Superhydrophobic polymethylsilsesquioxane pinned one dimensional ZnO nanostructures for water remediation through photo-catalysis

Abstract

ZnO nanostructures have been heavily explored for a variety of sensing properties and of late a major emphasis by researchers has been to find applications for ZnO materials in the domain of photo-catalysis. ZnO nanoparticles have been found as a better alternative to other materials for removing organic dyes from polluted water and the abolition of several hazardous materials etc. In this work we have developed ultra-dense high aspect ratio ZnO nano-forest like structures and explored their potential as photo-catalysts. The films formulated are superhydrophobic (contact angle ∼ 154°) in nature and have been evaluated as containing a high density of oxygen defects in the crystalline state of the ZnO (as validated through photoluminescence measurements). The samples were found to possess enhanced photo-catalytic properties, as measured through a dye degradation process using an UV-Vis spectrophotometer. These photo-catalytic properties may be due to the high defect density and also the enhanced area of the interactive surface as one goes from nano-particles to nano-rod like structures. The paper gives an insight into highly unique carpeted nano-wire bundles of ZnO and offers immense utility to the realization of high efficiency remediation filters.

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Introduction

Over the past several decades, a lot of exploration has been made into waste water treatment and the abolition of several pollutants, like organic and inorganic wastes and effluents etc. In line with this, there has been immense scope to explore the use of nano-materials for the purposes of the purification and disinfection of water and air. Due to the large enhancement of surface area provided by these materials, their role as a catalyst to accelerate remediation processes is of significant importance. Various materials (TiO_2, ZnO etc.) have been widely explored for better performance, with these materials being structured into various different shapes and sizes at mesoscopic scales. ZnO has been investigated successfully as an alternative to the conventional treatment for the removal of pollutant dyes from waste water. It is a low cost, high band gap material and it is absorbent of radiation in the UV region, absorbing more light quanta than even TiO₂.^1–4 When a source of UV light falls on ZnO, the material behaves as a semiconductor producing electron/hole pairs, thus increasing the overall conductivity of the material. The electron/hole pairs generated induce a complex series of reactions leading to the complete degradation of the dye pollutants adsorbed on the surface of this material. The degradation rate is highly enhanced if the ZnO material is nano-structured, as the one dimensional rod like structure of such a material leads to the easy release of electrons in the gap between the rods where the dye pollutant is present. Further, ZnO nanostructures offer a very high interfacial area to the fluid that is in direct contact with the nanostructures for remediation purposes. ZnO has been found to degrade most kinds of persistent organic pollutants, such as detergents, dyes, pesticides and volatile organic compounds, under UV irradiation.^5–9 Among various factors, such as aspect ratio, doping elements etc., the surface defects introduced in the ZnO nanostructures play a critical role in enhancing the photo-catalytic performance of the material. Defects on the ZnO nanostructures have been extensively investigated by many researchers and they have been found to play a major role in offering a high density of adsorption sites and they also offer enhanced surface reactivity. Zheng et al. showed that the oxygen vacancies and interstitial oxygen induced by solvothermal fabrication processes had a major impact on the photo-catalytic activity of ZnO.⁷ Wang et al. demonstrated the enhancement of the photo-catalytic activity of ZnO while exposed to the visible spectrum by narrowing the band gap through induced defects.⁸ Li et al. reported that tuning the relative concentration ratio of bulk defects to surface defects in TiO₂ nanocrystals could enhance the separation of photo-generated electron–hole pairs, thus improving the overall photo-catalytic efficiency of this material.⁹ ZnO has the ability to absorb a larger fraction of the solar spectrum (because of its band gap of 3.2 eV) vis à vis TiO₂, although its photo-catalytic reaction mechanism is similar to TiO₂.^10–12 Out of the variegated structures developed, viz. nano-belts, nano-wires, nano-cages, nano-combs, nano-springs, nano-rings^47–51 etc., one-dimensional ZnO nanostructures, such as nano-rods and nano-wires, have been investigated extensively due to their higher surface areas and their superior electrochemical properties, which are attributed to dimensional anisotropy. Therefore, a greater number of electrons and protons exist on the active sites of the nano-structured surfaces, resulting in a higher activity compared with other lower surface area nanoparticles. Although the vertical growth and associated surface area enhancement has been explored for ZnO nano-wires/rods by a lot of researchers,^13–18 very few attempts have been made that can fabricate and explore high aspect ratio vertically standing variable height nanostructures pinned to a highly nano-porous carpet. These structures would enhance the surface area multifold, thereby creating additional sites for binding onto dyes and chemicals so as to reduce them into harmless products and perform remediation.

In an earlier work,¹⁷ we reported a distinct way of fabricating ZnO nano-forests with a very high surface roughness and we explored their sensitivity towards the detection of trace gases. In this study, we explore their application for photo-catalysis with a much lower power UV source (10 W), making the material highly suitable for the coating of filter membranes, so that the remediation can be performed through the membranes. Further, the tall standing nanostructures are annealed and a comparative study is made between the annealed and unannealed samples, with respect to their photo-catalytic properties using Rhodamine AR dye. We have also tried to formulate a detailed understanding of the crystal defects within these structures that lead to an enhancement in their photo-catalytic properties.

