,8 per 100 000 population [2].For all these reasons, there is a need to control air quality so that the values do not exceed the limits imposed by the European Commission [3].
In the last years, devices that can measure and monitoring chemical-, physical-and biological-changes in the environment, with low cost, compact size, and low-power consumption have been developed under the form of sensors [4,5].Sensors are devices that provides an output signal usable in response to a specific measure, such as observable chemical reactions between certain materials [6].A gas sensor can be used to monitor potentially dangerous leaks and, if connected to an automatic control system, can be an advantage in detecting high concentrations of gas in a given environment [7,8].
These can be classified according to their operating principle, namely metal-oxide semiconductor sensors (MOS) based on conductivity variation [9], amperometric sensors based on solids or liquid electrolytes [10], or optical sensors using fluorescence or absorption of light [11].Metal oxides have been fabricated to monitor and detect VOCs for more than 50 years due to their semiconducting properties.Semiconducting metal oxides (SMOXs) are attractive for gas sensing applications since they are cheap, flexible to apply to different manufacturing methods and easy to use [12,13].
They may also be divided into specifics sensors, which provide specific information regarding a target compound, or, non-specific, that provide a global response to one or several families of chemical species (In section 3 the sensors for air monitoring are presented in more detail) [14].They play an important role in various fields of application such as environmental monitoring, industrial production and safety, and when integrated into measurement systems, they can detect changes that occur in the physical environment [14,15].Thus, due to its unique structure, uncommon chemical and physical properties, good conductivity and large specific surface area graphene based sensors performed well with good accuracy, rapidness, high sensitivity and selectivity, low detection limits, and long term stability [16].Nowadays, it is clear that graphene is a very promising candidate to integrate sensors for IAQ monitoring [17,18].However, pristine graphene is chemically inert to be useful in the detection of gases.As such, an approach to increase its chemical activity is required, through functionalization [19].
The present work focuses on recent advances in the field of graphene-based gas sensors, emphasizing the use of modified graphene that broadly expands the range of nanomaterials sensors and intends to be the starting point for the development of a functional prototype of the chemical gas sensor with high sensitivity, from the modified graphene structures.

Air Quality Requirements, Parameters and Assessment
Indoor air pollutants may originate from a different range of sources such as combustion for heating, material deterioration and VOCs emitted from paints, varnishes and preservatives.In addition, insoluble nanoparticles as well as biological particles present in indoor air, can also affect human health through direct toxicity, immune and infectious mechanisms [20][21][22][23][24][25][26][27][28][29][30].
For air quality assessment, numerous indices were proposed.The first index was the "Pollutant Standard Index" (PSI), which was modified and replaced by the "Air Quality Index" (AQI), both of which were developed and introduced by the United States Environmental Protection Agency (US-EPA) [24].In Europe, the Environment Directorate General of The European Commission ("DG Environment") has developed a legislation, which establishes health-based standards and objectives for a number of pollutants present in the air.These standards and objectives are summarized in the Table 1 [3].As people spend a substantial part of their time on buildings, maintaining indoor air quality levels becomes a challenge to overcome.Thus, the use of sensors and the development of new materials, such as graphene-based sensors, allow these levels to be monitored and controlled.
Conventional analytical instruments can be use accurately to measure the concentration of pollutants inside the buildings, but are not practical because of their complexity, volume and emitted noise.In addition, most analyzes require sample preparation, so a real-time analysis is difficult to obtain [21,25,26].Solid-state chemical sensors have been widely used, however, also their measurement accuracy is limited and Target value to be met as of 31.12.2012present a long-term stability problem [26].Nanotechnology becomes to fill existing flaws by providing numerous opportunities for the development of the next generation gas detector with enhanced sensor performance such as high specificity, fast response, ultra-high sensitivity at extremely low concentrations and coverage, low power consumption, room temperature operation and good reversibility [26,27].

