The idea of a mesocrystal was first presented by H. Cölfen and M. Antonietti in 2005^[1], defined as a superstructure of crystalline nanoparticles with a crystallographically ordered alignment on the scale of some hundred nanometers to micrometers^[1−3]. Over the last few years, the concept of mosocrystal has developed and classified to their structures^{[4, 5]}. The primary method used in mesocrystal synthesis is hydrothermal, in which the solution is kept at a proper temperature for a specific time^{[6, 7]}. After that, the resulting mesocrystals are washed to eliminate impurities. In some cases, mesocrystals also need further annealing.

Specifically, metal oxide mesocrystals are superstructures of accumulated metal oxide nanostructures with interesting optical, electronic, and electrical properties that make them useful for different applications, including optoelectronics, catalysis, and photodetection^{[8, 9]}. A general approach for various metal oxide mesocrystals syntheses has not been recognized yet, which limits the development of mesocrystals composed of two or more different types^[10]. Metal oxide mesocrystals are essential for an extensive range of applications, including catalysis^[11], sensing^[12], optoelectronics^{[13, 14]}, and solar energy conversion^{[15, 16]}, since they have the combined properties of the building metal oxide nanostructures as well as the properties of interfacial interactions between these nanoparticles^[17].

More specifically, zinc oxide (ZnO) has a wide bandgap as well as high excitonic energy at room temperature^[18]. Among the various ZnO nanostructures, mesocrystal ZnO nanorods (ZnONRs) have attracted substantial consideration due to their high stability and large specific surface area^[19]. However, using ZnO has limited photophysical applications due to some disadvantages, including particle aggregation during the photophysical process, its wide bandgap leading to restriction of ZnO usage in the visible region, and the quick recombination of charge recombination of the photogenerated electron−hole pairs^[20]. To face these problems, ZnO is incorporated with carbonaceous species, which causes the effective separation of photogenerated excitons on the ZnO surface and reduces the electron−hole recombination rate through the creation of an electrical field around hetero-interfaces^[21]. More specifically, reduced graphene oxide (rGO) is an exceptional carbonaceous material that improved photophysical properties due to its highly active surfaces and excellent carrier mobility^{[22, 23]}. As a result, both rGO and ZnO are attractive candidates for the concept of practical nanocomposites^[24].

In this work, mosocrystal ZnONRs with different sizes, ranging between several hundred nanometers and micrometers, are synthesized using a simple hydrothermal method. The structural properties, including crystal structure, morphological structure, and elemental structure, are investigated. After that, the ZnONRs/rGO nanocomposite superstructure is synthesized using the hydrothermal method. The incorporating mechanism between mesocrystal ZnONRs and rGO is investigated by studying the crystal, chemical, and morphological properties of the nanocomposite superstructure. Finally, optical transmittance, bandgap energy, band structure, electrical conductivity, and photoconductivity of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure are investigated using ultraviolet−visible (UV−Vis) spectroscopy and a four-point probe.

The graphite powder (12.01 g/mole), sodium nitrate (NaNO₃, 84.99 g/mole), potassium permanganate (KMnO₄, 158.034 g/mole), potassium hydroxide (KOH, 56.1056 g/mol), hydrazine hydrate (N₂H₄, 32.0452 g/mol), zinc nitrate hexahydrate (Zn(NO₃)_0.6H₂O, 297.5 g/mol), hexamethylenetetramine (HMT, C6H12N4, 140.186 g/mol) were purchased from Sigma-Aldrich.

Modified Hummer’s method was used to synthesize graphene oxide (GO). Graphite (2.50 g) and sodium nitrate (1.25 g) were mixed in 58 mL of sulfuric acid using a magnetic stirrer in an ice bath for 1 h. Afterward, potassium permanganate (7.50 g) was added drop by drop for about 30 min, as the temperature rose to 40 °C for another 30 min. 250 mL of deionized water and 25 mL of hydrogen peroxide (30%) were added to the solution. After that, the resultant powder was washed with deionized water several times. 100 mL of hydrochloric acid was added to the resultant powder, left overnight, and washed with deionized water. Finally, the resultant GO powder was dried at 60 °C overnight in a vacuum oven. For reduced GO (rGO) synthesis, the resultant GO powder was homogenized in deionized water using a sonication bath under ambient conditions at room temperature. After that, potassium hydroxide (0.60 g) and hydrazine hydrate (4.00 mL) were added to the solution. The temperature of the solution increases to 100 °C under a continuous stirrer for 24 h. Finally, the resultant rGO was washed with distilled water and ethanol and dried at 70 °C for 24 h.