Materials and methods

Silicon (100) p-type substrates (source: Logistic Inc. NY) were taken and thoroughly cleaned using AMD (acetone, methanol, and de-ionized water). 40% hydrofluoric acid (source: SDFCL, India) mixed in a 1 : 10 ratio with de-ionized water was used to passivate the silicon surface, resulting in a hydrophobic surface (contact angle ∼ 90°). The photo-degradation of the dye Rhodamine AR (M.W. 479.02, purchased from Thomas Baker) was studied to evaluate the efficacy of the zinc oxide nanostructures in our experiments. Zinc nitrate [Zn(NO₃)₂·6H₂O] (source: Merck Specialties Pvt. Ltd.) and hexamethylenetetramine [C₆H₆N₄] (source: Merck Specialties Pvt. Ltd.) were utilized for the growth of ZnO one dimensional rod like structures over catalyst nanoseeds of ZnO implanted in a porous bed of silica. The structural characterization was performed with FESEM (Zeiss Supra 40 V, Germany) and the morphology of the films with the one dimensional nanostructures growing out at different heights was determined. Further characterization of the nanostructures using X-ray diffraction (XRD) (X’Pert PRO, PAN analytical, Netherlands X-ray system) with Cu-Kα source radiation (having a wavelength of 1.54 Å) for visualizing the vertical growth along the c-axis, as well as the measurement of the built in stresses offered by the nano-template and a correlation to the oxygen defects using photoluminescence (Xenon Lamp Spectrofluorometer (Jobin Yvon, Fluorolog-3)), was carried out on these structures. We have also performed Raman characterization (by Wi Tec, Germany [Raman parameters: objective lens = 20×, laser wavelength = 532 nm, power = 12 mW, spot size = 500 nm]) for the analysis of the samples and a UV-Vis spectrophotometer (50 Bio Varian Cary spectrophotometer) was used to monitor the photo-catalytic degradation of Rhodamine AR in the presence of these ZnO carpets. The catalytic activity was demonstrated in HPLC grade water purchased from Merck Pvt. Ltd., India. The annealing of the ZnO films was carried out at a heating rate of 10 °C min⁻¹ in the presence of oxygen for 1 hour. The flow rate of oxygen was set to 300 sccm.

To fabricate the dense ZnO nanostructures, firstly, PMSSQ nanoparticles and PPG were dissolved together with the nanoseeds and formulated into a homogenous dispersion using PGMEA (solvent) in the following manner. PGMEA was mixed separately with PMSSQ (in 5 : 1 ratio by weight) by ultrasonication. Further, PPG was mixed with PGMEA with the help of a vortex shaker and ultrasonicator until complete mixing was ensured. 1 g of the resulting PMSSQ–PGMEA solution was mixed with 5 g of the PPG–PGMEA solution and it was again ultrasonicated to obtain a homogenous dispersion. For convenience this solution was named as “solution 1”. In another solution, PGMEA was mixed with the ZnO nanoseeds with the help of a vortex shaker. This solution was named as “solution 2”. ZnO nanoseeds were prepared with the help of zinc chloride (MERCK Specialties Pvt. Ltd.) (0.1 M in 20 ml methanol) and NaOH pellets (SAMIR TECH-CHEM Pvt. Ltd., India) (1.25 M), which were fully mixed with stirring. The solution was mixed properly until a milky white colour appeared. The solution was added drop wise through a syringe (with an attached 0.45 μm sieve) onto the silicon substrate until the uniform deposition of the precursor was observed. Upon heating this substrate at 200 °C for 20 minutes, the crystallites decomposed to form ZnO particles. The above process was repeated several times to ensure a uniform seed layer on the substrate surface. The deposited layer was then scraped off the silicon surface using a sharp razor edge and collected in powder form in a vial. After accumulating a sufficient weight of this powder, the nano-templating was performed. Another silicon wafer (100) was cleaned using an AMD protocol and hydrogen passivated by immersing it in a 1 : 10 HF : DI water solution for ∼10 seconds. Then, an equal amount of solutions 1 and 2 were taken and mixed completely. The solution prepared was poured drop by drop over the substrate with the help of a syringe (5 ml) (attached with a 0.45 micron sieve) until the whole area of the substrate was covered with the film. The obtained film was ramped up to 450 °C on a hotplate and held at this temperature for 5 minutes. The film was thus heated above the decomposition temperature of PPG (>200 °C), causing the evaporation of PPG and entropic disturbances in the dispersed nanoparticles (both PMSSQ and ZnO) leading to random collisions between them and eventually resulting in particle–particle cross linking. ZnO nanoparticles were entrapped within the PMSSQ matrix and dispersed heterogeneously through the substrate at various heights of the PMSSQ film. This highly seeded matrix was used as a template to promote the growth of the ZnO one dimensional nanostructures in a gravity fed convection oven (set at 90 °C) for over 24 hours by holding the matrix upside down over a solution of (0.01 M) zinc nitrate (Zn(NO₃)₂ and (0.05 M) hexamethylenetetramine (HMTA) in DI water. The chemical reaction which occurred in the solution was initiated with the hydrolysis of HMTA in water, releasing formaldehyde (HCHO) and ammonia (NH₃). The ammonia (NH₃) is slowly released and assists in the formation of hydroxyl ions (OH⁻), which leads to the minimization of bulk nucleation of the Zn²⁺ in the solution. The net output of the process is the formulation of Zn(OH)₂, which dehydrates to form ZnO.^17,18 The chemical steps involved are clearly illustrated in eqn (1)–(7).