Graphene as a Sensor Device
In the recent years, there has been a significant improvement in the construction of sensors with the incorporation of several nanomaterials, such as nanowires synthesized from metals, metal oxides, semiconductors, carbon nanotubes (CNT) and metal nanoparticles.Due to their conductive properties and high surface-to-volume ratio, nanomaterial thus contributing to the improvement of the analytical performance of such sensors [28][29][30].One of the advantages of semiconductor gas sensors is the ability of easily combining the functions of a sensitive element and signal converter and control electronics in the same device, greatly simplifying the design [26,31].Nanostructures have a high surface-to-volume ratio, which is one of the most important characteristics of a material to be used for gas sensing and provides large active surface area for the interaction of gas molecules.This strongly favors the adsorption of gases on nanostructures and leads to highly sensitive sensors performance [32].The most recent example of the application of new materials in this type of sensors are the studies of the use of graphene in sensors for air quality measurement [33,34].The two-dimensional (2D) materials have captured enormous interest after the first successful isolation of graphene in 2004 [28].
Graphene is an extremely diversified material, with exceptional characteristics that allows producing different materials with varied properties, leading to great technological advances in the most varied areas [16,35].Because silicon-based technology is close to the limit in terms of performance improvement, the electronic properties of graphene make it an excellent choice for the semiconductor industry [33,36].
Compared to other materials graphene showed great potential for the construction of sensors.The combination of extraordinary properties makes graphene highly sensitive to changes of local environmental conditions, which is an important advantage in the sensing field, since all carbon atoms interact directly with the analyses, thus promoting higher sensitivity [34,37].Fluctuations due to thermal movement of charges and defects limits the sensitivity of graphene.Each atom in the graphene is exposed to its environment, allowing it to sense individual events when a gas molecule attaches to graphene's surface [38].The adsorbed molecules change the concentration in graphene, which leads to changes in resistance.The achieved sensitivity is due to the fact that graphene is an electronically low-noise material, which makes it a promising candidate for gas sensors [39].
The synthesis of graphene can be divided into two main categories: physical-and chemical methods as shown in Table 2 and depends, among others, on the crystallinity, purity and desired size [40,41].
The chemical vapor deposition (CVD) process is the most common method to grow graphene.It consists of breaking the bonds of the molecules of a gas subjected to high temperatures, so that the atoms coming from the gas are deposited on a certain substrate and the graphene films synthesized by this method can be transferred to other substrates, facilitating their integration into various materials [30,43].
The deposition of high-quality graphene from CVD process is usually done onto various transition-metal substrates with copper (Cu) as the most popular metal to produce homogeneous single layer graphene in large-area [44][45][46].
Copper is one of the best catalyst options because, even at 1000°C, the solubility of carbon in copper is insignificant, so the carbon precursor forms graphene directly on copper surface during the growth step [47,48].

