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Article Theoretical and Experimental Research on Sensing Properties of Sulfur-Doped Oxide

Yao Yu 1,2,3,4 , Zhijia Liao 1, Fanli Meng 1,2,3,* and Zhenyu Yuan 1,2,3

1 The College of Information Science and Engineering, Northeastern University, Shenyang 110819, China; [email protected] (Y.Y.); [email protected] (Z.L.); [email protected] (Z.Y.) 2 Hebei Key Laboratory of Micro-Nano Precision Optical Sensing and Measurement Technology, Qinhuangdao 066004, China 3 Key Laboratory of Data Analytics and Optimization for Smart Industry, Northeastern University, Ministry of Education, Shenyang 110819, China 4 School of Electronic Information Engineering, Ningxia Institute of Science and Technology, Shizuishan 753000, China * Correspondence: [email protected]

Abstract: In this paper, gas sensing characteristics of sulfur-doped graphene oxide (S-GO) are firstly presented. The results of the sensing test revealed that, at room temperature (20 ◦C), S-GO has

the optimal sensitivity to NH3. The S-GO gas has a relatively short response and recovery time for the NH3 detection. Further, the sensing limit of ammonia at room temperature is 0.5 ppm. Theoretical models of graphene and S-doped graphene are established, and electrical properties of the graphene and S-doped graphene are calculated. The enhanced sensing performance was ascribed to the electrical properties’ improvement after the graphene was S-doped.   Keywords: ammonia sensor; room temperature; graphene oxide with sulfur Citation: Yu, Y.; Liao, Z.; Meng, F.; Yuan, Z. Theoretical and Experimental Research on Ammonia Sensing Properties of Sulfur-Doped 1. Introduction Graphene Oxide. Chemosensors 2021, Ammonia, as a harmful gas, not only damages the environment and endangers human 9, 220. https://doi.org/10.3390/ health, but is also one of the contributors to PM2.5. At present, the increase in ammonia chemosensors9080220 content in the environment is mainly due to direct or indirect human activities. According to the report of the European Union, the quality of ammonia emitted to the environment is Academic Editor: Eleonora Alfinito about 20 to 30 million tons every year. In recent years, with the further development of the chemical industry and agriculture, people’s awareness of environmental protection Received: 2 July 2021 has also been increasing, thus the demand for monitoring ammonia concentration is also Accepted: 10 August 2021 Published: 11 August 2021 increasing [1–3]. On the other hand, ammonia will do harm to human health. Research shows that the concentration of ammonia must be lower than 20 ppm in the environment

Publisher’s Note: MDPI stays neutral wherein humans work, so that it will not affect human health. Therefore, it is necessary to with regard to jurisdictional claims in detect low concentrations of ammonia [4–6]. Currently, the common ammonia detection published maps and institutional affil- methods are optical [7], calorimetric [8], gasphase chromatography, and acoustic methods. iations. These methods require special instruments and equipment, and problems exist such as high cost, large size, inconvenience in use, inability to monitor in real time, and difficulty in widespread. These methods require special instruments and equipment, which are costly, bulky, inconvenient, cannot be monitored in real time, and difficult to apply widely. The gas sensitive sensor can effectively overcome the problems of these traditional methods Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. and is a promising method for ammonia detection [9]. This article is an open access article Graphene is a two-dimensional planar material formed by the close arrangement distributed under the terms and of [10–12]. Since its discovery in 2004, graphene has attracted a lot of at- conditions of the Creative Commons tention owing to its excellent physical and chemical properties, and has been further Attribution (CC BY) license (https:// applied to many industries and research fields. Graphene has good electrical conductivity, creativecommons.org/licenses/by/ high mobility at room temperature (RT), high mechanical strength, and high 4.0/). strength and toughness in the discovered materials [13–15]. In terms of thermal properties,

Chemosensors 2021, 9, 220. https://doi.org/10.3390/chemosensors9080220 https://www.mdpi.com/journal/chemosensors Chemosensors 2021, 9, 220 2 of 15

graphene has high , which is the highest among known carbon ma- terials. Graphene is composed of carbon atoms, so its chemical properties are relatively stable. At the same time, it can adsorb and desorb many atoms and [16–18]. On the one hand, its characteristics can be changed by modification and doping [19–21]. On the other hand, it can be used as a sensitive material for gas sensor to adsorb and detect gas molecules. Because of the above excellent properties, graphene has been widely used in basic research [22], various [23], micro , new energy batteries [24], aerospace materials, and other fields [25]. Few studies exist on sulfur-doped graphene oxide . For example, Yang et al. reported S-doped GNs, derived from of graphene-oxide (GO) and benzyl disulfide, as a highly efficient metal-free catalyst for alkaline medium [26]. Choi et al. prepared N- and S-doped via the of cysteine [27]; however, the applicability of these approaches is significantly limited by the issues of high cost and inability to scale up. In this work, sulfur-doped graphene oxide (S-GO) was synthesized. The detection limit of ammonia was down to 0.5 ppm and the working temperature was room temperature. The synthesized materials were characterized by scanning electron microscopy (SEM) and Fourier transform absorption (FTIR), which proved the successful doping of sulfur. Then, the gas sensing performance test is carried out, including the sensitivity test of ammonia, concentration gradient test, and selectivity and stability test. The experimental results show that the material has a better response to ammonia and can detect ammonia at room temperature. At the same time, it has a lower response to other gases and can detect ammonia in a complex gas environment, which has a certain practical value.

2. Materials and Methods 2.1. Materials

Graphite powder (99%), concentrated (H2SO4, 98%), perman- ganate (KMnO4, 99%), concentrated hydrochloric acid (HCl, 35–36%), peroxide (H2O2, 30%), thiourea (CH4N2S, 99%), and (99%). All chemicals were purchased from Shandong Xiya reagent, and were of analytical grade and could be used without further purification.