Zn(NO₃)₂·6H₂O → Zn²⁺ + 2NO₃⁻ + 6H₂O (1)

Zn²⁺ + 4OH⁻ → Zn(OH)₄⁻² (2)

Zn²⁺ + 4NH₃ → Zn(NH₃)₄⁺² (3)

(CH₂)₆N₄ + 6H₂O → 4NH₃ + 6HCHO (4)

(CH₂)₆N₄ + 4H₂O → (CH₂)₆(NH)₄ + 4OH⁻ (5)

Zn(OH)₄⁻² → ZnO + H₂O + 2OH⁻ (6)

Zn(NH₃)₄⁺² + 2OH⁻ → ZnO + 4NH₃ + H₂O (7)

The Field Emission Scanning Electron Microscopy (FESEM) images show highly dense, vertical nanostructures grown out of the nanoporous matrices. The reason for the vertical growth of the ZnO nanostructures has been previously explained by Tasker (1979),²² where he described three different surface topologies of ZnO, of which the type (III) surface resembles the present case. Since ZnO exists in the wurtzite crystal form which has hexagonal unit cells with six non polar faces capped by polar oxygen and Zn atoms in basal planes, these polar faces are highly unstable, containing very high surface energies, and can be defined as Tasker (III) surfaces. The tall structure growth is initiated in order to minimize the overall system energy, thus becoming stable when the growth takes place mostly in the vertical direction. The as grown film in this work was characterized with XRD, which confirmed the vertical growth of the ZnO nanostructures. Fig. 5 (the XRD data) shows a large peak for the (002) plane, signifying a higher density in the vertical orientation. The optical characterization was also performed with UV-visible spectroscopy in order to evaluate the band gap of the zinc oxide structures. Photoluminescence spectra suggested the presence of a differential in the crystal defects, which causes the varied photo-catalytic rates which were observed as one used different structures of ZnO with and without annealing.

To study the photo-catalytic behaviour, firstly 50 ml Rhodamine AR dye in aqueous solution (conc. = 2 × 10⁻⁵ mol l⁻¹) was poured into a glass petri dish after thorough mixing. The ZnO film was then held inside this solution which was top irradiated with a 10 watts power UV lamp emitting at a 254 nm wavelength. The exposure interval of this dye was varied and each sample was repeatedly characterized with a UV-visible spectrophotometer after exposing the solution for different durations. The characterization was performed by aliquoting out a 2 ml solution into a quartz cuvette and then resuspending this aliquot of solution, after using the UV-visible spectrophotometer, back into the source solution to maintain the pH during the whole experiment at 10. The absorbance of the Rhodamine AR dye was measured, giving an absorbance peak at 554 nm.

Results and discussion

Morphological evaluation of the films (FESEM and TEM)

Fig. 1 shows some representative FESEM images (FESEM, Quanta 200, Zeiss, Germany), taken at different zones and various magnifications, of the vertically growing zinc oxide one-dimensional structures pinned in nano-structured carpets. To visualize the length of the one dimensional wire/rod-like structures, the SEM stage was inclined at 45 degrees and lengthwise images of the wires were captured. The wires varied in length ranging from 1–5 microns. The top view of the ZnO nano-wires shows the hexagonal crystal growth (which confirms the presence of the wurtzite crystal lattice of ZnO). The sides of the hexagonal cross-section ranged from ∼80–150 nm. There were no morphological changes in the dimensions of the annealed and unannealed samples, although there was a significant variation registered in the UV-Vis (Fig. 4), XRD (Fig. 5) and photoluminescence (Fig. 6) characterization of the annealed and unannealed samples (Fig. 2).

Fig. 1 FESEM images of the ZnO nano-forest at various locations and in different magnifications using normal/angled sample stages.

Fig. 2 (a and b) TEM images for ZnO nanoparticles, (c and d) the nano-forest scratched from the film and (e and f) SAED patterns of the nano-particles and nano-forest respectively.

Elemental mapping

In this section we characterize the films thoroughly for the elemental mapping and analysis related to structural defects. The elemental mapping and composition were studied with the help of an energy dispersive X-ray spectra analyzer (Oxford instruments) [Fig. 3(a)]. The results indicate the strong presence of Zn and O elements indicating a full coverage of the pinned carpet with entanglement and contact between the various vertical structures, as one would normally envision when they grow at high density along the vertical direction. Fig. 3(d) and (e) indicate the elemental presence of oxygen and zinc species in the film cross-sections, as indicated by green and red dots respectively.

Fig. 3 (a) The elemental peaks corresponding to each element present over the film. (b) The percentage of the elements over the film. (c) The selected region for identifying the elemental presence. (d and e) The elemental mapping showing the dispersion of the O and Zn particles respectively over the film.

Structural analysis of the films

UV-visible spectrometry. We performed UV-visible spectrometry on both unannealed and annealed samples and found a distinct change in the band gap for all samples. We hypothesize that the change in the band gap in this case is purely due to a substantial change in the density of the defect states. In our opinion, the defect states get significantly re-distributed uniformly due to the annealing of the samples. The band gap analysis is performed through a Tauc plot,³¹ according to which the optical band gap ‘E_g’, the photon energy ‘hν’ and the molar absorptivity at frequency ‘ν’ are provided by eqn (8), where D is a constant and n is an integer.

αhv = D(hv − E_g)ⁿ (8)

From the UV-visible spectra, we found that annealed samples possess a red-shift of the band gap, which can be seen in Fig. 4. The inset shows a plot of the photon energy against the molar absorptivity. The band gap decreases as we anneal the film at higher temperature (from ∼3.02 eV for the unannealed sample to ∼2.90 eV for the sample annealed at 300 °C and ∼2.85 eV for the sample annealed at 500 °C). The photo-catalytic activity, as reported later, is found to be the highest in the annealed sample at 500 °C as annealing ensures a distribution of the defects throughout the surface of the nanostructures and a lesser band gap formulated at 500 °C demonstrates an easier electron–hole release.