The ZnONRs stock solution was prepared by mixing zinc nitrate hexahydrate (2.975 g) and HMT (1.40 g) in 100 mL water using a magnetic stirrer for 2 h. The resultant stock solution was transferred to an autoclave under 95 °C for 6 h. The set of overall reactions to form ZnONRs is described as follows^[25],

1C6H12N4+6H2O→6CH2O+4NH3,

2NH3+H2O↔NH4++OH−,

32OH−+Zn2+→Zn(OH)2.

The resultant Zn(OH)2NRs were washed with ethanol and distilled water and dried at 50 °C for 24 h in air. Finally, Zn(OH)2NRs were annealed in air at 500 °C for 2 h to get ZnONRs,

4Zn(OH)2→HeatZnO+H2O.

For ZnONRs/rGO nanocomposite superstructure synthesis, 0.1 g rGO was dispersed in 20 mL ethanol using ultrasonication for 1 h under ambient conditions. On the other hand, 0.1 g ZnONRs were dispersed in 20 mL ethanol using ultrasonication for 1 h under ambient conditions. rGO and ZnONRs solutions were mixed using ultrasonication for 1 h under ambient conditions. ZnONRs/rGO nanocomposite solution was autoclaved at 180 °C for 24 h. The final step involves washing with distilled water and, after that, drying at 70 °C for 24 h. After that, the ZnONRs/rGO nanocomposite superstructure with a thickness of about 10 μm was dispersed in ethanol and deposited in the glass substrate using the casting method (Fig. 1).

FTIR microscope measurement (HYPERION 3000 Bruker) was used to investigate the chemical structure and properties. XRD patterns were obtained using powder XRD (Malvern Panalytical Ltd., Malvern, UK) using CuKα1 ray (λ = 0.1540598 nm). Surface morphology was performed using a scanning electron microscope (SEM, Quanta FEG 450). UV−Vis transmittance spectra were measured using a UV−Vis spectrometer (Shimadzu, U-3900H). The absorption coefficient spectra were calculated using α=(1/d)ln(1/T)^{[26, 27]}. The electrical conductivity was measured using a four-point probe (Microworld Inc.) connected with a Keithley 2450 source meter. The average conductivity of the film was calculated by averaging the resistance of 12 points across the film and considering the film thickness.

A mesocrystal is a superstructure involving nanoparticles from several hundred nanometers to micrometers. Unlike the traditional approach of a single crystal growth, non-traditional crystallization is used in the mesocrystals growth (Fig. 2(a)). Herin, we synthesized mesocrystal ZnONRs using a hydrothermal method by mixing zinc nitrate hexahydrate and hexamethylenetetramine. Then, we transferred the mixture into an autoclave at 95 °C for 6 h. Finally, ZnO nanorods are annealed in air at 500 °C for 2 h. The SEM image of the mesocrystal ZnONRs demonstrates that their sizes range between 500 nm and 5 µm (Fig. 2(b)). Additionally, the mesocrystal ZnONRs have a hexagonal rod-like structure (Fig. 2(c)). They are accumulated from small nanoparticles with average size ranges of 50−70 nm (Fig. 2(d)), indicating that the synthesized mesocrystal ZnONRs have a single-crystalline structure composed by oriented aggregation of ZnO nanocrystals. XRF scan demonstrates that the mesocrystal ZnONRs have about 96.5% Zn, whereas the impurities are about 3.5% (Fig. 2(e)). Additionally, the XRD pattern of mesocrystal ZnONRs confirms the hexagonal wurtzite crystal structure in the SEM images, according to the space group of P63mc and JCPSD 36−1451 (Fig. 2(f)).

