Engineering the defect distribution in ZnO nanorods through laser irradiation

In recent years, defect engineering has shown great potential to improve the properties of metal oxide nanomaterials for various applications thus received extensive investigations. While traditional techniques mostly focus on controlling the defects during the synthesis of the material, laser irradiation has emerged as a promising post-deposition technique to further modulate the properties of defects yet there is still limited information. In this article, defects such as oxygen vacancies are tailored in ZnO nanorods through nanosecond (ns) laser irradiation. The relation between laser parameters and the temperature rise in the ZnO due to laser heating was established based on the observation in the SEM and the simulation. Raman spectra indicated that the concentration of the oxygen vacancies in the ZnO is temperature-dependent and can be controlled by changing the laser fluence and exposure time. This is also supported by the absorption spectra and the photoluminescence spectra of ZnO NRs irradiated under these conditions. On the other hand, the distribution of the oxygen vacancies was studied by XPS depth profiling, and it was confirmed that the surface-to-bulk ratio of the oxygen vacancies can be modulated by varying the laser fluence and exposure time. Based on these results, four distinctive regimes containing different ratios of surface-to-bulk oxygen vacancies have been identified. Laser-processed ZnO nanorods were also used as the catalyst for the photocatalytic degradation of rhodamine B (RhB) dye to demonstrate the efficacy of this laser engineering technique.


Introduction
Metal oxides are members of a critical group of semiconductor materials that have been extensively studied for important new applications in optoelectronics [1,2], as functional sensors [3,4] and in photocatalysis [5][6][7].As nanotechnology has rapidly developed over the last ten years, and the scale size of components has extended into the nanometer range, metal oxides have been shown to exhibit a variety of unique properties that are different from those of these materials in bulk form [8]. Some of the initial studies have evaluated the use of titanium dioxide (TiO 2 ) nanomaterials for various applications [9,10], while the high electron mobility in zinc oxide (ZnO) nanomaterials has indicated that this material may be ideal in certain nano-devices [11,12].However, some metal oxides share common problems such as low photo-absorption in the visible region and a fast recombination rate of photo-generated electron-hole pairs.To mediate these limitations, one strategy is to decorate (or dope) the metal oxide nanomaterials with metal nanoparticles [13,14].For instance, Mukhopadhyay et al [15].have demonstrated that the absorption of TiO 2 and ZnO nanorods (NRs) in the visible spectral region can be enhanced by depositing gold nanoparticles on the surface of the NRs.This was shown to increase the photocatalytic degradation of methylene blue and congo red in solution over these materials.Another promising strategy has focused on tuning the physical properties of the metal oxide nanomaterials themselves.As the understanding of metal oxide nanomaterials has advanced, it is now recognized that point defects in metal oxide nanomaterials play a critical role in determining their optical, electrical and mechanical properties [16][17][18].While conventional techniques such as high-temperature quenching are used for tuning the properties of metal oxide nanomaterials through the engineering of point defects [19,20], electron-beam irradiation [21,22] and laser irradiation [23,24] have emerged as two new techniques for defect engineering since they can be used to adjust and optimize the defect concentration at specific locations within the nanostructure.The introduction of point defects through laser irradiation has been reported in both n and p-type metal oxide nanomaterials [23][24][25].One major strategy produces point defects by exposing the metal oxide nanomaterials in liquid to pulsed laser irradiation [26,27].For example, Lau et al [26].demonstrated that laser irradiation of ZnO and TiO 2 nanoparticles triggers both a pulsed laser melting in liquid (PLML) process and pulsed laser fragmentation in liquids (PLFL).Both PLML and PLFL have been shown to generate defects in the nanoparticles and result in improved photocatalytic activities [26].This is, however, accompanied by the generation of a high concentration of bulk defects during the cooling process.These defects can act as recombination sites for electron-hole pairs resulting in a reduction in photocatalytic efficiency [28,29].This may explain why it was found that additional thermal annealing after PLML resulted in better photocatalytic efficiency, as thermal annealing may remove some of the bulk defects in ZnO nanoparticles [26].On the other hand, laser irradiation of metal oxide nanomaterials in air can also generate point defects because of photodissociation of bonds in the irradiated material [30,31].This process can occur without significant thermal heating and melting, so that the nature of the point defects generated photochemically can be different from those introduced by PLML and PLFL.In both cases, the properties of point defects introduced by laser irradiation of metal oxides in air have not been thoroughly investigated.Many previous studies have focused on tailoring the concentration of the defects in these materials but there is little information currently available on whether these laser-induced defects are located at the surface or within the bulk of the metal oxide.Such information is critical in the optimization of the properties of the material for applications such as photocatalysis.Since the migration of defects is limited by activation energies and is therefore temperature dependent [32], it can be assumed that additional thermal energy introduced during laser irradiation will assist the diffusion of defects.Laser irradiation is then a two-step process involving the creation of defects followed by control over the migration of defects after production.
In this work, we demonstrate the capabilities of this twostep process in a study of the irradiation of ZnO nanorods (NRs) in air with nanosecond (ns) pulses from a 1064 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser.Laser irradiation is used to generate defects and then to modify the properties of these defects.We find that both laser fluence and exposure time (number of applied pulses) affect the concentration of defects as well as the overall heat input and temperature rise in ZnO NRs.A combination of these two processes can then be used to tailor the defect distribution and properties of defects in ZnO NRs.As an example of the efficacy of this laser-engineering technique, we have evaluated the use of this process in preparing ZnO NRs for the photocatalytic degradation of rhodamine B (RhB) dye.