Fig. 4 The UV-visible spectra of all the samples. Inset shows the Tauc plot for determining the band gap of the annealed and unannealed samples.

XRD analysis. We recorded the photoluminescence spectra (reported separately later in this manuscript) from which it is observed that there is an abundance of intrinsic defects within the ZnO crystal lattice. Crystallinity is a significant parameter that determines the photo-catalytic activity of a catalyst. Previous researchers, viz. Eufinger et al.²⁰ and Yubuta et al.,²¹ claimed that photo-catalytic activity increased with increased crystallinity in semiconductor metal oxides. Li et al.²⁴ concluded that the photo-catalytic activity of various ZnO powders occurred mainly due to the crystallinity rather than to the surface area. This group however was unable to predict a specific relationship between the photo-catalytic activity, the crystallinity and the surface area.

Fig. 5 shows the XRD data for all samples, in which all the peaks could be safely indexed as ZnO wurtzite structure as found in the standard reference data (JCPDS: 36 1451). The crystal sizes could be determined for these structures, and so could the deviation from crystallinity, through a line broadening analysis. The growth of the ZnO nanostructures in our case was performed through a nano-carpet entrapment process and hypothetically we can probably predict a highly pre-strained lattice structure from our method of fabrication of these structures. So, the most crucial parameter for our analysis is the strain induced line broadening and the XRD of the nano-structured samples should be demonstrative of lattice induced strains. Lattice strains in crystals generally manifest in two forms, uniform or non-uniform strains. It has been reported that the uniformly induced strains only cause a peak shift in the XRD whereas non-uniform strain is found to have a peak broadening effect. The causes of non-uniform strain can be many, for example point defects, poor crystallinity etc. Having no peak shifts observed in the samples confirms the presence of only non-uniform lattice strains. We have calculated the lattice size of the ZnO crystals by Debye–Scherer’s method [ESI S1†].^52,53

Fig. 5 The XRD data for the unannealed and annealed samples. Inset graph shows the difference in the peak width of the unannealed and annealed samples.

The average crystalline size of the ZnO film is 15.35 nm. The annealed samples at 300 °C have a crystalline size of 20.64 nm and that of 500 °C is 25.32 nm, which implies the sharpening of the peak width, thereby confirming that the crystallinity of the ZnO film is improved as the annealing temperature increases. Therefore, the annealed samples definitely possess low strain. In any event, the de-straining of a lattice would always lead to a better crystallinity of the samples. The lattice strains are further predicted by the Williamson–Hall method [see ESI eqn (S2)–(S5)†]. The predictions of the lattice strain through this method are summarized in Table 1 along with the lattice size. Generally, on annealing it is observed that there is an overall relaxation of the pre-strained lattice. In Fig. 5, the inset graph shows a comparison of the XRD data of the unannealed and annealed samples, indicating the difference in the peak broadening in the samples.

Table 1 Predicted crystal sizes and lattice strain values (calculation details reported in the ESI)

	Unannealed	Annealed at 300 °C	Annealed at 500 °C
Crystalline size (nm)	15.35 ± 5.44	20.64 ± 0.265	25 ± 5.44
Lattice strain	0.00 ± 0.430	0.258	0.170 ± 0.014

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Photoluminescence spectra. The ZnO nanostructures pinned on the silica carpet were provided with an excitation wavelength of 330 nm (3.76 eV). The unannealed film shows one huge peak at ∼380 nm and a second peak at ∼558 nm. The peak at ∼380 nm is due to strong near band edge UV emission. This UV emission is attributed to the direct recombination of excitons via an exciton–exciton collision process. The second peak is attributed to the broad green band which is obtained from the radial recombination of a photo generated hole with an e⁻ belonging to a single ionized oxygen vacancy present on the surface and is observed when ZnO is fabricated under oxygen deficient conditions.¹⁷ The possibility of any other impurities in the film is very much less, which would have otherwise caused the green emission as in the present case, as also confirmed by EDAX measurements earlier. As shown in Fig. 6, the UV emission of ZnO nanostructures increases when the sample is annealed at 500 °C. This effect can be attributed to the desorption of the surface adsorbed water and hydroxyl groups at high temperatures, which are unavoidable defects in such solution growth methodologies. On annealing the ZnO nano-wires at 500 °C in air, we observed the UV-emission decrease substantially. The visible emission however showed an overall increase due to the surface defects in the ZnO nanostructures. Also there is an observable blue shift in the annealed sample, with the green emission peak shifting from 560 nm to ∼520 nm on annealing, which could be due to corrections in crystal imperfections and lattice relaxations (Stokes shift).^23–26

Fig. 6 The PL spectra of the unannealed sample and the samples annealed at 300 and 500 °C.

