Influence of the Interface on the Photoluminescence Properties in ZnO

Article pubs.acs.org/JPCC

Influence of the Interface on the Photoluminescence Properties in ZnO Carbon-Based Nanohybrids E. Rauwel,† A. Galeckas,‡ M. Rosário Soares,§ and P. Rauwel*,† †

Tartu College, Tallinn University of Technology, Puiestee 78, 51008 Tartu, Estonia Department of Physics, University of Oslo. P.O. Box 1048, Blindern, 0316 Oslo, Norway § CICECO-Aveiro Institute of Materials, University of Aveiro, Aveiro, Portugal ‡

S Supporting Information *

ABSTRACT: We report on the synthesis of free-standing ZnO nanoparticles (NPs) and ZnO NPs-decorated multiwalled carbon nanotubes (MWCNTs) via nonhydrolytic sol−gel (NHSG) routes. The role of the hydrates in the zinc precursor on the structure and surface termination of ZnO NPs via NHSG routes has already been reported in our earlier study. In the present work, the synthesis has been carried out in ambient air without using a glovebox, whereupon the effects of the synthesis temperature have been studied systematically. The structure, morphology, and optical properties of the free-standing ZnO-NPs are compared with those of the carbon nanotube-based nanocomposites, ZnO:MWCNTs, synthesized under identical conditions. The observed efficient and broad luminescence covering the entire visible spectrum points toward the high potential of the hybrid nanocomposites in optoelectronic and photovoltaic applications. (NHSG) method.21,22 In this report, we applied a similar method of synthesis but without using a glovebox and a controlled atmosphere, making this synthesis easier and more cost effective. Photoluminescent ZnO-carbon nanocomposites are an emerging class of nanomaterials that has spurred a lot of interest due to their unique optical properties.23 It has been shown that these hybrid structures yield an enhanced UV to visible emission ratio due to improved surface-plasmonmediated emission and new defect emission.24,25 It has also been demonstrated that CNTs also enhance the photocatalytic performance of ZnO by the efficient separation of photoinduced charge carriers.26 The present study focuses on synthesizing ZnO NPs without using a glovebox by direct mixing of the precursors in ambient air and also on decorating multiwalled carbon nanotubes (MWCNTs) with these ZnO NPs in a one-pot synthesis. The importance of such a hybrid ZnO:MWCNT nanocomposite for optoelectronic applications can be understood by considering that even though electroluminescence in ZnO was demonstrated more than a decade ago,27 integration of ZnO into light-emitting devices still remains inhibited by problems related to p-type doping of ZnO.28 Moreover, the quantum yield of ZnO is also greatly affected by the crystalline imperfections and drops rapidly with the increase of defect-related nonradiative recombination

1. INTRODUCTION During the past decades, semiconductor nanoparticles (NPs) and, in particular, ZnO nanocrystals1 have been intensively studied for their high potential in many micro/nanoelectronic,2−4 magnetic,5 photovoltaic,6,7 and optical applications.8−10 More recently, ZnO_carbon nanotube hybrid structure have attracted considerable attention, and it was shown that the presence of carbon nanotube can influence the growth of ZnO nanomaterials. Various morphologies of ZnO nanostructure have been grown on the surface of CNTs using thin film deposition.11 Research targeted toward the preparation and characterization of nanocomposites combining ZnO nanoparticles with carbon-based nano-objects like carbon nanotubes (CNTs), graphene, and graphene oxide have then spurred a lot of interest toward the development of sensors and photovoltaics.12−14 The high refractive index, high thermal conductivity, and wide direct band gap (Eg ≈ 3.37 eV)15 make ZnO a promising candidate in the field of nanotechnology. Many chemical and physical methods are available for the synthesis of nanoparticles, and in certain cases these syntheses require specific equipment, ultrapure precursors, and solvents. For these reasons, during the past decade, green synthesis routes employing plant extracts have attracted attention as a versatile, cost-effective, and viable alternative for fabrication of metal and metal oxide nanoparticles.16,17 Recent reports show that green synthesis methods can also be successfully applied to create metal oxide nanoparticles, particularly ZnO NPs.18,19 We recently reported on the influence of hydrate species for the synthesis of ZnO nanoparticles20 using a nonhydrolytic sol−gel © XXXX American Chemical Society