Functionalization of Graphene by Atomic Layer Deposition (ALD)
The operating principle of graphene devices is similar to other solid-state sensors and is based on changes in their electrical conductivity due to gas molecules adsorbed on their surface, acting as donors or receivers [49,50].Depending on the size relationship between a chemical species and material, these interfaces may be superficial when chemical and/or physical adsorption occurs, or volume interactions, when the chemical absorption passes through the active layer of the sensor (bulk effect) [51].
Figure 1 represents the schematic diagram of a back-gated graphene device on top of an arbitrary substrate during NH3 exposure with also a representation of current-biased measurement setup [51].
The natural properties of graphene and functionalized graphene, such as massless charged carriers, and the high interaction with gas molecules, make these one of the most efficient materials for detecting gases.The type of interactions between atoms and molecules of graphene differ from the weak interactions of van der Waals with the strong covalent bonds, which leads to an intense change in the conductivity of the graphene [39,52,53].
Due to its high-quality crystal lattice, graphene has intrinsically low electrical noise and is capable of transmitting more charge fluctuations.Consequently, some additional electrons can create a noticeable change in the conductance of graphene.As all carbon atoms are located on the surface, small changes in the resistance of a graphene sheet, even if down the molecular level, are measurable [54] making graphene highly sensitive to any change in its surrounding environment [39,52].
One of the techniques that allow the manufacture of atomicscale materials and components either as thin film or as nanoparticles with accuracy that cannot be achieved by current CVD techniques is atomic layer deposition (ALD).As matter of fact, ALD has proved to be a technique of choice for the coating of nanostructured carbon materials and it's based on self-limiting surface reactions separated in gas phases [55,56].When two precursors A and B react on gas phases during the CVD deposition to produce a thin film on the surface of the substrate, the same precursors react separately in ALD with the substrate surface to produce a uniform coating.The principle is based on the splitting of the deposition reaction in two separated self-limiting reactions due to the deposition mechanism [57].
ALD of aluminum oxide (Al2O3) using trimethylaluminum (TMA) and water as precursor were developed as a model for ALD system.The surface reaction during the ALD deposition mechanism has one of the highest ALD reaction enthalpies [57,58].
Nowadays, with the down-scaling of semiconductor device dimensions, the requirement of nanotechnology has grown enormously and ALD has found new opportunities in microelectronics industry for the development of metal oxide semiconductor field effect transistors (MOSFETs) and highdensity memory devices with high-level integration [59,60].In this way, ALD has proved to be suitable for the deposition of metal oxides.In general, metal oxide semiconductors are used as active layers in resistive sensors [61,62].As a proof-of-concept, we synthesized CVD graphene on copper foils, in order to fabricate the platform for the sensing devices.The functionalization of CVD graphene via ALD is an interesting way to modify the surface properties of the asprepared graphene.As result, a nanostructure composed of graphene/metal oxide is obtained for detecting various gas species at room temperature.

Synthesis of Graphene by CVD
Graphene samples were synthesized using CVD method at 950 °C and at a pressure of 4,67 × 10 4 Pa.Copper with 25µm thickness was used as catalytic substrate.The heating step was started by increasing the furnace temperature from room temperature (≈ 20 °C) to 950 °C under 117 sccm (standard cubic centimeters per minute) N2 and 27 sccm H2 atmosphere.The annealing process was carried out under same conditions to remove oxide from the copper foil and to increase the grain size of Cu.After keeping temperature at 950 °C for 64 min, it was used an atmosphere rich in methane at a flow rate of 16 sccm in order to grow graphene for 5 min and, finally, the sample was cooled down at natural velocity.The experimental parameters (temperature profile, gas composition and time) are shown in Figure 2.

Transfer Process of Graphene
Graphene films were removed from the copper foils by etching in a solution of iron chloride III (FeCl3) with deionized water, as shown in Figure 3. Once the copper oxidation is complete, it is necessary to remove the floating graphene membrane so that it is subsequently transferred to an arbitrary substrate.The suspended films were transferred to deionized water (about 2 min) to remove any residual copper etchant.Afterwards the graphene was transferred for SiO2/Si (Figure 3.1), glass (Figure 3.2) and polymer (Figure 3.

3).
From Figure 3 it is also possible to notice that the graphene was successfully removed from the copper foils and it can be seen as grey contrast on the top of the glass, for example (Figure 3.2).This transfer step is crucial, concerning the integration of the graphene in the sensor device fabrication process.

ALD of Al2O3
After the transfer step, the graphene functionalization was done with Al2O3 coating by ALD in home-made cross-flow reactor using a stop valve feature.ALD of Al2O3 process is based on the reaction between the water (H2O) and trimethylaluminum (TMA) precursors at 100°C.To this end, the TMA and H2O were alternately pulsed on the graphene surface during deposition and the growth relies on two selflimiting surface reactions.Both precursors were kept at roomtemperature and pure nitrogen (N2) was used as carrier and purge gas.To ensure higher exposure time of precursor gas over the graphene surface network, the stop valve mode was used.Several samples were prepared, with different number of ALD cycles.
The as-prepared nanostructures were characterized by a large set of experimental techniques such XPS, Raman, and SEM.In particular, the Raman scattering spectroscopy is remaining as a major technique in the study of graphene, providing a quick and simple characterization of the graphene structural defects accordingly to the literature [63].SEM micrographs were obtained on a TM4000Plus microscope (Hitachi).The micrographs were recorder by a backscatter electrons detector for high contrast.Micro-Raman spectroscopy was performed using a Jobin Yvon (HORIBA) HR800 instrument, using a 530 nm laser wavelength as excitation source (Kimmon, IK series, Japan) and x100 objective (NA=0.9,Olympus, Japan).High resolution X-ray photoelectron spectroscopy (XPS) was recorded in an ultra-high vacuum system (base pressure of 200 Pa).The system combines a hemi-spherical electron energy analyzer (SPECS Phoibos 150), a delay-line detector and a monochromatic Al Kα X-ray source (1486,74 eV).The spectra were acquired at normal emission take-off angle and with a pass-energy of 20 eV.