2.2. Synthesis of S-GO Graphene oxide was synthesized by the improved Hummer method [28,29]. As shown in Figure1a, with continuous stirring in an bath, 3 g of powder was added into 150 mL of 98% concentrated sulfuric acid, then 9 g of was slowly added to the solution while the temperature of the solution was lower than 5 ◦C. In the next step, the mixed solution was transferred to the water bath system and stirred at 40 ◦C for 30 min. Then, 300 mL distilled water was gradually added to the solution and kept at 95 ◦C for 15 min under continuous stirring. Then, 500 mL distilled water and 50 mL were added to stop the chemical reaction. The obtained substance was washed several times with diluted HCl solution and pure water until the PH was close to 7. The obtained graphene oxide was dispersed in water to obtain graphene oxide aqueous solution. Finally, the obtained aqueous solution was centrifuged to remove the unreacted graphite. Chemosensors 2021, 9, x FOR PEER REVIEW 3 of 15

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(a)

(b)

Figure 1. Schematic illustration of (a) the synthesis of graphene oxide and S-doped graphene andand ((b)) fabricated gasgas sensorsensor based onon S-dopedS-doped graphene.graphene.

After oxidation treatment, waswas intercalated by strong oxidant toto form graphene oxide flakes,flakes, and hydroxyl and epoxyepoxy groupsgroups werewere randomlyrandomly distributeddistributed onon graphene oxideoxide flakes.flakes. A A stable, stable, light brown brown yellow yellow single-layer single-layer graphene graphene oxide oxide suspension suspen- wassion formedwas formed in water. in water. Owing to to the the reducibility reducibility of of the the sulfur sulfur source, source, the thehydrothermal hydrothermal method method at high at hightem- temperatureperature and andhigh high pressure pressure will willlead lead to the to thereduction reduction reaction reaction of graphene of graphene oxide oxide and and the thenumber number of -containing of oxygen-containing groups groups on the on thesurface surface will will be begreatly greatly reduced, reduced, resulting resulting in inthe the decrease decrease in gas in gasadsorption capacity. capacity. At the At same the time, same the time, conditions the conditions at high temper- at high temperatureature and high and pressure high pressure will lead will to the lead agglomeration to the agglomeration of graphene, of graphene, which will which reduce will its reducespecific itssurface specific area. surface Thus, area. we choose Thus, weto add choose the sulfur to add source the sulfur into sourcegraphene into oxide graphene solu- oxidetion at solution room temperature at room temperature and stir it andfor a stir long it fortime a longto make time the to sulfur make theelement sulfur adhere element to adherethe surface to the of graphene surface of oxide graphene to obtain oxide S-GO to obtain as S-1. S-GO The specific as S-1. synthesis The specific method synthesis is as methodfollows: istake as 30 follows: mL graphene take 30 mLoxide graphene solution oxide of 1 mg/mL, solution disperse of 1 mg/mL, it evenly disperse by , it evenly byadd ultrasound, excessive thiourea, add excessive stir it thiourea, vigorously stir itfo vigorouslyr 12 h at room for 12 temperature, h at room temperature, then wash thenand washcentrifuge and centrifugethe mixed solution, the mixed finally solution, get the finally muddy get thesubstance, muddy and substance, carry out and the carry next outgas thesensitivity next gas test. sensitivity test. 2.3. Sensor Fabrication and Gas Sensor Measurement System 2.3. Sensor Fabrication and Gas Sensor Measurement System The schematic illustration of sensor fabrication can be depicted in Figure1b. The as- The schematic illustration of sensor fabrication can be depicted in Figure 1b. The as- prepared GO and S-GO were dispersed in ethanol and dropped on tube substrate prepared GO and S-GO were dispersed in ethanol and dropped on ceramic tube substrate with gold electrodes by rubber tipped dropper separately. When the materials were dried at room temperature, the ceramic tube was connected to the test system for testing.

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Chemosensors 2021, 9, 220 4 of 15 with gold electrodes by rubber tipped dropper separately. When the materials were dried at room temperature, the ceramic tube was connected to the test system for testing. The test system used in this paper is shown in Figure 2. The main components include syntheticThe testair systemcylinder, used gas in chamber, this paper micro is shown injector, in Figure data 2source. The main table, components and computer. include The syntheticfunction of air synthetic cylinder, air gas is chamber, to clean the micro gas injector, chamber data at the source beginning table, and of the computer. experiment, The functionremove the of syntheticimpurity airgas isin to the clean gas thechamber, gas chamber and then at blow the beginning in after the of test the experiment,to eliminate removethe test gas. the impurity The gas chamber gas in the is gas a closed chamber, space and with then a blowvolume in afterof 1 L. the By test blowing to eliminate synthetic the testair and gas. a The certain gas chambervolume of is measured a closed space gas, withthe gas a volume environment of 1 L. with By blowing different synthetic concentra- air andtions a is certain simulated. volume The of data measured source gas,table the and gas computer environment provide with test different power concentrationsand record the isexperimental simulated. results. The data source table and computer provide test power and record the experimental results.

Figure 2. Gas sensing test system.