The most common defects associated with ZnO formed under lower temperatures (<100 °C) are oxygen vacancies (V_O), interstitial Zinc (Zn_i) and Zinc vacancies (V_Zn). We have reported earlier that there is an inversion of the majority carriers over a temperature of 275 °C and the ZnO changes from n- to p-type.¹⁷ Here however the formation temperature is less than 100 °C and thus the majority carrier still remains n-type at this low temperature. We have considered the possibility of having V_O, Zn_i, V_Zn defects in our nanostructures. Considering the energy level diagrams for ZnO crystals³⁰ [ESI Fig. S1†] and the observation from the PL spectra of all the visible emission due to defects falling in the green region (with an energy interval of 2.24–2.46 eV), there are two possibilities which can occur. First is the transition from the conductance band to V_Zn, which is located ∼0.8 eV above the valence band, hence resulting in an emission of ∼2.5 eV, which is somewhat close to the observations made in our case. Second is due to hole capture by which occurs in three states (uncharged, singly charged and doubly charged) and occupies the energy levels from ∼2.17–2.5 eV above the valence band. The presence of the zinc interstitial Zn_i can be ruled out. Firstly, because the formation energy of Zn_i is quite large compared to other defects. Secondly, if Zn_i really existed we would have observed a transition from Zn_i to the top of the valence band with an emission energy corresponding to ∼2.8 eV but no such emission has been observed in the spectra. To rule out one of the other two possibilities we can observe the results from the annealing of ZnO nano-wires in an oxygen atmosphere. The annealing was carried out with a heating rate of 10 °C min⁻¹ and an annealing time of 1 hour, the cooling was performed using furnace cooling. The flow rate of oxygen used in the annealing was set to 300 sccm. To understand the formation of defects in the crystal lattice, the defect formation at higher temperatures can be written as:

The concentration of the vacancies formed can be estimated by

where ε_o and ε_Zn are formation energies of and respectively (a Schottky defect), k is the Boltzmann constant and T is thermodynamic temperature, which was considered, approximately, as the annealing temperature. The dependence on the partial pressure of O₂ (p_O2) can be summarized as in the equations below.

From the above equations, it can be seen that oxygen vacancies should decrease with increases in p_O2 and the zinc vacancies should increase with increases in p_O2 and vice versa. Fig. 6 shows a comparison of the spectra with oxygen annealing at 300 °C and 500 °C. In both cases, it is observed that the emission intensity decreases in the case of an O₂ atmosphere, i.e. an increase in p_O2 caused a decrease in the emission intensity. Therefore we can conclude that the one of main defects in the ZnO crystal is V_O. The oxygen vacancy can exist in three possible charged states, viz. neutral , singly ionized and doubly ionized . The singly ionized state is found to be thermodynamically unstable by calculations based on first principles. However some researchers suggest that plays a crucial role in the green emission of the samples. At higher temperatures (500 °C) conditions favourable to formation are achieved due to local lattice relaxations. The absorption of a photo-generated hole by a neutral oxygen vacancy under relatively low excitation results in the formation of the metastable which emits in the green wavelength. The metastable state will eventually, after some time, convert to the more stable .^23–30 This explains the entire room temperature PL phenomenon observed in the samples and establishes the presence of oxygen vacancies in the crystal of ZnO.

Fig. 7 The Raman spectra (a) for the ZnO film in an unannealed condition and (b) the ZnO films annealed at 300 and 500 °C.

Raman characterization. In order to understand the properties of the nanostructures, Raman characterization was performed and some peaks which may be possibly due to phonon confinement effects, local heating, stresses etc. were observed (Fig. 7). An inclusive study of the phonons in ZnO and their temperature dependence is illustrated below on a ZnO single crystal grown by the aforementioned approach. The data shown in the figure indicates an intense E₂ (high) mode, which confirms the perfect Wurtzite crystal structure of ZnO. The wurtzite crystal belongs to space group C_6v with two formula units in the primitive cell. The zone centre optical phonons can be classified according to the following representations: T_opt = A₁ + E₁ + 2E₂ + 2B₁, where the B₁ modes are silent modes, the A₁ and E₁ modes are the polar modes, and the E₂ modes are non polar, Raman active only and consist of low and high frequency phonons. The E2 modes are associated with the vibration of the oxygen atoms and the Zn sub-lattice.^32–36 The E₁ (LO) peaks are associated with several defects, such as oxygen vacancies, Zn_i (zinc interstitials)^40–43 etc. The unannealed sample has a stronger peak, indicating a higher presence of oxygen vacancies. Once annealing in the presence of oxygen is performed, followed by another round of Raman characterization, these peaks diminish, confirming the reduction of oxygen vacancies in the ZnO crystal lattice. It is interesting to note that the E₁(LO) phonon mode in the high-temperature-annealed samples undergoes a line width broadening and down shift in frequency with an asymmetric line shape. Although this may be due to strong coupling of the free carriers with the E₁ (LO) mode, the influence of the change in particle size due to the high temperature annealing cannot be ruled out here.

Using the Gaussian–Lorenz fitting, there is a weak shoulder peak in the low frequency direction of the intense vibration at 439 cm⁻¹, which corresponds to the E₁ (LO) mode. Moreover, the acoustic phonon overtone and optical phonon overtone with A₁ symmetry are located at 203 and 331 cm⁻¹, respectively. As per the research reported earlier, the acoustic combination of A₁ and E₂ was observed around 1101 cm^−1. Our result also shows a broad band between 1060 and 1200 cm⁻¹, which confirms the above-mentioned report. The broad peaks at 334, 663, 1145 cm⁻¹ should belong to the multi-phonon processes.^43–46

Contact angle measurement. We have also measured the contact angle to examine the nature of the film surface (Fig. 8). These films are found to be superhydrophobic in nature, again probably due to the tall standing structure of the zinc oxide films. The contact angle with a water droplet was found to be ∼154° and that with glycerol was found to be ∼156°. The contact angle hysteresis was also evaluated to be ∼4.2°, which proves the superhydrophobic nature of the film⁵⁴ [as shown in Fig. S3 of ESI†]. Superhydrophobic films of porous silica have already demonstrated very high capacitance, as reported by Bok et al.⁵⁵ We hypothesize that this superhydrophobicity may be the reason for the high charge storage inbetween the tall nano-wire structures, thus enabling the accelerated dye remediation as detailed later.