RESULTS AND DISCUSSION
In Figure 4, it is possible to observe the differences between the Raman spectra of the as-prepared graphene (red line) and the functionalized graphene after 50 ALD cycles of Al2O3 (green line).Three important features were observed in the Raman spectra and they are common to both samples; Dband (around 1300 cm -1 ) is usually associated with the density of defects present in the graphene network, that is, the intensity of the D band is directly related to the concentration of defects.On the other hand, the G-band (around 1580 cm   of peaks at BEs related to carbon-oxygen species indicate a good quality sp 2 carbon and a low of the structure due to the transfer process.After ALD functionalization FWHM of C 1s peaks (black drawn) increase slightly, 0,63 eV on Cu and 0,69 eV on SiO2, and it appears a small shoulder at higher BE due to adventitious carbon contamination, more easily attached to Al2O3 than to Gr.
Figure 5B shows the energy regions corresponding to Al 2p and Al 2s before (red) and after (black) 10 cycles of ALD functionalization.In the case of Gr on SiO2 (down) symmetric peaks show up centered at 74,6 eV and 119,6 eV confirming the successful deposition of aluminum oxide [64].The case of Gr on Cu (top) is not so obvious due to the overlapping of Al 2s and Al 2p with Cu 3s and Cu 3p regions, respectively.Anyway, it is clear that the intensity of the component at the BE position of Al increase while the intensity coming from Cu peaks decrease.
ALD technique allows the control over the coverage of the sample.As it can be seen in Figure 5C, the increment of ALD cycles increase progressively the coverage with aluminum oxide until a point (50 cycles) at which the Cu substrate cannot be detected by XPS.
Figure 6A-B shows the difference between SEM performed on graphene sample and on SiO2/Si with a growth time of 5 min.In Figure 6A can notice that the graphene film (darker contrast) are continuous, uniform and clean without noticeable particles, but grain boundaries, Cu surface steps, and wrinkles are observed.Figure 6B shows that the graphene sheet has a much rougher surface, with some impurities and cracks.This roughness contrast can be explained as the topographic contrast that is the most frequent application of the SEM.
The results clearly demonstrate that graphene was successfully synthesized and transferred to the substrate.
Accordingly to the XPS studies, Al2O3 was successfully deposited on the graphene surface demonstrating the following two features: i) the graphene 2D nanomaterial is a suitable platform for the elaboration of nanostructures  comprised of graphene/metal oxide and ii) the ALD technique is a non-destructive approach for the graphene surface functionalization.The combination of these features provides the means to develop new nanocomposites for gas sensing applications.
From this perspective, a laboratory prototype device based on measuring electrical properties of the graphene sample as a function of the gas absorption is under development.The basic concept to create a system using a graphene layer to detect the presence of gases in the atmosphere is based on the fact that the graphene surface can absorb these gases and the gas particles will change its resistivity value.The first step was to create a suitable device to measure the resistance and its variation in a graphene layer.This must be cheap, affordable, easy to replicate and a flexible measuring system that can guarantee a good accuracy and low noise levels.Figures 7A shows our experimental device and 7B the connections with three cables for the graphene sensor to detect CO2 in the atmosphere.In order to create a sensor, it is important to supply the graphene layer with very low currents and voltages to avoid burning or damaging the sensor.
The measuring system (Figure 8) consists of a chip-board to provide the desired current and voltage signal, and hardware to connect the I/O of the board with the graphene sensor.The board chosen is a Cypress CY8CKIT-059 PSoC® 5LP that has been programmed using the PSOC Creator 4.2 software.
To reduce the noise, a real low-pass filter was built in the electric circuit on the bread-board, connecting a capacitor between the voltage output of the sensor and the ground.A led was also added in the circuit to signal the presence of CO2 in case the resistance value was going under a minimum threshold.
The possible solutions initially considered for the measurement hardware system were a four-points probe and a Wheatstone bridge architecture.The four-points probe is commonly used to measure sheet resistance of thin films,  Author: Marco Machesi.
particularly semiconductor thin films.An advantage for accurate measurement of low resistance values is the separation of current and voltage electrodes that eliminate the lead and contact existence from the measurement.Because, at this point it is not relevant to know with high accuracy the absolute value of the resistance but it is important to detect with low ratio noise/signal the resistivity variation of the graphene sample, the solution adopted was a simple three cables connection to the sensor (I+, I-and ground), as shown in Figure 7b.This method is easy to build, gives good results, is portable and cheap.
As we have demonstrated before, graphene was transferred to three substrates, however the experimental tests were made in glass (sensor 1) and plastic film (sensor 2).These features are explained in Table 3.
The perfect environment to perform the tests on the sensors would be a sealed box where it was possible to introduce specific amounts of CO2 or other gases, however, in this preliminary stage of the tests, they were performed in an open space (at the laboratory).
In order to analyze the results, the SerialChart application was used.It was connected to the same ComPort of the board and printed a graph of resistance value (blue) and voltage value (red) in real time (Figure 9).In brown is the current input.The tests were the same for both the sensors.The first step was to measure the resistivity under normal conditions and record their value.A glass bell was then placed on the top of the sensor and checked that no variations was presented on the resistivity.After that, the presence of CO2 and H2O vapor was tested.The tests were performed on different days and in two different laboratories.It was found that, due to the electrical sensitivity of the graphene, the resistivity value changed from one laboratory to the another and from one day to the next due to different atmospheric conditions.All the measurements were made with a current input of 5 µA.This value is high enough for a low noise signal rate and low enough not to degrade the samples.Because of the noise present in the system, values in an estimated range of ±120 Ω were considered.
Tests A and B were done using sensor 2 (graphene on thermal release tape) and test C using sensor 1 (graphene on glass).Table 4 show the sample resistance under normal conditions when the CO2 bell is applied and with CO2 and H2O vapor.
Test A was performed in the first laboratory and the sensor resistance under normal conditions was 9000 (±120) Ω.
Applying the glass bell with CO2, the value decreased immediately and after 15 seconds reached the stable value of 7400 (±120) Ω, representing a decrease of 17,8%.When the bell was removed, it took 4 minutes for the sensor to reach the initial value.Once the initial value was restored, the bell with CO2 and H2O vapor was placed, the resistance value started to increase and after 18 seconds reached the stable value of 10900 (±120) Ω.It increased approximately 21%.
When the bell was removed, more than 20 min were required to the sensor to reach the initial value.Author: Marco Machesi.
Test B was done with the same sensor but in the second laboratory.The sensor resistance in normal conditions was 8700 (±120) Ω. Applying the bell with CO2 the value was decreasing and reached the normal stable value after 12 seconds.The value decreased by 12,6% to 7600 (±120) Ω.
After the bell was removed, approximately 4 min were necessary to the sensor to reach the initial value.When the bell with CO2 and H2O vapor was placed, the value started to increase and after 18 seconds it reached the stable value of 10600 (±120) Ω, increasing approximately 22%.Again, when the bell was removed, more than 20 min were necessary to the sensor to reach the initial value.
The last test (test C) was performed again in the first laboratory with sensor A (graphene on glass) with an initial resistance of 3600 (±120) Ω. Applying the bell with CO2 and after 12 seconds it reached the stable value of 3200 (±120) Ω, decreasing approximately 11%. 3 min after the bell was removed, the sensor reached the initial value.Once the initial value has been restored, the bell with CO2 and H2O vapor has been placed and the value increase immediately.After 20 seconds it reached the stable value of 4300 (±120) Ω, an increase of 19,4% After the bell was removed more than 15 minutes were necessary to the sensor to reach the initial value.