Sample preparation and pulsed laser irradiation
20 mg of powder consisting of single crystalline ZnO nanorods (99.9%,US Research Nanomaterials Inc.) was dispersed in 2 ml of ethanol (99.9%,Sigma-Aldrich).The solution was sonicated for 10 min in an ultrasonic cleaner to achieve more uniform dispersion and then the mixture was deposited on a 2.5 × 2.5 cm quartz glass sheet by drop-cast.A Nd:YAG laser with a pulse duration of 4 ns and a wavelength of 1064 nm operating at 30 Hz was used for the irradiation.The diameter of the beam on the sample surface is ∼2.5 mm.Indexing grids were drawn on the back of the quartz glass sheet to divide it into 64 individual 3 mm × 3 mm cells and the ZnO nanorods in each cell were then irradiated individually.The laser fluences were 200, 240, and 300 mJ cm −2 while exposure times at each fluence were 1.5, 3.5, and 5 min.After laser irradiation, the irradiated nanorods were removed from the quartz glass sheet using a single-edged razor blade and collected in a petri dish.

Characterization
The structural properties of the ZnO nanorods were examined by scanning electron microscopy (SEM, Zeiss FESEM 1530), transmission electron microscopy (TEM, Talos 200X) and x-ray diffraction (XRD, PANalytical X'Pert Pro) while the optical properties were determined from diffuse reflectance spectroscopy with the integrating sphere installed (DRS, Shimadzu UV-2501) and photoluminescence spectroscopy with an excitation wavelength of 325 nm (Edinburgh Instruments FL920).The elemental composition and chemical bonding in the irradiated ZnO NRs were determined by Raman spectroscopy (Renishaw micro-Raman spectrometer) with an excitation wavelength of 633 nm and x-ray photoelectron spectroscopy (XPS, Thermo VG Microlab 350).Baseline characterization of all parameters was carried out for each sample before laser irradiation.

Photodegradation of RhB dye
5 mg of ZnO nanorod powder was weighted and deposited on the quartz glass first and their morphologies were examined by SEM.The size distribution was analyzed by ImageJ so that the average size and the content of the ZnO nanorods can be maintained approximately the same between different samples.The nanorod power was then dispersed in a 30 ml RhB dye solution whose concentration was 5 ppm.The mixture was then sonicated for 30 sec and then placed in an enclosure for 30 min under dark conditions to reach the adsorptiondesorption equilibrium [33,34].A magnetic stir bar was used to stir the mixture during this period.After 30 min, 1 ml of the mixture was taken from the solution and the UV-vis absorption spectrum was measured.Subsequently, the mixture was exposed to a xenon light source (PerfectLight).This illumination with broadband light was used for photocatalysis.During this procedure, the beaker containing the mixture was immersed in a water bath to minimize evaporation of the solution.1 ml of the mixture was extracted from the solution every four minutes and subjected to centrifugation at 10 000 rpm for 30 s to precipitate the ZnO nanorods so that they did not affect the measurement of the optical absorption of RhB.The degradation of RhB dye was studied from an analysis of these absorption spectra.To relate these absorption spectra to the concentration of RhB, solutions containing 1 ppm, 2 ppm, 3 ppm, 4 ppm and 5 ppm of RhB were prepared and evaluated.