Fig. 8 The contact angles for the ZnO nano-forest in which (a) shows the contact angle (∼154°) with a water drop and (b) the contact angle with glycerol (∼156°), with the film depicting a hydrophobic nature.

Photo-catalytic study. The photo-catalytic efficiency of the PMSSQ embedded ZnO films was tested for the degradation of the Rhodamine AR dye, whose molecular formula is C₂₈H₃₁ClN₂O₃. A schematic of the photo-catalytic mechanism undergone by the dye in the presence of the ZnO one dimensional nanostructures is shown in Fig. 9. This reaction has been used as a model reaction for the evaluation of the photo-catalytic efficiency of various metal oxide nanoparticles (TiO₂, ZnO and so forth).¹⁹ In an aqueous photo-catalytic system, the induced hole is used to oxidise the water to a powerful oxidising radical species (the hydroxyl radical mainly) which further helps in the oxidisation of an organic/model compound. Usually, the electron produced on UV irradiation is either taken up by an electron acceptor, such an oxygen molecule (O₂), or by a metal ion. If the pH is greater than the point of zero charge, a superoxide radical is formed, when it is taken up by the O₂. This further reacts with water to form hydrogen peroxide (H₂O₂) – which on further oxidation generates OH˙ free radicals. These OH˙ radicals further attack the reactant species (dye molecule) to degrade them into carbon dioxide (CO₂), water and minerals (i.e. mineralisation). The metal ion (if any) can be reduced to its lower valence state and can be deposited on the surface of the catalyst if the induced electron is taken by a metal ion with a redox potential more than the band gap of the photo-catalyst. Because of the preferential ability of oxygen to react other competent species, the oxidation of metal ions is rare. Overall, the electron transfer process becomes more efficient if the reaction species are pre-adsorbed on the surface of the catalyst. In short, the induced electron–hole pair helps in the formation of hydroxyl radicals (a primary oxidant) which are ultimately used in oxidising the organic and inorganic species present in the reaction system.^37–39

Fig. 9 A schematic diagram showing the photo-catalytic phenomena.

Fig. 10 shows the UV-visible spectra of the degrading dye molecule after several periods of time for the unannealed sample and those annealed at 300 °C and 500 °C. The absorbance of the dye exposed to UV light is recorded up to the instant that the dye completely degrades. In the ESI, Fig. S2† shows the corresponding colour change due to dye degradation following UV exposure after various time intervals have elapsed. Fig. 11 shows the degradation rate of the dye, representing the reaction rates of all the samples. From the behaviour of the oxygen annealed sample, it is obvious that there is an enhanced reaction rate. We hypothesize that this additional treatment minimizes the release of oxygen as it replaces atoms within the ZnO lattice, thus creating more surface defects on the one dimensional nanostructures. A significant difference occurs in the Rhodamine dye degradation rate when correlated with the different morphologies and crystallinity at both UV wavelengths. The log-scale in the inset shows the linear relationship with the irradiation time, indicating that the photo-degradation process of the Rhodamine AR dye demonstrates pseudo-first-order kinetics. As per the reaction kinetics, the slope represented in the inset is representative of the reaction rate constant. The rate constants of the annealed samples are found to be higher than that of the unannealed sample.

Fig. 10 The photo-catalytic response of unannealed (a) and annealed samples at 300 °C (b) and 500 °C (c).

Fig. 11 The degradation rate of the Rhodamine dye with UV irradiation time. C_o is the initial concentration of the dye and C is the reaction concentration with time. Inset is the log plot of C_o/C with respect to time providing information on the reaction rate constant of the dye degradation process.

Reusability of the photo-catalyst. We performed an experiment to check the cycling behaviour of the film elements in order for this to qualify for commissioning within a filter. Cycle 1 took 380 min, cycle 2: 395 min, cycle 3: 410 min and cycle 4: 435 min. The degradation time of the dye is plotted with the number of uses, shown in Fig. 12.

Fig. 12 The reusability test for the sample, in which the sample is tested four times and dye degradation time is measured each time.

Conclusion

We present a distinct approach to produce ultrahigh surface area ZnO nanostructures which can undoubtedly provide a higher number of active sites for any kind of application, viz. biosensing, gas sensing, photo-catalysis etc., where the analyte of interest, while being in contact with the nanostructures, positively affects the process of degradation. The nanostructures created have varied morphologies and we have explored the photo-catalytic nature of the nanostructures using very low (10 W) UV irradiation. Further, we have developed an overall scheme through which the nanostructures are pinned down to a carpet which allows them to be patterned as high density water remediation filters. It can be concluded that defects in the one-dimensional ZnO nanostructures get enhanced significantly as the structures are pinned to a surface as a carpet. Such an increase is advantageous to the photo-catalytic degradation effects of these one-dimensional structures and could enhance charge separation and reduce electron–proton recombination losses, as well as also better serving as highly dense active sites for dye degradation reactions.

Acknowledgements

Authors gratefully acknowledge the DST Unit on Soft Nano-fabrication, at I.I.T. Kanpur for providing characterization facilities and the National Programme for Materials and Smart Structures (NPMASS) for providing financial support for this work.