Firstly, thethe sensorsensor waswas connectedconnected toto thethe systemsystem andand stablestable resistanceresistance inin thethe airair waswas obtained, thenthen thethe syringe syringe sucked sucked a certaina certain concentration concentration of gasof gas to be to measured be measured and injectedand in- itjected into it the into gas the chamber, gas chamber, the resistance the resistance of the of sensor the sensor changed, changed, and finally and finally the computer the com- recordedputer recorded the response the response curve tocurve the to gas. the All gas. gas All sensitivity gas sensitivity tests were tests carriedwere carried out at out room at temperatureroom temperature (20 ◦C) (20 and °C) 30% and relative 30% relative humidity. humidity. The measured gas gas concentration concentration is is calculated calculated by by Equation Equation (1). (1). C isC theis target the target gas con- gas concentration,centration, Vi isV thei is thegas gasvolume volume in the in thesyringe, syringe, VC isV Ctheis thegas gaschamber chamber volume, volume, P0 isP 0theis ◦ thevapor vapor pressure pressure of the of thetarget target gas gasat room at room temperature temperature (20 (20°C), C),and and Pa isP atheis the standard standard at- atmosphericmospheric pressure. pressure. Put Put the the saturated saturated solution solution of organic of organic gas gasinto into the reagent the reagent bottle bottle and andwait waitfor a for long a long enough enough time, time, the saturated the saturated vapor vapor will willform form inside inside the reagent the reagent bottle. bottle. Ac- Accordingcording to the to the formula, formula, the the gas gas volume volume required required for fordifferent different gasgas concentrations concentrations in the in thegas gaschamber chamber can be can calculated. be calculated. A certain A certain volume volume of gas ofcan gas be canextracted be extracted using a usingmicro asyringe micro syringeand injected and injectedinto the intogas chamber, the gas chamber, that is, it that is convenient is, it is convenient to test the to organic test the gas organic of different gas of differentconcentrations. concentrations. P ×V C = 0 i (1) P P×0V×Vi C=a c (1) The response of the gas sensor is definedP asa×V Equationc (2). S is the response to the gas sensor.TheR responsea is the stable of the resistance gas sensor of is the defined sensor as exposed Equation to (2). air. SR isg isthe the response resistance to the of thegas sensor in target gas. The response time is the time between the gas in and the resistance sensor. Ra is the stable resistance of the sensor exposed to air. Rg is the resistance of the reachingsensor in 90%target of gas. the The stable response resistance. time is The the time time between between thethe airgas in in andand thethe resistanceresistance recoveringreaching 90% to 90%of the of stable the original resistance. resistance The time in air between is defined the as air the in recovery and the time.resistance re- covering to 90% of the original resistance in air is defined as the recovery time. Ra−Rg S = ×100% (2) Ra

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3. Results and Discussion 3.1. Characterization The scanning (SEM) model is SSX-550, which is manufactured by Shimadzu company of Japan. Main specifications and technical indicators: (1) accelerating voltage: 0.5–30 kV, 10 V/step; (2) magnification: 20–300,000 times. It is mainly used for the surface microscopic observation and composition analysis of inorganic solid materials such as , metals, and fiber composites, and the surface morphology is observed by secondary electron image. The surface micro region was analyzed qualitatively and quantitatively by backscattered electron image and X-ray energy spectrum. The model of Fourier transform infrared spectrometer (FTIR) is VERTEX 70, which is developed and manufactured by Bruker company in Germany. Main technical indicators: (1) spectral measurement range: 7800~230 cm−1 (1.5~50 µm); (2) spectrometer resolution: better than 0.16 cm−1; (3) low temperature measurement temperature: 10 K to room tem- perature, ±0.2 K controllable. It is mainly used for structural identification and quantitative analysis of optical thin film materials and materials. As shown in Figure3, the synthesized sulfur-doped graphene was characterized by SEM and EDS. Figure3a is the SEM of graphene oxide. It can be seen that graphene has a planar structure [30,31]. At the same time, under the action of surface tension, the surface of graphene oxide forms wrinkles. Because graphene is very thin, the SEM image was translucent. Figure3b shows the graphene oxide after doping sulfur. Compared with the original graphene oxide, the wrinkles in S-1 increased. However, compared with the hydrothermal method, the synthesis method under normal temperature and pressure does not lead to agglomerated and broken materials, so it may be helpful for the detection of ammonia. Figure3c,d show the EDS sampling points and the corresponding element contents, respectively. Carbon and oxygen are the components of graphene oxide, and element sulfur was detected, which indicated the successful synthesis of sulfur-doped graphene oxide. In addition, the reason for the detection of was that the dispersed samples need to be coated on the silicon for characterization, so element silicon was detected. Table1 lists the content percentage of different elements.

Table 1. Element content of S-GO.

Element Weight% % C 69.42 83.05 O 3.39 3.05 Si 27.10 13.86 S 0.09 0.04 Total 100.00 100.00

The FTIR image of GO and S-1 is shown in Figure4. The black curve represents the spectrum of GO and the blue curve represents the spectrum of S-1. It can be seen from the figure that strong characteristic peaks can be observed at 1600 cm−1, which is the characteristic of C=C vibration [32]. At the same time, strong characteristic peaks can also be seen around 1300 cm−1, which is the result of C-OH vibration [33]. Combined with the electron microscope, it can be shown that the main component of the synthesized material is graphene oxide. However, at 1250 cm−1, there is a characteristic peak in S-1 that is not found in GO, which is the result of S-O vibration [34], indicating that the combination of sulfur atoms and materials is successful. On the one hand, the intensity of the characteristic peak is small, which indicates that the sulfur content is low; on the other hand, the incorporation of sulfur does not affect the characteristics of materials significantly. The characteristic peaks in FTIR are summarized in Table2. Chemosensors 2021, 9, x FOR PEER REVIEW 5 of 15

