SnO 2 Nanoparticles Decorated 2 D Wavy Hierarchical Carbon Nanowalls with Enhanced Photoelectrochemical Performance

Two-dimensional carbon nanowall (2D-CNW) structures were prepared by hot wire assisted plasma enhanced chemical vapor deposition (hw-PECVD) system on silicon substrates. Controlled variations in the film structure were observed with increase in applied rf power during deposition which has been established to increase the rate of dissociation of precursor gases. The structural changes resulted in the formation of wavy-like features on the 2D-CNW, thus further enhancing the surface area of the nanostructures. The FESEM results confirmed the morphology transformation and conclusively showed the evolution of the 2D-CNW novel structures while Raman results revealed increase in ID/IG ratio indicating increase in the presence of disordered domains due to the presence of open edges on the 2D-CNW structures. Subsequently, the best 2D-CNW based on the morphology and structural properties was functionalized with tin oxide (SnO2) nanoparticles and used as a working electrode in a photoelectrochemical (PEC) measurement system. Intriguingly, the SnO2 functionalized 2D-CNW showed enhancement in both Mott-Schottky profiles and LSV properties which suggested that these hierarchical networks showed promising potential application as effective charge-trapping medium in PEC systems.


Introduction
Research interests on carbon based nanomaterials from 0Dfullerene, 1D-carbon nanotube (CNT), 2D-carbon nanowall (CNW), and graphene have tremendously increased in the past decades.This is mainly due to the novel properties of carbon based nanostructures such as better conductivity, chemical compatibility, and wide electrochemical stability, in addition to the mechanical and thermal characteristic that are typically exceptional compared to the bulk carbon material [1,2].In particular, the study on 2D-carbon nanostructures has bloomed upon the discovery of graphene which has increased the curiosity of researchers on investigating interesting properties shown by nanosystems with graphene-like features.
To date, a plethora of fascinating 2D carbon based nanostructures had been successfully developed such as nanoflakes, nanohorns, nanopetals, and carbon nanowalls.Commonly, most of these 2D nanowalls are composed of few layers of graphene sheets that are loosely stacked together.There are reports claiming that these wall-like carbon nanostructures are actually graphene nanowalls with layers of vertical graphene sheets demonstrating congruent properties [3].Ma et al. have reported that free standing CNWs possess very high surface area and proved to be an excellent candidate for surface interaction due to the abundance of active reaction sites available compared to that of fully embedded nanowall in horizontal manner [4].There is also a reported study on free standing nanowalls which are aligned perpendicularly to the substrates surface without requiring auxiliary lateral support [5].However, relatively fewer reports are found on 2D-carbon nanowalls (2D-CNWs) grown by hot wire radio frequency assisted plasma enhanced chemical vapor deposition, a physical deposition technique which is known to reproducible and introduces fewer impurities into the film structure.
In this work, the CNW thin films were grown on silicon dioxide substrates using a home-built hw-PECVD at different applied radio frequency (rf) power.The morphology, structural properties, and composition of 2D-CNW thin films were systematically characterized.The novelty of the current study is the ability to fabricate CNWs with free standing wavy structures by hw-PECVD.These nanowalls structures are unique due to the expanded outward feature displaying flaky structures with abundance of active edges.Such features enable these CNWs to serve as matrix for the decoration of SnO 2 nanoparticles to be modified into advanced nanocomposites, which can be employed as a highly reactive active material for photoelectrode application.Photoelectrochemical measurements reflect that these nanocomposites exhibit improved photoelectrochemical properties in contrast to that of its single-component counterpart.To the best of our knowledge, such special hybrid nanocomposite has never been reported yet and hence it may serve as alternative routes for the advancement of nanomaterials research as well as in field of photoelectrochemistry.