Structural properties
Previous studies have shown that quenching ZnO nanomaterials after melting results in the production of many bulk defects [26,27], indicating that melting in ZnO nanomaterials strongly influences the overall defect distribution.Melting in nanostructures is generally not uniform due to the concentration of thermal energy in 'hotspots' during laser irradiation, so it is important to examine the morphology of ZnO NRs after irradiation with different fluences and exposure times to determine the locations where melting has occurred.Some SEM images of ZnO NRs after irradiation under different conditions are shown in figure 1.It can be seen (figures 1(a)-(c)) that no significant melting of ZnO NRs occurs after irradiation for up to 5 min at a fluence of 200 mJ cm −2 .On the other hand, ZnO NRs showed little change in morphology after irradiation at 240 mJ cm −2 for 1.5 (figure 1(d)) and 3.5 min (figure 1(e)) but extending the exposure time to 5 min generated globules at the end of some ZnO NRs (figure 1(f)).These globules are formed by localized melting at locations where the ZnO NRs experienced high temperatures excursions during laser irradiation.The delay in the appearance of melting (5 min versus 3.5 min) indicates that repetitive application of laser pulses eventually raises the overall temperature of the NRs.This occurs because the temperature of individual NRs stacks between laser pulses and cannot reset to 300 K [35,36].When the laser fluence is increased to ∼300 mJ cm −2 , melting occurs even for exposure times as short as 1.5 min and many globules appear in the nanostructure (figure 1(g)).This happens when the instantaneous peak temperature during one laser pulse exceeds the melting temperature of ZnO NRs.This temperature is likely less than the melting temperature of bulk ZnO due to surface energy effects in such nano-systems [37,38].This conclusion is supported by the appearance of extensive melting in the following irradiation for 3.5 (figure 1(h)) and 5 min (figure 1(i)).Under these conditions (high fluence and long irradiation time) some of the short nanorods are completely melted while others exhibit significant changes in shape.
To study the relation between the temperature in the ZnO NRs and the laser fluence, finite element method (FEM) simulations were conducted to estimate the temperature rise in the ZnO NWs due to the laser irradiation.Since the pulse duration of the ns laser is much longer than the electronphonon coupling time of ZnO, the lattice and the electronic systems are in thermal equilibrium during the laser pulse [39,40].It can then be assumed that incident laser energy is instantaneously converted into heat [41,42], so that heat transfer is described by the equation: where r (kg m −3 ) is the density of the material, ) is the source term at a depth z (m) in the material at time t (s), which is obtained from the laser intensity distribution and the Lambert-Beer law: where a (m −1 ) and R are the absorption coefficient and reflectivity of the material, respectively.I z t , ( ) (W m −2 ) is the laser intensity normal to the material surface at a depth z in the material at time t.This formalism assumes that an individual NR is irradiated with a plane wave incident normal to the long axis of the NR.Values of the relevant parameters used in simulations are given in table S1.The time-dependent temperature in an isolated ZnO NR during one laser pulse at fluences of 200, 240 and 300 mJ cm −2 is given in figures 2(a)-(c), respectively.In this approximation, which ignores the effect of aggregation of NRs and any concentration of the electric field in hotspots, the peak temperature within an individual ZnO NR reaches 1667 K at a laser fluence of 200 mJ cm −2 , 1907 K at a fluence of 240 mJ cm −2 , and 2248 K at a fluence of 300 mJ cm −2 which reaches the melting temperature of bulk ZnO [43].Given the assumptions inherent in these simulations, these data can only be used to show that thermal heating to temperatures close to the melting point within a NR during laser irradiation increases along with incident laser fluence.As noted above, the overall ZnO NR structure will also accumulate heat between laser pulses [35,36], so that localized melting may occur at specific locations as seen in figures 1(h) and (i).The simulation results reflecting such accumulation of heat after multiple laser pulses are given in figure S1 in the supporting information.It is obvious that the temperature in the ZnO NR elevates with the increase of exposure time.
To further investigate the morphological changes of the ZnO NRs irradiated under various conditions, TEM analysis was carried out and the high-resolution TEM (HRTEM) images are given in figure 3. Figure 3(a) shows the HRTEM image of the unprocessed ZnO NR and it is well-crystallized without noticeable defects.200 mJ cm −2 for 1.5 min barely changes the FWHM of the diffraction peaks.On the other hand, irradiation at 240 mJ cm −2 for 3.5 min results in noticeable increasement in the FWHM of all diffraction peaks.It is broadly accepted that this broadening in the FWHM is the combined effects of defects in the crystalline and the changes in the lattice size [45,46].Extending the exposure time to 5 min offsets these increasements in the FWHM and even causes reduction at an 2θ angle of 47.7°, indicating that the defects are possibly removed through the thermal annealing.Increasing the laser fluence to 300 mJ cm −2 while maintaining the exposure time of 3.5 min also increases the FWHM of all diffraction peaks but with smaller amplitudes.