References

1. J. H. Lim, et. al., UV Electroluminescence Emission from ZnO Light-Emitting Diodes Grown by High-Temperature Radiofrequency Sputtering, Adv. Mater., 2006, 18, 2720–2724.
2. A. B. G. Lansdown and A. Taylor, Zinc and titanium oxides: promising UV absorbers but what influence do they have on the intact skin?, Int. J. Cosmet. Sci., 1997, 19, 167–172.
3. F. Akira, T. N. Rao and D. A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol., C, 2000, 1–21.
4. M. Kunal, J. Kumar and A. Sharma, TiO₂-nanoparticles-impregnated photocatalytic macroporous carbon films by spin coating, Nanomater. Energy, 2013, 2, 121–133.
5. A. M. Ali, E. A. C. Emanuelsson and D. A. Patterson, Photocatalysis with nanostructured zinc oxide thin films: the relationship between morphology and photocatalytic activity under oxygen limited and oxygen rich conditions and evidence for a Mars Van Krevelen mechanism, Appl. Catal., B, 2010, 97, 168–181.
6. M. Soltaninezhad and A. Aminifar, Study nanostructures of semiconductor zinc oxide (ZnO) as a photocatalyst for the degradation of organic pollutants, Int. J. Nano Dimens., 2011, 2, 137–145.
7. Y. Zheng, et. al., Luminescence and Photocatalytic Activity of ZnO Nanocrystals: Correlation between Structure and Property, Inorg. Chem., 2007, 46, 6675–6682.
8. J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang and Y. Dai, Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO, ACS Appl. Mater. Interfaces, 2012, 4(8), 4024–4030.
9. C.-C. Lin and Y.-Y. Li, Synthesis of ZnO nanowires by thermal decomposition of zinc acetate dihydrate, Mater. Chem. Phys., 2009, 113, 334–337.
10. S. Sakthivel, B. Neppolian, M. V. Shankar, B. Arabindoo, M. Palanichamy and V. Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO₂, Sol. Energy Mater. Sol. Cells, 2003, 77(1), 65–82.
11. I. T. Peternel, et. al., Comparative study of UV/TiO₂, UV/ZnO and photo-Fenton processes for the organic reactive dye degradation in aqueous solution, J. Hazard. Mater., 2007, 148(1–2), 477–484.
12. N. Daneshvar, D. Salari and A. R. Khataee, Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO₂, J. Photochem. Photobiol., A, 2004, 162(2–3), 317–322.
13. H. Wang, et. al., Comparison of dye degradation efficiency using ZnO powders with various size scales, J. Hazard. Mater., 2007, 141(3), 645–652.
14. A. Akyol, H. C. Yatmaz and M. Bayramoglu, Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions, Appl. Catal., B, 2004, 54(1), 19–24.
15. C. Ma, Z. Zhou, H. Wei, Z. Yang, Z. Wang and Y. Zhang, Rapid large-scale preparation of ZnO nanowires for photocatalytic application, Nanoscale Res. Lett., 2011, 6(1), 1–5.
16. L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. Zhang, R. J. Saykally and P. Yang, Low-temperature wafer-scale production of ZnO nanowire arrays, Angew. Chem., Int. Ed., 2003, 42(26), 3031–3034.
17. G. Ankur, S. S. Pandey, M. Nayak, A. Maity, S. Basu Majumder and S. Bhattacharya, Hydrogen sensing based on nanoporous silica-embedded ultra dense ZnO nanobundles, RSC Adv., 2014, 4(15), 7476–7482.
18. G. Ankur, S. S. Pandey and S. Bhattacharya, High aspect ZnO nanostructures based hydrogen sensing, in Proceeding of International Conference On Recent Trends In Applied Physics And Material Science: RAM 2013, AIP Publishing, 2013, vol. 1536, no. 1, pp. 291–292.
19. Y. J. Jang, C. Simer and T. Ohm, Comparison of zinc oxide nanoparticles and its nano-crystalline particles on the photocatalytic degradation of methylene blue, Mater. Res. Bull., 2006, 41(1), 67–77.
20. K. Eufinger, et. al., Effect of microstructure and crystallinity on the photocatalytic activity of TiO₂ thin films deposited by dc magnetron sputtering, J. Phys. D: Appl. Phys., 2007,(17), 5232.
21. K. Yubuta, et. al., Structural characterization of ZnO nano-chains studied by electron microscopy, J. Alloys Compd., 2007, 436(1–2), 396–399.
22. P. W. Tasker, The stability of ionic crystal surfaces, J. Phys. C: Solid State Phys., 1979, 12(22), 4977.
23. P. Erhart, K. Albe and A. Klein, First-principles study of intrinsic point defects in ZnO: role of band structure, volume relaxation, and finite-size effects, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 73(20), 205203.
24. D. Li and H. Haneda, Morphologies of zinc oxide particles and their effects on photocatalysis, Chemosphere, 2003, 51(2), 129–137.
25. G. Yinyan, T. Andelman, G. F. Neumark, S. O’Brien and I. L. Kuskovsky, Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification, Nanoscale Res. Lett., 2007, 2(6), 297–302.
26. P. Zu, Z. K. Tang, G. K. L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma and Y. Segawa, Ultraviolet spontaneous and stimulated emissions from ZnO microcrystallite thin films at room temperature, Solid State Commun., 1997, 103(8), 459–463.
27. D. Banerjee, J. Y. Lao, D. Z. Wang, J. Y. Huang, Z. F. Ren, D. Steeves, B. Kimball and M. Sennett, Large-quantity free-standing ZnO nanowires, Appl. Phys. Lett., 2003, 83(10), 2061–2063.