Received: March 31, 2017 Revised: June 14, 2017 Published: June 27, 2017 A

DOI: 10.1021/acs.jpcc.7b03070 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

before transferring the solution into an autoclave for the reaction synthesis. Characterization. X-ray diffraction (XRD) patterns were collected using a Panalytical Empyrean diffractometer with a Cu Kα1 radiation source (λ = 0.15406 nm). Scanning electron microscopy (SEM) images were recorded on a FEI Quanta 200FEG. High-resolution transmission electron microscopy (HRTEM) was carried out on a probe-corrected Titan G2 80− 200 kV operating at 200 kV in TEM mode and providing a point to point resolution of 2.4 Å. Optical absorption properties were derived from the diffuse reflectance measurements performed at room temperature using a ThermoScientific EVO-600 UV−vis spectrophotometer. Photoluminescence (PL) was investigated at 300 K temperature by employing the 325 nm wavelength of a cw He− Cd laser with an output power of 10 mW as an excitation source. The emission was collected by a microscope and directed to a fiber optic spectrometer (Ocean Optics USB4000, spectral resolution 2 nm). All PL study have been performed on powder samples at 300 K under air or vaccuun. The density of the compact powders with and without CNTs is estimated to be approximately the same in all measurements.

centers. The nature of a broad visible luminescence in ZnO, also known as the deep-level emission (DLE) band, is related to a variety of optically active intrinsic defects. In particular, the characteristic green emission is usually associated with singly ionized oxygen vacancies,3,29,30 whereas the blue−violet band is linked with zinc vacancies or interstitials,31 although certain controversy in particular assignments still exists.32 The sharp luminescence band in the UV region originates from the exciton annihilation along with recombination processes via shallow impurities and collectively is referred to as the nearband edge (NBE) emission. Since the two principal carrier recombination routes, intrinsic NBE and defect-related DLE, compete during the photoluminescence (PL) process, the intensity ratio of NBE and DLE in a PL spectrum is commonly used as a figure of merit to define the purity or crystallinity of ZnO. Despite certain difficulties in achieving stable green and blue emission, ZnO nanocrystals are considered as promising candidates for many applications due to their nontoxic character and stability in air.33 In the present study, we address the possibility of producing carbon-based nanocomposites using a simplified one-pot NHSG method with an ultimate goal of investigating and relating structural, morphological, and optical properties of such hybrid nanocomposites. The synthesis of ZnO NPs using the NHSG method has already been reported in the literature;20,34 however, to the best of our knowledge, no investigations on one-pot synthesis of carbon-based nanocomposites in ambient air have been published yet. The paper is organized as follows: first, we present results on the carbonbased nanocomposite formation via a one-pot synthesis followed by structural and morphological analysis. Next, optical emission and absorption properties of the free-standing ZnO NPs are compared with those of the hybrid ZnO:MWCNTs nanocomposites taking into consideration the size distribution and morphology of NPs at different synthesis temperatures. The comparative analysis also takes account of the PL emission yield and stability in different ambient (air and vacuum) bearing in mind that ambient largely determines the surface state. For instance, the environment change may lead to activation or passivation of the surface defects, which in turn will affect band bending at the surface and, consequently, spectral content as well as quantum efficiency. Furthermore, certain variations in DLE and NBE emissions are expected once ZnO NPs are anchored to the metallic carbon nanotubes, mainly due to the interfacial effects of CNT-ZnO, which tend to modify the surface defects.