After describing the mesocrystal ZnONRs' structure, the crystal, chemical, and morphological properties of ZnONRs/rGO nanocomposite superstructure will be presented. First, XRD pattern demonstrates that ZnONRs have polycrystalline hexagonal wurtzite crystal structure with diffraction peaks at 31.4°, 34.1°, 35.9°, 47.2°, and 56.3°, corresponding to diffraction planes of (100), (002), (101), (102), and (110), respectively, which agrees with JCPSD 36−1451 (Fig. 3(a)). The lattice constants of the hexagonal structure were calculated by a=4/3d100 and c=2d002, where dhkl is the interplanar distance calculated from Bragg’s law (n=2dhklsinθhkl)^[28]. The lattice constants of ZnONRs are a=3.29A and c=5.26A. Additionally, the XRD pattern of rGO exhibits three diffraction angles at 8.86°, 24.04°, and 42.88°, corresponding to GO (002), rGO (002), and rGO (111), respectively^[29]. This finding shows that the reduction of GO was successfully performed with a small ratio of unreduced GO. On the other hand, the XRD also demonstrates that ZnONRs/rGO nanocomposite superstructure exhibits the diffraction peaks for both of ZnONRs and rGO, with absence of the diffraction peak of GO (002), indicating that incorporating rGO with ZnONRs reduces the oxygen from the residual GO in the ZnONRs/rGO nanocomposite superstructure. Additionally, incorporating rGO with ZnONRs shifts the diffraction peaks to higher diffraction angles, indicating that the lattice constants of ZnONRs in the ZnONRs/rGO nanocomposite superstructure decreases to a=3.24A and c=5.20A. Moreover, Williamsons−Hall (WH) method was used to calculate the crystallite size of ZnONRs and ZnONRs in ZnONRs/rGO nanocomposite superstructure, as described in the literature^{[26, 30]}. The crystallite size of ZnONRs is 63 nm, whereas the crystallite size of ZnONRs in ZnONRs/rGO nanocomposite superstructure is 27 nm.

The FTIR spectrum of ZnONRs exhibits three main vibrational bands of stretching Zn−O bonds (460, 635, and 776 cm⁻¹) (Fig. 3(b)). In addition, the vibrational bands in the range of 800−1200 cm⁻¹ are associated with the acetate group, the residuals from the starting materials^[31]. H impurities in the ZnONRs were determined by stretching ZnO∶H bonds (~3450 cm⁻¹). On the other hand, the FTIR spectrum of rGO exhibits several vibrational bands at 3388 cm⁻¹ (stretching −OH bond in C−OH group, carboxylic acids, and water), 2900 cm⁻¹ (stretching −CH₂), 1680 cm⁻¹ (stretching C=O of the carboxylic functionalities (−COOH) presumably located at the sheet edges), 1545 cm⁻¹ (stretching C=C), 1382 cm⁻¹ (bending C−OH hydroxyl groups), and 1000 cm⁻¹ (bending C−O), which is accepted with literature^[32]. Additionally, the ZnONRs/rGO nanocomposite superstructure exhibits the same vibrational bands for both ZnONRs and rGO. The SEM micrograph of rGO illustrates that rGO exhibits thin sheets randomly aggregated, with distinct edges and wrinkled surfaces (Figs. 3(c) and 3(d)). On the other hand, the ZnONRs/rGO nanocomposite superstructure exhibits thin sheets randomly aggregated with ZnONRs (Fig. 3(e)). Additionally, by enlarging the SEM image to a scale of 500 nm, we can see that the rGO thin sheets contain ZnO nanostructures (Fig. 3(f)) that are different from rGO in Fig. 3(d).

The optical and electrical properties of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure were investigated by studying the absorption coefficient, bandgap energy, band structure, and electrical conductivity. The optical transmittance of the mesocrystal ZnONRs in the visible region is about 65% (Fig. 4(a)), which is lower than the transmittance values for other ZnO nanostructures^{[33, 34]}. This can be attributed to the internal scattering between the ZnO aggregation and primary mesocrystal ZnONRs. Incorporated mesocrystal ZnONRs with rGO reduced the transmittance values to about 57% (Fig. 4(a)) and increased the absorption coefficient values (Fig. 4(b)). Additionally, the high absorption edge located before the incident wavelength of 400 nm shifted to the red region as mesocrystal ZnONRs incorporated with rGO, indicating the decreasing of the bandgap energy. The bandgap energy was calculated using a Tauc plot according to the equation (αhv)2=β(hv−Eg)^[35]. Therefore, the bandgap energy of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure is 3.31 and 2.96 eV, respectively. The decrease in the bandgap energy can be attributed to light absorption by heterogeneous bands of Zn−O−C. Additionally, the chemical interaction between ZnONRs and rGO or the formation of a joint electronic system between them results in bandgap energy narrowing^[36].