Raman spectroscopy
Raman spectra of ZnO NRs after laser irradiation under different conditions are given in figure 5.The intense peak at ∼439 cm −1 represents the nonpolar E 2 high optical phonon mode associated with the hexagonal wurtzite structure [47,48], while the broad feature at ∼565 cm −1 is typically ascribed to defect modes involving oxygen vacancies while the peak at ∼775 cm −1 is the LA + LO mode with A1 symmetry.The effect of laser irradiation on the spectrum can be seen in figure 5. Previous studies have shown that a decrease in the strength of the E 2 high peak accompanied by a reduction in the energy of this peak indicates that the concentration of oxygen vacancies has increased [27].It can be seen in figure 5(a) that the intensity of the E 2 high peak is reduced as the exposure time at a fluence of 200 mJ cm −2 increases from 1.5 to 5 min while the peak energy decreases by up to 2 cm −1 compared to that obtained in the unprocessed sample.This suggests that the concentration of oxygen vacancies continues to increase with exposure under these conditions.When the fluence is increased to 240 mJ cm −2 , the intensity of the E 2 high peak decreases as before up to an exposure of 3.5 min (figure 5(b)), then resets to a value close to that of the unprocessed ZnO NRs when the exposure time reaches 5 min.This indicates that high exposure under irradiation at a fluence of 240 mJ cm −2 raises the temperature of the sample into the range where defects associated with oxygen vacancies can be annealed out.The annealing temperature in these ZnO NRs will be less than the melting temperature which is consistent with the images in figure 1(f) which shows that there is little melting of the sample after 5 min of irradiation at 240 mJ cm −2 .Since melting is evident when the ZnO NRs are irradiated at 300 mJ cm −2 (figures 1(g)-(i)), one would expect that the oxygen vacancy concentration would continue to be reduced on irradiation at this fluence.However, the spectra shown in figure 5(c) indicates that this process does not occur when the fluence is raised to 300 mJ cm −2 .While the intensity of the E 2 high peak decreases somewhat for exposures up to 3.5 min at this fluence, the intensity of this peak increases again for an exposure of 5 min.This complex behavior suggests that the oxygen vacancy concentration is the result of a balance between the defect formation rate driven by photo-absorption, and a reverse reaction involving thermally activated recombination.
At high photo-absorption rates (high fluence), this equilibrium shifts toward a higher oxygen vacancy concentration while the rate of defect formation is compensated by the enhanced recombination rate at higher temperatures.The recombination of defects in ZnO NRs has been shown to involve an activation energy associated with the migration of vacancies in the ZnO lattice [32].In a low dimensional system such as an NR this migration likely involves the diffusion of defects towards the surface since the formation energy of the defects are often lower in the surface regions [51].Recovery of the intensity of the E 2 high peak under high exposure conditions (300 mJ cm −2 , 5 min exposure) suggest that laser-induced annealing does not only require a high temperature, but that this increase in temperature also needs to be sustained for an extended period of time.