3. RESULTS AND DISCUSSION Structural Characterization. XRD measurements performed on free-standing ZnO nanoparticles indicate no structural variation with the increase of synthesis temperature. Figure 1 shows XRD patterns of ZnO NPs synthesized at

Figure 1. X-ray diffraction patterns of ZnO nanoparticles synthesized at different temperatures.

temperatures ranging from 200 to 300 °C with no signs of secondary phases; the particles are highly crystalline with a hexagonal wurtzite (P63mc) crystal structure (a = 3.25 Å and c = 5.20 Å).35 In general, XRD patterns appear similar to those of ZnO NPs synthesized under similar conditions but in a glovebox at the precursor mixing step;20 however, peaks are sharper in the case where no glovebox is used, indicating larger nanoparticles. It is reasonable to assume that addition of MWCNTs into the solution of precursors during the synthesis may affect the size and shape of the nanoparticles during the growth process. Figure 2 presents XRD patterns of ZnO NPs synthesized in the presence of MWCNTs under similar conditions showing neither visible differences from the free-standing NPs nor any signs of secondary phase. According to the Scherrer method36 applied to 100 reflections, most of the ZnO NPs have an average diameter at around 100 nm (Table 1). Note, however,

2. MATERIALS AND METHOD Synthesis. The procedure for synthesizing ZnO NPs was carried out under air. In a typical synthesis, zinc acetate (3.41 mmol) (99.99%, Aldrich) was added to 20 mL (183 mmol) of benzyl amine (≥99.0%, Aldrich). The reaction mixture was transferred into a stainless steel autoclave and carefully sealed. Thereafter, the autoclave was taken out of the glovebox and heated in a furnace at temperatures ranging from 200 to 300 °C for 2 days. The resulting milky suspensions were centrifuged; the precipitates were thoroughly washed with ethanol and dichloromethane and subsequently dried in air at 60 °C. In the case of the carbon nanohybrid synthesis, NANOCYL NC7000 MWCNT with an average diameter of 10 nm and length of 1.5 μm was used. For the synthesis of carbon-based ZnO nanocomposites, the MWCNTs were directly homogeneously dispersed into the solution of zinc acetate and benzylamine B


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Figure 3. (a) TEM micrograph of ZnO nanoparticles synthesized at 200 °C. (b) Electron diffraction pattern indicating pure hexagonal wurtzite (P63mc) crystal structure with a few lower order reflections indexed.

Figure 2. X-ray diffraction patterns of nanocomposites of ZnOdecorated MWCNTs synthesized at different temperatures.

that XRD peaks of the ZnO NPs can only give information on the size of the nanoparticles and the Scherrer equation only applied to spherical nanoparticles. It is noteworthy that the shape of ZnO NPs can vary depending on the method of synthesis. In order to estimate more accurately the size of the ZnO nanoparticles, size distribution histograms were plotted in Figure S1 from the Supporting Information. An average size of 80−125 nm depending on the conditions of synthesis was calculated for the ZnO nanoparticle size (Table 1). These values in some cases differ slightly from the particle sizes estimated from Scherrer’s formula via X-ray diffraction, which may be due to the nonspherical shape of the ZnO nanoparticles. These data were obtained from TEM images for more than 60 nanocrystals per sample. TEM. TEM of Free-Standing ZnO Nanoparticles. HRTEM study of ZnO NPs synthesized at 200 °C is presented in Figure 3, revealing a mostly tetragonal morphology of the particles mixed with a lower fraction of hexagonal-shaped NPs. The overview of the ZnO NPs size distribution in Figure 3a indicates a relatively good degree of monodispersion with an average size of the particles estimated to be close to 100 nm. The electron diffraction (ED) performed on a selected area (Figure 3b) confirms the high crystallinity of the ZnO NPs and their hexagonal P63mc structure. In our previous study, ZnO NPs grown in similar conditions, but using a glovebox, have a diameter of 30−70 nm with mainly spherical shape.20 It appears that the nanoparticles grown in the present conditions are larger but remain highly crystalline. Figure 4 illustrates that increasing the temperature of synthesis from at 200 to 240 °C leads to formation of smaller