The band structure of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure was plotted (Fig. 4(c)) after calculating the valence band energy and conduction band energy, as well as the internal disorder bands, according to the literature^{[37, 38]}. The valence and conduction band energies of mesocrystal ZnONRs are 2.88 and −0.43 eV, respectively, shifting to 0.08 and 3.04 eV after incorporating with rGO. The internal disorder bands of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure are also 0.18 to 0.39 eV, respectively. In the ZnONRs/rGO nanocomposite superstructure, ZnO and rGO act as electron donors and acceptors, respectively. In which, the photoexcited electrons in the ZnO conduction band move to the rGO surface through the interface, resulting in a shorter electron transfer pathway in the nanocomposite superstructure (Fig. 3(d))^[39]. Under photoexcitation, the defect concentration and vacancies have been increased, leading to transitions from levels to conduction band^[18].

The electrical conductivity maps of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure were measured using a 4-point probe at 12 points across 1 cm × 1 cm of the samples. The electrical conductivity map of mesocrystal ZnONRs has a homogeneous electrical distribution across the sample with a range of 0.075−0.326 S∙cm⁻¹, and an average value of 0.145 S∙cm^{−1[28, 40, 41]}. This can be attributed to the high electron ability in mesocrystal ZnONRs due to the intimate contact between the adjacent ZnO nanostructures through the mesocrystal ZnONRs^{[2, 42]}. On the other hand, the electrical conductivity map of the ZnONRs/rGO nanocomposite superstructure has a homogeneous electrical distribution across the sample with a range of 62.672−63.148 S∙cm⁻¹, and an average value of 62.9 S·cm⁻¹ (Fig. 4(f)). The increase in the electrical conductivity upon incorporating with rGO can be attributed to shorter electron transfer pathways in the ZnONRs/rGO nanocomposite superstructure^[39].

UV sensing generally monitors the electrical conductivity variation using a 4-point probe when UV light irradiates (365 nm, 5 W) (Fig. 5(a)). Under dark, the oxygen molecules from the surrounding media are trapped on the nanostructure surface, forming a depletion layer near the surface, resulting in low electrical conductivity^[43]. However, under UV irradiation, electron−hole pairs are generated by light absorption and transfer through the depletion layers, consequently increasing the electrical conductivity. The electrical conductivity of mesocrystal ZnONRs increases from 0.145 to 0.599 S·cm⁻¹ under UV-irradiation. On the other hand, the electrical conductivity of the ZnONRs/rGO nanocomposite superstructure increases from 62.9 to 589.354 S·cm⁻¹ under UV-irradiation. The UV-photosensitivity determines the efficiency of the UV detectors and is given by: Photosensitivity=(σLight−σDark)/σDark, where σLight and σDark are the electrical conductivities under irradiation and in the dark, respectively. Therefore, the photosensitivity of the ZnONRs is ~3.13, which increases after incorporating with rGO to ~8.37. Compared to ZnO film, with a photosensitivity of ~1.05^[44], and ZnONPs, with photosensitivity of ~1.29^[45], the proposed mesocrystal ZnONRs exhibit higher photosensitivity due to the combination properties of the building metal oxide nanostructures as well as the properties of interfacial interactions between these nanoparticles^[17]. In addition, the photosensitivity of ZnO/rGO film is about 6.35^[44], whereas the photosensitivity of ZnONPs/rGO nanostructure is about 5.32^[45]. Additionally, the photoresponsivity of the photodetector can be evaluated using Resp=σph/P0 S·W^‒1·cm^‒1[46]. The photoresponsivity of mesocrystal ZnONRs is 0.12 S·W^‒1·cm^‒1, which is higher than ZnO film (6.10×10−4 S·W^‒1·cm^‒1)^[44] and ZnONPs (0.06 S·W^‒1·cm^‒1)^[45]. On the other hand, the photoresponsivity of the ZnONRs/rGO nanocomposite superstructure is 117.87 S·W^‒1·cm^‒1, which is higher than the photoresponsivity of ZnO/rGO film and ZnONPs/rGO nanostructure^{[44, 45]}.