Optical properties
Further information on the nature of defects in ZnO can be obtained from optical spectra, as defect states introduce energy levels between the valence and conduction bands [52,53].Absorption features associated with these defect states can be readily characterized by DRS and photoluminescence (PL) spectroscopy.The diffuse reflection data from DRS has been converted into absorption by using Kubelka-Munk equation [54].Figure 6(a) shows the absorption spectrum of ZnO NRs irradiated at 200 mJ cm −2 for different times.The effect of laser irradiation is to introduce broadband absorption between 360 and 700 nm which increases with exposure time.Similar enhancements in broadband absorption can also be observed when ZnO NRs are irradiated for 1.5 and 3.5 min at a fluence of 240 mJ cm −2 (figure 6(b)).It can be seen that laser irradiation under these conditions increases the intensities of the absorption spectra in the visible region and shift the absorption edges to higher wavelengths, which are often considered as the result of increasing oxygen vacancies [55].However, these enhancements in absorption virtually disappear when the exposure time is extended to 5 min.This effect can be attributed to thermal heating of the nanostructures by laser irradiation which enables the removal of defect centers via annealing.As discussed in section 3.1, increasing the laser fluence to 300 mJ cm −2 also results in significant thermal heating in ZnO NRs even over short exposure times.Absorption spectra of ZnO NRs irradiated at 300 mJ cm −2 show some enhancement in broadband absorption when the exposure time is 1.5 min and this increases for an exposure of 3.5 min (figure 6(c)), but the broadband absorption between 360 and 700 nm has disappeared entirely at an exposure of 5 min.As discussed above, this behavior is postulated to arise as an equilibrium between the formation of defects by photo-absorption and the recombination of defects facilitated by thermal heating.
Bandgap narrowing caused by defects in ZnO and other oxides can be attributed to optical transitions involving charge carriers trapped at energy levels within the bandgap [56,57].These traps were verified and investigated by techniques such as photocurrent transients [23].For instance, et al were able to probe and determine these trap levels in ZnO using photocurrent transients under various light illuminations [58].On the other hand, the overall reduction in the effective bandgap can also be obtained from a Tauc plot of v h 2 ( ) ( ) a n versus hn [59], where v ( ) a is the absorption coefficient (m −1 ), h is Planck's constant and ν is the light frequency.Photoluminescence (PL) spectroscopy is also used to characterize states of defects in ZnO.The PL spectrum of ZnO nanostructures typically consists of a strong UV emission peak related to near-band edge transitions of excitons, together with a broad peak in the visible corresponding to emission from defect centers [60][61][62].Figure 7 shows PL spectra of ZnO NRs after laser irradiation under different conditions.To facilitate comparison, PL intensities are normalized to that of the UV emission peak.With this normalization, it is apparent that the intensity of the broad peak at visible wavelengths is only weakly affected by laser irradiation for 1.5 min at a fluence of 200 mJ cm −2 .The UV-to-vis ratio of the peak intensities decreases slightly from ∼9.1 to ∼8.3.On the other hand, the intensity of the broadband peak is dramatically enhanced after laser irradiation for 3.5 min at 240 mJ cm −2 and the UV-to-vis ratio decreases significantly to a value of ∼1.1.This increase in intensity in the visible region is accompanied by a shift in the peak of the emission band from ∼358 to ∼361 nm, which is often considered as the near-band-edge emission [63,64].These changes are consistent with a significant increase in defect concentration under these conditions and a corresponding decrease in bandgap energy [65][66][67][68].Extending the exposure time to 5 min, while maintaining the laser fluence at 240 mJ cm −2 decreases the amplitude of the broad PL peak due to a reduction of the defect concentration.An intermediate modification of the ZnO NR sample occurs after irradiation at a fluence of 300 mJ cm −2 for 3.5 min.And the UV-to-vis ratio of the peak intensities decreases to 3.2.The full dataset of PL spectra of ZnO NRs processed at more conditions is given in figure S3 in the supporting information.