Figure 4. HRTEM micrographs of ZnO nanoparticles synthesized at 240 °C: (a) low-magnification overview of the samples; (b) highmagnification image of a few smaller nanoparticles.

spherically shaped ZnO nanoparticles of 30 nm in diameter (Figure 4a). However, bigger ZnO NPs with an average size of 110 nm (Figure 4b) are also present and, similarly to the ZnO NPs synthesized at 200 °C, have tetragonal and hexagonal morphologies. TEM study shows that the ratio of tetragonally shaped ZnO NP is higher compared to ZnO NPs synthesized at 200 °C. Figure 5 depicts the morphologies of ZnO NP synthesized at 300 °C. Apparently, a higher synthesis temperature facilitates formation of smaller, nanometer-sized ZnO NPs. One can observe a distinct fraction of particles as small as 10 nm alongside the subset of bigger NPs of 120 nm. Figure 5a provides an overview of the sample containing large NPs of tetragonal or hexagonal shapes and with distinct facets, some

Table 1. Size of ZnO Nanocrystallites Calculated Using Scherrer Equation Applied to XRD Peak Widths and Measured by HRTEM Studya sample label ETCZn001 ETCZn002 ETCZn006 ETCZn004 ETCZn003 ETCZn005 a

composition ZnO NPs ZnO NPs ZnO NPs ZnO NPs + MWCNTs ZnO NPs + MWCNTs ZnO NPs + MWCNTs

temperature (°C)

nanocrystallites size (XRD) (nm)

nanocrystallites size (HRTEM) (nm)

integrated PL yield (normalized % max)

200 240 300 200

122 112 141 146

115 115 80 90

7 26 100 10

240

88

125

44

300

124

80

83

PL yield for each specimen is indicated as % with regard to maximal quantum efficiency. C


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Figure 6. (a)TEM micrograph of ZnO nanoparticles produced at 200 °C in the presence of MWCNTs; (b) high-magnification image showing isolated MWCNTs after synthesis of ZnO NPs at 240 °C.

size as the free-standing ZnO NPs. Similarly to free-standing ZnO NPs, the fraction of tetragonal-shaped nanoparticles is increased at 240 °C growth. ZnO NPs present sharp edges and overall increased average particle size; however, the size distribution is more uniform compared to the synthesis without CNTs. The overview image in Figure 7a shows that ZnO NPs are in direct contact with the surface of MWCNTs and thus capable of promoting charge transfer from the ZnO nanoparticles and the MWCNTs network.

Figure 5. HRTEM micrographs of ZnO nanoparticles produced at 300 °C: (a) an overview showing ZnO NPs; (b) high-resolution image of ZnO NPs that grows on the surface of a larger ZnO NPs; inset shows a highly crystalline grain boundary; (c) lower magnification images of ZnO NPs with smaller NP surrounding the larger ones; (d) HRTEM image highlighting the growth of new highly crystalline ZnO seeds or NP on the surface of the bigger NPs.