CONCLUSION AND FUTURE WORK
With ever-growing environmental concerns, the detection and monitoring of various gaseous species are of critical importance.Indoor air pollutants might be originated from a range of sources.Most of these pollutants, namely CO, CO2, NOx, VOCs, PM10, PM2.5, relativity humidity and temperature are inhaled and affect human health.That way, the present paper reflects a preliminary study on the importance and the interest of the incorporation of modified graphene in sensors for gas monitoring, air quality and detection of potentially dangerous leaks.
Graphene has significant applications in electronics, presenting itself as a strong candidate in the replacement of silicon in future solid-state devices.However, there are barriers to be transposed, as is the case of the absence of an energy gap (bandwidth energy prohibited).Without the gap, you cannot turn semiconductor devices on and off.In this sense, it is highly desirable to introduce a banned energy band onto graphene in order to shape its transport properties.An adjustable band gap would then be desirable because it would allow great flexibility in design and optimization of such devices, particularly if such adjustment by applying a variable external electric field.
The results clearly demonstrate that graphene was successfully synthesized and transferred to the substrate and the successfully deposition of Al2O3 via ALD on the graphene surface and further demonstrating that the graphene surface functionalization is feasible.An easy way to improve the performance of the sensors presented will be by using insulated cables and better quality connections to reduce the noise present in the system.Although the tests were done in a non-optimal environment with multiple influencing factors and disturbances, it is possible to clearly detect the variations of gases in the air around the sensor, in particular CO2 and H2O vapor.These sensors cannot detect small variations of gas particles in the air but they are a starting point for further work toward this goal.From this perspective, a laboratory prototype device based on measuring electrical properties of the sample as a function of the gas absorption is under development.Also, as future work, additional research on the functionalization of graphene will be performed to improve the sensitivity and selectivity of the sensor for air pollutants such as for example VOCs, PM and CO.Finally, it should be noted that a method for the mass production of graphene has not yet been identified being also a future work to be considered.