XPS depth profiling
The elemental composition of defects in ZnO NRs and their depth profile can be obtained from XPS measurements.Highresolution spectra in the Zn 2p and O 1s regions at a depth of 0.5 nm in the ZnO samples after laser irradiation under different conditions are shown in figure 8.In the Zn 2p region, the peaks at binding energies of ∼1044.1 and ∼1021 eV are assigned to the Zn 2p 1/2 and Zn 2p 3/2 peaks of Zn 2+ oxidation state of ZnO, respectively [27].It can be seen that the binding energies of these peaks changed little after laser irradiation under various conditions and the binding energy difference between the doublets (23.1 eV) remained the same (figures 8(a)-(e)).In the O 1s region (figures 8(f)-(j)), the major peak centered at ∼530.1 eV corresponds that of oxygen bonded to Zn atoms i.e. lattice oxygen in the wurtzite structure [69,70].The second peak with binding energies of ∼532 eV can be deconvoluted to two peaks at ∼531.6 eV and ∼532.1 eV which are associated with oxygen vacancies and  chemically adsorbed oxygen species in the ZnO, respectively [70][71][72].The atomic percentage of different oxygen species is obtained by calculating the area of these peaks followed by normalization and the results at various depth are presented in table 1.The detailed XPS results in the O 1s region at various depths for these ZnO samples are presented in figure S2 and the depth profiling results of unprocessed ZnO sample are given in figure S3 as a reference of the baseline.Table 1(A) shows the percentage of oxygen atoms in each bonding environment at different depths in the ZnO after laser diation for 1.5 min at 200 mJ cm −2 .These data show that, while oxygen vacancies are concentrated in the surface (depth = 0 nm) and sub-surface regions (depth = 0.5-3 nm), they are also present deeper inside the samples (depth > 6 nm).The effect of laser irradiation on the concentration of these oxygen vacancies throughout the samples is clearly evident.The distribution of oxygen vacancies throughout the ZnO samples after laser irradiation can be attributed to the fact that ZnO is only weakly absorbing at the laser wavelength (1064 nm) allowing laser radiation to penetrate inside the sample.Table 1(B) demonstrates the percentage of each oxygen species at selected depths in the ZnO after laser irradiation for 3.5 min 240 mJ cm −2 .The fraction of oxygen vacancies in the surface and sub-surface regions (depth < 3 nm) is higher than the corresponding values shown in table 1(A).Under this set of laser irradiation conditions, the highest oxygen vacancy concentration (12.04%) appears at a depth of 1.5 nm.This is accompanied by a decrease in oxygen vacancy concentration at depths greater than 3 nm indicating that irradiated by laser under this condition is not effective in generating oxygen vacancies in the bulk region.Table 1(C) shows the distribution of oxygen vacancies when ZnO is irradiated for 5 min at a fluence of 240 mJ cm −2 .It is apparent that the concentration of oxygen vacancies is reduced at all locations while the percentage of lattice oxygen is larger at all depths compared to the percentages recorded in tables 1(A) and B. This effect is consistent with thermal annealing of oxygen vacancies induced by laser heating.Variations in the concentration of chemisorbed oxygen in these samples reflect changes in the surface activity of ZnO related to the concentration of oxygen vacancies, as it is well known that surface and sub-surface defects facilitate surface adsorption of atmospheric oxygen [27].Irradiation of ZnO samples at a higher fluence (300 mJ cm −2 ) dramatically increases the concentration of oxygen vacancy centers at depths up to 24 nm indicating that more bulk oxygen vacancies were generated after laser irradiation under these conditions and that these defects survive annealing (table 1(D)).
Based on results shown in the previous sections, we have developed a heuristic mechanism to trace the effect of laser irradiation on the production and annealing of defects in ZnO.The effect of laser irradiation at different fluence and exposure times can be pictured as shown schematically in figure 9.In the low excitation regime, the lattice temperature does not rise significantly (i.e.ZnO NRs processed at 200 mJ cm −2 for 1.5 min), intrinsic and laser-induced defect centers have limited mobility so that oxygen vacancies are distributed throughout the sample, both in the bulk and in the surface regions.When laser excitation is sufficient to exceed some thermal threshold such as the migration energy barriers of ZnO ( ∼1.7 eV) [51], either through an increase in incident laser fluence or by prolonging the exposure time, migration and recombination of defects is facilitated via an Arrhenius  activated reaction.The preferential migration of lattice defects toward the surface in ZnO NRs under these conditions is indicated by the study of Deng et al [51].This process can be associated with the second processing regime in which surface defects are dominant (i.e.ZnO irradiated at 240 mJ cm −2 for 3.5 min).If laser irradiation then continues for a longer time (e.g. 5 min), the temperature in the ZnO rises to the point at which annealing removes both surface and bulk defects.The last processing regime occurs when the temperature reaches or surpasses the melting point of ZnO NRs (i.e.irradiated at 300 mJ cm −2 for 3.5 min).In this case, the lattice is subject to localized softening or melting causing amorphization and a reduction in short range order [73,74], which causes re-distribution of defects.Because of the rapid cooling that occurs following excitation with ns laser pulses, defects can be frozen in both the bulk and also the surface of the NRs.A summary of these effects, relating laser fluence and exposure time to defect distribution, is given in figure 10.