surrounded by smaller NPs. In Figure 5b, a NP of 20 nm growing on a larger NP is examined in detail, indicating epitaxial growth at the crystalline interface as shown in the inset. In Figure 5c, 5 nm sized and spherical in appearance NPs are shown alongside the larger ∼50 nm in size ZnO NPs. The fraction of tetragonal shape ZnO NPs grows with the increase of the synthesis temperature, and a new diamond shape can also be observed. The 5 nm sized NPs growing on the surfaces of larger ZnO NP are shown in Figure 5d and of good crystalline quality. It is noteworthy at this point that the presence of such nanopyramids on the surface of the bigger ZnO NPs may have an effect on the luminescence properties,37 particularly considering nanoparticle size-dependent PL yield. TEM of ZnO Nanoparticles Synthesized in the Presence of MWCNTs. TEM observations suggest that the presence of MWCNTs in the precursor mixture apparently does not affect growth of the ZnO nanoparticles, which is in agreement with our XRD results. The low-magnification TEM image in Figure 6a shows that the embedded nanoparticles in ZnO:MWCNTs are of similar shape and size as the free-standing ZnO NPs synthesized under similar conditions. Figure 6b shows a high-magnification image of a single isolated MWCNT introduced in the precursor mixture after ZnO synthesis at 240 °C and centrifugation. The surface of the MWCNT is mostly clean with the exception of a hint of amorphous carbon but without organic coating, confirming the inertness of the MWCNTs during the synthesis. Nevertheless, damage to the sidewalls as a result of ZnO:MWCNTs synthesis cannot be ruled out. However, the nanotubes show bends and twists, also topological defects already existing in the as grown MWCNT. HRTEM study shows that ZnO nanoparticles synthesized at 240 °C in the presence of MWCNTs have similar shape and

Figure 7. (a) HRTEM micrographs of ZnO nanoparticles produced at 240 °C in the presence of MWCNTs; (b) high magnification of a ZnO nanoparticle with stacking faults in direct contact with a MWCNT. Red arrows indicate stacking faults, while yellow arrow indicates partial dislocation bordering the stacking faults.

One can also observe that ZnO NPs are highly crystalline, suggesting that MWCNTs do not affect their crystallinity during the growth process. Figure 7b shows a highmagnification image with a part of a hexagonally shaped ZnO NP in contact with MWCNT. The ZnO NP harbors a large number of stacking faults and dislocations, which are Zn vacancy or Zn interstitial related, and accordingly may play an important role in the PL properties of the nanocomposite. Stacking faults creating local strains in the crystal lattice are the most common planar defects present in the Wurtzite hexagonal structure of ZnO that are easily observed via TEM contrast analysis.38 Moreover, both types, Zn vacancy and interstitial-mediated stacking faults, have been observed via TEM of ZnO structures.39 Moreover, contact of highly defective NP with CNT tends to modify surface defects to a large extent as will be described in the forthcoming PL section. Figure 8 presents TEM observations of the hybrid ZnO:MWCNT nanocomposite produced at 300 °C. The HRTEM micrograph in Figure 8a shows that ZnO nanoD


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Figure 8. (a) TEM micrographs of ZnO nanoparticles produced at 300 °C in the presence of MWCNTs. (b) HRTEM micrographs of one ZnO NPs surrounded by MWCTs showing pyramidal particles. (c) ZnO NPs in direct contact with facets MWCNTs.

Figure 9. Diffuse-reflectance spectroscopy of the free-standing ZnO NPs and hybrid ZnO:MWCNT nanocomposites synthesized at (a) 200, (b) 240, and (c) 300 °C temperature. Absorbance spectra are represented by the equivalent Kubelka−Munk functions.

particles have tetragonal, hexagonal, and spherical shapes and that all ZnO NPs are in contact with MWCNTs. The TEM image in Figure 8b shows specifically a pyramidal-shaped ZnO nanoparticle surrounded by MWCNTs, which demonstrates that ZnO NPs are growing right within the MWCNTs network, thus promoting direct contact between NPs and MWCNTs. The higher magnification image in Figure 8c shows no visible stacking faults and defects inside the ZnO nanoparticles in contrast to the former case. Hence, based on our TEM observations, it is fair to conclude that the presence of MWCNTs during the synthesis does not affect the ZnO nanoparticles growth in any manner, even at high temperature; however, the crystalline quality of the NPs is improved at higher temperature. Furthermore, the tiny nanoparticles that were present on the surface of larger NP in the case of the synthesis without CNT are no longer visible in the hybrid nanocomposites. Optical Characterization. The diversity of shapes and sizes of the free-standing and CNT-embedded ZnO nanoparticles revealed by TEM infers that it is reasonable to expect just as variable optical properties. There are several potential reasons for that, most relevant among which is a combination of ZnO being a polar material with dimensions reduced to a nanoscale. More specifically, a significant increase of surface-tovolume ratio in a nanoparticle compared to that of a bulk crystal leads to accordingly enhanced manifestation of surfacerelated defects in optical emission/absorption measurements. The importance of the particle’s shape, on the other hand, spawns from the wurtzite structure of ZnO crystal, providing a variety of polar and nonpolar surfaces, which may be exposed