Additionally, the photoconductivity response of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure was measured by turning on/off the UV light (Figs 5(b) and 5(c)). It is observed that the photoconductivity of ZnONRs/rGO nanocomposite superstructure is higher than that of ZnONRs. The higher sensitivity of ZnONRs/rGO nanocomposite superstructure reveals a chemical interaction between ZnONRs and rGO sheets on electron transfer. Incorporating mesocrystal ZnONRs with rGO sheets enables carrier transportation in nanocomposite superstructure, which improves electrical conductivity and photoconductivity under UV irradiation. Moreover, the photoconductivity and photosensitivity improvement of ZnONRs/rGO nanocomposite superstructure is described by UV-induced carrier generation and transport through the formation of ZnO−C bonding. Additionally, decreasing the bandgap energy results in a shorter electron transfer pathway to the rGO through carboxyl groups, where the excited electrons from the conduction band of ZnO take a low resistive path through the nanocomposite interfaces^{[39, 47]}. In this work, suitable photosensitivity was obtained by employing a simple technique for preparing ZnONRs/rGO superstructure composite. According to the literature^[48−50], both ZnO and rGO composites are thermally stable at temperatures below 300 °C, where the photoconduction mechanism occurs. On the other hand, ZnO and rGO composites are very stable materials in a moisture environment^{[51, 52]}. Therefore, we can conclude that the ZnONRs/rGO superstructure composite is suitable for photodetection applications and is highly stable under temperature and moisture fluctuations.

In this work, we reported an effective method for synthesizing mesocrystal ZnONRs using a simple hydrothermal method. The mesocrystal ZnONRs have hexagonal rod-like structures with size ranges between 500 nm and 5 µm, assembled from small nanoparticles with average size ranges of 50−70 nm. On the other side, we synthesized the rGO using a modified Hummer’s method. The synthesized rGO exhibits thin sheets randomly aggregated, with distinct edges and wrinkled surfaces. The ZnONRs/rGO nanocomposite superstructure was synthesized after that using the hydrothermal method. The ZnONRs/rGO nanocomposite superstructure exhibits thin sheets randomly aggregated with ZnONRs. By enlarging the SEM image to the scale of 500 nm, we can also see that the rGO thin sheets contain ZnO nanostructures that are different from rGO. The bandgap energy of mesocrystal ZnONRs and ZnONRs/rGO nanocomposite superstructure is 3.31 and 2.96 eV, respectively. The electrical conductivity map of mesocrystal ZnONRs has a homogeneous electrical distribution across the sample with a range of 0.075−0.326 S·cm⁻¹, and an average value of 0.145 S·cm⁻¹. The electrical conductivity map of the ZnONRs/rGO nanocomposite superstructure has a homogeneous electrical distribution across the sample with a range of 62.672−63.148 S·cm⁻¹, and an average value of 62.9 S·cm⁻¹. The electrical conductivity of mesocrystal ZnONRs increases from 0.145 to 0.599 S·cm⁻¹ under UV-irradiation. On the other hand, the electrical conductivity of the ZnONRs/rGO nanocomposite superstructure increases from 62.9 to 589.354 S·cm⁻¹ under UV-irradiation. The photosensitivity of the ZnONRs is ~3.13, which is increasing after incorporating with rGO to ~8.37. Additionally, the photoresponsivity of mesocrystal ZnONRs is 0.12 S·W^‒1·cm^‒1, whereas for ZnONRs/rGO nanocomposite superstructure, the photoresponsivity is 117.87 S·W^‒1·cm^‒1. Compared to other studies, we report a simple and inexpensive technique for preparing ZnONRs/rGO nanocomposite superstructure with high UV sensitivity suitable for UV detector applications.