Ra-Rg S= ×100% (2) Ra

3. Results and Discussion 3.1. Characterization The scanning electron microscope (SEM) model is SSX-550, which is manufactured by Shimadzu company of Japan. Main specifications and technical indicators: (1) acceler- ating voltage: 0.5–30 kv, 10 V/step; (2) magnification: 20–300,000 times. It is mainly used for the surface microscopic observation and composition analysis of inorganic solid ma- terials such as ceramics, metals, and fiber composites, and the surface morphology is ob- served by secondary electron image. The surface micro region was analyzed qualitatively and quantitatively by backscattered electron image and X-ray energy spectrum. The model of Fourier transform infrared spectrometer (FTIR) is VERTEX 70, which is developed and manufactured by Bruker company in Germany. Main technical indica- tors: (1) spectral measurement range: 7800~230 cm−1 (1.5~50 µm); (2) spectrometer resolu- tion: better than 0.16 cm−1; (3) low temperature measurement temperature: 10 K to room Chemosensors 2021, 9, x FOR PEER REVIEWtemperature, ±0.2 K controllable. It is mainly used for structural identification and quan- 6 of 15 titative analysis of optical materials and semiconductor materials. As shown in Figure 3, the synthesized sulfur-doped graphene was characterized by SEM and EDS. Figure 3a is the SEM of graphene oxide. It can be seen that graphene has a planar structure [30,31]. At the same time, under the action of surface tension, the surface of graphene oxide forms wrinkles. Because graphene is very thin, the SEM image was translucent. Figure 3b shows the graphene oxide after doping sulfur. Compared with the original graphene oxide, the wrinkles in S-1 increased. However, compared with the hy- drothermal method, the synthesis method under normal temperature and pressure does not lead to agglomerated and broken materials, so it may be helpful for the detection of ammonia. Figure 3c,d show the EDS sampling points and the corresponding element con- tents, respectively. Carbon and oxygen are the components of graphene oxide, and ele- ment sulfur was detected, which indicated the successful synthesis of sulfur-doped gra- phene oxide. In addition, the reason for the detection of silicon was that the dispersed Chemosensors 2021, 9, 220 samples need to be coated on the silicon wafer for characterization, so element silicon was6 of 15 detected. Table 1 lists the content percentage of different elements.

Figure 3. SEM images of (a) GO and (b) S-GO. (c) EDS spectrum and (d) element content of S-GO.

Table 1. Element content of S-GO.

Element Weight% Atom% C 69.42 83.05 O 3.39 3.05 Si 27.10 13.86 S 0.09 0.04 Chemosensors 2021, 9, x FOR PEER REVIEW Total 100.00 100.006 of 15

The FTIR image of GO and S-1 is shown in Figure 4. The black curve represents the

spectrum of GO and the blue curve represents the spectrum of S-1. It can be seen from the figure that strong characteristic peaks can be observed at 1600 cm−1, which is the charac- teristic of C=C vibration [32]. At the same time, strong characteristic peaks can also be seen around 1300 cm−1, which is the result of C-OH vibration [33]. Combined with the electron microscope, it can be shown that the main component of the synthesized material is gra- phene oxide. However, at 1250 cm−1, there is a characteristic peak in S-1 that is not found in GO, which is the result of S-O vibration [34], indicating that the combination of sulfur atoms and materials is successful. On the one hand, the intensity of the characteristic peak is small, which indicates that the sulfur content is low; on the other hand, the incorpora- tion of sulfur does not affect the characteristics of materials significantly. The characteris- FigureFigure 3. 3.SEM SEMtic images imagespeaks of ofin ( a( a)FTIR) GO GO andand are ((b bsummarized)) S-GO.S-GO. (c) EDS inspectrum spectrum Table and and2. ( (dd) )element element content content of of S-GO S-GO..

Table 1. Element content of S-GO.

Element Weight% Atom% C 69.42 83.05 O 3.39 3.05 Si 27.10 13.86 S 0.09 0.04 Total 100.00 100.00

The FTIR image of GO and S-1 is shown in Figure 4. The black curve represents the spectrum of GO and the blue curve represents the spectrum of S-1. It can be seen from the figure that strong characteristic peaks can be observed at 1600 cm−1, which is the charac- teristic of C=C vibration [32]. At the same time, strong characteristic peaks can also be seen around 1300 cm−1, which is the result of C-OH vibration [33]. Combined with the electron microscope, it can be shown that the main component of the synthesized material is gra- phene oxide. However, at 1250 cm−1, there is a characteristic peak in S-1 that is not found in GO, which is the result of S-O vibration [34], indicating that the combination of sulfur atoms and materials is successful. On the one hand, the intensity of the characteristic peak is small, which indicates that the sulfur content is low; on the other hand, the incorpora- tion of sulfur does not affect the characteristics of materials significantly. The characteris- tic peaks in FTIR are summarized in Table 2. FigureFigure 4. FTIR 4. FTIR spectra spectra of of sGO sGO and and S-1.

Table 2. Detected signals in the FTIR spectra.

Wavenumber (cm−1) 1250 1300 1600 Chemical bond S-O C-OH C=C

Figure 4. FTIR spectra of sGO and S-1.

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Table 2. Detected signals in the FTIR spectra.

Wavenumber (cm−1) 1250 1300 1600 Chemosensors 2021, 9, 220 Chemical bond S-O C-OH C=C 7 of 15