Photocatalysis
The large database on the properties of the dye RhB makes this material ideal for studying the mechanism of the photodegradation of RhB in the presence of ZnO nanomaterials [75,76].Upon illumination with broadband light, photoexcited electrons migrate to the surface and combine with adsorbed oxygen to form the anion O ,

2
-while photo-generated holes will react with H 2 O or OH − to produce the OH • radical.These species are then active in the degradation of RhB dye molecules forming a variety of chemical products.The lifetime of the electron-holes pairs, as well as their proximity to the surface, is critical in determining the overall efficiency of the photo-degradation reaction.The formation of oxygen vacancies during laser irradiation introduces additional energy levels into the bandgap thereby enhancing the production of photo-generated charge carriers that assist in the degradation process [77].However, not all of the defects generated by laser processing assist in photocatalysis, as defect centers deeper in the ZnO NRs can act as traps for photogenerated carriers [78].Given this limitation, it is expected that only defects in the surface or near-surface regions will provide the charge carriers that result in the formation of OH • radicals and O 2 -anions at the surface.As a result, efforts have been made to adjust the defect concentration within the surface region relative to that of defects in the bulk region to obtain optimized performance in photocatalytic applications [79,80].As we have seen, the concentration and distribution of surface and bulk oxygen vacancies in ZnO NRs is temperature-dependent and can be controlled by laser irradiation parameters.The effect of laser irradiation on the photocatalytic properties of ZnO NRs has been evaluated by dispersing laser processed samples in 30 ml of a 5 ppm RhB solution.The photo-degradation of RhB can be ascertained by the effect on the visible absorption spectrum   irradiation conditions increases the concentration of oxygen vacancies within the ZnO NRs that act to trap photogenerated charge carriers.

Conclusions
We find that the laser irradiation of ZnO NRs under controlled conditions can be used to tailor the composition and distribution of lattice defect centers that are important in determining optical and photocatalytic properties.Control over these properties is obtained within a parameter space consisting of laser fluence and exposure time.Optimization of these properties has been used to prepare ZnO NRs with enhanced photocatalytic characteristics as demonstrated through controlled experiments on the photodegradation of RhB:ZnO NRs solutions.We show that ns laser irradiation of ZnO NR samples in air produces discrete temperature-dependent distributions of oxygen vacancies in surface and bulk sites as well as changes in morphology.Using standard analytical techniques, we have determined that these changes arise from a combination of the photochemical generation of defect centers together with thermal heating induced by a series of overlapping laser pulses.Four distinctive regimes containing different ratios of surface-to-bulk oxygen vacancies have been identified in irradiated samples offering the possibility of laser engineering ZnO NR materials to enhance specific material properties.As an example of this capability, the photo-degradation of RhB dye was carried out using laser-processed ZnO NRs as the catalyst.A laser processing window was identified that leads to substantial improvements in catalytic activity for the photodegradation of RhB in the presence of ZnO NRs.
Figure 3(b) shows the HRTEM picture of