28. S. C. Lyu, Y. Zhang, H. Ruh, H.-J. Lee, H.-W. Shim, E.-K. Suh and C. J. Lee, Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires, Chem. Phys. Lett., 2002, 363(1), 134–138.
29. B. Lin, Z. Fu and Y. Jia, Green luminescent center in undoped zinc oxide films deposited on silicon substrates, Appl. Phys. Lett., 2001, 79(7), 943–945.
30. L. Schmidt-Mende and J. L. MacManus-Driscoll, ZnO – nanostructures, defects, and devices, Mater. Today, 2007, 10(5), 40–48.
31. B. Kumar, H. Gong, S. Y. Chow, S. Tripathy and Y. Hua, Photoluminescence and multiphonon resonant Raman scattering in low temperature grown ZnO nano-structures, Appl. Phys. Lett., 2006, 89, 7.
32. M. Rajalakshmi, A. K. Arora, B. S. Bendre and S. Mahamuni, Optical phonon confinement in zinc oxide nanoparticles, J. Appl. Phys., 2000, 87, 2445.
33. P. Wang, G. Xu and P. Jin, Size dependence of electron-phonon coupling in ZnO nanowires, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 113303.
34. H. M. Cheng, H. C. Hsu, Y. K. Tseng, L. J. Lin and W. F. Hsieh, Raman scattering and efficient UV photoluminescence from well-aligned ZnO nanowires epitaxially grown on GaN buffer layer, J. Phys. Chem. B, 2005, 109, 8749.
35. J. F. Scott, UV resonant Raman scattering in ZnO, Phys. Rev. B: Condens. Matter Mater. Phys., 1970, 2, 1209.
36. K. A. Alim, V. A. Fonoberov and A. A. Balandin, Origin of the optical phonon frequency shifts in ZnO quantum dots, Appl. Phys. Lett., 2005, 86, 053103.
37. K. Kabra, R. Chaudhary and R. L. Sawhney, Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis: a review, Ind. Eng. Chem. Res., 2004, 43(24), 7683–7696.
38. C.-C. Chen, Degradation pathways of ethyl violet by photocatalytic reaction with ZnO dispersions, J. Mol. Catal. A: Chem., 2007, 264(1–2), 82–92.
39. M. R. Hoffmann, et. al., Environmental Applications of Semiconductor Photocatalysis, Chem. Rev., 1995, 95(1), 69–96.
40. S. Y. Li, P. Lin, C. Ying Lee and T. Y. Tseng, Field emission and photofluorescent characteristics of zinc oxide nanowires synthesized by a metal catalyzed vapor–liquid–solid process, J. Appl. Phys., 2004, 95(7), 3711–3716.
41. B. Kumar, H. Gong, S. Y. Chow, S. Tripathy and Y. Hua, Photoluminescence and multiphonon resonant Raman scattering in low temperature grown ZnO nano-structures, Appl. Phys. Lett., 2006, 89, 7.
42. R. Wahab, S. G. Ansari, Y. S. Kim, H. K. Seo, G. S. Kim, G. Khang and H.-S. Shin, Low temperature solution synthesis and characterization of ZnO nano-flowers, Mater. Res. Bull., 2007, 42(9), 1640–1648.
43. P. Wang, G. Xu and P. Jin, Size dependence of electron-phonon coupling in ZnO nanowires, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 113303.
44. H. M. Cheng, H. C. Hsu, Y. K. Tseng, L. J. Lin and W. F. Hsieh, Raman scattering and efficient UV photoluminescence from well-aligned ZnO nanowires epitaxially grown on GaN buffer layer, J. Phys. Chem. B, 2005, 109, 8749.
45. J. F. Scott, UV resonant Raman scattering in ZnO, Phys. Rev. B: Condens. Matter Mater. Phys., 1970, 2, 1209.
46. K. A. Alim, V. A. Fonoberov and A. A. Balandin, Origin of the optical phonon frequency shifts in ZnO quantum dots, Appl. Phys. Lett., 2005, 86, 053103.
47. F.-Q. He and Y.-P. Zhao, Growth of ZnO nanotetrapods with hexagonal crown, Appl. Phys. Lett., 2006, 88, 193113.
48. W. Cheng, P. Wu, X. Zou and T. Xiao, Study on synthesis and blue emission mechanism of ZnO tetrapod like nanostructures, J. Appl. Phys., 2006, 100(5), 054311.
49. Q. F. He and Y. P. Zhao, Growth and optical properties of peculiar ZnO tetrapods, J. Phys. D: Appl. Phys., 2009, 39, 2105–2108.
50. S. Huang, Q. Yang, B. Yu, D. Li, R. Zhao, S. Li and J. Kang, Controllable synthesis of branched ZnO/Si nanowire arrays with hierarchical structure, Nanoscale Res. Lett., 2014, 9(1), 1–9.
51. D. Deng, S. T. Martin and S. Ramanathan, Synthesis and characterization of one-dimensional flat ZnO nanotower arrays as high-efficiency adsorbents for the photocatalytic remediation of water pollutants, Nanoscale, 2010, 2(12), 2685–2691.
52. A. K. Zak, W. H. Abd Majid, M. Ebrahimizadeh Abrishami and R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson–Hall and size–strain plot methods, Solid State Sci., 2011, 13(1), 251–256.
53. V. D. Mote, Y. Purushotham and B. N. Dole, Williamson–Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles, J. Theor. Appl. Phys., 2012, 6(1), 1–8.
54. S. A. Kulinich and M. Farzaneh, Effect of contact angle hysteresis on water droplet evaporation from super-hydrophobic surfaces, Appl. Surf. Sci., 2009, 255(7), 4056–4060.
55. S. Bok, A. A. Lubguban, Y. Gao, S. Bhattacharya, K. Gillis, S. Gupta, P. R. Sharp and S. Gangopadhyay, Synthesis of electrochemically active Silica matrix bound carbon based electrode materials via Sol-Gel process, J. Electrochem. Soc., 2008, 155(5), K91–K95.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02938d

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