to a different degree depending on a particular shape and morphology (tetragonal, hexagonal, diamond, spherical) of a nanoparticle. In this section, we outline optical absorption and emission properties and present a comparative analysis of the free-standing ZnO NPs and hybrid ZnO:MWCNT nanocomposites. Absorption Properties from Diffuse-Reflectance Measurements. The absorption properties of the free-standing ZnO NPs and hybrid ZnO:MWCNT nanocomposites were deduced from the UV−vis diffuse-reflectance spectroscopy (DRS) measurements carried out at room temperature. It is apparent from the comparison of DRS spectra summarized in Figure 9 that optical absorption in hybrid ZnO:MWCNT nanocomposites is fully dominated by the embedded ZnO NPs, which share the same key features with the free-standing NPs. Such a dominance is not surprising bearing in mind the specifics of DRS measurements, where both the volumetric ratio of the nanocomposite constituents (ZnO NPs vs MWCNT) and efficient light-scattering from multifaceted NPs clearly favor DRS response from the nanoparticles. It is also noteworthy in this context that mostly metallic in nature MWCNTs tend to bundle in natural conditions (air ambient), thus further diminishing the likelihood of observing specific optical signature of CNTs. On the other hand, the absorbance spectra of the freestanding and embedded NPs represented by Kubelka−Munk functions in Figure 9 reveal certain dissimilarities in the belowgap region. The free-standing ZnO NPs exhibit a broad absorption shoulder stretching from the band edge (∼3.3 eV) down to ∼2 eV, which is indicative of a significant presence of E


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Figure 10. Photoluminescence spectra obtained at 300 K in air ambient (a−c) and in vacuum (d−f) from the free-standing ZnO NPs and hybrid ZnO:MWCNT nanocomposites synthesized at 200 (a and d), 240 (b and e), and 300 °C (c and f), respectively. Note different PL intensity scale (×4) in c and f. PL spectral components of the hybrid nanocomposites and free-standing NPs are presented in b and c, respectively (shaded areas represent Gaussian deconvolution fits; vertical dashed markers indicate the positions of the common spectral components).

components varies considerably in the free-standing ZnO NPs and hybrid nanocomposites for given ambient parameters, suggesting that MWCNTs in contact with NPs nonetheless play a significant role in the photogenerated carrier transport. The key elements of PL spectra in Figure 10 are associated with optical activity of ZnO intrinsic defects contributing to deeplevel emission (DLE) in the visible range and with excitonic near-band-edge (NBE) emission in the UV region of spectra. Indeed, a number of optically active point defects may form during the synthesis of ZnO NPs, including oxygen vacancies (VO), Zn vacancies (VZn), oxygen interstitials (Oi) and antisites (OZn), Zn interstitials (Zni), as well as their complexes.41 While the intensity ratio of NBE to DLE bands is commonly considered as a measure of ZnO crystallinity, the mutual ratios of multiple components contributing to DLE reveal actual balance of intrinsic defects, which depends on the synthesis conditions and postfabrication treatment, whereas in the case of ZnO NPs it might be also affected by nanoparticle size and shape factors. Furthermore, the enhanced role of near-surface traps and surface-adsorbed species due to the high surface-tovolume ratio in NPs may offer a range of alternative, radiative and nonradiative, recombination pathways for the photogenerated carriers. One can observe in Figure 10 a general trend for both the free-standing ZnO NPs and the hybrid ZnO:MWCNTs that total quantum efficiency (integrated PL intensity) increases with increasing synthesis temperature, which is indicative of suppressed competition from the nonradiative recombination pathways. Interestingly, this agrees well with a similar trend of decreasing DLA with increasing synthesis temperature observed in absorption spectra.