Synthesis of 2D-CNW Thin Films.
A home-built hw-PECVD system (Kejuruteraan Wing Hung, Kuala Lumpur, Malaysia), equipped with a radio frequency source for generating the plasma, was used to fabricate 2D-CNW with Au nanoparticles (Particular GmbH, Burgdorf, Germany) as catalyst.A seven-turn coil of 8 mm diameter made from tungsten wire of 1 mm diameter was used as the hot filament (R. D. Mathis Company, Long Beach, CA, USA).Silicon dioxide (SiO 2 ) substrates with size of 1 × 2 cm were placed 10 mm from the filament.The deposition parameters used in the deposition process of the 2D-CNWs are as follows: hydrogen flow rate ratio of methane: 20 : 5 sccm, deposition pressure: 2.15 mbar, substrate temperature: 420 ∘ C, hot filament temperature: 1850 ∘ C, and deposition time: 35 minutes and the rf power was fixed at 0, 20, 40, 60, 80, and 100 W for the different sets of samples studied in this work.Sigma-Aldrich, St. Louis, MO, USA), were subsequently loaded into a 250 mL four-neck round-bottom flask.Under constant magnetic stirring, the mixture was heated to 120 ∘ C and saturated under nitrogen blanket for 1 hour.The mixture was then raised to 287 ∘ C at 4 ∘ C/min of ramping rate and further refluxed for 5 hours.Under vigorous stirring, a change in the solution from clear to pale yellow indicates the formation of SnO 2 nanoparticles.The SnO 2 nanoparticles were directly deposited onto the as-grown 2D-CNW using spin coating method.The SnO 2 nanoparticles were spin-cast at a rotation speed of 3000 rpm during 30 seconds.Then, the sample was annealed at 350 ∘ C for 1 hour and rapidly cooled to the room temperature.Two different mass fractions (w%) were used in the preparation of the two nanocomposite samples studied in this work, namely, 2.5 and 5.0 w%.

Characterization of 2D-CNW and SnO 2 Decorated CNW
Thin Films.The as-obtained SnO 2 integrated 2D-CNW films were carefully observed using field emission scanning electron microscope (FESEM) using a FEI Quanta 200 FESEM (FEI Company, Hillsboro, OR, USA).A high resolution transmission electron microscope (HR-TEM, JEM 2100-F) was used to determine the high resolution images of the functionalized nanostructures.The bonding properties of the functionalized 2D-CNW thin films were ascertained by X-ray photoemission spectroscopy (XPS) at the beam line, BL3.2 (a) and (b) of the Synchrotron Light Research Institute in Thailand.The Raman spectra were obtained by using Raman Microscope (Renishaw inVia).The optical properties of the films were measured by UV-Vis-NIR spectrophotometer (Cary 5000, Agilent Technologies).The PEC measurements done on the SnO 2 decorated 2D-CNW thin films were carried out on a three-electrode electrochemical cell system equipped with platinum coil as counter electrode and Ag/AgCl as reference electrode, respectively.Xenon arc lamp (300 W) containing solar mass filter is used as a light source for the PEC system that connected to a potentiostat (PGSTAT204, Metrohm Autolab) to probe the electrical signal generated throughout the irradiation.