3.2. Gas Sensing Properties 3.2. GasAs shown Sensing in Properties Figure 5, the gas sensing properties of GO and S-GO were tested. Firstly, it can be seen that the resistance of the S-GO is high. This is because the surface of gra- As shown in Figure5, the gas sensing properties of GO and S-GO were tested. Firstly, phene oxide is rich in oxygen-containing groups, which will greatly increase the resistance it can be seen that the resistance of the S-GO is high. This is because the surface of graphene ofoxide S-GO. is richThe inroom oxygen-containing temperature synthesis groups, whichmethod will used greatly can avoid increase the the agglomeration resistance of S-GO.phe- nomenonThe room on temperature the surface synthesis of graphene, method but graphe used canne avoidoxide theis not agglomeration reduced, so the phenomenon resistance ofon the the material surface is of high. graphene, Figure but 5b grapheneshows the oxide response is not curve reduced, of material so the S-GO resistance to 10 ofppm the ammoniamaterial isat high. room Figure temperature.5b shows It the can response be seen curve that the of material corresponding S-GO to value 10 ppm is ammonia73%. The responseat room temperature.time represents It canthe betime seen required that the for corresponding the resistance valueto reach is 73%.a stable The state response after thetime gas represents is injected, the and time the required recovery for time the repres resistanceents the to reachtime required a stable statefor the after resistance the gas to is recoverinjected, after and the the synthetic recovery air time is injected. represents As the shown time in required Figure 5b, for the resistanceresponse time to recover is 112 safter and thethe syntheticrecovery airtime is injected.is 33 s, which As shown are improved in Figure 5comparedb, the response with the time characteristics is 112 s and the of GO.recovery Figure time 5a isshows 33 s, whichthe response are improved curve of compared GO to 100 with ppm the ammonia. characteristics It can of be GO. seen Figure that the5a showsS-GO has the better response sensitivity, curve of response GO to 100 time, ppm and ammonia. recovery It time can bethan seen GO, that so theS-doped S-GO improvehas better the sensitivity, sensitivity response to ammonia. time, and recovery time than GO, so S-doped improve the sensitivity to ammonia.

(a) (b)

FigureFigure 5. 5. ResponseResponse curves curves comparison comparison of of ( (aa)) GO GO and and ( (bb)) S-1. S-1.

FigureFigure 6 shows the gradient test of S-1 for differentdifferent concentrationsconcentrations ofof ammoniaammonia andand thethe detection detection limit limit of of S-1 S-1 for for ammonia. ammonia. As As shown shown in in Figure Figure 66a,a, at room temperature, firstfirst injectinject 0.5 0.5 ppm ppm ammonia, ammonia, the the material material resistance resistance decreases decreases slightly, slightly, and and then then continuously continuously increaseincrease the the ammonia ammonia concentration concentration until until th thee ammonia ammonia concentration concentration in in the the gas gas chamber chamber reachesreaches 1010 ppm. ppm. ItIt can can be be seen seen that that the the sensor sensor responds responds to different to different concentrations concentrations of am- of monia.ammonia. Figure Figure 6b 6showsb shows the the lower lower detection detection limit limit of of S-1 S-1 material. material. The The lowest ammonia concentrationconcentration of of 0.5 0.5 ppm ppm can change the material resistance,resistance, andand thethe correspondingcorresponding sensi- sen- sitivitytivity is is 5%. 5%. The The lowest lowest ammonia ammonia concentration concentration detected detected by by GO GO is 1is ppm, 1 ppm, which which indicates indi- catesthat sulfurthat sulfur doping doping can reduce can reduce the detection the detection limit limit of the of sensor. the sensor.

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(a) (b)

(c)

FigureFigure 6. (6.a )(a Response) Response curve curve of of S-1 S-1 to to gradient gradient concentrations concentrations ofof ammonia, ( (bb)) detection detection limit limit test test of of S-1 S-1 to to ammonia, ammonia, and and (c)( sensitivityc) sensitivity curve curve of of S-1 S-1 to to different different concentrations concentrations of of ammonia. ammonia.

SelectivitySelectivity isis alsoalso importantimportant for gas gas sensors. sensors. In In practical practical application, application, the the gas gas environ- environ- mentment of of the the sensor sensor isis moremore complex,complex, and ther theree may may be be a a variety variety of of gases. gases. For For the the ammonia ammonia sensor,sensor, it it should should be be ableable toto detect ammonia from from a a variety variety of of volatile volatile gases, gases, which which requires requires thethe sensor sensor to to havehave excellentexcellent selectivity. Therefore, Therefore, the the selectivity selectivity of of sulfur-doped sulfur-doped graphene graphene isis tested tested in in this this chapter,chapter, andand thethe testtest result resultss are are shown shown in in Figure Figure 7a.7a. Considering Considering the the types types ofof gases, gases, they they can can be be divided divided into into alcohols, alcohols, ketones, ketones, aldehydes, aldehydes, acids, acids, , benzenes, and and so so on, soon, the so control the control gases gases for testing for testing are formic are form acid,ic ethanol, acid, ethanol, methanal, methanal, , benzene, and 2-butanone. and 2- Thebutanone. sensitivity The of sensitivity S-1 to the of same S-1 concentrationto the same concentration of ammonia isof 73%,ammonia and the is 73%, other and gases the are lessother than gases 20%, are so less it has than good 20%, selectivity so it has good for ammonia. selectivity Thefor ammonia. six gases (formicThe six acid,gases ammonia,(formic ethanol,acid, ammonia, methanal, ethanol, benzene, methanal, and 2-butanone) benzene, and are 2-butanone) mixed and injectedare mixed into and the injected gas chamber, into andthe thegas responsechamber,curve and the is shownresponse in Figurecurve is7b. sh Itown can in be Figure seen that 7b. theIt can sensor be seen has that a certain the responsesensor has to thea certain mixed response gas, but to other the mixed interfering gas, but gases other have interfering a certain gases impact have on a the certain ability ofimpact S-1 to on detect the ability ammonia, of S-1 and to detect the sensitivity ammonia, to and ammonia the sensitivity is reduced. to ammonia Figure7b is shows reduced. that S-1Figure can distinguish7b shows that the S-1 existence can distinguish of ammonia the existence in different of ammonia interfering in different gases. interfering gases.

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(a) (b) (a) (b) Figure 7. (a) Selectivity test of S-1, (b) response curve of S-1 to mixed gas. FigureFigure 7. 7.( (aa)) SelectivitySelectivity test of S-1, S-1, ( (bb)) response response curve curve of of S-1 S-1 to to mixed mixed gas. gas.