Figure 1 .
Figure 1.SEM pictures of ZnO NRs irradiated for a range of times at different laser fluence.The dashed circles in (h) and (i) indicate regions of significant melting and morphological change.These appear only under conditions of extreme exposure to incident laser radiation.

Figure 3 (
d) presents the HRTEM image of the ZnO NR after irradiation at 300 mJ cm −2 for 3.5 min.The slightly rougher surface of the ZnO NR might be the result of melting and resolidification processes under this condition.The defects are apparent to see and they locate at regions as deep as 17 nm.The crystalline structures of ZnO nanorods irradiated under various conditions were also investigated by XRD and the results are shown in figure 4. The diffraction peaks located at 2θ angles of 32°, 34.5°, 36.2°,47.7°and 56.8°are corresponding to the (100), (002), (101), (102) and (110) planes of ZnO, respectively.These XRD patterns agree well with the previous reported values for ZnO wurtzite structures[27].It can be seen from figure4that the diffraction peaks do not experience any noticeable shifts after laser irradiation.On the other hand, the intensities of the diffraction peaks are compared in figure4(b).Irradiation at 200 mJ cm −2 for 1.5 min slightly decreases the intensities of all diffraction peaks while such decrease is more noticeable after laser irradiation at 240 mJ cm −2 and 300 mJ cm −2 for 3.5 min.Previous studies attributed this decrease in peak intensities to the degenerated crystalline caused by the introduction of defects[44].Extending the exposure time to 5 min while keeping the fluence at 240 mJ cm −2 reset the intensities to the values close to the unprocessed ZnO NRs, indicating that the defects were removed under this condition.The corresponding values of the full width at half maximum (FWHM) of the diffraction peaks are given in figure 4(c).Irradiation at

Figure 3 .
Figure 3. HRTEM images of (a) unprocessed ZnO NR, and ZnO NRs irradiated at (b) 240 mJ cm −2 for 3.5 min, (c) 240 mJ cm −2 for 5 min, and (d) 300 mJ cm −2 for 3.5 min.The defects are indicated by the red circles in the figure.

Figure 4 .
Figure 4. (a) XRD results of ZnO irradiated under conditions indicated in the figure.The corresponding (b) intensity and (c) FWHM of the diffraction peaks are also shown, respectively.

Figure 6 .
Figure 6.Absorption spectra of ZnO NRs samples before and after irradiation at fluences (a) 200 mJ cm −2 , (b) 240 mJ cm −2 and (c) 300 mJ cm −2 for different exposure times.Tauc plots showing the bandgap energy of the ZnO NRs before and after irradiation at fluences (d) 200 mJ cm −2 , (e) 240 mJ cm −2 and (f) 300 mJ cm −2 for different exposure times.

Figure 7 .
Figure 7. Photoluminescence spectra of ZnO NRs samples after laser irradiation under different conditions.

Table 1 .
(A) The percentage concentration of different oxygen species at various depth in the ZnO sample irradiated at 200 mJ cm −2 for 1.5 min.(B) The atomic percentage of different oxygen species at various depth in the ZnO NR irradiated at 240 mJ cm −2 for 3.5 min.(C) The atomic percentage of different oxygen species at various depth in the ZnO NR irradiated at 240 mJ cm −2 for 5 min.(D) The atomic percentage of different oxygen species at various depth in the ZnO NR irradiated at 300 mJ cm −2 for 3.5 min.Atomic percentage of oxygen species (%) Depth (nm)

Figure 9 .
Figure 9.The distribution of defects with respect to different regimes.

Figure 10 .
Figure 10.Correlation between laser parameters and the identified processing regimes for ZnO NR samples.