Results and Discussions
3.1.Formation of Hierarchical 2D-CNW.The FESEM images of the films as a function of different rf power, P rf , are represented in Figure 1.Without applying P rf, , one can observe that there is no distinct formation of carbon nanostructures as seen in Figure 1(a).The thermal energy sourced from deposition temperature, whether from the substrate or the hot wire heating, is insufficient to induce the formation of 2D-CNW or any form of film.Notably, it is seen that the formation of nanostructured carbon films began to be observed at P rf as low as 20 W. This implies that plasma power is a vital factor to induce the growth of CNWs which turn out to be an initial stage of nucleation for the development of 2D-CNWs.At this stage, the low decomposition rate could have reduced the number of reactive species such as ion (CH 1 + , CH 2 + , CH 3 + , etc.) and hydrocarbon radicals present in the plasma environment due to the lower kinetic energy of the active species.Moreover, these active species tend to produce high surface energy and eventually agglomerate together before reaching suitable growth sites.When P rf was increased to 40 W, a sharp morphological change is observed and distinct sheet-like 2D-CNWs started to form only at P rf of 60 W which appear to be deposited layer-by-layer forming a larger foam-like matrix covering the entire SiO 2 substrate [6].These 2D-CNW structures are typically made up of vertical sheets with apparent spatial distribution.Conclusively, further increment in P rf could have resulted in the enhancement of momentum, density, and temperature of electrons that facilitated the mobile state of active ions in the gaseous phase and thus increased the active site density for the formation of these novel nanostructures.It can possibly be deduced that the primary and secondary reactions occurring in this system are governed by the decomposition rate of precursor which can influence the formation of various active species in the plasma.The formation of these vertical structures is induced by the effect of local electric field polarity of the plasma resulting from anisotropic polarizability [7,8].Further increment of P rf to 80 W renders increase of the wall-to-wall distance and the distribution of the network vertical sheetlike nanostructures as observed in Figure 1(e).At P rf of 100 W, it is deduced that the effective ion bombardment increases the kinetic energy of active species which leads to faster growth rate of secondary wavy-like structures [9].Such intriguing free standing vertically oriented wall structures offer greater active surface area for materials functionalization, which can provide effective electron pathways and thus can be good candidates as ideal photoelectrode for PEC application.One may see that the individual layers of the 2D-CNW structures are interconnected to each other to form the wavylike 2D-CNW with thickness ∼10 nm while the measured lattice fringes are about 0.36 nm, which correspond to typical lattice spacing of graphitic materials [10].
Raman spectroscopy was used to characterize the structural properties of the 2D-CNW films prepared at different P rf .Figure 3 shows typical Raman spectra with corrected baseline and the resulting spectra were deconvoluted into six components [11,12].The six components are D, G, and 2D bands which were individually deconvoluted by using Lorentzian curve fitting, while D  , D + D  , and D + G bands were deconvoluted via Gaussian curve fitting.The Raman spectra of the as-fabricated thin films reveal four major peaks at around 1350, 1590, 2700, and 2940 cm −1 which can be assigned to D, G, 2D, and D + G bands, respectively.The G band is accompanied by a shoulder peak assigned to D  band at 1620 cm −1 while a D + D  shoulder peak can be found at around 2480 cm −1 [9].The calculated FWHM, peak positions for different bands, and  D / G ratios for the corresponding spectra of the films are summarized in Table 1.
The appearance of G band at around 1590 cm −1 is mainly attributed to the stretching vibration mode (E 2g ) of a hexagonal carbon lattice which confirms the formation of graphitized structure [12].As reported elsewhere, G peak position for graphitic carbon materials can be seen at around 1580 cm −1 [13].However, the G peak position of the asfabricated films grown at 0 W and 20 W has been shifted to 1598 and 1596 cm −1 , respectively.This shifting and wide FWHM G suggest that the structures may be due to the indigenousness of nanocrystalline graphitic features of 2D-CNW [14].Indeed, the corresponding G peak position becomes less prominent as P rf increases which indicates the formation of larger sized graphitic clusters as evidenced from FESEM results.
The strong peak observed at ∼1350 cm −1 can be assigned to D band which originates from the intervalley double resonance while the presence of D  band at around 1620 cm is contributed by the intravalley double resonance process.These resonance is produced from the breathing modes of sixfold rings indicating the activation of defects in graphite structure.Such defects include vacancies, strain chain, and/or ring distortion in the 2D-CNW structures.It is worth noting that the large amount of open edges observed on 2D sheets due to the formation of multilayered wall-like structures results in the more prominent intensities of the D peak as compared to the G peak intensities as observed in all the thin films.
The existence of 2D and D + G bands is due to the presence of highly ordered structures with graphene-like domain that gives rise to second-order Raman vibration modes [15].The presence of 2D band, however, is strongly correlated with the number of layers in the graphene structure [16,17].The 2D peak position of 2D-CNW thin films consistently appears at around 2700 cm −1 as P rf increased.The large value of FWHM 2D indicates that these structures have similar features of the multilayered graphene structures reported by Ferrari and Basko and also confirmed by previously observed HRTEM images [18].
The  D / G value varies in range of 1.7 to 2.7 which is close to the  D / G value of 2D-CNW features previously reported in the literature [9,19,20].This may be due to the enhancement in atomic arrangement as a result of the removal of weak C-C bonds which contribute to the disorder in the film structure.Notably, it is found that the  D / G ratio increases to a maximum of 2.7 at P rf of 80 W but at the same time narrows the FWHM of the G band indicating that there exist two competing mechanisms at this P rf .This suggests that the CNWs prepared at P rf of 80 W consist of small crystallites with relatively high degree of graphitization which correspond well to the study reported by Rout et al. and Kurita et al. [21,22].At maximum P rf ,  D / G reduced again suggested that the number of ordered structures increased and this is perhaps due to the reduction of defects along the edges of 2D-CNW nanostructures.
The elemental composition of the films was ascertained from XPS measurements.The XPS spectra show the types of bonds are affiliated to the carbon and oxygen atoms present in the films.Narrow scan spectra of C 1s and O 1s as a function of P rf are shown in Figures 4(a) and 4(b), respectively.These two regions represent the binding energy in range of 280-296 eV and 524-540 eV [23].It appears that the intensity of C 1s peak is appreciably lower for the film prepared without applying P rf .On the other hand, the corresponding O 1s peak intensity is significantly higher compared to films prepared at P rf of 20 to 100 W.These spectra were then deconvoluted by using Gaussian fitting with nonlinear background extraction for further investigation.
As shown in Figure 4(c), the C 1s peak of 2D-CNW thin film prepared at 80 W can be perfectly deconvoluted into   [25].When the samples were exposed to air, the presence of oxygen not only was associated with the oxygen or water vapor physical absorption on 2D-CNW surface, but also indicates the presence of oxygen atoms that are chemically bonded to the structures [26].
In this work, although the composition appears to be constant throughout the range of P rf applied, further analysis suggests that the difference in the applied power brings about the change in sp 2 /sp 3 ratio as a function of P rf as represented in Figure 4(e).Interestingly, the trend shown in sp 3 /sp 2 ratio is similar to the Raman  D / G results.The sp 3 /sp 2 ratio decreases initially when P rf is increased from 20 W to 60 W and then increases significantly at 80 W. The initial decrease in sp 3 /sp 2 ratio supports the earlier suggestion in the Raman results analysis which is attributed to the enhancement in the atomic arrangement.The increase in the number of heterogeneous nucleation sites by reactive H atom in the plasma results in preferential growth of stronger sp 2 C=C bonds in the film.The increase in sp 3 /sp 2 for film deposited at P rf of 80 W can be due to the effects produced by the change in the morphology of the film.The increase in defects along the edges of the walls which is significant for these films allows for the preferential formation of sp 3 bonds along these edges.This is supported by the percentage of oxygen related components obtained from analysis of O 1s peak in Figure 4(e).