- 1 )
is associated to the in-plane vibration of the sp 2 carbon atoms.Finally the 2D-band (around 2700 cm -1 ) results from a second-order process [41].These results indicate high-quality graphene grown on Cu emphasized by the less pronounced presence of the D band when compared with the other two bands (G and 2D bands).Most importantly, the ALD of Al2O3 does not affect the intrinsic properties of the graphene demonstrated by the similarities of the Raman measurements.This finding also suggests that the ALD is a non-destructive technique to functionalize this type of nanomaterials.These modes are present in all graphene-based materials, however, their frequencies, intensities, and line widths are influenced by other factors, such as the number of graphene layers, extern doping or laser excitation energy.

Figure 4 :
Figure 4: Raman spectra of the as-prepared graphene (red line) and after 50 ALD cycles with Al2O3 (green line).The Raman spectra was acquired at different points of the samples.Author: Ricardo Silva.

Figure 7 :
Figure 7: A) Experimental device, B) the connection with three cables for the graphene sensor.Authors: Maria J. Hortigüela and Gonzalo Otero-Irurueta.

Figure 8 :
Figure 8: The bread-board integrating the controller-board and the electric circuit.

Figure 9 :
Figure 9: Chart print in the SerialChart program in real time.

Table 2 : Comparisons on Different Aspects of Methods for Graphene Synthesis [42] Synthesis methods Precursors Layer characters Advantages Disadvantages
dimension in several µm Low cost; large-scale production; high quality (plasma etching) Time-consuming; complicated process Electrochemical method Graphite Single and multiple dimension; hundreds of nm to ca. 10 µm Low cost; high quality Low yields Total organic synthesis PAHs Single layer, dimension less than 20 nm High quality precisely defined structures High cost; limited size range; complicated process