deep centers in the band gap related to intrinsic defects and, possibly, adsorbed species. From the comparison of Figures 9a−c, one can also notice a trend of gradually decreasing deeplevel absorption (DLA) with the increase of temperature during synthesis of ZnO NPs. It is interesting to note that in the case of hybrid ZnO:MWCNT nanocomposites, a similar DLA shoulder is only observed for the nanocomposite synthesized at 240 °C (Figure 9b), whereas DLA becomes hardly noticeable at 300 °C (Figure 9c) and comprises an entirely different band centered at around 1.5 eV in nanocomposite synthesized at 200 °C (Figure 9a). It is noteworthy that the latter development, featuring a gradual increase of absorbance for longer wavelengths approaching the near-infrared, could be a manifestation of the building up of the free-carrier absorption (intraband transitions) expected within the framework of the Drude model.40 Emission Properties from Photoluminescence Measurements. The photoluminescence properties were investigated at 300 K in air ambient and in vacuum. Figure 10 summarizes PL results from the free-standing ZnO NPs put against ZnO:MWCNT nanocomposites with the key elements of emission represented by Gaussian deconvolution fits. One can observe certain similarity of all PL plots featuring a repeatable set of the same key spectral components as indicated by vertical markers in Figure 10. The cause of such similarity is that PL originates solely from ZnO NPs, be it free-standing or embedded in nanocomposite, since the metallic nature of the MWCNT precludes this constituent from contributing to emission (neither to absorption, albeit for different reason, i.e., bundling in air ambient as mentioned earlier). However, the total PL quantum efficiency and intensities of particular spectral F


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emission efficiency being quenched to almost 50% of that in the air ambient.

Referring to the intensity ratios of NBE to DLE in the PL spectra, ZnO nanoparticles synthesized at 200 and 240 °C demonstrate superior crystallinity both in the case of freestanding NPs and in embedded ZnO:MWCNTs. Concurrently, ZnO NPs synthesized at 200 °C exhibit the lowest total quantum efficiency (integrated PL yield) compared to other growth temperatures, inferring strongest nonradiative losses (Table 1). By contrast, the most luminous are ZnO NPs synthesized at 300 °C apparently as a result of suppressed nonradiative losses and also due to increased DLE component. In fact, the broad DLE band, which dominates in the roomtemperature PL spectra of all nanostructures, is a superposition of several emissions. Among these, the green luminescence centered at around 2.5 eV is associated with singly ionized oxygen vacancies, suggesting that only very few surface defects are quenched after ZnO NPs decorate the MWCNT. Moreover, considering the presence of extended defects, as proven by TEM, it is reasonable to assume that red DLE component centered at ∼1.9 eV, which appears most prominent in the ZnO:MWCNT grown at 240 °C, is related to zinc vacancy (VZn). Such association with extended defects acting as a source of intrinsic ones is further supported by the occurrence of intense blue emission band (3 and 2.8 eV deconvolution peaks in Figure 10b) solely in the case of 240 °C nanocomposites and taking into consideration that this spectral region is known for optical manifestation of zinc vacancies and interstitials.41 For all samples, coupling of the ZnO NPs with MWCNTs enhances to a variable extent both the NBE and the green DLE component (∼2.5 eV), whereas the red DLE component (∼1.9 eV) appears less susceptible in this regard. The green luminescence comprises two closely positioned peaks at ∼2.5 and ∼2.3 eV related to optical transitions via singly ionized oxygen vacancies Vo+ capturing different charges.42,43 The peak at ∼2.5 eV is due to a singly ionized oxygen vacancy turning neutral by capturing an electron from the conduction band and recombining with a hole in the valence band. This phenomenon tends to occur within the volume of the nanoparticle.43 On the other hand, the emission at 2.3 eV results from Vo+ capturing a hole from the surface charges and turning into doubly ionized Vo2+ and is therefore a more surface-related phenomenon. In all cases, upon coupling of ZnO NPs with MWCNTs, the emission at 2.5 eV is largely enhanced compared to the emission at 2.3 eV, which clearly suggests that the CNTs tend to passivate the surface states and suppress the oxygen vacancy-related emission component. PL measurements conducted in both air ambient and vacuum conditions provide means to modify surface states.44,45 Typically, in air ambient conditions adsorbates like O2 molecules undergo chemisorption on the NPs surface by capturing a free electron from the n-type ZnO resulting in an upward band bending. In turn, this leads to increased separation of the photogenerated electrons and holes and creation of singly charged Vo+ in the depletion region at the surface, which can be monitored by PL via an enhanced green DLE band at the expense of NBE. The upward band bending may also affect quantum efficiency by preventing electrons from reaching nonradiative recombination centers at the surface. The desorption of O2 in vacuum results in opposite band bending effects as can be seen in Figure 10, where a general decrease of the quantum efficiency in vacuum for all samples is apparent. This phenomenon is most conspicuous for the ZnO NPs and ZnO:MWCNT synthesized at 300 °C, showing the PL