AsAsAs shown shownshown in in Figure Figure Figure 8,8,8, thethe the repeatability repeatability ofof of S-1S-1 S-1 waswas was tested tested tested in in inRT RT RTand and and 30% 30% 30% RH. RH. RH. First, First, First, 1 1 ppm1ppm ppm ammonia ammonia ammonia was was was injected. injected. injected. AfterAfter After the thereaction reaction waswas was stable,stable, stable, air air was was air blown was blown blown in, in, and and in, the andthe re- re- the sistanceresistancesistance of of ofthe the the sensor sensor sensor waswas was restored.restored. restored. Then, Then, 11 ppm 1ppm ppm ammoniaammonia ammonia was was was injected injected injected and and and the the the process process process waswaswas repeated repeatedrepeated three threethree times. times. TheThe testtest test result result showed showed thatthat that this this this kind kind kind of of of gas gas gas sensor sensor sensor has has has excellent excellent excellent repeatability.repeatability.repeatability.

Figure 8. Repeatability test of S-1. FigureFigure 8. 8. RepeatabilityRepeatability test test of of S-1. S-1. 3.3. Theoretical Basis and Computational Method 3.3.3.3. Theoretical Theoretical Basis Basis and and Computational Computational Method Method The energy band diagram and density of states diagram of the two materials were The energy band diagram and density of states diagram of the two materials were obtainedThe energy by simulation band diagram and theoretical and density calculation. of states From diagram the perspectiveof the two materialsof theoretical were obtainedobtainedcalculation, by by thesimulation simulation gas sensing and and mechanism theoretical theoretical of calculation. calculation.the two materials From From isthe the analyzed. perspective perspective On the of of one theoretical theoretical hand, calculation,calculation,the calculation the the gasresults gas sensing sensing can bemechanism mechanism verified by of of the the the experimental two two materials materials results; is is analyzed. analyzed. on the Onother On the the hand, one one hand, the hand, thetheexperimental calculation calculation directionresults results can cancan be bebe verified verifiedguided by by thethe the calculationexperimental experimental results. results; results; on on the the other other hand, hand, the the experimentalexperimentalThe traditional direction direction gas can can sensing be be guided guided mechanism by by the the is calculation calculation interpreted results. results. as oxygen anion theory, which can TheonlyThe traditional traditionalqualitatively gas gas describe sensing sensing and mechanism mechanism explain the is is interpretedinteraction interpreted between as as oxygen oxygen the anion anionsensitive theory, theory, material which which cancan only only qualitatively qualitatively describe describe and and explain explain the the interaction interaction between between the the sensitive sensitive material material and the measured gas. The first principle uses quantum mechanics to analyze the properties of atoms and molecules by calculating the interaction between nuclei and , and

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and the measured gas. The first principle uses quantum mechanics to analyze the proper- Chemosensors 2021, 9, 220 10 of 15 ties of atoms and molecules by calculating the interaction between nuclei and electrons, and finally obtains the properties of macroscopic materials. From the micro point of view of the material analysis and calculation, without the help of empirical parameters, we can finallyobjectively obtains reflect the the properties characteristics of macroscopic of the material materials. after doping. From the By micro comparing point the of viewcalcu- oflation the material results of analysis the material and calculation,before and afte withoutr doping, the helpwe can of obtain empirical the parameters,influence of dop- we caning objectively on the material reflect characteristics. the characteristics In addition, of the material the electrical after doping. properties By comparing of the material the calculationafter gas adsorption results of thecan material be calculated, before so and as afterto analyze doping, the we adsorption can obtain process the influence and sensi- of dopingtive mechanism on the material of thecharacteristics. material to the Intarget addition, gas. the electrical properties of the material after gasIn adsorptionthis work, the can CASTEP be calculated, module so as of to Materi analyzeals theStudio adsorption 8.0 is used process to model and sensitive and cal- mechanismculate the sulfur-doped of the material graphene to the target [35,36], gas. and the corresponding energy band diagram, densityIn this of state work, (DOS), the CASTEP and partial module wave of density Materials of state Studio diagram 8.0 is (PDOS) used to are model obtained. and calculateThe results the of sulfur-doped the calculation graphene and experiment [35,36], and show the correspondingthat doping can energy improve band the diagram, semicon- densityductor ofproperties state (DOS), of graphene. and partial At wave the same density time, of statethe adsorption diagram (PDOS) relationship are obtained. between The am- resultsmonia of and the the calculation two materials, and experiment as well as show the corresponding that doping can energy improve band the diagram, semiconductor density propertiesof state, and of graphene. partial wave At thedensity same of time, state the diagram adsorption of the relationship two materials between after adsorption, ammonia andwere the calculated. two materials, Compared as well aswith the correspondingthe original graphene, energy band the diagram, doped densitygraphene of state,has a andstronger partial adsorption wave density capacity of state for ammonia, diagram of which the two indicates materials that afterthe doped adsorption, elements were en- calculated.hance the ability Compared of graphe withne the to originaldetect ammonia. graphene, the doped graphene has a stronger adsorption capacity for ammonia, which indicates that the doped elements enhance the ability3.4. Calculations of graphene and to Analyses detect ammonia. 3.4. CalculationsAs shown andin Figure Analyses 9, the models of GO and sulfur-doped graphene oxide( S-GO) were established for optimization [37–39]. The carbon atom is gray, the oxygon atom is As shown in Figure9, the models of GO and sulfur-doped graphene oxide( S-GO) red, the hydrogen atom is white, and the sulfur atom is yellow. First of all, the bond length were established for optimization [37–39]. The carbon atom is gray, the oxygon atom is red, between sulfur atoms and carbon atoms changes after the optimization. From the side the hydrogen atom is white, and the sulfur atom is yellow. First of all, the bond length view, it can be seen that the original plane structure of graphene has changed. Sulfur at- between sulfur atoms and carbon atoms changes after the optimization. From the side oms protrude slightly from the plane. This is because sulfur atoms are larger than carbon view, it can be seen that the original plane structure of graphene has changed. Sulfur atoms atoms, so the force between atoms protrudes from the plane, resulting in the S-C bond protrude slightly from the plane. This is because sulfur atoms are larger than carbon atoms, length (1.739 Å) being longer than the C-C bond (1.420 Å). It is slightly larger, but it does so the force between atoms protrudes from the plane, resulting in the S-C bond length (1.739not affect Å) being the stabilit longery than of the the structure. C-C bond (1.420 Å). It is slightly larger, but it does not affect the stability of the structure.