Tin Oxide Anchored 2D-CNWs and Photoelectrochemical
Measurement.The progression in 2D-CNWs formation from carbon nanostructures at low P rf into 2D-CNWs network structures at higher P rf has been realized in microscopic studies done in the previous section.The growth of CNW thin films (Figure 1) shows that the applied P rf in the 2D-CNWs deposition gives significant influence to their morphological properties.Thus, the choice of P rf for CNW growth is crucial for a study devoted to the fabrication of 2D-CNWs with large wall-to-wall distance network structure suitable for functionalization of SnO 2 nanoparticles for PEC studies.Architecture of semiconductor photoelectrode is of fundamental aspect in the perspective of PEC system performance [27].Therefore, in this section, 2D-CNWs thin film prepared at 80 W is selected for functionalization of SnO 2 nanoparticles using simple spin coating technique to be used as photoelectrode for PEC measurements.
FESEM image of SnO 2 and SnO 2 nanoparticles functionalized 2D-CNWs (SnO 2 -CNWs) thin films is presented in Figure 5.It is observed that the distribution of SnO 2 nanoparticles is not uniform on the smooth surface of SiO 2 bare substrate.Comparatively, it was found that 2.5 w% SnO 2 nanoparticles tend to be distributed uniformly onto 2D-CNWs thin film.This could be due to porosity of the 2D-CNWs covering the c-Si substrate that renders SnO 2 nanoparticles easily incorporated into the CNW matrix.Further functionalization of 5.0 w% SnO 2 nanoparticles on the 2D-CNWs results in agglomeration of SnO 2 on the surface of the CNWs.
The EDX results of 2D-CNW and SnO 2 -CNWs thin films are presented in Figure 6  gaps of CNW and SnO 2 are 1.35 and 3.74 eV, respectively, which is close to the value reported by Kawai et al. [29,30].In contrast, the energy gap of the SnO 2 -CNWs nanocomposite thin film is found to be in the range of 1.82 to 2.80 eV and the value increases with increase in the concentration of SnO 2 nanoparticles.The combination of both 2D-CNWs and SnO 2 can potentially narrow the bandgap of the SnO 2 -CNWs nanocomposite thin film photoelectrode because of electronic interaction between CNWs and SnO 2 that leads to improvement of light harvesting properties of PEC under visible irradiation [31,32].
In order to evaluate the surface charges for the corresponding films, Mott-Schottky (MS) measurements characteristic plots are illustrated in Figure 7.The donor density of the films can be obtained by using Mott-Schottky equation: where  is the capacitance of the space charge region,  is the elementary electron charge,  is the interfacial area,   is donor density of the films,  is the static dielectric constant of the films,   is the permittivity of free space,  bias is the applied potential,   is the flat band potential,  is Boltzmann's constant, and  is the absolute temperature [33].From the slope in the MS plot   for the films are determined as shown in    and 5.0 w% show great enhancement in the anodic current.This may be due to the increase in donor density in the functionalized films.More interestingly, under illumination of simulated solar source on the sample, the anodic current increases for 2.5 and 5.0 w% SnO 2 -CNWs nanocomposite thin films.For instance, the current increases from 7.8 mA to 11.8 mA at 1.0 V for 5.0 w% SnO 2 -CNWs.Improvement PEC properties of SnO 2 -CNWs nanocomposite thin films can be ascribed to the fact that SnO 2 nanoparticles incorporated on wavy-like structures provide larger surface area.The 2D-CNWs network serves as direct route of charge transport which enhance the separation of photogenerated charge carrier which greatly enhanced the PEC response [35].It is also can be deduced that the density of SnO 2 incorporated onto 2D-CNW structures contributes to the enhancement of PEC performance as a result of the increased photon absorption in hybrid materials.