4. SUMMARY AND CONCLUSIONS We demonstrated the feasibility of directly producing carbonbased nanocomposites by using a one-pot NHSG method to synthesize ZnO NPs in the presence of MWCNTs. The effects of glovebox utilization on the ZnO NP synthesis via the NHSG route were studied. With a glovebox, the ZnO NPs tend to be more uniformly sized and shaped compared to those synthesized in air ambient. In the latter process, moisture tends to incorporate into the precursor disrupting nonaqueous conditions, whereupon both the size and the shape of NPs are more difficult to control. ZnO NP synthesis without a glovebox produces somewhat larger nanoparticles with an average diameter ranging from 10 to 140 nm. The morphology of NPs is also influenced by direct synthesis in air promoting two main shapes: tetragonal with sharp edges and hexagonal, besides the spherical shape of smaller NPs if such are produced. TEM study revealed that the fraction of tetragonally shaped ZnO NPs directly depends on the synthesis temperature. The hexagonally shaped NPs comprise two polar and six nonpolar facets,46 with the latter, nonpolar surface, known to be more prone to oxygen vacancies.47 The PL emission of all investigated nanostructures is dominated by the oxygen defect-related green luminescence at ∼2.3 eV. The carbonbased ZnO:MWCNT nanocomposites demonstrate PL properties interrelated with ZnO NP morphology that is in turn dependent on the synthesis temperature. The total PL efficiency and intensities of certain spectral components vary noticeably in the free-standing NPs and hybrid nanocomposites. This suggests that despite the metallic nature of MWCNTs precluding contribution to luminescence on their own, MWCNTs in contact with ZnO NPs or with other metal oxide NPs in general nevertheless play a significant role in the photogenerated carrier transport.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03070. Size distribution histograms (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (372) 620 4806. ORCID

E. Rauwel: 0000-0001-8950-1415 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the European Regional Development Fund project TK134 (TAR 16019). Financial support from the Estonian Research Council (grant PUT431), the Estonian Road Map infrastructure, and the NAMUR projects, MENESR and MAEDI French ministries (Parrot program no. 33787YJ). This work was developed in the scope of the project CICECO-Aveiro Institute of Materials (ref. no. FCT UID/CTM/50011/2013), financed by national funds through FCT/MEC and cofinanced by FEDER under the G


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The Journal of Physical Chemistry C PT2020 Partnership Agreement. Partial financial support by the Research Council of Norway and the University of Oslo through the FUNDAMENT project (FRINATEK program) is gratefully acknowledged.

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