(a) (b)

FigureFigure 9. 9.(a ()a Planform) Planform of GOof GO module module after after geometry geometry optimization optimization and (b and) planform (b) planform of S-GO of module S-GO aftermodule geometry after geometry optimization. opti- mization. Figure 10a,b show the band structure diagram of GO and S-GO. The conduction band and valenceFigure band 10a,b of show intrinsic the band graphene structure intersect diagram near of the GO Fermi and S-GO. level, whichThe conduction indicates thatband intrinsicand valence graphene band is of a intrinsic zero band graphene gap material intersect and near is not the suitable Fermi aslevel, a sensitive which indicates material forthat gasintrinsic detection. graphene When is sulfur a zero atoms band are gap added, material the and energy is not levels suitable increase as aobviously, sensitive material which indicatesfor gas thatdetection. a large When number sulfur of impurity atoms energyare added, levels the are energy added andlevels more increase electronic obviously, states arewhich provided, indicates which that is a conducivelarge number to theof impurity electron transferenergy levels between are theadded material and more and gaselec- molecules;tronic states secondly, are provided, a band which gap of is about conducive 0.557 eV to appearsthe electron at the transfer intersection between ofconduction the material band and valence band, which indicates that sulfur-doped graphene is a semiconductor and can be used as a sensitive material for gas detection; finally, the Fermi level drops to the valence band, which shows N-type doping. Ammonia as a kind of reducing gas

Chemosensors 2021, 9, x FOR PEER REVIEW 11 of 15

and gas molecules; secondly, a of about 0.557 eV appears at the intersection of Chemosensors 2021, 9, 220 conduction band and valence band, which indicates that sulfur-doped graphene is a sem-11 of 15 iconductor and can be used as a sensitive material for gas detection; finally, the Fermi level drops to the valence band, which shows N-type doping. Ammonia as a kind of re- ducing gas will lead to the decrease of the resistance of N-type materials, which is con- willsistent lead with to the the decrease actual experimental of the resistance results. of N-type materials, which is consistent with the actual experimental results.

(a) (b)

(c) (d)

FigureFigure 10. 10.Band Band structure structure diagram diagram of of (a )(a GO) GO and and (b ()b S-GO;) S-GO; (c ()c DOS) DOS diagrams diagrams of of GO GO and and S-GO; S-GO; ( d(d)) TDOSTDOS diagramdiagram ofof S-GO.S- GO. The density of states (DOS) analysis is an important method to analyze the electronic propertiesThe density of materials of states before (DOS) and analysis after doping. is an important The density method of states to analyze represents the electronic the density ofproperties energy level of materials distribution. before and As after a semiconductor doping. The density material, of states it mainly represents depends the den- on the intensitysity of energy of the densitylevel distribution. of states near As a the semi Fermiconductor level. Asmaterial, shown it in mainly Figure depends 10c, the on density the of statesintensity of intrinsic of the density graphene of states and near sulfur-doped the Fermi graphenelevel. As shown is calculated. in Figure Overall, 10c, the thedensity density of states of intrinsic graphene and sulfur-doped graphene is calculated. Overall, the den- of states of S-GO is higher than that of GO, which indicates that many electronic states are sity of states of S-GO is higher than that of GO, which indicates that many electronic states produced by doping, which is consistent with the appearance of many impurity energy are produced by doping, which is consistent with the appearance of many impurity en- levels in the energy band diagram. At the Fermi level, the density of states of S-GO is ergy levels in the energy band diagram. At the Fermi level, the density of states of S-GO larger, which indicates that the band gap is generated. In the valence band, the valence is larger, which indicates that the band gap is generated. In the valence band, the valence band of S-GO is widened, which can cause more electrons in the state to be excited, which is helpful for the conductivity of the material. The partial wave density of states (PDOS) shown in Figure 10d also shows that the partial wave density of states of C-2p and S-2p orbitals is strong, and the combined action

makes the total density of states produce a strong peak. It also shows that the between carbon and sulfur atoms is strong, which is consistent with the results of energy band diagram and charge transfer analysis. In general, the doping of sulfur atoms enhances the semiconductor properties and conductivity of graphene, which has a significant effect on its ability as a sensitive material. Chemosensors 2021, 9, x FOR PEER REVIEW 12 of 15

band of S-GO is widened, which can cause more electrons in the state to be excited, which is helpful for the conductivity of the material. The partial wave density of states (PDOS) shown in Figure 10d also shows that the partial wave density of states of C-2p and S-2p orbitals is strong, and the combined action makes the total density of states produce a strong peak. It also shows that the covalent bond between carbon and sulfur atoms is strong, which is consistent with the results of energy band diagram and charge transfer analysis. In general, the doping of sulfur atoms Chemosensors 2021, 9, 220 12 of 15 enhances the semiconductor properties and conductivity of graphene, which has a signif- icant effect on its ability as a sensitive material. As shown in Figure 11a, the band diagram of S-GO changed after ammonia adsorp- tion. AsFirstly, shown the in band Figure gap 11a, decreased the band diagramfrom 0.502 of S-GO eV to changed 0.403 eV, after the ammonia band gap adsorption. became smaller,Firstly, the and band the conduct gap decreased band and from valence 0.502 band eV to were 0.403 closer. eV, the The band electrons gap became in the smaller,valence bandand the were conduct more easily band andexcited valence to the band conduc weret band, closer. which The electronsshowed that in the thevalence conductivity band ofwere the morematerial easily increased excited and to the the conduct resistance band, decreased, which showed which was that consistent the conductivity with the of de- the creasematerial in increasedthe sensor and resistance the resistance in the actual decreased, experiment. which wasIt is consistentalso proven with that the the decrease graphene in dopedthe sensor with resistance sulfur is inn-type the actual semiconductor. experiment. At Itthe is alsosame proven time, more that the impurity graphene levels doped are observed,with sulfur which is n-type can semiconductor.enhance the interaction At the same between time, materials more impurity and gases, levels which are observed, proves thatwhich the can sensitivity enhance to the ammonia interaction is higher. between materials and gases, which proves that the sensitivity to ammonia is higher.