Conclusion
In summary, 2D carbon network has been successfully synthesized using a home-built hot wire assisted plasma enhanced chemical vapor deposition system.The rf power applied is shown to modify the morphology of the 2D-CNW structures.The formation of novel graphitic structures was confirmed by Raman spectroscopy and HRTEM analysis.The CNWs were further decorated with SnO 2 nanoparticles employing a simple spin coating technique.From the results, it is anticipated that 2D-CNW structures provide direct pathways of charge transport mitigating recombination of photogenerated electron-hole pairs in the SnO 2 functionalized 2D-CNW thin film.This method could therefore provide a new channel in the preparation of novel carbon nanostructures for advanced hybrid materials that could potentially be used in PEC applications.

Figure 2
illustrates a series of magnification TEM views of 2D-CNWs obtained at 80 W. Figure 2(c) demonstrates high resolution TEM image, where the wavy-like 2D-CNW multilayered structures of overlapping plane are clearly visible.

Figure 2 :
Figure 2: HRTEM images of the CNW prepared at P rf of 80 W.

Figure 5 :
Figure 5: FESEM image of SnO 2 nanoparticle on bare Si substrate (a) and SnO 2 nanoparticles decorated CNW at 2.5 and 5.0 w% for (b) and (c), respectively.
(a).These spectra detected C, O, and Si elements on the film with 2D-CNWs and additional Sn element for the SnO 2 -CNWs nanocomposite film.Microstructural properties can be realized by the Raman spectra of both 2D-CNWs and SnO 2 -CNWs nanocomposite films which are shown in Figure 6(b).The Raman spectrum of CNW thin film reveals four peaks at around 1400, 1590, 2840, and 3030 cm −1 assigned to D, G, 2D, and D + G bands, respectively.While SnO 2 -CNWs nanocomposite thin film presents three additional peaks at around 358, 475, and 630 cm −1 ascribed to E u , E g , and A 1g Raman active mode of SnO 2 material [28].

Figure 7 :
Figure 7: Mott-Schottky plot of the measured capacitance of the SnO 2 (a), CNW (b), and the SnO 2 decorated CNW thin films with SnO 2 concentration of 2.5 w% (c) and 5.0 w% (d).

Figure 8 :
Figure 8: Linear sweep voltammograms of the SnO 2 , CNW, and the SnO 2 decorated CNW thin films with SnO 2 concentration of 2.5 and 5.0 w% without (a) and under illumination (b).

Table 1 :
FWHM, peak position of D, G, and 2D bands, and  D / G ratio of 2D-CNW thin films.
Rf power (W)FWHM (cm −1 ) P e a k p o s i t i o n ( c m −1 )  D / G

Table 3
. The   values of SnO 2 and CNW films are almost the same.However, for SnO 2 -CNWs nanocomposite

Table 2 :
[34]gy gap of SnO 2 , CNW, and SnO 2 -CNW thin films.SnO 2 concentration of 2.5 w%, the value increases to 1.16 × 10 15 cm −3 and increases further to 1.30 × 10 17 for the film with SnO 2 concentration of 5.0 w%.The key improvement in this case can be attributed to the effective electron transfer between SnO 2 and CNW which in turn improves the conductivity of the photoelectrode[34].The linear sweep voltammogram (LSV) of the SnO 2 , CNW, and the SnO 2 -CNWs nanocomposite thin films with SnO 2 concentration of 2.5 and 5.0 w% determined in the presence of 0.1 M Na 2 SO 4 without and with illumination are presented in Figures8(a) and 8(b), respectively.It can be observed that, at positive bias voltage, the anodic current response increases with increase in bias voltage.SnO 2 -CNWs nanocomposite thin films with different concentration of 2.5