(a) (b)

Figure 11. (a) Band structurestructure diagramdiagram of of S-G S-G after after ammonia ammonia adsorption; adsorption; (b ()b TDOS) TDOS diagram diagram of S-Gof S-G after after ammonia ammonia adsorption. adsorp- tion. Figure 11b shows the DOS and the PDOS of the adsorbed ammonia by S-GO. In the totalFigure state 11b density shows the diagram, DOS and the the density PDOS of of the the state adsorbed near theammonia Fermi by level S-GO. increases, In the totalwhich state indicates density that diagram, the band the gap density of the of doped the state material near increases the Fermi and level the increases, ability to absorbwhich indicatesammonia that gas the is enhanced. band gap Inof thethe PDOSdoped of material S-G, the increases top of the and valence the ability band to is absorb close to am- the moniaFermi energygas is enhanced. level. In the In range the PDOS of −5 of eV S-G, to 0 the eV, wetop canof the see valence that the densityband is ofclose N-2p to andthe FermiS-2p has energy a strong level. and In widethe range peak. of It − shows5 eV to that 0 eV, the we covalent can see bond that the strength density and of compositeN-2p and S-2pstrength has a formed strong betweenand wide sulfur peak. atomIt shows and th nitrogenat the covalent atom are bond higher strength at the and top composite of valence strengthband, so formed it is shown between that itsulfur has high atom sensitivity and to ammonia atom are gas higher in the at actualthe top experiment. of valence band,This shows so it is that shown sulfur that doping it has increases high sensitivity a large amountto ammonia of impurity gas in energythe actual levels, experiment. provides Thismore shows electronic that statessulfur for doping the material, increases and a enhanceslarge amount its conductivity, of impurity which energy is inlevels, line withpro- videsthe calculation more electronic results ofstates the bandfor the structure. material, and enhances its conductivity, which is in line with the calculation results of the band structure. 3.5. Performance Comparison to Other Graphene-Based NH3 Gas Sensors

3.5. PerformanceThe sensing Comparison properties to of Other chemical Graphene-Based resistance sensors NH3 Gas based Sensors on GO reported in [40–45] are shown in Table3. This sensor based on S-GO has better sensing appearance in the The sensing properties of chemical resistance sensors based on GO reported in [40– aspect of sensitivity and reaction/recovery time. 45] are shown in Table 3. This sensor based on S-GO has better sensing appearance in the aspect of sensitivity and reaction/recovery time. Table 3. NH3-sensing performance of graphene-based sensors at room temperature.

Concentration of Response/Recovery Materials Sensitivity (%) Reference Ammonia (ppm) Time (s) GO/ester 100 12 55/80 [40] rGO/SnO2 50 30 60/120 [42] Py/rGO 100 22 134/310 [45] rGO/P3HT 50 13 92/415 [44] TiO2/rGO 50 5.5 Slow/slow [43] ZnO/rGO 50 3.05 84/216 [41] Sulfer/GO 10 73 112/33 This work Chemosensors 2021, 9, 220 13 of 15

4. Conclusions The S-GO nanocomposites were successfully synthesized and used to fabricate am- ◦ monia sensors to sense NH3 at RT (20 C). According to the experimental results, the S-GO sensor has better sensing properties. Compared with the original graphene, the S-GO sensor exhibited an excellent response value of 73% to 10 ppm ammonia and the test limit is 0.5 ppm at RT. In addition, the response and recovery speed revealed an immense improvement as compared with pristine graphene. The S-GO sensor responded to 10 ppm NH3 in 112 s and recovered in 33 s. Moreover, the S-GO sensor also showed excellent selectivity to different kinds of gases. Theoretical calculation based on first-principles was executed and completed. These improvements in sensing properties can be ascribed to the improvement of band structure and DOS after sulfur doping.

Author Contributions: Conceptualization, F.M. and Y.Y.; methodology, Y.Y. and Z.Y.; software, Y.Y.; validation, Z.Y., F.M. and Z.L.; formal analysis, Y.Y. and Z.L.; investigation, Y.Y.; resources, F.M.; data curation, Z.L.; writing—original draft preparation, Y.Y.; writing—review and editing, Y.Y.; visualization, Z.L.; supervision, F.M.; project administration, F.M.; funding acquisition, F.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (62033002, 61833006, 62071112, and 61973058), the 111 Project (B16009), the Fundamental Research Funds for the Central Universities in China (N180408018, N2004019, and N2004028), the Liao Ning Revitalization Talents Program (XLYC1807198), the Liaoning Province Natural Science Foundation (2020-KF-11-04), and the Hebei Natural Science Foundation (No. F2020501040). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data of our study are available upon request. Acknowledgments: Thanks to Shen Yanbai, Department of Minerals processing engineering, School of Resources and Civil Engineering, Northeastern University, China, for the support in the technical analysis of software applications. Conflicts of Interest: The authors declare no conflict of interest.

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